04 Tm2110eu01tm 0004 Radio Interfaces

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GPRS Radio Interface Siemens

TM2110EU01TM_0004 2002 Siemens AG

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Contents

1 The Radio Interface (Layer 1) 31.1 Layer 1 of the GSM-/GPRS-Radio Interface Um 41.2 Channel Bundling, Sharing of Channels 61.3

Radio Block 8

1.4 Coding Schemes: 101.5 Logical GPRS Radio Channels 141.6 Multiframes in GPRS 182 Exercises 213 Solutions 25

GPRS Radio Interface


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Siemens GPRS Radio Interface

TM2110EU01TM_0004

2002 Siemens AG

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1 The Radio Interface (Layer 1)

The Radio Interface Um

(Layer 1)

GPRS:

Interfaces

Fig. 1


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1.1 Layer 1 of the GSM-/GPRS-Radio Interface Um

By introducing GPRS services into the GSM-PLMN, worldwide modifications arenecessary also in the area of physical transmission (layer1) via the air or radio inter-face Um. The tasks of layer 1 radio interface relate to the transmission of user andsignaling data as well as to the measuring of receiver performance, cell selection, de-termination and updating of the delayed MS transmission (timing advance TA), powercontrol PC and channel coding.

In the GPRS, a decisive difference to the realization of the connection-oriented ser-vices (circuit-switched services) relates to the fact that a physical channel and a so-called packet data channel can be used by several mobile stations at the same time.One packet data channel is allocated per radio block, i.e. for four consecutive TDMAframes and not for a specific time interval. This means that signaling and the packetdata traffic of several mobile stations can be statistically multiplexed into one packetdata channel. Furthermore, the packet data channel can be seized asymmetrically.

On the other hand it is also possible for a mobile station to use more than one packetdata channel at the same time, i.e. to combine several physical channels of one radiocarrier. In principle, up to 8 packet data channels can be seized simultaneously. Thenumber of channels that are combined for reception (DL) and transmission (UL) canbe different to achieve asymmetric data rates for certain applications (e.g. file transferprotocol FTP, internet surfing).

The assignment of radio resources can be done dynamically or in a fixed allocation.In case of the fixed allocation a message with a bit pattern is sent downlink to indi-cate which channels can be used by this MS for UL transmission.

If dynamic allocation is applied the MS will be receive a temporary flow identifier (TFI)and an uplink state flag (USF) for each of the time slots it is allowed to use. The TFIis part of the control information in the DL packet and identifies the "owner" of thepacket. Each packet also includes an USF that indicates which of the MSs (that hasbeen assigned to use this time slot UL) is allowed to transmit the next radio block UL.


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GSM RF:

GPRS Layer 1 (Um)

L1-

tasks

Transmission

of user &

signaling data

determinate &

actualise

Timing Advance

Cell SelectionMeasure

signal strength

Power Control

functions Resource optimization:1 physical channel to be used

by many MSs simultaneously !!

asymmetrical trafficUL / DL possible !!

High data rate trafficup to 171.2 kbit/s:

combining 1..8 PDCH for 1 MS !!

Allocation of physical channel(Packet Data Channel PDCH)

dynamically: 1 or 4 Radio Blocks

(1 Radio Block = 4 Normal Burst

in 4 consecutive TDMA-frames)

User & signaling data of several MSs

statistically to be multiplexed into 1 PDCH

(also fixed allocation possible) 29 multislot classes

Fig. 2 Tasks of the GSM air interface, layer 1 (GSM RF)


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1.2 Channel Bundling, Sharing of Channels

Sharing of Resources in a Cell:GSM circuit switched (CS) users will share the timeslots in a BTS with the GPRS packet switched (PS)users. A physical channel can ei-ther be used for GSM CS or GPRS PS traffic but not for both at the same time. De-pending on the traffic load in the cell there will be more or less channels available forGPRS, CS connections are dealt with priority.

Sharing of Physical Channels:It is a characteristic of a CS connection that thephysical resource (the time slot) is reserved for one subscriber. Therefore theGSM CS users cannot share their channels with others. In contrast GPRS PS sub-scribers can share physical channels. The handling of the channels, the multiplexingof subscribers onto the same time slots is done by software (protocol, MAC) andhardware (PCU). Packet oriented connections are not only carried out through thecore network by usage of an appropriate hardware (ATM switches) and software(protocols) but also on the air interface. This is an important feature of GPRS with re-gard to an optimized usage of resources on Um, which is the limiting bottleneck in thePLMN.

Multislot Classes: The subscribers for GPRS will have different needs (applications,data rates) and therefore the MS will have more or less capabilities. The network(PCU) will have to identify these different MSs by their multislot class, which indicateshow many time slots (channels) can be bundled by the MS uplink and downlink. Acheap GPRS mobile will be a GSM mobile that is able to handle the protocols andcoding schemes of GPRS. This will be multislot class 1: one time slot UL and one

time slot downlink can be "bundled". The other extreme is multislot class 29 whichwill be able to receive and to transmit in eight time slots UL and DL simultaneously. Inconsequence such a MS has to have two synthesizers, and a high battery capacitybecause this is more or less continuous transmission and reception. The MS willsend its multislot class and the PCU will only assign time slot combinations which canbe handled by this equipment.


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Channel Bundling, Sharing of Channels

Radio Blocks

Subsriber A

Radio Blocks

Subscriber B

Radio Blocks

Subscriber C

Radio Block

Subscriber D

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

UL DL

Fig. 3 Channel bundling, sharing of channels


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1.3 Radio Block

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

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

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

a Block Check Sequence BCS.

The functional blocks (radio blocks) are protected in the framework of convolutionalcoding against loss of data. Usually, this means inserting redundancy.

Furthermore, channel coding includes a process of interleaving, i.e. different ar-rangement in time. The convolutional radio blocks are interleaved to a specific num-ber of bursts/burst blocks. In the case of GPRS, interleaving is carried out across fournormal bursts NB in consecutive TDMA frames and, respectively, to 8 burst blocks

with 57 bit each.

Four new coding schemes were introduced for GPRS (Rec. 03.64): CS-1 to CS-4.These can be used alternatively depending on the information to be transferred andon the radio interfaces quality.


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Radio Block Strucure

collect

user data

signaling

Radio Block

RLC Data Block BCSMAC Header

RLC/MAC Control Block BCSMAC Header

BCS: Block Code Sequence

(for error recognition)

MAC: Medium Access Control RLC: Radio Link Control

One Radio Block = 4 normal bursts

Fig. 4 Radio block

Convolutional

coding

(not CS-4)

Radio Block

Radio Block

(Redundancy !)rate 1/2convolutionalcoding

Radio Block (456 Bits)

puncturingPuncturing

(only CS-2, CS-3)

Interleaving 57 Bit8 Burst-

blocks57 Bit 57 Bit 57 Bit57 Bit

Channel Coding4 new Coding Schemes:

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

Um: Allocation of PDCH for 1 / 4 Radio Blocks = 4 / 16 Normal Bursts Fig. 5 Channel coding schemes


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1.4 Coding Schemes:

CS-1:CS-1 uses the same coding scheme as specified by Rec. 05.03 for theSDCCH. It comprises a half rate convolutional code for FEC forward error correction.CS-1 corresponds to a data rate of 9.05 kbit/s.

CS-2and CS-3 are punctured version of the same half rate convolutional code asCS-1. The coded bits are numbered starting from 0 and certain punctured bits areremoved.

CS-2:With CS-2 the punctured bits have numbers 4 i + 3 with i = 3,...,146 (excep-tion: i = 9, 21, 33, 45, 57, 69, 81, 93, 105, 117, 129, 141). This means that none ofthe first 12 bits is punctured. CS-2 corresponds to a data rate of 13.4 kbit/s. Remark:For CS-2 the puncturing pattern must be adapted to the future new TRAU frame for-

mat in order to be used via the Abis interface (e.g. more bits must be punctured tomake space for RLC signaling).

CS-3:With CS-3 the punctured bits have numbers 6 i + 3 and 6 i + 5 with i =2,...,111. CS-3 corresponds to a data rate of 15.6 kbit/s.

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

By bundling up to 8 packet data channels of one carrier into one MS, transmissionrates up to 171.2 kbit/s are possible.


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9,05 kbit/s 13,4 kbit/s 15,6 kbit/s 21,4 kbit/s

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

(FEC)

Quality Um

Coding

Scheme

Code

Rate

Radio

Block*

Coded

Bits

Punctured

Bits

Data Rate

kbit/s

CS-1 1 / 2 181 456 0 9,05

CS-2 2 / 3 268 588 132 13,4

CS-3 3 / 4 312 676 220 15,6

CS-4 1 428 456 0 21,4

Channel Coding: Coding Schemes

* Radio Block without

Uplink State Flag USF &

Block Check Sequence BCS Fig. 6 Coding schemes of GPRS, CS1 with high redundancy, CS4 no redundancy, radio blocks


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GPRS Channel Coding

Existing channel coding procedures have been modified with a view to introducingthe GPRS. New coding schemes CS 1-4 were specified from ETSI 4. Basically, they

make it possible to transmit 9.05 kbit/s (CS-1), 13.4 kbit/s (CS-2), 15.6 kbit/s (CS-3)and 21.4 kbit/s (CS-4) per timeslot, respectively.

On the Abis interface, transport capacity is restricted to 16 kbit/s owing to the fact thatexisting TRAU frames are used. The transmission of data for CS-3 and CS-4 wouldrequire larger transport capacities via Abis and would thus involve serious modifica-tions in the existing network architecture. For this reason, only coding schemes CS-1and CS-2 are supported in GR2.0/BR5.5. Of these two, CS-1 is particularly important.Due to the unrestricted redundancy in data transmission, CS-1 is well suited to serveas a safe basic coding for RLC/MAC data and control blocks. With a high-quality ra-dio interface CS-1 data transmission rates of up to 8 kbit/s are possible. Even if the

air interface quality (the C/I ratio) decreases, the rate of transmission decreases veryslowly.

Under favorable radio transmission conditions, CS-2 achieves higher transmissionrates, with a maximum at 12 kbit/s. However, the rate of transmission depends morestrongly on the C/I ratio than with CS-1.

This is even truer of coding schemes CS-3 and CS-4, respectively, whose transmis-sion rates are considerably higher than those of CS-1 and CS-2 under good radiotransmission conditions; but they rapidly decrease if the quality of the radio transmis-sion interface gets worse.


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CS 1 - 4: Bit Rate Comparison

18 17 16 15 14 13 12 11 10 9 8 7 6

NetThrough

put(kbit/s)

0

2

4

6

8

10

12

14

16

18

20

CS1

CS2

CS3

CS4

5

Channel Coding Introduction: CS-1 (9,05 kbit/s & CS-2 (13,4 kbit/s)

CS-1: basic coding for RLC/MAC data & control blocks

no CS-3 (15,6 kbit/s), CS-4 (21,4 kbit/s)

Abis limitation (current TRAU frames: 16 kbit/s)

Carrier / Interference C/I (dB) Fig. 7 Comparison of the efficiency of the four coding schemes under realistic circumstances of the air interface


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1.5 Logical GPRS Radio Channels

Use of "classical" logical channels for GSM-CSA Logical channel is used for a special purpose/contents. For example the MSs haveto find out if this cell is a suitable one (operated by the "right" network operator),

which features are offered (e.g. HR/FR/EFR, GPRS, ...), what is the structure of Um(channel combination), ... This is provided by the BCCH which is naturally onlytransmitted in the downlink. Some resources have to be given for initial access for theMS (RACH). For these reasons logical channels have been defined to fulfill all tasks,

which are necessary in a GSM network on the air interface (see figure 13).

The GPRS subscribers will share the air interface with the circuit switched users. Onthe other hand the protocol structure of GPRS is different from "classical" GSM-CS.

Therefore the user traffic and (part of) the signaling will have to be separated. Beforethis separation can take place the different MS (GPRS/non-GPRS) have to be han-dled by signaling procedures for access (channel assignment. There are two solutionof this problem. The first one is to use (some of) the logical channels for GSM-CS:

The GPRS-MS detects the BCCH of this particular cell and looks for the system in-formation to find out if GPRS is available. If this is a cell belonging to the same rout-ing area the MS can choose this cell and wait for paging or for the user to use theRACH for activating a PDP. In case that the user wants to run an PS application theGPRS MS will use an access burst (RACH) which indicates that this is a GPRS MSand the request will be answered by the PCU assigning resources for packetswitched traffic (time slots reserved for GPRS). Signaling (e.g. for authentication) willthen take place using these resources indicated by the message in the AGCH.

So GPRS uses some of the logical channels of GSM-CS. On one hand this can bean advantage if the resources are sufficient. On the other hand if in the future moreand more GPRS traffic has to be handled, separate logical channels reserved forGPRS MS will have to be given. This is the second solution. In any case the GPRSMS will have to look for the BCCH of the cell to find out if GPRS is available. If thesecond solution has been chosen the GPRS MS will also read information where aPBCCH (Packet Broadcast Control Channel) is to be found (which time slot). Thissecond solution will be explained in figure 14.


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Allocation of dedicated signalling channel

Dedicated signaling MS BTSE (Call

Setup, LUP, Security, SMS, CBCH,...)

Signaling

Traffic

User Data

CGI, FR/EFR/HR, GPRS available

frequency hopping, channel combination,...)

Time synchronisation + BSIC, TDMA-No.

Traffic Channel/H

DL

DL

UL

UL + DL

DL

UL

+

BCCH

FCCH

SCH

PCH

AGCH

RACH

SDCCH

SACCH

FACCH

TCH/F

TCH/H

frequency synchronisation

Paging / Searching (MTC)

Request for access

Measurement Report,

TA, PC, cell parameters,...

Signaling instead of TCH

BCH

CCCH

DCCH

User traffic (Full Rate)

User traffic (Half Rate)

Logical Channel(for GSM Circuit Switched)

Synchronisation Channel

Frequency Correction Channel

Access Grant Channel

Random Access Channel

Paging Channel

Broadcast Control Channel

Stand Alone Dedicated

Control Channel

Broadcast Channel

Slow Associated

Control Channel

Fast Associated

Control Channel

Traffic Channe/Fl

Dedicated Control Channel

Common Control Channel

NCHNotification Channel

Notifying MSs

Fig. 8 "Classical" logical channels of GSM may be used by GPRS users too


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Use of new logical channels for GPRS

In addition to the nine existing logical radio channels used for signaling (BCCH, SCH,FCCH, PCH, RACH, AGCH as well as SDCCH, SACCH and FACCH) and the Traffic

Channel (TCH) for circuit switched user information, a new set of logical channelswas defined for GPRS.

Packet traffic is realized by means of the Packet Traffic Channel (PTCH), which in-cludes the following:

Packet Data Traffic Channel PDTCH.

Packet Associated Control Channel PACCH

Packet Timing advance Control Channel PTCCH

The PDTCH is temporarily assigned to the mobile stations MS. Via the PDTCH, user

data (point-to-point or point-to-multipoint) or GPRS mobility management and sessionmanagement GMM/SM information is transmitted.

The PACCH was defined for the transmission of signaling (low level signaling) to adedicated GPRS-MS. It carries information relating to data confirmation, resource al-location and exchange of power control information.

New GPRS signaling channels are mainly specified analogously to GSM Phase1/2.

ThePacket Common Control Channel PCCCH has been newly defined. It consistsof a set of logical channels, which are used for common control signaling to start the

connection set-up:Packet Random Access Channel PRACH

Packet Paging Channel PPCH

Packet Access Grant Channel PAGCH

Packet Notification Channel PNCH

PRACH and PAGCH fulfill GPRS-MS functions, which are analogue to the classicallogical channels RACH and AGCH for non-GPRS-users. The PNCH is used for theinitiation of point-to-multipoint multicast (PtM multicast).

For the transmission of system information to the GPRS mobile stations, the

Packet Broadcast Control Channel PBCCH

was defined analogue to the classical BCCH.

In a physical channel all different types of logical channels can be contained (noseparation into traffic and signaling channels respectively as is done in conventionalGSM). The differentiation of channel contents is carried out per radio block using theMAC header, i.e. contents are specified for the four normal bursts of a radio blocksent in each case.

The MAC function, which distributes the physical channel to the various mobile sta-tions and allocates radio resources to an MS can also use the conventional logicalchannels in GSM.


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Logical channels

for GPRS

PBCCH

PRACH

PPCH

PNCH

PAGCH

PACCH

PTCCH/U

PTCCH/D

PDTCH

Packet Broadcast

Control Channel

Packet Random

Access Channel

Packet Paging

Channel

Packet Notification

Channel

Packet AccessGrant Channel

Packet Associated

Control Channel

Packet Timing Advance Control

Channel Uplink/Downlink

Packet Data

Traffic Channel

Packet

Signaling

Packet

Traffic

Broadcast channel

Common

Control

channels

Dedicated channels

Packet System

Information

Access request for

UL packet data

transmission

Paging GPRS-MS

(PtP)

Paging GPRS-MS

(PtM)

Resource allocation

Dedicated signaling

MS-network,

e.g.power control

Timing advance

Determination and

Control

Transmission of

User data

UL

DL

DL

DL

UL&DL

UL&DL

UL

Fig. 9 New logical channels for GPRS


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1.6 Multiframes in GPRS

The GPRS packet data traffic is arranged in 52-type multiframes (GSM Rec. 03.64).52 TDMA frames in each case are combined to form one GPRS traffic channel multi-frame, which is subdivided into 12 blocks with 4 TDMA frames each. One block(B0-B11) contains one radio block each (4 normal bursts, which are related to eachother by means of convolutional coding). Every thirteenth TDMA frame is idle. In theidle frame the PTACCH is sent. The idles frames are used by the MS to be able todetermine the various base station identity codes BSIC, to carry out timing advanceupdates procedures or interference measurements for the realization of power con-trol.

For packet common control channels PCCH, conventional 51-type multiframes canbe used for signaling or 52-type multiframes. The GPRS users can use "classical"common control channels of GSM before they will be directed onto their PTCHs. Allmobiles will read the BCCH anyway. Either in case of GSM mobiles to fulfill the sametasks as before and for GPRS equipment this logical channel will indicate weatherGPRS service is available and if extra logical channels (PBCCH, PPCH, ...) are used.

GSM CS traffic and GPRS subscribers are clearly separated so that there is no con-flict due to different signaling or multiframe structure.

It is important that there are no "visible" changes for "GSM only mobiles" due to theintroduction of GPRS. GSM CS connections will use for example the same 26 multi-frame structure for TCH and the 51 multiframe structure for signaling.


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iB0 B1 B2 B3 B4 B5 i B6 B7 B8 i B9 B10 B11 i

52 TDMA Frames = PDCH Multiframe

4 Frames 1 Frame

New multiframe

for GPRS PDCH follows 52 multiframe structure

52 Multiframe: 12 Blocks 4 TDMA-frames

PCCCHs: classical 51er Multiframesor 52er Multiframes

B0 - B11 = Radio Blocks (Data / Signaling)

i = Idle frame (PTCCH)

BCCH indicates PDCHwith PBCCH (in B0)

DL: this PDCH bears PDCCH & PBCCH

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

PBCCH indicates PCCCH blocks & further PDCHs with PCCCH

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

PDCH without PCCCH: PDTCH & PACCH only

Idle frame: Identification of BSICs

Timing Advance Update Procedure

Interference measurements

for Power Control

Fig. 10 Multiframes for GPRS consist of a certain time slot in 52 consequent TDMA frames


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4. What does puncturing mean?

process of interleaving

process of adding redundancy

some bits are removed

bust block building

5. True/False

CS 4 cannot be used because of the unreliable Air interface

CS cannot be used because the TRAU cannot read CS 4

CS cannot be used because 21.4 kbit/s cannot be transported on the air inter-face

CS cannot be used because 21.4 kbit/s cannot be transported on Abis Interface

6. How many subscribers can send data in one 52 Multiframe?

1

4

12

52

7. Which kind of logical channels will be used in the beginning of GPRS?

PBCCH, PPCH, PNCH

PBCH, PAGCH, PACCH

PACCH, PDTCH, PTACCH

PBCCH, PNCCH, PRACH

8. If one subscriber gets 3 TS. In which way are the radio blocks sent over the air?

The radio blocks are spread over three TS

Each radio block is always sent within the "same" TS

One radio block is sent in one TS


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3 Solutions


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Solution

Title:Radio Interface

Objectives: The participant is able to understand the organization on the airinterface

Pre-requisite: none

1. One radio Block is sent within:

4TS

8TS

12TS

52TS

2. One radio block consists of:

MAC Header, RLC Data Block, RLC Control Block

MAC Header, MAC control Block, BCS

MAC Control Block, RLC Data Block, BCS

MAC Header, RLC Data Block, BCS

3. What is sent with priority?

CS

PS

4. What does puncturing mean?

process of interleaving

process of adding redundancy

some bits are removed

bust block building


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5. True/False

CS 4 cannot be used because of the unreliable Air interface

CS cannot be used because the TRAU cannot read CS 4

CS cannot be used because 21.4 kbit/s cannot be transported on the air inter-face

CS cannot be used because 21.4 kbit/s cannot be transported on Abis Interface

6. How many subscribers can send data in one 52 Multiframe?

1

4

12

52

7. Which kind of logical channels will be used in the beginning of GPRS?

PBCCH, PPCH, PNCH

PBCH, PAGCH, PACCH

PACCH, PDTCH, PTACCH

PBCCH, PNCCH, PRACH

8. If one subscriber gets 3 TS. In which way are the radio blocks sent over the air?

The radio blocks are spread over the three TS

Each radio block is always sent within the "same" TS

One radio block is sent in one TS

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04 Tm2110eu01tm 0004 Radio Interfaces

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