gprs air interface_1

SYSTEM TRAINING

GPRS Air Interface

Training Document

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© Nokia Networks Oy 1 (51)

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Copyright © Nokia Networks Oy 2007. All rights reserved.

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Contents

1 Module objectives....................................................................5

2 Introduction...............................................................................62.1 The GPRS radio interface: key functions...................................6

3 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)...................................13

4 Additional GPRS channels in GSM.......................................144.1 Channel organisation in GSM/GPRS.......................................144.1.1 Physical channel and TDMA-Frame.........................................144.1.2 Bursts.......................................................................................154.2 GSM – logical channels and their mapping in physical channels164.2.1 GSM – logical channels............................................................164.2.2 Multiframes in GSM..................................................................184.2.3 Radio block...............................................................................204.3 GSM – additional logical channels and their mapping in physical

channels...................................................................................204.3.1 GPRS – additional logical channels.........................................204.3.2 Additional Multiframes with GPRS............................................22

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5 GPRS multislot capabilities...................................................24

6 Channel coding (optional topic)............................................256.1 GPRS.......................................................................................256.1.1 CS-1.........................................................................................276.1.2 CS-2.........................................................................................276.1.3 CS-3.........................................................................................286.1.4 CS-4.........................................................................................286.1.5 CS selection and identification.................................................296.1.6 Multislot handsets.....................................................................296.2 Air interface performance.........................................................30

7 Radio resource management................................................337.1 Available resources for GPRS..................................................337.2 GPRS resource for subscribers: Uplink resource allocation... .34

8 Data transfer...........................................................................378.1 Mobile originated packet transfer.............................................388.2 Mobile terminated packet transfer............................................39

9 Modulation..............................................................................439.1 GMSK.......................................................................................439.2 EDGE.......................................................................................439.2.1 EDGE coding schemes............................................................459.2.2 Incremental Redundancy and Link adaptation.........................46

10 Key points...............................................................................48

11 Review questions...................................................................49

References................................................................................................51


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2 Module objectives

At the end of the module the participant will be able to:

Explain the functions of the air interface in the Physical, MAC and RLC layers

Differentiate between physical and logical GPRS channels

List and describe the GPRS air interface logical channels and their functions

Explain the GPRS TDMA frame, multiframe and superframe structure

List and compare four different coding schemes and the puncturing concept

Describe multiple timeslot usage

Describe briefly the process of channel allocation, in the uplink and downlink

without using any references.

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3 Introduction

All communication between the mobile station (MS) and the GPRS/GSM network takes place over the air interface. It is the most important interface in the current mobile network as it is the cause of the bottleneck in current GPRS network performance.

MS GSM/GPRS NetworkUm

Uplink Direction

Downlink Direct ion

Figure 1. The air interface

3.1 The GPRS radio interface: key functions

The GPRS air interface consists of asymmetric and independent uplink and downlink channels. It is asymmetric because the radio resources allocated to an MS in the uplink and downlink may be different. The downlink carries data from 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 over the air interface perform a number of functions as summarised below:

Modulation is the process of converting binary signals into a transmittable signal using a carrier frequency. The physical RF layer performs this function. The GMSK modulation scheme is used in GSM/GPRS and the 8-PSK scheme is used in EGPRS.

Timing advance is needed because as the distance between the transmitter and receiver vary, timing advance is used to estimate the time at which the mobile stations in a cell should transmit signals so that they arrive at the base station (BTS) in time synchronisation and without any collisions.

Synchronisation deals with synchronisation between the transmitter and the receiver so that the receiver can know the rate and time at which to sample incoming bit stream. This is a physical layer function.


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Power control is the process of controlling the transmitted power by an MS so as to maintain a good radio link but at the same minimise the interference 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 1s and 0s are converted into a format which maximises the data throughput through the air interface. In the GPRS standard, four coding schemes CS 1-4 are defined. In the EGPRS standard, 9 coding schemes MCS 1-9 are defined.

Puncturing is the intentional removal of a number of bits at predefined positions in a radio block so as to reduce the number of bits to size 456 bits. Puncturing is used in CS-2, 3.

Interleaving is a technique used to protect information transmitted over the air interface. It uses the idea of "not carrying all your eggs in one basket". By distributing information to be transmitted over a number of containers, 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 for GSM and GPRS since one physical channel can be shared by a number of GPRS users.

Medium access control (MAC) is used when a number of mobile stations are trying to access a medium in an orderly manner.

Segmentation involves breaking up of variable size large data blocks into fixed size smaller blocks for efficient transmission over the air interface. Segmented data has to be reassembled at the other end of the air interface. RLC and SNDCP layer perform segmentation and reassembly.

Congestion control procedures are needed for detection and recovery from congestion on the air interface. This function is implemented in the LLC layer.

Ciphering is the process of converting transmitted information into a ciphered data that can only be read by authorised persons. All user data transmitted on the air interface is ciphered for security purposes. The LLC layer in the MS and SGSN performs this function.

Multiplexing is the process of combining a number of signals together for transmission over a channel. Time division multiplexing is used in the GSM/GPRS air interface. Multiplexing of data from a number of sources is also performed at the SNDCP layer.

Signal measurements: A mobile station is continuously monitoring the signal strength received from the BTS and other cells. These measurements are used for several purposes.

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Handover is the process of changing from one BTS to another. All of the handover signalling takes place over the air interface.


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4 Air interface layering

The GPRS air interface can be modelled as a hierarchy of layers as shown in Figure 2. Layering is an important concept in the development of communication protocols. Each layer performs a specific function and provides services to the layers above it and uses the services provided by the layers below it.

BSS (PCU, CCU)MS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

GSM RFphy. link & RF

IP / X.25

LLC

Um

RLCRadio Link

Control

MACMedium Access

Control

GSM RFphy. link & RF

•LLC segmentation/ re-assembly•acknowledged/ unacknowledged mode

•Backward Error Correction BEC

•Access signalling procedures•physical channel bundling•sub-multiplexing

•physical channel organisation•channel coding•GSMK

Figure 2. GPRS air interface layered model

RLC/MAC layer and the Physical layer are important layers. The Physical layer is 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 to enable communication over the air interface. These layers are described in Sections 4.2 and 4.3.

The RLC/MAC layer refers to the RLC and MAC layers of the protocol architecture. It provides services for communication over the GPRS radio interface. The MAC layer also controls access to the shared medium and contention resolution between a number of mobile stations and the network. The RLC/MAC layer uses the services of the Physical Link layer. The MAC layer functions may allow a single MS to use several physical channels simultaneously. The RLC function defines the procedures for selective

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retransmission of unsuccessfully delivered RLC data blocks. These layers are described in 4.4 and 4.5.

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.

4.1 Operation

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

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 the information is transmitted through the air interface. At the receiver, each layer extracts the relevant data and sends it to the higher layer.

SNDCP PDU (SN-PDU)

LLC-PDU

RLC Block

MAC Block

Network PDU (NPDU) e.g. IP-packet

SNDCP

LLC

RLC

MAC

Phys. Link

Phys. RF

Network

LLC-PDU

RLC Block

Burst Burst Burst Burst

channel coding

Figure 3. GPRS protocol data units

The network protocol data units (N-PDUs) are sent to SNDCP layers. Here they are 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 into


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one or several LLC frames/PDUs. At the RLC layer, the LLC PDUs are segmented into one or more RLC data blocks to which a RLC and MAC header may be added. At the RLC/MAC layer, a selective ARQ (Automatic Repeat Request) protocol (including block numbering) between the MS and the network provides retransmission of erroneous RLC data blocks. When a complete LLC frame is successfully transferred across the RLC layer, it is forwarded to the LLC layer above it. The radio blocks are normally carried by four normal bursts in GPRS and EGPRS. Though, there are some exceptions to this rule. The format of the radio block will be discussed later.

4.2 Physical RF layer (optional topic)

The Physical RF layer is the lowest layer of the GPRS protocol stack across the GPRS air interface. It performs two functions:

Modulation of the physical waveforms based on the sequence of bits received from the Physical Link layer.

Demodulation of received waveforms into a sequence of bits. These bits are transferred to the Physical Link layer for interpretation.

Modulation techniques used in GPRS and EGPRS are described in Section 10, and demodulation techniques are also covered in GSM 5 Series Specifications (5.01 - 5.04…).

4.3 Physical Link layer (optional topic)

The Physical Link layer operates above the Physical RF layer. It provides all of the services needed for information transfer over the physical channel between the MS and the network. These functions include:

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

Channel coding: Conversion of binary 1s and 0s into a format that maximises 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 and adjusting of timing advance for MS to correct for variances in propagation delay.

Procedures for monitoring and evaluation of radio link signal quality.

Procedures for cell (re-) selection.

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Procedures for transmitter power control and battery power conservation procedures, for example, Discontinuous Reception (DRX) procedures.

Detailed information about this functionality can be found in the subsequent sections and in GSM 5 Series Specifications (5.01 - 5.04…).

4.4 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 for mobile originated channel access

Scheduling of access attempts, including queuing of packet accesses for mobile terminated channel access

Handling priority of data and control messages.

Different radio block structures are used for GPRS and EGPRS data transfer and control message. A GPRS radio block for data transfer consists of one MAC header, one RLC header, and one RLC data block. It is always carried in four 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 uplink and downlink directions.

RLC header contains a variable length control field which is different for 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/MAC header, a header check sequence, one or two RLC data blocks, and BCS.

Figure 4. EGPRS radio block structure for data transfer


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RLC/MACHeader

HeaderCheck

Sequence

RLC Data BCSRLC/MACHeader

HeaderCheck

Sequence

RLC Data BCS


For GPRS and EGPRS control messages, a radio block for control message transfer consists of a MAC header, RLC/MAC control message, and the BCS.

Figure 5. RLC and MAC Radio Blocks

4.5 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 MAC layer

Segmentation and re-assembly of LLC-PDUs into RLC data blocks

Backward Error Correction (BEC) procedures for selective re-transmission of incorrect code words in the acknowledged mode of transmission

During a transmission, the coding schemes can be adjusted to the radio channel conditions.

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RLC Data

BCS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

user data

LLC PDU

& segmentation

RLC D……C Data

BCS = Block Check Sequence

RLC Data BCS

RLC Data BCSMACHeader

radio link signalling &control data

RLC/MAC ControlMessages

MACHeader

RLCHeader

RLCHeader

RLC Data

BCS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

user data

LLC PDU

& segmentation

RLC D……C Data

BCS = Block Check Sequence

RLC Data BCS

RLC Data BCSMACHeader

radio link signalling &control data

RLC/MAC ControlMessages

MACHeader

RLCHeader

RLCHeader


5 Additional GPRS channels in GSM

5.1 Channel organisation in GSM/GPRS

In GSM, 25 MHz spectrum has been frequency divided into 124 bands, each having a bandwidth of 200 kHz. On each of the 200 kHz bands a carrier can be transmitted at the centre frequency of the band. So the carriers are frequency division multiplexed.

Figure 6. FDD and FDMA organisation in GSM

Each carrier is further time divided into timeslots (TSL) and each timeslot is referred 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. These are referred to as logical channels.

5.1.1 Physical channel and TDMA-Frame

In 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). Each PDCH is a shared medium between multiple MSs and the network. In GPRS, different packet logical channels can be transported in the same physical channel (PDCH) in the same way as in the traditional GSM air interface.


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UPLINK DOWNLINK

GSM900: 890 MHz - 915 MHz 935 MHz - 960 MHz


1 2 3 ...

Channel 1 - 1241 - 374

200 kHz

1 2 3 ...

Duplex frequency 45 MHz / 95 MHz

guard band

UPLINK DOWNLINK



1 2 3 ...

Channel 1 - 1241 - 374

200 kHz

1 2 3 ...

Duplex frequency 45 MHz / 95 MHz

guard band


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 577s).

TDMA frame= 8 timeslots

01

23

45

76

01

23

45

76

01

23

45

200 kHz

Physcial channel, e.g. allocatedto one

subscriber with FR voice &no frequency hopping

frequency

time

TDMA frame

Figure 7. Physical channel and TDMA frame

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

In GSM, 51 (26-frame) multiframes or 26 (51-frame) multiframes go to make up 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 (577s * 8 * 52* 25 * 2048). There are 2 662 400 frames in a hyperframe. This represents the maximum value of the frame number, since the TDMA frame number (FN) is repeated once per hyperframe.

5.1.2 Bursts

Channels and frames represent the organisation of the radio interface resources. A burst is an electro-magnetical “impuls, which is used to transmit user

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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 information. Between successive burst, there is a guard interval of 30 microseconds that is equivalent to 8.25 bits.

There are five types of bursts:

Normal burst carries traffic and controls channels in the uplink and downlink direction. It contents include: 3 tail bits, 57 encrypted bits,1 flag bit, 26-bit training sequence, 1 flag bit, 57 encrypted bits, 3 tail bits, and a guard period of 8.25-bit length.

Frequency correction burst is broadcasted in the BCH and is called the FCCH. It serves as the BTS beacon. The contents of this burst are 3 tail bits, 142 fixed bits (all coded as 0), 3 tail bits, and a guard period.

Synchronising burst is used on the synchronising channel SCH to transmit information that is used to time-synchronise the MS with the GSM network. This burst contains a long training sequence as well as the TDMA 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 the timing 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 which consist of 3 tail bits, 58 mixed bits, 26 bit training sequence, 58 mixed bits, 3 tail bits and a guard period of 8.25-bit length.

5.2 GSM – logical channels and their mapping in physical channels

5.2.1 GSM – logical channels

Logical channels imply partial use of physical channels by many sources. Thus each physical channel can contain a number of logical channels. Each logical channel performs a well-specified task. In GSM, a number of logical channels are defined:

Traffic channels (TCH) that are used to carry GSM data and speech in both directions. There are two types of TCH, namely TCH/F and TCH/H.


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Control channels (CCH) perform all of the control functions and are subdivided into BCH, CCCH, and DCCH.

Frequency correction channel (FCCH) is a downlink, broadcast, signalling channel that is used for carrying information that allows MS 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 to MS in a cell.

Common control channels (CCCH) are bi-directional, point-to-multipoint, signalling channels that are used to establish dedicated channel. There are three types of CCCHs:

Paging channels (PCH) are downlink, broadcast channels, which are 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 assign an MS to a specific DCCH in response to a RACH.

BCCH

FCCH Frequency correction

Signallingand Control

Traffic

CCCH

DCCH

SCH Frame synchronisation + BSIC

PCH Paging mobiles

RACH Requesting dedicated channel

AGCH Allocating dedicated/ traffic CH

Broadcast of cell information,e.g. channel combination

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

SACCH Measurements, TA, PC, ...

FACCH Extra signalling within 26 TDMA Multiframe

TCH/ F full rate traffic channel

TCH/ H half rate traffic channel

BCH

DL

UP

DL

DL

DL & UP

DL & UP

Logical channelsare usedto transmit a well defined content

Figure 8. Logical Channels in GSM

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Dedicated Control Channels (DCCH) are channels, which are exclusively allocated to a MS to exchange signalling and control information with the PLMN network.

- Standalone dedicated control channels (SDCCH) are used for exchanging signalling information (MS authentication, location updates, and TCH assignment) between MS and BTS before a TCH is allocated.

- Slow associated control channels (SACCH) are used mainly for the transmission of radio link control information between the MS and the BTS. For instance, measurement reports are sent uplink and power control and timing advance commands downlink.

- Fast associated control channels (FACCH) are used on traffic channel resources (26 multiframes). They are used for instance during the handover process.

5.2.2 Multiframes in GSM

The 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 be transmitted on a physical channels

Multiframesspecify, at which position within a physical channel a specific

logical channel information is transmitted

TDMA Frame

26 TDMA Frame

e.g. used forGSM speech

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

TCH

TCH

SACCH

idle

Figure 9. 26 TDMA Multiframe


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Figure above demonstrates how for full rate speech is transmitted via the radio interface. In this example, TS 6 was allocated to the mobile subscriber. This timeslot is the physical channel resource for the mobile subscriber. Speech transmission is organised over 26 TDMA frames. Of course, the mobile subscriber in our example is only using TS 6 in each TDMA frame. The first 12 TDMA 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) is than used for radio link management. TA and PC commands are transmitted downlink, and uplink, we can find measurement reports here. This information is transmitted via the SACCH. The next 12 TDMA frames are used for speech again, and then there is an idle frame, where the mobile phone as time to make measurements in the neighbourhood. Then, the next 26 TDMA multiframe begins.

In the figure below, you can see an example of a 51 TDMA multiframe

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

F = FCCHS = SCHB = BCCHC = PCH + AGCHR = RACHI = IDLE FRAME

All on timeslot zeroof successive TDMAframes.

0

10

20

30

40

50

F

F

F

S

B

C

S

C

C

S

C

CFS

C

C

S

C

C

F

I

Downlink Uplink51-TDMA-Frame

time

1 Radio Block= 4 Frames= 456 info. bits

Figure 10. 51 TDMA Multiframe example: CCCH multiframe

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5.2.3 Radio block

GSM uses radio blocks for signalling (see also figure above). Hereby a specific content is transmitted via four consecutive TDMA frames in the same timeslot position. All logical channels, which were specified additionally with GPRS, use the radio block structure.

Figure 11. Radio block: four bursts in consecutive frames

5.3 GSM – additional logical channels and their mapping in physical channels

5.3.1 GPRS – additional logical channels

GPRS introduces several new logical channels to the GSM air interface. There are no dedicated signalling channels as in GSM. The PDCH are used for data and signalling.

Packet broadcast control channel (PBCCH) is a downlink-only channel for broadcasting packet data (GPRS) specific system information messages to all GPRS-enabled mobile stations in a cell. If the PBCCH is not allocated, the packet-data-specific system information is broadcast on the BCCH.

Packet common control channel (PCCCH) consists of logical channels used for common control signalling for packet data. There are four types of PCCCH:


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Radio Block

TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6




Frame 0

Frame 1

Frame 2

Frame 3

Radio Block





Frame 0

Frame 1

Frame 2

Frame 3

TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6




Frame 0

Frame 1

Frame 2

Frame 3


Packet random access channel (PRACH) is an uplink-only channel, which the MSs use for uplink traffic channel request and for obtaining the timing advance. The normal GSM RACH can also be used for this, in case there is no PCCCH allocated in the cell.

Packet paging channel (PPCH) is a downlink-only paging channel used to page the MS prior to downlink packet transfer. The PPCH can be used for paging of both CS and PS data services. The normal GSM PCH can be used for GPRS in case there is no PCCCH allocated in the cell.

Packet access grant channel (PAGCH) is a downlink-only channel used for resource assignment during the packet transfer establishment phase. The normal GSM AGCH can be used in case there is no PCCCH allocated in the cell.

Packet notification channel (PNCH) (only in GPRS Phase 2) is a downlink-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 transfer. A PDTCH corresponds to the resource allocated to a single MS on one physical channel for user data transmission. In multislot operation, one MS may use multiple PDTCHs in parallel for individual packet transfer. PDTCH are uni-directional as opposed to TCH in GSM.

Packet associated control channel (PACCH) (bi-directional) is a signalling channel dedicated for a certain MS. The signalling information could include acknowledgements, power control, resource assignments, or reassignment messages

Packet timing advance control channel (PTCCH) is used in uplink direction for the transmission of random access bursts to estimate the timing advance for one mobile. In the downlink direction one PTCCH is used to transmit timing advance information to several MSs. PTCCH information is transmitted in positions 12 and 38 of the 52-multiframe structure.

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PTCCH/D

PTCCH/U

PBCCH

Signallingand Control

PacketTraffic Channel

PCCCH

PPCH

PRACH MS initiates uplink transfer

PAGCH Resource assignment to an MS

PNCH Notifying PtM Packet Transfer

Broadcast of packet dataspecific information

PDTCH Packet Data Transfer; (multislot)

PACCH

DL

UP

DL

DL

DL

DL & UPPTCH

Signalling: resource allocation,acknowledgements, PC, TA, etc.

Paging MSs for packet dataand circuit switched services

Used by MS to send random burst to BSS for timing advanceUsed to send timing advanceInformation to MSs of one PDCH

UP & DL

UL

DL

PDCCH

Figure 12. Additional logical channels with GPRS

5.3.2 Additional Multiframes with GPRS

GPRS means the introduction of a new TDMA multiframe: the 52 TDMA multiframe. Each multiframe consists of 416 (52 x8) bursts. Even if an operator only offers GPRS services (and no circuit switched services, two multiframe types are required:

51 TDMA multiframe (duration 235.38) used for signalling, and

52 TDMA multiframe used for user traffic and – optionally – for signalling

All GPRS operators offer of course circuit switched services, so that in this case, 26, 51, and 52 TDMA multiframes can be found in one cell. They can even co-exist on the same TRX.

If one TSL is allocated for GPRS, then one multiframe of 52 frames will contain:

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.


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

Radio Block= 4 TS in consecutive

TDMA frames

idle frame= 1 frame

52 TDMA Frame = PDCH Multiframe

Uplink on one PDCH:Multiplexing of• PDTCH & PACCH, or• PDTCH, PACCH & PRACH

Downlink 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 direction by assigning different radio blocks of one PDCH to different MSs. Since the GPRS radio interface consists of asymmetric and independent uplink and downlink channels, we need to be some mechanism for multiplexing and resource sharing. This is covered in radio channel allocation.

The MAC function defines the procedures that enable multiple MSs to share a common transmission medium, which may consist of several physical channels. The MAC function provides arbitration between multiple MSs attempting to transmit simultaneously, and provides collision avoidance, detection and recovery procedures. The downlink carries packets from the network to multiple MSs and does not require contention arbitration. The uplink is shared among multiple MSs and requires contention control procedures.

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6 GPRS multislot capabilities

One requirement, which had to be met with GPRS, was to over increased data rates to the subscribers. Two solutions for increased data rates were introduced in GPRS:

New coding schemes: coding schemes are algorithms to add redundancy to the user information. By adding redundancy, the reliability of the transmission via the radio interface can be increase. But the more redundancy is added, the less user data can be transmitted during a certain period 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 a subscriber wants to download a huge file, several downlink physical channels can be allocated to him, and only one physical channel for uplink data transmission. The number of physical channels is limited to 8, because all physical channels allocated to one subscriber must be located on the same TRX. The vast majority of GPRS-MS supports only channel bundling of up to 3 physical channels in one direction. Why?Since each MS has only one transponder, the start of the TDMA frame on the uplink is delayed by three timeslot periods from the corresponding start of the TDMA frame in the downlink as shown below. This allows the mobile to receive, process, and transmit using the same timeslot number. The time between the transmission and reception is also used for performing measurements on the signal quality from neighbouring cells for handover purposes. If the MS has two timeslots, then the uplink and downlink will be separated by a smaller gap. The maximum number of timeslots that a single transponder MS can use is thus limited to three.

Figure 14. Gap between uplink and downlink transmissions


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7 Channel coding (optional topic)

7.1 GPRS

Coding scheme CS-1 is used in GSM. In the GPRS standards, there are four possible air-interface-coding schemes namely CS1, CS2, CS3, and CS4. Coding scheme 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 to CS-4 offer higher throughput rates at the cost of less protection against transmission errors.

CS-1 CS-2 CS-3 CS-4

Increasing data throughput rates

Increasing protection against errors

Figure 15. Comparison of coding schemes

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

RLC Data Block+ MAC header

Convolutional CodeIn: 228 bits

Out: 456 bits

Cyclic Coding +Tail

16 + 4 bits

Fire Code + TailIn: 184 bits

Out: 228 bits

Reordering,Partioning,

Adding StealingFlages

Interleaving

CS-1

CS-2, 3, 4 Convolutional CodeAnd Puncturing

In: x bitsOut: 456 bits

CS-4

Information bits Interleaved bits

Figure 16. Coding processing

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The processing used in GPRS channel coding and interleaving is depicted in Figure 16. The RLC data blocks are coded with a systematic block code for error-detection purposes. CS-1 is coded with a fire code and CS-2, 3, and 4 are coded with a cyclic redundancy coding scheme (CRC). Both these schemes add parity bits to the RLC data block. Tail bits are also added. For error-correction purposes, the resulting data blocks are encoded with a 1/2-rate convolution code, except in CS-4, and punctured if necessary to fit into 456-bit radio blocks structure. 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 Convolution Coding Stage

Puncturing Stage

456 bits

USF BCS40/16 bits

MAC Header

Tail(4 bits)

Precoded USF3/6/12 bits

USF(3 bits)

MAC(5 bits)

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

Cyclic or Fire Coding

Figure 17. Radio block structure for CS1 to CS3

Table 1. Coding parameters for the GPRS coding schemes

Scheme Code rate

USF Pre-coded USF

Radio Block excl. USF and BCS

BCS Tail Coded bits Punctured bits

Data rate kb/s

CS-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 15.6

CS-4 1 3 12 428 16 - 456=428+12+16 0 21.4


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7.1.1 CS-1

CS-1 scheme in GPRS is identical to the CS-1 scheme used in GSM, which is used for signalling on the SDCCH, SACCH, and FACCH channels. In GPRS, CS 1 is used for Packet Random Access Channel (PRACH) and Packet Timing Advance Control Channel on Uplink (PTCCH/U).

In CS-1 you start with a MAC data or control block of 181 bits, which contains a 176-bit RLC block and a 5-bit MAC header. The USF has eight states, which are represented by a binary 3-bit field. The 3-bit USF header is added to the MAC block and the 184-bit block is sent to the fire coder, which adds the block check sequence of 40 bits and a 4-bit tail field of 0000. The resulting 228-bit field 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 in four consecutive bursts of 114 bits each as discussed earlier. Puncturing is not used in CS-1. The effective throughput in CS-1 is calculated as follows for a 456-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-coding USF (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.

7.1.2 CS-2

In CS-2 you start with a MAC data block of 268 bits, which contains a 263-bit RLC data block and a 5-bit MAC header. The 3-bit USF header is pre-coded for extra protection and extended to 6 bits in CS-2. To the pre-coded 6-bit USF and 268 data block, a 16-CRC-bit field, and a 4-bit tail block is appended to give a total of 294 bits. The 16-bit CRC for BCS is calculated over the whole uncoded MAC data block. This 294-bit block containing pre-coded USF, MAC data block, CRC, and tail is then input to the 1/2-rate convolutional coder that produces an output code of 588 encoded bits. It is not possible to fit this encoded 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 positions from the output bit sequence. Any coding scheme that uses puncturing is more vulnerable to errors in data transmission. This coding rate is referred to as approximately 2/3 because the input to the encoder was 294 bits and the output after puncturing was 456 bits. The effective throughput in CS-2 is calculated as follows for a 456-bit radio block:

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7.1.3 CS-3

The MAC data block in CS-3 is 312 bits, which contains a 307-bit RLC data block and a 5-bit MAC header. As in CS-2, the 3-bit USF header is pre-coded and 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 to the 1/2-rate convolutional coder that produces an output code of 676 encoded bits. Puncturing is used to reduce the size of the encoded block to 456 bits by deleting 220 bits from pre-defined positions. This coding rate is referred to as approximately 3/4 or 338/456. The effective throughput in CS-3 is calculated as follows for a 456 bit radio block:






7.1.4 CS-4

The input to CS-4 is a 428-MAC data block that consists of 5-bit MAC header and 423-bit RLC data block. A 12-bit pre-coded USF field and a 16-bit CRC field 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-4 as shown below, which implies that there is no forward error correction. Most amount of protection against transmission errors is given to the USF field only for backward error correction purposes.


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Figure 18. Radio block structure for CS-4

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





Percentage of overhead = 28/456= 6.1%

7.1.5 CS selection and identification

The dynamic selection of the coding scheme to be used is dependent on the reception quality, error rate, and the equipment being used. The CS can also change during a transaction. Furthermore, all active MSs in a GPRS cell have to decode the downlink information being transmitted. Thus a method to identify the 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 being used.

7.1.6 Multislot handsets

In GSM, the MS typically uses one channel (timeslot) for uplink and one for downlink. In GPRS it is possible to have a multislot MS, for example a 3-slot MS with the same or different (asymmetric) uplink and downlink capability. GPRS allows up to eight air interface timeslots to be combined together to give higher rate connections. In the 8-TSL MS, a single GPRS user has exclusive use of all eight timeslots.

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

Channel Coding Scheme

CS-1 CS-2 CS-3 CS-4

Single TSL Data Rate

9.05 kbit/s 13.4 kbit/s 15.6 kbit/s 21.4 kbit/s

3-TSL Data Rate 27.15 kbit/s 40.2 kbit/s 46.8 kbit/s 64.2 kbit/s

8-TSL Data Rate 72.0 kbit/s 107.2 kbit/s 124.8 kbit/s 171.2kbit/s

7.2 Air interface performance

The figure below shows some results of Nokia simulations to determine the air interface throughput rate for each coding scheme in different carrier-to-interference ratios (C/I). The simulations were made for two cases: With one timeslot and three timeslots allocated for GPRS. Remember that several users could share the throughput.

Depending on the value of C/I ratio, the CS that produces the best throughput can 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 current networks CS-1 and CS-2 are the viable options.

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25

C/I

Kbi

t/s

CS-1

CS-2

CS-3

CS-4

0

10

20

30

40

50

0 5 10 15 20 25

C/I

Kbi

t/s

CS-1

CS-2

CS-3

CS-4

Minimum Average

Typical NW C/I

Minimum Average

Typical NW C/I

1 Timeslot 3 Timeslots

Figure 19. Simulated network throughput of user data for GPRS coding schemes (non-frequency hopping, polling interval = 18 blocks)


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At low values of C/I ratio, CS-1 performs best. At around 15 dB, CS-2 performance is better than CS-1. Above C/I ratio of 18 dB, CS-3 is better than CS-1. A similar analysis can be performed for three-timeslots case.

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Table 3. Throughput for C/I values of 12, 15, and 20dB

C/I ratio CS-1 CS-2 CS-3 CS-4 Best performance

12dB 8 kb/s 6 kb/s 4 kb/s 0.5 kb/s CS-1

15dB 8 kb/s 10kb/sec 8 kb/sec 1 kb/s CS-2

20dB 8kb/sec 12 kb/sec 14 kb/sec 5 kb/sec CS-3


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8 Radio resource management

8.1 Available resources for GPRS

GSM timeslots are used for used for circuit switched (CS) traffic and assigned by the GSM network, whereas timeslots for packet switched (PS) traffic are assigned by the PCU. One question that arises is how many timeslots are to be reserved for each type of service? Circuit switched traffic has priority over packet switched traffic. But when there are idle GSM timeslots, one would like to 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 CS traffic can be transmitted on them. If congestion occurs for circuit switched traffic, then only dedicated GPRS traffic channels can carry PS traffic.

Default timeslots are by default for GPRS traffic channels that can be dynamically configured to handle CS load if needed. The default timeslots are always switched to the PCU when allowed by the CS traffic load.

Additional timeslots by default carry CS traffic but can be dynamically configured into a GPRS timeslot when required. During peak GPRS traffic periods, additional channels are switched to GPRS use, but only if the CS traffic load permits that to occur.

All full rate or dual rate traffic channels are capable of carrying GPRS traffic channels. The operator can set the following:

GPRS capacity cell by cell and TRX by TRX

Amount of dedicated timeslots

Amount of default timeslots

Amount 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 move dynamically.

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Figure 20. Radio Resource Management

8.2 GPRS resource for subscribers: Uplink resource allocation

When 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 referred to as medium access control (MAC). The MAC layer of the GPRS protocol performs this function. GSM/GPRS uses Slotted Aloha in that each MS station can only start transmitting at the beginning of a TSL interval.

Medium access control is relevant to the uplink direction only. It is not relevant to data transmitted in the downlink direction from a single BTS to all the MSs in a cell. This is because there is no contention when there is only one source in the downlink direction. A further complication is added because in GPRS one PDCH is shared amongst many MSs. So there is a need for the BSS (PCU) to indicate the radio blocks that are reserved for each active GPRS user for uplink transmission.

There are three uplink resource allocation methods defined in the GPRS standards:

Fixed allocation of uplink radio resources

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 list of allocated radio blocks per timeslot in which the MS station can transmit. The MS also needs to know when it can transmit on the allocated resources. This is usually stated in terms of the absolute frame number. This resource allocation method is mandatory for the network and the MS.

For dynamic resource allocation in the uplink direction, a field called Uplink State Flag (USF) is used. The USF is a 3-bit field that is transmitted in the MAC header of every block in the downlink direction. Each MS that wants to


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transmit 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 active MS then checks the USF value of each radio block that it is transmitted by the BTS. The following rule is applied:

"If the USF value xyz occurs in the MAC header of downlink block j, it identifies 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 of PDCHs and one corresponding USF value for each PDCH. The MS monitors the USF values in downlink transmission on the assigned PDCHs. The MS may transmit in uplink direction in the radio blocks that currently have the same USF value 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 user on the right has been given USF value 1 (binary 001). The user on the left has been given USF value 2 (binary 010) in this particular PDCH. The USFs are set so that the user on the right may use radio blocks B0 to B4, and the user on the left may use radio blocks B5 to B9. There is a parameter called USF Granularity, which, if set to 1 in a downlink radio block j, allows MS to transmit in the j+1 uplink block and the next 4 uplink block.

USF=1:B0- B4

USF=2:B5- B9

Figure 21. Usage of Uplink State Flag (USF)

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The USF has only eight values (3 bits) so, in theory, only eight (23) users can simultaneously share one PDCH physical channel in uplink direction. The binary pattern 111 can be reserved for indicating PRACH blocks, that is, for mobiles to send resource allocation requests in the uplink direction. If the binary pattern 111 is reserved for PRACH, then only up to seven MS can share a PDCH.

Downlink multiplexing of radio blocks destined for different MSs is enabled with another identifier called Temporary Flow Identifier (TFI), which is included in each radio block.


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9 Data transfer

The Problem

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 Block belongs 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 flow must be uniquely identified. Unlike circuit switched data transfer, packet data transfer is unidirectional, asymmetric, and independent. Consequently, a unique identification is required both for uplink and downlink traffic.

The Temporary Flow Identity (TFI) is a 5-bit field allocated by the PCU that is part of each data block transmitted across the air interface. The TFI uniquely identifies a data transfer session1 in the uplink or downlink direction. Each TFI is unique for the allocated PDCHs. But the same TFI may be used in the uplink and 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 ARQ mechanism to acknowledge correctly received RLC data blocks. These data blocks are numbered with unique sequence numbers called a block sequence number (BSN). The sender transmits data blocks using a sliding window scheme. The receiver sends ACK or NACK to identify the last correctly received RLC data block up to an indicated BSN. Every time an ACK or NACK is received, the size of the sending sliding window is modified and the erroneous blocks are retransmitted. There is an acknowledgement procedure in the LLC layer.

Unacknowledged mode for RLC/MAC operation does not use ACK and NACK or retransmission of erroneous data blocks. It uses forward-error-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 the uplink to initiate uplink transfer for sending data or signalling information.

Packet Paging Channel (PPCH) is used to page an MS prior to downlink packet transfer.

1 A data transfer session is not a PDP session! It refers only to a set of RLC blocks to be transmitted. A TFI can change quite often during an active PDP context (= end user “session”).

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Packet Access Grant Channel (PAGCH) is used in the packet transfer establishment phase to send resources assignment to an MS.

Packet Data Traffic Channel (PDTCH) is a channel allocated for data transfer either in the uplink (PDTCH/U) or downlink (PDTCH/D) direction.

9.1 Mobile originated packet transfer

Mobile originated packet transfer can begin after a mobility management (MM) and one or several PDP contexts have been established. Let us assume that the subscriber is running a bursty application and has already sent some data via the air interface. Now the user wants to continue the data transfer. To do so, the subscriber temporarily needs some resources. So the MS sends a request to the PCU for radio resources and the PCU responds with a radio resource assignment message. Thereafter data transmission begins and positive acknowledgement (ACK) and negative acknowledgements (NACK) are sent by the peer entity. The sequence of events that take place are shown in Figure 22 and described below:

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

1. Packet Channel Request:The uplink packet transfer is initiated by a Packet Channel Request. This can be done on the RACH or PRACH.

2. Packet Immediate Assignment:On the network side, resources for data transfer have to be allocated to the 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 GPRS service is demanded and the network can assign uplink resources


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on one or two PDCHs. This might not be enough. Therefore, the mobile originated packet transfer is split into two phases. The AGCH is used for the Packet Immediate Assignment.

Using a PRACH, the MS can deliver more adequate information about the requested resources. Consequently, one or more PDCHs can be allocated to the subscriber. The PAGCH is used for the Packet 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, the one-phase access has been completed.

The two-phase access then is optional. It is initiated by the MS, when it is not satisfied with the uplink resources allocated to it.

3. Packet Resource Request:This message is used to carry the complete description of the requested resources for the uplink transfer.

4. Packet Resource Assignment:This message is the network’s response, indicating the resources reserved for 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 on a PACCH.

9.2 Mobile terminated packet transfer

When a packet is received from an external network by the GGSN, it contains a source and destination IP address. The GGSN has to translate the destination IP address to a PDP context TID or establish a tunnel with SGSN serving MS. The packet is then tunnelled to the SGSN using the GTP protocol. The SGSN translates TID into a TLLI and NSAPI, which identifies a logical connection between SGSN and MS. Thereafter the SGSN sends the packet using the SNDCP protocol to the MS.

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41 ©NOKIA

Radio block identification Downward multiplexing of radio blocks is done using Temporary Flow Identifier (TFI)

What is TFI?• TFI is assigned in a resource assignment message prior to transmission LLC layer frames

between MS and BSS• TFI is unique among concurrent processes• TFI is preferable to MS identity which is a very long number• TFI is included in every RLC frame header

Radio blocks:

• several subscribers served on onphysical channel

• up to 8 channels can be used by one phone

Figure 23 Downlink multiplexing of data

Mobile terminated packet transfer is only possible if MS is in the Ready state and is initiated by the network using the Packet Resource Assignment message as shown in Figure 25. In case there is a PCCCH allocated in the cell, the Packet Resource Assignment is transmitted on the PAGCH. In case there is no PCCCH allocated in the cell, the Packet Resource Assignment is transmitted on the AGCH. The Packet Resource Assignment message includes the list of PDCH(s) that will be used for downlink transfer as well as the PDCH carrying the PACCH. The MS will have to monitor all the PDCH and identify its downlink data using the TFI, which is part of each downlink data block.


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42 ©NOKIA

Radio

Block

TFI g

Radio

Block

TFI f

Radio

Block

TFI e

Radio

Block

TFI d

Radio

Block

TFI c

Radio

Block

TFI b

Radio

Block

TFI a

Radio

Block

TFI g

0 71 2 3 4 5 6

0 71 2 3 4 5 6

0 71 2 3 4 5 6

0 71 2 3 4 5 6

Frame 0

Frame 1

Frame 2

Frame 3

Multislot allocation example

TFI and Radioblocks

Figure 24 Multislot use in downlink

The TFI is an identifier that is included in every radio link control (RLC) header belonging to a particular temporary block flow (TBF) and in the control messages associated to the LLC frame transfer in order to address the peer RLC entities. The more often a TFI allocated to specific user is included in the downlink RLC header the higher the DL bit rate will be. Theoretically a user can thus have all eight slots in a TDMA frame/multiframe structure. In practice there are other limitations such as MS capability. The timing advance and power control information is also included, if available. Otherwise, the MS may be requested to respond with an access burst.

Figure 25. Mobile terminated packet transfer

The release of radio resources is initiated by the network by terminating the downlink transfer and polling the MS for a final Packet Ack/Nack.

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Figure 26. Downlink data transfer


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10 Modulation

Modulation is the process of encoding binary data onto a carrier of frequency Fc. All modulation schemes modify the amplitude, frequency, or phase of the carrier. The input to the modulation scheme is the digital data (or modulating data) that is to be transmitted and is usually measured in bits per second. The modulator output is the modulated signal and is usually measured in symbols per second.

MODULATORMODULATOR

Digital databits/sec

Modulated datasymbols/sec

Figure 27. Modulation

10.1 GMSK

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

10.2 EDGE

To enhance data service GSM can use an additional technique called Enhanced Data rates for GSM Evolution (EDGE). EDGE is a radio-based high-speed mobile data standard. EDGE improves network capacity and data rates, for both circuit switched and packet switched data.

EDGE uses 200 kHz radio channels, which are the same as current GSM channel widths. From a technical perspective,

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EDGE allows the GSM and GPRS network to offer a set of new radio access bearers to its core network. EDGE is designed to improve spectral efficiency through link quality control. EDGE requires wider transmission channel widths and features flexible time slots to mix and match all forms of communications, including voice, data, and video. Although EDGE boosts the GSM and GPRS network, introducing EDGE to the existing network has little technical impact, since it is fully based on GSM and requires relatively small changes to network hardware and software. Thus operators do not have to make any changes to the network structure or invest in new regulatory licenses.

The 8PSK modulation scheme is used for EGPRS. This is one of the improvements EDGE brings since the throughput of this modulation scheme is three times higher than of GMSK. In this scheme, the transmitted symbols are one of eight sinusoids, which have the same amplitude and frequency but differ in phase. The digital data bits are combined into groups of three bits. Thus there are eight possible combinations starting from (0,0,0) to (1,1,1). Each of the 3-bit patterns is then matched to one of 8PSK symbols. The mapping is done in such a way that there is a single bit difference between adjacent symbols. This is referred to as Gray coding. It ensures that if a symbol is received in error as an adjacent 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

(1,0,0) 6 -/2

(1,1,0) 7 -/4


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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)

I

Q

Figure 28. PSK modulation scheme

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

10.2.1 EDGE coding schemes

Nine modulation and coding schemes, MCS-1 to MCS-9, are defined for the EGPRS packet data traffic channels, and these are tabulated below. For all EGPRS packet control channels the corresponding GPRS control channel coding is used. ETSI standards state that MCS-1 to MCS-9 are mandatory for MSs supporting EGPRS. However, an EPGRS network may support only some of the MCSs.

Table 4. Coding parameters for the EGPRS coding schemes

Scheme Code rate

Header code rate

Mod RLC blocks per

RB

Bits in a radio block

Family BCS

T

A

I

L

H

C

S

Data ratekb/s

MCS-9 1.0 0.36

8PS

K

2 2x592 A 2x12 2x6

8

59.2

MCS-8 0.92 0.36 2 2x544 A 54.4

MCS-7 0.76 0.36 2 2x448 B 44.8

MCS-6 0.49 1/3 1 592

544+48

A

12 6

29.6

27.2

MCS-5 0.37 1/3 1 448 B 22.4

MCS-4 1.0 0.53 GM

1 352 C 17.6

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SK

MCS-3 0.80 0.53 1 296

272+24

A 14.8

13.6

MCS-2 0.66 0.53 1 224 B 11.2

MCS-1 0.53 0.53 1 176 C 8.8

NOTE: The italic captions indicate the padding

10.2.2 Incremental Redundancy and Link adaptation

Incremental Redundancy (IR) is an efficient combination of two techniques, Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC). In the ARQ method, when the receiver detects the presence of errors in a received RLC block, it requests and receives a re-transmission of the same RLC block from the transmitter. The process continues until an uncorrupted copy reaches the destination.

The Forward Error Correction (FEC) method adds redundant information to the user information at the transmitter, and the receiver uses the information to correct 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 sent right away. Rather, only a small amount is sent first, which yields a high user throughput if the decoding is successful. However, if decoding fails, a re-transmission takes place according to the ARQ method.

Using IR, the transmitter transmits a different set of FEC information from the same 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 of EGPRS. Supporting IR, the receiver is able to combine the necessary amount of error correcting information. Since the combination includes more information than any individual transmission, the probability of correct reception is increased. IR co-operates with link adaptation, which selects the amount of redundancy information transmitted in each transmission. The benefits of IR are increased throughput due to better and automatic adaptation to different and varying channel conditions and reduced sensitivity to link quality measurements.

EDGE not only increases efficiency and speed, but also improves data protection through link quality control. The system uses various measurements of the past link to predict up coming channel quality. This prediction determines the relevant protection of the information to be sent. The Link Adaptation (LA) mechanism works to provide the highest throughput and lowest delay available by adapting the protection of the information to be sent, according to the link quality. Enabling LA requires accurate link quality measurements and a set of modulation and coding schemes (MCSs) with different degrees of protection.


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The IR and LA benefits can be combined. While IR improves throughput by automatically adapting the total amount of transmitted redundancy to the radio channel conditions, LA selects the amount of redundancy for each individual transmission. This helps reduce the number of re-transmissions, and thus keeps the transfer delay reasonably low.

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11 Key points

The GPRS radio interface consists of asymmetric and independent uplink and downlink channels. In addition to sharing the downlink channels, the MSs can also share a single timeslot in the uplink direction. The MAC function on the air interface defines procedures that control the multiplexing of several MSs on the same transmission medium.

Entities communicating over the air interface have to perform a number of functions: framing, channel coding, modulation, congestion control, segmentation, medium access control, synchronisation, multiplexing, timing advance, power control, handover, ciphering, interleaving, signal measurements, puncturing, etc.

The GPRS protocol stack contains Physical RF, Physical Link, MAC and RLC, LLC and SNDCP layers. Each layer performs a well-defined function. 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 GPRS signalling 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 block is 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 usage provides data rates from 9 to 170 kbps. Depending on the CS and # of TSL, different throughput rates can be obtained. At present CS-1 and 2 are the viable options. Higher data rates can be obtained using EGPRS.

Packet Resource Assignment and Reassignment messages play an important role in controlling uplink and downlink data transfer.

The USF flag is used for MAC in the uplink direction. It allows up to seven 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 be seamlessly integrated with the existing GSM CS channels, allowing the operator to configure the radio timeslots as per requirement and CS traffic load.


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12 Review questions

1. Which fields are used for medium access control and multiplexing multiple 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 PDCH multiframe?

4. What is the purpose of PTCCH?

5. Which layer is responsible for segmentation and reassembly of LLC PDUs 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?

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References

Nokia DX200 SGSN Product Description

Nokia GPRS Charging Gateway Product Description

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 Radio Interface. 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) – Base Station System (BSS) interface; Radio Link Control/Medium Access Control (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 Dependent Convergence 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 reception


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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










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