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Internship Report

June – July 2014

At

Idea Cellular Ltd, Noida

Subject 2G and 3G RF Planning

Department Network Planning

Supervisor Mr. Vishwas Yadav

Mentor Mr. Inderjeet Yadav

Submitted by:

Bhavyai Gupta, B.Tech. III Year, ECE

Delhi Technological University, Shahbad Daulatpur

Main Bawana Road, Delhi-42

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Certificate

This is to certify that the project report entitled “2G and 3G RF Planning” is a

bona fide record of Seminar submitted by Bhavyai Gupta as the record of

the work carried out by him under my guidance. It is being accepted in

fulfillment of the Summer Internship, in the department of Network Planning,

Idea, Noida.

Supervisor

Mr. Vishwas Yadav

Deputy General Manager

Network Planning

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Acknowledgement

Simply put, I could not have done this work without the lots of help I received

cheerfully from the whole of Idea. The work culture in Idea really motivates.

Everybody is such a friendly and cheerful companion here that work stress is

never comes in way.

I would specially like to thank Mr. Manish Rastogi, the AGM – HR of Idea,

Noida for providing me a platform for the internship. For me it was a unique

experience to be in Idea.

I would also like to thank Mr. Vishwas Yadav for providing the nice ideas to

work upon. I am also highly indebted to my mentor Mr. Inderjeet Yadav, who

seemed to have solutions to all my problems.

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Abstract

There are now over 500 million GSM users worldwide. GSM is the most widely

used network service.

This report covers the basics of GSM and related technologies and their

architectures. The concentration of this report is the Network Planning, how

the GSM network is planned, how site is surveyed and installed. Then the

optimization of the network is briefed.

Then our focus shift towards the evolving technologies and the architecture

of 3G.

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Table of Contents

S No. Topic Page No.

01 Introduction to GSM 7

02 Introduction to TDMA 9

03 Evolution of GSM 10

04 Open Interfaces in GSM 11

05 Registration and Databases 12

06 2G Network Architecture 15

07 Network Switching Subsystem (NSS) 15

08 Base Station Subsystem (BSS) 21

09 Network Management System (NMS) 25

10 Channels 26

11 Access Technology and Modulation 30

12 GSM Frame Structure 33

13 Bursts 37

14 Signaling 38

15 OSI Model 45

16 Location Update 49

17 Call Set up in GSM 50

18 Handover 52

19 Charging 57

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S No. Topic Page No.

20 Services 60

21 Problems in Air Interface 63

22 Abis 66

23 Multiplexing 67

24 Network Planning 69

25 Optimization 95

26 3rd Generation 101

27 3G Network Structure 107

28 3G Network Architecture 112

29 Differences between 2G and 3G 116

30 Cell Site Visit 117

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Introduction to GSM

What is GSM?

Global System for Mobile communications (GSM) is a standard developed by

the European Telecommunications Standards Institute (ETSI) to describe

protocols for second generation (2G) digital cellular networks used by mobile

phones. It is the de facto global standard for mobile communications with

over 90% market share, and is available in over 219 countries and territories.

Originally, GSM stood for Groupe Spécial Mobile, a group formed by the

Conference of European Posts and Telegraphs (CEPT) in 1982 to research the

merits of a European standard for mobile telecommunications. The GSM is

now commonly known as Global System for Mobile.

The USA, South America, in general and Japan had made a decision to

adopt other types of mobile systems which are not compatible with GSM.

However, in the USA the Personal Communication System (PCS) has been

adopted which uses GSM technology with a few variations.

The GSM standard was developed as a replacement for first generation (1G)

analog cellular networks as it was developed using TDMA technology.

Objectives of GSM

At that time, the objectives of the GSM network were-

the system must be pan European

the system must maintain a good speech quality

the system must use radio frequencies as efficiently as possible

the system must have high/adequate capacity

the system must be compatible with other data communication

specifications

the system must contain good security concerning both subscriber and

transmitted information

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Advantages of GSM

uses radio frequencies efficiently

new services offered compared to analog cellular systems

quality of speech is better than analog systems

encryption of speech

international roaming possible

lower prices due to competition

The system must be pan European

GSM Phase II+ Features

With improvements in computing and radio access technology, GSM offered

continuous improvement and more services. In 1995 the “Phase 2”

recommendations were frozen. The GSM 900 and GSM 1800 specifications

were merged and additional supplementary services were defined, the short

message service was improved and improvements in radio access and SIM

cards were introduced.

After the Phase 2 recommendations, GSM continues to evolve at full speed.

Many new features are being introduced to GSM and the number of

improvements is so large that together they are called "Phase 2+" features.

Support for dual band handsets

High Speed Circuit Switched Data (HSCSD) services

General Packet Radio Services (GPRS)

Support for hierarchical cell structures

Supplementary services support when roaming

Enhanced full rate coding

Enhancements to SMS

Call line identity and restriction

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Call waiting

Call hold

Multi party communication

Closed user groups

Advice of charge

Unstructured supplementary services for data for operator

Operator determined barring

Introduction to TDMA

What is TDMA?

Time division multiple access (TDMA) is a channel access method for shared

medium networks. It allows several users to share the same frequency

channel by dividing the signal into different time slots. The users transmit in

rapid succession, one after the other, each using its own time slot. This allows

multiple stations to share the same transmission medium while using only a

part of its channel capacity.

Characteristics of TDMA

Shares single carrier frequency with multiple users

Non-continuous transmission makes handover simpler

Slots can be assigned on demand in dynamic TDMA

Less stringent power control than CDMA due to reduced intra cell

interference

Pulsating power envelope: Interference with other devices

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Evolution of GSM

1982

CEPT initiated a new cellular system. The European Commission

(EC) issued a directive which required member states to reserve

frequencies in the 900MHz band for GSM to allow for roaming.

1985 CEPT made decision on time schedule and action plan.

1986 CEPT tested eight experimental systems in Paris.

1987 Memorandum of Understanding (MoU). Allocation of the

frequencies. [890-915 Uplink; 935-960 Downlink]

1988

European Telecommunications Standard Institute (ETSI) was

created includes members from administrations, industry and

user groups.

1989 Final recommendations and specifications for GSM Phase 1.

1990 Validation systems implemented and the 1st GSM World

congress in Rome with 650 participants.

1991 First official call in the world with GSM on 1st July.

1992

World’s first GSM network launched in Finland. By December

there were 13 networks operating in 7 areas. New frequency

allocation for GSM 1800 (DCS 1800). [1710-1785 Uplink; 1805-1880

Downlink]

1993

GSM demonstrated for the first time in Africa at Telkom '93 in

Cape Town. Roaming agreements between several operators

are established. By December 1993 there were 32 GSM networks

operating in 18 areas.

1994

The first GSM network in Africa was launched in South Africa. The

GSM Phase 2 data/fax bearer services were launched. By

December 1994 there were 69 GSM networks in operation. The

GSM World Congress was held in Madrid with 1400 participants

1995

There were 117 GSM networks operating around the world. Fax,

data and SMS roaming was implemented. The first GSM 1900

network is implemented in the USA.

1996 By December 1996 there were 120 GSM networks operating

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Open Interfaces in GSM

When an interface is open, it defines strictly what is happening through the

interface and what kind of functions must be implemented between the

interfaces.

The two truly open interfaces are-

between Mobile Station and Base Station, called Air Interface

between Mobile Services Switching Centre and Base Station Controller,

called A interface

Fig: Air Interface and A Interface

To prevent excessive load on a central system, intelligence is distributed

throughout the network by dividing the network into three separate

subsystems-

Network Switching Subsystem

Base Station Subsystem

Network Management Subsystem

The actual network needed for establishing calls is composed of the NSS and

the BSS. The BSS is responsible for radio path control and every call is

connected through the BSS. The NSS takes care of call control functions. Calls

are always connected by and through the NSS. The NMS is the operation and

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maintenance related part of the network and it is needed for the control of

the whole GSM network.

Mobile Station (MS)

Mobile Station (MS), or sometimes also called User Equipment (UE) is a

combination of terminal equipment and subscriber data. The subscriber data

is stored in a separate module called SIM (Subscriber Identity Module).

Registration and Databases

A connection through the mobile network is possible only if there is a point to

point connection between the caller and the person who is called. Therefore,

it is absolutely necessary that the network knows the subscriber’s location. The

network keeps track of the subscribers’ location with the help of various

databases.

Subscriber Identity Module (SIM)

It is an integrated circuit that securely stores the international mobile

subscriber identity (IMSI) and the related key used to identify and

authenticate subscribers on mobile telephony devices (such as mobile

phones and computers).

It is –

database inside the mobile

contains user specific information

identification number of the user

list of subscribed services

list of available networks

tools for authentication and ciphering

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storage for messages and contacts

PIN and PUK passwords

Each SIM is internationally identified by its integrated circuit card identifier

(ICCID). ICCIDs are stored in the SIM cards and are also engraved or printed

on the SIM card body during a process called personalisation.

The number is composed of the following subparts:

Issuer Identification Number

Individual account identification

Check digit

International Mobile Subscriber Identity (IMSI)

It is used to identify the user of a cellular network and is a unique

identification associated with all cellular networks. It is stored as a 64 bit field

and is sent by the phone to the network. It is also used for acquiring other

details of the mobile in the home location register (HLR) or as locally copied

in the visitor location register.

To prevent eavesdroppers identifying and tracking the subscriber on the

radio interface, the IMSI is sent as rarely as possible and a randomly

generated TMSI (Temporary Mobile Subscriber Identity) is sent instead. TMSI is

reallocated after every successful authentication verification.

IMSI is composed of the following subparts:

Mobile Country Code (MCC)

Mobile Network Code (MNC)

Mobile Subscriber Identification Number (MSIN)

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Mobile Subscriber ISDN Number (MSISDN)

It is the number used for routing calls to the subscriber. MSISDN is the number

normally dialled to connect a call to the mobile phone. A SIM has a unique

IMSI that does not change, while the MSISDN can change in time, i.e.

different MSISDNs can be associated with the SIM.

It is composed of the following subparts:

Country Code (CC)

National Destination Code (NDC)

Subscriber Number (SN)

MSISDN is not used to identifying subscribers because-

Country Code is of different length for different countries. A length

indicator would be needed.

MSISDN identifies the service used. Therefore one subscriber would

need several MSISDNs depending on the type of services used.

Mobile Station Roaming Number (MSRN)

The serving MSC/VLR generates a temporary MSRN and associates it with the

IMSI. The roaming number is used in initiating the connection and it has the

following structure-

It has following subparts –

Country Code (CC)

National Destination Code (NDC)

Subscriber Number (SN)

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2G Network Architecture

The GSM network is divided into three subsystems – Network Switching

Subsystem (NSS), Base Station Subsystem (BSS), and Network Management

Subsystem (NMS). These three subsystems, different network elements, form

the GSM network architecture.

Fig: 2G Network Architecture

Network Switching Subsystem (NSS)

NSS is the component of a GSM system that carries out call switching and

mobility management functions for mobile phones roaming on the network

of base stations. It is owned and deployed by mobile phone operators and

allows mobile devices to communicate with each other and telephones in

the wider public switched telephone network (PSTN). The architecture

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contains specific features and functions which are needed because the

phones are not fixed in one location.

Elements of NSS

1. Mobile Switching Center (MSC)

2. Home Location Register (HLR)

3. Variable Location Register (VLR)

4. Authentication Center (AC)

5. Equipment Identity Register (EIP)

Mobile Switching Center (MSC)

The mobile switching center (MSC) is the primary service delivery node for

GSM, responsible for routing voice calls and SMS as well as other services such

as conference calls, FAX and circuit switched data.

Fig: MSC

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The MSC sets up and releases the end-to-end connection, handles mobility

and hand-over requirements during the call and takes care of charging and

real time pre-paid account monitoring.

Gateway Mobile Switching Center (GMSC)

The Gateway MSC (G-MSC) is the MSC that determines which visited MSC the

subscriber who is being called is currently located at. It also interfaces with

the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed

through a G-MSC. The term is only valid in the context of one call since any

MSC may provide both the gateway function and the Visited MSC function,

however, some manufacturers design dedicated high capacity MSCs which

do not have any BSSs connected to them. These MSCs will then be the

Gateway MSC for many of the calls they handle.

Home Location Register (HLR)

The HLR is a central database that contains details of each mobile phone

subscriber that is authorized to use the GSM core network. There can be

several logical, and physical, HLRs per public land mobile network (PLMN),

though one international mobile subscriber identity (IMSI)/MSISDN pair can be

associated with only one logical HLR (which can span several physical nodes)

at a time.

The HLRs store details of every SIM card issued by the mobile phone operator.

Each SIM has a unique identifier called an IMSI which is the primary key to

each HLR record.

Data stored include –

1. MSISDN Mobile Subscriber ISDN Number

2. IMSI International Mobile Subscriber Identity

3. VLR address Current location of the Subscriber

4. Data Subscriber Data stored permanently

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Visitor Location Register (VLR)

The visitor location is a database of the subscribers who have roamed into

the jurisdiction of the MSC which it serves. Each main base station in the

network is served by exactly one VLR, hence a subscriber cannot be present

in more than one VLR at a time.

The data stored in the VLR has either been received from the HLR, or

collected from the MS. In practice, for performance reasons, most vendors

integrate the VLR directly to the V-MSC and, where this is not done, the VLR is

very tightly linked with the MSC via a proprietary interface. Whenever an MSC

detects a new MS in its network, in addition to creating a new record in the

VLR, it also updates the HLR of the mobile subscriber, apprising it of the new

location of that MS.

Data stored include –

1. IMSI International Mobile Subscriber Identity

2. LAC Location Area Code

3. Data Subscriber Data stored temporarily

4. MSRN Mobile Station Roaming Number

Authentication Center (AC)

The authentication center (AC) is a function to authenticate each SIM card

that attempts to connect to the GSM core network (typically when the

phone is powered on). Once the authentication is successful, the HLR is

allowed to manage the SIM and services described above. An encryption

key is also generated that is subsequently used to encrypt all wireless

communications (voice, SMS, etc.) between the mobile phone and the GSM

core network.

It is a procedure used in checking the validity and integrity of subscriber data.

Proper implementation of security in and around the AC is a key part of an

operator's strategy to avoid SIM cloning.

The authentication procedure is based on an identity key, K i that is issued to

each subscriber when his data are established in the HLR. The authentication

procedure verifies that the Ki is exactly the same on the subscriber side as on

the network side.

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Authentication is performed by the VLR at the beginning of every call

establishment, location update and call termination (at the called subscriber

side).

In order to perform the authentication, the trick is to compare the Ki stored in

the mobile with the one stored in the network without actually having to

transmit it over the radio air interface.

The GSM uses 3 algorithms for the purposes of authentication and ciphering.

These are A3, A5, and A8.

A3 – authentication

A5 – ciphering

A8 – generating ciphering key

A3 and A8 are located in the SIM module and in the Authentication Center

(AC). A5 is located in the MS and in the BTS.

The mobile subscriber is created in the Authentication Center, before he

starts to use the security functions. The following information is required in

creating the subscriber:

IMSI of the subscriber

Ki of the subscriber

algorithm version used

The same information is also stored in the Mobile Subscriber's SIM. The basic

principle of GSM security functions is to compare the data stored by the

network to the data stored in the subscriber’s SIM. The IMSI number is the

unique identification of the mobile subscriber. K i is an authentication key with

a length of 32 hexadecimal digits. The algorithms A3 and A8 use these digits

as a basic value in authentication.

The Authentication Center generates information that can be used for all the

security purposes during one transaction. This information is called an

Authentication Triplet.

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The authentication triplet consists of three numbers:

RAND

SRES

Kc

RAND – Random Number

SRES – Signed Response is a result that A3 produces

Kc – Ciphering key that A8 generates

A certain RAND inserted to the algorithms with a certain Ki always produces a

certain SRES and a certain Kc.

When the VLR has this kind of three-value combination and the Mobile

Subscriber authentication procedure is initiated, the VLR sends the random

number RAND through the BSS to the SIM in the mobile station. As the SIM has

(or it should have) exactly the same algorithms as used in triplet generation

on the network side, the RAND number that the SIM receives and inserts to

the algorithm should produce exactly the same SRES value as the one

generated on the network side.

The speech of the user and the ciphering key, Kc, are processed by the

ciphering algorithm (A5) which produces the coded speech signal.

Equipment Identity Register (EIP)

The equipment identity register is often integrated to the HLR. The EIR keeps a

list of mobile phones (identified by their IMEI) which are to be banned from

the network or monitored. This is designed to allow tracking of stolen mobile

phones. In theory all data about all stolen mobile phones should be

distributed to all EIRs in the world through a Central EIR.

The EIR data does not have to change in real time, which means that this

function can be less distributed than the function of the HLR. The EIR is a

database that contains information about the identity of the mobile

equipment that prevents calls from stolen, unauthorized or defective mobile

stations. Some EIR also have the capability to log Handset attempts and store

it in a log file.

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Functions of NSS

1. Call Control identifies subscriber, establishes call and clears after

conversation is over

2. Charging collects charging information about services used

and transfers it to Billing Center

3. Mobility maintains information about location of subscriber

Management

4. Signaling signaling with other networks and the BSS and PSTN

5. Data Handling permanent storage in HLR and variable data in VLR

6. Locating locates subscriber before establishing call

Subscriber

Base Station Subsystem (BSS)

The base station subsystem (BSS) is the section of a traditional cellular

telephone network which is responsible for handling traffic and signaling

between a mobile phone and the network switching subsystem. The BSS

carries out transcoding of speech channels, allocation of radio channels to

mobile phones, paging, transmission and reception over the air interface and

many other tasks related to the radio network.

Elements of BSS

Base Transceiver Station (BTS)

Base Station Controller (BSC)

Transcoder (TC)

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Base Transceiver Station (BTS)

The BTS contains the equipment for transmitting and receiving radio signals

(transceivers), antennas, and equipment for encrypting and decrypting

communications with the base station controller (BSC). Typically a BTS for

anything other than a picocell will have several transceivers (TRXs) which

allow it to serve several different frequencies and different sectors of the cell

(in the case of sectorised base stations).

A BTS is controlled by a parent BSC via the "base station control function"

(BCF). The BCF is implemented as a discrete unit or even incorporated in a

TRX in compact base stations. The BCF provides an operations and

maintenance (O&M) connection to the network management system (NMS),

and manages operational states of each TRX, as well as software handling

and alarm collection.

By using directional antennas on a base station, each pointing in different

directions, it is possible to sectorise the base station so that several different

cells are served from the same location. Typically these directional antennas

have a beamwidth of 65 to 85 degrees. This increases the traffic capacity of

the base station (each frequency can carry eight voice channels) whilst not

greatly increasing the interference caused to neighboring cells (in any given

direction, only a small number of frequencies are being broadcast). Typically

two antennas are used per sector, at spacing of ten or more wavelengths

apart. This allows the operator to overcome the effects of fading due to

physical phenomena such as multipath reception. Some amplification of the

received signal as it leaves the antenna is often used to preserve the

balance between uplink and downlink signal.

Picocell

A picocell is a small cellular base station typically covering a small area, such

as in-building (offices, shopping malls, train stations, stock exchanges, etc.), or

more recently in-aircraft. In cellular networks, picocells are typically used to

extend coverage to indoor areas where outdoor signals do not reach well, or

to add network capacity in areas with very dense phone usage, such as train

stations or stadiums. Picocells provide coverage and capacity in areas

difficult or expensive to reach using the more traditional macrocell

approach.

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Base Station Controller (BSC)

The BSC provides the intelligence behind the BTSs. Typically a BSC has tens or

even hundreds of BTSs under its control. The BSC handles allocation of radio

channels, receives measurements from the mobile phones, and controls

handovers from BTS to BTS (except in the case of an inter-BSC handover in

which case control is in part the responsibility of the anchor MSC, from where

handover has been initiated). A key function of the BSC is to act as a

concentrator where many different low capacity connections to BTSs (with

relatively low utilization) become reduced to a smaller number of

connections towards the MSC.

Fig: BSC

The databases for all the sites, including information such as carrier

frequencies, frequency hopping lists, power reduction levels, receiving levels

for cell border calculation, are stored in the BSC. This data is obtained directly

from radio planning engineering which involves modelling of the signal

propagation as well as traffic projections.

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Transcoder (TC)

To enable the efficient transmission of the digital speech information over the

radio Air Interface the digital speech signal is compressed.

For transmission over the air interface, the speech signal is compressed by the

MS to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm

is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP).

However, the standard bit rate for speech in the PSTN is 64Kbits/s. Therefore, a

converter has to be provided in the network to change the bit rate from one

to another. This is called Transcoder.

Functions of BSS

1. Radio Path BSS takes care of Radio resources like radio channel

Control allocation and quality of radio connection

2. BTS and TC BSCs maintain the BTS. BSC is capable of separating

Control BTS from network and collects alarm information

from BTS and TC.

3. Synchronization MSC synchronizes the BSC and BSC synchronizes BTS

associated with it. Synchronization is controlled by

BSC inside a BSS.

4. Interface Air and A interface signaling. MS must have a

Signaling connection through the BSS.

5. Connection b/w Connection may be of either signaling type or traffic

MS and NSS type.

6. Mobility Different cases of handovers

Management

7. Collection of Statistical Data is collected and sent to NMS for post

Data processing purposes.

A Base Station Subsystem is controlled by an MSC. Typically, one MSC

contains several BSSs. A BSS may consists of many cells.

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Cell is area covered by one or more frequency resources. Each cell identified

by Cell Global Identity (CGI). It has following subparts –

Mobile Country Code (MCC)

Mobile Network Code (MNC)

Location Area Code (LAC)

Cell Identity (CI)

Paging

Paging is a signal that is transmitted by all the cells in the Location Area (LA).

It contains the identification of the subscriber. All the mobile stations in the LA

receive the paging signal, but only one of them recognizes the identification

and answers to it. As a consequence of this answer, a point to point

connection is established.

Network Management System (NMS)

Its purpose is to monitor various functions and elements of the network for

slow and failing components. These tasks are carried out by the NMS/2000

which consists of a number of Work Stations, Servers and a Router which

connects to a Data Communications Network (DCN).

The operator workstations are connected to the database and

communication servers via a Local Area Network (LAN). The database server

stores the management information about the network. The communications

server takes care of the data communications between the NMS and the

equipment in the GSM network known as “Network Elements”.

These communications are carried over a Data Communications Network

(DCN) which connects to the NMS via a router. The DCN is normally

implemented using an X.25 Packet Switching Network.

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Functions of NMS

1. Fault Management Its purpose is to ensure the smooth operation of

the network and rapid correction of any kind of problems that are

detected. It provides the network operator with information about

the current status of alarm events and maintains a history database

of alarms.

2. Configuration Management It maintains up to date information

about the operation and configuration status of network elements.

It includes management of radio network, software and hardware

management of the network elements, time synchronization and

security operations.

3. Performance Management NMS collects measurement data from

individual network elements and stores it in a database. On the

basis of these data, the network operator is able to compare the

actual performance of the network with the planned performance

and detect both good and bad performance areas within the

network.

Channels

TDMA divides one radio frequency channel into consecutive periods of time,

each one called a "TDMA Frame". Each TDMA frame contains eight shorter

periods of time known as “Time Slots”. TDMA timeslots are called "Physical

Channels" as they are used to physically move information from one place to

another.

The radio carrier signal between the MS and the BTS is divided into a

continuous stream of timeslots which in turn are transmitted in a continuous

stream of TDMA frames.

When MS is turned on

1. The MS scans all the radio frequencies and measures them

2. It selects the frequency with the best quality and tunes to it

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3. With the help of synchronization signal in a TDMA Frame, the mobile

synchronizes itself to the network

The synchronisation information required by this process is broadcast by the

network and analysed by the mobile.

In terms of Channels, when MS is turned on,

Searches for BTS to connect to

Scans entire frequency band or uses list containing the allocated

carrier frequencies for this operator

Search for particular BCCH

And the BCCH contains-

Current LA identity

Synchronization information

Network identity

Without above information, a MS cannot work with a network. This

information is broadcasted at regular intervals, leading to term Broadcast

Channels (BCH). All Broadcast Channels are downlink, and point to

multipoint.

1. FCCH- downlink, point to multipoint

BTS transmits a carrier frequency. MS identifies BCCH carrier by the

carrier frequency and synchronizes with the frequency.

2. SCH- downlink, point to multipoint

BTS transmits information about the TDMA frame structure in a cell and

BTS identity (Base Station Identity Code). MS synchronizes with the

frame structure within a particular cell, and ensures that the chosen BTS

is a GSM BTS. BSIC can only be decoded by an MS if the BTS belongs to

a GSM network.

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3. BCCH- downlink, point to multipoint

BTS broadcasts some general information such as LAI, maximum output

power allowed in the cell and the identity of BCCH carriers for

neighbouring cells. MS receives LAI and will signal to the network as

part of the Location Update procedure. MS sets its output power level

based on the information received on the BCCH. The MS stores the list

of BCCH carrier frequencies on which RX level measurement is done for

Handover Decision.

When the MS has finished analysing the information on a BCH, it then has all

the information required to work with a network. However, if it roams to

another cell, it must repeat the process of reading FCCH, SCH, and BCCH in

the new cell.

If the mobile subscriber then wishes to make or receive a call, then Common

Control Channels must be used.

1. PCH- downlink, point to point

BTS transmits a paging message to indicate an incoming call or short

message. The paging message contains the identity number of the

mobile subscriber that the network wishes to contact. At certain time

intervals, MS listens to the PCH. If it identifies its own mobile subscriber

identity number on the PCH, it will respond.

2. RACH- uplink, point to point

BTS receives access request from MS for call set-up, location update or

SMS. MS answers paging message on the RACH by requesting a

signalling channel.

3. AGCH- downlink, point to point

BTS assigns a signalling channel (SDCCH) to the MS. MS receives

signalling channel assignment (SDCCH).

At this stage, MS and BSS are ready to begin call set-up procedures. For this,

Dedicated Channels must be used.

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1. SDCCH- bi-link, point to point

BTS switches to the assigned SDCCH, used for call set-up signalling. TCH

is assigned on here. (SDCCH is also used for SMS messages to MS). The

MS switches to the assigned SDCCH. Call set-up is performed. The MS

receives a TCH assignment information (carrier and time slot).

2. CBCH- downlink, point to multipoint

BTS uses this logical channel to transmit short message service cell

broadcast. MS receives cell broadcast messages.

3. SACCH- bi-link, point to point

BTS instructs the MS on the allowed transmitter power and parameters

for time advance. SACCH is used for SMS during call. MS sends

averaged measurements on its own BTS (signal strength and quality)

and neighbouring BTS’s (signal strength). The MS continues to use

SACCH for this purpose during a call.

4. FACCH- bi-link, point to point

BTS transmits handover information. MS transmits necessary handover

information in access burst.

Once call set-up procedures have been completed on the control physical

channel, the MS tunes to a traffic physical channel. It uses the TCH logical

channel. Types of Traffic Channels-

1. Full Rate-

Transmits full rate speech (13Kb/s). It occupies one physical channel.

2. Half Rate-

Transmits half rate speech (5.6Kb/s). Two Half Rate TCH can share one

physical channel, thus doubling the capacity of a cell.

30

Another form of traffic channel is the Enhanced Full Rate (EFR) Traffic

Channel. The speech coding in EFR is still done at 13Kbits/s, but the coding

mechanism is different than that used for normal full rate traffic. Traffic

channels can transmit both speech and data and are bi-directional

channels.

Call to a MS

1. The MSC/VLR knows which LA the MS is located in. A paging message is

sent to the BSC that is controlling the LA.

2. The BSC distribute the paging message to the BTS in the desired LA. The

BTS transmits the message over the Air interface using PCH.

3. When the MS detects a PCH identifying itself, it sends a request for a

signalling channel using RACH.

4. The BSC uses AGCH to inform the MS of the signalling channel (SDCCH

and SACCH) to use.

5. SDCCH and SACCH are used for call set-up. A TCH is allocated and the

SDCCH is released.

6. The MS and BTS switch to the identified TCH frequency and time slot.

The MS generates ring tone. If the subscriber answers, the connection is

established. During the call, signals can be sent and received by the

MS using SACCH.

Access Technology and Modulation

In a mobile communications network, part of the transmission connection

uses a radio link and another part uses 2Mbit/s PCM links.

Radio transmission is used between the Mobile Station and the Base

Transceiver Station and the information must to be adapted to be carried

over 2Mbit/s PCM transmission through the remainder of the network.

31

Frequency Ranges of -

GSM 900 Uplink 890MHz to 915MHz

Downlink 935MHz to 960MHz

GSM 1800 Uplink 1710MHz to 1785MHz

Downlink 1805MHz to 1880MHz

Uplink – MS to BTS

Downlink – BTS to MS

Difference between GSM-900 and GSM-1800

GSM 900 provides 124 RF channels (channels numbers 1 to 124) spaced at

200 KHz. Duplex spacing of 45 MHz is used. Guard Bands 100 kHz wide are

placed at either end of the range of frequencies.

GSM 1800 provides 374 channels (channels number 512 to 885). Duplex

spacing is 95 MHz.

Radio transmission is used between the Mobile Station and the Base

Transceiver Station and the information must to be adapted to be carried

over 2Mbit/s PCM transmission through the remainder of the network.

In GSM 900 the duplex frequency (the difference between uplink and

downlink frequencies) is 45 MHz and in GSM 1800 it is 95 MHz

The total number of carriers in GSM 900 is 124, whereas in GSM 1800 the

number of carriers is 374.

Digital transmission in GSM is implemented using two methods known as

Frequency Division Multiple Access (FDMA) and Time Division Multiple Access

(TDMA).

Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access or FDMA is a channel access method

used in multiple-access protocols as a channelization protocol. FDMA gives

users an individual allocation of one or several frequency bands, or channels.

FDMA can be used with both analog and digital signal. Each user transmits

and receives at different frequencies as each user gets a unique frequency

slots.

32

Crosstalk may cause interference among frequencies and disrupt the

transmission.

Time Division Multiple Access (TDMA)

In the GSM system, the synchronization of the mobile phones is achieved by

sending timing advance commands from the base station which instructs the

mobile phone to transmit earlier and by how much. This compensates for the

propagation delay resulting from the light speed velocity of radio waves. The

mobile phone is not allowed to transmit for its entire time slot, but there is a

guard interval at the end of each time slot. As the transmission moves into the

guard period, the mobile network adjusts the timing advance to synchronize

the transmission.

Initial synchronization of a phone requires even more care. Before a mobile

transmits there is no way to actually know the offset required. For this reason,

an entire time slot has to be dedicated to mobiles attempting to contact the

network (known as the RACH in GSM).

The mobile attempts to broadcast at the beginning of the time slot, as

received from the network. If the mobile is located next to the base station,

there will be no time delay and this will succeed. If, however, the mobile

phone is at just less than 35 km from the base station, the time delay will

mean the mobile's broadcast arrives at the very end of the time slot. In that

case, the mobile will be instructed to broadcast its messages starting nearly a

whole time slot earlier than would be expected otherwise. Finally, if the

mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a

neighbouring time slot and be ignored.

It is this feature, rather than limitations of power, that limits the range of a GSM

cell to 35 km when no special extension techniques are used. By changing

the synchronization between the uplink and downlink at the base station,

however, this limitation can be overcome.

Each timeslot on a TDMA frame is called a Physical Channel. There are 8

physical channels per carrier frequency in GSM.

33

Gaussian Minimum Shift Keying (GMSK)

GSM uses a phase modulation technique over the air interface known as

Gaussian Minimum Shift Keying (GMSK).

In digital communication, Gaussian minimum shift keying or GMSK is a

continuous-phase frequency-shift keying modulation scheme. It is similar to

standard minimum-shift keying (MSK); however the digital data stream is first

shaped with a Gaussian filter before being applied to a frequency

modulator. This has the advantage of reducing sideband power, which in

turn reduces out-of-band interference between signal carriers in adjacent

frequency channels. However, the Gaussian filter increases the modulation

memory in the system and causes intersymbol interference, making it more

difficult to differentiate between different transmitted data values and

requiring more complex channel equalization algorithms such as an adaptive

equalizer at the receiver. GMSK has high spectral efficiency, but it needs a

higher power level than QPSK, for instance, in order to reliably transmit the

same amount of data.

The radio air interface has to cope with many problems such as variable

signal strength due to the presence of obstacles along the way, radio

frequencies reflecting from buildings, mountains etc. with different relative

time delays and interference from other radio sources. With such levels of

interference, complex equalisation techniques are required with GMSK.

GSM Frame Structure

Two types of Multi Frames –

26 TDMA frame multi frame – used to carry TCH, SACCH, and FACCH

51 TDMA frame multi frame – used to carry BCCH, CCCH, SDCCH, and

SACCH.

SDCCH is divided into 8 groups D0-D7 so that it can serve 8 MS concurrently.

A0-A7 are the corresponding SAACH channel groups which are used for TX.

Power Control, TA correction if necessary while the MS is located in SDCCH.

TDMA with 8 basic physical channels per carrier. The carrier separation is 200

kHz. A physical channel is therefore defined as a sequence of TDMA frames,

a time slot number, and a frequency hopping sequence.

34

The longest recurrent time period of the structure is called hyperframe and

has a duration of 12533.76 seconds.

1 hyperframe is divided into 2048 superframes which have a duration of 6.12

seconds. Superframe is divided into multiframes. 1 superframe has 1326 TDMA

frames.

There are four types of multi frames-

26 - multiframe (51 per superframe) with a duration of 120ms,

comprising of 26 TDMA frames.

51- multiframe (26 per superframe) with a duration of 235.

52 – multiframe (25.5 per superframe)

Duration of one slot =576.92 us

Duration of one frame =4.61 ms [4.61 / 8]

Duration of one 26-multiframe =120 ms [26 * 4.16]

Duration of one 51-multiframe =235.38 ms [51 * 4.16]

Duration of one Superframe =6.12 s [51 * 120]

Duration of one Hyperframe =12533.76 s [6.12 * 2048]

Fig: Channel Configuration

35

• Multiframe is used for distribution of logical channels

• Superframe is used for Mobile synchronization

• Hyperframe is used for signaling procedures and Ciphering

Fig: 26 Frame Traffic Channel Multiframe

The 12th frame (no. 13) in the 26-frame traffic channel multiframe is used by

the Slow Associated Control Channel (SACCH) which carries link control

information to and from the MS–BTS. Each timeslot in a cell allocated to traffic

channel usage will follow this format, that is, 12 bursts of traffic, 1 burst of

SACCH, 12 bursts of traffic and 1 idle.

36

The duration of a 26-frame traffic channel multiframe is 120ms (26 TDMA

frames) .When half rate is used, each frame of the 26-frame traffic channel

multiframe allocated for traffic will now carry two MS subscriber calls (the

data rate for each MS is halved over the air interface). Although the data

rate for traffic is halved, each MS still requires the same amount of SACCH

information to be transmitted, therefore frame 12 WILL BE USED as SACCH for

one half of the MSs and the others will use it as their IDLE frame, and the same

applies for frame 25, this will be used by the MSs for SACCH (those who used

frame 12 as IDLE) and the other half will use it as their IDLE frame.

Fig: 51 Frame Control Channel Multiframes

37

The 51-frame structure used for control channels is considerably more

complex than the 26-frame structure used for the traffic channels. The 51-

frame structure occurs in several forms, depending on the type of control

channel and the network provider’s requirements.

Bursts

Types of Bursts

1. Normal

Used to carry information on traffic and control channels.

2. Frequency Correction

Used for frequency synchronization of the mobile.

3. Synchronization

Used for frame synchronization of the mobile.

4. Access

Used for Random and Handover access.

5. Dummy

Used when no other channel requires a burst to be sent and carries no

information.

Bursts Used

Frequency Correction Burst FCCH

Synchronization Burst SCH

Access Burst FACCH and RACH

Normal Burst All Others

38

Signalling

Signalling in telecommunication systems is basically a set of messages used

for setting up, supervising and clearing the call.

Functions of Signalling

To set up a call

To supervise a call

To clear a call

Due to differences in signalling standards, the international governing body

for telecommunications, ITU, recommended the Channel Associated

Signalling System (CAS) as the standard.

Drawbacks of CAS

Suitable only for the cases where traffic is low

Not possible to send signalling in the absence of a call

Wastes bandwidth

Common Channel Signalling

The ITU came up with a new recommendation which was the Common

Channel Signalling System Number 7, abbreviated as SS7.

SS7 is a Common Channel Signalling System with a signalling path bandwidth

of 64Kbit/s. It is modular in design although the modules are not as clearly

defined as is the case with the OSI 7-layer model, which it pre-dates.

39

It consists of two parts- first part was responsible for transferring the message

within a signalling network and the second part was the user of these

messages.

Message Transfer Part (MPT) – responsible for transferring messages

Telephone User Part (TUP) – user of messages

Message Transfer Part (MTP)

The entire SS7 is built on the foundation of this MTP which consists of three sub-

layers.

Layer 1 Physical Connections

Lowest Level defines physical and electrical characteristics

Layer 2 Data Link Control

Mid-Level helps in error free transmission of signalling messages

between adjacent elements

Layer 3 Signalling Message Handling

Highest Level responsible for taking the message from any element in a

signalling network to any other element within the same

network

Fig: MTP Layers

40

Telephone User Part (TUP)

User who receives, sends, and acts on these messages.

Small variations in messages within one country were allowed, which were

now called National User Part (NUP).

With the introduction of the Integrated Services Digital Network (ISDN), which

has a broader capability than the PSTN, some extra sets of messages were

required. These became known as the ISDN User Part (ISUP). Whether it’s TUP,

NUP or ISUP they are all doing the same job in helping to set up a call.

Fig: Protocol stack of MTP and TUP/NUP/ISUP

Signalling Connection and Control Part (SCCP)

It was realised that the TUP/MTP combination alone was not sufficient when

"virtual connections" became necessary. MTP guarantees the transfer of

messages from any "signalling point" in the signalling network to any other

"signalling point", safely and reliably.

But, each message could reach the destination signalling point by using

different paths. This may cause situations where the order of messages that

are received, are different from the original sequence. When this order is

important, there is need for establishing a "virtual connection".

Virtual Connections use a "Connection Oriented" protocol that will provide

sequence numbers to enable the messages to be placed in the correct

order at the distant end.

41

MTP is capable of routing a message within one network only. The case of

setting up a call across multiple networks is not the same as signalling across

the same network. The signalling goes leg by leg according to the call. But in

the absence of a call, MTP cannot route a signalling message across multiple

networks.

Solution to above problems-

Creation of another protocol layer on top of MTP which was called the

Signalling Connection and Control Part (SCCP). SCCP takes care of virtual

connections and connectionless signalling.

TUP and SCCP both use services of MTP and hence parallel to each other.

At the moment there is no other protocol in SS7 for PSTN exchanges.

Fig: Location of SCCP

Other Applications of SS7 in GSM

A continuous tracking of the mobile station is required which results in what is

known as the Location Update procedure. Additional sets of standard

messages are required to fulfil the signalling requirements of GSM networks.

42

The additional protocol layers are-

1. Base Station Subsystem Application Part (BSSAP)

2. Mobile Application Part (MAP)

3. Transaction Capabilities Application Part (TCAP)

Base Station Subsystem Application Part (BSSAP)

It is used when an MSC communicates with the BSC and the MS. Since the MS

and MSC have to communicate via the BSC, there must be a virtual

connection, therefore the service of SCCP is also needed.

The authentication verification procedure and assigning a new TMSI all take

place with the standard sets of messages of BSSAP. Communication between

MSC and BSC also uses the BSSAP protocol layer.

Therefore, BSSAP serves two purposes-

MSC-BSC signalling

MSC-MS signalling

Fig: Location of BSSAP in SS7

43

Mobile Application Part (MAP)

While a mobile terminated call is being handled, the MSRN has to be

requested from the HLR without routing the call to HLR. Therefore, for these

cases another protocol layer was added to the SS7 called Mobile Application

Part (MAP). MAP is used for signalling communication between NSS elements.

The MSC-MSC communication using MAP is used only in case of non-call-

related signalling. For routing a call from one MSC to another MSC, TUP or

ISUP is still used.

Transaction Capabilities Application Part (TCAP)

In MAP signalling, one MSC sends a message to an HLR, and that message

requests (or invokes) a certain result. The HLR sends the result back, which

may be the final result or some other messages might also follow (or it might

not be the last result). These invocations and results that are sent back and

forth between multiple elements using MAP need some sort of secretary to

manage the transactions. This secretary is called the Transaction Capabilities

Application Part (TCAP).

Fig: MAP and TCAP

44

Protocol Name Function

MTP Message Transfer Part

Responsible for transferring an SS7

message from one network element to

another within the same signaling

network

TUP

NUP

ISUP

Telephone User Part

National User Part

ISDN User Part

User parts of MTP. They send, receive,

analyze and act on the messages

delivered by MTP. All of these are Call

Control Messages that help in setting up,

supervising and clearing a call

SCCP Signaling Connection

and Control Part

Protocol layer responsible for making

virtual connections and making

connectionless signaling across multiple

signaling networks

BSSAP Base Station Subsystem

Application Part

Protocol layer responsible for

communicating GSM specific messages

between MSC & BSC, and MSC & MS

MAP Mobile Application

Part

A GSM specific protocol for non-call-

related applications between NSS

elements

TCAP

Transaction

Capabilities

Application Part

Protocol layer responsible for providing

service to MAP by handling the MAP

transaction messages between multiple

elements.

SS7 Requirements for individual GSM elements

Protocol Stack in MSC

The MSC is the element in GSM networks which is responsible for call

control, therefore, TUP/ISUP sits on top of MTP for that purpose. The

MSC/VLR is also responsible for location updates and communication

with the BSC and the HLR. For this reason it also needs to have BSSAP

and MAP which sit on top of SCCP. The MSC also has TCAP to provide

services for MAP. It can be seen therefore, the MSC/VLR has all the SS7

protocol stacks implemented in it.

Protocol Stack in HLR

MTP, SCCP, TCAP and MAP as the signalling protocols in the HLR.

45

Protocol Stack in BSC

The BSC only needs BSSAP, but since BSSAP needs the services of the

SCCP which in turn needs the MTP, the BSC contains MTP, SCCP and

BSSAP.

Other Signalling Protocols in GSM

Between the BSC and the BTS, a signalling protocol is used known as LAP-D

(Link Access Procedure for the ISDN "D" channel). This is the same protocol

that is used in ISDN networks between the customer and the network.

Between the mobile station and the BTS, the same signalling protocol is used

with small modifications to cope with the characteristics of the radio

transmission medium. This protocol is known as LAP-Dm where the "m"

denotes modified.

The LAP-D message structure is similar to SS#7 but it does not support

networking capabilities, therefore, it is used for point to point connections.

Protocols for Radio Resource (RR) management are passed using LAP-Dm

and LAP-D. Other protocols for Mobility Management (MM) and Connection

Management (CM) are passed between the Mobile Station and the MSC.

A Virtual Connection uses packet type switching principles and the

connection only exists when packets or messages are being transferred. In

the simplest form of packet switching each packet is regarded as a

complete transaction in itself. This is known as “Connectionless” mode as

there is no sense of a connection being set up before communication begins

and the network treats each packet independently.

Open Systems Interconnection Model (OSI Model)

The Open Systems Interconnection model (OSI) is a conceptual model that

characterizes and standardizes the internal functions of a communication

system by partitioning it into abstraction layers.

46

The model groups communication functions into seven logical layers. A layer

serves the layer above it and is served by the layer below it. For example, a

layer that provides error-free communications across a network provides the

path needed by applications above it, while it calls the next lower layer to

send and receive packets that make up the contents of that path. Two

instances at one layer are connected by a horizontal connection on that

layer.

Fig: OSI Model

a) Physical Layer

It defines the electrical and physical specifications of the data

connection. It defines the relationship between a device and a

physical transmission medium (e.g., a copper or fibre optical cable).

This includes the layout of pins, voltages, line impedance, cable

specifications, signal timing, hubs, repeaters, network adapters, host

bus adapters (HBA used in storage area networks) and more.

It defines the protocol to establish and terminate a connection

between two directly connected nodes over a communications

medium.

It may define the protocol for flow control.

It defines a protocol for the provision of a (not necessarily reliable)

connection between two directly connected nodes, and the

modulation or conversion between the representation of digital data in

user equipment and the corresponding signals transmitted over the

47

physical communications channel. This channel can involve physical

cabling (such as copper and optical fiber) or a wireless radio link.

b) Data Link Layer

The data link layer provides a reliable link between two directly

connected nodes, by detecting and possibly correcting errors that

may occur in the physical layer.

The data link layer is divided into two sublayers:

Media Access Control (MAC) layer - responsible for controlling how

computers in the network gain access to data and permission to

transmit it.

Logical Link Control (LLC) layer - control error checking and packet

synchronization.

c) Network Layer

The network layer provides the functional and procedural means of

transferring variable length data sequences (called datagrams) from

one node to another connected to the same network. A network is a

medium to which many nodes can be connected, on which every

node has an address and which permits nodes connected to it to

transfer messages to other nodes connected to it by merely providing

the content of a message and the address of the destination node

and letting the network find the way to deliver ("route") the message to

the destination node.

d) Transport Layer

The transport layer provides the functional and procedural means of

transferring variable-length data sequences from a source to a

destination host via one or more networks, while maintaining the quality

of service functions.

The transport layer controls the reliability of a given link through flow

control, segmentation/desegmentation, and error control. Some

protocols are state- and connection-oriented. This means that the

transport layer can keep track of the segments and retransmit those

that fail. The transport layer also provides the acknowledgement of the

successful data transmission and sends the next data if no errors

48

occurred. The transport layer creates packets out of the message

received from the application layer. Packetizing is a process of dividing

the long message into smaller messages.

e) Session Layer

The session layer controls the dialogues (connections) between

computers. It establishes, manages and terminates the connections

between the local and remote application. It provides for full-duplex,

half-duplex, or simplex operation, and establishes checkpointing,

adjournment, termination, and restart procedures. The OSI model

made this layer responsible for graceful close of sessions, which is a

property of the Transmission Control Protocol, and also for session

checkpointing and recovery, which is not usually used in the Internet

Protocol Suite. The session layer is commonly implemented explicitly in

application environments that use remote procedure calls.

f) Presentation Layer

The presentation layer establishes context between application-layer

entities, in which the application-layer entities may use different syntax

and semantics if the presentation service provides a mapping between

them. If a mapping is available, presentation service data units are

encapsulated into session protocol data units, and passed down the

TCP/IP stack.

This layer provides independence from data representation (e.g.,

encryption) by translating between application and network formats.

The presentation layer transforms data into the form that the

application accepts. This layer formats and encrypts data to be sent

across a network. It is sometimes called the syntax layer

g) Application Layer

The application layer is the OSI layer closest to the end user, which

means both the OSI application layer and the user interact directly with

the software application. This layer interacts with software applications

that implement a communicating component. Such application

programs fall outside the scope of the OSI model. Application-layer

functions typically include identifying communication partners,

determining resource availability, and synchronizing communication.

When identifying communication partners, the application layer

determines the identity and availability of communication partners for

an application with data to transmit. When determining resource

49

availability, the application layer must decide whether sufficient

network or the requested communication exists.

Location Update

MS constantly receives information sent by the network, which includes ID of

VLR address of current area. MS stores that ID. Every time ID is broadcasted,

MS compares the ID stored with the new ID. Whenever there is a change, MS

sends a registration enquiry to the area it has just entered. The network

registers the MS in new VLR area and the HLR is informed about the new VLR

address.

There are 3 types of location updates-

location registration

generic

periodic

Location Registration takes place when MS is turned on. It is also called IMSI

attach because as soon as MS turns on, it informs VLR that it is back in service.

As a result, network sends MS LAI (Location Area Identity Number) and TMSI

(Temporary Mobile Subscriber Identity Number). TMSI is transmitted so that

IMSI is not transmitted over Air Interface for security reasons.

Generic location update is performed if the stored LAI is different from the

received LAI (MS keeps receiving data through control channels). The MS

starts a Location Update process by accessing the MSC/VLR that sent the

location data. A channel request message is sent that contains the subscriber

identity and LAI stored in SIM card.

When the target MSC/VLR receives the request, it reads the old LAI which

identifies the MSC/VLR that has served the mobile phone up to this point. A

signaling connection is established between the two MSC/VLRs and the

subscriber’s IMSI is transferred from the old MSC to the new MSC. Using this

IMSI, the new MSC requests the subscriber data from the HLR and then

updates the VLR and HLR after successful authentication.

50

Periodic Location Update carried out when network does not receive any

location update from the MS in specified time. If the subscriber is moving

within a single location area, there is no need to send a location update

request.

The network broadcasts the timer value so that a MS knows the periodic

location update timer values. Therefore, when the set time is up, the MS

initiates a registration process by sending a location update request signal.

The VLR receives the request and confirms the registration of the mobile in the

same location area.

Locating the Subscriber

GMSC is connected to the serving MSC/VLR. Now we have to set-up

connection to the called subscriber. Since the exact location of the called

subscriber is unknown, we have to conduct an entire search in the MSC/VLR

area unless area is divided into smaller areas. Therefore, the MSC/VLR area is

divided into smaller areas. These are called Location Areas (LA) and they are

managed by the MSC/VLR.

Each LA is identified by a Location Area Identity (LAI). Location Area Identity

Code has following subparts –

Mobile Code Country (MCC)

Mobile Network Code (MNC)

Location Area Code (LAC)

Call Set-up in GSM

1. MSISDN is dialed.

2. PSTN analyzes MSISDN. Result of analysis is the routing information

required to find mobile network. Mobile network is identified on the

basis of NDC, and then PSTN accesses nearest GMSC.

3. GMSC analyzes MSISDN. Result of analysis is that GMSC obtains HLR

address of the subscriber. GMSC sends MSISDN to HLR.

51

4. HLR determines current location of subscriber, as it has VLR address of

the subscriber

5. HLR interrogates the MSC/VLR that is currently serving MS/Subscriber.

Interrogation is done instead of connecting right away so that-

avoid setting call to a switched off MS

we need to have information that enables the GMSC to route the

call to the target MSC.

6. The servicing MSC/VLR is the destination of the call in terms of routing.

The servicing MSC/VLR generates a temporary MSRN and associates it

with the IMSI. This MSRN is used in initiating the connection.

7. MSRN and MSISDN have same structure but used for different purposes.

MSISDN is used for interrogating HLR whereas is the response given by

the servicing MSC/VLR and is used for routing the call.

MSRN-

identify the subscriber

points to exchange so that all intermediate exchanges know where

the call is routed

8. MSC/VLR sends MSRN to HLR. HLR does not interrogates MSRN because

MSRN is used for traffic transactions and HLR does not handle traffic.

HLR forwards MSRN to GMSC.

9. GMSC analyzes the MSRN. MSRN identifies the location of the called

subscriber. Result of this analysis is a routing information which identifies

the destination of the call.

10. The final phase of the routing process is taken care of by the serving

MSC/VLR. In fact, the serving MSC/VLR also has to receive the roaming

number so that it knows that this is not a new call, but one that is going

to terminate here – i.e. a call to which it has already allocated an

MSRN. By checking the VLR, it recognizes the number and so it is able

to trace the called subscriber

52

11. To locate the subscriber, a Paging process is initiated in the Location

Area. The mobile phone of the called subscriber recognizes the paging

signal and answers it.

Fig: Simplified Steps in Call Set-up

Handover

Handover or Handoff refers to the process of transferring an ongoing call or

data session from one channel connected to the core network to another

channel.

Purpose for handover

when the phone is moving away from the area covered by one cell

and entering the area covered by another cell the call is transferred to

the second cell in order to avoid call termination when the phone gets

outside the range of the first cell

53

when the capacity for connecting new calls of a given cell is used up

and an existing or new call from a phone, which is located in an area

overlapped by another cell, is transferred to that cell in order to free-up

some capacity in the first cell for other users, who can only be

connected to that cell

when the channel used by the phone becomes interfered by another

phone using the same channel in a different cell, the call is transferred

to a different channel in the same cell or to a different channel in

another cell in order to avoid the interference

The decision to perform a handover is always made by the BSC that is

currently serving the subscriber, except for the handover for traffic reasons. In

case of traffic reasons, the MSC takes the decision.

Factors Determining Handovers

Reported by Mobile on SACCH uplink –

RX Level Downlink

RX Quality Downlink

Reported by BTS –

RX Level Uplink

RX Quality Uplink

Timing Advance

Possible Additional Factors –

BTS Load

Recent Handovers

Neighbor Priority

54

Timing advance

Timing advance value corresponds to the length of time a signal takes to

reach the base station from a MS. Each user transmits periodically for less

than one-eighth of the time within one of the eight timeslots. Since the users

are at various distances from the base station and radio waves travel at the

finite speed of light, the precise arrival-time within the slot can be used by the

base station to determine the distance to the mobile phone. The time at

which the phone is allowed to transmit a burst of traffic within a timeslot must

be adjusted accordingly to prevent collisions with adjacent users. Timing

Advance (TA) is the variable controlling this adjustment.

Types of Handovers

1. Intra Cell – Intra BSC

Subscriber is handed over to another traffic channel (generally in

another frequency) within the same cell. In this case the BSC controlling

the cell makes the decision to perform handover.

2. Inter Cell – Intra BSC

The subscriber moves from cell 1 to cell 2. In this case the handover

process is controlled by BSC. The traffic connection with cell 1 is

released when the connection with cell 2 is set up successfully.

3. Inter Cell – Inter BSC

The subscriber moves from cell 2 to cell 3, which is served by another

BSC. In this case the handover process is carried out by the MSC, but,

the decision to make the handover is still done by the first BSC.

4. Inter MSC

The subscriber moves from a cell controlled by one MSC/VLR to a cell in

the domain of another MSC/VLR. The MSC/VLR currently serving the

subscriber (also known as the anchor MSC), contacts the target

MSC/VLR and the traffic connection is transferred to the target

MSC/VLR. As both MSCs are part of the same network, the connection

is established smoothly. It is important to notice, however, that the

target MSC and the source MSC are two telephone exchanges. The

call can be transferred between two exchanges only if there is a

telephone number identifying the target MSC.

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Types of Handovers

Other classification of Handovers can be –

1. Preventive

Improve call quality or minimize interference

Power Budget – Based on Downlink Signal Levels

Distance – Based on Timing Advance

2. Rescue

Prevent dropped calls or poor quality calls

Level – Triggered by low signal on either downlink or uplink

Quality – Triggered by poor quality on either downlink or uplink

3. Power Budget

Based on downlink signal levels

Uses power budget handover margin

If neighbor is better than serving cell by more than the margin, initiate

handover

In a well performing system, most handovers will be caused by power

budget

4. Distance

Based on timing advance

Uses distance handover margin – can be negative

If serving cell timing advance is greater than a set distance, and

neighbor cell is better than the serving cell by more than the margin,

then initiate handover

Distance handovers are not generally used extensively, as they can

cause ping-ponging

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5. Level

Triggered by both uplink and downlink

Uses level handover margin – generally set less than power budget

margin

If serving cell RX Level (uplink and downlink) is lower than a set value,

and neighbor cell is better than the serving cell by more than the

margin, then initiate handover

Level handovers are intended to handover the call before the signal

level gets so low that quality is affected

6. Quality

Triggered on both uplink and downlink

Uses quality handover margin – generally set less than level margin –

can be negative

If serving cell RX Quality (uplink and downlink) is lower than a set value,

and neighbor cell is better than the serving cell by more than the

margin, then initiate handover

Quality handovers are intended to handover the call before the quality

level gets so low that the customer notices

Handover Strategy

Power Budget margin is set to 6dB

Level margin is set to 3dB

Quality margin is set to 0dB

In high signal areas, Power Budget is always looking for a handover, but the

neighbor cell must be must stronger than the serving cell for the handover to

happen.

In low signal areas (worse than the level trigger), the neighbor cell only needs

to be somewhat stronger than the serving cell for the handover to happen.

In poor quality areas (worse than the quality trigger), the neighbor cell only

needs to be as strong as the serving cell for the handover to happen.

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This structure allows different behavior in different without extensive

optimization of each individual cell.

Handover Number (HON)

The anchor MSC/VLR receives the handover information from the BSS. It

recognizes that the destination is within the domain of another MSC and

sends a Handover Request to the target MSC via the signaling network. The

target MSC answers by generating a HON and sends it to the anchor

MSC/VLR, which performs a digit analysis in order to obtain the necessary

routing information. This information allows the serving MSC/VLR to connect

the target MSC/VLR. When the two MSCs are connected, the call is

transferred to a new route.

The Handover Number has following subparts –

Country Code (CC)

National Destination Code (NDC)

Subscriber Number (SN)

Charging

In addition to a standard fee, subscribers have to pay for the calls they make

and the services they use. The actual charging practices vary considerably

from one network operator to another.

1. Subscription Charge: To cover the costs of operations like receiving of

SIM card, recording of basic information to the HLR, network operators

often charge the subscriber an initial subscription charge.

2. Renting of Service: Subscriber is usually charged for the availability of

the network services and the right to use them. This is a regular fee

which is charged irrespective of whether the subscriber makes any calls

or not.

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3. Charge for using Network: list of parameters that can be used as a

basis for charging the subscribers-

type of service e.g. Speech, SMS

duration of call

time of day the call was made

destination of call

origin of call

use of network e.g. the PSTN

use of supplementary services like call barring, call forwarding

use of radio services

international roaming leg

Whom to Charge

If the called subscriber is registered in a location area belonging to his home

network, the connection is established as explained in the previous chapter

and the calling subscriber pays for the call.

If the called subscriber is outside the service area of his home network and is

connected to another network, then the call has to be routed to him using

the services of one or more foreign networks. In such a case, the charge will

be shared according to the following principle-

The calling subscriber pays for the connection to the number he dialed

The called subscriber pays for the international roaming leg.

International Roaming Leg refers to the connection between the home

network and subscriber via a foreign network.

The same principle is applied when the mobile subscriber has forwarded

incoming calls to another number. The called subscriber pays for the

forwarded call.

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Collect call is the case in which the called subscriber pays for the call. In the

Collect Call, called subscriber has to accept the call, after which he is

responsible for all the costs.

Charging Procedure

Charging is normally determined by collecting metering pulses, by which the

exchange can calculate the price of the call. It is called time charging.

The record containing the information about one chargeable event is called

the charging record. These records are stored primarily as charging files in the

MSC or HLR and then transferred to a separate billing center. The serving

operator controls the entire charging process. The process begins when a call

is set up and at the same time, a charging record is opened in the serving

MSC/VLR. In general the first and the last MSC involved in a call set up,

collect the charging record.

When the subscriber moves and inter MSC handover is performed, the

charging record is not transferred to the new MSC during handover. Instead,

first MSC keeps record of the call as long as it lasts.

When a sufficient number of charging records have been accumulated they

are sent to a billing center in one bulk via an X.25 or Ethernet connection.

Distributed Charging

In order to produce bills for each subscriber, Billing Centers should collect

detailed charging data from all the MSCs within the PLMN.

With International Roaming, this operation should be extended covering all

the PLMNs where a Roaming Contract is signed. Charging information must

be collected from the Billing Centers (BC) of all the networks that subscribers

have been visited and passed to the Billing Center of the home network.

When two GSM operators sign a “roaming contract”, they agree how often

they will transfer charging data between each other.

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Services

Services can be grouped as speech services, where the transmitted data is

speech and data services which covers the rest of the information types such

as text, facsimile (fax), etc.

Services can also be grouped as-

Basic Services which are individual functions and may be automatically

available and included in the basic rights of the subscriber as soon as

he registers

Supplementary Services which are extra services that are not included

as basic features, but are associated with the basic services by

enhancing and/or adding extra features to the basic services

When a user subscribes for more than one basic service, he will have a

different MSISDN for every basic service to which he subscribes.

Standard Classification of Services-

Teleservices which provide the full communication capacity by means

of terminals and network functions as well as those provided by

dedicated centers

Bearer Services which provide the capability of transmitting signals

between a GSM network access point and an appropriate access

point in the terminating network

Speech and Emergency Calls

These are the most common teleservices used in the GSM network.

Speech is also the basic service that each subscriber is guaranteed to. The

normal security procedures apply to all such calls except in the case of

emergency calls which are processed regardless of possible security

violations.

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Short Message Service (SMS)

The Short Message Service (SMS) is a service enabling the mobile subscriber

to receive and/or send short (max. 160 characters) messages in text format.

This service requires a dedicated equipment called Short Message Service

Center (SMSC) which may be located in the NSS or outside the GSM network,

but it always has signaling connections to MSC. The SMSC acts as a

temporary storing and forwarding center if the Mobile Station is unreachable.

The tasks of an SMSC can be described as

1. Reception of text messages (SMS) from wireless network users

2. Storage of text messages

3. Forwarding of

4. Delivery of text messages (SMS) to wireless network users

5. Maintenance of unique time stamps in text messages

When a user sends a text message (SMS message) to another user, the

message gets stored in the SMSC (short message service centre) which

delivers it to the destination user when they are available. This is a store and

forward option.

An SMS message is stored temporarily in the SMS centre if the recipient mobile

phone is unavailable. It is possible on most mobile handset to specify an

expiry period after which the SMS message will be deleted from the SMS

centre.

The SMS sender needs to set a flag in the SMS message to notify the SMS

centre that he wants the status report about the delivery of this SMS message.

This status report is sent to the SMS sender in the form of an SMS.

The services of SMSC are not required in cell broadcasting, as the BSC is

equipped with the necessary SMSC functions. The maximum length of a cell

broadcast SMS is 93 characters.

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Facsimile (Fax) Transmission

Facsimile transmission is a teleservice that sets requirements for terminal

equipment and their adaptation. There is one predefined case in which the

Mobile Station needs to be interfaced with a computer equipped with a fax

modem. However, because it is used for data transmission, there has to be a

provision for the bearer service in order to define the characteristics of the

bearer such as data transmission rate and Air Interface error correction

protocol.

In the case of T61 Facsimile transmission, the receiver is either not aware that

the incoming call is addressed to the fax and so he has to establish the

nature of the call by talking with the calling party first, or the receiver knows

that it is a facsimile call but still wants to talk with the calling party. In both

cases, the nature of the transmitted information is data (group 3 facsimile)

and speech alternately (during the same call).

The T62 automatic facsimile is an automatic fax service where the receiver

has a different MSISDN for the fax service and all calls to this number are

purely data transmission calls.

Supplementary Services

Supplementary services enhance or supplement the basic

telecommunication services.

Advice of Charge

Alternate Line Service

Barring of all incoming calls

Barring of all incoming calls when roaming outside the HPLMN

Barring of incoming calls when abroad

Barring of outgoing calls

Barring of outgoing International calls

Barring of outgoing international calls excluding those directed to the

HPLMN country

Call forwarding on mobile subscriber busy

Call forwarding on no answer

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Call forwarding unconditional

Call Hold

Call Waiting

Calling line identification presentation

Calling line identification restriction

Conference call

Explicit Call Transfer

Operator Determined Barring

Problems in Air Interface

Multi path Propagation

It is quite rare that there is a direct "line of sight" transmission between the

mobile station and the base transceiver station. In the majority of cases, the

signals arriving at the mobile station have been reflected from various

surfaces. Thus a mobile station (and the base transceiver station) receives the

same signal more than once. Depending on the distance that the reflected

signals have travelled, they may affect the same information bit or corrupt

successive bits.

Solutions to Multi path Propagation

1. Viterbi Equalisation –

This is generally applicable for signals that have been reflected from far

away objects. When either the BTS or MS transmits user information, the

information contained in the burst is not all user data. There are 26 bits

which are designated for a "training sequence" included in each TDMA

burst transmitted. Both the MS and BTS know these bits and by

analysing the effect the radio propagation on these training bits, the

air interface is mathematically modelled as a filter. Using this

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mathematical model, the transmitted bits are estimated based on the

received bits. The mathematical algorithm used for this purpose is

called "Viterbi equalisation".

2. Channel Coding –

Channel coding (and the following solutions) is normally used for

overcoming the problem caused by fading dips. In channel coding,

the user data is coded using standard algorithms. This coding is not for

encryption but for error detection and correction purposes and

requires extra information to be added to the user data. In the case of

speech, the amount of bits is increased from 260 per 20ms to 456 bits

per 20ms. This gives the possibility to regenerate up to 12.5% of data

loss.

3. Interleaving –

Interleaving is the spreading of the coded speech into many bursts. By

spreading the information onto many bursts, we will be able to recover

the data even if one burst is lost. (Ciphering is also carried out for

security reasons).

4. Frequency Hopping –

With Frequency Hopping, the frequency on which the information is

transmitted is changed for every burst. Frequency hopping generally

does not significantly improve the performance if there are less than

four frequencies in the cell.

Call is transmitted through several frequencies in order to

• average the interference (interference diversity)

• minimize the impact of fading (frequency diversity)

5. Antenna Receiver Diversity –

In this case two physically separated antennas receive and process the

same signal. This helps to eliminate fading dips. If a fading dip occurs at

the position of one antenna, the other antenna will still be able to

receive the signal. Since the distance between two antennas is a few

metres, it can only be implemented at the BTS.

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Shadowing

Hills, buildings and other obstacles between antennas cause shadowing (also

called Log Normal Fading). Instead of reflecting the signal these obstacles

attenuate the signal.

Shadowing is generally a problem in the uplink direction, because a BTS

transmits information at a much higher power compared to that from the MS.

Solution to Shadowing

The solution adopted to overcome this problem is known as adaptive power

control. Based on quality and strength of the received signal, the base station

informs the mobile station to increase or decrease the power as required. This

information is sent in the Slow Associated Control Channel (SACCH).

Propagation Delay

Information is sent in bursts from the mobile station to the Base Transceiver

Station (BTS). These bursts have to arrive at the base transceiver station such

that they have to map exactly into their allocated time slots. However, the

further away the mobile station is from the BTS then the longer it will take for

the radio signal to travel over the air interface. This means that if the mobile

station or base station transmits a burst only when the time slot appears, then

when the burst arrives at the other end, it will cross onto the time domain of

the next timeslot, thereby corrupting data from both sources. This problem is

called Propagation Delay.

Solution to Propagation Delay

The solution used to overcome this problem is called "adaptive frame

alignment". The Base Transceiver Station measures the time delay from the

received signal compared to the delay that would come from a mobile

station that was transmitting at zero distance from the Base Transceiver

Station. Based on this delay value, the Base Transceiver Station informs the

mobile station to either advance or retard the time alignment by sending the

burst slightly before the actual time slot. The base station also adopts this time

alignment in the down link direction.

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Abis

The transmission between the other network elements, in particular from Base

Transceiver Station to Base Station Controller (BSC) is called Abis.

Base Transceiver Station

It is physical site from where the radio transmission in both the downlink and

uplink direction takes place. The particular hardware element inside the Base

Transceiver Station (BTS) responsible for transmitting and receiving these radio

frequencies is appropriately named "Transceiver (TRX)". These TRXs are then

configured into one, two or three cells. If a BTS is configured as one cell it is

called an "Omnidirectional BTS" and if it is configured as either two or three

cells it is called a "Sectorized BTS". In an omnidirectional BTS the maximum

number of TRXs is ten, and in a sectorized BTS the maximum number of TRXs is

four per sector.

Transmission between BSC and BTS

There are three alternative methods to provide the connections between a

BSC and several BTSs. There are three options available: point-to-point

connection, multidrop chain and multidrop loop.

Point to point connection indicates that the Base Station Controller (BSC) is

connected directly to every BTS with a 2Mbit/s PCM line. This is a simple and

effective method particularly in cases when the distance between BSC and

BTS is short.

One PCM line has ample capacity to transfer data to several BTSs

simultaneously. Therefore, it is possible to draw just one BSC - BTS connection

and link the BTSs as a chain. This technique is called Multidrop Chain. The BSC

sends all the data in one 2Mbit/s PCM line and each BTS in turn analyses the

signal, collects the data from the correct timeslots assigned for itself and

passes the signal to the next BTS.

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Fig: BTS – BSC Connections

In Multidrop Loop, instead of a chain we connect the BTSs in the form of a

loop. The flow of the signal is similar to the signal flow in multidrop chain,

except that a BTS will change the “listening” direction if the signal from one

side fails. This ensures that the BTSs always receive information from the BSC

even if the connection is cut off at some point in the loop.

Multiplexing

According to GSM 900 and GSM 1800 specification, the bit rate in the air

interface is 13 Kbits/s and the bit rate at the Mobile Services Switching Centre

(MSC) and PSTN interface is 64 Kbits/s. This means that the bit rate has to be

converted at some point after the signal has been received by the BTS and

before it is sent to other networks.

The actual hardware which does the conversion from 13 Kbps to 64 Kbps and

vice versa is called a transcoder. In theory this piece of equipment belongs to

the Base Transceiver Station. However, by putting the transcoder at a

different place we can take some advantages in reducing the transmission

costs.

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If the transcoder is placed at the BTS site (in the BSC interface), then the user

data rate from BTS to Base Station Controller (BSC) would be 64 Kbps. The

transmission for this would be similar to standard PCM line transmission with 30

channels per PCM cable. The same would also apply between BSC and

MSC.

If we put the transcoder somewhere else, say just after MSC, then also we

cannot get significant advantage. This is because although after transcoding

the bit rate reduces to 13 kbps we still have to use the PCM structure to send

the traffic channels, with 8 bits per time slot. However since after transcoding

we have a bit rate of 13 Kbps and an additional 3 Kbps (making 16 Kbps)

only two bits per time slot will be used. The other 6 bits are effectively wasted.

Independent from its actual position, the transcoder belongs to the BSS even

if it is placed next to the MSC. (When the TC is placed away from the BTS it is

called a Remote TC according to the GSM recommendations).

We saw that from the MSC data comes out at 64Kbits/s rate and from the

Transcoder it comes out at 16Kbits/s. Each PCM channel (time slot) has 2 bits

of information. It appears that we are able to put in data from other 3 PCM

lines also here by multiplexing. However there are other issues as well such as

Common Channel Signalling information, OMC data and some other

network information which cannot be transcoded. Thus we are able to

multiplex 3 PCM lines and send 90 channels in one PCM line from MSC

(transcoder) towards the BSC. The BSC is able to switch 2 bits per time slot (or

1 bit) to the correct direction.

Fig: Transcoder and Sub multiplexer

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Network Planning

A good geographical coverage is the basis for providing network services.

Careful network planning is thus a primary aspect of implementing GSM

networks.

The goal is to achieve optimum use of resources and maximum revenue

potential whilst maintaining a high level of system quality. Full consideration

must also be given to cost and spectrum allocation limitations. A properly

planned system should allow capacity to be added economically when

traffic demand increases.

By doing a proper RF Planning by keeping the future growth plan in mind we

can reduce a lot of problems that we may encounter in the future and also

reduce substantially the cost of optimization. On the other hand a poorly

planned network not only leads to many Network problems, it also increases

the optimization costs and still may not ensure the desired quality.

Requirements that must be taken into consideration in the early stages of the

planning process:

1. Cost of building the network

2. Capacity of the network

3. Coverage

4. Maximum congestion allowed

5. Quality of calls

6. Further development of the network

The main steps of a Network Planning process are as follows:

1. Collection of all relevant information such as topographical map and

statistical books

2. Network Dimensioning based on coverage and capacity requirements

3. Selection of Base Station sites

4. Survey of intended sites

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5. Use of computer aided design system for coverage prediction,

interference analysis and frequency planning

Site Survey

A wireless site survey, sometimes called an RF site survey or wireless survey, is

the process of planning and designing a wireless network. The survey usually

involves a site visit to test for RF interference, and to identify optimum

installation locations for access points. This requires analysis of building floor

plans, inspection of the facility, and use of site survey tools. Interviews with IT

management and the end users of the wireless network are also important to

determine the design parameters for the wireless network.

A radio frequency (RF) site survey is the first step in the deployment of a

Wireless network and the most important step to ensure desired operation. A

site survey is a task-by-task process by which the surveyor studies the facility to

understand the RF behaviour, discovers RF coverage areas, checks for RF

interference and determines the appropriate placement of Wireless devices.

Why Site Survey is done?

To provide a wireless solution that will deliver the required wireless coverage,

data rates, network capacity, roaming capability and Quality of Service

(QoS).

In a Wireless network, many issues can arise which can prevent the radio

frequency (RF) signal from reaching all parts of the facility. Examples of RF

issues include mulitpath distortion, hidden node problems, and near/far

issues.

In order to address these, you need to find the regions where these issues

occur. A site survey helps you to do this. A site survey helps define the

contours of RF coverage in a particular facility. It helps us to discover regions

where mulitpath distortion can occur, areas where RF interference is high and

find solutions to eliminate such issues. A site survey that determines the RF

coverage area in a facility also helps to choose the number of Wireless

devices that a firm needs to meet its business requirements.

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How Site is surveyed?

Wireless site surveys are typically conducted using computer software that

collects and analyses WLAN metrics and/or RF spectrum characteristics.

Before a survey, a floor plan or site map is imported into a site survey

application and calibrated to set scale. During a survey, a surveyor walks the

facility with a portable computer that continuously records the data. The

surveyor either marks the current position on the floor plan manually, by

clicking on the floor plan, or uses a GPS receiver that automatically marks the

current position if the survey is conducted outdoors. After a survey, data

analysis is performed and survey results are documented in site survey reports

generated by the application.

All these data collection, analysis, and visualization tasks are highly

automated in modern software. In the past, however, these tasks required

manual data recording and processing.

Types of Site Survey

There are three main types of wireless site surveys: passive, active, and

predictive.

During a passive survey, a site survey application passively listens to

WLAN traffic to detect active access points, measure signal strength

and noise level. However, the wireless adapter being used for a survey

is not associated to any WLANs. For system design purposes, one or

more temporary access points are deployed to identify and qualify

access point locations. This used to be the most common method of

pre-deployment wifi survey.

During an active survey, the wireless adapter is associated with one or

several access points to measure round-trip time, throughput rates,

packet loss, and retransmissions. Active surveys are used to

troubleshoot wifi networks or to verify performance post-deployment.

During a predictive survey, a model of the RF environment is created

using simulation tools. It is essential that the correct information on the

environment is entered into the RF modeling tool, including location

and RF characteristics of barriers like walls or large objects. Therefore,

temporary access points or signal sources can be used to gather

information on propagation in the environment. Virtual access points

are then placed on the floor plan to estimate expected coverage and

adjust their number and location. The value of a predictive survey as a

design tool versus a passive survey done with only a few access point is

that modeled interference can be taken into account in the design.

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Depending on the survey type, a number of software and software/hardware

options are available to WLAN surveyors.

Software

Passive and active surveys are performed using software and typically require

only a compatible off-the-shelf Wi-Fi adapter; no additional specialized

hardware is required. Predictive surveys require no hardware at all, as no

wireless data collection is needed. Currently, professional-level site survey

applications exist primarily for Microsoft Windows. Some site survey

applications for other platforms, including iOS and Android, also exist,

however they are limited in functionality due to the limitations of the

underlying platform API. For example, signal level measurements cannot be

obtained on iOS without jailbreaking.

Hardware

Unlike passive and active surveys, RF spectrum surveys require specialized RF

equipment. There are various types of spectrum analyzers ranging from large

and expensive bench-top units to portable ("field units") and PC-based

analyzers. Because portability is a decisive factor in conducting wireless site

surveys, PC-based spectrum analyzers in CardBus and USB form factors are

widely used today. WLAN chipset manufacturers are starting to incorporate

spectrum analysis into their chipset designs; this functionality is integrated into

some high-end enterprise-class 802.11n access points

After all the installation sites have been surveyed, a detailed network plan

can be made. This includes the design of a transmission network which is

usually supplied by existing operators (leased PCM lines), or by microwave

links.

The radio environment has to be measured and tested to ensure its proper

operation and coverage after installation.

In sparsely populated areas we use powerful BTS’s which are usually mounted

on high ground to provide maximum unobstructed coverage to all directions.

This type of BTS is called Omnidirectional BTS.

In urban areas, where traffic volume is higher, the size of a cell is much smaller

and the distance between BTS’s is shorter. The standard type of BTS is also

different: the cell is divided into three sectors that have a few frequencies

each. This is called Sectorised BTS.

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Types of Towers –

1. Roof Top Tower (RTT)

Telecom service provider installs a cellular tower on the roof of a

building, paying the rent for the space used.

2. Roof Top Pole (RTP)

Telecom Service Provider installs a pole/antenna on the roof of a

building, paying the rent for the space used.

3. Pole (POL)

Telecom Service Provider pays the rent for the ground and erects his

pole from the ground level.

4. Cell On Wheels (COW)

Used where a very large temporary gathering is organized. For

temporary providing of signals.

Tools Used for RF Planning –

1. Network Planning Tool

2. Propagation Test Kit

3. Traffic Modelling Tool

4. Project Management Tool

Network Planning Tool

Network planning tool is used to assist engineers in designing and optimizing

wireless networks by providing an accurate and reliable prediction of

coverage, doing frequency planning automatically, creating neighbour lists

etc.

With a database that takes into account data such as terrain, clutter, and

antenna radiation patterns, as well as an intuitive graphical interface, the

Planning tool gives RF engineers a state-of-the-art tool to:

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Design wireless networks

Plan network expansions

Optimize network performance

Diagnose system problems

Propagation Test Kit

The propagation test kit contains of –

Test Transmitter

Antenna

Receiver to scan RX Level

Computer to collect data

GPS to get altitude and longitude

Cables and accessories

wattmeter

The transmitted power levels are then measured and collected by the Drive

test kit. This data is then loaded on the Planning tool and used for tuning

models.

Traffic Modelling Tool

Traffic modelling tool is used by the planning engineer for Network modelling

and dimensioning. It helps the planning engineer to calculate the number of

network elements needed to fulfil coverage, capacity and quality needs.

Project Management Tool

Though not directly linked to RF Design Planning, it helps in scheduling

the RF Design process and also to know the status of the project.

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Site database: This includes RF data, site acquisition, power, civil, etc.

Inventory Control

Fault tracking

Finance Management

MapInfo

MapInfo Professional is the tool used in Idea. It is a desktop geographic

information system (GIS) software product produced by Pitney Bowes

Software (formerly MapInfo Corporation) and used for mapping and location

analytics. MapInfo allows users to visualise, analyse, edit, interpret,

understand and output data to reveal relationships, patterns, and trends.

MapInfo allows users to explore spatial data within a dataset, symbolize

features, and create maps. It is used along with Google Earth to view clutter

and plan sites accordingly.

MapInfo Professional is used by proficient GIS users or analysts for complex

spatial analysis, building reports that describe their conclusions, and making

decisions based on those conclusions. It is used for a wide range of business

applications in many industries.

Features in Map Info –

1. Layering: One of the most frequently used features of MapInfo

Professional is its ability to combine data from widely different sources,

even with different formats and projections, in the same map window.

Once combined in the map window, relationships that only exist

geographically between the different data sets can be visualised and

queried. Layers can be vector and raster together.

2. Thematic Mapping: Allows the user to shade maps, present bar & pie

charts, graduated symbols, dot density, and grids. In addition, the Prism

thematic feature that allows regions of the map to be extruded to give

the impression of height.

3. SQL Selection with Geographic Extensions: Build and save SQL queries

that access and integrate data from multiple tables. Frequently

performed queries can be written once, re-used and distributed to

others.

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4. Charts & Graphs: Interactive graphs and charts including 3D, bubble,

column, histogram, surface, area, bar, line and pie scatter charts.

Select graph templates from thumbnail sketches. Graphing style

control includes position, tilt, rotation and pie explosion.

5. Hotlinks: Any object in a map can now contain a link to a document

(URL, .doc, .xls, .ppt, .tab, .wor, .mdb, etc.) that will automatically

launch when clicked.

6. 3D Visualization: 3D viewing allows for freehand tilt and rotations of

maps as well as for traditional panning and zooming.

7. Raster Image Support: Use raster images such as scanned paper maps,

satellite images, photographs and logos to provide detailed content

layers for your maps.

8. Linked Views: View and/or edit data in multiple linked views (including

rows and columns, graphs and maps) simultaneously.

9. Buffers around Objects: Perform detailed geographic searches with

buffering and area selection tools.

10. Geographic Searches: Integrate geographic criteria into database

queries (contains, intersects, within, etc.).

11. Drag and Drop Tool: Improve presentations by "dragging and

dropping" a map into other applications such as Microsoft Word, Excel

and PowerPoint and Corel Draw or export maps directly into

Photoshop.

12. Crystal Reports: Use the built-in report writer from Crystal Reports to

provide additional support for your visual analysis.

13. Conflict Management: Manage discrepancies in data when multiple

users write to server based data files.

14. Universal Translator: Translate bi-directional between MapInfo

Professional and other mapping environments including AutoCAD, ESRI

and Intergraph/Bentley. Formats include DWG, DXF, DGN, Shape and

E00 and so on.

15. Rotate Map Window Utility: Rotate the map window a specific number

of degrees.

16. Easy Loader: Upload MapInfo TAB files into database.

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MapInfo Snapshots

Fig: Area served by a particular BTS. Area in brown colour

is served by sector displayed in brown colour, and so on.

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Fig: Other BTS along with the their coverage areas

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Google Earth Snapshots

Fig: Direction of antennas and clutter served by respective sectors

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Fig: A more detailed picture showing coverage areas

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Fig: Easily distinguishable clutter and open areas

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Path Loss

The propagation attenuation, or path loss, is defined as the ratio between the

transmitted and received powers on each end of a radio link:

L = PT/PR

Path loss is defined in such a way that it is always greater than one. It is the

general formula for Path Loss.

Path loss can further be divided into two factors: free space loss and

additional loss. The free space loss comes from the fact that the power from

the transmitter radiates in all direction as a spherical wave.

Additional losses come from the fact that the radio waves are usually not

propagating in ideal free space. There is a nearby earth plane, precipitation

like rain, hail and snow, natural obstacles like hills, mountains and forests and

man-made obstacles like buildings and vehicles.

RX Level

RX Level means Received Level, it is the level which MS receives. It is

calculated by –

RX Level (dBm) = EIRP (dBm) – Path Loss (dB)

where EIPR is Effective Isotropically Radiated Power

EIRP (dBm) = Pt (dBm) – Lc (dB) + Ga (dBi),

where Pt is Output power of transmitter,

Lc is Cable Losses, and

Ga is Antenna Gain

Path Loss (dB) = 20 log(4πd) – 20 log(λ)

It is the formula of Path Loss used by radio and antenna engineers.

where d is distance, and

λ is wavelength

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Link Budget

A link budget is accounting of all of the gains and losses from the transmitter,

through the medium (free space, cable, waveguide, fiber, etc.) to the

receiver in a telecommunication system. It accounts for the attenuation of

the transmitted signal due to propagation, as well as the antenna gains,

feedline and miscellaneous losses. Randomly varying channel gains such as

fading are taken into account by adding some margin depending on the

anticipated severity of its effects. The amount of margin required can be

reduced by the use of mitigating techniques such as antenna diversity or

frequency hopping.

A simple link budget equation looks like:

Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

A link budget equation including all the effects,

PRX = PTX + GTX – LTX – LFS – LM + GRX - LRX

where, PRX = received power (dBm)

PTX = transmitter output power (dBm)

GTX = transmitter antenna gain (dBi)

LTX = transmitter losses (coax, connectors...) (dB)

LFS = free space loss or path loss (dB)

LM = miscellaneous losses (fading margin, body loss, polarization

mismatch, other losses...) (dB)

GRX = receiver antenna gain (dBi)

LRX = receiver losses (coax, connectors...) (dB)

Radio Propagation Model

A radio propagation model, also known as the Radio Wave Propagation

Model or the Radio Frequency Propagation Model, is an empirical

mathematical formulation for the characterization of radio wave

propagation as a function of frequency, distance and other conditions. A

single model is usually developed to predict the behaviour of propagation for

all similar links under similar constraints. Created with the goal of formalizing

the way radio waves are propagated from one place to another, such

models typically predict the path loss along a link or the effective coverage

area of a transmitter.

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As the path loss encountered along any radio link serves as the dominant

factor for characterization of propagation for the link, radio propagation

models typically focus on realization of the path loss with the auxiliary task of

predicting the area of coverage for a transmitter or modelling the distribution

of signals over different regions.

Because each individual telecommunication link has to encounter different

terrain, path, obstructions, atmospheric conditions and other phenomena, it

is intractable to formulate the exact loss for all telecommunication systems in

a single mathematical equation. As a result, different models exist for different

types of radio links under different conditions. The models rely on computing

the median path loss for a link under a certain probability that the considered

conditions will occur.

Different models have been developed to meet the needs of realizing the

propagation behaviour in different conditions.

a) Okumura Model

The Okumura model for urban areas is a Radio propagation model

that was built using the data collected in the city of Tokyo, Japan.

The model is ideal for using in cities with many urban structures but

not many tall blocking structures. The model served as a base for

the Hata Model.

Okumura model was built into three modes. The ones for urban,

suburban and open areas. The model for urban areas was built first

and used as the base for others.

Coverage

Frequency = 150–1920 MHz

Mobile station antenna height: between 1 m and 10 m

Base station antenna height: between 30 m and 1000 m

Link distance: between 1 km and 100 km

Mathematical formulation

The Okumura model is formally expressed as:

L = LFSL + AMU – HMG – HBG – ∑Kcorrection

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where, L = median path loss in dB

LFSL = free space loss in dB

AMU = Median attenuation in dB

HMG = Mobile station antenna height gain factor.

HBG = Base station antenna height gain factor.

Kcorrection = Correction factor gain (such as type of

environment, water surfaces, isolated obstacle

etc.)

Okumura's model is one of the most widely used models for signal

prediction in urban areas. This model is applicable for frequencies in

the range 150–1920 MHz (although it is typically extrapolated up to

3000 MHz) and distances of 1–100 km. It can be used for base-station

antenna heights ranging from 30–1000 m.

b) Hata Model

This model is based on Hata Model. . It is turn has developed separate

models for varying environments:

Hata Model for Urban Areas

Hata Model for Suburban Areas

Hata Model for Open Areas

Hata model for urban areas

In wireless communication, the Hata model for urban areas, also known as

the Okumura–Hata model for being a developed version of the Okumura

model, is the most widely used radio frequency propagation model for

predicting the behaviour of cellular transmissions in built up areas. This model

incorporates the graphical information from Okumura model and develops it

further to realize the effects of diffraction, reflection and scattering caused

by city structures. This model also has two more varieties for transmission in

suburban areas and open areas.

Applicable to/under conditions

This particular version of the Hata model is applicable to the radio

propagation within urban areas.

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This model is suited for both point-to-point and broadcast transmissions and it

is based on extensive empirical measurements taken.

PCS is another extension of the Hata model. The Walfisch and Bertoni model is

further advanced.

Coverage

Frequency: 150–1500 MHz

Mobile Station Antenna Height: 1–10 m

Base station Antenna Height: 30–200 m

Link distance: 1–10 km.

Mathematical formulation

The Hata model for urban areas is formulated as following:

LU = 69.55 + 26.16 log(f) – 13.82 log(hB) – CH + [44.9 – 6.55 log(hB)]log(d)

For small or medium sized city,

CH = 0.8 + (1.1 log(f) – 0.7)hM – 1.56log(f)

and for large cities,

CH = 8.29 [ log(1.54* hM) ]2 – 1.1 for 150 ≤ f ≤ 200

CH = 3.20 [ log(11.75* hM) ]2 – 4.97 for 200 ≤ f ≤ 1500

where LU = Path loss in urban areas in dB

hB = Height of base station antenna meters

hM = Height of mobile station antenna in meters

f= Frequency of transmission in MHz

CH = Antenna height correction factor

d = Distance between the base and MS in km

Hata model for suburban areas

The Hata model for suburban areas, also known as the Okumura–Hata model

for being a developed version of the Okumura model, is the most widely used

model in radio frequency propagation for predicting the behavior of cellular

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transmissions in city outskirts and other rural areas. This model incorporates the

graphical information from Okumura model and develops it further to better

suite the need. This model also has two more varieties for transmission in

urban areas and open areas.

The Hata model predicts the total path loss along a link of terrestrial

microwave or other type of cellular communications. And is a function of

transmission frequency and the average path loss in urban areas.

Applicable to/under conditions

This particular version of Hata model is applicable to the transmissions just out

of the cities and on rural areas where man-made structures are there but not

so high and dense as in the cities. To be more precise, this model is suitable

where buildings exist, but the mobile station does not have a significant

variation of its height. This model is suited for both point-to-point and

broadcast transmissions.

Coverage

Frequency: 150 MHz – 1.50 GHz

Mathematical formulation

Hata model for suburban areas is formulated as,

LSU = LU – 2[ log(f/28) ]2 – 5.4

where, LSU = Path loss in suburban areas in dB

LU = Average path loss in urban areas for small sized city in dB

f = Frequency of transmission in MHz

Hata model for open areas

The Hata model for open areas, also known as the Okumura–Hata model

from its origins in the Okumura model, is the most widely used model for

predicting the behavior of cellular radio transmissions in open areas. It further

exploits the graphical information from the Okumura model. Two additional

varieties for transmission are urban areas and suburban areas.

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The Hata model for open areas predicts the total path loss along a link of

terrestrial microwave or other type of cellular communications. It is a function

of transmission frequency and the median path loss in urban areas.

Applicable to/under conditions

This version of Hata model is applicable to the transmissions in open areas

where no obstructions block the transmission link. It is suited for both point-to-

point and broadcast transmissions.

Coverage

Frequency: 150 MHz to 1.5 GHz

Mathematical formulation

The Hata model for open areas is formulated as:

LO = LU – 4.78 [ log(f) ]2 + 18.33 log(f) – 40.94

where LO = Path loss in open area in dB

LU = Path loss in urban areas for small sized city in dB

f = Frequency of transmission in MHz

Dimensioning Cells

A cell is the basic ‘construction block’ of a GSM network. One cell is the

geographical area covered by one BTS. Cells are grouped under Base

Station Controllers (BSC).

Erlang is the measuring unit of network traffic. One Erlang equals the

continuous use of a mobile device for one hour.

X Erlangs = (Calls/hour) * (Avg Conversation Time)/3600 sec

Amount of traffic is independent of the observation duration.

If one hundred six-minute calls are received on a group of such circuits, then

the total traffic in that hour is six hundred minutes or 10 Erlangs.

When used to represent carried traffic, a value followed by “erlangs”

represents the average number of concurrent calls carried by the circuits (or

other service-providing elements), where that average is calculated over

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some reasonable period of time. The period over which the average is

calculated is often one hour, but shorter periods (e.g., 15 minutes) may be

used where it is known that there are short spurts of demand and a traffic

measurement is desired that does not mask these spurts.

When used to describe offered traffic, a value followed by “erlangs”

represents the average number of concurrent calls that would have been

carried if there were an unlimited number of circuits (that is, if the call-

attempts that were made when all circuits were in use had not been

rejected). The relationship between offered traffic and carried traffic

depends on the design of the system and user behavior. Three common

models are (a) callers whose call-attempts are rejected go away and never

come back, (b) callers whose call-attempts are rejected try again within a

fairly short space of time, and (c) the system allows users to wait in queue until

a circuit becomes available.

A third measurement of traffic is instantaneous traffic, expressed as a certain

number of Erlangs, meaning the exact number of calls taking place at a

point in time. In this case the number is an integer. Traffic-level-recording

devices, such as moving-pen recorders, plot instantaneous traffic.

Erlang-B (sometimes also written without the hyphen Erlang B), also known as

the Erlang loss formula, is a formula for the blocking probability that describes

the probability of call losses for a group of identical parallel resources

(telephone lines, circuits, traffic channels, or equivalent).

The Erlang B formula applies under the condition that an unsuccessful call,

because the line is busy, is not queued or retried, but instead really vanishes

forever. It is assumed that call attempts arrive following a Poisson process, so

call arrival instants are independent. Further, it is assumed that the message

lengths (holding times) are exponentially distributed (Markovian system),

although the formula turns out to apply under general holding time

distributions.

Grade of Service (GOS) is the maximum congestion allowed. Supposing that

GOS is 5 % - which means that during a certain observation period (usually 1

hour) 5 out of 100 calls fail due to lack of resources.

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Fig: Erlang B Table

Find number of channels required for communication if Erlangs and GOS is

known by using the Erlang B Table. Suppose the channels required are 16.

Since each carrier supports 8 channels, we can make estimation that this cell

must be equipped with 2 carriers, i.e., 2 TRX.

Frequency Reuse

There is a limited number of frequencies available to each Base Station

Subsystem and they must be distributed between the cells to ensure a

balanced coverage throughout the BSS.

The GSM network includes a specification of the Frequency reuse pattern.

The next step involves the dimensioning of the Location Areas. This is carried

out according to the traffic characteristics of each area. The final phase is

the dimensioning of the Fixed Network on the basis of the traffic requirements

and dimensioning of the entire radio network.

The elements that determine frequency reuse are the reuse distance and the

reuse factor.

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The reuse distance, D is calculated as

D = R (3N)1/3

Two types of sites-

Coverage Sites- for providing coverage of network

Capacity Sites- to share traffic

Traffic that can be handled by a particular site is determined by the number

of TRX in that BTS. In earlier days, there was a limit to the BTSs that there can

be maximum of 4 TRX in each sector of the BTS. In general, for 3 sectors, that

would mean 12 TRX in each BTS and hence 12 TRX at each site. In case to

handle more traffic, 2 BTS were installed in parallel, side by side. But newer

BTSs support more TRX in each sector, may be up to 8 TRX for each sector;

and hence 24 TRX at one BTS.

Each frequency is divided into 8 slots, each having one TCH of Full rate. If TCH

of Half rates are used, then each slot would have 2 TCH of half rate. Erlang B

Table consider TCH of Full rates only. Grade of Service (GOS) is the threshold

of percentage of Call Blocks.

Traffic that can be handled by one BTS with a particular number of TRX with

specified GOS (usually 2%) is determined by Erlang Table. It is theoretical

traffic value that BTS can handle. With the use of Half rate TCH, capacity to

handle traffic would increase, but the quality of service would decrease.

Idea implements Nokia Architecture in BSS. BTS that Idea use were

manufactured by a joint venture of Nokia and Seimens called Nokia Seimens

Network. But after the possession of Idea by Microsoft, it is done by Nokia

only.

It is very much possible that BSS is manufactured by some company and NSS

is manufactured by other company. In that case, NMS for both will be

different. But the BSS and the NSS will still be compatible so that they can

communicate with each other and the OSS.

Drive Testing

Drive Testing is a method of measuring and assessing the coverage, capacity

and Quality of Service (QoS) of a mobile radio network.

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Fig: 2G drive testing

The technique consists of using a motor vehicle containing mobile radio

network air interface measurement equipment that can detect and record a

93

wide variety of the physical and virtual parameters of mobile cellular service

in a given geographical area.

By measuring what a wireless network subscriber would experience in any

specific area, wireless carriers can make directed changes to their networks

that provide better coverage and service to their customers.

Fig: 3G drive testing

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Drive testing requires a mobile vehicle outfitted with drive testing

measurement equipment. These equipments are usually highly specialized

electronic devices that interface to OEM mobile handsets. This ensures

measurements are realistic and comparable to actual user experiences.

Data Collected during Drive Testing

Drive test equipment typically collects data relating to the network itself,

services running on the network such as voice or data services, radio

frequency scanner information and GPS information to provide location

logging.

The data set collected during drive testing field measurements can include

information such as:

1. Signal intensity

2. Signal quality

3. Interference

4. Dropped calls

5. Blocked calls

6. Anomalous events

7. Call statistics

8. Service level statistics

9. Quality of Service information

10. Handover information

11. Neighbouring cell information

12. GPS location co-ordinates

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Site Audit

The act of conducting a review, examination and reconciliation of

Telecom, Wireless and Network customer service records, invoicing and

contract agreements in order to ensure the accuracy of budgetary

forecasting.

Independent review and examination of records and activities to

assess the adequacy of system controls, to ensure compliance with

established policies and operational procedures, and to recommend

necessary changes in controls, policies, or procedures.

Analysis of invoices, lines, rates, tariffs, taxes, plans, usage, call volume,

systems, and contracts resulting in cost reduction, proper invoicing and

optimization of telecommunication systems often conducted by an

independent telecommunications consultant or firm.

Optimization

Reasons for Optimization –

Maintain/Improve QoS

Attract new customers

Maximise revenue-generating service

Maximize efficiency of network functional elements

Original design information has changed

Flawed original design information

Congestion may exist in certain areas and by prudent optimisation,

additional capacity can be generated with no additional infrastructure

investment.

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Performance Management Cycle

Fig: Performance Management Cycle

The initial step in performance management is to define a set of QoS (Quality

of Service) parameters such as dropped call rates and call success rates. Key

metrics are derived from data collected from sources such as drive tests,

statistical data, customer complaints and field engineer reports and are used

to measure the performance of the network. These metrics are analysed and

compared to the QoS targets in order to identify any performance

degradation in the network.

Key Performance Indicators (KPIs)

KPIs should be maintained within threshold for good performance of the

network.

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KPIs to be monitored –

SDD Standalone Dedicated Drop

TCHD Traffic Channel Drop

SDB Standalone Dedicated Block

TCHB Traffic Channel Block

HOSR Handover Success Rate

Fig: KPIs

What is Dropped Call?

All cell resources are available but calls are failing, then we have a call drop

scenario. This could be caused by software errors, congestion, C7 link failures,

HW problems or many other reasons.

If a call is abnormally disconnected, a Clear Request is sent to the MSC .If the

Call is disconnected in a normal Fashion then Clear Message with cause

code Call Control is sent. It is important to establish what types of calls are

failing, and over what percentage of the network it is occurring.

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SD Blocking

SD blocking means that you are not getting SD resource for the call

origination. When MS connects with NW then RACH and AGCH are provided.

After AGCH, SDCCH is provided but if SDCCH is not provided at this time due

some problems or due to unavailable of SD by BSC, it’s called as SD Blocking.

There are no of reasons for that. If such a case arises the customer will not be

able to originate any call.

If all the SD resources are full and not available for SD assign then it comes

under congestion. If at a particular time call is attempted and it fails then it

known as Blocking.

Solutions to SD Blocking

Check the No. of SDCCH channel Available, if less then increase SD

channel taking care that there is no TCH Blocking.

Check LAC boundary, if location update is more, then change the LAC

of that site and set C2 and HYS.

Use of Dynamic SDCCH (It is a BSC parameter and will be applied on

whole BTS).

Hardware check / shift SD to new time slot.

Use report number 182 in the OSS to analyses SD Blocking reasons and

130 for SD congestion.

SD Drop

It occurs between allocation of SD and before TCH allocation. Sometimes SD

drop occurs because queuing is not activated in the system.

If SD drop is high, look on parameters like – overshooting, shift the SD time slot,

may be hardware issue, interference, change the values of RXP, PMAX, may

be issue of uplink or downlink issue in that cells for UL put a TMA in that cell

and for DL provide tilt, re-orient that antenna.

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Solutions to SD Drop

Check the BCCH Plan.

MapInfo to find out proper frequency to reduce interference.

The best way to find the real issues for Interference makes Drive Test.

Check interference by Interference scanning.

Check clean BCCH by frequency scanning.

Use report ZEOL to find the alarms.

Use 208 for Path loss analysis.

Use 196 for UL-DL Interference.

Use 163 report for SD drop.

TCH Blocking

When TCH is not allocated to the user after SD allocation, it is TCH Blocking. It

is the failed call attempts which the MS user can notice. It takes place due to

lack of TCH Resource.

Solutions to TCH Blocking

Implement half rate or Dual rate.

Add another TRX.

If TRX addition not possible, try to share the traffic of that cell with the

neighboring cell by changing tilt or orientation.

Use report number 135 TCH Congestion.

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TCH Drop

Drop during conversation is known as TCH drop.

Solutions to TCH Drop

Check the BCCH Plan.

If a cell is picking call from long distance, check the sample log

according to TA.

Site Orientation.

Effective tilt should be check.

Mount position should be check

Handover Success Rate

If HOSR will be good TCH drop will also be good.

If Handover success rate degrades, call drop rate will take place.

Solutions to HOSR

Try to retune neighbours.

The best way to find the real issues for HO fail make Drive Test. By DT it is

very easy to find the fail between cells.

153 reports for HO fail between two cells.

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3rd Generation

The third generation, 3G, is expected to complete the globalisation process

of the mobile communication. The trend is that 3G will mostly be based on

GSM technical solutions due to the reason: the GSM as technology

dominates the market and great investments made to GSM should be utilised

as much as possible.

Access to the Internet will become more important and executives will want

to access corporate databases from virtually anywhere. New services will be

required in addition to speech and data, therefore network operators will

offer video and other multimedia applications.

Specification Process for 3G

As the 3G system is expected to be global, world-wide and generic, the

Specification Bodies related are also global ones as the following list

indicates. In addition to the Specification Bodies, the specification process

includes operators and manufacturers co-operation.

There are four international standardisation bodies acting as “generators” for

3G specification work, these being:

1. ITU-T (International Telecommunication Union): This organisation

provides in practise all the telecommunication branch specifications

that are official in nature. Hence, these form all the guidelines required

by the manufacturers and country-specific authorities. ITU-T has finished

its development process for IMT2000, International Mobile Telephone –

2000 and the specification work is transferred to the 3GPP.

2. ETSI (European Telecommunication Standard Institute): This

organisational body has had a very strong role when GSM

Specifications were developed and enhanced. ETSI is divided into

workgroups named as SMG (number) and every workgroup has a

specific area to be developed. Because of the GSM background ETSI is

in a relatively dominant role in this specification work. In Europe, the

further developed IMT-2000 is called 3G.

102

Fig: 3G Specification Bodies

3. ARIB (Alliance of Radio Industries and Business): ARIB provides

commercially oriented contributions for the specification process from

the Australiasian area. It has remarkable experience, both commercial

and technical, in the new selected 3G Air Interface technology and

several variants of it.

4. ANSI (American National Standard Institute): ANSI is the American

specification body defining telecommunication-related issues in that

part of the world. ANSI’s role is relatively small as far as 3G concerned

because of some political points of view. ANSI is mainly concentrating

on a competing 3G Air Interface technology selection called as

cdma2000.

3rd Generation Partnership Project (3GPP)

In order to maintain globality and complete control of the 3G specifications,

a separate Specification Body called 3GPP (3rd Generation Partnership

Project) takes care of the specification work in co-operation with previously

listed institutes. The outcome of the 3GPP work is a complete set of

specifications defining the 3G-network functionality, procedures and service

aspects.

103

The initial scope of 3GPP was to make a globally applicable third-generation

(3G) mobile phone system specification based on GSM specifications within

the scope of the International Mobile Telecommunications-2000 project of

the International Telecommunication Union (ITU). The scope was later

enlarged to include the development and maintenance of:

the Global System for Mobile Communications (GSM) including GSM

evolved radio access technologies (e.g. General Packet Radio Service

(GPRS) and Enhanced Data Rates for GSM Evolution (EDGE))

an evolved third Generation and beyond Mobile System based on the

evolved 3GPP core networks, and the radio access technologies

supported by the Partners (i.e., UTRA both FDD and TDD modes).

an evolved IP Multimedia Subsystem (IMS) developed in an access

independent manner

Operator Harmonization Group (OHG)

Global system means global business and this is why there has been a lot of

pressure to select or emphasise certain solutions more than others. This

political debate actually delayed the specification work remarkably and

finally an organisation taking care of harmonisation issues was established.

This organization, OHG aims to find a common understanding concerning the

global issues. The results of this organisation are used as inputs in 3GPP work as

well as in 3G future implementations.

The aim of OHG work is to affect the specifications so that all the radio

access variants are compatible with all the variants meant for switching; this

will ensure true globality for 3G.

UMTS (Universal Mobile Telephony System) is the name for the

European, ETSI driven 3G variant. It emphasises the interoperability and

backward compatibility between the 3G implementation and GSM.

IMT-2000 (International Mobile Telephony-2000) is the ITU-T name for the

3rd generation cellular system. The Japanese view of 3G is based on

the IMT-2000. The switching part of this variant is quite open issue but it is

expected to be based on the existing GSM technology. The Radio

Access is almost similar to the European variant but some

enhancements/extensions are made.

104

IMT-2000 / cdma2000 are the names for the American 3G variant. In

the American variant, the TDMA (Time Division Multiple Access)

scenario which is at least partially based on the enhanced GSM (EDGE)

is also considered as an alternative.

GSM systems will evolve towards the UMTS by progressively introducing new

techniques to provide higher bandwidth. These steps are as follows –

High Speed Circuit Switched Data (HSCSD)

General Packet Radio Services (GPRS)

Enhanced Data rates for GSM Evolution (EDGE)

High Speed Circuit Switched Data (HSCSD)

Traditionally TDMA timeslot provided a bit rate of 9.6 Kbps; however a new

modified air interface brings the speed up to 14.4 Kbps. With HSCSD, a

combination of up to four TDMA timeslots could be used to provide data

transfer rate at 57.6 Kbps.

General Packet Radio Services (GPRS)

General packet radio service (GPRS) is a packet oriented mobile data

service on the 2G and 3G cellular communication system. GPRS allows users

to be charged for the actual amount of data they transfer. The mobile user

doesn't have to connect to the network each time he wants to transfer data,

he can stay connected all day.

With the higher transmission speeds provided by GPRS, end users will find that

file downloads are faster, applications that were previously not possible now

become possible and the overall attractiveness of the data services will

increase.

GPRS was originally standardized by European Telecommunications

Standards Institute (ETSI). It is now maintained by the 3rd Generation

Partnership Project (3GPP)

In order to offer package switched data service, there should be some

modifications done in the GSM network architecture. Data packages are

handled with the help of two new network elements –

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SGSN (Serving GPRS Support Node)

GGSN (Gateway GPRS Support Node)

The Serving GPRS Support Node (SGSN) is a router that maintains the location

information of the mobile station and the Gateway GPRS Support Node

(GGSN) enables the data packets to be passed on to other packet switching

networks.

Enhanced Data rates for GSM Evolution (EDGE)

EDGE will provide a bridge from GSM into the 3rd Generation mobile networks.

It will use an advanced GSM modulation technique to provide data speeds

of 384Kbits/s but still using the existing 200 kHz GSM channel.

The extra capacity is achieved by increasing the data capacity of a single

GSM timeslot from 9.6 Kbps to 48 Kbps and possibly up to nearly 70 Kbps

under good radio conditions.

EDGE can be used for any packet switched application, such as an Internet

connection.

Wireless Application Protocol (WAP)

Wireless Application Protocol (WAP) is a technical standard for accessing

information over a mobile wireless network. A WAP browser is a web browser

for mobile devices such as mobile phones that uses the protocol. Before the

introduction of WAP, mobile service providers had limited opportunities to

offer interactive data services, but needed interactivity to support Internet

and Web applications.

Wireless Markup Language (WML), based on XML, is a markup language

intended for devices that implement the Wireless Application Protocol (WAP)

specification, such as mobile phones. It provides navigational support, data

input, hyperlinks, text and image presentation, and forms, much like HTML

(HyperText Markup Language).

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High-Speed Downlink Packet Access (HSDPA)

HSDPA is an enhanced 3G (third-generation) mobile-telephony

communications protocol in the High-Speed Packet Access (HSPA) family,

also dubbed 3.5G, 3G+, or Turbo 3G, which allows networks based on

Universal Mobile Telecommunications System (UMTS) to have higher data-

transfer speeds and capacity. As of 2013 HSDPA deployments can support

down-link speeds of up to 42.3 Mbit/s. HSPA+ offers further speed increases,

providing speeds of up to 337.5 Mbit/s with Release 11 of the 3GPP standards.

For HSDPA, a new transport layer channel, High-Speed Downlink Shared

Channel (HS-DSCH), has been added to 3GPP release 5 and further

specification. It is implemented by introducing three new physical layer

channels: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-Shared

Control Channel (HS-SCCH) informs the user that data will be sent on the HS-

DSCH, 2 slots ahead. The Uplink High Speed-Dedicated Physical Control

Channel (HS-DPCCH) carries acknowledgment information and current

channel quality indicator (CQI) of the user. This value is then used by the base

station to calculate how much data to send to the user devices on the next

transmission. The High Speed-Physical Downlink Shared Channel (HS-PDSCH) is

the channel to which the above HS-DSCH transport channel is mapped that

carries actual user data.

Hybrid automatic repeat-request (HARQ)

Data is transmitted together with error correction bits. Minor errors can thus

be corrected without retransmission; see forward error correction.

If retransmission is needed, the user device saves the packet and later

combines it with retransmitted packet to recover the error-free packet as

efficiently as possible. Even if the retransmitted packets are corrupted, their

combination can yield an error-free packet. Retransmitted packet may be

either identical (chase combining) or different from the first transmission

(incremental redundancy).

Since HARQ retransmissions are processed at the physical layer, their 12 ms

round-trip time is much lower compared to higher layer retransmissions.

Fast packet scheduling

The HS-DSCH downlink channel is shared between users using channel-

dependent scheduling to make the best use of available radio conditions.

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Each user device continually transmits an indication of the downlink signal

quality, as often as 500 times per second. Using this information from all

devices, the base station decides which users will be sent data in the next

2ms frame and how much data should be sent for each user. More data can

be sent to users which report high downlink signal quality.

The amount of the channelization code tree, and thus network bandwidth,

allocated to HSDPA users is determined by the network. The allocation is

"semi-static" in that it can be modified while the network is operating, but not

on a frame-by-frame basis. This allocation represents a trade-off between

bandwidth allocated for HSDPA users, versus that for voice and non-HSDPA

data users. The allocation is in units of channelization codes for Spreading

Factor 16, of which 16 exist and up to 15 can be allocated to the HS-DSCH.

When the base station decides which users will receive data in the next

frame, it also decides which channelization codes will be used for each user.

This information is sent to the user on one of up to 4 HS-SCCHs, which are not

part of the HS-DSCH allocation previously mentioned, but are allocated

separately. Thus, for a given 2ms frame, data may be sent to a number of

users simultaneously, using different channelization codes.

Adaptive modulation and coding

The modulation scheme and coding are changed on a per-user basis,

depending on signal quality and cell usage. The initial scheme is quadrature

phase-shift keying (QPSK), but in good radio conditions 16QAM and 64QAM

can significantly increase data throughput rates. With 5 Code allocation,

QPSK typically offers up to 1.8 Mbit/s peak data rates, while 16QAM offers up

to 3.6 Mbit/s. Additional codes (e.g. 10, 15) can also be used to improve

these data rates or extend the network capacity throughput significantly.

3G Network Structure

The obvious lack of GSM systems is and was the bandwidth offered to the

enduser. In the beginning the bandwidth offered to the end-user was

reasonable but later on when the technology developed and the end-user

requirements increased and new services such as the Internet became more

common the bandwidth became inadequate.

This was the main reason for starting the specification for the next generation

cellular networks.

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Fig: 3G Network Principle Diagram

The abbreviation RAN comes from the words Radio Access Network and the

term CN means Core Network. The multiple access method used in RAN is

Wideband Code Division Multiple Access, WCDMA. The RAN is limited with

open interfaces in order to guarantee multi-vendor scenarios. Also the

interfaces within the CN and between the CN and the other networks can be

considered as open but there may be several national limitations /

enhancements / extensions present. The 3G network can also be presented

as a collection of Management Layers, which cover certain parts of the

network.

The Radio Resource Management is completely covered between the RAN

and the UE and it involves managing how the channels are allocated. The

Mobility Management, Session Management and Call Control are

maintained by the Core Network Domains and there function is dependent

upon the domain is the CS (circuit switched) or PS (packet switched). The

higher layer functions performed between the UE and CN are often called as

CM, Communication Management. The CM entity covers the topics like Call

Control (CC), Supplementary Services (SS) and Short Message Service (SMS).

The 3G network will have the means and readiness for data transfer in all

forms. The traffic to be delivered through 3G can be divided into two

categories being Circuit Switched and Packet Switched. The Circuit Switched

traffic normally has a high real-time requirement (i.e. no delay or the delay

occurring must be constant). Normal speech and Video Phoning are

examples of this kind of traffic. The Packet Switched traffic normally does not

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have such exact real-time requirements and a good example of this kind of

traffic is an Internet connection.

W-CDMA (Wideband-Code Division Multiple Access) will be employed on the

air interface mainly for wide area applications and will use paired frequency

bands, one for the uplink and one for the downlink. This is commonly referred

to as Frequency Division Duplex (FDD).

UMTS will also employ TD-CDMA (Time Division-Code Division Multiple Access)

for low mobility indoor applications using Time Division Duplex (TDD) similar to

cordless technologies. Together, these two elements of the air interface (FDD

and TDD) are known as UTRA (UMTS Terrestrial Radio Access).

The evolution of UMTS progresses according to planned releases. Each

release is designed to introduce new features and improve upon existing

ones.

Release 99

Bearer Services

64 kbps circuit switch

384 kbps switched

Location Services

Call Service: compatible with GSM, based on USIM

Voice Quality Features

Release 4

Edge Radio

Multimedia Messaging

Improved Location Services

IP Multimedia Services

TD – SCDMA

Release 5

IP Multimedia Subsystem

IPv6, IP transport in UTRAN

Improvements in GERAN, MExE, etc

Release 6

WLAN integration

Multimedia broadcast and multicast

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Improvements in IMS

HSUPA

Fractional DPCH

Release 7

Enhanced L2

64 QAM, MIMO

Voice over HSPA

CPC – continuous packet connectivity

FRLC – Flexible RLC

Release 8

Dual Cell HSDPA

Release 9

Dual Cell HSUPA

In FDD, transmit on one frequency and receive on another frequency.

Fig: FDD

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In TDD, TX and RX is on the same frequency but on different times.

Fig: TDD

Code Division Multiple Access (CDMA)

Code Division Multiple Access is a technique that allows many different

mobile telephones to use the same frequency at the same time but with

each phone assigned a unique code sequence known as a "spreading

code".

CDMA is a form of "spread spectrum" where the information is spread across

the available bandwidth of the radio channel.

The spreading code is used to encode an information bearing digital signal.

The receiver uses the same code to decode the signal and recover the

information data. As the bandwidth of the code signal is chosen to be much

larger than the bandwidth of the information signal, the encoding process

enlarges (spreads) the spectrum of the signal. This spectral spreading of the

transmitted signal gives CDMA its multiple access capability.

Wideband Code Division Multiple Access (W-CDMA)

For the 3rd generation mobile systems, a high bit rate is required for multi-

media data. Therefore, the spreading code must be of a higher bit rate.

CDMA uses a bandwidth of 1.25MHz but the W-CDMA systems for UMTS will

occupy a bandwidth of approximately 5MHz.

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In the W-CDMA system the spreading codes are used to spread out the data

signal to cover the whole wideband spectrum which is allocated for the data

transfer.

The data rates of 144Kbits/s and 384Kbits/s are achievable within this

bandwidth and can provide reasonable capacity 2Mbit/s peak rate under

limited conditions.

The large 5MHz bandwidth can resolve more multipaths than narrower

bandwidths. This will increase diversity and improve performance. Wider

bandwidths of 10, 15 and 20MHz may be proposed in the future to support

high data rates more effectively.

3G Network Architecture

UMTS can in many aspects be looked upon as an extension to GSM and

GPRS. The greatest changes are related to the access part of the network.

The access network, called UMTS Terrestrial Radio Network (UTRAN), consists

of base stations and base stations controllers.

The base stations are called Node B. A Node B can support FDD mode, TDD

mode or dual-mode operation. Several base stations are managed by a

Radio Network Controller (RNC).

The RNC is responsible for the Handover decisions that require signalling to

the UE.

Node B

Node B is a term used in UMTS equivalent to the BTS (base transceiver station)

description used in GSM.

The utilization of WCDMA technology allows cells belonging to the same or

different Node Bs and even controlled by different RNC to overlap and still

use the same frequency (in fact, the whole network can be implemented

with just one frequency pair). The effect is utilized in soft handovers.

Since WCDMA often operates at higher frequencies than GSM (2,100 MHz as

opposed to 900 MHz for GSM), the cell radius can be considerably smaller for

WCDMA than for GSM cells as the path loss is frequency dependent. WCDMA

now has networks operating in the 850–900 MHz band. In these networks, at

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these frequencies, the coverage of WCDMA is considered better than that of

the equivalent GSM network.

Fig: 3G Network Architecture

A full cell site has a cabinet, an antenna mast and actual antenna. An

equipment cabinet contains e.g. power amplifiers, digital signal processors

and backup batteries.

Node B Setup –

A full cell site has a cabinet, an antenna mast and actual antenna. An

equipment cabinet contains e.g. power amplifiers, digital signal processors

and backup batteries. What you can see by the side of a road or in a city

center is just an antenna. However, the tendency nowadays is to

camouflage the antenna (paint it the colour of the building or put it into an

RF-transparent enclosure). Smaller indoor nodes may have an antenna built

into the cabinet door.

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A Node B can serve several cells, also called sectors, depending on the

configuration and type of antenna. Common configuration include omni cell

(360°), 3 sectors (3×120°) or 6 sectors (3 sectors 120° wide overlapping with 3

sectors of different frequency).

Radio Network Controller (RNC)

The Radio Network Controller (or RNC) is a governing element in the UMTS

radio access network (UTRAN) and is responsible for controlling the Node Bs

that are connected to it. The RNC carries out radio resource management,

some of the mobility management functions and is the point where

encryption is done before user data is sent to and from the mobile.

The logical connections between the network elements are known as

interfaces. The interface between the RNC and the Circuit Switched Core

Network (CS-CN) is called Iu-CS and between the RNC and the Packet

Switched Core Network is called Iu-PS. Other interfaces include Iub (between

the RNC and the Node B) and Iur (between RNCs in the same network). Iu

interfaces carry user traffic (such as voice or data) as well as control

information, and Iur interface is mainly needed for soft handovers.

Media Gateway (MGW)

A media gateway is a translation device or service that converts digital

media streams between disparate telecommunications network. Because

the media gateway connects different types of networks, one of its main

functions is to convert between different transmission and coding techniques.

Soft Handover

Soft handover or soft handoff refers to a feature used by the CDMA and W-

CDMA standards, where a cell phone is simultaneously connected to two or

more cells (or cell sectors) during a call. If the sectors are from the same

physical cell site (a sectorised site), it is referred to as softer handoff. This

technique is a form of mobile-assisted handover, CDMA cell phones

continuously make power measurements of a list of neighboring cell sites,

and determine whether or not to request or end soft handover with the cell

sectors on the list.

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Hard handover is a typical Handover mechanism in a communication

network which is designed to work by first breaking off from the initial

connection with a base station before switching to another base station. This

is done in order to retain communications in a session for mobile users after

incurring a non-perceptible and insignificant brief interruption. A Hard

handoff is also referred to as “Break-before-Make” handover.

Cell Radius

Hata’s Empirical formula

Path Loss =

= 69.55 + 26.16 * log(fc) – 13.82 * log(hb) + [44.9 – 6.55 * log(hb)] log(R) –a(hm)

where,

hb is the base station effective antenna height in meters

fc is the carrier frequency in MHz

hm is the mobile station effective antenna height in meters

PL is the propagation loss or path loss EIRP in dB

a(hm) is the correction factor for the mobile station antenna height,

hm in meter. A = 0 for hm = 1.5 m.

R is the cell radius in kilometers

From above equation, cell radius R can be calculated.

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Differences between 2G and 3G

Parameter 2G 3G

Cost

License fees is low. Network

construction and

maintenance is also low.

License fees is high. Network

construction and

maintenance is also high.

Data

Transmission

Lower data speeds, and less

compatible with functions of

smartphone.

High data speeds, and more

compatible with newer

technology.

Data

Speed 236 kbps (UL and DL) 21Mbps(DL) and 5.7Mbps (UL)

Features Basic services and

supplementary services

Mobile TV, video transfers and

GPS

Frequency

Band Width 200kHz 5MHz

Security Low High

Modulation GMSK QPSK, 16QAM, 64QAM, BPSK

Channel

Access FDMA with TDMA FDD with WCDMA

Frequency 900MHz and 1800MHz 2100MHz

No. of Sites

Required less for a particular

area as frequency of

coverage is less

Required more for a particular

area as frequency of

coverage is more

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A Visit to the Cell Site

During the course of the internship, I was allowed to visit the Base Transceiver

Station (BTS), and inside its shelter, and I was briefed about all the network

equipments.

Inside the Shelter

1. Base Transceiver Station (BTS)

Fig: BTS

The BTS of used here was manufactured by NSN. This model was called

Flexi Edge BTS. It can support a maximum of 24 TRX.

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2. Power Interface Unit

It stops the fluctuations of AC. Its output is smooth AC. Its output is fed

to SMPS.

3. SMPS

Fig: SMPS

To convert AC to DC. It has many modules for conversion. If power

conversion limit exceeds, new modules need to be added to convert

more power. Power converter by the SMPS is supplied to BTS and its

TRXs. For connections, feeder cables are used. MCB is used in the SMPS

for protection of the circuit.

4. Battery bank

Its output is -48V. Battery Bank is used to supply power in the time

interval of main power cut and switching to the power supplied by

Diesel Generator. It can supply power for 4-5 hours. -48V is kept as a

standard for “Cathodic Protection”.

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Fig: Battery Bank

5. Duplexer

A duplexer is a device that allows bi-directional (duplex)

communication over a single path. It isolates the receiver from the

transmitter while permitting them to share a common antenna.

6. ACs and Fans

The AC prevent the overheating of all the instruments of the shelter.

Fans provide immediate backup in case AC goes off.

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On the Tower

Fig: GSM Tower

1. Microwave Antenna

Fig: Microwave Antenna

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To transmit or receive data from other sites, like from a BTS to other BTS

or from a BTS to a BSC. Frequencies has no relation with size of the

microwave antennas. Microwave antennas used for Line of Sight

communications only.

2. GSM Antennas

Fig: GSM Antenna

These antennas are used for providing network coverage. Signals

received in the MS are due to them.

3. Grounding

Grounding used to protect the site from current leakage. Site always

installed on a proper concrete support pillar.

4. Shape of the Tower

Triangular or Straight shape doesn’t matter. Neither does three tower

pillars or four. It all depends on design and cost management and load

of antennas it should handle.