4 GPRS Radio Interface

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T E K nsultTelecom Engineering and Consulting

GPRS Training

GPRS Radio Interface

1

ORASCOM Telecom Tunisia

Tunis 2004


Contents1 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 81.4 Coding Schemes 101.5 Logical GPRS Radio Channels 141.6 Multiframes in GPRS 18




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Copyright TEKonsult Munich

All rights reserved

November 2004




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1 The Radio Interface (Layer 1)

GPRS:Interfaces

The Radio Interface Um

(Layer 1)

Fig. 1




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1.1 Layer 1 of the GSM-/GPRS-Radio InterfaceUm

Due to the introduction of the GPRS network, some modifications have to beimplemented on the GSM Core and Radio subsystems. The radio subsystemincludes all the equipments serving for the radio resources management and indirect relation with the mobile user (the SIM card, the terminal, the base stationsand the base station controllers).The creation of new radio sites needs a lot of time and is expensive, thus the GPRSnetworks would have to use the existing sites.

More, the PGRS cells shouldn’t be smaller than the GSM cells; else coverage holeswill appear.However, the integration of the GPRS into the GSM networks requires a lot ofmodifications at the level of the radio subsystem and the mobile terminalequipment:

- A new SIM card is required to support the new GPRS services;- A new terminal adapted to the characteristics of the GPRS services

especially the higher transmission rates;- A new radio layer between the mobile terminal and the base stations to

enable the air interface to support the packet data transfer.

- New radio channels are integrated and are allocated in a flexible manner(sharing of time intervals between different users);- The base stations are modified in order to support the GPRS radio

characteristics.- Some modifications have to be involved in the base station controllers.

The GPRS radio interface needs some modifications consisting in theimplementation of a new Media Access Control radio layer able to transmit packetdata. In addition, this interface has to be enough flexible for the mobile terminaland the network operator. The number of time intervals allocated to the GPRSnetwork is variable and the number of intervals used during a packet transmissionis negotiated between the mobile terminal and the base station. This permits theupdate of the system easily based on the carried traffic quantity.Note that some GSM physical characteristics such as the modulation, thefrequency spectrum sharing and the bursts use are kept.

In addition to the tasks of carrying user and signaling data, the radio layer has toreport to the mobile terminal and the network specific performance measurementssuch as the measuring of receiver performance, cell selection, determination and




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updating of the delayed MS transmission (timing advance TA), power control PC

and channel coding.

A major difference between the circuit-switched and packet-switched servicesconsists in the fact that contrarily to circuit data channel that is used by only onemobile terminal a packet data channel can be shared by several mobile terminals.

The packet data channel is allocated per radio block (4 consecutives TDMAframes) and not for a specific time interval. One packet channel can now be seizedsymmetrically or asymmetrically and carry the user and signaling data of severalmobile stations.

If the quantity of traffic to carry is higher that the capacity of a packet datachannel, a mobile terminal can use several packet data channels simultaneouslyand combine several physical channels of one radio carrier. The GPRS standardspecifies that up to 8 packet data channels can be combined together, it precise alsothat the number of combined channels on the downlink can be different of thenumber of combined channels on the uplink, this permits the achievement ofasymmetric data rates for specific applications such as the HTTP and the FTP.

The radio resources assignment can be dynamic or static:- Static assignment: in this case a bit pattern sent on the downlink will

indicate to the mobile terminal which channels it can use for its packet datatransmission.

Dynamic assignment: per each used timeslot, the mobile terminal will receive atemporary flow identifier (TFI) that will identify the owner of the packet,accompanied by an uplink state flag (USF) that will indicate which of the assignedmobile terminals to use the time slot UL will be allowed to transmit the next radioblock over the Uplink direction.




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GSM RF:

GPRS Layer 1 (Um)

L1-

tasks

Measuresignal strength

Transmission

of user &

signaling data

Cell Selection

determinate &actualise

Timing Advance

Power Controlfunctions

Resource optimization:

1 physical channel to be used

by many MSs simultaneously!!

asymmetrical traffic

UL / DL poss ible !!

High data rate traffic

up to 171.2 kbit/s:

comb ining 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)

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)

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: the timeslots of a BTS will be shared between theGSM and GPRS subscribers. Note that a traffic channel cannot be used to carrycircuit switched traffic and packet switched traffic simultaneously. Thedistribution of the radio resources between the GSM and GPRS subscribers willdepend of the cell traffic load with keeping the circuit switched traffic prior.

Sharing of Physical Channels: in the GSM networks one timeslot can be used byonly one subscriber and no resources sharing can pe performed. In contrast, theGPRS subscriber can share the resources between each other. This operation will

require the integration of a new MAC protocol and a new hardware: the packetcontrol unit.In order to limit the bottlenecks in the PLMN, and in addition to the hardware andprotocol changes in the core network, new features responsible for the resourcesusage optimization must be implemented on the Um air interface.

Multislot Classes: The needs of the GPRS subscribers differ based on the desiredquality of service required by the service and the capabilities of the mobileterminal. Based on this needs, a mulitslot class indicating how many timeslots canbe bundled by the mobile terminal uplink and downlink, will be associated to each

mobile station and the differentiation between these different mobile terminalswill be the responsibility of the PCU, which will assign the combinations of thetimeslots that will be handled by the equipment based on the multislot classrequested by the mobile.In a multislot class 1, one timeslot for the uplink and one time slot for thedownlink can be bundled. This multislot will be used with a GSM mobile stationhaving the capability of handling GPRS protocols and coding schemes.

The multislot corresponding to the maximum extreme will belong to the class 29and reserve simultaneously eight time slots for the transmission and reception

over the uplink and downlink. In this case, the mobile terminal should beequipped with two synthesizers, and a high battery capacity because this is moreor less continuous transmission and reception.




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Channel Bundling, Sharing of ChannelsUL DL

Radio Blocks

Subsriber A

Radio Blocks

Subsriber B

Radio Blocks

Subsriber C

Radio Blocks

Subsriber D

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS8 TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS8

Fig. 3 Channel bundling, sharing of channels




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1.3 Radio Block

The GSM recommendation 03.64 explains the modifications achieved on thechannel coding procedures for GPRS purposes. Before being encoded the digitalinformation to transfer is divided into transferable blocks. Each block contain inaddition of the user information contained in an RLC Data Block, a MAC headerused for the Medium Access Control, an RLC/MAC Signaling Block and a BlockCheck Sequence.All these blocks are protected against loss by the use of convolutional codingconsisting in the insertion of redundancy information. They will be theninterleaved to four normal bursts NB in consecutive TDMA frames and,

respectively, to 8 burst blocks with 57 bit each.

The GSM Recommendation 03.64 specifies the characteristics of the four newcoding schemes that were defined especially for the GPRS purposes. These codingschemes are known as CS-1, CS2, CS3 and CS4, and are used alternativelydepending on the type of the information to be transferred and the quality of theradio interface.




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One Radio Block = 4 normal bursts

Radio Block Strucure

Radio Block

MAC Header RLC Data Block

MAC Header RLC/MAC Control Block

collect

user data

signaling

BCS: Block Code Sequence

(for error recognition)MAC: Medium Access Control RLC: Radio Link Control

Fig. 4 Radio block

Channel Coding

Radio Block

Radio Block

Radio Block(456 Bits)

Convolutional

coding

(not CS-4)

Puncturing

(only CS-2,CS-3)

rate 1/2 convolutional coding

Puncturing

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: uses the same coding scheme specified for the SDCCH in therecommendation GSM 05.03. It offers a data rate up to 9.05 kbit/s and uses a halfrate convolutional code for the Forward Error Correction (FEC).CS-1 is well suited to serve a safe basic coding for RLC/MAC and control data(unrestricted redundancy information). What’s remarkable with this codingscheme is that even if the radio interface quality decreases the data transmissionrate decreases slowly.

CS-2 and CS-3: these two coding schemes are punctured version of the same half

rate convolutional code used for CS-1. The coded bits are numbered starting from0 and certain punctured bits are removed.

CS-2: With CS-2 some of the bits will be punctured. They will have the numbers 4* i + 3, i can take the values in the range [3, 146] except {9, 21, 33, 45, 57, 69, 81, 93,105, 117, 129, 141}. Note that the first punctured bit will be the fifteenth bit. Withthis coding scheme, the transmission rate can reach 13.4 kbit/s. Note that withCS2, the puncturing pattern must be adapted to the TRAU frame format in orderto be used via the Abis interface. An example of adaptation is the puncturing ofmore bits to reserve more space for the RLC signaling.

CS-2 offers higher transmission rates than the CS-1 but is more strongly dependentof the radio interface quality.

CS-3: With CS-3 the punctured bits have numbers 6 * i + 3 and 6 * i + 5, with ivarying in the range [2, 111], and the transmission rate reaches 15.6 kbit/s.

CS-4: CS-4 has no redundancy and does not perform any forward error correction.With CS4 the data rate reaches 21.4kbit/s and by bundling up to 8 packet datachannels of one carrier into one mobile terminal, transmission rates up to 171.2kbit/s becomes possible.

CS-3 and CS-4 offer higher transmission rates, than those of CS-1 and CS-2, butthese rates are highly dependent of the radio interface quality and decreasesquickly when the C/I increases.




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Channel Coding: Coding Schemes

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

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

Different

Redundancy

(FEC)

Quality Um

* Radio Block without

Uplink State Flag USF &

Block Check Sequence BCS

21.404564281CS-4

15.6220676312≈ 3 /4CS-3

13.4132588268≈ 2 / 3CS-2

9.0504561811 / 2CS-1

Data

Rate

kbit/s

Punctured

Bits

Coded

Bits

Radio

Block*

Code

Rate

Coding

Scheme

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

In the first implementations of GPRS networks, only CS-1 and CS-2 will be used.This is due to the fact that the implementation of CS-3 and CS-4 would require thereservation of higher transport resources via the Abis; which is not possiblebecause the Abis capacity is constant (16 kbit/s). Thus serious modifications of the

existing network architecture become mandatory.




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Fig. 7 Comparison of the efficiency of the four coding schemes under realisticcircumstances of the air interface

Channel Coding

• Introduct ion: CS-1 (9,05 kbit/s & CS-2 (13,4 kbit/s)• CS-1: basic coding fo r RLC/MAC data & control blocks• no CS-3 (15,6 kbit /s), CS-4 (21,4 kbit /s)

→

Ab is limi tat ion (cur rent TRAU frames: 16 kb it /s)

CS 1 - 4: Bit Rate Comparison

0

2

4

6

8

10

12

14

16

18

20

Carrier / Interference C/I (dB)

18 17 16 15 14 13 12 11 10 9 8 7 6 5

CS-1

CS-2

CS-3

CS-4

Net Throughput (kbit/s)




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1.5 Logical GPRS Radio Channels

Use of "classical" logical channels for GSM-CS

In GSM, the radio channels are based on a TDMA structure that is implemented onmultiple frequency sub-ands (TDMA/FDMA). Each base station is equipped witha certain number of these pr-assigned frequency/time channels.CEPT has made available two frequency bands be used by the GSM system. Theseare: 890-915 MHz for the direction mobile to base station, and 935-960 MHz f or thedirection base station to mobile terminal. These bands are divided into 124 pairs ofcarriers spaced by 200 kHz, starting with the pair 890.2 MHz.

The assigned spectrum of 200 kHz per channel is segmented in time by using afixed allocation, time-division multiple access (TDMA) scheme. The time axis isdivided into eight time slots of length 0.577 ms. The slots numbered from timeslot0 to 7 form a frame with length 4.615 ms. The recurrence of one particular time slotin each frame makes up one physical channel.

GSM defines a variety of traffic and signaling/control channels of different bitrates. These channels are assigned to logical channels derived from multiframestructuring of the basic eight slotted TDMA frames just discussed. For this

purpose, two multiframe structures have been defined: one consisting of 26 timeframes (resulting in a recurrence interval of 120 ms), and one comprising 51 timeframes (or 236 ms). The 26 multiframe is used t o define traffic channels (TCH),and their slow and fast associated control channels (SACCH and FACCH) thatcarry link control information between the mobile and the base stations. The TCHhave been defined to provide six different forms of services, that is, full-ratespeech or data channels supporting effective bit rates of 13 kb/s (for speech), 2.4,4.8, and 9.6 kb/s; and the half-rate channels with effective bit-rates of 6.5 (forspeech) and kb/s, 2.4 kb/s, and 4.8 kb/s for data (note that the gross bit rates onthese channels are higher due to required channel coding, 22. 8 k b/ s f or full-rate

speech). The full-rat e TCHs are implemented on 24 frames of the multiframe, witheach TCH occupying one time slot from each frame. The SACCH is implementedon frame 12 (numbered from 0), providing eight SACCH channels, one dedicatedto each of the eight TCH channels. Frame 25 in the multiframe is currently idle andreserved to implement the additional eight SACCH required when half-ratespeech channels become a reality. The FACCH is obtained on demand by stealingfrom the TCH, and is used by either end for signaling the transfer characteristics ofthe physical path, or other purposes such as connection handover control




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messages. The stealing of a TCH slot for FACCH signaling is indicated through a

flag within the TCH slot.The 51-frame multiframe has a more complex structure and we will refer thereader to GSM Recommendation 05.0 for the specific positions of the variouslogical channels in the multiframe.The 51-frame structure, however, is used to derive the following signaling andcontrol channels.SDCCH — Stand-alone dedicated control channel is used for the transfer of callcontrol signaling to and from the mobile during call setup. Like the TCHs, theSDCCH has its own SACCH and is released once call setup is complete.BCCH — Broadcast control channel is used in the BSS to mobile direction to

broadcast system information such as the synchronization parameters, availableservices, and cell ID. This channel is continuously active, with dummy burstssubstituted when there is no information to transmit, because its signal strengthsare monitored by mobiles for handover determination.SCH — Synchronization channel carries information from the BSS for framesynchronization. FCCH — Frequency control channel carries information from the BSS for carriersynchronization.CCCH — Common control channels are used for transferring signalinginformation between all mobiles and the BSS for call origination and call-pagingfunctions. There are three common con-trol channels:

• PCH: paging channel used to call (page) a mobile from the system.• RACH: random access channel used by the mobiles trying to access the

system. The mobile phone uses this channel for requesting a DCCH from thesystem at call initiation.

• AGCH: access grant channel used by the system to assign resources to amobile such as a DCCH channel.

Note that the AGCH and the PCH are never used by a mobile at the same time,and therefore are implemented on the same logical channel. All the controlsignaling channels, except the SDCCH, are implemented on time slot 0 in different

TDMA frames of the 51 multiframes using a dedicated RF carrier frequencyassigned on a per cell basis.

The multiframe structure for the SDCCH and its associated slow associated controlchannel (SACC) is implemented on one of the physical channels (TDM slots andRF carriers) selected by the system operator.

GPRS was conceived to work jointly with GSM, thus the GPRS subscribers willhave to share the air interface with the circuit switched users. But the differences




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between the GSM and GPRS protocol structures will require a separation of the

user and signaling traffic. In reality, and as it will be seen later, only a part of thesignaling information will be separated.

During the access step and before separation of traffic flows, the different types ofusers have to be handled by signaling procedures. Two solutions were proposed torealize this:

- The first one consists in using some of the GSM logical channels. The GPRSmobile terminal has to detect the BCCH of the cell where it is located andget the system information. It will then verify the availability of GPRS. If

this is a cell belonging to the same routing area the mobile terminal canchoose this cell and wait for paging messages or use the RACH to activate aPDP. This RACH indication an access request will be treated by the PCU,which will assign resources for the packet switched traffic in form oftimeslots reserved for the GPRS service. The Signaling steps and all relatedprocedures such as the authentication will be performed using the resourcesspecified in an AGCH message. As you can see, GPRS will share someresources with GSM, this can be advantageous if enough resources exist butif in the future GPRS will ask for more resources, separate resources have tobe reserved to it (this is the second solution).

- The second one consists in reserving separate logical channels for the GPRSusers. The GPRS mobile terminal will continue to look first to the BCCH ofthe cell to verify the availability of GPRS. In case it will detect that specificresources are reserved for the GPRS users it will try to find the time slotover which is carried the PBCCH (Packet Broadcast Control Channel).




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Logical Channel

(for GSM Circuit Switched)

Signaling

BCHBroadcast

channel

CCCH

CommonControl

Channel

DCCH

Dedicated

Control

Channel

Traffic

User Data

User traffic (Full Rate)

User traffic (Half Rate)

DL

BCCH:Broadcast Control Channel

FCCH:Frequency Correction Channel

SCH:Synchronisation Channel

CGI, FR/EFR/HR, GPRS availablefrequency hopping, channel combination,...)

frequency synchronisation,...)

Time synchronisation + BSIC, TDMA-No.

DL

UL

PCH:Paging Channel

AGCH: Access Grant Channel

NCH:Notification Channel

RACH:Random Access Channel

Paging / Searching (MTC)

Allocation of dedicated signalling channel

Notifying MSs

Dedicated signaling MS ↔ BTSE (CallSetup, LUP, Security, SMS, CBCH,...)

DL

ULSDCCH:Stand Alone DedicatedControl Channel

SACCH:Slow AssociatedControl Channel

FACCH:Fast Associated

Control Channel

Measurement Report,TA, PC, c ell parameters,...

Signaling instead of TCH

Request for access

UL

DL TCH/H:Traffic Channel/H

TCH/F:Traffic Channe/Fl

+

+

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 existing GSM signaling and user circuit switched trafficchannels, the GPRS standard defines a specific new set of logical channels.

These logical channels are named Packet Data Channels (PDCH) and include

The packet traffic is carried over the Packet Traffic Channels (PTCH). Thesechannels include:

- Packet Data Traffic Channel PDTCH: this channel carries the point topoint and point to multipoint user data, and GPRS mobility managementand session management information.

- Packet Associated Control Channel PACCH: this channel serves to carrylow level signaling information, such as resource allocation and exchange ofpower control information, to the mobile terminal.

- Packet Timing advance Control Channel PTCCH

Similarly to the GSM standard, new GPRS signaling channels have been defined inaddition to the packet traffic channels. These channels are named Packet CommonControl Channels and include a set of logical channels, which are used forcommon 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: this channel is used for the specificpurpose of initiating a point-to-multipoint multicast connection.

Note that the functions performed by PRACH and PAGCH are very similar tothose of the logical channels RACH and AGCH for non-GPRS-users.

The system information will be transmitted over specific channel named Packet

Broadcast Control Channel PBCCH similar to the GSM BCCH channel.

Note that like in GSM networks, all types of logical channels can be combined in aphysical channel and no separation between traffic and signaling channels is done.

Note that the MAC function responsible for the distribution of physical channels tothe various mobile terminals and the allocation of a radio resource to an MS cancontinue using the classical GSM channels.Note also that the differentiation between logical channels and their contents isperformed by radio block based on the MAC header content.




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Logical Channel

for GPRS

PacketSignaling

BCH

Broadcastchannel

CCCH

Common

Control

Channel

DCCH

Dedicated

Control

Channel

Traffic

Packet

Transmission of User data

DL PBCCH:PacketBroadcast Control ChannelPacket SystemInformation

DL

UL

PPCH: Packet Paging Channel

PAGCH: Packet Access Grant Channel

PNCH: Packet Notification Channel

PRACH:Random Access Channel

Paging GPRS-MS

)PtP)

Resource allocation

Paging GPRS-MS

)(PtM

Dedicated signaling MS↔ BTSE

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

DL

UL &

DL

PACCH:Packet Associated

Control Channel

PTCCH/D

Timing advance

Determination and Control

Signaling instead of TCH

Access request for

UL packet data transmission

ULPTCCH/U:Packet Timing Advance ControlChannel Uplink/Downlink

UL &

DL

PDTCH:Packet Data Traffic Channel

Fig. 9 New logical channels for GPRS




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1.6 Multiframes in GPRS

The GSM Recommendation 03.64 stipulates that the GPRS packet data traffic hasto be arranged in multiframes including each 52 TDMA frames and forming GPRStraffic channel multiframes. Each 4 consecutives TDMA frames constitute a radioblock (4 normal bursts, which are related to each other by means of convolutionalcoding) and each 12 blocks (B0-B11) constitute a 52-type multiframes.

The thirteenth TDMA frame is always idle and serves to send the traffic carriedover the PTACCH. This traffic includes information concerning the base stationidentity codes (BSIC), timing advance, update procedures and interference

measurement needed for the realization of power control.

The GPRS standard specifies that for carrying signaling traffic over packetcommon control channels (PCCH) two types of multiframes can be used: theconventional multiframes containing 51 TDMA frames or the 52-type. The GPRSusers can use the classical GSM common control channels before they will bedirected onto their PTCHs and in all the cases they will read the BCCH. Thislogical channel will serve to the indication of the availability of the GPRS serviceand if extra logical channels (PBCCH, PPCH, ...) are used, in addition the classicaltasks performed in the GSM networks.

Note that the separation between the GSM circuit switching traffic and the GPRSpacket switching traffic ensures that no conflict will happen due to a difference inthe signaling or multiframe structure.Note also that for the users using only GSM services no changes will be remarkeddue to the GPRS introduction, in fact the GSM circuit switching traffic willcontinue to use 26 multiframe structure for TCH and the 51 multiframe structurefor signaling.




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New multiframe for GPRS

• PDCH follows 52 multiframe structure

• 52 Multiframe: 12 Blocks à 4 TDMA-frames

• PCCCHs: „ classical“ 51er Multiframes

or 52er Multiframes

52 TDMA Frames = PDCH Mult iframe

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

4 Frames 1 Frame

B0 - B11 = Radio Blocks (Data / Signaling)

i = Idle frame (PTCCH)

• BCCH indicates PDCH with PBCCH (in B0)

• DL: this PDCH bears PDCCH & PBCCH

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

PBCCH indicates PCCCH blocks & further PDCHs

with PCCCH• UL: PDCH with PCCCH: all blocks to be used for

PRACH, PDTCH, PACCH

PDCH without PCCCH: PDTCH & PACCH only

Idle frame:

• Identification of BSICs

• Timing Advance Update

Procedure

• Interference measurements

for Power Control

Fig. 9 New logical channels for GPRS

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