01 mn1780eu09mn 0001 sbs overview

SBS Overview Siemens

MN1780EU09MN_0001 © 2002 Siemens AG

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Contents

1 Basics 3

1.1 Global System for Mobile Communication GSM 4

1.2 Public Land Mobile Network PLMN 5

1.3 Radio Cell 5

1.4 Mobile Station MS 6

1.5 Subscriber Identity Module SIM 7

1.6 Example: Location Registration 7

2 Operation and Maintenance System 9

3 Core Network: Switching Subsystem 13

3.1 Mobile Services Switching Center MSC 15

3.2 Home Location Register HLR and Visitor Location Register VLR 16

3.3 Authentication Center AC 18

3.4 Equipment ID Register EIR 19

3.5 Serving GPRS Support Node SGSN 22

3.6 Gateway GPRS Support Node GGSN 22

4 Radio Access Network: Radio Subsystem 25

4.1 Base Station Controller BSC 28

4.2 Base Transceiver Station Equipment BTSE 30

4.3 Transcoder and Rate Adapter Unit TRAU 32

4.4 Local Maintenance Terminal LMT 34

4.5 Documentation 35

5 Radio Interface Um 37

5.1 GSM900/GSM1800/GSM1900 Frequency Bands 39

5.2 Radio Frequency Carriers RFC 40

5.3 Physical Channels 42

5.4 Logical Channels 44

5.5 Signaling Channels 46

5.6 Channel Combinations 52

5.7 Multiframes 53

SBS Overview

Siemens SBS Overview

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5.8 Timeslot Structure 56

6 GSM Basic Features 59

6.1 Frequency Hopping 60

6.2 Antenna Diversity 64

6.3 Power Control 65

6.4 Handover 66

6.5 Multiband Operation 68

6.6 Hierarchical Cell Structure 70

6.7 GSM-Railway GSM-R 71

7 Data Transmission in GSM Phase 2+ 73

7.1 High Speed Circuit Switched Data HSCSD 76

7.2 General Packet Radio Service GPRS 80

7.3 Enhanced Data Rates for GSM Evolution EDGE 90

8 Location Services 99

8.1 General 101

8.2 ETSI Architecture 102

8.3 SBS Implementation 104

9 Exercises 111



3

1 Basics


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1.1 Global System for Mobile Communication GSM

The Global System for Mobile Communications GSM is a ("2nd generation") standard for mobile communication. This standard was developed by the European Telecommunication Standards Institute ETSI (founded in 1988).

Within ETSI, special work groups called Special Mobile Groups (SMG), are responsible for the GSM standard.

The following table gives a few milestones of GSM history:

Date Event

1982 "Groupe Spécial Mobile" expert team (CEPT)

1987 Memorandum of Understanding (13 countries)

1989 Renaming: "Global System for Mobile Communication"

1990 GSM900 standard "phase1" adopted

1992 Large European operators start commercial operation

1993 GSM "phase 2" (e.g. HR, supplementary services)

1997 "Phase 2+": Annual releases (HSCSD, GPRS, EDGE, CAMEL ...)

GSM Phase 1

GSM Phase2

GSM Phase 1

GSM Phase 2+

GSM Phase 2

GSM Phase 1

1990 1993 1997

Backwards compatibility

Fig. 1 GSM Development



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1.2 Public Land Mobile Network PLMN

A Public Land Mobile Network is a network, established and operated by its licensed operator(s), for the specific purpose of providing land mobile communication services to the public.

1.3 Radio Cell

A radio cell is the smallest service area in a PLMN. The term “cell” comes from the (idealized) honeycomb shape of the areas into which the PLMN coverage area is divided. A cell consists of a base station transmitting over a small geographic area that is represented as a hexagon. The whole PLMN area is covered by a great number of radio cells.

A cell is also called Base Transceiver Station BTS.

The nominal radius of a cell may be

��up to 35 km radius for the GSM900 system and

��up to 8 km radius for the GSM1800/GSM1900 system.

Radio cell or cell or BTS

Fig. 2 Radio cells


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1.4 Mobile Station MS

The Mobile Station is the radio equipment needed by a subscriber to access the services provided by the PLMN. The MS may be a fixed station (e.g. installed in a vehicle) or a portable handheld station.

Mobile Subscriber Base Station Mobile Subscriber

Fig. 3 Mobile subscriber



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1.5 Subscriber Identity Module SIM

The Subscriber Identity Module SIM provides the mobile equipment ME with a subscriber's identity. The mobile equipment is not operational (except for emergency calls) without a SIM. The SIM must be inserted into the mobile equipment before it can be used.

The SIM is a smart card with a microprocessor and memory. The SIM may be the size of a credit card or even smaller for portable phones.

1.6 Example: Location Registration

After inserting the SIM card into the mobile equipment and switching on the mobile equipment, the mobile station initiates a series of signaling activities. When this signaling is completed, the current location is stored on the SIM card and the network is aware of the subscriber’s current location.

Short messages received from the network may also be stored on the SIM card.

To protect the SIM from unauthorized use, the subscriber must enter a four-digit personal identification number PIN. The PIN is stored on the SIM; if the wrong PIN is entered three consecutive times, the card blocks itself, and may be unblocked only with an eight-digit code that is also stored on the SIM.

ME + SIM = MS

+ =

Fig. 4 Mobile Station


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9

2 Operation and Maintenance System


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The major subsystems of a Public Land Mobile Network are:

��Operation and Maintenance Subsystem ("management system"))

��Switching Subsystem ("core network")

��Radio Subsystem ("radio access network")

The Operation and Maintenance Subsystem OMS perform monitoring of the network components of the SSS and the BSS centrally.

The OMS is made up of operation and maintenance centers OMC divided into OMC-B for the administration of BSS network elements and OMC-S for the administration of SSS network elements.

Operation and maintenance for SSS and BSS may be performed independently (to a large degree). The OMC-B and OMC-S may be co-located (and may also supervise 3rd generation network elements).



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GSM900/GSM1800/GSM1900 System

Operation & Maintenance Subsystem

Switching Subsystem Radio Subsystem

RSSSSS

OMS

SIEMENS

N IXDORF

Fig. 5 Major subsystems of a PLMN

GSM900/GSM1800/GSM1900 System

Operation & Maintenance Subsystem

Radio Subsystem Switching Subsystem

BTSEHLR

OMC

EIRVLR

MSC

AC

BTSE TRAU

GSN

BTSE BSC

SI EMENS

NIXDORF

Fig. 6 Operation and maintenance subsystem


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13

3 Core Network: Switching Subsystem


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The network components of the Switching Subsystem SSS for circuit-switched services are:

��Mobile Services Switching Center MSC

��Home Location Register HLR

��Visitor Location Register VLR

��Authentication Center AC

��Equipment Identification Register EIR

For packet-switched services, additional network elements are required in the core network:

��GPRS Support Nodes GSN

��GPRS Register GR

Note: GSM standards refer to Switching Subsystem (and Radio Subsystem). In 3rd generation standards the (equivalent) terms core network and radio access network are used. As GSM phase 2+ contains 3rd generation network elements, terminology varies accordingly.

Switching Subsystem (SSS)

HLR(GR) AC

MSC

VLR

GSN

EIR

Fig. 7 Network elements of the SSS



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3.1 Mobile Services Switching Center MSC

The Mobile Services Switching Center is responsible for establishing traffic channel connections

��to the BSS

��to other MSC

��to other networks (e.g. public switched telephone network PSTN, "Gateway MSC").

The MSC database contains information for the routing of traffic channel connections and handling of the basic and supplementary services. The MSC also performs administration of cells and location areas.

RSS Switching Subsystem (SSS)

BTSEBSC

BTSETRAU

MSC MSC

PSTN

Fig. 8 Mobile services switching center


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3.2 Home Location Register HLR and Visitor Location Register VLR

In the PLMN, the mobile subscriber is not permanently connected to a single MSC. Instead, depending on the mobile subscriber's location, the "serving" MSC may change in time.

Therefore, the PLMN contains a network component called Home Location Register HLR that administers the subscriber data. The HLR is a database where the mobile subscribers are created, deleted, and barred by the operator. It contains all permanent subscriber identities as well as the services that a mobile subscriber is authorized to use.

The GPRS subscriber database, called GPRS Register GR, is an extension of the HLR.

The Visitor Location Register VLR contains the relevant data of all mobile subscribers currently located in the service area of a MSC. The permanent data are the same as the data found in the HLR. Mirroring the data in the VLR, as well as in the HLR, reduces the data traffic to the HLR because it is not necessary to ask for this data every time it is needed.

If a mobile station is located in its own MSC, it still uses the two different registers (to simplify procedures).

During a location update (when the subscriber moves to a different VLR service area), the new (current) VLR requests subscriber data from the HLR.



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BTSEBSC

BTSETRAU

MSC MSC

PSTN

VLR

Restriction

None

Service

Tele

Subscriber

234-8000

VLR

HLRSubscriber Data

Restriction

None

Barred

None

None

Service

Tele

Tele

Tele/Fax

Tele/Fax

Subscriber

234-8000

234-8200

234-2340

234-2256

Fig. 9 Home and Visitor Location Register


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRSubscriber Data

Subscriber DataRequest

PSTN

Fig. 10 Visitor location register: data request


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3.3 Authentication Center AC

Authentication is done to protect the network against unauthorized access. Subscriber authentication is performed at each registration and at each call set-up attempt (mobile originating or terminating).

For checking the SIM, the VLR uses authentication parameters, called triples that are generated by the Authentication Center AC. The triples consist of

��RAND (Random Number),

��SRES (Signed Response) and

��Kc (Cipher Key).

The network element AC is associated with the HLR. Upon request from the VLR, the AC provides the VLR with these triples.

For the authentication, the RAND is sent to the MS ("challenge"). The MS uses the RAND with the algorithm A3 and the Authentication Key stored on the SIM to also calculate the Signed Response.

The MS sends the SRES to the MSC/VLR ("response") where it is compared with the value calculated by the MSC/VLR.

If the SRES sent from the MS matches the SRES calculated by the MSC/VLR, access to the network is granted and the Cipher Key Kc is sent to the BTSE for use in encrypting and decrypting messages to and from the MS.

If the SRES do not match, the MS is denied access to the network.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLR

PSTN

ACRAND SRES Kc

RAND

SRES

Fig. 11 Authentication



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3.4 Equipment ID Register EIR

The Equipment Identification Register is a database that stores the International Mobile Equipment Identity IMEI numbers for all registered Mobile Equipment ME. The IMEI uniquely identify all registered ME. There is generally one EIR per PLMN. It interfaces to the various HLR in the PLMN.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRAC

PSTN

IMEIEIR

Fig. 12 Equipment identification register


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The structure of the IMEI is as follows:

TAC is the type approval code.

This 6-character code allows approved types to be distinguished from non-approved types. The TAC is administered centrally.

FAC is the final assembly code.

This two-character code identifies the place of manufacture and designates the final assembly. The manufacturer administers the FAC.

SNR is the serial number.

This 6-character code is an individual code allocated to all equipment with the same TAC and FAC. The manufacturer of the mobile station issues the SNR.

The Spare Bit SP is always zero.

IMEI

Type

Approval

Code

(TAC)

Final

Assembly

Code

(FAC)

Serial

Number

(SNR)

Spare

Bit

(SP)

15 Digits

Fig. 13 International mobile equipment identity



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IMEI are assigned to one of three classes ("lists") stored in the database:

��the black list for barred mobile stations (e.g. mobiles that have been reported stolen).

��the gray list for mobile stations to be observed

��the white list for approved mobile stations

The EIR checks whether the IMEI fits into one of the three categories and passes the result to the MSC. If, for example the ME belongs to the black list, the call is blocked.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRAC

PSTN

EIR

Black List

Unapproved

equipment,

because it is

stolen or

defective.

Grey List

Approved but

suspicious

equipment.

Stolen?

Defective?

EIR

? ? ?

IMEI

White List

Approved,

error-free,

unsuspicious

equipment.

Fig. 14 Black, gray and white lists


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3.5 Serving GPRS Support Node SGSN

The Serving GPRS Support Node SGSN (in the packet-switched part of the core network) is the counterpart to the Visited MSC VMSC (in the circuit-switched part).

SGSN

��is the node serving GPRS mobile stations in a region;

��traces the location of the respective GPRS MS ("routing area");

��is responsible for the paging of MS;

��performs security functions and access control (authentication/cipher setting procedures,...).

��performs routing/traffic management;

��collects charging data;

��provides interfaces to GGSN (Gn), Packet Control Unit PCU (Gb), other PLMN, HLR, VLR, SMS-GMSC, and EIR.

3.6 Gateway GPRS Support Node GGSN

Gateway GPRS Support Node GGSN is comparable to a Gateway MSC.

GGSN

��is the node allowing contact/interworking between a GSM PLMN and a packet data network PDN (Gi interface);

��contains the routing information for GPRS subscribers available in the PLMN;

��has a screening function;

��can inquire about location information from the HLR (optional)

��transfers data/signaling to SGSN.



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Mobile DTE

SGSNServing GPRS

Support Node

PSTN

Internet

Intranet

X.25

GGSNGateway GPRS

Support Node

VMSC /

VLRGMSC

HLR

ISDNPCU

BSS

GPRS subscription data

(GPRS Register GR)

GPRS Support Nodes

SGSN:Interface to Base Station System

GGSN:Interface to Packet Data Network

Fig. 15 Packet Switches in the Core Network


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4 Radio Access Network: Radio Subsystem


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The Radio Subsystem RSS is composed of:

��Mobile Station MS

��Base Station System BSS

��Radio Interface Um

The Mobile Station and the Base Station System communicate across the Um interface ("air interface" or "radio link").

The BSS includes the:

��Base Station Controller BSC

��Base Transceiver Station Equipment BTSE

��Transcoder and Rate Adapter Unit TRAU

Note: The Siemens realization of GSM's BSS is called Siemens Base Station System SBS.

The connections between the network elements are typically PCM30 (or PCM24) lines running via copper lines, coaxial cables or microwave links.

The following table summarizes the (terrestrial) RSS interfaces and their associated PCM lines:

Interface PCM line Protocol Used

A PCMA CCSS#7 (open interface to circuit-switched core network)

Asub PCMS LAPD ("LPDLS", proprietary)

Abis PCMB LAPD ("LPDLM & LPDLR", proprietary)

Gb PCMG BSSGP (open interface between SGSN and Packet Control Unit PCU (located in BSC))



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Radio Subsystem (RSS)

BSS

To/from

SSS

To/from

OMS

Mobile

Station

Um

Fig. 16 Radio Subsystem RSS

Transcoder

and

Rate Adapter

Unit

TRAU

Base Station

Controller

BSC

Abis Asub

Base Station System BSS

A

S

G

S

N

M

S

C

Gb

PCU

Base

Transceiver

Station

Equipment

BTSE

Fig. 17 Base Station Subsystem BSS


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4.1 Base Station Controller BSC

The Base Station Controller is the "brain" of the Base Station System.

The BSC

��switches traffic channels from the core network to the MS (via BTSE),

��handles the signaling between access and core network,

��supervises the complete BSS (central Operation & Maintenance).

The BSC is composed of max 2 shelves (in an indoor rack).

BSC capacity figures are given in the following table:



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Lamp Panel

X

T

L

P

8

X

T

L

P

7

X

T

L

P

6

P

P

L

D

14

P

P

L

D

13

P

P

L

D

12

P

P

L

D

11

P

P

L

D

10

P

P

L

D

9

X

T

L

P

5

X

T

L

P

4

X

T

L

P

3

P

P

L

D

8

P

P

L

D

7

P

P

L

D

6

P

P

L

D

5

P

P

L

D

4

P

P

L

D

3

X

T

L

P

2

X

T

L

P

S

1

P

W

R

S

1

P

W

R

S

0

P

L

L

H

0

X

T

L

P

1

P

P

C

C

1

P

P

L

D

1

P

L

L

H

1

X

T

L

P

0

P

W

R

S

1

P

W

R

S

0

X

T

L

P

S

0

P

P

C

C

0

P

P

L

D

2

P

P

L

D

0

D

K

40

0

I

X

L

T

0

U

B

E

X

0

S

N

X

X

0

T

D

P

C

0

M

E

M

T

0

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P

C

C

0

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P

C

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1

M

E

M

T

1

T

D

P

C

1

S

N

X

X

1

U

B

E

X

1

I

X

L

T

1

D

K

40

1

EXPANSION

Fuse and Alarm

Panel

Base

Fig. 18 BSC Rack Layout (Example: BR5.5 Hardware without PCU)


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4.2 Base Transceiver Station Equipment BTSE

The Base Transceiver Station Equipment comprises the radio transmission and reception equipment, including the antennas, and also the signaling processing for the radio interface.

The BTSE provides up to 24 transceivers (TRX) and serves up to 24 cells (in picoBTS, BS24x the number of cells is 12). Max 8 racks in total (max 3 of which provide TRX) make up a BTSE.

Max 2 PCMB lines terminate on BTSE.

Two (main) product lines are in use:

��The ("classic") BTSone and

��the (new) BTSplus family.

BTSE Type Product Line

Max Number of TRX

Max Total No of Racks

(Radio/Service Racks)

BS40, 41 BTSplus 4 5 (1/4)

BS240, 241 BTSplus 24 8 (3/5)

BS240XL BTSplus 24 7 (2/5)

BS242 BTSplus 24 1

BS82 BTSplus 8 2 ("cabinets")

BS20, 21, 22 BTSone 2 1

BS60, 61 BTSone 6 2 (1/1)

BTSE names indicate if they are used

��indoor (-5 ... +55 �C, last digit "0"),

��outdoor (-45 ... +55 �C, last digit "1") or

��indoor and outdoor (perhaps with some limitation, last digit "2").

The first digits give the number of TRX available.



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Fig. 19 BS240 (indoor, left) and BS241 (outdoor, right) Base Racks

Fig. 20 E-Micro BTS ("BS82") for indoor and outdoor use


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4.3 Transcoder and Rate Adapter Unit TRAU

The Transcoder and Rate Adapter Unit performs de-/coding (for speech calls) and rate adaptation (for circuit-switched data calls).

The two main functional units of the TRAU are:

��Transcoder for speech coding/compression

��Rate Adapter for data rate adaptation

Although the TRAU is (logically) part of the Base Station System (and controlled from the BSC), it is usually located near the MSC to save transport capacity on PCMS (typical PLMN speech takes 16 kbit/s bandwidth vs. 64 kbit/s for PSTN speech).

The TRAU (shelf) has a capacity of 120 speech channels. Up to four (independent) shelves can be housed in the same rack.



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T

P

W

R

0

T

R

A

C

0

T

R

A

C

1

T

R

A

C

2

M

S

C

I

0

B

S

C

I

0

B

S

C

I

1

M

S

C

I

1

T

R

A

C

3

T

R

A

C

4

T

R

A

C

5

T

P

W

R

1

Fuse and Alarm Panel 0




TRAU-0

TRAU-1

TRAU-2

TRAU-3

Fig. 21 TRAU Rack (Note: Dedicated TPWR modules are not required for the latest TRAU hardware.)


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4.4 Local Maintenance Terminal LMT

The LMT software runs on a desktop or a laptop computer. The LMT is indispensable for commissioning BSC, BTSE and TRAU and for local maintenance.

For (central) operation and supervision of larger networks the OMC's graphical user interface is better suited.

The O interface between OMC-B and BSC is either a X.25 connection realized as a dedicated link via a PSPDN or embedded within the Asub and A interfaces via a semi permanent connection through the MSC ("nailed-up connection").

The LMT's T interface is based on X.21+V.11 and HDLC+ proprietary layer specifications (CCITT) using the LAPB protocol and is used for the BSC, TRAU and all BTSE.

BTSE TRAUBSC

SIEMENSNIXDORF

SIEMENSNIXDORF

LAN

OMC

O interface

LMT

T-Interface T-InterfaceT-Interface

Fig. 22 Operation & Maintenance Interfaces on Base Station System



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4.5 Documentation

SBS documentation may be divided into the following main categories:

System Descriptions

providing information on network architecture and functions for e.g. GSM-PLMN or GPRS-PLMN.

Technical Descriptions TED

explaining functions, features, hardware and software architecture as well as external and internal interfaces.

Operator Guidelines OGL

giving LMT and OMS-B handling information.

Operation Manuals OMN

containing the most relevant (LMT and OMS-B) operating procedures.

Maintenance Manuals MMN

providing replacement procedures for all SBS modules.

Installation and Test Manuals ITMN

explaining the step-by-step commissioning (incl. e.g. switch settings).

Command Manuals CML

referencing all available commands (and input parameter ranges) for LMT and OMS-B.

Output Manuals OML

referencing all error messages and suggesting repair actions.

Installation Manuals IMN

dealing with the hardware related aspects of installation e.g. physical setup and cabling.

Performance Measurements PM

providing information on performance counters and signaling message flows.


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The documentation is provided electronically on CD-ROM as PDF (portable data format) files. The operating documentation includes the following manuals (in alphabetical order, for release O/BR5.5 more than 80 documents in total):

Manual Abbreviation Contents

Abbreviations Abbreviations

Command Manuals CML For BSC, BTSE, OMS-B, TRAU

Delta Manual New features (relative to the previous release)

Glossary Explanation of terms

Installation Manuals IMN Hardware related information, for BSC, BTSE (all types), OMS-B, TRAU

Installation and Test Manuals

ITMN Step-by-step commissioning, for BSC, BTSE (all types), OMS-B, TRAU

Maintenance Manuals

MMN Replacement procedures incl. hardware related information, for BSC, BTSE (all types), OMS-B, TRAU

Network Integration Manual

NIMN Testing network configuration (incl. database) "as planned"

Technical Description OMS-B

TED:OMS-B Technical background information on OMS-B

Operator Guidelines OGL For LMT, OMS-B (commands, description, graphical panels, interactive panel architect, main menu, states and repertories, tools)

Output Manuals OML All error messages for BSC, BTSE and TRAU

Operation Manuals OMN For BSC (via LMT) and OMS-B

Performance Measurements

PM SBS counters and SBS message flows

System Descriptions For GPRS PLMN, GSM PLMN, GSM-R, Network System Concept, WAP/MIA for Mobile Solutions

Technical Descriptions

TED Separate documents for "Common" and BS40/41, BS240/241, BS240XL, BS242, BS82, BS2x/6x/11 information (hardware related)

Terminology Guide to documentation



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5 Radio Interface Um


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The Um interface is the radio interface between the BTSE (antenna) and the mobile station.

Mobile Station

Uplink

Downlink

Um

BTSE

Fig. 23 Um interface



39

5.1 GSM900/GSM1800/GSM1900 Frequency Bands

A specific frequency range is assigned to GSM900/GSM1800/GSM1900 systems.

Each frequency range is divided into two sub-bands:

1. Uplink UL for the radio transmission between the MS and the BTSE,

2. Downlink DL for the radio transmission between the BTSE and the MS.

The fact that different frequency bands are used for uplink and downlink transmission is called Frequency Division Duplex FDD.

GSM 900

Uplink Downlink

GSM 1800

Uplink Downlink

GSM 1900

Uplink Downlink

GSM 1800 (DCS):

1710-1785 & 1805-1880 MHz

Duplex Distance:

95 MHz

Primary GSM:

890-915 & 935-960 MHz

Extended GSM:

880-915 & 925-960 MHz

Railway GSM:

876-915 & 921-960 MHz

Duplex Distance:

45 MHz

GSM 1900 (PCS):

1850-1910 & 1930-1990 MHz

Duplex Distance:

80 MHz

1850 199019301910

MHz

1710 188018051785

MHz

880 960925915

MHz

Fig. 24 Frequency ranges for GSM900, GSM1800 and GSM1900


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5.2 Radio Frequency Carriers RFC

Both sub-bands (Uplink and Downlink) are divided into Radio Frequency Carriers with a bandwidth of 200 kHz each (Frequency Division Multiple Access FDMA).

The differences between GSM900, GSM1800 (DCS) and GSM1900 (PCS) relate to:

��Operating frequency

��Bandwidth of the sub-bands

��Number of Radio Frequency Carriers RFC available.

(Extended)

GSM 900

Uplink Downlink

GSM

1800 (DCS)

GSM

1900 (PCS)

880

MHz915

MHz

925

MHz

960

MHz

1710

MHz1785

MHz

1805

MHz

1880

MHz

1850

MHz1910

MHz

1930

MHz

1990

MHz

C C C C. . . CCCC . . .

200

kHz

200

kHz

300 RFC (299 used)

375 RFC (374 used)

175 RFC (174 used)

Fig. 25 Radio frequency carriers for GSM900, GSM1800 (DCS) and GSM1900 (PCS)



41

Depending on the traffic volume, every radio cell uses one or more RFC. Since the number of RFC is limited, the same RFC must be used several times. To avoid co-channel interference, a safe distance is required between the BTSE using the same RFC. This safe distance is called reuse distance.

The size of a single cell depends on topology and on traffic volume. In hilly regions or in densely populated areas with high traffic volume the cell radius is kept small. A small cell radius can be achieved by reducing the output power of the base station

RFC 24

RFC 32

RFC 8

RFC 5

RFC 2

RFC 17

RFC 13

RFC 47

RFC 40RFC 24

RFC 32

RFC 11

RFC 47

RFC 8

RFC 5

RFC 2

reuse distance (~ 4 r)

Fig. 26 Radio frequency carriers and reuse distance


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5.3 Physical Channels

One RFC is divided into eight time slots (Time Division Multiple Access TDMA). This is comparable to Pulse Code Modulation (transmission) technology where PCM30/24 frames are composed of 32/24 time slots, respectively.

RFC x

RFC yRFC x

RFC x

RFC y

TDM

A Fra

me

(4.6

15 m

s)

200 kHz 200 kHz

0 15 31

TDMA Frame

(125 �s)

PCM 30

RFC x

Fig. 27 Time division multiple access TDMA

RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

101

2

3

4

5

6

7RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

201

2

3

4

5

6

7RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

301

2

3

4

5

6

7 RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

174

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

174

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

2

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

1

Uplink Downlink

FDMA FDMA

f

t

TDMATDMA

FDD

Fig. 28 Frequency division multiple access and time division multiple access (t=time, f=frequency)



43

A physical channel is defined by the timeslot number (in the TDMA frame) on a specific carrier (RFC) in the uplink band and the corresponding carrier in the downlink band.

A physical channel may carry only one or several logical channels ("multiplexing").

RFC RFC

Physical Channel

Uplink Downlink

corresponding

Traffic

Channel

Traffic

Channel

Fig. 29 Physical channel


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5.4 Logical Channels

Logical channels carry payload (speech or data) or signaling.

For circuit-switched CS traffic there is a clear separation between the physical channels used for payload and those used for signaling.

For packet-switched PS traffic, however, the same physical channel may carry both signaling and payload.

5.4.1 Logical Channels for CS Services

Logical ChannelsLogical Channels

Traffic ChannelsTraffic Channels

Full Rate

TCH/F

Half Rate

TCH/H

Signaling ChannelsSignaling Channels

Broadcast

Control

Channels

BCCH

Broadcast

Control

Channels

BCCH

Common

Control

Channels

CCCH

Common

Control

Channels

CCCH

Dedicated

Control

Channels

DCCH

Dedicated

Control

Channels

DCCH

Fig. 30 Logical channels for circuit-switched traffic



45

5.4.2 Logical Channels for PS Services

Logical ChannelsLogical ChannelsLogical Channels

Traffic ChannelsPacket Traffic

ChannelsPacket Traffic

Channels

Packet

Associated

Control

Channel

Signaling ChannelsPacket Signaling

ChannelsPacket Signaling

Channels

Packet Broadcast

Control Channel

PBCCH

Packet Broadcast

Control Channel

PBCCH

Packet

Timing

Advance

Contr. Channel

Packet

Data

Traffic

Channel

Packet Common

Control Channel

PCCCH

Packet Common

Control Channel

PCCCH

Fig. 31 Logical channels for packet-switched traffic


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5.5 Signaling Channels

Three types of signaling channels are used:

��Broadcast Control Channels (CS and PS)

��Common Control Channels (CS and PS)

��Dedicated Control Channels (CS only)

5.5.1 Broadcast Control Channels

The broadcast control channels are defined for the BTSE to MS direction (downlink) only and are subdivided into the:

1. Broadcast Control Channel BCCH (defined per cell) which informs the mobile station about various cell parameters including country code, network code, local area code, PLMN code, RF channels used within the cell where the mobile is located, surrounding cells, and frequency hopping sequence number.

2. Frequency Correction Channel FCCH carrying information for frequency correction of the MS downlink ("fine tuning" of MS to BTS).

3. Synchronization Channel SCH providing information about the frame number and BTS identification code BSIC.

4. Cell Broadcast Channel CBCH used by the cell broadcast service for distributing e.g. weather information.

5.5.2 Common Control Channels

Common Control Channels are specified as unidirectional channels, either on the downlink or the uplink. The following sub-channels are distinguished:

1. Paging Channel PCH which is used DL to page mobile stations,

2. Access Grant Channel AGCH, also used DL, to assign a dedicated channel to a mobile station,

3. Random Access Channel RACH which is an UL channel to indicate a mobile station’s request for a dedicated channel,

4. Notification Channel NCH, which is used in ASCI (Advanced Speech Call Items, paging MS using Voice Group Call Service / Voice Broadcast Service VBS).



47

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Broadcast Control

Channel BCCHBroadcast Control

Channel BCCH

Frequency Correction Channel FCCH

Synchronization Channel SCH

Broadcast Control Channel BCCH

Cell Broadcast Channel CBCH

DL

DL

DL

DL

Common Control

Channel CCCHCommon Control

Channel CCCH

Random Access Channel RACH

Access Grant Channel AGCH

Paging Channel PCH

Notification Channel NCH

UL

DL

DL

DL

Dedicated Control

Channel DCCHDedicated Control

Channel DCCH

Stand-alone Dedicated

Control Channel SDCCH

Slow Associated

Control Channel SACCH

Fast Associated

Control Channel FACCH

DL & UL

DL & UL

DL & UL

Fig. 32 Signaling channels


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BTSE

BCCH

cell identity: 262041422

Fig. 33 Example: Broadcast Channel information

BTSE

RACH

Request of a SDCCH

12142341234

Fig. 34 Example: Random Access Channel



49

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The BSC assigns a SDCCH to the MS via the Access Grant Channel AGCH.

BTSE

AGCH

SDCCH provided

12142341234

Fig. 35 Example: Access Grant Channel


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All of the signaling information (i.e. dialed digits, authentication, traffic channel assignment) for call setup is exchanged over the SDCCH.

BTSE

SDCCH

Control Information

Fig. 36 Example: Stand-alone Dedicated Control Channel



51

5.5.3 Dedicated Control Channels

Dedicated Control Channels are full duplex (bi-directional) channels. They are subdivided into the

1. Slow Associated Control Channel SACCH, which is always associated with a TCH or SDCCH (embedded within the same frame structure). The SACCH is used for the transmission of radio link measurement data.

2. Fast Associated Control Channel FACCH, which is always associated with a TCH and is used for the transmission of signaling data, after the set-up of the call, when the SDCCH has been already released. The FACCH data are inserted into the TCH burst instead of traffic data (bit stealing), indicated by a "stealing flag" (i.e., the FACCH may contain handover information).

3. Stand-alone Dedicated Control Channel SDCCH which is normally assigned after the MS access request has been granted and is used for signaling purposes (set-up of the calls etc.)


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5.6 Channel Combinations

The following combinations of logical channel types are specified (GSM Rec. 05.02, version 8.3.1):

1. TCH/F + FACCH/F + SACCH/TF

2. TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)

3. TCH/H(0,0) + FACCH/H(0,1) + SACCH/TH(0,1) + TCH/H(1,1)

4. FCCH + SCH + BCCH + CCCH

5. FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/C4(0...3)

6. BCCH + CCCH

7. SDCCH/8(0...7) + SACCH/C8(0...7)

8. TCH/F + FACCH/F + SACCH/M

9. TCH/F + SACCH/M

10. TCH/FD + SACCH/MD

11. PBCCH + PCCCH + PDTCH + PACCH + PTCCH

12. PCCCH + PDTCH + PACCH + PTCCH

13. PDTCH + PACCH + PTCCH

where CCCH=PCH+RACH+AGCH+NCH and PCCCH=PPCH+PRACH+PAGCH+PNCH

14. CTSBCH + CTSPCH + CTSARCH + CTSAGCH

15. CTSPCH + CTSARCH + CTSAGCH

16. CTSBCH

17. CTSBCH + TCH/F + FACCH/F + SACCH/CTS

18. E-TCH/F + E-IACCH/F + E-FACCH/F + SACCH/TF

19. E-TCH/F + E-IACCH/F + E-FACCH/F + SACCH/M

20. E-TCH/F + E-IACCH/F + SACCH/M

21. E-TCH/FD + E-IACCH/F + SACCH/MD

22. CFCCH + CSCH + CPBCCH + CPCCCH + PDTCH + PACCH + PTCCH

23. CPCCCH + PDTCH + PACCH + PTCCH

Note 1: For SMSCB the CBCH replaces SDCCH no.2 in 5) and 7) above.

Note 2: A combined CCCH/SDCCH allocation (5) may only be used when no other CCCH is allocated.

Note 3: 8) – 10) are used in multislot configurations.

Note 4: 14) – 17) are used in CTS.

Note 5: 22) – 23) are used in EGPRS Compact.



53

5.7 Multiframes

5.7.1 Traffic Channel Multiframe (CS)

For circuit-switched traffic, a TCH is always allocated together with its associated slow-rate channel (SACCH). Twenty-six TDMA frames (=120 ms) are grouped together to form one multiframe for speech/data. TCH are sent on 24 timeslots; one slot is used for SACCH signaling information and one slot remains unused (for full rate).

Not only subscriber information (speech/data) can be transmitted in a traffic channel TCH. If the signaling requirement increases (e.g. for a handover), signaling information FACCH is transmitted instead of TCH. A so-called stealing flag in the normal burst indicates this.

T

7

0

1

TT T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

TT

0 1 62 3 4 7 8 9 10 11 12 135 15 16 17 18 19 20 21 22 23 24 2514

SSS

S

ST T

T

8 bursts per cycle time

T TCH

S SACCH

unused

Fig. 37 Traffic channel multiframe


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5.7.2 Signaling Channel Multiframe (CS)

For circuit-switched traffic, fifty-one TDMA frames (=235.4 ms) are grouped together to form one signaling channel multiframe.

As an example the following figure shows the basic combination including (downlink direction) a FCCH, SCH, BCCH, and AGCH/PCH all on the same timeslot (i.e., 0). The uplink direction contains a RACH.

The BCCH with the AGCH/PCH uses 40 slots per multiframe. These 40 timeslots are grouped together as 10 groups of four. The BCCH use the first four timeslots of the first group of 10. The remaining timeslots are used by the AGCH/PCH.

The FCCH is sent on timeslot 0 in frames 0, 10, 20, 30 and 40, while the SCH is sent in timeslot 0 on frames 1, 11, 21, 31 and 41.

T

7

0

1

8 B P c y c le t im e

0 1 0 2 0 3 0 4 01

621 1

1 22 1

2 23 1

3 24 1

4 2

F C C H

S C H

B C C H

A G C H /P C Hd o w n lin k

R A C HU P L IN KUPLINK

DOWNLINK

Fig. 38 Signaling channel multiframe (including FCCH, SCH, BCCH and AGCH/PCH)



55

5.7.3 Multiframes in GPRS

The GPRS packet data traffic is arranged in 52-type multiframes (GSM Rec. 03.64). 52 TDMA frames are combined to form one GPRS traffic channel multiframe, which is subdivided into 12 blocks with 4 TDMA frames each. One block (B0-B11) contains one radio block in each case (4 normal bursts, which are related to each other via convolutional coding). Every thirteenth TDMA frame is idle. The idle frames are used by the MS to determine the various base station identity codes BSIC, to carry out timing advance updates procedures or interference measurements for power control.

For packet common control channels PCCH, conventional 51-type multiframes can be used for signaling or 52-type multiframes.

iB0 B1 B2 B3 B4 B5 i B6 B7 B8 i B9 B10B11 i

52 TDMA Frames = PDCH Multiframe

4 Frames 1 Frame

B0 - B11 = Radio Blocks (Data / Signalling)i = Idle frame

• BCCH indicates PDCH with PBCCH (in B0)

• DL: this PDCH bears PDCCH & PBCCH

PBCCH in B0 (+ max. 3 further blocks; indicated in B0)

PBCCH indicates PCCCH blocks & further PDCHs with PCCCH

• UL: PDCH with PCCCH: all blocks to be used for PRACH, PDTCH, PACCH

PDCH without PCCCH: PDTCH & PACCH only

Idle frame:• Identification of BSICs

• Timing Advance Update Procedure

• Interference measurements

for Power Control

Fig. 39 GPRS multiframes


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5.8 Timeslot Structure

The RF signal sent in a timeslot is called "burst". Depending on the type of logical channel transmitted, different kinds of bursts are used:

��Normal burst

��Access burst

��Frequency correction burst

��Synchronization burst

��Dummy burst

The modulation is applied for the useful duration of the burst. In general, the useful duration of the burst is equivalent to 148 bits excluding the access burst that has a useful duration equivalent of 88 bits. To minimize interference, the mobile station is required during the guard periods to attenuate its transmission amplitude and to adjust possible time shifts and amplitudes of the bursts.

The normal burst is used to carry information on traffic channels and on signaling channels (exception: Random Access Channel, Synchronization Channel and Frequency Correction Channel). Its structure is shown below:

��T = Tail Bits: This burst section consists of three bits (always coded with "000") and is needed for synchronization at the receive side.

��Coded (encrypted) bits: There are two burst sections, which contain the coded and encrypted traffic information (speech or data) in 2 x (57 +1) bits. The 1 bit is called stealing flag and indicates whether the 57 bits are really user data or FACCH signaling information.

��Training Sequence: This burst section contains 26 bits used for synchronization. Eight different training sequences have been defined by GSM.

��GP = Guard Period: These "8.25" bits serve to guard phase deviations (due to moving MS) and to reduce the transmission power.

The access burst has an extended guard period that helps to control the initial time lag of the signals due to the distance between mobile station and BTSE. Once the time lag has been corrected (timing advance), the remaining time lag resulting from the alteration of distance of a moving mobile station is controlled with the aid of the normal guard period of 8.25 bit duration.

Frequency correction bursts are sent by the BTSE and are used by the mobile station to adjust its receiver and transmitter frequencies (frequency synchronization).

Synchronization bursts are used to establish an initial bit and frame synchronization (time synchronization).

Dummy bursts are inserted if TCH timeslots on the BCCH carrier are not filled with user data to reach a constant level of the BCCH carrier. This is necessary because the level of the BCCH carrier is evaluated for handover decisions and also for the decision, which is the serving cell (for call set up).



57

7 0 1 2 3 4 5 6 7 0 1 . . .

T

3

Coded Bits

58

Training Sequence

26

Coded Bits

58

T

3

GP

8.25

. . .

Normal Burst

Fig. 40 Normal burst

3

T

57

Encrypted

1

S

26

Training1

S

57

Encrypted

3

T

8.25

GP

8

T

41

Synch. Sequence

36

Encrypted3

T

68,25

GP

3

T

142

Fixed Bits

3

T

8.25

GP

3

T

39

Encrypted

64 Extended

Training Sequence39

Encrypted

3

T

8.25

GP

3

T

58

Mixed Bits

26

Training

58

Mixed Bits

3

T

8.25

GP

Normal Burst

Access Burst

Frequency Correction Burst

Synchronization Burst

Dummy Burst

Bursts on Um interface:

Duration 577 ��s or 156.25 bit

Fig. 41 Burst formats


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59

6 GSM Basic Features


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6.1 Frequency Hopping

While a single transmit signal leaves the sender (BTSE or MS), several signals arrive at the receiver (MS or BTSE), not only the "original" but additional (delayed and distorted) signals which are due to multipath propagation as

��there is a direct path between the BTSE and the MS (indicated by 1 in the following picture), and there are

��delayed paths due to reflections at different obstacles, like buildings, rocks etc. (indicated by 2 in the following picture)

The superpositions of the different receive signals results in positive or negative interference that is strongly dependent on the MS position. Worst case is a call drop, when the level of the receiver signal falls below the threshold of the required RX sensitivity.

Note: In addition to this "fast fading" there is also "slow fading" of the RX signal due to electromagnetic attenuation increasing with the distance between sender and receiver.



61

SIEMENS

MS

BTSE

1

2

2

Fig. 42 Multipath propagation

Rx- Sensitivity

Distance

Receive

Level approx. 17 cm at 900 MHz

(λ/2)

(Fast)

Fading Dips

Fig. 43 RX level due to multipath propagation (fast fading)


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Frequency Hopping, applied both UL and DL, reduces the losses due to multipath propagation by "averaging out" the "bad" frequencies.

It is performed by assigning a sequence of different frequencies to a radio channel. Subsequent timeslots in a generic frame follow a specified hopping sequence (cyclic or random). Thus, the assigned radio channel is seen as "changing" or "hopping" from one frequency to the next.

There are two kinds of Frequency Hopping:

��Base band Hopping

��Synthesizer Hopping.

With Base band Hopping each carrier unit transmits at a fixed ARFCN, its output frequency is not changed. The switching of the channel to the desired TRX is performed in the base band parts of the BTS.

The advantage of Base band Frequency Hopping is that all RF parts operate at fixed frequencies, so any type of combiner may be used. However, the number of carrier units available limits the number of hopping frequencies.

With Synthesizer Frequency Hopping the RF part of each TRX switches between hopping frequencies. Its advantage is that the number of carrier units available does not limit the number of hopping frequencies. However, filter combiners do not support Synthesizer Hopping.

With the help of frequency hopping an improvement of

��the S/N ratio in noise limited systems (rural areas)

��the C/I ratio in interference limited systems (urban areas)

is achieved.

The logical channels are divided in two groups regarding the frequency hopping:

��non-hopping channels which don't change their frequency (BCCH, CCCH)

��hopping channels which may change their frequency (TCH, SDCCH)



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frame 0 frame 1 frame 2 frame 3 frame 4 frame 5

RFC 1

RFC 2

RFC 3

RFC 4

RFC 5

TDMA frame

4,615 ms

Timeslot

0,577 ms

Fig. 44 Frequency hopping with 5 frequencies


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6.2 Antenna Diversity

Antenna Diversity is used to improve the receiver performance and in particular the uplink quality under fading conditions:

Two spatially separated (BTS) receiving antennas are installed. Each antenna provides an independent RX signal. Diversity Combining of the two receive signals is performed in the carrier unit.

Diversity Combining

(CU/TPU)

Rx Path#0 Rx Path#1

RX Signal

Ca. 20 �

Fig. 45 Horizontal antenna diversity



65

6.3 Power Control

Power Control is used to adapt the RF output power (of the MS and the BTS) dynamically to the propagation conditions on the radio path. The aim is to use the lowest possible TX power still yielding acceptable transmission quality. Thus,

��the power consumption of the Mobile Station and

��the total interference level in the radio system

are minimized.

Power Control for the MS is based on uplink measurements, gathered by the BTS. Power Control for the BTS is based on downlink measurements, gathered by the MS.

The criteria for power control decisions are

��the received signal strength

��the received signal quality.

Threshold comparisons, preceded by a measurement averaging process, are made to determine whether a power increase or decrease is required. The measurement averaging process and the power control decision algorithm for each call in progress are performed by the BTS.

BTSE

SIEMENS

Distance D2

Distance D1

Power Out 1

Power Out 2

Fig. 46 RF power control


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6.4 Handover

Handover is required to maintain an ongoing call when the Mobile Station passes from one cell coverage area to another. Handover means that a call in progress is switched over seamlessly from one radio channel to another.

(Note: In Adaptive Multirate Handover, the change is between different "codecs".)

Handover takes place between radio channels that may belong

��to the same BTS (intracell handover) or

��to different BTS (intercell handover).

If the handover is controlled by the BSC or MSC, it is called inter-BSC or inter-MSC handover, respectively.

The handover due to radio criteria is determined from

��uplink measurement results (gathered by the BTS) and

��downlink measurement results (gathered by the MS).

In detail, the radio criteria are

a) received signal strength

b) received signal quality (determined from Bit Error Rate)

c) MS - BTS distance (determined from timing advance)

d) better cell (power budget of serving cell relative to adjacent cells)

In contrast to the first three criteria, which are imperative, the better cell criterion is optional. It derives from signal strength measurements carried out by the MS on the BCCH carriers broadcast by the adjacent BTS. In a well-planned network, "better cell" is the overwhelming HO cause determining the cell boundaries.

In addition, the following network criteria may be evaluated:

a) serving cell congestion (directed retry for call setup, HO due to BSS resource management criteria)

b) MS - BTS distance (in extended cells)

c) RX level and (optional, in addition) MS - BTS distance (in concentric cells)

The decision whether a handover is required for a call in progress derives from threshold comparisons that are applied to the respective (averaged) measurement results.

If an intercell handover is to be initiated, the power budget criterion is applied to set up a list of preferred handover target cells in a decreasing order of priority. This target cell list forms the basis for the final handover decisions made in the BSC or the MSC.

The measurement averaging processes, the threshold comparison processes as well as the target cell list compilation for each call in progress are performed by the BTS. The final handover decision and handover execution, however, are made by the BSC or MSC.



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BSC 2

1

2

3

BSC 1

MSC1

1 intracell handover

2 intercell handover – intra-BSC

3 intercell handover – inter-BSC

Fig. 47 Types of handovers


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6.5 Multiband Operation

Multiband Operation means that the same network operator provides GSM services in both GSM900 and GSM1800 frequency bands.

Therefore, the operation of "dual band" MS as well as handover between cells belonging to different bands is also supported, with an appropriate management of the target cell list.

The transceiver equipment for both frequency bands is located in the same BTSE. This allows a network operator to use an already existing network infrastructure when expanding a GSM900 network with GSM1800 equipment and vice versa.

Multiband operation may even feature a "Common BCCH", i.e. a situation when one BTS (with a single BCCH) offers carrier frequencies in both GSM900 and GSM1800 bands.

BTS-5

GSM1800

BSC

BTS-4

GSM 1800

BTS-3

GSM 900

BTS-2

GSM 900BTS-1

GSM 900

GSM1800

BTS-0

GSM 900

Fig. 48 Network with mixed cell configuration GSM900/ GSM1800



69

6.5.1 Concentric Cells

A concentric cell is subdivided into

��inner area and

��complete area.

The output power of the TRX serving the inner area (with small cell radius) is strongly reduced (or belongs to GSM1800).

The concentric cell feature is used to introduce an additional frequency re-use pattern for the frequencies in the inner area with a shorter re-use distance resulting in increased network capacity.

A particular type of intracell handover (between inner and outer area) is available.

Concentric cells are particularly suited for multiband operation (perhaps with Common BCCH).

f5

f2

f6

f3

f4

f1

BTSE

complete areainner area

Fig. 49 Sectored concentric cells with inner and complete area


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6.6 Hierarchical Cell Structure

In a typical network evolution, the network organization is driven first by coverage considerations and later on by capacity requirements. In the first stage, coverage of the network area is achieved easily by using umbrella cells and macro cells. With the evolution of the network implementing small cells, i.e. micro cells and pico cells, increases capacity. Thus, fully developed networks are characterized by a layered structure of hierarchical cells (perhaps in multiband operation).

Overlay and macro cells are characterized by

��high transmit power,

��wide area coverage (large cell radius),

��antenna above rooftop level,

��fast-moving MS (speed-sensitive HO),

��redundant capacity (for traffic overflow from micro cells).

Micro and pico cells have

��low transmit power,

��small area coverage (e.g. hot spots in city centers or airports),

��antenna below rooftop level (or even indoor),

��slow-moving MS.

Umbrella Cell

GSM 900

Layer 5

Macro Cell

GSM 900

Layer 4

Small Cell

GSM 900

Layer 3

Pico Cell

GSM 900/1800

Layer 1

Micro Cell

GSM 900/1800

Layer 2

Fig. 50 Multiple layer network



71

6.7 GSM-Railway GSM-R

Railway companies in Europe have a lot of different communication requirements for which several incompatible systems are used in parallel. To overcome this coexistence and to simplify communication an adaptation of the GSM standard was developed, GSM-Railway GSM-R.

Compared to traditional GSM900, the GSM-R standard has the following main features:

��The frequency range of 4 MHz in the Uplink and Downlink is located below the frequency range of the extended GSM band.

��Since the speed of the trains can be up to 250 - 400 km/h the Doppler frequency shift due to fast moving MS is bigger than in conventional GSM.

��Redundancy has high priority: two BSC with BTSE connected in a loop configuration may be serving the same track; redundant O-Links may be used between OMC-B and BSC.

SIEM ENS

S IEME NS

S IEM ENS

BSC

MSC/ VLR

HLR

Railway PBA

NetworkATC Center

OMC-S OMC-B

BTSE

BTSE

BTSE

TRAU

Cell #1

Cell #2

Cell #3

Fig. 51 GSM-Railway network structure


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73

7 Data Transmission in GSM Phase 2+


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To increase the data transmission rates, in GSM phase 2+ new bearer services with rates exceeding those of ISDN are defined:

��High Speed Circuit Switched Data HSCSD

��General Packet Radio Service GPRS

��Enhanced Data Rates for GSM (Global) Evolution EDGE

High Speed Circuit Switched Data HSCSD

HSCSD (Rec. 02.34) is a circuit switched data service (only point-to-point) for applications with higher bandwidth demands and continuous data stream, e.g. video streaming or video telephony. The higher bandwidth is achieved by combining 1-8 physical channels for one subscriber. Additionally, the data transmission codec is changed such that a maximum of 14.4 kbit/s instead of 9.6 kbit/s is available per physical channel. Thus, HSCSD theoretically supports transmission rates up to 115.2 kbit/s. For implementing HSCSD, merely the GSM-PLMN software requires modification. More problematic is the high demand on (radio) resources.

General Packet Radio Service GPRS

With GPRS it is possible to combine 1-8 physical channels for one user, just as with HSCSD. Various new coding schemes with transmission rates of up to 21.4 kbit/s per physical channel enable theoretical transmission rates of up to 171.2 kbit/s. As opposed to HSCSD, GPRS is a packet-switched bearer service, meaning that several subscribers can use the same physical channel. GPRS is resource efficient for applications with a short-term need for high data rates ("bursty traffic" e.g. surfing the Internet, E-mail, ...). GPRS also enables point-to-multipoint transmission and volume-dependent charging. However, extensions of the GSM network and protocol architecture are required for GPRS implementation.

Enhanced Data rates for GSM Evolution EDGE

EDGE (Release`99) supports up to 69.2 kbit/s per physical channel by changing the GSM modulation procedure (8PSK instead of GMSK). Theoretically, transmission rates of up to 553.6 kbit/s (meeting 3G requirements) would be possible by combining up to 8 channels. A combination of GPRS and EDGE offers high bandwidth and highly economical frequency resource utilization at the same time.



75

Data transmission

in GSM Phase 2+

TS 1 •••• 8

Phase 1/2:

max 9.6 kbit/s (per TS)

HSCSD, GPRS, EDGE:

combining 1 - 8 TS

HSCSD (Circuit-Switched): 14.4 kbit/s

no HW changes, but high demand on radio resources

GPRS (Packet-Switched): 9.05 ... 21.4 kbit/s

HW and protocol changes, resource efficient

EDGE (CS / PS): max 43.2 / 59.2 kbit/s

new modulation type: 8-PSK instead of GMSK

Fig. 52 Data transmission in Phase 2+

115.2 kbit/s

High-

Speed

Circuit

Switched

Data

14.4 kbit/s

General

Packet

Radio

Service 171.2 kbit/s

21.4 kbit/s

345.6 kbit/s

473.6 kbit/s

Enhanced

Data

Rates for

GSM

Evolution

ECSD

EGPRS

43.2 kbit/s

59.2 kbit/s

Fig. 53 Netto transmission rates for HSCSD, GPRS and EDGE


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7.1 High Speed Circuit Switched Data HSCSD

High Speed Circuit Switched Data provides circuit switched data transmission using max. 8 TCH simultaneously (according to GSM; actually, the MSC architecture and the available MS support multislot operation on max 4 TCH).

HSCSD is particularly suited for real-time applications e.g. video communication.

High Speed Circuit Switched Data provides the following kinds of services:

14.4 kbit/s data (in a single time slot).

Transparent / Non-transparent services

For transparent HSCSD connections the BSC is not allowed to change the user data rate, but the BSC may modify the number of TCH used by the connection (in this case the data rate per TCH changes accordingly).

For non-transparent HSCSD connections the BSC is also allowed to change the user data rate (data compression on the air i/f, on fixed network side -to and from modem of internet provider - no compression at data rates 19.2 kbit/s, 38.4 kbit/s or 64 kbit/s in later versions).

Symmetric and pseudo-asymmetric services

In symmetric service the timeslot allocation for downlink and uplink is symmetric and the same data rates are used in downlink and uplink direction.

In pseudo-asymmetric service the timeslot allocation for downlink and uplink is symmetric but the data rate used in uplink direction is lower than in downlink direction.



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9.6 kbit/s /

14.4 kbit/s

28.8 kbit/s

57.6 kbit/s

High-Speed Circuit Switched Data HSCSD

Advantages:

+ high data rates

+ multislot operation

+ no new network elements

+ only software modification

Disadvantages:

- inefficient for bursty traffic

- new MS required

Fig. 54 High-speed circuit switched data - main features


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The following conditions characterize an HSCSD connection:

��All n radio timeslots are located on the same TRX.

��The same frequency hopping sequence and training sequence is used.

��Only TCH/F are used.

��In symmetric configuration individual signal level and quality reporting for each channel is applied.

��For an asymmetric HSCSD configuration individual signal level and quality reporting is used for those channels, which have uplink SACCH associated with them.

��The quality measurements reported on the main channel are based on the worst quality measured on the main and the unidirectional downlink timeslots used.

��In both symmetric and asymmetric HSCSD configurations the neighbor cell measurements are reported on each uplink channel used.

Handovers

All TCH used in an HSCSD connection are handed over simultaneously. The BSC may modify the number of timeslots used for the connection and the channel coding when handing over the connection to the new channels.

All kinds of handovers are supported.

Flexible air resource allocation

The BSC is responsible for flexible air resource allocation. It may modify the number of TCH/F as well as the channel coding used for the connection. Reasons for the change of the resource allocation may be either a lack of radio resources, handover and/or maintenance of the service quality.

The change of the air resource allocation is done by the BSC using new service level upgrading and downgrading procedures.

SBS implementation

The SBS provides max 4 TCH/F (at 14.4 kbit/s each) for one HSCSD call.



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ME45

GPRS capable,

Dual Band

S40

HSCSD capable,

Triple Band

Fig. 55 GPRS and HSCSD Phones


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7.2 General Packet Radio Service GPRS

GPRS offers packet-switched data transmission but requires new network elements making up the packet-switched core network:

7.2.1 Serving GPRS Support Node

Serving GPRS Support Node SGSN is on the same hierarchic level as an MSC and handles functions comparable to a Visited MSC.

SGSN

��is the node serving GPRS mobile stations in a region assigned to it;

��traces the location of the respective GPRS MS (Mobility Management);

��is responsible for paging the MS;

��performs security functions and access control (authentication/cipher setting procedures,...); procedures are based on the same algorithm, ciphers and criteria as in the former GSM. Ciphering algorithms have been optimized for the transmission of packet data;

��performs routing/traffic management;

��collects charging data;

��realizes the interfaces to GGSN (Gn), PCU (Gb), other PLMNs (Gp). Further interfaces may be added (Gr, Gs, Gf).

7.2.2 Gateway GPRS Support Node

Gateway GPRS Support Node GGSN realizes functions comparable to those of a gateway MSC.

GGSN

��performs interworking between a GSM PLMN and a packet data network PDN (Gi-interface);

��contains the routing information for GPRS subscribers available in the PLMN. Routing information serves to contact the respective SGSN in the providing area of which an MS is momentarily located;

��has a screening function;

��can inquire about location information from the HLR via the optional Gc interface.



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GGSN

SGSN & GGSN

SGSN

Serving GPRS Support Node SGSN� serves MSs in SGSN area

� Mobility Management functions, e.g

� Update Location, Attach, Paging,..

� Security and access control:

� Authentication, Cipher setting, IMEI Check...

� New cipher algorithm

� Routing / Traffic-Management

� collecting charging data

� realises Interfaces: Gn, Gb, Gd, Gp, Gr, Gs, Gf

Gateway GPRS Support Node SGSN� Gi-,Gn-Interface: Interworking PLMN � PDN

� Routing Information for attached GPRS user

� Screening / Filtering

� collecting charging data

� optional Gc interface

Fig. 56 SGSN and GGSN


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7.2.3 Interfaces

New interfaces are defined for GPRS, in addition to interfaces A-G already defined in the GSM-PLMN:

��Gb - between an SGSN and a BSS; Gb allows the exchange of signaling and user data: Unlike the A-interface, in which a user is assigned a certain physical resource for the entire/full duration of a connection, on Gb a resource is only assigned in case of activity (i. e. when data are being transmitted/received). A large number of subscribers use the same physical resources. The same holds for interfaces Gi, Gn and Gp.

��Gc - between a GGSN and an HLR

��Gd - between an SMS-GMSC / SMS-IWMSC and an SGSN

��Gf - between an SGSN and an EIR

��Gi - between GPRS and an external packet data network PDN

��Gn - between two GPRS support nodes GSN within the same PLMN

��Gp - between two GSN located in different PLMN. The Gp interface allows the supporting of GPRS services over an area of cooperating GPRS PLMN.

��Gr - between an SGSN and an HLR

��Gs - between an SGSN and an MSC/VLR; serves to support an MS using both GPRS and circuit switched services ("Common Mobility Management" e.g. update of location information).



83

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MS BSS SGSN GGSN PDN TE

GGSN

other PLMN

SMS-GMSC

SMS-IWMSC

MSC/VLR HLR/(GR)

SM-SC

EIRSGSN

GpGf

Gi

GrGc

Gn

GbUm

Gs

Gd

E C

A

D

GPRS - Architecture:

Interfaces Signaling only

Signaling &

Data transmission

Gn

Fig. 57 GPRS (and GSM) Interfaces


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7.2.4 Packet Control Unit PCU

In the BSS, the PCU

��manages GPRS radio channels (Radio Channel Management), e.g. power control, congestion control, broadcast control information

��allocates resources for UL and DL packet data transfer

��performs access control, e.g. access request and grants

��converts protocols (between interfaces Gb and Um).

In principle, the PCU may be placed in BTS, BSC or SGSN (GSM Rec. 03.60). Locating the PCU in the BSC is the most practical solution, however, used also for SBS.

7.2.5 Channel Codec Unit CCU

The CCU performs

��Channel coding (including forward error correction FEC and interleaving) and

��Radio channel measurements (including received quality and signal level, timing advance measurements)

7.2.6 GPRS Mobile Stations MS

A GPRS MS can work in three different operational modes. The operational mode depends on the service an MS is attached to (GPRS or GPRS and other GSM services) and on the mobile station’s capacity of handling GPRS and other GSM services simultaneously.

��"Class A" operational mode: The MS is attached to GPRS and other GSM services and the MS supports the simultaneous handling of GPRS and other GSM services.

��"Class B" operational mode: The MS is attached to GPRS and other GSM services, but the MS cannot handle them simultaneously.

��"Class C" operational mode: The MS is attached exclusively to GPRS services.



85

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Packet Control Unit PCU

• Channel Access Control functions

• Radio Channel Management functions

(Power Control, Congestion Control,...)

• scheduling data transmission (UL/DL)

• protocol conversion (Gb<-->Um)

GPRS-MS: operational mode

• Class-A: MS attached to GPRS and

non-GPRS, simultaneous handling

• Class-B: as A, not simultaneously

• Class-C: GPRS only

Um Abis

CCU

CCU

BTS BSC site GSN site

PCUMS

Gb

Channel Codec Unit CCU

• Channel Coding (FEC, interleaving,..)

• Radio Channel Measurement functions

(received quality & signal level, TA,..)

Fig. 58 PCU, CCU, MS


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7.2.7 Radio Interface

The tasks of layer 1 radio interface relate to the transmission of user and signaling data as well as to the measuring of receiver performance, cell selection, determination and updating of the delayed MS transmission (timing advance TA), power control and channel coding.

In GPRS, a main difference to circuit-switched services is that several mobile stations in parallel can use a physical channel and a packet data channel. One packet data channel is allocated per radio block, i.e. for four consecutive TDMA frames and not for a specific time interval. This means that signaling and the packet data traffic of several mobile stations can be statistically multiplexed into one packet data channel. Furthermore, the packet data channel can be seized asymmetrically.

On the other hand it is also possible for a mobile station to use more than one packet data channel at the same time, i.e. to combine several physical channels of one radio carrier. In principle, up to 8 packet data channels can be seized simultaneously.

In a GPRS cell, one or several physical channels can be allocated to GPRS transmission. These physical channels (Packet Data Channels PDCH) are shared by GPRS mobile stations and are taken from the common/shared pool of all available physical channels of the cell.

Distribution of the physical channels for various logical packet data channels is based on blocks of 4 normal bursts each. UL and DL for GPRS packet data are assigned separately (consideration of asymmetrical traffic peaks). Allocation of circuit switched services and GPRS is achieved dynamically, depending on what capacities are required ("capacity on demand“). PDCH need not be allocated permanently. However, it is possible for the operator to reserve a number of physical channels for GPRS traffic.



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GSM RF:

GPRS Layer 1 (Um)

L1-

tasks

Transmission

of user &

signaling data

determinate &

actualize

Timing Advance

Cell SelectionMeasure

signal strength

Power Control

functions Resource optimization:1 physical channel to be used

by many MSs simultaneously !!

asymmetrical trafficUL / DL possible !!

High data rate trafficup to 171.2 kbit/s:

combining 1..8 PDCH for 1 MS !!

Allocation of physical channel(Packet Data Channel PDCH)

dynamically: 1 or 4 Radio Blocks

(1 Radio Block = 4 Normal Burst

in 4 consecutive TDMA-frames)

�� User & signalling data of several MSs

statistically to be multiplexed into 1 PDCH

(also fixed allocation possible)

Fig. 59 Radio Interface


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7.2.8 Channel Coding

Channel coding is modified substantially for GPRS purposes (GSM Rec. 03.64). Channel coding starts with the division of digital information into transferable blocks. These radio blocks, i.e. the data to be transferred (prior to encoding) comprise:

��a header for the Medium Access Control MAC (MAC Header),

��signaling information (RLC/MAC Signaling Block) or user information (RLC Data Block) and

��a Block Check Sequence BCS.

The functional blocks (radio blocks) are protected by convolutional coding against loss of data. Usually, this means inserting redundancy.

Furthermore, channel coding includes a process of interleaving, i.e. re-arrangement in time. The convolutional radio blocks are interleaved to a specific number of bursts/burst blocks. In the case of GPRS, interleaving is carried out across four normal bursts in consecutive TDMA frames (and, respectively, to 8 burst blocks with 57 bit each).

Coding Schemes

New GPRS coding schemes - CS1 - CS4 - have been defined for the transmission of packet data traffic channel PDTCH (Rec. 03.64). Coding schemes can be assigned as a function of the quality of the radio interface. Normally, groups of 4 burst blocks each are coded together.

CS-1 makes use of the same coding scheme as specified for SDCCH (GSM Rec. 05.03). It consists of a half rate convolutional code for forward error correction FEC. CS-1 corresponds to a data rate of 9.05 kbit/s.

CS-2 corresponds to a rate of 13.4 kbit/s, while

CS-3 corresponds to a data rate of 15.6 kbit/s.

CS-2 and CS-3 represent punctured versions of the same half rate convolutional code as CS-1.

CS-4 has no redundancy in transmission (no FEC) and corresponds to a data rate of 21.4 kbit/s.

In principle, 1 to 8 time slots of a TDMA frame can be combined dynamically for a user for the transmission of GPRS packet data. Theoretically it is thus possible to achieve peak performances of up to 171.2 kbit/s.



89

9,05 kbit/s

13,4 kbit/s

15,6 kbit/s

21,4 kbit/s

CS-1

CS-2

CS-3

CS-4

CS & PS (GPRS):

“capacity on demand”

Physical channel of one cell

GPRS-MSs:

sharing physical channel

GPRS-MSs:

combining 1-8 TS

Common coding of 4 Bursts

(Radio Block)

Up to

171,2 kbit/s(theoretically)

1 - 8

channel

GPRS-MSs:

asymmetric UL / DL

Fig. 60 Coding Schemes

collect

user data

signaling

RLC Data Block BCSMAC Header

RLC/MAC Control Block BCSMAC Header

Convolutional

coding

(not CS-4)

Radio Block

Radio Block

BCS: Block Code Sequence

(for error recognition)

Radio Block

(Redundancy !)rate 1/2 convolutional coding

Radio Block (456 Bits)

puncturingPuncturing

(only CS-2, CS-3)

MAC: Medium Access Control

RLC: Radio Link Control

Interleaving 57 Bit8 Burst-

blocks57 Bit 57 Bit 57 Bit57 Bit

•••

Um: Allocation of PDCH for 1 / 4 Radio Blocks = 4 / 16 Normal Bursts

Fig. 61 Channel coding


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7.3 Enhanced Data Rates for GSM Evolution EDGE

7.3.1 Basics

EDGE (GSM 10.59, GSM 05.04,...) is an alternative concept to increase data transmission rates. As new coding schemes cannot significantly increase performance (GPRS uses 21.4 kbit/s net transmission rate with 22.8 kbit/s gross transmission rate), and no more than 8 timeslots are available on a carrier, EDGE changes the modulation used on the radio interface. In the same time interval, which it takes in "ordinary" GSM to send a bit, in EDGE a symbol representing 3 bits is transmitted.

EDGE nearly triples the data transmission rates (because of protocol overhead the factor is not exactly 3). Similar to HSCSD and GPRS, the new modulation technique is more sensitive to interference and therefore requires good radio conditions.

Since EDGE is based on a different concept, it can be used together with HSCSD and GPRS. The respective variants are called:

��Enhanced Circuit Switched Data ECSD and

��Enhanced General Packet Radio Service EGPRS.

A similar concept is used in the American market for enhancing the capabilities of the D-AMPS networks. The standardization of the UWC-136 HS system is done within the UWCC (Universal Wireless Communication Consortium), which closely cooperates with the ETSI in this regard.



91

T T GP57+1 information bits 57+1 information bitsTS

T T GP57+1 information symbols 57+1 information symbolsTS

every symbol

contains 3 bits

TS training sequence

T tail bits

GP guard period

x 3

Fig. 62 Normal bursts in GSM (upper) and EDGE (lower)

I

Q

1

0

Gaussian Minimum Shift Keying

1 bit per symbol

Symbol rate: 270.833 ksym/s

Payload/burst: 114 bit

Gross rate: 22.8 kbit/s

I

Q

(0,0,0)(0,1,1)

(0,1,0)

(1,1,1)

(1,1,0)

(0,0,1)

(1,0,1)

(1,0,0)

8-Phase Shift Keying

3 bit per symbol

Symbol rate: 270.833 ksym/s

Payload/burst: 346 bit

Gross rate: 69.2 kbit/s

Fig. 63 Comparison of GSM modulation (left) and EDGE modulation (right)


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7.3.2 Enhanced Circuit Switched Data ECSD

The main features introduced with ECSD are:

��New channel coding schemes allow higher data transmission rates with different levels of protection by check bits,

��new logical channels make use of these coding schemes and send additional information in signaling channels,

��fast power control as add-on to the "normal" power control mechanism

Channel Coding

Three new coding schemes are implemented with EDGE for circuit switched traffic channels: ECSD TCS-1, ECSD TCS-2 and ECSD TCS-3.

The code rate describes numerically how much redundancy is used in a coding scheme. It is defined as the ratio of the (synthetically) radio interface rate to the gross transmission rate. The last column shows the user data rate, which can be achieved with one timeslot.

Channel type

Modulation Gross rate Radio inter-face rate

Code rate Net trans-mission rate

TCH/F2.4 GMSK 22.8 kbit/s 3.6 kbit/s 0.16 2.4 kbit/s




ECSD TCS-1 8PSK 69.2 kbit/s 29.0 kbit/s 0.42 28.8 kbit/s





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Logical Channels

New logical channels are defined, using the ECSD coding schemes. 4 new channel combinations are introduced.

The abbreviations have the following meaning:

��E-TCH/F: ECSD Traffic CHannel Fullrate using one of the three new coding schemes.

��E-IACCH: ECSD Inband Associated Control CHannel

��E-FACCH/F: ECSD Fast Associated Control CHannel for Fullrate

��SACCH/TF: Slow Associated Control CHannel for Traffic channel Full rate

��SACCH/M: Slow Associated Control CHannel for Multislot traffic channels

��E-TCH/FD: ECSD Traffic CHannel Full rate only for Downlink direction

��SACCH/MD: Slow Associated Control CHannel for Multislot traffic channels used only in Downlink

Fast Power Control

Fast power control FPC is used in ECSD connections, replacing the conventional power control mechanisms used in standard GSM connections. FPC uses the new logical channel E-IACCH (ECSD Inband Associated Control Channel) to report the measurement results to the base station system in uplink and to transmit power control commands in downlink.

The E-IACCH uses 24 bits in every block of 20 ms, thus achieving a transmission rate of 1.2 kbit/s in contrast to the "normal" power control via SACCH multiframes with 0.8 kbit/s and a block time of 480 ms.

Normal power control and fast power control are running simultaneously, i.e. power control commands are sent from the base station in the E-IACCH and the SACCH and measurement results are reported in both, the E-IACCH and the SACCH, but if FPC is activated, the messages in the SACCH are ignored. This mechanism allows a fast transition between the two power control modes.

Power level measurements, which are important input for cell selection, cell reselection and handover decisions, are based on the BCCH carrier of neighboring cells. Therefore care has to be taken if on this BCCH carrier some timeslots are transmitting 8PSK-modulated data, since the power envelope does not remain constant in this case.


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E

7.3.3 Enhanced GPRS EGPRS

For Enhanced GPRS, the following main features are relevant:

��new coding schemes allow higher data transmission rates because of 8PSK modulation with different levels of protection by check bits,

��link quality control ensures an adaptation of data transmission rates depending on the condition of the air interface, i.e. in case of e.g. high interference level the data transmission rate is dynamically reduced to an optimum balance between speed and error correction capabilities, and

��quality of service QoS in EDGE phase 1 and phase 2.

7.3.3.1 Channel Coding

9 new coding schemes have been developed for EGPRS:

Coding Scheme

Modu-lation

User data rate (kbit/s)

Code rate

Puncturing schemes

Useful bits Family

CS-1 GMSK 9.05 0.50 -- 181 --

CS-2 GMSK 13.4 0.66 -- 268 --

CS-3 GMSK 15.6 0.75 -- 312 --

CS-4 GMSK 21.4 1.00 -- 428 --

MCS-1 GMSK 8.8 0.53 2 176 C

MCS-2 GMSK 11.2 0.66 2 224 B

13.6 296 MCS-3 GMSK

14.8

0.80 3

272+24

A (padding)

MCS-4 GMSK 17.6 1.00 3 352 C

MCS-5 8PSK 22.4 0.37 2 448 B

27.2 592 MCS-6 8PSK

29.6

0.49 2

544+48

A (padding)

MCS-7 8PSK 44.8 0.76 3 2*448 B

MCS-8 8PSK 54.4 0.92 3 2*544 A

MCS-9 8PSK 59.2 1.00 3 2*592 A

Logical Channels

The logical channels, which can be used with EGPRS, are the same as for "ordinary" GPRS.



95

7.3.3.2 Link Quality Control

Link quality control is implemented by two different means:

��link adaptation (mandatory) and

��incremental redundancy (optional).

Link Adaptation

Link adaptation is based on the MS measurements of the bit error rate. Depending on the number of bit errors a quality level is determined and reported to the base station system. The base station system decides, based on this quality level, whether a change of the coding scheme increases the performance and informs the MS about the new coding scheme to be used.

Changes of coding schemes are only possible within the same family, even if the modulation technique is different.

An example of the achievable data rates in dependence of the Modulation and Coding Scheme MCS used and the Carrier/Interference ratio is shown in the figure below (GSM 900, TU50, without frequency hopping).

0

10

20

30

40

50

60

10 20 3025155

Through-

put in kbit/s

C/I in dB

MCS-1 (C)MCS-2 (B)MCS-3 (A)MCS-4 (C)

MCS-5 (B)

MCS-6 (A)

MCS-7 (B)

MCS-8 (A)MCS-9 (A)

Fig. 64 Link adaptation


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Incremental Redundancy

Incremental redundancy compensates short-term fading effects. It uses the fact that within one coding scheme different puncturing schemes are defined. If a data packet has been received but cannot be decoded successfully, the originator may transmit a different punctured version of the data packet in the same coding scheme.

Receivers can operate in two different modes:

��Either the receiver discards the data block received in the first step and tries to decode the repeated data block, or

��the receiver keeps the first received data packet in its memory and combines it with the information received in the second step (joint decoding).

Joint detection must be implemented in EGPRS mobiles, for base station systems it is an optional solution.

An example for the gain obtained by incremental redundancy is shown in the figure below for the MCS-9 coding scheme (TU3 with frequency hopping).

Fig. 65 Incremental redundancy



97

Quality of Service

EDGE will be introduced in several phases. In EGPRS phase 1, only certain Quality of Service QoS profiles are supported:

��Interactive services (e.g. web browsing or accessing company data servers). They require preserving the payload content, but the delay time of packets is not too critical.

��Background services (e.g. e-mail transmission and SMS). Preserving the payload/content is important, however the transmission is uncritical regarding the delay time.

With EDGE phase 2 additional QoS profiles are defined:

��Conversational (bi-directional services like speech transmission (telephony), but also Voice over IP and videoconferences): The highest priority here is to keep the delay time short and to maintain the order of data packets.

��Streaming (for unidirectional services like real-time audio and video transmission): Like in the conversational QoS class the short delay time and the order of data packets have highest priority.



99

8 Location Services

��r r '

r

Fig. 66



101

8.1 General

Location Services (LCS) offer the opportunity to deploy new value added services based on the known position of the mobile station. Among these services are

��Location dependent billing (e.g. home zone),

��Safety services (e.g. emergency calls, localization of vehicles),

��Tracking services (e.g. fleet management, navigation),

��Information services (e.g. tourist information, restaurant finder).

Beside offering commercial benefits, LCS are driven by regulatory requirements (US Federal Communication Commission requires location of emergency calls by end of 2001).

The positioning methods introduced are

��Cell-ID/Timing Advance (with low accuracy),

��Enhanced Observed Time Difference (E-OTD, medium accuracy),

��Time/Angle of Arrival (TOA/AOA, medium accuracy),

��Enhanced Cell ID with Network Measurement Results (medium accuracy) and

��Assisted-Global Positioning System GPS (with high accuracy).

The accuracy of some of the positioning methods depend on cell size, shape and cell planning.

8.1.1 Comparison

Positioning Method

Cell ID / TA E-OTD TOA

Accuracy low

(500 m)

medium

(100 ... 200 m)

medium

(100 ... 200 m)

MS requirements

all MS E-OTD capable MS only

all MS

Network investment

low medium ... high medium … high

SBS support yes yes (planned) no


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8.2 ETSI Architecture

Location Services provide MS positions in universal latitude and longitude coordinates to location applications ("LCS clients").

The new LCS network entities are

Location Measurement Unit LMU

The LMU perform assistance measurements with special receivers, e.g. radio interface timing measurements. There are two types of LMU available:

��LMU type A (remote or co-located), connected like a "normal" MS via Um on a dedicated (temporary or permanent) SDCCH with its own IMSI and subscription profile in HLR and

��LMU type B connected via Abis.

The TOA LMU measure the Time of Arrival of access bursts.

The E-OTD LMU provides (pseudo-)synchronization in the network.

Gateway Mobile Location Center G-MLC

��supports access to LCS by external applications (e.g. via TCP/IP) or from other PLMN,

��stores LCS subscription info on a per LCS client basis,

��requests info from HLR about the MS to be located,

��manages subscriber privacy,

��receives the final location estimate and determines whether they satisfy the requested QoS (retry, reject),

��generates LCSD related charging and billing data.

Serving Mobile Location Center S-MLC

��manages the overall coordination required to perform MS positioning,

��determines the positioning method to be used based on the QoS, the network capabilities and the MS location capabilities,

��controls LMU for obtaining radio interface measurements,

��stores info about LMU (e.g. positions, capabilities),

��calculates the final location estimate and accuracy and returns it in a location response to the requesting G-MLC.



103

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

BTS(LMU

type B)

BSCMSC /

VLR

G-

MLC

External

LCS Client

LMUType A

CBCS-

MLC

S-

MLC

HLRLMUType B

G-

MLC

Abis

Abis

A

Lg

Lg Le

Lp

Lh

LbCBC-

BSC

CBC-

SMLC

Um

Um

D

MLC Mobile Location Center (Serving/Gateway)

LMU Location Measurement Unit

Lx LCS Interfaces

Different

PLMN

Ls

Fig. 67 Location Services (Architecture acc. to ETSI)


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8.3 SBS Implementation

The BSS centric architecture is supported: the S-MLC is logically connected via the Lb interface to the BSC. The difference to the NSS architecture, where the S-MLC is connected to the MSC via Ls, is the function split between S-MLC and MSC.

SBS supports the positioning methods

��Cell ID / Timing Advance and

��Enhanced Observed Time Difference E-OTD (planned).

For E-OTD, LMU type B is supported, i.e. LMU a module integrated in BTSE, connected via Abis to BSC. The handset-assisted mode is supported, i.e. no assistance data are provided by the network and no positioning calculation is done in the MS.

While the S-MLC is logically connected to BSC, physically it is connected to MSC. One S-MLC supports several BSC via intermediate MSC.



105

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BTS BSCMSC /

VLR

G-

MLC

External

LCS Client

LMUType B

S-

MLC

S-

MLC

HLRG-

MLC

Abis A

Lg

Lg Le

Lp

Lh

Lb

Um

D

MLC Mobile Location Center (Serving/Gateway)

LMU Location Measurement Unit

Lx LCS Interfaces

Abis

Fig. 68 LCS Architecture supported by SBS


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8.3.1 Positioning Methods

Cell ID / Timing Advance TA

As a generic handling procedure to assist all positioning methods and as a fallback procedure the BSC provides cell ID of the serving cell and actual TA value of the served mobile station to the S-MLC. The maximum accuracy is the range of one TA value, i.e. 550 m as a ring of width r

�

� on a circle respectively a sector of a circle in case of sectorized cells.

MS in dedicated mode

The cell ID and the initial TA value for a served MS are known by the serving BSC at start of a circuit switched connection. Later, the BSC requests the actual TA from the BTS and conveys it to the S-MLC.

MS in idle mode

The MS is paged or sets up a call (e.g. emergency call). The BSC transmits the TA value in addition to the cell ID when the circuit switched connection is established.

Different applications (e.g. weather report or traffic on motorways) do not require absolute high accuracy. For these applications the accuracy provided by cell ID and TA transfer is deemed sufficient.

Requirements

"Ordinary" mobile stations can be used. No HW changes in the SBS are required.

S-MLC/G-MLC have to be deployed.



107

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BTS

MS

r

Measurement

error margin

r

�

�

• to assist other positioning

methods („fall back“) or e.g.

for home zone billing

• transfer of TA (from serving BTS)

and Cell ID

• in idle and dedicated mode

• result is ring (omni cell) or

ring sector (sectorized)

TA=1 (bit) corresponds to a

distance of 550 m

Fig. 69 Cell ID / Timing Advance - Basics


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108

Enhanced Observed Time Difference E-OTD

E-OTD is based on the determination of the time difference between received signals. The time difference is measured in the downlink where the mobile has to listen to signals emitted by different BTS. The measurements are performed on the BCCH frequency, the carrier where the Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) are located. The mobile measures the timing difference of received signals between the serving BTS and at least two neighboring BTS. The result is the Observed Time Difference (OTD). Accuracy is downgraded if the BTS are in line (e.g. on a highway) or insufficient BTS are available. In rural areas with only one BTS available, no E-OTD positioning can be performed (fallback is Cell ID / TA).

The involved BTS have to be synchronized, i.e. Real Time Difference RTD equal to zero, or pseudo-synchronized, i.e. Real Time Differences must be known.

For the determination of Real Time Differences between BTS, E-OTD Location Measurement Units (E-OTD LMU) are deployed in the network. The task of the E-OTD LMU is to determine the offset between different BTS by measuring the time difference between the arrival of bursts from different base stations. The measured signals are downlink bursts, e.g. normal bursts or synchronization bursts. The reporting frequency of actual RTD values to the network depends on the drifting speed of RTD values.

LMU are situated in known positions that can be base station sites or other suitable places. The number of E-OTD LMU per BTS depends on the network configuration, network deployment and on the required performance. Field trials have shown that in the average one E-OTD LMU is sufficient for two BTS sites. For sectored sites , the sectors have to be synchronized (this is true as long as all sectors belong to the same controller). Non-synchronized sectors require additional LMU.

For controlling the measurement and for network centric calculation of the mobile’s position, the S-MLC is associated to the mobile network. The position computation of the mobiles is performed by the S-MLC by using the following data:

��Observed Time Difference between different BTS at the mobile (determined by the mobile)

��Real Time Differences between base stations (determined by E-OTD LMU)

��Geographical position of base stations and LMU and other data as antenna heights.

The S-MLC stores information about the position of base stations and LMU and receives the RTD values from the LMU. Additionally the S-MLC obtains OTD measurements from each MS being located. The position of the mobile is calculated via hyperbolic multilateration.

E-OTD can have two variants (mobile based E-OTD positioning if the final position computation is performed inside the MS, or mobile assisted E-OTD positioning where the position computation is performed inside the LCS network).



109

BTS

BTS

LMU

SYNC

Measurement

error margin

MS

RTDs

SYNC

OTDs

BTS

• MS receives synch bursts from min 3 BTS

and reports Observed Time Differences

• LMU receives synch bursts from same BTS

as MS and reports their Real Time Differences

• S-MLC calculates MS position by

intersection of 2 hyperbolas

(input: OTD from MS, RTD from LMU,

BTS positions)

Fig. 70 Enhanced Observed Time Difference - Basics

For mobile assisted E-OTD Positioning, the OTD measurements are transferred to the S-MLC, where in combination with RTD measurements and BTS coordinates the position is calculated.

(For mobile based E-OTD Positioning, RTD measurements (obtained by the LMU and collected in the S-MLC) are transferred together with BTS coordinates towards the target MS. It is not planned to support mobile based E-OTD.)

Requirements

E-OTD capable mobile stations are needed.

E-OTD LMU and S-MLC/G-MLC have to be deployed.



111

9 Exercises



113

��What means GSM?

��What means PLMN?

��What are the major subsystems in a PLMN?

��Which subsystem is responsible for monitoring of the BSS and SSS?

��List the major functions of the network components of the BSS and SSS.

��Name the most important interfaces defined by GSM?

��What is transcoding and for which types of calls is it used?

��What is a radio cell?

��What influences the size of a radio cell and what is the maximum size of a radio cell in GSM900?

��Find out the correlation between the guard period of an access burst and the maximum size of a GSM cell.

��What is the frequency range for GSM900 (extended band)?

��How many radio frequency carriers are available in GSM900?

��Can a cell have more than one RFC?

��What is meant by TDMA, FDMA, and FDD?

��What type of burst is used for traffic channels?

��Which manual provides background information on SBS features?

��Where do you find information on the BS240 commissioning procedure?

��Why is power control performed?

��Which types of frequency hopping are available?

��What is the name for operating TRX of different bands in the same cell?

��Name the main features of GSM-R.

��What does HSCSD stand for?

��What are transparent and non-transparent modes of operation?

��What are the differences between GPRS and HSCSD?

��What is the (theoretical) maximum data rate for GPRS?

��How many logical channels are used in GPRS?

��Explain the difference between GMSK and 8-PSK.

��Name the main differences between HSCSD, GPRS and ECSD, EGPRS.

��What is Link Adaptation?

��Which methods are used for Location Services? What are their main differences?

01 mn1780eu09mn 0001 sbs overview

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