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GPRSSYS

GPRS Air InterfaceTraining Document6-64444Issue 5.0 Nokia Networks Oy 1 (50)

GPRS Air InterfaceThe information in this document is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This document is intended for theuse of Nokia's customers only for the purposes of the agreement under which the document issubmitted, and no part of it may be reproduced or transmitted in any form or means withoutthe prior written permission of Nokia. The document has been prepared to be used byprofessional and properly trained personnel, and the customer assumes full responsibilitywhen using it. Nokia welcomes customer comments as part of the process of continuousdevelopment and improvement of the documentation.The information or statements given in this document concerning the suitability, capacity, orperformance of the mentioned hardware or software products cannot be considered bindingbut shall be defined in the agreement made between Nokia and the customer. However,Nokia has made all reasonable efforts to ensure that the instructions contained in thedocument are adequate and free of material errors and omissions. Nokia will, if necessary,explain issues which may not be covered by the document.

Nokia's liability for any errors in the document is limited to the documentary correction oferrors. NOKIA WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THISDOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDINGMONETARY LOSSES), that might arise from the use of this document or the information in it.This document and the product it describes are considered protected by copyright accordingto the applicable laws.NOKIA logo is a registered trademark of Nokia Oyj.Other product names mentioned in this document may be trademarks of their respectivecompanies, and they are mentioned for identification purposes only.Copyright Nokia Oyj 2004. All rights reserved.2 (50) Nokia Networks Oy 6-64444Issue 5.0

Contents

Contents1 Module objectives................................................................................52 Introduction 62.1 The GPRS radio interface: key functions.............................................63 Air interface layering............................................................................93.1 Operation 103.2 Physical RF layer (optional topic)......................................................113.3 Physical Link layer (optional topic).....................................................113.4 Medium Access Control layer (optional topic)....................................123.5 Radio Link Control layer (optional topic)............................................134 Additional GPRS channels in GSM...................................................144.1 Channel organisation in GSM/GPRS.................................................144.1.1 Physical channel and TDMA-Frame...............................................144.1.2 Bursts 15

4.2 GSMlogical channels and their mapping in physical channels.......164.2.1 GSMlogical channels..................................................................164.2.2 Multiframes in GSM........................................................................184.2.3 Radio block204.3 GSMadditional logical channels and their mapping in physicalchannels......................................................................204.3.1 GPRSadditional logical channels................................................204.3.2 Additional Multiframes with GPRS..................................................225 GPRS multislot capabilities...............................................................246 Channel coding (optional topic)........................................................256.1 GPRS 256.1.1 CS-1 27

6.1.2 CS-2 276.1.3 CS-3 28

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6.1.4 CS-4 286.1.5 CS selection and identification........................................................296.1.6 Multislot handsets...........................................................................296.2 Air interface performance..................................................................307 Radio resource management............................................................337.1 Available resources for GPRS...........................................................337.2 GPRS resource for subscribers: Uplink resource allocation..............348 Data transfer 378.1 Mobile originated packet transfer.......................................................388.2 Mobile terminated packet transfer......................................................396-64444Issue 5.0 Nokia Networks Oy 3 (50)

GPRS Air Interface

9 Modulation 429.1 GMSK 429.2 EDGE 429.2.1 EDGE coding schemes...................................................................449.2.2 Incremental Redundancy and Link adaptation................................4510 Key points 47

11 Review questions.............................................................................484 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

1 Module objectivesAt the end of the module the participant will be able to: Explain the functions of the air interface in the Physical, MAC and RLClayers Differentiate between physical and logical GPRS channels List and describe the GPRS air interface logical channels and theirfunctions Explain the GPRS TDMA frame, multiframe and superframe structure

List and compare four different coding schemes and the puncturingconcept Describe multiple timeslot usage Describe briefly the process of channel allocation, in the uplink anddownlinkwithout using any references.6-64444Issue 5.0 Nokia Networks Oy 5 (50)

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2 IntroductionAll communication between the mobile station (MS) and the GPRS/GSMnetwork takes place over the air interface. It is the most important interface inthe current mobile network as it is the cause of the bottleneck in current GPRSnetwork performance.MS Um GSM/GPRS NetworkUplink DirectionDownlink Direct ionFigure 1. The air interface

2.1 The GPRS radio interface: key functionsThe GPRS air interface consists of asymmetric and independent uplink anddownlink channels. It is asymmetric because the radio resources allocated to anMS in the uplink and downlink may be different. The downlink carries datafrom the network to multiple MSs and does not require contention arbitration.The uplink resources are shared among multiple MSs and require contention

resolution for orderly use of the radio resources. Entities communicating overthe air interface perform a number of functions as summarised below:

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Modulation is the process of converting binary signals into atransmittable signal using a carrier frequency. The physical RF layerperforms this function. The GMSK modulation scheme is used inGSM/GPRS and the 8-PSK scheme is used in EGPRS. Timing advance is needed because as the distance between thetransmitter and receiver vary, timing advance is used to estimate the timeat which the mobile stations in a cell should transmit signals so that theyarrive at the base station (BTS) in time synchronisation and without anycollisions. Synchronisation deals with synchronisation between the transmitter andthe receiver so that the receiver can know the rate and time at which tosample incoming bit stream. This is a physical layer function.6 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

Power control is the process of controlling the transmitted power by anMS so as to maintain a good radio link but at the same minimise theinterference with neighbouring cells reusing the same frequency. Channel coding is needed because information transmitted over the air

interface is corrupted by noise, interference and fading. Thus binary 1sand 0s are converted into a format which maximises the data throughputthrough the air interface. In the GPRS standard, four coding schemes CS1-4 are defined. In the EGPRS standard, 8 coding schemes MCS 1-8 aredefined. Puncturing is the intentional removal of a number of bits at predefinedpositions in a radio block so as to reduce the number of bits to size 456bits. Puncturing is used in CS-2, 3, and 4. Interleaving is a technique used to protect information transmitted overthe air interface. It uses the idea of "not carrying all your eggs in onebasket". By distributing information to be transmitted over a number ofcontainers, the chances of getting data through the air interface are better.This is interleaving. This function is performed in the MS and the BTS. Framing involves packing of information into time bursts, frames,hyperframes, radio blocks, etc. Different framing structure is used forGSM and GPRS since one physical channel can be shared by a number ofGPRS users. Medium access control (MAC) is used when a number of mobilestations are trying to access a medium in an orderly manner. Segmentation involves breaking up of variable size large data blocksinto fixed size smaller blocks for efficient transmission over the airinterface. Segmented data has to be reassembled at the other end of theair interface. RLC and SNDCP layer perform segmentation and

reassembly. Congestion control procedures are needed for detection and recoveryfrom congestion on the air interface. This function is implemented in theLLC layer. Ciphering is the process of converting transmitted information into aciphered data that can only be read by authorised persons. All user datatransmitted on the air interface is ciphered for security purposes. TheLLC layer in the MS and SGSN performs this function. Multiplexing is the process of combining a number of signals togetherfor transmission over a channel. Time division multiplexing is used in theGSM/GPRS air interface. Multiplexing of data from a number of sourcesis also performed at the SNDCP layer.

Signal measurements:A mobile station is continuously monitoring thesignal strength received from the BTS and other cells. These

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measurements are used for several purposes.6-64444Issue 5.0 Nokia Networks Oy 7 (50)

GPRS Air Interface

Handover is the process of changing from one BTS to another. All of thehandover signalling takes place over the air interface.8 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

3 Air interface layeringThe GPRS air interface can be modelled as a hierarchy of layers as shown inFigure 2. Layering is an important concept in the development ofcommunication protocols. Each layer performs a specific function and providesservices to the layers above it and uses the services provided by the layersbelow it.MS BSS (PCU, CCU)SNDCPRLCRadio LinkControl

MACMedium Access

Control

GSM RFphy. link & RF

IP / X.25LLC

UmRLCRadio LinkControl

MACMedium AccessControl

GSM RFphy. link & RF

LLC segmentation/ re-assemblyacknowledged/ unacknowledgedmodeBackward Error Correction BEC

Access signalling proceduresphysical channel bundlingsub-multiplexingphysical channel organisationchannel codingGSMK

Figure 2. GPRS air interface layered model

RLC/MAC layer and the Physical layer are important layers. The Physical layeris the lowest layer of the hierarchy and is divided into two distinct sub-layers,namely Physical RF layer and the Physical Link layer. Its primary role is toenable communication over the air interface. These layers are described inSections 3.2 and 3.3.The RLC/MAC layer refers to the RLC and MAC layers of the protocol

architecture. It provides services for communication over the GPRS radiointerface. The MAC layer also controls access to the shared medium andcontention resolution between a number of mobile stations and the network.The RLC/MAC layer uses the services of the Physical Link layer. The MAClayer functions may allow a single MS to use several physical channelssimultaneously. The RLC function defines the procedures for selective6-64444Issue 5.0 Nokia Networks Oy 9 (50)

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retransmission of unsuccessfully delivered RLC data blocks. These layers aredescribed in 3.4 and 3.5.The LLC layer above RLC/MAC layer uses the services of the RLC/MAC.Now we shall have a look at the functions of each of the layers.

3.1 OperationThe access to the GPRS uplink uses a Slotted-Aloha based reservation protocol.

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Each layer of the GPRS protocol architecture performs three functions: receives data (protocol data unit or PDU) from the layer above it performs some processing on it sends it to the layer below it.This operation carried on until the lowest layer, Physical RF layer, where theinformation is transmitted through the air interface. At the receiver, each layerextracts the relevant data and sends it to the higher layer.SNDCP PDU (SN-PDU)LLC-PDURLC BlockMAC BlockNetwork PDU (NPDU) e.g. IP-packetSNDCPLLCRLCMACPhys. Link

Phys. RFNetworkLLC-PDURLC Block

Burst Burst Burst Burstchannel coding

Figure 3. GPRS protocol data units

The network protocol data units (N-PDUs) are sent to SNDCP layers. Here theyare segmented into one or more subnetwork protocol data units (SN-PDUs).The SN-PDUs are then sent to the LLC layer where they are encapsulated into10 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

one or several LLC frames/PDUs. At the RLC layer, the LLC PDUs aresegmented into one or more RLC data blocks to which a RLC and MAC headermay be added. At the RLC/MAC layer, a selective ARQ (Automatic RepeatRequest) protocol (including block numbering) between the MS and thenetwork provides retransmission of erroneous RLC data blocks. When acomplete LLC frame is successfully transferred across the RLC layer, it isforwarded to the LLC layer above it. The radio blocks are normally carried byfour normal bursts in GPRS and EGPRS. Though, there are some exceptions tothis rule. The format of the radio block will be discussed later.

3.2 Physical RF layer (optional topic)The Physical RF layer is the lowest layer of the GPRS protocol stack across theGPRS air interface. It performs two functions: Modulation of the physical waveforms based on the sequence of bitsreceived from the Physical Link layer. Demodulation of received waveforms into a sequence of bits. These bitsare transferred to the Physical Link layer for interpretation.Modulation techniques used in GPRS and EGPRS are described in Section 9,and demodulation techniques are also covered in GSM 5 Series Specifications(5.01 - 5.04).

3.3 Physical Link layer (optional topic)The Physical Link layer operates above the Physical RF layer. It provides all ofthe services needed for information transfer over the physical channel betweenthe MS and the network. These functions include:

Data unit framing: Placement of data into bursts, frames, radio blocks,superframes, etc.

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Channel coding: Conversion of binary 1s and 0s into a format thatmaximises the data throughput. Detection and correction of errors due to noise in the physical medium. Procedures for detecting congestion on the air interface. Procedures for synchronising MS and network including determining andadjusting of timing advance for MS to correct for variances inpropagation delay. Procedures for monitoring and evaluation of radio link signal quality. Procedures for cell (re-) selection.6-64444Issue 5.0 Nokia Networks Oy 11 (50)GPRS Air Interface

Procedures for transmitter power control and battery power conservationprocedures, for example, Discontinuous Reception (DRX) procedures.Detailed information about this functionality can be found in the subsequentsections and in GSM 5 Series Specifications (5.01 - 5.04).

3.4 Medium Access Control layer (optional topic)The Medium Access Control (MAC) layer operates above the Physical Link

layer. Its functions are the following: Uplink and downlink multiplexing of data and control signalling Handling contention resolution, collision detection, and recovery formobile originated channel access Scheduling of access attempts, including queuing of packet accesses formobile terminated channel access Handling priority of data and control messages.Different radio block structures are used for GPRS and EGPRS data transferand control message. A GPRS radio block for data transfer consists of oneMAC header, one RLC header, and one RLC data block. It is always carried infour normal bursts (discussed later).The descriptions of the radio block structure fields are given below:

MAC header contains an 8-bit control field which is different for uplinkand downlink directions. RLC header contains a variable length control field which is differentfor uplink and downlink directions. RLC data field contains one or more LLC PDUs. Block check sequence (BCS) is used for error detection and correction.The Physical Link layer appends BCS.For EGPRS, a radio block for data transfer consists of a combined RLC/MACheader, a header check sequence, one or two RLC data blocks, and BCS.Figure 4. EGPRS radio block structure for data transfer12 (50) Nokia Networks Oy 6-64444Issue 5.0

RLC/MAC

HeaderHeaderCheckSequenceRLC Data BCSKey points

For GPRS and EGPRS control messages, a radio block for control messagetransfer consists of a MAC header, RLC/MAC control message, and the BCS.Figure 5. RLC and MAC Radio Blocks

3.5 Radio Link Control layer (optional topic)The GPRS RLC function is responsible for the following actions:

Transfer of Logical Link Control layer PDUs (LLC-PDU) to the MAClayer

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Segmentation and re-assembly of LLC-PDUs into RLC data blocks Backward Error Correction (BEC) procedures for selective retransmissionof incorrect code words in the acknowledged mode oftransmission During a transmission, the coding schemes can be adjusted to the radiochannel conditions.6-64444Issue 5.0 Nokia Networks Oy 13 (50)

RLC DataBCSSNDCPRLCRadio LinkControl

MACMedium AccessControl

LLCPhy. RFPhy. Link

user dataLLC PDU& segmentation

C Data RLC DBCS = Block Check SequenceRLC Data BCSMAC RLC Data BCSHeader

radio link signalling &control dataRLC/MAC ControlMessagesMACHeaderRLCHeaderRLC

HeaderGPRS Air Interface

4 Additional GPRS channels in GSM4.1 Channel organisation in GSM/GPRSIn GSM, 25 MHz spectrum has been frequency divided into 124 bands, eachhaving a bandwidth of 200 kHz. On each of the 200 kHz bands a carrier can betransmitted at the centre frequency of the band. So the carriers are frequencydivision multiplexed.Figure 6. FDD and FDMA organisation in GSM

Each carrier is further time divided into timeslots (TSL) and each timeslot isreferred to as a physical channel as information can be transmitted in it. It is

possible to share a physical channel amongst many processes or users. Theseare referred to as logical channels.

4.1.1 Physical channel and TDMA-FrameIn GSM, the physical channel is a timeslot offering a data rate of 22.8 kbits/sec.The GPRS physical channel is called a packet data channel (PDCH). EachPDCH is a shared medium between multiple MSs and the network. In GPRS,different packet logical channels can be transported in the same physicalchannel (PDCH) in the same way as in the traditional GSM air interface.14 (50) Nokia Networks Oy 6-64444Issue 5.0

UPLINK DOWNLINKGSM900: 890 MHz - 915 MHz 935 MHz - 960 MHz

GSM1800: 1710 MHz - 1785 MHz 1805 MHz - 1880 MHz1 2 3 ...

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Channel 1 - 1241 - 374200 kHz

1 2 3 ...Duplex frequency 45 MHz / 95 MHz

guard bandKey points

A TDMA frame is defined as a grouping of eight bursts or TSs which are

numbered 0 to 7 as shown below. It has duration of 4.615ms (8 x 577 s).TDMA frame= 8 timeslots012345760

1234576012345

200 kHzPhyscial channel,e.g. allocatedto onesubscriber with FR voice &no frequency hopping

frequencytimeTDMA frame

Figure 7. Physical channel and TDMA frame

TDMA frames are transmitted one after another. Every TDMA frame isallocated a frame number. Frame numbers are broadcasted by BTS on thesynchronising channel (SCH) and this is used for frame level synchronisationbetween the MS and BSS. The numbering repeats every hyperframe, which hasduration of 3 hours, 28 minutes, 53 seconds, and 760 milliseconds. Framenumbers are also used for ciphering thus making it difficult for hackers todecipher messages being transmitted.

In GSM, 51 (26-frame) multiframes or 26 (51-frame) multiframes go to makeup a superframe of duration 6.12 seconds. 2048 superframes go to make up a

hyperframe of duration 3 hours 28 minutes 53 seconds 760 ms (577 s * 8 *52* 25 * 2048). There are 2 662 400 frames in a hyperframe. This representsthe maximum value of the frame number, since the TDMA frame number (FN)is repeated once per hyperframe.

4.1.2 BurstsChannels and frames represent the organisation of the radio interface resources.A burst is an electro-magnetical impuls, which is used to transmit user6-64444Issue 5.0 Nokia Networks Oy 15 (50)

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information. Within GSM, the transmission of bursts must be synchronised to

the channel and frame organisation.Each burst has duration of 577 milliseconds and contains 148 bits of

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information. Between successive burst, there is a guard interval of 30microseconds that is equivalent to 8.25 bits.There are five types of bursts: Normal burst carries traffic and controls channels in the uplink anddownlink direction. It contents include: 3 tail bits, 57 encrypted bits,1 flag bit, 26-bit training sequence, 1 flag bit, 57 encrypted bits, 3 tailbits, and a guard period of 8.25-bit length. Frequency correction burst is broadcasted in the BCH and is called theFCCH. It serves as the BTS beacon. The contents of this burst are 3 tailbits, 142 fixed bits (all coded as 0), 3 tail bits, and a guard period. Synchronising burst is used on the synchronising channel SCH totransmit information that is used to time-synchronise the MS with theGSM network. This burst contains a long training sequence as well as theTDMA Frame Number (FN) and the Base Station Identity Code (BCIC).It contains 3 tail bits, 39 encrypted bits, 64 bit synchronising sequence,39 encrypted bits, 3 tail bits, and a guard period of 8.25-bit length. Access burst is used for Random Access by an MS. It has a longer guard

period as the mobile could be far away from the BTS and not know thetiming advance required to work the base station. Its contents include3 tail bits, 41-bit synchronising sequence, 36 encrypted bits, 3 tail bits,and a guard period of 68.25-bit length. Dummy burst carries no information and uses a fixed bit pattern whichconsist of 3 tail bits, 58 mixed bits, 26 bit training sequence, 58 mixedbits, 3 tail bits and a guard period of 8.25-bit length.

4.2 GSMlogical channels and their mapping in physicalchannels4.2.1 GSMlogical channelsLogical channels imply partial use of physical channels by many sources. Thus

each physical channel can contain a number of logical channels. Each logicalchannel performs a well-specified task. In GSM, a number of logical channelsare defined: Traffic channels (TCH) that are used to carry GSM data and speech inboth directions. There are two types of TCH, namely TCH/F and TCH/H.16 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

Control channels (CCH) perform all of the control functions and aresubdivided into BCH, CCCH, and DCCH. Frequency correction channel (FCCH) is a downlink, broadcast,signalling channel that is used for carrying information that allowsMS to tune in to the BTS.

Synchronising channel (SCH) is a downlink, broadcast,signalling channel that is used for carrying the identity of a BTS(BSIC) and frame-synchronisation (RFN) between MS and BTS. Broadcast common channel (BCCH) is a downlink, broadcast,signalling channel that is used to covey cell specific information toMS in a cell. Common control channels (CCCH) are bi-directional, point-tomultipoint,signalling channels that are used to establish dedicatedchannel. There are three types of CCCHs: Paging channels (PCH) are downlink, broadcast channels, whichare used to page for subscribers for mobile terminated calls. Random access channel (RACH) is an uplink channel that is used

by MS to request a dedicated control channel. Access grant channels (AGCH) are downlink channels used to

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assign an MS to a specific DCCH in response to a RACH.BCCHFCCH Frequency correctionSignallingand ControlTraffic

CCCHDCCHSCH Frame synchronisation + BSICPCH Paging mobilesRACH Requesting dedicated channelAGCH Allocating dedicated/ traffic CHBroadcast of cell information,e.g. channel combination

SDCCH Signalling between MS and BTSe.g. Authentication, SMS, LUP

SACCH Measurements, TA, PC, ...FACCH Extra signalling within26 TDMA Multiframe

TCH/ F full rate traffic channelTCH/H half rate traffic channelBCHDLUPDLDLDL & UPDL & UPLogicalchannelsareusedtotransmit a

well definedcontentFigure 8. Logical Channels in GSM6-64444Issue 5.0 Nokia Networks Oy 17 (50)

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Dedicated Control Channels (DCCH) are channels, which areexclusively allocated to a MS to exchange signalling and controlinformation with the PLMN network.- Standalone dedicated control channels (SDCCH) are used forexchanging signalling information (MS authentication, locationupdates, and TCH assignment) between MS and BTS before a TCHis allocated.- Slow associated control channels (SACCH) are used mainly for the

transmission of radio link control information between the MS andthe BTS. For instance, measurement reports are sent uplink andpower control and timing advance commands downlink.- Fast associated control channels (FACCH) are used on trafficchannel resources (26 multiframes). They are used for instanceduring the handover process.

4.2.2 Multiframes in GSMThe conventional GSM multiframes are either 26 TDMA frames s (duration 120 ms) used for TCH, or 51 TDMA frame (duration 235.38) used for signalling.Multiframes describe, how logical channel information is multiplexed/

organised via a physical channel.The logical channel information must betransmitted on a physical channels

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Multiframesspecify, at which positionwithin a physical channel a specificlogical channel information istransmittedTDMA Frame26 TDMAFramee.g.used for

GSMspeechTS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7TCHTCHSACCHidle

Figure 9. 26 TDMA Multiframe18 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

Figure above demonstrates how for full rate speech is transmitted via the radiointerface. In this example, TS 6 was allocated to the mobile subscriber. Thistimeslot is the physical channel resource for the mobile subscriber. Speechtransmission is organised over 26 TDMA frames. Of course, the mobilesubscriber in our example is only using TS 6 in each TDMA frame. The first 12TDMA frames within a 26 TDMA multiframe are used for speech transmission.As a consequence, TCH/F can be found here. TDMA frame 13 (or TS 13) isthan used for radio link management. TA and PC commands are transmitteddownlink, and uplink, we can find measurement reports here. This informationis transmitted via the SACCH. The next 12 TDMA frames are used for speechagain, and then there is an idle frame, where the mobile phone as time to makemeasurements in the neighbourhood. Then, the next 26 TDMA multiframebegins.In the figure below, you can see an example of a 51 TDMA multiframeRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR

F = FCCHS = SCHB = BCCHC = PCH + AGCHR = RACH

I = IDLE FRAMEAll on timeslot zeroof successive TDMAframes.01020304050FFFSBCSCCSCCFSCC

SCCFI

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Downlink 51-TDMA-Frame Uplinktime1 Radio Block= 4 Frames= 456 info. bits

Figure 10. 51 TDMA Multiframe example: CCCH multiframe6-64444Issue 5.0 Nokia Networks Oy 19 (50)

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4.2.3 Radio blockGSM uses radio blocks for signalling (see also figure above). Hereby a specificcontent is transmitted via four consecutive TDMA frames in the same timeslotposition. All logical channels, which were specified additionally with GPRS,use the radio block structure.Figure 11. Radio block: four bursts in consecutive frames

4.3 GSMadditional logical channels and their mapping inphysical channels4.3.1 GPRSadditional logical channelsGPRS introduces several new logical channels to the GSM air interface. There

are no dedicated signalling channels as in GSM. The PDCH are used for dataand signalling. Packet broadcast control channel (PBCCH) is a downlink-onlychannel for broadcasting packet data (GPRS) specific system informationmessages to all GPRS-enabled mobile stations in a cell. If the PBCCH isnot allocated, the packet-data-specific system information is broadcast onthe BCCH. Packet common control channel (PCCCH) consists of logical channelsused for common control signalling for packet data. There are four typesof PCCCH:20 (50) Nokia Networks Oy 6-64444Issue 5.0

RadioBlock

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7Frame 0Frame 1Frame 2Frame 3Key points

Packet random access channel (PRACH) is an uplink-onlychannel, which the MSs use for uplink traffic channel request andfor obtaining the timing advance. The normal GSM RACH canalso be used for this, in case there is no PCCCH allocated in thecell. Packet paging channel (PPCH) is a downlink-only pagingchannel used to page the MS prior to downlink packet transfer. ThePPCH can be used for paging of both CS and PS data services. Thenormal GSM PCH can be used for GPRS in case there is noPCCCH allocated in the cell. Packet access grant channel (PAGCH) is a downlink-onlychannel used for resource assignment during the packet transferestablishment phase. The normal GSM AGCH can be used in casethere is no PCCCH allocated in the cell. Packet notification channel (PNCH) (only in GPRS Phase 2) is adownlink-only channel used for the PTM-M notifications to a

group of MSs before PTM-M packet transfer. Packet data traffic channel (PDTCH) is reserved for GPRS packet data

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transfer. A PDTCH corresponds to the resource allocated to a single MSon one physical channel for user data transmission. In multislot operation,one MS may use multiple PDTCHs in parallel for individual packettransfer. PDTCH are uni-directional as opposed to TCH in GSM. Packet associated control channel (PACCH) (bi-directional) is asignalling channel dedicated for a certain MS. The signalling informationcould include acknowledgements, power control, resource assignments,or reassignment messages Packet timing advance control channel (PTCCH) is used in uplinkdirection for the transmission of random access bursts to estimate thetiming advance for one mobile. In the downlink direction one PTCCH isused to transmit timing advance information to several MSs. PTCCHinformation is transmitted in positions 12 and 38 of the 52-multiframestructure.6-64444Issue 5.0 Nokia Networks Oy 21 (50)GPRS Air Interface

PTCCH/DPTCCH/U

PBCCHSignallingand ControlPacketTraffic ChannelPCCCHPPCHPRACH MS initiates uplink transferPAGCH Resource assignment to an MSPNCH Notifying PtM Packet TransferBroadcast of packet dataspecific information

PDTCH Packet Data Transfer; (multislot)

PACCHDLUPDLDLDLDL & UPPTCHSignalling: resource allocation,acknowledgements, PC, TA, etc.Paging MSs for packet dataand circuit switched servicesUsed by MS to send randomburst to BSS for timing advanceUsed to send timing advanceInformation to MSs of one PDCH

UP & DLULDLPDCCHFigure 12. Additional logical channels with GPRS

4.3.2 Additional Multiframes with GPRSGPRS means the introduction of a new TDMA multiframe: the 52 TDMAmultiframe. Each multiframe consists of 416 (52 x8) bursts. Even if anoperator only offers GPRS services (and no circuit switched services, twomultiframe types are required:

51 TDMA multiframe (duration 235.38) used for signalling, and 52 TDMA multiframe used for user traffic andoptionallyfor

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signallingAll GPRS operators offer of course circuit switched services, so that in thiscase, 26, 51, and 52 TDMA multiframes can be found in one cell. They caneven co-exist on the same TRX.If one TSL is allocated for GPRS, then one multiframe of 52 frames willcontain: 12 radio blocks that can used packet data channels (PDCH) 2 idle frames that are used for interference measurements 2 frames for PTCCH that are used for timing advance control.22 (50) Nokia Networks Oy 6-64444Issue 5.0Key points

B0 B1 B2 T B3 B4 B5 i B6 B7 B8 T B9 B10 B11 iRadio Block

= 4 TS in consecutiveTDMA framesidle frame= 1 frame52 TDMA Frame = PDCH MultiframeUplink on one PDCH:

Multiplexing of PDTCH & PACCH, or PDTCH, PACCH & PRACHDownlink on one PDCH:Multiplexing of PDTCH, PACCH PDTCH, PACCH & PCCCH, incl.PBCCH (indicated by BCCH) PDTCH, PACCH and PCCCH(indicated by (P)BCCH)PTCCH

Figure 13. The multiframe structure of the packet data channel (PDCH)

A number of MSs can share a single timeslot in uplink and downlink directionby assigning different radio blocks of one PDCH to different MSs. Since the

GPRS radio interface consists of asymmetric and independent uplink anddownlink channels, we need to be some mechanism for multiplexing andresource sharing. This is covered in radio channel allocation.The MAC function defines the procedures that enable multiple MSs to share acommon transmission medium, which may consist of several physical channels.The MAC function provides arbitration between multiple MSs attempting totransmit simultaneously, and provides collision avoidance, detection andrecovery procedures. The downlink carries packets from the network to multipleMSs and does not require contention arbitration. The uplink is shared amongmultiple MSs and requires contention control procedures.6-64444Issue 5.0 Nokia Networks Oy 23 (50)

GPRS Air Interface

5 GPRS multislot capabilitiesOne requirement, which had to be met with GPRS, was to over increased datarates to the subscribers. Two solutions for increased data rates were introducedin GPRS:

New coding schemes: coding schemes are algorithms to add redundancy to

the user information. By adding redundancy, the reliability of thetransmission via the radio interface can be increase. But the moreredundancy is added, the less user data can be transmitted during a certainperiod of time. Coding schemes are discussed in the next section.

Channel bundling: Not only one physical channel, but up to 8 physical

channels can be allocated to one MS. This can be done asymetrically, i.e. if asubscriber wants to download a huge file, several downlink physical

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channels can be allocated to him, and only one physical channel for uplinkdata transmission. The number of physical channels is limited to 8, becauseall physical channels allocated to one subscriber must be located on the sameTRX. The vast majority of GPRS-MS supports only channel bundling of upto 3 physical channels in one direction. Why?Since each MS has only one transponder, the start of the TDMA frame onthe uplink is delayed by three timeslot periods from the corresponding startof the TDMA frame in the downlink as shown below. This allows the mobileto receive, process, and transmit using the same timeslot number. The timebetween the transmission and reception is also used for performingmeasurements on the signal quality from neighbouring cells for handoverpurposes. If the MS has two timeslots, then the uplink and downlink will beseparated by a smaller gap. The maximum number of timeslots that a singletransponder MS can use is thus limited to three.DOCUMENTTYPETypeUnitOrDepartmentHereTypeYourNameHere TypeDateHere

1-slot2-slot3-slot0 1 2 3 4 5 6 7 0 15 6 7 0 1 2 3 4 5 60 1 2 3 4 5 6 7 0 15 6 7 0 1 2 3 4 5 60 1 2 3 4 5 6 7 0 15 6 7 0 1 2 3 4 5 6DownlinkUplink

MonitorDownlinkUplinkMonitorDownlinkUplinkMonitor

Figure 14. Gap between uplink and downlink transmissions24 (50) Nokia Networks Oy 6-64444Issue 5.0

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6 Channel coding (optional topic)6.1 GPRSCoding scheme CS-1 is used in GSM. In the GPRS standards, there are fourpossible air-interface-coding schemes namely CS1, CS2, CS3, and CS4. Codingscheme CS1 has the highest error correction and the lowest data throughput,while CS4 has no error correction but the highest data throughput. Thus CS-2 toCS-4 offer higher throughput rates at the cost of less protection againsttransmission errors.

CS-1 CS-2 CS-3 CS-4Increasing data throughput ratesIncreasing protection against errorsFigure 15. Comparison of coding schemes

ETSI standards require that all coding schemes (CS-1 to CS-4) are mandatoryfor mobile stations supporting GPRS. However, for a network supportingGPRS, only CS1 is mandatory. In Nokia GPRS Release 1, the coding schemesCS1 and CS2 are supported. The network selects the coding scheme to be used.

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RLC Data Block+ MAC headerConvolutional CodeIn: 228 bitsOut: 456 bitsCyclic Coding +Tail

16 + 4 bitsFire Code + TailIn: 184 bitsOut: 228 bitsReordering,Partioning,

Adding StealingFlagesInterleavingCS-1CS-2, 3, 4 Convolutional Code

And PuncturingIn: x bitsOut: 456 bitsCS-4Information bits Interleaved bits

Figure 16. Coding processing6-64444Issue 5.0 Nokia Networks Oy 25 (50)

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The processing used in GPRS channel coding and interleaving is depicted inFigure 16. The RLC data blocks are coded with a systematic block code forerror-detection purposes. CS-1 is coded with a fire code and CS-2, 3, and 4 arecoded with a cyclic redundancy coding scheme (CRC). Both these schemes addparity bits to the RLC data block. Tail bits are also added. For error-correctionpurposes, the resulting data blocks are encoded with a 1/2-rate convolutioncode, except in CS-4, and punctured if necessary to fit into 456-bit radio blocksstructure. The block structures of the coding schemes are shown in CS-1 toCS-3 are shown in Figure 17. The final radio block size is 456 bits for CS-1 to

CS-4. The composition of the radio block is tabulated.Rate 1/2 Convo lut ion Cod ing Stage

Punctu ring Stage456 bitsUSF BCS40/16 bitsMAC HeaderTail(4 bits)Precoded USF3/6/12 bits

USF(3 bits)MAC(5 bits)

RLC Data/Control Block(176/288/307 bits)

Cycl ic or Fire CodingFigure 17. Radio block structure for CS1 to CS3Table 1. Coding parameters for the GPRS coding schemesScheme CoderateUSF PrecodedUSFRadio Blockexcl. USFand BCSBCS Tail Coded bits Punctured

bitsData rate

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kb/sCS-1 1/2 3 3 181 40 4 456=2*(3+181+40+4) 0 9.05

CS-2 2/3 3 6 268 16 4 588=2*(6+268+16+4) 132 13.4

CS-3 3/4 3 6 312 16 4 676=2*(6+312+16+4) 220 15.6CS-4 1 3 12 428 16 - 456=428+12+16 0 21.426 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points6.1.1 CS-1CS-1 scheme in GPRS is identical to the CS-1 scheme used in GSM, which isused for signalling on the SDCCH, SACCH, and FACCH channels. In GPRS,CS 1 is used for Packet Random Access Channel (PRACH) and Packet TimingAdvance Control Channel on Uplink (PTCCH/U).In CS-1 you start with a MAC data or control block of 181 bits, which containsa 176-bit RLC block and a 5-bit MAC header. The USF has eight states, whichare represented by a binary 3-bit field. The 3-bit USF header is added to theMAC block and the 184-bit block is sent to the fire coder, which adds the blockcheck sequence of 40 bits and a 4-bit tail field of 0000. The resulting 228-bitfield is then input to the 1/2-rate convolution coder that produces an output code

of 456 encoded bits. This 456-bit block is then transmitted in a radio block infour consecutive bursts of 114 bits each as discussed earlier. Puncturing is notused in CS-1. The effective throughput in CS-1 is calculated as follows for a456-bit radio block: Number of data bits in one radio block = 181 bits Duration of radio block = 20 ms Effective throughput rates = 181/20 ms = 9.05 kbits/sec Number of overhead bits = 456-181= 275 bits Percentage of overhead = 275/456= 60%The first step of the coding procedure is to add a Block Check Sequence (BCS)for error detection. For CS-1 to CS-3, the second step consists of pre-codingUSF (except for CS-1), adding four tail bits and a half-rate convolution coding

for error correction that is punctured to give the desired coding rate.6.1.2 CS-2In CS-2 you start with a MAC data block of 268 bits, which contains a 263-bitRLC data block and a 5-bit MAC header. The 3-bit USF header is pre-coded forextra protection and extended to 6 bits in CS-2. To the pre-coded 6-bit USF and268 data block, a 16-CRC-bit field, and a 4-bit tail block is appended to give atotal of 294 bits. The 16-bit CRC for BCS is calculated over the whole uncodedMAC data block. This 294-bit block containing pre-coded USF, MAC datablock, CRC, and tail is then input to the 1/2-rate convolutional coder thatproduces an output code of 588 encoded bits. It is not possible to fit thisencoded block into a 456-bit radio block so puncturing is used to reduce the

size of the encoded block. Thus 132 bits are deleted from pre-defined positionsfrom the output bit sequence. Any coding scheme that uses puncturing is morevulnerable to errors in data transmission. This coding rate is referred to asapproximately 2/3 because the input to the encoder was 294 bits and the outputafter puncturing was 456 bits. The effective throughput in CS-2 is calculated asfollows for a 456-bit radio block:6-64444Issue 5.0 Nokia Networks Oy 27 (50)

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Number of data bits in one radio block = 268 bits Duration of radio block = 20 ms Effective throughput rates = 268/20 ms = 13.4 kbits/sec Number of overhead bits = 456-268= 188 bits Percentage of overhead = 188/456= 41%

6.1.3 CS-3The MAC data block in CS-3 is 312 bits, which contains a 307-bit RLC data

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block and a 5-bit MAC header. As in CS-2, the 3-bit USF header is pre-codedand extended to 6 bits, to which a 312 MAC block, a 16 parity bit field and a4-bit tail block is appended to give a total of 338 bits. This block is then input tothe 1/2-rate convolutional coder that produces an output code of 676 encodedbits. Puncturing is used to reduce the size of the encoded block to 456 bits bydeleting 220 bits from pre-defined positions. This coding rate is referred to asapproximately 3/4 or 338/456. The effective throughput in CS-3 is calculated asfollows for a 456 bit radio block: Number of data bits in one radio block = 312 bits Duration of radio block = 20 ms Effective throughput rates = 312/20 ms = 15.6 kbits/sec Number of overhead bits = 456-312= 144 bits Percentage of overhead = 144/456= 32%

6.1.4 CS-4The input to CS-4 is a 428-MAC data block that consists of 5-bit MAC headerand 423-bit RLC data block. A 12-bit pre-coded USF field and a 16-bit CRCfield is added to give a 456-bit block. The 16-bit CRC field is computed from

the MAC data block. No convolutional coding or puncturing is applied in CS-4as shown below, which implies that there is no forward error correction. Mostamount of protection against transmission errors is given to the USF field onlyfor backward error correction purposes.28 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

blockcode no coding456 bitsUSF BCSRadio Block

Figure 18. Radio block structure for CS-4

The effective throughput in CS-4 is calculated as follows for a 456-bit radio

block: Number of data bits in one radio block = 428 bits Duration of radio block = 20 ms Effective throughput rates = 312/20 ms = 21.4 kbits/sec Number of overhead bits = 456-428= 28 bits Percentage of overhead = 28/456= 6.1%

6.1.5 CS selection and identificationThe dynamic selection of the coding scheme to be used is dependent on thereception quality, error rate, and the equipment being used. The CS can alsochange during a transaction. Furthermore, all active MSs in a GPRS cell have todecode the downlink information being transmitted. Thus a method to identifythe CS being currently used in a radio block is needed. The stealing flags, which

occur within four bursts of one radio block, are used to identify the CS beingused.

6.1.6 Multislot handsetsIn GSM, the MS typically uses one channel (timeslot) for uplink and one fordownlink. In GPRS it is possible to have a multislot MS, for example a 3-slotMS with the same or different (asymmetric) uplink and downlink capability.GPRS allows up to eight air interface timeslots to be combined together to givehigher rate connections. In the 8-TSL MS, a single GPRS user has exclusive useof all eight timeslots.6-64444Issue 5.0 Nokia Networks Oy 29 (50)

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Table 2. Comparison of coding schemes CS-1 to CS-4

Channel CodingSchemeCS-1 CS-2 CS-3 CS-4

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Single TSL DataRate9.05 kbit/s 13.4 kbit/s 15.6 kbit/s 21.4 kbit/s3-TSL Data Rate 27.15 kbit/s 40.2 kbit/s 46.8 kbit/s 64.2 kbit/s8-TSL Data Rate 72.0 kbit/s 107.2 kbit/s 124.8 kbit/s 171.2kbit/s

6.2 Air interface performanceThe figure below shows some results of Nokia simulations to determine the airinterface throughput rate for each coding scheme in different carrier-tointerferenceratios (C/I). The simulations were made for two cases: With onetimeslot and three timeslots allocated for GPRS. Remember that several userscould share the throughput.Depending on the value of C/I ratio, the CS that produces the best throughputcan be found. For C/I of around 15 dB, CS-2 would give a little above 10 kbit/s,CS-1 and CS-3 around 8 kbit/s, and CS-4 1 kbit/s per timeslot. For currentnetworks CS-1 and CS-2 are the viable options.024

68101214160 5 10 15 20 25C/IKbit/sCS-1CS-2CS-3CS-40

10203040500 5 10 15 20 25C/IKbit/s

CS-1CS-2CS-3CS-4Minimum AverageTypical NW C/IMinimum Average

Typical NW C/I1 Timeslot 3 Timeslots

Figure 19. Simulated network throughput of user data for GPRS codingschemes (non-frequency hopping, polling interval = 18 blocks)30 (50) Nokia Networks Oy 6-64444Issue 5.0Key points

At low values of C/I ratio, CS-1 performs best. At around 15 dB, CS-2performance is better than CS-1. Above C/I ratio of 18 dB, CS-3 is better thanCS-1. A similar analysis can be performed for three-timeslots case.6-64444Issue 5.0 Nokia Networks Oy 31 (50)

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Table 3. Throughput for C/I values of 12, 15, and 20dBC/I ratio CS-1 CS-2 CS-3 CS-4 Best

performance12dB 8 kb/s 6 kb/s 4 kb/s 0.5 kb/s CS-115dB 8 kb/s 10kb/sec 8 kb/sec 1 kb/s CS-2

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20dB 8kb/sec 12 kb/sec 14 kb/sec 5 kb/sec CS-332 (50) Nokia Networks Oy 6-64444Issue 5.0

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7 Radio resource management7.1 Available resources for GPRSGSM timeslots are used for used for circuit switched (CS) traffic and assignedby the GSM network, whereas timeslots for packet switched (PS) traffic areassigned by the PCU. One question that arises is how many timeslots are to bereserved for each type of service? Circuit switched traffic has priority overpacket switched traffic. But when there are idle GSM timeslots, one would liketo transmit as much PS traffic on it.GPRS timeslots are classified into dedicated, default and additional timeslots: Dedicate timeslots are exclusively reserved for GPRS traffic and no CStraffic can be transmitted on them. If congestion occurs for circuitswitched traffic, then only dedicated GPRS traffic channels can carry PStraffic. Default timeslots are by default for GPRS traffic channels that can bedynamically configured to handle CS load if needed. The defaulttimeslots are always switched to the PCU when allowed by the CS trafficload. Additional timeslots by default carry CS traffic but can be dynamicallyconfigured into a GPRS timeslot when required. During peak GPRStraffic periods, additional channels are switched to GPRS use, but only ifthe CS traffic load permits that to occur.All full rate or dual rate traffic channels are capable of carrying GPRS trafficchannels. The operator can set the following: GPRS capacity cell by cell and TRX by TRXAmount of dedicated timeslots

Amount of default timeslotsAmount of additional timeslots BCCH TRX or non-BCCH TRX is preferred for GPRS.Figure 20 shows how the boundary between CS and PS territory can movedynamically.6-64444Issue 5.0 Nokia Networks Oy 33 (50)

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Figure 20. Radio Resource Management

7.2 GPRS resource for subscribers: Uplink resourceallocationWhen a number of mobile stations (MSs) are trying to access a shared medium,there is a need for orderly access to that medium so that no two stations transmit

at the same time resulting in a collision of data packets. This function is referredto as medium access control (MAC). The MAC layer of the GPRS protocolperforms this function. GSM/GPRS uses Slotted Aloha in that each MS stationcan only start transmitting at the beginning of a TSL interval.Medium access control is relevant to the uplink direction only. It is not relevantto data transmitted in the downlink direction from a single BTS to all the MSsin a cell. This is because there is no contention when there is only one source inthe downlink direction. A further complication is added because in GPRS onePDCH is shared amongst many MSs. So there is a need for the BSS (PCU) toindicate the radio blocks that are reserved for each active GPRS user for uplinktransmission.There are three uplink resource allocation methods defined in the GPRSstandards: Fixed allocation of uplink radio resources

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Dynamic allocation of uplink radio resources Extended dynamic allocation of uplink.In fixed resource allocation, the MS is simply given a list of timeslots and a listof allocated radio blocks per timeslot in which the MS station can transmit. TheMS also needs to know when it can transmit on the allocated resources. This isusually stated in terms of the absolute frame number. This resource allocationmethod is mandatory for the network and the MS.For dynamic resource allocation in the uplink direction, a field called UplinkState Flag (USF) is used. The USF is a 3-bit field that is transmitted in theMAC header of every block in the downlink direction. Each MS that wants totransmit data requests a USF value in the PRACH message and is allocated a3-bit USF value (xyz) in PAGCH message for a particular PDCH. Each active34 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

MS then checks the USF value of each radio block that it is transmitted by theBTS. The following rule is applied:"If the USF value xyz occurs in the MAC header of downlink block j, itidentifies that the MS (xyz) may transmit on the corresponding uplink block

j+1".What if there are more than one PDCH available for GPRS data transmission?In the initial assignment message on the PAGCH (AGCH), the MS gets a list ofPDCHs and one corresponding USF value for each PDCH. The MS monitorsthe USF values in downlink transmission on the assigned PDCHs. The MS maytransmit in uplink direction in the radio blocks that currently have the same USFvalue that was given to it earlier in the assignment message.An example of the USF usage is shown in Figure 21. In the example, the useron the right has been given USF value 1 (binary 001). The user on the left hasbeen given USF value 2 (binary 010) in this particular PDCH. The USFs are setso that the user on the right may use radio blocks B0 to B4, and the user on theleft may use radio blocks B5 to B9. There is a parameter called USFGranularity, which, if set to 1 in a downlink radio block j, allows MS totransmit in the j+1 uplink block and the next 4 uplink block.USF=1USF=3 USF=2USF=1:B0- B4USF=2:B5- B9B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11B9 B8 B7 B6 B5B3 B2 B1 B0B4

Figure 21. Usage of Uplink State Flag (USF)

The USF has only eight values (3 bits) so, in theory, only eight (23) users cansimultaneously share one PDCH physical channel in uplink direction. Thebinary pattern 111 can be reserved for indicating PRACH blocks, that is, for6-64444Issue 5.0 Nokia Networks Oy 35 (50)

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mobiles to send resource allocation requests in the uplink direction. If the binarypattern 111 is reserved for PRACH, then only up to seven MS can share aPDCH.Downlink multiplexing of radio blocks destined for different MSs is enabledwith another identifier called Temporary Flow Identifier (TFI), which isincluded in each radio block.36 (50) Nokia Networks Oy 6-64444Issue 5.0

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8 Data transferThe Problem

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The user data packets of many subscribers are transmitted on the same TRX.But how can the receiver decide, to whom a radio block or RLC Data Blockbelongs to? Two problems can be observed:

Several subscribers can use (more or less)

simultaneously the same physical channel.

The user data of one subscriber can be transmitted on

several physical channels of the same carrier.Therefore, when radio resources are dedicated to the subscriber, the data flowmust be uniquely identified. Unlike circuit switched data transfer, packet datatransfer is unidirectional, asymmetric, and independent. Consequently, a uniqueidentification is required both for uplink and downlink traffic.The Temporary Flow Identity (TFI) is a 5-bit field allocated by the PCU thatis part of each data block transmitted across the air interface. The TFI uniquelyidentifies a data transfer session1 in the uplink or downlink direction. Each TFIis unique for the allocated PDCHs. But the same TFI may be used in the uplinkand downlink direction since these directions are independent of each other.

There are two modes of packet data transfer over the air interface: Acknowledged mode for RLC/MAC operation uses selective ARQmechanism to acknowledge correctly received RLC data blocks. Thesedata blocks are numbered with unique sequence numbers called a blocksequence number (BSN). The sender transmits data blocks using asliding window scheme. The receiver sends ACK or NACK to identifythe last correctly received RLC data block up to an indicated BSN. Everytime an ACK or NACK is received, the size of the sending slidingwindow is modified and the erroneous blocks are retransmitted. There isan acknowledgement procedure in the LLC layer. Unacknowledged mode for RLC/MAC operation does not use ACKand NACK or retransmission of erroneous data blocks. It uses forwarderror-

correction technique to recover the original data blocks.The important logical channels that are used for data transfer are the following: Packet Random Access Channel (PRACH) is used by the MS in theuplink to initiate uplink transfer for sending data or signallinginformation. Packet Paging Channel (PPCH) is used to page an MS prior todownlink packet transfer.1A data transfer session is not a PDP session! It refers only to a set of RLC blocks to betransmitted. A TFI can change quite often during an active PDP context (= end usersession).6-64444Issue 5.0 Nokia Networks Oy 37 (50)

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Packet Access Grant Channel (PAGCH) is used in the packet transferestablishment phase to send resources assignment to an MS. Packet Data Traffic Channel (PDTCH) is a channel allocated for datatransfer either in the uplink (PDTCH/U) or downlink (PDTCH/D)direction.

8.1 Mobile originated packet transferMobile originated packet transfer can begin after a mobility management (MM)and one or several PDP contexts have been established. Let us assume that thesubscriber is running a bursty application and has already sent some data via theair interface. Now the user wants to continue the data transfer. To do so, thesubscriber temporarily needs some resources. So the MS sends a request to thePCU for radio resources and the PCU responds with a radio resourceassignment message. Thereafter data transmission begins and positiveacknowledgement (ACK) and negative acknowledgements (NACK) are sent by

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the peer entity. The sequence of events that take place are shown in Figure 22and described below:MS NetworkPacket Channel RequestPacket Immediate AssignmentPacket Resource RequestPacket Resource Assignment

PRACH or RACHPAGCH or AGCHPACCHPACCH(Optional)(Optional)

Figure 22. Mobile originated packet transfer (access and allocation)

1. Packet Channel Request:The uplink packet transfer is initiated by a Packet Channel Request. Thiscan be done on the RACH or PRACH.2. Packet Immediate Assignment:On the network side, resources for data transfer have to be allocated tothe subscriber. The reservation considers the resources, demanded with

the Packet Channel Request. If the MS used a RACH, it could only be indicated that a GPRSservice is demanded and the network can assign uplink resources38 (50) Nokia Networks Oy 6-64444Issue 5.0

Key points

on one or two PDCHs. This might not be enough. Therefore, themobile originated packet transfer is split into two phases. TheAGCH is used for the Packet Immediate Assignment. Using a PRACH, the MS can deliver more adequate informationabout the requested resources. Consequently, one or more PDCHscan be allocated to the subscriber. The PAGCH is used for thePacket Immediate Assign. Power control (PC) and timing advance

(TA) information are included in this message.With the Packet Channel Request and the Packet Immediate Assignment, theone-phase access has been completed.The two-phase access then is optional. It is initiated by the MS, when it is notsatisfied with the uplink resources allocated to it.3. Packet Resource Request:This message is used to carry the complete description of the requestedresources for the uplink transfer.4. Packet Resource Assignment:This message is the networks response, indicating the resources reservedfor uplink transfer. Power control (PC) and timing advance (TA)information are included in this message.

Both Packet Resource Request and Packet Resource Assignment are realised ona PACCH.

8.2 Mobile terminated packet transferWhen a packet is received from an external network by the GGSN, it contains asource and destination IP address. The GGSN has to translate the destination IPaddress to a PDP context TID or establish a tunnel with SGSN serving MS. Thepacket is then tunnelled to the SGSN using the GTP protocol. The SGSNtranslates TID into a TLLI and NSAPI, which identifies a logical connectionbetween SGSN and MS. Thereafter the SGSN sends the packet using theSNDCP protocol to the MS.Error! Objects cannot be created from editing field codes.

Figure 23 Downlink multiplexing of dataMobile terminated packet transfer is only possible if MS is in the Ready state

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and is initiated by the network using the Packet Resource Assignment messageas shown in Figure 25. In case there is a PCCCH allocated in the cell, thePacket Resource Assignment is transmitted on the PAGCH. In case there is noPCCCH allocated in the cell, the Packet Resource Assignment is transmitted on6-64444Issue 5.0 Nokia Networks Oy 39 (50)GPRS Air Interface

the AGCH. The Packet Resource Assignment message includes the list ofPDCH(s) that will be used for downlink transfer as well as the PDCH carryingthe PACCH. The MS will have to monitor all the PDCH and identify itsdownlink data using the TFI, which is part of each downlink data block.Error! Objects cannot be created from editing field codes.Figure 24 Multislot use in downlink

The TFI is an identifier that is included in every radio link control (RLC) headerbelonging to a particular temporary block flow (TBF) and in the controlmessages associated to the LLC frame transfer in order to address the peer RLCentities. The more often a TFI allocated to specific user is included in thedownlink RLC header the higher the DL bit rate will be. Theoretically a usercan thus have all eight slots in a TDMA frame/multiframe structure. In practice

there are other limitations such as MS capability. The timing advance andpower control information is also included, if available. Otherwise, the MS maybe requested to respond with an access burst.MS NetworkPacket Resource AssignmentPACCH orPAGCH orAGCH

Figure 25. Mobile terminated packet transfer

The release of radio resources is initiated by the network by terminating thedownlink transfer and polling the MS for a final Packet Ack/Nack.40 (50) Nokia Networks Oy 6-64444Issue 5.0

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Data Blocktemporary Packet Ack/NackAccess and AssignmentMS NetworkPDTCHPACCHPacket Resource ReassignmentPacket Resource Reassignment AckPACCHPACCHPACCHfinal Packet Ack/NackData BlockPDTCHData BlockPDTCHData Block (polling)PDTCHData BlockPDTCHData BlockPDTCHData BlockPDTCHData BlockPDTCHData BlockPDTCHData Block (last, polling)PACCH

Figure 26. Downlink data transfer6-64444Issue 5.0 Nokia Networks Oy 41 (50)

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9 Modulation

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Modulation is the process of encoding binary data onto a carrier of frequencyFc. All modulation schemes modify the amplitude, frequency, or phase of thecarrier. The input to the modulation scheme is the digital data (or modulatingdata) that is to be transmitted and is usually measured in bits per second. Themodulator output is the modulated signal and is usually measured in symbolsper second.MMOODDUULLAATTOORRDigital databits/secModulated datasymbols/sec

Figure 27. Modulation

9.1 GMSKThe GMSK modulation scheme is used for GSM and GPRS as it providesminimum spectral requirements and constant output power. In this scheme, eachbit is represented by one symbol. The symbol rate is approximately 270.8ksymbols per second, which corresponds to 270.833 kbit/s.

9.2 EDGETo enhance data service GSM can use an additional techniquecalled Enhanced Data rates for GSM Evolution (EDGE). EDGE isa radio-based high-speed mobile data standard. EDGE improvesnetwork capacity and data rates, for both circuit switched andpacket switched data.EDGE uses 200 kHz radio channels, which are the same ascurrent GSM channel widths. From a technical perspective,42 (50) Nokia Networks Oy 6-64444Issue 5.0Key points

EDGE allows the GSM and GPRS network to offer a set of newradio access bearers to its core network. EDGE is designed toimprove spectral efficiency through link quality control. EDGErequires wider transmission channel widths and featuresflexible time slots to mix and match all forms ofcommunications, including voice, data, and video. AlthoughEDGE boosts the GSM and GPRS network, introducing EDGE tothe existing network has little technical impact, since it is fullybased on GSM and requires relatively small changes to networkhardware and software. Thus operators do not have to makeany changes to the network structure or invest in newregulatory licenses.The 8PSK modulation scheme is used for EGPRS. This is one of theimprovements EDGE brings since the throughput of this modulation scheme is

three times higher than of GMSK. In this scheme, the transmitted symbols areone of eight sinusoids, which have the same amplitude and frequency but differin phase. The digital data bits are combined into groups of three bits. Thus thereare eight possible combinations starting from (0,0,0) to (1,1,1). Each of the 3-bitpatterns is then matched to one of 8PSK symbols. The mapping is done in sucha way that there is a single bit difference between adjacent symbols. This isreferred to as Gray coding. It ensures that if a symbol is received in error as anadjacent symbol, only one of the bits will be in error.Digital bits Symbol Phase(1,1,1) 0 0

(0,1,1) 1 /4

(0,1,0) 2 /2

(0,0,0) 3 3 /4(0,0,1) 4

(1,0,1) 5 -3 /4

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(1,0,0) 6 - /2

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

(0,0,0) (0,1,0)(0,1,1)(1,1,1)(1,1,0)(1,0,0)IQFigure 28. PSK modulation scheme

The 8PSK symbols are continuously rotated with 3 /8 radians per symbolbefore pulse shaping. The symbol rate is approximately 270.833 ksymbols/sec,which corresponds to 812.5 kbit/sec.

9.2.1 EDGE coding schemesNine modulation and coding schemes, MCS-1 to MCS-9, are defined for theEGPRS packet data traffic channels, and these are tabulated below. For allEGPRS packet control channels the corresponding GPRS control channelcoding is used. ETSI standards state that MCS-1 to MCS-9 are mandatory forMSs supporting EGPRS. However, an EPGRS network may support only someof the MCSs.Table 4. Coding parameters for the EGPRS coding schemesScheme CoderateHeadercode rateMod RLCblocks per

RBBits in aradio blockFamily BCSTAILHCSData ratekb/s

MCS-9 1.0 0.368PSK2 2x592 A 2x12 2x6859.2MCS-8 0.92 0.36 2 2x544 A 54.4MCS-7 0.76 0.36 2 2x448 B 44.8MCS-6 0.49 1/3 1 592544+48

A12 629.627.2

MCS-5 0.37 1/3 1 448 B 22.4MCS-4 1.0 0.53 1 352 C 17.6

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Key points

GMSKMCS-3 0.80 0.53 1 296272+24

A 14.813.6MCS-2 0.66 0.53 1 224 B 11.2MCS-1 0.53 0.53 1 176 C 8.8NOTE: The italic captions indicate the padding

9.2.2 Incremental Redundancy and Link adaptationIncremental Redundancy (IR) is an efficient combination of two techniques,Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC). In theARQ method, when the receiver detects the presence of errors in a receivedRLC block, it requests and receives a re-transmission of the same RLC blockfrom the transmitter. The process continues until an uncorrupted copy reachesthe destination.The Forward Error Correction (FEC) method adds redundant information to the

user information at the transmitter, and the receiver uses the information tocorrect errors caused by disturbances in the radio channel. In the IR scheme(also known as Type II Hybrid ARQ scheme), all the redundancy is not sentright away. Rather, only a small amount is sent first, which yields a high userthroughput if the decoding is successful. However, if decoding fails, a retransmissiontakes place according to the ARQ method.Using IR, the transmitter transmits a different set of FEC information from thesame RLC block. These sets are called puncturing schemes, and there are two(P1 and P2) or three (P1, P2 and P3) of them in each of the nine MCSs ofEGPRS. Supporting IR, the receiver is able to combine the necessary amount oferror correcting information. Since the combination includes more informationthan any individual transmission, the probability of correct reception is

increased. IR co-operates with link adaptation, which selects the amount ofredundancy information transmitted in each transmission. The benefits of IR areincreased throughput due to better and automatic adaptation to different andvarying channel conditions and reduced sensitivity to link qualitymeasurements.EDGE not only increases efficiency and speed, but also improves dataprotection through link quality control. The system uses various measurementsof the past link to predict up coming channel quality. This prediction determinesthe relevant protection of the information to be sent. The Link Adaptation(LA) mechanism works to provide the highest throughput and lowest delayavailable by adapting the protection of the information to be sent, according tothe link quality. Enabling LA requires accurate link quality measurements and a

set of modulation and coding schemes (MCSs) with different degrees ofprotection.6-64444Issue 5.0 Nokia Networks Oy 45 (50)

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The IR and LA benefits can be combined. While IR improves throughputby automatically adapting the total amount of transmittedredundancy to the radio channel conditions, LA selects theamount of redundancy for each individual transmission. Thishelps reduce the number of re-transmissions, and thus keepsthe transfer delay reasonably low.46 (50) Nokia Networks Oy 6-64444Issue 5.0Key points

10 Key points The GPRS radio interface consists of asymmetric and independent uplink

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and downlink channels. In addition to sharing the downlink channels, theMSs can also share a single timeslot in the uplink direction. The MACfunction on the air interface defines procedures that control themultiplexing of several MSs on the same transmission medium. Entities communicating over the air interface have to perform a numberof functions: framing, channel coding, modulation, congestion control,segmentation, medium access control, synchronisation, multiplexing,timing advance, power control, handover, ciphering, interleaving, signalmeasurements, puncturing, etc. The GPRS protocol stack contains Physical RF, Physical Link, MAC andRLC, LLC and SNDCP layers. Each layer performs a well-definedfunction. It accepts data from a higher layer, performs processing on,adds a header, and passes it to the layer below it. GPRS introduces several new logical channels dedicated for GPRSsignalling and data transfer, mapped onto physical channels (PDCH). The mapping of logical channels is done over a multiframe comprising52 TDMA frames, divided into 12 radio blocks (each consisting of four

TDMA frames), two PTCCH frames, and two idle frames. A radio blockis a set of four consecutive bursts from/to a given mobile station,transmitted over four successive TDMA frames. Different coding schemes (CS1, CS2, CS3 and CS4) and multislot usageprovides data rates from 9 to 170 kbps. Depending on the CS and # ofTSL, different throughput rates can be obtained. At present CS-1 and 2are the viable options. Higher data rates can be obtained using EGPRS. Packet Resource Assignment and Reassignment messages play animportant role in controlling uplink and downlink data transfer. The USF flag is used for MAC in the uplink direction. It allows up toseven mobile stations to share a timeslot in the uplink direction. The TFI field is a 5-bit field that is used for multiplexing in the uplink

and downlink directions. Due to flexible radio resource management, GPRS channels can beseamlessly integrated with the existing GSM CS channels, allowing theoperator to configure the radio timeslots as per requirement and CStraffic load.6-64444Issue 5.0 Nokia Networks Oy 47 (50)

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11 Review questions1. Which fields are used for medium access control and multiplexingmultiple users on the uplink and downlink PDCH?2. How many users can share the same Packet Data Channel (PDCH)timeslot in the uplink direction?3. How many frames, radio blocks, and bursts are there in a PDCHmultiframe?4. What is the purpose of PTCCH?5. Which layer is responsible for segmentation and reassembly of LLCPDUs and Backward Error Correction (BEC) procedures?6. Which coding scheme has adopted the same coding as used for SDCCH?7. Which layer uses the functionality of USF?8. Which coding scheme does not use FEC?9. Which logical channels can be used for resource assignment?48 (50) Nokia Networks Oy 6-64444Issue 5.0

References

References Nokia DX200 SGSN Product Description Nokia GPRS Charging Gateway Product Description

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Nokia GN2500 GGSN Product Description Nokia GPRS Solution Description Nokia GPRS System Description GSM 01.04 (ETR 350): Digital cellular telecommunications system(Phase 2+); Abbreviations and acronyms GSM 02.60: Digital cellular telecommunications system (Phase 2+);General Packet Radio Service (GPRS); Stage 2 GSM 03.60: Digital cellular telecommunications system (Phase 2+);Stage 2 Service Description of the General Packet Radio Service (GPRS) GSM Specification 03.64 (Overall Description of the GPRS RadioInterface. R.99) GSM 04.04: Digital cellular telecommunications system; Layer 1;General requirements GSM 04.07: Digital cellular telecommunications system (Phase 2+);Mobile radio interface signalling layer 3 General aspects GSM 04.08: Digital cellular telecommunications system (Phase 2+);Mobile radio interface layer 3 specification

GSM 04.60: Digital cellular telecommunications system(Phase 2+);General Packet Radio Service (GPRS); Mobile Station (MS)BaseStation System (BSS) interface; Radio Link Control/Medium AccessControl (RLC/MAC) protocol GSM 04.64: Digital cellular telecommunications system(Phase 2+);General Packet Radio Service (GPRS); Logical Link Control (LLC) GSM 04.65: Digital cellular telecommunications system (Phase 2+);General Packet Radio Service (GPRS); Subnetwork DependentConvergence Protocol (SNDCP) GSM 05.01: Digital cellular telecommunications system (Phase 2+);Physical layer on the radio path, General description GSM 05.02: Digital cellular telecommunications system (Phase 2+);

Multiplexing and multiple access on the radio path GSM 05.03: Digital cellular telecommunications system (Phase 2+);Channel coding GSM 05.04: Digital cellular telecommunications system (Phase 2+);Modulation GSM 05.05: Digital cellular telecommunications system (Phase 2+);Radio transmission and reception6-64444Issue 5.0 Nokia Networks Oy 49 (50)GPRS Air Interface

GSM 05.08: Digital cellular telecommunications system (Phase 2+);Radio subsystem link control GSM 05.10: Digital cellular telecommunications system (Phase 2+);

Radio subsystem synchronisation GSM Specification 07.60 GSM Specification 07.70 GSM Specification 08.14 GSM Specification 08.16 GSM Specification 08.18 GSM Specification 09.02 GSM Specification 09.16 GSM Specification 09.18 GSM Specification 09.60 GSM Specification 12.1550 (50) Nokia Networks Oy 6-64444Issue 5.0