part 5. gsm (global system for mobile communications)sdma/elec6040_2010/part 5... · ds-cdma,...

ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKUp. 1

Part 5. GSM (Global System for Mobile Communications)


Global GSM Subscribers

20072006200520042001199819961994

500

1000

1500

2000

2500

1 50 100Num

ber o

f GSM

Sub

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ers

(Mill

ion)

0Year

3000

2008


GroupeSpecialeMobile

established by CEPT

Several proposals for GSM multiple access:

wideband TDMA, narrowband TDMA, DS-CDMA, hybrid

CDMA/FDMA, narrowband FDMA

Eight prototype systems tested in

CNET laboratories in France;

Permanent nucleus set up

Basic transmission principles selected: 8-slot TDMA, 200-kHz

carrier spacing, frequency hopping;

MoU signed

GSM phase 1 specifications frozen (drafted 1987-1990)

GSM1800 standardization began

GSM 1800 specifications frozen; commercial

operation delayed due to the lack of terminals;

1982 1984 1986 1987

1990 1991

History (1)

GSM (Global System for Mobile communications) became an ETSI

technical committee

1988


1996

History (2)

Enhanced full rate (EFR) speech codec standard ready;

14.4-Kbps standard ready; GSM1900 commercial

operation started

GSM submitted as a PCS technology candidate to the

United States; PCS1900 standard adopted in the

United States

1995

GSM900 commercial operation started;GSM phase 2+

development started

1992

HSCSD standard ready; GSM cordless system (home base station) standardization

started; EDGE standardization started

GPRS standard ready; WCDMAselected as the

third generation air interface

EDGE standard

ready

GSM standardization moved to 3GPP

and TSG GERAN established

1997 1998 1999 2000


Some Facts

For providing telephone and ISDN services.

Initially designed to unite mobile communication standards that were previously used over European countries. Huge success achieved afterwards. Became deployed globally.

Advantages over 1G analog systems from users’ viewpoint:– Higher digital voice quality– Lower cost – Introduction of Subscriber Identity Module (SIM): a memory device

that stores information such as subscriber’s identification number, networks that can be used, and other user-specific information; offering convenience to users.

– Improved on-the-air privacy due to data encryption.


GSM System Architecture

All copied from Rappaport’sWireless Communications

Mobile Station

Base Transceiver

Station

Base Station

Controller

Mobile Switching

Center

Home Location Register

Visitor Location Register

Authentication Center

Operation and Maintenance

Center


Terminologies (1)BSS– Consists of many BSCs

which connect to a single MSC

– Provides and manages radio transmission paths between MSs and MSCs.

– Also manages the radio interface between MSsand other GSM subsystems.

BSC– May co-locate with a BTS– Connects to remote BTSs by microwave links, leased lines or optical fibers.– Controls up to several hundred BTSs.– Controls handovers when BTSs involved are under the control of the same BSC,

thereby alleviating the burden of the MSC in handling handovers.


Terminologies (2)

NSS– Manages the

switching functions of the system.

– Allows MSCs to communicate with other networks such as PSTN and ISDN.

MSC–Is the central unit in NSS.–Controls the traffic among all BSCs


1

2

3

4

5

6When the car moves from the cell covered by BTS1 to the cell covered by BTS2, a so-called handoff operation is needed to transfer the communication with the MS from BTS1 to BTS2. Which function blocks are involved in this operation? What about when the car moved from BTS3 to BTS4?

Example


Terminologies (3)

HLR– Is a database that

contains subscriber information and location information for each user who resides in the same city as the MSC.

– Contains a unique International Mobile Subscriber Identity(IMSI) for each user, used to identify each home user.


Terminologies (4)

VLR– Is a database which

temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC.

– Is linked between severaladjoining MSCs in a particular market or geographic region.

– [When a roaming mobile is logged onto a particular VLR, the MSC sends the necessary information to the visiting subscriber’s HLR so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user’s HLR.]


AUC– Is a strongly protected

database that handles the authentication and encryption keys for every single subscriber in the HLR and VLR.

– Contains a register called the Equipment Identity Register (EIR) which identifies stolen or fraudulently altered phones that transmit identity data that do not match with information contained in either the HLR or VLR.

Terminologies (5)


Example

Suppose that you are a local customer of GSM services provided by Hutchison in Hong Kong. In which function block of Hutchison’s local GSM network will your subscriber information and location information be stored? If you travel to Paris and enjoy the international roaming mobile service, in which function block of the network in Paris will your information be stored? If your phone is lost and Hutchison disables your phone number as you requested, where will this number be stored in the network?


Terminologies (6)

OSS– Supports the

operation and maintenance of GSM.

– Allows engineers to monitor, diagnose and troubleshoot the system.

Main functions:– to maintain all

telecom hardware and network operations with a particular market– to manage all charging and billing procedures– to manage all mobile equipment in the system


All copied from Rappaport’s Wireless Communications

Radio Air InterfaceAbis Interface– carries traffic and

maintenance dataA Interface– uses an SS7 protocol

called the signaling correction control part (SCCP)

– supports communication between the MSC and the BSS, and network messages between the individual subscribers and the MSC

SS7 (Signaling System 7)– is a set of telephony signaling protocols which are used to set up the vast

majority of the world's public switched telephone network telephone calls.

GSM System Interfaces


Radio Interface (1)Frequency domain– Two bands of 25MHz– Each band divides into a number of 200kHz wide channels called

ARFCNs (Absolute Radio Frequency Channel Numbers)– Each channel is time shared by 8 subscribers using TDMA

Multiple Access Method– Combination of TDMA and FDMA

f (MHz)890 915 935 960

25MHz 25MHz

for reverse link (Uplink) for forward link (Downlink)

200kHz

45MHz


Radio Interface (2)Time domain– 8 time slots (TSs) per frame; each frame occupies 4.615ms; each time

slot has 576.92µs.

modulation data rate:156.25bits 270.83kbps576.92 sμ

=


Radio Interface (3)Collision of received signals– MSs at different distances from the BS: different round trip delay– Collision of the signals from mobiles assigned to adjacent time slots


Radio Interface (4)Solution: Time Advance– Transmit and receive time spacing: 3 time slots – Time Advance: BS commands MSs to advance their transmission

by the round trip delay

0 1 2 3 4 5 6 7

0

TA0/2 TA0/2

3 timeslot -TA0

1

TA1/2 TA1/2

3 timeslot -TA1

0 1

TA0>TA1

Receivedsignals alligned

BS

MS0

MS1


Radio Interface Summary

FDD (frequency division duplex)

ARFCN (Absolute Radio Frequency Channel Number)– Specifies the

carrier frequency that is used.

Each radio channel occupies 200kHz.

Copied from Rappaport’s Wireless Communications


Channel Types

Physical channel:– Is the combination of a TS number and an ARFCN.– Is the channel that is used by a user.– E.g., user 3 is assigned a pair of physical channels TS2-ARFCN3 and

TS7-ARFCN1000

Logical channel:– Each specific time slot of frame may be dedicated to either handling

traffic data, signaling data, or control channel data.– links the physical layer with the data link layer– Traffic channel (TCH): is used to carry digitized speech or data for a

user.– Control channel (CCH): Carries signaling or synchronizing

commands between the BS and MS.


Traffic Channels (1)Multiframe structure for TCH– 26 frames per multiframe; each multiframe occupies 120ms– data are broken up every 13 frames by SACCH or idle frames– the 26th frame contains idle bits when full-rate TCHs are used and

contains SACCH data when half-rate TCHs are used


Traffic Channels (2)

Half-rate is an optional feature of GSM– Used when the cell is nearly congested or when MS has low battery since

it saves 30% more energy– Could double the network capacity for voice traffic, at the expense of

quality

carrying speech:

carrying data:

Full-Rate TCH: channel data rate: 22.8kbps; Full-Rate Speech Channel (TCH/FS): carries user speech digitized at a raw data rate of 13kbps

Half-Rate TCH: channel data rate: 11.4kbps; Half-Rate Speech Channel (TCH/HS): carries digitized speech sampled at a rate half that of a full-rate channel, 6.5kbps

Data Channel for 9600bps (TCH/F9.6) Data Channel for 4800bps (TCH/F4.8)Data Channel for 2400bps (TCH/F2.4)


Traffic Channels (3)

Example of system state at arbitrary time


Three main control channelsBroadcast Channel (BCH)– operates on the forward link of a specific ARFCN within each cell, and

transmits data only in TS0– other seven timeslots for the same ARFCN are available for TCH data or

DCCH data– provides synchronization for all mobiles within the cell

Common Control Channel (CCCH)– occupies TS0 that is not otherwise used by the BCH or the Idle frame– most commonly used control channels– used to page specific subscribers, assign signaling channels to specific

users, and receive mobile requests for serviceDedicated Control Channel (DCCH)– may exist in any time slot and on any ARFCN except TS0 of the BCH

ARFCN– bidirectional, forward and reverse links

Control Channels


Broadcast Control Channel (BCCH)– occupies frame 2 to frame 5 of a control channel multiframe (illustrated

later)– broadcast information: cell and network identity, operating characteristics

of the cell (current control channel structure, channel availability and congestion)

– broadcast a list of channels that are currently in use within the cellFrequency Correction Channel (FCCH)– occupies TS0 for the very first frame (frame 0) and is repeated every ten

frames within a control channel multiframe– allows each MS to synchronize its local oscillator to the exact frequency

of BSSynchronization Channel (SCH)– occupies TS0 of the frame immediately following the FCCH frame– allows each MS to frame synchronize with the BS– broadcast frame number (FN), base station identity code (BSIC), time

advancement commands

BCH


Paging Channel (PCH) (forward)– provides paging signals from the base station to all mobiles in the cell– notifies a specific mobile of an incoming call which originates from the

PSTN– provides cell broadcast text message to all subscribers, SMS feature

Random Access Channel (RACH) (reverse)– used by a MS to acknowledge a page from the PCH– used by a MS to originate a call– uses a slotted ALOHA access scheme

Access Grant Channel (AGCH) (forward)– used by the BS to respond to a RACH sent by a MS – carries data which instructs the MS to operate in a particular physical

channel (time slot and ARFCN)– the final CCCH message sent by the BS before the MS is moved off the

control channel

CCCH


Standalone Dedicated Control Channel (SDCCH)– bidirectional, ensures that the MS and the BS remain connected while the BS

and MSC verify the MS and allocate resources for the MS– an intermediate and temporary channel – used to send authentication and alert messages

Slow Associated Control Channel (SACCH)– associated with a TCH or a SDCCH– forward link: send slowly but regularly changing control information, e.g.,

transmit power level instructions and specific timing advance instructions– reverse link: carries information about the received signal strength and the

quality of the TCHFast Associated Control Channel (FACCH)– carries urgent messages, same type of information as the SDCCH – replaces all or part of a TCH when there is a need for some heavy-duty

signaling (e.g., a handoff request)

DCCH


Example of Control Channel Multiframe

channel combination: FCCH+SCH+CCCH+BCCH


Examples of How a MS Behaves ― Synchronization with the Network

When a MS is turned onSearch for the BCH: BCH operates at a specific ARFCN, just search for the freq. channel with the highest power levelSearch for the FCCH: BS transmits, during certain known intervals, a pure sine wave for the period of exactly one time slot; the received signal exhibits a tone at 1/4th of the GSM symbol rate, i.e., 67kHz Search for the SCH: SCH is in the time slot following FCCHRead broadcast info. in BCCH: location of the cell, how to access BS, etc..

1.

2. FCCH


Registration

A procedure that the MS informs its presence to the network when the MS is switched on.

Copied from An Introducation to GSM

RACH

AGCHSDCCHSDCCHSDCCH

SDCCH

SDCCH

SDCCH


Call Establishment (MOC)

Mobile-originated call

(MOC)

Mobile-terminated call

(MTC)

– Procedure almost identical


RACHAGCHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCH

FACCH

FACCHFACCHFACCH

TCH


Call Establishment (MTC)

Mobile-originated call

(MOC)

Mobile-terminated call

(MTC)

– Procedure almost identical



Speech Transmission



Speech coding– Based on Residually Excited Linear Predictive Coding (RELP)– Yields 260 bits per 20ms block of speech ⇒ Gives a bit rate of 13kbps.

Discontinuous Transmission Mode (DTX)– Exploits the fact that a normal person speaks for only 40% of time.– No transmission during silent period => a longer subscriber battery life

and less instantaneous radio interferenceRelative importance of speech-coder outputs– Ia bits: the most important 50 bits out of 260 bits of speech– Ib bits: the 132 bits that are less important than Ia bits– Type II bits: the rest of 78 bits out of 260 bits

Physical Layer (1)


Channel coding for the speech signal:– 50 Ia bits appended with 3

parity check bits– 50 Ia bits & 3 parity bits &

132 Ib bits are altogether coded by a convolutional code with rate 1/2 and constraint length 5.

– The 78 Type-II bits are unprotected.

– The resultant gross data rate of the GSM speech with channel coding is 22.8kbps.

Physical Layer (2)



Channel coding for data channel: half rate punctured convolutional codeChannel coding for control channel: a shortened binary cyclic fire code followed by a half rate convolutional codeInterleaving– To mitigate the effect of sudden fade.– 456 encoded bits for each 20ms speech frame are broken into eight 57-bit sub-

blocks. The 8 sub-blocks are transmitted over eight consecutive traffic-channel (TCH) time slots.

– Each TCH time slot carries two 57-bit blocks for two segments of 20ms speech.


Physical Layer (3)


Burst formatting– Adds binary data to the cipher text to help synchronization and

equalization.Ciphering– Security achieved by encryption– Encryption algorithm changed from call to call.

Modulation– 0.3 GMSK (Gaussian minimum shift keying) [0.3 means the 3dB

bandwidth of the Gaussian pulse shaping filter with relation to the bit rate is BT = 0.3.]

– Binary ones and zeros are represented by shifting the RF carrier by ±67.708kHz, one fourth of the channel data rate of GSM (270.833333kbps).

Physical Layer (4)

Refer to Proakis’sDigital Communications


Frequency hopping– To avoid persistent staying at a bad channel.– Maximum hopping rate is 217.6 hops per second– Maximum number of hopping frequencies = 64

Physical Layer (5)


Channel equalization– To combat against the adverse effects due to multipath propagation and

fading– With the help of training sequences in every time slot.

Physical Layer (6)

D D D

h(t,τ0) h(t,τ1) h(t,τ2) h(t,τL-1)

Transmitted Signal

Received Signal

Multipath Channel: H(t,τ)

Received Signal

Channel Equalization

Recovered Signal

inverse the channel effect: H-1(t,τ)


Handover (1)

The MS continuously monitors the neighboring cells’ perceived power levels.The home BS gives the MS a list of channels of other BSs for measurement.The measurement report is periodically sent from the MS back to the home BS.Different types of handover situations:– Handover between BTSs under the control of the same BCS. [The

MSC is not troubled to handle handover, so that MSC is relieved in the handover burden. But the MSC has to be notified about the switching of BTSs.]

– Handover between BTSs under the control of different BSCs but the same MSC. [The MSC has to control the handover procedure.]

– Handover between BTSs under the control of different MSCs (and, implicitly, different BSCs). [Communications between different MSCsrequired.]


Handover (2)



Handover (3)


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