01_mn1780eu10mn_0003_sbs_overview

SBS overview Siemens

MN1780EU10MN_0003 © 2002 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 39 4.6 Logical channels for PS services 44 4.7 Multiframes 48 4.8 Timeslot structure 52 5 Abis and Asub interface 55 5.1 Asub and A interfaces 56 5.2 Abis and Um interface 60 6 GSM basic features 65 6.1 Power control 66 6.2 Cell (re)selection 68 6.3 Handover 70 6.4 Multiband operation 74 7 Data transmission in GSM phase 2+ 77 7.1 High Speed Circuit Switched Data HSCSD 80 7.2 General Packet Radio Service GPRS 84 7.3 Enhanced data rates for GSM evolution EDGE 92

SBS overview

Siemens SBS overview

MN1780EU10MN_0003© 2002 Siemens AG

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8 Location services 101 8.1 Architecture 104 8.2 Positioning methods 106 9 Exercises 113



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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 European Telecommunication 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 that was established in December 1998. The collaboration agreement brings together a number of telecommunications standards like ARIB, CCSA, ETSI, ATIS, TTA, and TTC.

Public Land Mobile Network PLMN A Public Land Mobile Network is a network, established and operated by its licensed operators, for the specific purpose of providing land mobile communication services to 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 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.

Mobile Station MS and SIM 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. The Subscriber Identity Module SIM provides the 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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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 following functional 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 Centers OMC; in the Siemens solution:

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

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



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RC SC

MSC

HLR VLR

EIRAC

BSC

BTSTRAU

MobileStation

MS

Radio SubSystem

RSS =

Base StationSubsystem

BSS

Network Switching Subsystem

NSS

+

other networks

Operation SubSystem OSS

PSTN

ISDN

Data NetworksMS =

ME + SIM

Fig. 2 GSM PLMN



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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 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/fromSSS

To/fromOMS

MobileStation

Um

Fig. 3 Radio Subsystem RSS

TranscoderandRate Adapter Unit

TRAU

Base StationController

BSC

Abis Asub

Base Station System BSS

A

S

G

S

N

M

S

C

Gb

PCU

BaseTransceiverStationEquipment

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 four PCMB lines in case of the BTSplus family. These multiple links support load-sharing and fault recovery. 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 in the BSC database they are called LPDLM (although they may carry both LPDLM and LPDLR type signaling). Minimum for each PCMB line at least one LAPDLE/LPDLM is required.



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BTSE Object: LAPDLEBSC Object: LPDLM

BTSplus

BSC

PCMB#0

PCMB#1

PCMB#2

PCMB#3

4 PCMB linesbetween

BSC - BTSplus

1 LAPD linkper

PCMB line

Up to 8 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:

BR5.5 BR6.0 BSC HC 1st Step

BR7.0 HC BSC 2nd Step

Controlled TRXs* 250 500 900

Controlled Cells 150 250 400

Controlled BTSE 100 200 200

Controlled TRAU 20 32 36

PCM lines 46 72 120

LAPD (Abis+Asub) up to 112 up to 240 up to 240

GPRS channels 2x64 1536 3072

SS7L 8 8 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 step one 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 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")

EMICROII BTSplus 6 3 ("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. 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) 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 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 indispensable for 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 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 (ITU-T) using the LAPB protocol and is used for the BSC, TRAU and all 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 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.

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, 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, BS242, BS82, BS2x/6x/11 information (hardware related)

TEDTechnical Descriptions

Guide to documentationTerminology

SBS counters and SBS message flowsPMPerformance Measurements

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 Test Manuals

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

Explanation of termsGlossary

For BSC, BTSE, OMS-B, TRAUCMLCommand Manuals

AbbreviationsAbbreviations

ContentAbb.Manual

Fig. 10 Document types



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27

4 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. 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 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


GSM 1900 (PCS):

1850-1910 & 1930-1990 MHz


1850 199019301910MHz

1710 188018051785MHz

880 960925915MHz

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 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

GSM1800 (DCS)

GSM1900 (PCS)

880MHz

915MHz

925MHz

960MHz

1710MHz

1785MHz

1805MHz

1880MHz

1850MHz

1910MHz

1930MHz

1990MHz

C C C C. . . CCCC . . .

200kHz

200kHz

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 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 24RFC 32

RFC 8RFC 5RFC 2

RFC 17RFC 13

RFC 47RFC 40

RFC 24RFC 32

RFC 11RFC 47

RFC 8RFC 5RFC 2

reuse distance (~ 4 r)

Fig. 14 Radio frequency carriers and reuse distance



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4.3 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 y

RFC yRFC x

RFC x

RFC y

TDMA Frame

(4.61

5 ms)

200 kHz 200 kHz

0 15 31

TDMA Frame(125 µs)

PCM 30

RFC x

Fig. 15 Time division multiple access TDMA

RFC1RFC

1RFC1RFC

1RFC1RFC

1RFC1RFC

10

12

34

56

7RFC1RFC

1RFC1RFC

1RFC1RFC

1RFC1RFC

20

12

34

56

7RFC1RFC

1RFC1RFC

1RFC1RFC

1RFC1RFC

30

12

34

56

7 RFC1RFC

1RFC1RFC

1RFC1RFC

1RFC1RFC

174

RFC3RFC

3RFC3RFC

3RFC3RFC

3RFC174

RFC3RFC

3RFC3RFC

3RFC3RFC

3RFC3

RFC3RFC

3RFC3RFC

3RFC3RFC

3RFC2

RFC3RFC

3RFC3RFC

3RFC3RFC

3RFC1

Uplink Downlink

FDMA FDMA

f

t

TDMA TDMA

FDD

Fig. 16 Frequency division multiple access and time division multiple access (t=time, f=frequency)



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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

TrafficChannel

TrafficChannel

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 physical channels used for payload and those used for signaling.

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)

TIP Channel Combinations

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



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

Logical ChannelsLogical Channels

Traffic ChannelsTraffic Channels

Full RateTCH/F

Half RateTCH/H

Signaling ChannelsSignaling Channels

BroadcastControl

ChannelsBCCH

BroadcastControl

ChannelsBCCH

CommonControl

ChannelsCCCH

CommonControl

ChannelsCCCH

DedicatedControl

ChannelsDCCH

DedicatedControl

ChannelsDCCH

Fig. 18 Logical channels for circuit-switched traffic



36

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 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.

4.4.2 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 (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 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.)



37

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

Broadcast Control Channel BCCH


Frequency Correction Channel FCCH

Synchronization Channel SCH


Cell Broadcast Channel CBCH

DL

DL

DL

DL

Common Control Channel CCCH

Common Control Channel CCCH

Random Access Channel RACH

Access Grant Channel AGCH

Paging Channel PCH

Notification Channel NCH

UL

DL

DL

DL

Dedicated Control Channel DCCH

Dedicated Control Channel DCCH

Stand-alone DedicatedControl Channel SDCCH

Slow AssociatedControl Channel SACCH

Fast AssociatedControl Channel FACCH

DL & UL

DL & UL

DL & UL

Fig. 19 Signaling channels



38



39

4.5 Example for signaling channels The BTS periodically broadcasts common system information via BCCH logical channel. The information contained in the BCCH is received and used by all MS that camp the BTS covering area.



40

BTSE

BCCHcell identity: 262041422

Fig. 20 Example: Broadcast Channel information

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 the information 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 exchange between the MS and the network as long as an TCH is allocated to the MS, everything via SDCCH channel too.



41

The MS requests dedicated channel though the RACH also when paged (mobile terminating 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 the dedicated channel request cause.

BTSE

RACHRequest of a SDCCH

12142341234

Fig. 21 Example: Random Access Channel



42

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

BTSE

AGCHSDCCH provided

12142341234

Fig. 22 Example: Access Grant Channel



43

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

BTSE

SDCCHControl Information

Fig. 23 Example: Stand-alone Dedicated Control Channel



44

4.6 Logical channels for PS services For packet-switched PS traffic, the same physical channel may carry both signaling and payload. The packet data logical channels are mapped onto the physical channels that are dedicated to packet data. The physical channel dedicated to packet data traffic is called 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 MS or to a group of MSs in the PTM-M case. In the multislot operation, one MS may use multiple PDTCHs in parallel for individual packet transfer. All packet data traffic channels are unidirectional, either uplink (PDTCH/U), for a mobile originated packet transfer or downlink (PDTCH/D) for a mobile terminated packet 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/H respectively. A PDTCH, when used for multislot operation shall be full-rate



45

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

Logical ChannelsLogical ChannelsLogical Channels

Traffic ChannelsPacket Traffic Channels

Packet Traffic Channels

Packet Associated

ControlChannel

Signaling ChannelsPacket Signaling Channels

Packet Signaling Channels

Packet Broadcast Control Channel

PBCCH

Packet Broadcast Control Channel

PBCCH

Packet Timing

AdvanceContr. Channel

PacketData

TrafficChannel

Packet CommonControl Channel

PCCCH


PCCCH

Fig. 24 Logical channels for packet-switched traffic



46

4.6.2 Packet Broadcast Control Channels (PBCCH) 1. Packet Broadcast Control Channel PBCCH broadcast packet data specific

System Information. If PBCCH is not allocated, the packet data specific system information 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 data services. 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 packet transfer. 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. This channel is used downlink only.

4.6.4 Packet Dedicated Control Channels (PDCCH) 1. Packet Associated Control Channel PACCH conveys signaling information

related to a given MS. The signaling information includes e.g. acknowledgements and power control information. The PACCH shares resources with PDTCHs, that are currently assigned to one MS.

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



47

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


PCCCH


PCCCH

Packet BroadcastControl Channel

PBCCH

Packet BroadcastControl Channel

PBCCHPacket 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 DedicatedControl Channel

PDCCH

Packet DedicatedControl Channel

PDCCH

Packet Associated Control Channel PACCH

Packet TA Control Channel PTCCH

DL & UL

DL & UL

Fig. 25 Packet switched Signaling channels



48

4.7 Multiframes There are 8 timeslots per TDMA frame. The TDMA frames are forming the multiframes. 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 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.



49

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

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



50

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 (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

701

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 li n k

R A C HU P L IN KUPLINK

DOWNLINK

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



51

4.7.3 Packet switched Multiframes The 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 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, PACCHPDCH without PCCCH: PDTCH & PACCH only

Idle frame:• Identification of BSICs• Timing Advance Update Procedure• Interference measurements

for Power Control

Fig. 28 GPRS multiframes



52

4.8 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).



53

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).

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

3T

57Encrypted

1S

26Training

1S

57Encrypted

3T

8.25GP

8T

41Synch. Sequence

36Encrypted

3T

68,25GP

3T

142Fixed Bits

3T

8.25GP

3T

39Encrypted

64 ExtendedTraining Sequence

39Encrypted

3T

8.25GP

3T

58Mixed Bits

26Training

58Mixed Bits

3T

8.25GP

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



54



55

5 Abis and Asub interface



56

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 the PCMS 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 signaling information. This signaling requires a transmission rate of 64 kbit/s. One of the 4 PCMA 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 lines remain empty. Note: Not every TRAU carries CCSS7 signaling information. An OMAL signaling link defined as AINT (nailed up connection) is, e.g. put in timeslot 30. OMAL is transmitted with 64 kbit/s on a single PCMA line. Correspondingly, three timeslots 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.



57

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h dg cf be a

0163031

TRAUBSC

e a

f b

g c

h d01630

31

TRAU MSC

Asub

AX

X

X

X

X

X

X

empty

CCSS7

LPDLS

OMAL

CCSS7

SW/FAW

X

SW/FAW

OMAL

LPDLS

X

X

X

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



58

5.1.1 TRAU matrices, multislot connections and pooling Several time slots on Um / Abis / Asub can be combined and finally mapped into a single 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 the combination of max 4 (sub) slots on Um / Abis / Asub. The most commonly used time slot distribution between PCMS and PCMA is defined in the BSC database as "not_compatible_with_cross_connect" (former TRAU Matrix 1). The sorting rules are:

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

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

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

TIP Note: 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 hardware limitations, however, NUC through TRAU require that time slots on PCMA and PCMS are identical, resulting in an auxiliary sorting rule.



59

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



60

a

5.2 Abis and Um interface The higher data rates for packet switched services introduced in BR7.0 require an enhanced Abis capacity. The flexible Abis Allocation Strategy (FAAS) is based on Abis pools and appropriate Abis subpools, which can be configured per base station site via O&M procedures. The pool concept no longer assigns a fixed relation between the air interface and the appropriate 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 subpool belongs to a single PCM line, routed together with one associated LAPD link to manage 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 appropriate number of Abis resources from the common Abis pool, associates them to the radio channel and signals their association to radio channels and Abis resources to the BTS. The amount of allocated 16kbit/s abis resources per radio time slot depends on several factors, first of all the service type ( e.g. GPRS coding scheme CS4 requires 2x16kbit/s TS, while EGPRS coding scheme MSC7 needs on Abis side 4x16kbit/s TS).



61

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18 14

LAPD 10 2 0

TS pool forBTSM 2

LAPD

LAPD

SW/FAW

TS poolforBTSM 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



62

Dynamic Abis Resource allocation is applied both to packet switched services and to 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 capacity to 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 defined on the same PCM line

• Subpools can be distributed over all PCM lines belonging to a base station site (at least 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 to guarantee that the subslots are adjacent.

• Overlapping between pools and subpools are forbidden With the common pool concept any radio timeslot is dynamically associated to an appropriate number of Abis resources from the Abis pool.



63

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BSCsubpool subpool

CELL_1

CELL_2

CELL_3

CORE

CORE

CELL_4

CELL_5

CELL_6

CORE

CORE

Abis Pool forBTSM:1

BTSM:1

BTSM:2

subpool

subpool

Abis Pool forBTSM:2

Fig. 33 Abis pools per BTSM



64



65

6 GSM basic features



66

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 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. Power Control can be set independently for uplink and downlink. In addition, for circuit switched services it can be , if set, either static or adaptive. The last one comprises adaptive power correction step sizes dependent on measured values for the signal level and quality. Optionally the Service Dependent Power Control offers to define for the fourteen different service groups different threshold levels.



67

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BTSE

SIEMEN S

Distance D2

Distance D1

Power Out 1

Power Out 2

Fig. 34 RF power control



68

6.2 Cell (re)selection

Idle Mode MS measures the received level on each RF carrier. Based on these measurements one can estimate whether a cell will be an appropriate serving cell from the radio propagation 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 more appropriate 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 current serving cell:

• Receive level (neighbor cell) > Receive level (serving cell) The inter-system cell reselection from GSM to UMTS allows a MS to reselect to the UMTS system. For intersystem cell reselection, the Received Signal Code Power is the 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 MS in 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 (packet switched) cell.

TIP Handover is only used for circuit switched connections and the BSS is deciding about the HO execution.



69

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MS direction of movement

Cell Reselection

Radius of Cell 2for selection

BTS1

BTS1

BTS2

BTS2

Receive Level

Radius ofserving cell 1

Fig. 35 Cell (re)selection



70

6.3 Handover

6.3.1 Intra GSM HO 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. 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.



71

BSC 2

1

23

BSC 1

MSC1

1 intracell handover 2 intercell handover – intra-BSC 3 intercell handover – inter-BSC

Fig. 36 Types of handovers

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.



72

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 this case always the MSC is executing the HO. The BSS initiates a handover to UMTS according due to radio criteria in one of the following 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)



73

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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



74

6.4 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". For example a concentric BTS is configured. The inner area is the GSM1800 frequency and the outer area is the GSM 900 frequency. This BTS can be supplied with a single BCCH and offers carrier frequencies in both GSM900 and GSM1800 bands.



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BTS-5GSM1800

BSC

BTS-4GSM 1800

BTS-3GSM 900

BTS-2GSM 900

BTS-1GSM 900GSM1800

BTS-0GSM 900

Fig. 38 Network with mixed cell configuration GSM900/ GSM1800



76



77

7 Data transmission in GSM phase 2+



78

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.



79

Data transmissionin 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-SpeedCircuit SwitchedData

14.4 kbit/s

GeneralPacketRadio Service 171.2 kbit/s

21.4 kbit/s

345.6 kbit/s

473.6 kbit/s

EnhancedDataRates forGSMEvolution

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 using up 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 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. 41 High-speed circuit switched data - main features



82

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.



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Asub A

BSC

TRAU0

TRAU1

TRAU2

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 HSCSD2 TS on air IF

64 TS used for HSCSD

2*14,4 HSCSD

4*14,4 HSCSD4 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 elements making 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 and handles 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 a gateway MSC. For example performs interworking between a GSM PLMN and a packet data network PDN, contains the routing information or can inquire about location 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 GPRSSupport Node

PSTN

InternetIntranet

X.25

GGSNGateway GPRSSupport Node

VMSC /VLR

GMSC

HLR

ISDNBSS

GR

GPRS subscription data(GPRS Register GR)

GPRS Support NodesSGSN:

Interface to Base Station SystemGGSN:

Interface to Packet Data Network

SSS

PCUGb GiGn

Fig. 43 Packet Switches in the Core Network



86

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 CCU performs:

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

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

GPRS Mobile Station 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.



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PCU• Channel Access Control functions• Radio Channel Management (Power Control, CongestionControl,...)• 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



88

7.2.3 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 packet switched traffic a main difference to circuit-switched services is that several mobile 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 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. 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. The advantage is that the MS get the maximum possible capacity. In case of vertical allocation strategy several MS are multiplexed on the same timeslots. This leads to the advantage that the scare resources of the air interface are saved. Distribution of the physical channels for various logical packet data channels is based on blocks of 4 normal bursts each, called radio blocks. This means that signaling and the packet data traffic of several mobile stations can be statistically multiplexed into one packet data channel. UL and DL for GPRS packet data are assigned separately. Therefore the packet data channel can be seized asymmetrically.



89

Several MS multilplexedone 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

VerticalAllocation

Fig. 45 Multiplexing and TS combining

L1-tasks

Transmission of user & signaling data

Timing Advance

Cell SelectionMeasuresignal strength

Power Control

Allocation of physical channel(Packet Data Channel PDCH)

dynamically: 1 or 4 Radio Blocks

(1 Radio Block = 4 Normal Burstin 4 consecutive TDMA-frames)

Fig. 46 Radio Interface



90

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 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 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 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.



91

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 - 8channel

Fig. 47 Coding Schemes

collectuser data

signaling

RLC Data Block BCSMAC Header

RLC/MAC Control Block BCSMAC Header

Convolutionalcoding

(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 Bit 8 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



92

7.3 Enhanced data rates for GSM evolution EDGE 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. In SBS up to this release the EGPRS is implemented. 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.



93

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 sequenceT tail bitsGP 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/sPayload/burst: 114 bitGross 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/sPayload/burst: 346 bitGross rate: 69.2 kbit/s

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



94

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 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

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

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GMSK 8-PSK

Family A

Family B

Family C

MCS-2

MCS-3

MCS-4

MCS-5

MCS-6M

CS-1

MCS-7

MCS-8

MCS-9

CS-1

CS-2

CS-3

CS-4

GPRS

~ 22,8 kbit/s

~ 69 kbit/sD

ata

Rat

e (p

er ti

me

slot

)

Fig. 51 EGPRS Coding Schemes



96

7.3.2 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. 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).



97

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

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



98

7.3.3 Architecture The EGPRS architecture includes an EDGE capable GSM mobile station (MS), which is 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 the Packet 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 (E- CUs) featuring the new 8-PSK modulation technique and/or "normal" carrier units (G- CUs) supporting GMSK modulation. The number of E-CU and G-CU can be chosen depending on the capacity demands for Edge. E-CU are only supported for BTSE types belonging to the BTSplus generation. 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 switched traffic handling providing the Packet Control Unit functionality of the Type "PPXX" modules. The FAAS (Flexible Abis allocation Strategy) feature guarantees the higher capacity requirement on Abis. This is realized by the BR7.0 software.

Edge Mobile Station Two classes of mobile stations are provided. One class of mobile station is able to apply the 8PSK modulation in both the uplink and the downlink directions, which means that they support advanced facilities and capabilities. The other class applies 8PSK modulation in the downlink direction and GMSK modulation in the uplink direction.



99

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Siemens

Mobile DTE

BSC

Visited MSC/VLR

Gateway MSC

Serving GSN

packet switched: GPRS

Gateway GSN

BTSE

HLR/GR

PPXX modulesin BSC

BTSE with E-CU

IS D N

P S T N

InternetIntranet

P S P D NFAAS with BR7.0

PCU

Fig. 53 Changes in Existing Network Infrastructure



100



101

8 Location services

∆ r r '

r

Fig. 54 Location Services



102

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



103

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104

8.1 Architecture The new network entities are:

Serving Mobile Location Center (SMLC)

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

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

• calculates the final location estimate and accuracy and it in a location response to a 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 when receiving an LCS request to identify the requesting LCS client and to authorize it to use 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 the requested QoS (retry, reject)

• generates LCS related charging and billing rates



105

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BTSE BSC MSC / VLR

ExternalLCS Client

SMLC

HLRGMLC

Abis A

Lg

Lg Le

Lh

Lb

Um

DifferentPLMN

GMLC

Fig. 55 LCS network architecture



106

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 cell planning.

SMLC positioning functions The Common PRCF (Positioning Radio Coordination Function) determines the positioning method to be used (CITA, E-CITA, A-GPS …) and manages the resources required by the chosen method. The E-CITA PCF calculates position received from the E-CITA PRCF (including collection of the required prediction data from the SMLC database) and returns back the calculated location estimate. As examples two important methods (E-CITA and A-GPS) are described in more detail 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 serving cell. Combining to this information the Timing Advance value of the MS, the CITA method is realized. The E-CITA is a positioning method that uses the CITA enhanced by a comparison of predicted receive levels and measured receive levels.

In GSM every MS that is in dedicated mode measures and reports continuously the actual radio conditions to the network. These reports contain information about reception 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. In case of a positioning request, the SMLC will compute the location by matching the measured data with the prediction data. The output of the SMLC is shape, for example a point on the surface of the earth with an uncertainty circle. Beside the position, the algorithm is able to estimate the positioning error described by an uncertainty circle or ellipse for a given confidence level.



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BTS 1

MS

Cell ID

BTS 2

BTS 3

SignalStrength 1

SignalStrength 2

SignalStrength 3

BSC

Fig. 57 MS measurements

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

16

PositionX, Y, r

BTS measurements

MS measurements

Comparison

CGI, TA, RXLEV

Measurementreports

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 GPS method in mobile networks the large time to first fix, low sensitivity (no coverage indoors or in urban canyons), and excessive battery consumption are the disadvantages. To overcome these drawbacks, network assisted GPS (A-GPS) is used. The basic idea behind network assisted GPS (A-GPS) is that the GPS receiving system is distributed:

• MS with GPS receiver

• Reference GPS network The GPS reference network consists of GPS receiving stations , tracking all GPS satellites at all times. This data is used to model the satellite orbits and clocks well into 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 satellites signals for the MS. The MS measures only the times of arrival (called “pseudoranges”) which enables a lower signal level. Therefore the A-GPS method requires a handset with integrated GPS receiver. The Siemens mobile A-GPS functionality is integrated into the SMLC. The SMLC is determining the position with use of the GPS data. In the SMLC the A-GPS positioning method can be provided along with other lower accuracy positioning methods like CITA and E-CITA.



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GPS ReferenceNetwork

BSC

Ass. Data & position

calculation

SMLC

Measurements +

Assist data

Fig. 59 MS measurements and assistance data

SBS

A-GPS MS

Sattelite Signals

SMLC

GPS Reference data

GPS data

GPSPosition Calculation

Assistance data

Pseudo rangesPosition

X, Y, rCITA, 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 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? 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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