01 Mn1780eu11mn 0001 Sbs Overview

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SBS overview Siemens

MN1780EU11MN_0001 © 2005 Siemens AG

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Contents

1 Global system for mobile communication GSM 3

2 Network elements for PLMN 7

3 Radio access network: radio subsystem 11

3.1 Base Station Controller BSC 16

3.2 Base Transceiver Station Equipment BTSE 18

3.3 Transcoder and Rate Adapter Unit TRAU 20

3.4 Local Maintenance Terminal LMT 22

3.5 Documentation 24

4 Radio interface Um 27

4.1 GSM900/GSM1800/GSM1900 frequency bands 29 4.2 Radio Frequency Carriers RFC 30

4.3 Physical channels 32

4.4 Logical channels for cs services 34

4.5 Example for signaling channels 38

4.6 Logical channels for PS services 42

4.7 Multiframes 46

4.8 Timeslot structure 50

5 Abis and Asub interface 53

5.1 Asub and A interfaces 54 5.2 Abis and Um interface 58

6 GSM basic features 63

6.1 Power control 64

6.2 Cell (re)selection 66

6.3 Handover 68

6.4 Multiband operation 72

7 Data transmission in GSM phase 2+ 75

7.1 High Speed Circuit Switched Data HSCSD 78

7.2 General Packet Radio Service GPRS 82 7.3 Enhanced data rates for GSM evolution EDGE 90

SBS overview



Siemens SBS overview

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8 Location services 99

8.1 Architecture 102

8.2 Positioning methods 104

9 Exercises 111





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1 Global system for mobile communication

GSM




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Standardization

The Global System for Mobile Communications GSM is a ("2nd generation") standard

for mobile communication. This standard was developed by the EuropeanTelecommunication Standards Institute ETSI (founded in 1988).

Today the Specifications for the GERAN (GSM and Edge Radio Access Network)can be found in the internet under www.3gpp.org.

The 3rd Generation Partnership Project (3GPP) is a collaboration agreement thatwas established in December 1998. The collaboration agreement brings together anumber of telecommunications standards like ARIB, CCSA, ETSI, ATIS, TTA, andTTC.

Public Land Mobile Network PLMN

A Public Land Mobile Network is a network, established and operated by its licensedoperators, for the specific purpose of providing land mobile communication servicesto the public.

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 isdivided. A cell consists of a base station transmitting over a small geographic areathat is represented as a hexagon. The whole PLMN area is covered by a greatnumber 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.

Mobile Station MS and SIM

The Mobile Station is the radio equipment needed by a subscriber to access theservices provided by the PLMN. The MS may be a fixed station (e.g. installed in a

vehicle) or a portable handheld station. The Subscriber Identity Module SIM providesthe mobile equipment ME with a subscriber's identity.





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Maximum cell size

GSM900 35 km

(100 km)

8 kmGSM1800

Cellular Network

Fig. 1 Radio cells




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7

2 Network elements for PLMN




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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")

Network Elements

The subsystems functions are grouped into functional units or network elements.Functional units may be realized either as standalone Hardware HW units or associated with other GSM functional units in one HW unit.

The Radio SubSystem RSS consists of the Mobile Stations MS and the Base

Station Subsystem BSS, which is composed of the following functional units:• Base Station Controller BSC

• Base Transceiver Station BTS

• Transcoding and Rate Adaption Unit TRAU

The Network Switching Subsystem NSS (Phase ½) consists of the followingfunctional units:

• Mobile services Switching Center MSC

• Visitor Location Register VLR

• Home Location Register HLR

• Authentication Center AC

• Equipment Identity Register EIR.

The Operation SubSystem OSS consists of Operation & Maintenance CentersOMC; in the Siemens solution:

• Operation & Maintenance Center for the Base Station Subsystem is the RadioCommander (RC)

• Operation & Maintenance Center for the Switching Subsystem is the SwitchCommander (SC).





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RC SC

MSC

HLR VLR

EIRAC

BSC

BTS

TR

A

U

Mobile

Station

MS

RadioSubSystem

RSS =

Base Station

Subsystem

BSS

Network SwitchingSubsystem

NSS

+

other

networks

Operation SubSystem OSS

PSTN

ISDN

Data

NetworksMS =

ME + SIM

Fig. 2 GSM PLMN




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11

3 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 Uminterface ("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 SystemSBS.

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 associatedPCM 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 PacketControl Unit PCU (located in BSC))





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Radio Subsystem (RSS)

BSS

To/from

SSS

To/from

OMS

Mobile

Station

Um

Fig. 3 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. 4 Base Station Subsystem BSS




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PCMB and LAPD configuration

Time slots (signaling/traffic) can be assigned to a BTSE on up to eight PCMB lines in

case of the BTSplus family. These multiple links support load-sharing and faultrecovery.

The LAPD signaling timeslots may be 16 kbit/s sub slots (if max 2 TRX are used) or 64 kbit/s time slots. On the BTSM side, these time slots are called LAPDLE while inthe BSC database they are called LPDLM (although they may carry both LPDLM andLPDLR type signaling).

Minimum for each PCMB line at least one LAPDLE/LPDLM is required.





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BTSE Object: LAPDLE

BSC Object: LPDLM

B

T

S

pl

u

s

B

SC

PCMB#0

PCMB#1

PCMB#2

PCMB#3

8 PCMB lines

between

BSC - BTSplus

1 LAPD link

per

PCMB line

Up to 12

objects LPDLM

per BTSE

Fig. 5 Multiple Abis LAPD links




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3.1 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 for different BSC releases are given in the following table:

BR6.0 BSC HC1st Step

BR7.0 HC BSC2nd Step

BR8.0 HC BSC2nd Step

Controlled TRXs* 500 900 900

Controlled Cells 250 400 400

Controlled BTSE 200 200 200

Controlled TRAU 32 48 48

PCM lines 72 120 120

LAPD (Abis+Asub) up to 240 up to 240 up to 240

GPRS channels 1536 3072 3072

SS7L 8 16 16

* Given capacity is obtained under the assumption:

• PCM30 lines are used

• network planning is properly done and

• normal network operational conditions are normal conditions.

BSC HC is the abbreviation for the BSC High Capacity. The BSC high capacity stepone is also called BSC/72. The BSC high capacity step two is also called BSC/120.





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Fig. 6 BSC/120 rack layout




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3.2 Base Transceiver Station Equipment BTSE

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

The BTSE provides up to 48 transceivers (TRX) with the Flex CU and serves up to12 cells in case of BS240. Max 8 racks in total (max 3 of which provide TRX) makeup a BTSE.

Up to 8 PCMB lines terminate on BTSE.

Two (main) product lines are in use:

• The ("classic") BTSone and

• the (new) BTSplus family.

BTSE Type ProductLine

Max Number of TRX

Max Total No of Racks

(Radio/Service Racks)

BS40, 41 BTSplus 8 5 (1/4)

BS240, 241 BTSplus 48 8 (3/5)

BS240XL BTSplus 48 7 (2/5)

BS240XS BTSplus 6 1

BS82 BTSplus 8 2 ("cabinets")

BS82II BTSplus 12 3 ("cabinets")

BS288 BTSplus 12 1

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 often a hint to the number of Carrier Unit slots available.





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Fig. 7 BS240




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3.3 Transcoder and Rate Adapter Unit TRAU

The Transcoder and Rate Adapter Unit performs de-/coding (for speech calls) andrate 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 fromthe 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 eight (independent)shelves can be housed in the same rack.





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Fig. 8 High Capacity TRAU Rack




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3.4 Local Maintenance Terminal LMT

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

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

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

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





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BTSE TRAUBSC

SIEMENSNIXDORF

SIEMENSNIXDORF

LAN

OMC

O interface

T-InterfaceT-Interface

LMT

T-Interface

Fig. 9 Operation & Maintenance Interfaces on Base Station System




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3.5 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 externaland 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 ITMNexplaining 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.

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 dataformat) files. The operating documentation includes the following manuals (inalphabetical order, approximately 80 documents in total):

For GPRS PLMN, GSM PLMN, GSM-R, Network System Concept,

WAP/MIA for Mobile Solutions

SYDSystem Descriptions

Separate documents for "Common" and BS40/41, BS240/241,

BS240XL, BS82, BS82II, BS2x/6x, … information (hardware related)

TEDTechnical Descriptions

Guide to documentationTerminology

SBS counters and SBS message flowsPMPerformanceMeasurements

For BSC (via LMT) and OMS-BOMNOperation Manuals

All error messages for BSC, BTSE and TRAUOMLOutput Manuals

For LMT, OMS-B (commands, description, graphical panels, interactive

panel architect, main menu, states and repertories, tools)

OGLOperator Guidelines

Testing network configuration (incl. database) "as planned"NIMNNetwork Integration Manual

Replacement procedures incl. hardware related information, for BSC,

BTSE (all types), OMS-B, TRAU

MMNMaintenance Manuals

Step-by-step commissioning, for BSC, BTSE (all types), OMS-B, TRAUITMNInstallation and TestManuals

Hardware related information, for BSC, BTSE (all types), OMS-B, TRAUIMNInstallation Manuals

Explanation of termsGlossary

For BSC, BTSE, OMS-B, TRAUCMLCommand Manuals

Abbreviations Abbreviations

ContentAbb.Manual

Fig. 10 Document types




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4 Radio interface Um




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

Mobile Station

Uplink

Downlink

Um

BTSE

Fig. 11 Um interface





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4.1 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 transmissionis 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. 12 Frequency ranges for GSM900, GSM1800 and GSM1900




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4.2 Radio Frequency Carriers RFC

Both sub-bands (Uplink and Downlink) are divided into Radio Frequency Carrierswith 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. 13 Radio frequency carriers for GSM900, GSM1800 (DCS) and GSM1900 (PCS)





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Depending on the traffic volume, every radio cell uses one or more RFC. Since thenumber 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. Asmall 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 47RFC 40

RFC 24

RFC 32

RFC 11

RFC 47

RFC 8RFC 5

RFC 2

reuse distance (~ 4 r)

Fig. 14 Radio frequency carriers and reuse distance







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

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

RFC RFC

Physical Channel

Uplink Downlink

corresponding

Traffic

Channel

Traffic

Channel

Fig. 17 Physical channel




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4.4 Logical channels for cs services

Logical channels carry payload (speech or data) or signaling.For circuit-switched CS traffic there is a clear separation between the physicalchannels used for payload and those used for signaling.

Signaling channels

Three types of signaling channels are used:

z Broadcast Control Channels (CS and PS)

z Common Control Channels (CS and PS)

z Dedicated Control Channels (CS only)

TIPChannel Combinations

The allowed channel combinations of logical channel types are specified in GSMRec. 05.02.





35

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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. 18 Logical channels for circuit-switched traffic




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4.4.1 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 mobilestation about various cell parameters including country code, network code, localarea code, PLMN code, RF channels used within the cell where the mobile islocated, surrounding cells, and frequency hopping sequence number.

2. Frequency Correction Channel FCCH carrying information for frequencycorrection 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.

4.4.2 Common control channels

Common Control Channels are specified as unidirectional channels, either on thedownlink 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 (Adv. Speech Call Items,paging MS using Voice Group Call Service / Voice Broadcast Service VBS).

4.4.3 Dedicated control channels

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

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

2. Fast Associated Control Channel FACCH, which is always associated with aTCH and is used for the transmission of signaling data, after the set-up of thecall, when the SDCCH has been already released. The FACCH data are insertedinto the TCH burst instead of traffic data (bit stealing), indicated by a "stealingflag" (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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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. 19 Signaling channels




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4.5 Example for signaling channels

The BTS periodically broadcasts common system information via BCCH logicalchannel. The information contained in the BCCH is received and used by all MS thatcamp the BTS covering area.

BTSE

BCCH

cell identity: 262041422

Fig. 20 Example: Broadcast Channel information





39

Mobile originated call

An example to illustrate the task of the Um interface logical channels is a MOC.

• MS requests the assignment of the dedicated signaling channel via RACH •The network assigns a dedicated signaling channel (SDCCH) and sends theinformation which channel is assigned to MS via AGCH

• MS provides information on the requested service and transmits the subscriber identity via SDCCH. A number of access control messages are then exchangebetween the MS and the network as long as a TCH is allocated to the MS, everythingvia SDCCH channel too. The MS requests dedicated channel though the RACH also when paged (mobileterminating call MTC), when an emergency call needs to be set up, due to LU or IMSI

attach/detach. This logical channel contains the MS Random reference and thededicated channel request cause.

BTSE

RACH

Request of a SDCCH

12142341234

Fig. 21 Example: Random Access Channel




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

BTSE

AGCH

SDCCH provided

12142341234

Fig. 22 Example: Access Grant Channel





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All of the signaling information (i.e. dialed digits, authentication, traffic channelassignment) for call setup is exchanged over the SDCCH. The SDCCH is allocated tothe MS by the network whenever the RACH is received and SDCCH resources are

available.

BTSE

SDCCH

Control Information

Fig. 23 Example: Stand-alone Dedicated Control Channel




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4.6 Logical channels for PS services

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

The packet data logical channels are mapped onto the physical channels that arededicated to packet data. The physical channel dedicated to packet data traffic iscalled a Packet Data Channel (PDCH).

• GPRS introduces the following new types of logical channels:

• PBCCH (packet broadcast control channels)

• PCCCH (packet common control channels)

• PDCCH (packet dedicated control channels)

• PDTCH (packet data traffic channels)

4.6.1 Packet Data Traffic Channels (PDTCH)

PDTCH is a channel allocated for data transfer. It is temporarily dedicated to one MSor to a group of MSs in the PTM-M case. In the multislot operation, one MS may usemultiple PDTCHs in parallel for individual packet transfer.

All packet data traffic channels are unidirectional, either uplink (PDTCH/U), for amobile originated packet transfer or downlink (PDTCH/D) for a mobile terminatedpacket transfer.

A PDTCH when used for single timeslot operation may be either full-rate (PDTCH/F)or half-rate (PDTCH/H) depending on whether it is carried on a PDCH/F or PDCH/Hrespectively. A PDTCH, when used for multislot operation shall be full-rate





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Logical ChannelsLogical ChannelsLogical Channels

Traffic ChannelsPacket Traffic

ChannelsPacket Traffic

Channels

Packet

Associated

Control

Channel

Signaling ChannelsPacket Signaling

ChannelsPacket Signaling

Channels

Packet Broadcast

Control ChannelPBCCH

Packet Broadcast


Packet

Timing

Advance

Contr. Channel

Packet

Data

Traffic

Channel

Packet Common

Control ChannelPCCCH

Packet Common

Control ChannelPCCCH

Fig. 24 Logical channels for packet-switched traffic




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4.6.2 Packet Broadcast Control Channels (PBCCH)

1. Packet Broadcast Control Channel PBCCH broadcast packet data specificSystem Information. If PBCCH is not allocated, the packet data specific systeminformation is broadcast on BCCH. This channel is used downlink only.

4.6.3 Packet Common Control Channel (PCCCH)

1. Packet Random Access Channel PRACH is used by MS to initiate uplink transfer for sending data or signaling information. This channel is used uplink only.

2. Packet Paging Channel PPCH is used to page an MS prior to downlink packet

transfer. PPCH can be used for paging of both circuit switched and packet dataservices. This channel is used uplink only.

3. Packet Access Grant Channel PAGCH is used in the packet transfer establishment phase to send resource assignment to an MS prior to packettransfer. This channel is used downlink only.

4. Packet Notification Channel PNCH is used to send a PTM-M (Point To Multipoint- Multicast) notification to a group of MSs prior to a PTM-M packet transfer. Thischannel is used downlink only.

4.6.4 Packet Dedicated Control Channels (PDCCH)

1. Packet Associated Control Channel PACCH conveys signaling informationrelated to a given MS. The signaling information includes e.g. acknowledgementsand power control information. The PACCH shares resources with PDTCHs, thatare currently assigned to one MS.

2. Packet Timing advance Control Channel PTCCH is necessary to allow estimationof the timing advance for one MS in packet transfer mode.





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Packet Common

Control Channel

PCCCH

Packet Common

Control Channel

PCCCH

Packet Broadcast

Control Channel

PBCCH

Packet Broadcast


Packet Broadcast Control Channel PBCCH

DL

Packet Random Access Channel PRACH

Packet Access Grant Channel PAGCH

Packet Paging Channel PPCH

Packet Notification Channel PNCH

UL

DL

DL

DL

Packet Dedicated

Control Channel

PDCCH

Packet Dedicated

Control Channel

PDCCH

Packet Associated Control Channel PACCH

Packet TA Control Channel PTCCH

DL & UL

DL & UL

Fig. 25 Packet switched Signaling channels




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4.7 Multiframes

There are 8 timeslots per TDMA frame. The TDMA frames are forming themultiframes. There are different multiframes in case of:

• Circuit switched Traffic channels

• Circuit switched signaling channels

• Packet switched channels

4.7.1 Traffic channel multiframe (CS)

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

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





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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. 26 Traffic channel multiframe




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4.7.2 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 (downlinkdirection) 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 aregrouped together as 10 groups of four. The BCCH use the first four timeslots of thefirst 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 sentin timeslot 0 on frames 1, 11, 21, 31 and 41.

T

7

0

1

8 B P c y c l e 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 l i n k

R A C HU P L I N KUPLINK

DOWNLINK

Fig. 27 Signaling channel multiframe (including FCCH, SCH, BCCH and AGCH/PCH)





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4.7.3 Packet switched Multiframes

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

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

iB0 B1 B2 B3 B4 B5 i B6 B7 B8 i B9 B10 B11 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. 28 GPRS multiframes




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4.8 Timeslot structure

The RF signal sent in a timeslot is called "burst". Depending on the type of logicalchannel 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 usefulduration of the burst is equivalent to 148 bits excluding the access burst that has auseful duration equivalent of 88 bits. To minimize interference, the mobile station isrequired during the guard periods to attenuate its transmission amplitude and toadjust possible time shifts and amplitudes of the bursts.

The normal burst is used to carry information on traffic channels and on signalingchannels (exception: Random Access Channel, Synchronization Channel andFrequency 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 andencrypted traffic information (speech or data) in 2 x (57 +1) bits. The 1 bit is calledstealing flag and indicates whether the 57 bits are really user data or FACCHsignaling 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 tomoving MS) and to reduce the transmission power.

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

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

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





51

Dummy bursts are inserted if TCH timeslots on the BCCH carrier are not filled withuser data to reach a constant level of the BCCH carrier. This is necessary becausethe level of the BCCH carrier is evaluated for handover decisions and also for the

decision, which is the serving cell (for call set up).

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

T3

Coded Bits58

Training Sequence26

Coded Bits58

T3

GP8.25

. . .

Normal Burst

Fig. 29 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 µs or 156.25 bit

Fig. 30 Burst formats




MN1780EU11MN_0001

© 2005 Siemens AG

52





53

5 Abis and Asub interface




MN1780EU11MN_0001

© 2005 Siemens AG

54

5.1 Asub and A interfaces

Timeslot 0 of every PCM line carries the Service Word / Frame Alignment Word(SW/FAW) which is used e.g. for synchronization and is therefore unavailable for payload.

Often, timeslot 31 is used for LPDLS signaling between TRAU and BSC on thePCMS line. As a result, four timeslots on the TRAU's 4 PCMA lines remain empty.

Timeslot 16 on the Asub interface is commonly used to carry CCSS7 signalinginformation. This signaling requires a transmission rate of 64 kbit/s. One of the 4PCMA lines carries the signaling information to the MSC. Therefore, timeslot 16 of one of the PCMA lines is occupied, whereas three timeslots on the other PCMA linesremain empty.

Note: Not every TRAU carries CCSS7 signaling information.

An OMAL signaling link defined as AINT (nailed up connection) is, e.g. put in timeslot30. OMAL is transmitted with 64 kbit/s on a single PCMA line. Correspondingly, threetimeslots on the other PCMA lines remain empty.

The traffic timeslots on the PCMA line, containing 64 kbit/s of speech, are sub-multiplexed to one sub slot with 16 kbit/s on the PCMS line.





55

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h dg cf b

e a01630

31

TRAUBSC

e a

f b

g c

h d

01630

31

TRAU MSC

Asub

A X

X

X

X

X

X

X

empty

C

CS

S

7

L

P

D

L

S

O

M

A

L

C

C

S

S

7

S

W

/

F

A

W

X

S

W

/F

A

W

O

M A

L

L

PD

L

S

X

X

X

Fig. 31 Time slot assignment on Asub and A interfaces (basic pattern, w/o HSCSD or NUC)




MN1780EU11MN_0001

© 2005 Siemens AG

56

5.1.1 TRAU matrices, multislot connections and pooling

Several time slots on Um / Abis / Asub can be combined and finally mapped into asingle 64 kbit/s time slot on A interface in a (circuit-switched) multi slot connection.Currently, mobile stations (and SBS, at least for CS connections) support thecombination of max 4 (sub) slots on Um / Abis / Asub.

The most commonly used time slot distribution between PCMS and PCMA is definedin the BSC database as "not_compatible_with_cross_connect" (former TRAUMatrix 1). The sorting rules are:

• All channels related to multi slot connections (from all PCMA) with max 4TSL/connection are assigned to the first time slots on Asub.

• All channels related to multi slot connections (from all PCMA) with max 2TSL/connection are assigned next on Asub.

• All ordinary channels are assigned in an optimal order without wasting space on Asub.

TIPNote: TSLA, which cannot be assigned to a TSLS, must be configured as "no_def".

For every change in multi slot configuration, the matrix is rearranged for the (new)optimal distribution. Thus, every modification may change the time slot distribution.

Nailed-up connections are available in BSC and TRAU. Due to hardwarelimitations, however, NUC through TRAU require that time slots on PCMA and PCMSare identical, resulting in an auxiliary sorting rule.





57

Example: "Not compatible with cross-connect" (matrix type 1)

TS no. PCMS (sub slots) PCMA#0 PCMA#1 PCMA#2

1 0-10 0-10 0-10 0-10 1 1 1

2 0-5 0-5 2-15 2-15 2 2 2

3 0-1 1-1 2-1 3-1 3 3 3

4 0-2 1-2 2-2 3-2 4 4 4

5 0-3 1-3 2-3 3-3 MSL x 2 5 5

6 0-4 1-4 2-4 3-4 6 6 6

7 1-5 2-5 3-5 0-6 7 7 7

8 1-6 2-6 3-6 0-7 8 8 8

9 1-7 2-7 3-7 0-8 9 9 9

10 1-8 2-8 3-8 0-9 MSL x 4 10 10

11 1-9 2-9 3-9 1-10 11 11 11

12 2-10 3-10 0-11 1-11 12 12 12

13 2-11 3-11 0-12 1-12 13 13 13

14 2-12 3-12 0-13 1-13 14 14 14

15 2-13 3-13 0-14 1-14 15 15 MSL x 2

16 CCSS#7 CCSS#7 16 16

17 2-14 3-14 0-15 1-15 17 17 17

18 3-15 1-16 2-16 3-16 18 18 18

19 0-17 1-17 2-17 3-17 19 19 19

20 0-18 1-18 2-18 3-18 20 20 20

21 0-19 1-19 2-19 3-19 21 21 21

22 0-20 1-20 2-20 3-20 22 22 22

23 0-21 1-21 2-21 3-21 23 23 23

24 NUC NUC 24 24

25 0-22 1-22 2-22 3-22 25 25 25

26 0-23 1-23 2-23 3-23 26 26 26

27 1-24 2-24 3-24 0-25 27 not used not used

28 1-25 2-25 3-25 0-26 not used not used not used

29 1-26 2-26 3-26 0-27 not used not used not used

30 OMAL OMAL not used not used

31 LPDLS not used not used not used




MN1780EU11MN_0001

© 2005 Siemens AG

58

a

5.2 Abis and Um interface

The higher data rates for packet switched services require an enhanced Abiscapacity. The flexible Abis Allocation Strategy (FAAS) is based on Abis pools andappropriate Abis subpools, which can be configured per base station site via O&Mprocedures.

The pool concept no longer assigns a fixed relation between the air interface and theappropriate Abis . An Abis pool is the amount of 16kbit/s subslots, which is defined per base station site. An Abis pool is composed by one or several subpools. Each subpoolbelongs to a single PCM line, routed together with one associated LAPD link tomanage a correct fault propagation from the LAPD link to the Abis resources.

The dimension of the pool is defined by the operator and can be changed via OAM

commands.When a user requires radio timeslots in a cell, the BSC selects the appropriatenumber of Abis resources from the common Abis pool, associates them to the radiochannel and signals their association to radio channels and Abis resources to theBTS.

The amount of allocated 16kbit/s abis resources per radio time slot depends onseveral factors, first of all the service type (e.g. GPRS coding scheme CS4 requires2x16kbit/s TS, while EGPRS coding scheme MSC7 needs on Abis side 4x16kbit/sTS).





59

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18 14

L

A

P

D 10 2 0

TS pool for

BTSM 2

L

A

P

D

L

A

P

D

S

W

/

F

A

W

TS pool

for

BTSM 1

7 6 5 4 3 2 1

TRX-1-0

7 6 5 4 3 2 1

MCS7

CS4

Abis Allocation Strategy

BTSM: 1

BTSM: 27 6 5 4 3 2 1

7 6 5 4 3 2 1

7 6 5 4 3 2 1

7 6 5 4 3 2 1

TRX:0

TRX:1

TRX:2

TRX:3

TRX:0

TRX:1

Fig. 32 FAAS




MN1780EU11MN_0001

© 2005 Siemens AG

60

Dynamic Abis Resource allocation is applied both to packet switched services andto circuit switched services. According to the service applied, the appropriate number of Abis resources is dynamically allocated. As the capacity of each air interface

timeslot can vary during runtime, the dynamic Abis allocation adopts the Abis capacityto the required air interface capacity.

Abis pools and Abis subpools have the following properties and relations:

• Different Abis subpools, belonging to the same or different Abis pools can be definedon the same PCM line

• Subpools can be distributed over all PCM lines belonging to a base station site (atleast one subpool per line)

• The Abis subslots allocated to a radio channel may be distributed over different

subpools and consequently over different PCM lines. It is not necessary toguarantee that the subslots are adjacent.

• Overlapping between pools and subpools are forbidden

With the common pool concept any radio timeslot is dynamically associated to anappropriate number of Abis resources from the Abis pool.





61

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BSC

subpool subpool

CELL_1

CELL_2

CELL_3

C

O

R

E

C

O

R

E

CELL_4

CELL_5

CELL_6

C

O

R

E

C

O

R

E

Abis Pool for

BTSM:1

BTSM:1

BTSM:2

subpool

subpool

Abis Pool for

BTSM:2

Fig. 33 Abis pools per BTSM




MN1780EU11MN_0001

© 2005 Siemens AG

62





63

6 GSM basic features




MN1780EU11MN_0001

© 2005 Siemens AG

64

6.1 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 thelowest 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 madeto determine whether a power increase or decrease is required. The measurementaveraging process and the power control decision algorithm for each call in progressare performed by the BTS.

Power Control can be set independently for uplink and downlink. In addition, for circuit switched services it can of the type static or adaptive. The last one comprises

adaptive power correction step sizes dependent on measured values for the signallevel and quality.

Optionally the Service Dependent Power Control offers to define for the fourteendifferent service groups different threshold levels.





65

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BTSE

SIEMENS

Distance D2

Distance D1

Power Out 1

Power Out 2

Fig. 34 RF power control




MN1780EU11MN_0001

© 2005 Siemens AG

66

6.2 Cell (re)selection

Idle Mode

MS measures the received level on each RF carrier. Based on these measurementsone can estimate whether a cell will be an appropriate serving cell from the radiopropagation point of view, i.e. whether there will be a sufficient “link quality”.

While moving within the radio network in idle mode, another cell may be moreappropriate to serve the MS. Therefore, cell reselection may be performed.

From the radio propagation point of view it is worth to select a new (neighbor) cell if the received level from that neighbor cell exceeds the received level of the currentserving cell:

• Receive level (neighbor cell) > Receive level (serving cell)

The inter-system cell reselection from GSM to UMTS allows a MS to reselect to theUMTS system. For intersystem cell reselection, the Received Signal Code Power isthe measure for receive level in case of UMTS neighbor cells.

Packet switched Connections

From a serving GPRS cell the MS is executing the cell reselection to a neighbor GPRS cell. This so called Cell Reselection (packet switched) is performed by the MSin the same way as for circuit-switched connected MS in idle mode.

Also it is possible to do cell reselection from a GPRS cell to a UMTS (packetswitched) cell.

TIPHandover is only used for circuit switched connections and the BSS is deciding aboutthe HO execution.





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MS direction of movement

Cell Reselection

Radius of Cell 2

for selection

BTS1

BTS1

BTS2

BTS2

ReceiveLevel

Radius of

serving cell 1

Fig. 35 Cell (re)selection




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6.3 Handover

6.3.1 Intra GSM HO

Handover is required to maintain an ongoing call when the Mobile Station passesfrom one cell coverage area to another. Handover means that a call in progress isswitched over seamlessly from one radio channel to another. In Adaptive MultirateHandover, 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 isoptional. It derives from signal strength measurements carried out by the MS on theBCCH 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 resourcemanagement 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 fromthreshold comparisons that are applied to the respective (averaged) measurementresults.





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BSC 2

1

23

BSC 1

MSC1

1 intracell handover 2 intercell handover – intra-BSC3 intercell handover – inter-BSC

Fig. 36 Types of handovers

If an intercell handover is to be initiated, the power budget criterion is applied to setup 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 wellas 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 BSCor MSC.




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6.3.2 HO to UMTS

Calls started in the GSM system are handed over to the UMTS system when the user moves into the area with better UMTS coverage or the UMTS coverage only. In thiscase always the MSC is executing the HO.

The BSS initiates a handover to UMTS according due to radio criteria in one of thefollowing reasons:

• Emergency Handover

• Better cell

• Sufficient UMTS coverage

In addition, the following network criteria may be evaluated:• serving cell congestion (directed retry for call setup)





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MSC / VLRU-MSC / VLR

RNC BSC

Node-B Node-B BTS / CCU BTS / CCU

HLR

Location Area Location Area

Iu (cs) A

Iub Abis

UE/MS

Fig. 37 Combined UMTS / GSM network architecture




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6.4 Multiband operation

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

Therefore, the operation of "dual band" MS as well as handover between cellsbelonging 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 whenexpanding a GSM900 network with GSM1800 equipment and vice versa.

Multiband operation may even feature a "Common BCCH".

As an example a concentric BTS is configured. The inner area is the GSM1800frequency and the outer area is the GSM 900 frequency. This BTS can be suppliedwith a single BCCH and offers carrier frequencies in both GSM900 and GSM1800bands.

Another solution is to define a dual band standard cell that is also featuring theCommon BCCH.





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BTS-1

GSM1800

BSC

GSM 1800

GSM 900

GSM1800

BTS-0

GSM 900

GSM900

BTS-1

concentric

BTS-0

Dual band

standard

BTS-0

GSM 900

Fig. 38 Network with mixed cell configuration GSM900/ GSM1800




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7 Data transmission in GSM phase 2+




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To increase the data transmission rates, in GSM phase 2+ new bearer services withrates 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. videostreaming or video telephony. The higher bandwidth is achieved by combining 1-8physical channels for one subscriber. Additionally, the data transmission codec ischanged 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 115.2kbit/s. For implementing HSCSD, merely the GSM-PLMN software requiresmodification. 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 withHSCSD. 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. Asopposed to HSCSD, GPRS is a packet-switched bearer service, meaning thatseveral 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. surfingthe Internet, E-mail, ...). GPRS also enables point-to-multipoint transmission andvolume-dependent charging. However, extensions of the GSM network and protocolarchitecture 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 theGSM modulation procedure (8PSK instead of GMSK). Theoretically, transmissionrates of up to 553.6 kbit/s (meeting 3G requirements) would be possible bycombining up to 8 channels. A combination of GPRS and EDGE offers high

bandwidth and highly economical frequency resource utilization at the same time.





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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/sno HW changes, but high demand on radio resources

GPRS (Packet-Switched): 9.05 ... 21.4 kbit/sHW and protocol changes, resource efficient

EDGE (CS / PS): max 43.2 / 59.2 kbit/snew modulation type: 8-PSK instead of GMSK

Fig. 39 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. 40 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 usingup to max. 8 TCH simultaneously.

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 datarate, but the BSC may modify the number of TCH used by the connection (in thiscase 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 modemof internet provider - no compression at data rates 19.2 kbit/s, 38.4 kbit/s or 64 kbit/sin later versions).

Symmetric and pseudo-asymmetric services

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

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





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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. 41 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 eachchannel is applied.

• For an asymmetric HSCSD configuration individual signal level and qualityreporting is used for those channels, which have uplink SACCH associated withthem.

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

• In both symmetric and asymmetric HSCSD configurations the neighbor cellmeasurements are reported on each uplink channel used.

Handovers

All TCH used in an HSCSD connection are handed over simultaneously. The BSCmay modify the number of timeslots used for the connection and the channel codingwhen 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 thechange 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 levelupgrading and downgrading procedures.





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Asub A

BSC

TRAU

0

TRAU

1

TRAU

2

MSC

64 TS

used for HSCSD

4*14,4 HSCSD

BTSE

BTSE

4*14,4 HSCSD

Abis

H H H H

H H

Air IF

2*14,4 HSCSD2*14,4 HSCSD

2 TS on air IF

64 TSused for HSCSD

2*14,4 HSCSD

4*14,4 HSCSD

4 TS on air IF

Fig. 42 HSCSD transmission examples




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7.2 General Packet Radio Service GPRS

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

7.2.1 Core network elements

Serving GPRS Support Node SGSN is on the same hierarchic level as an MSC andhandles functions comparable to a Visited MSC. For example Mobility Management;paging, tracing the location, it performs security functions, access control,routing/traffic management.

Gateway GPRS Support Node GGSN realizes functions comparable to those of agateway MSC. For example performs interworking between a GSM PLMN and apacket data network PDN, contains the routing information or can inquire aboutlocation information from the HLR

Interfaces

New interfaces are defined for GPRS. The most important are:

• Gb - between an SGSN and a BSS; Gb allows the exchange of signaling and user data:

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

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





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SGSNServing GPRS

Support Node

PSTN

InternetIntranet

X.25

GGSNGateway GPRS

Support Node

VMSC /

VLRGMSC

HLR

ISDNBSS

G

R

GPRS subscription data

(GPRS Register GR)

GPRS Support Nodes

SGSN:Interface to Base Station System

GGSN:Interface to Packet Data Network

SSS

PCUGb GiGn

Fig. 43 Packet Switches in the Core Network




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7.2.2 Radio network units

Packet Control Unit PCU

In the SBS, the PCU is realized by the PPXX module in the BSC. 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.

Channel Codec Unit CCU

In the SBS the CCU is included in the Carrier Unit module of the BTSE. The CCUperforms:

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

• Radio channel measurements (including received quality and signal level, timingadvance measurements)

GPRS Mobile Station

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

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

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

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





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PCU• Channel Access Control functions

• Radio Channel Management

(Power Control, Congestion

Control,...)

• scheduling data transmission

(UL/DL)

• protocol conversion (Gb<-->Um)

GPRS-MS:• Class-A

• Class-B

• Class-C

Um Abis

CCU

CCU

BTSE BSC GSN site

PCU

Gb

CCU• Channel Coding (FEC, interleaving,..)

• Radio Channel Measurement

functions(received quality & signal level,

TA,..)

Fig. 44 PCU, CCU, MS




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7.2.3 Radio interface

The tasks of layer 1 radio interface relate to the transmission of user and signalingdata 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 packet switched traffic a main difference to circuit-switched services is that severalmobile stations in parallel can use a physical channel and a packet data channel.This is named multiplexing.

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

The allocation can be done in two ways:

• Horizontal Allocation

• Vertical Allocation

In case of horizontal allocation strategy the MS are placed in separate timeslots. Theadvantage is that the MS get the maximum possible capacity.

In case of vertical allocation strategy several MS are multiplexed on the sametimeslots. This leads to the advantage that the scare resources of the air interface aresaved.

Distribution of the physical channels for various logical packet data channels is basedon blocks of 4 normal bursts each, called radio blocks. This means that signalingand the packet data traffic of several mobile stations can be statistically multiplexedinto one packet data channel.

UL and DL for GPRS packet data are assigned separately. Therefore the packet datachannel can be seized asymmetrically.





87

Several MS

multilplexed

one TS

Several TS

allocated to

one MS

Up to 8 time slots of a TDMA frame can be

combined dynamically for a user for the

transmission of GPRS packet data.

Horizontal Allocation

Vertical

Allocation

Fig. 45 Multiplexing and TS combining

L1-

tasks

Transmission of user & signaling data

Timing Advance

Cell SelectionMeasure

signal strength

Power Control

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)

Fig. 46 Radio Interface




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7.2.4 Channel coding

Channel coding is modified substantially for packet switched purposes (GSM Rec.03.64). Channel coding starts with the division of digital information into transferableblocks. 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 DataBlock) 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-arrangementin 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 with57 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 assignedas a function of the quality of the radio interface. Normally, groups of 4 burst blockseach 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 user data rate of 9.05 kbit/s.

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

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

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

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

Common coding of 4 Bursts

(Radio Block)

Up to171,2 kbit/s(theoretically)

1 - 8

channel

Fig. 47 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

Radio Block

(Redundancy !)rate 1/2 convolutional coding

Radio Block (456 Bits)

puncturingPuncturing

(only CS-2, CS-3)

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. 48 Channel coding




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7.3 Enhanced data rates for GSM evolution EDGE

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

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

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

• Enhanced Circuit Switched Data ECSD and

• Enhanced General Packet Radio Service EGPRS.

In SBS up to this release the EGPRS is implemented.

A similar concept is used in the American market for enhancing the capabilities of theD-AMPS networks. The standardization of the UWC-136 HS system is done withinthe UWCC (Universal Wireless Communication Consortium), which closelycooperates 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. 49 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 bitGross rate: 69.2 kbit/s

Fig. 50 Comparison of GSM modulation (left) and EDGE modulation (right)




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E

7.3.1 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 onthe condition of the air interface, i.e. in case of e.g. high interference level the datatransmission rate is dynamically reduced to an optimum balance between speedand error correction capabilities, and

Channel Coding9 new coding schemes have been developed for EGPRS:

CodingScheme

Modu-lation

User datarate (kbit/s)

Coderate

Puncturingschemes

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 296MCS-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 592MCS-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.





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

GMSK 8-PSK

Family A

Family B

Family C

M

C

S

-

2

M

C

S

-

3

M

C

S

-

4

M

C

S

-

5

M

C

S

-

6M

C

S

-

1

M

C

S-

7

M

C

S

-

8

M

C

S

-

9

C

S

-

1

C

S

-

2

C

S

-

3

C

S

-

4

GPRS

~ 22,8 kbit/s

~ 69 kbit/s

D a t a R a t e ( p e r t i m e s l o t )

Fig. 51 EGPRS Coding Schemes




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7.3.2 Link adaptation

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

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





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

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. 52 Link adaptation




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7.3.3 Architecture

The EGPRS architecture includes an EDGE capable GSM mobile station (MS), whichis connected via the air interface (Um) to the E-CU that supplies EDGE functionality.

The packet switched traffic output from the E-CU in the BTS is transmitted to thePacket Control Unit (PCU) of the BSC from where it is routed to the GPRS backbone.

BSS Configuration with EDGE

The BTSE can be equipped with Edge capable carrier units featuring the new 8-PSKmodulation technique and/or "normal" carrier units (G- CUs) supporting GMSKmodulation. In the BTSone EDGE is not supported.

To reach the high data rates that are foreseen by the MCS, it is necessary to support

Concatenated PCU frames. This requires peripheral processors for packet switchedtraffic handling providing the Packet Control Unit functionality of the Type "PPXX"modules.

The FAAS (Flexible Abis allocation Strategy) feature guarantees the higher capacityrequirement on Abis. This is realized by the BR7.0 software and higher.

Edge Mobile Station

Two classes of mobile stations are provided. One class of mobile station is able toapply the 8PSK modulation in both the uplink and the downlink directions, whichmeans that they support advanced facilities and capabilities. The other class applies8PSK modulation in the downlink direction and GMSK modulation in the uplinkdirection.





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S i e m e n s

Mobile DTE

B

S

C

Visited

MSC/VLR

Gateway

MSC

Serving

GSN

packet switched: GPRS

Gateway

GSN

BTSE

HLR/GR

PPXX modules

in BSC

BTSE with

E-CU

ISDN

PSTN

Internet

Intranet

PSPDN

FAAS with BR7.0

PCU

Fig. 53 Changes in Existing Network Infrastructure




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99

8 Location services

∆ r r '

r

Fig. 54 Location Services




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Location Services (LCS) offer the opportunity to deploy new value added servicesbased 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 (USFederal Communication Commission requires location of emergency calls by end of 2001).

Location Services (LCS) provide MS positions to location applications ("LCS clients").To describe the position of the MS universal latitude and longitude are usedaccording to a defined geodetic reference system (e. g. WGS 084 coordinatesReference System).





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8.1 Architecture

The new network entities are:

Serving Mobile Location Center (SMLC)

• manages the overall coordination and scheduling of resources required to performMS positioning in a PLMN

• determines the positioning method to be used based on the QoS, the networkcapabilities and the MS location capabilities

• calculates the final location estimate and accuracy and it in a location response toa requesting Gateway Mobile Location Center (GMLC)

• there may be more than one SMLC in one PLMN

Gateway Mobile Location Center (GMLC)

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

• stores LCS subscription information on a per-LCS-client basis. This is used whenreceiving an LCS request to identify the requesting LCS client and to authorize it touse the specified request

• requests info from HLR about the MS to be located

• manages subscriber privacy

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

• generates LCS related charging and billing rates





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BTSE BSCMSC /

VLR

External

LCS Client

SMLC

HLRGMLC

Abis A

Lg

Lg Le

Lh

Lb

Um

Different

PLMN

GMLC

Fig. 55 LCS network architecture




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8.2 Positioning methods

The positioning methods introduced are

• (Enhanced) Cell-ID/Timing Advance

• Enhanced Observed Time Difference (E-OTD)

• Time/Angle of Arrival (TOA/AOA)

• (Uplink) Time Difference of Arrival (U-TDOA)

• Assisted-Global Positioning System GPS (A-GPS)

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

SMLC positioning functions

The Common PRCF (Positioning Radio Coordination Function) determines thepositioning method to be used (CITA, E-CITA, A-GPS …) and manages theresources required by the chosen method.

The E-CITA PCF calculates position received from the E-CITA PRCF (includingcollection of the required prediction data from the SMLC database) and returns backthe calculated location estimate.

As examples two important methods (E-CITA and A-GPS) are described in moredetail in the following.





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OMA SUPL:

LSU

SMLC

+ LMUs

None

5s

50 - 125

U-TDOA

SMLC

+ LMUs

Yes

(SW)

5s

50 - 125

E-OTD

3GPP:

SMLC+ Reference

network

SMLCSMLCNoneImpact on

network

5-10s5s5s3sFix Time

Yes

(GPS chip,

antenna)

NoneNoneNoneImpact on

handset

5 – 75100 - 200~50010-

30,000 Accuracy

[ m ]

A-GPSE-CI/TACI/TACI

Fig. 56 Comparison of Positioning Methods




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8.2.1 Enhanced CITA

The basic location information in cellular network is the actual Cell ID of the servingcell. Combining to this information the Timing Advance value of the MS, the CITAmethod is realized. The E-CITA is a positioning method that uses the CITA enhancedby a comparison of predicted receive levels and measured receive levels.

In GSM every MS that is in dedicated mode measures and reports continuously theactual radio conditions to the network. These reports contain information aboutreception power levels (RXLEV) of the serving cell and up to six neighboring cells.

To realize this method the SMLC has to be filled with prediction power level data. Incase of a positioning request, the SMLC will compute the location by matching themeasured data with the prediction data.

The output of the SMLC is shape, for example a point on the surface of the earth withan uncertainty circle.

Beside the position, the algorithm is able to estimate the positioning error describedby an uncertainty circle or ellipse for a given confidence level.





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BTS 1

MS

Cell ID

BTS 2

BTS 3

Signal

Strength 1

Signal

Strength 2

SignalStrength 3

BSC

Fig. 57 MS measurements

0 2 4 6 8 10 12 14 16 18 20

0

2

4

6

8

10

12

14

16

Position

X, Y, r

BTS

measurements

MS

measurementsComparison

CGI, TA, RXLEV

Measurement

reports

Field strength

predictions

SMLCSBS

Fig. 58 Location estimation




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8.2.2 Assisted GPS

Conventional GPS is a known solution for positioning. In case of using the GPSmethod in mobile networks the large time to first fix, low sensitivity (no coverageindoors or in urban canyons), and excessive battery consumption are thedisadvantages. To overcome these drawbacks, network assisted GPS (A-GPS) isused.

The basic idea behind network assisted GPS (A-GPS) is that the GPS receivingsystem is distributed:

• MS with GPS receiver

• Reference GPS network

The GPS reference network consists of GPS receiving stations, tracking all GPSsatellites at all times. This data is used to model the satellite orbits and clocks wellinto the future. The MS obtains this so called assistance data from the radio network.It consists of a list of visible satellites to facilitate the acquisition of the satellitessignals for the MS.

The MS measures only the times of arrival (called “pseudoranges”) which enables alower signal level. Therefore the A-GPS method requires a handset with integratedGPS receiver.

The Siemens mobile A-GPS functionality is integrated into the SMLC. The SMLC isdetermining the position with use of the GPS data. In the SMLC the A-GPS

positioning method can be provided along with other lower accuracy positioningmethods like CITA and E-CITA.





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GPS ReferenceNetwork

BSC

Ass. Data &

position

calculation

SMLC

M e a s u r e m e n

t s +

A s s i s t d a t a

Fig. 59 MS measurements and assistance data

SBS

A-GPS MS

Sattelite

Signals

SMLC

GPS

Reference

data

GPS data

GPS

Position Calculation

Assistance data

Pseudo rangesPosition

X, Y, r CITA, ECITA

PCF

Fig. 60 Location estimation




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9 Exercises




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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

Name the most important interfaces defined by GSM?

What is a radio cell?

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

Find out the correlation between the guard period of an access burst and themaximum 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?

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

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?




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