6.5.7 Timing Advance

03.643GPPGeneral Packet Radio Service (GPRS)Overall description of the GPRS radio interfaceStage 2TS

NOTE: The text in this subclause is informative. The normative text is in GSM 04.60 [7] and GSM 05.10 [16]. Where there is a conflict between these descriptions, the normative text has precedence.

The timing advance procedure is used to derive the correct value for timing advance that the MS has to use for the uplink transmission of radio blocks. This procedure is not used in dual transfer mode, in which case the timing advance procedures for dedicated mode are used.

The timing advance procedure comprises two parts:

– initial timing advance estimation;

– continuous timing advance update.

6.5.7.1 Initial timing advance estimation

The initial timing advance estimation is based on the single access burst carrying the Packet Channel Request. The Packet Uplink Assignment or Packet Downlink Assignment then carries the estimated timing advance value to the MS. This value shall be used by the MS for the uplink transmissions until the continuous timing advance update provides a new value (see subclause 6.5.7.2.). Two special cases exist:

– when Packet Queuing Notification is used the initial estimated timing advance may become too old to be sent in the Packet Downlink (/Uplink) Assignment

– when Packet Downlink (/Uplink) Assignment is to be sent without prior paging (i.e., in the Ready state), no valid timing advance value may be available.

Then the network has three options:

– Packet Polling Request can then be used to trigger the transmission of Packet Control Acknowledgement. This message can be formatted as four access burst from which the timing advance can be estimated.

– Packet Downlink (/Uplink) Assignment can be sent without timing advance information. In that case it is indicated to the MS that it can only start the uplink transmission after the timing advance is obtained by the continuous timing advance update procedure.

– The poll bit in the Packet Downlink (/Uplink) Assignment message can be set to trigger the transmission of Packet Control Acknowledgement. This can be used if System information indicates that acknowledgement is access bursts.

For the case where timing advance information is not provided in the assignment message, the mobile is not allowed to send normal bursts on the uplink until it receives a valid timing advance either in Packet Timing Advance/Power Control message or through the continuous timing advance procedure.

6.5.7.2 Continuous timing advance update

MS in Packet transfer mode shall use the continuous timing advance update procedure. The continuous timing advance update procedure is carried on the PTCCH allocated to the MS.

For uplink packet transfer, within the Packet Uplink Assignment, the MS is assigned Timing Advance Index (TAI) and the PTCCH.

For downlink packet transfer, within the Packet Downlink Assignment, the MS is assigned Timing Advance Index (TAI) and the PTCCH.

The TAI specifies the PTCCH sub-channel used by the MS.

On the uplink, the MS shall send in the assigned PTCCH access burst, which is used by the network to derive the timing advance.

The network analyses the received access burst and determines new timing advance values for all MSs performing the continuous timing advance update procedure on that PDCH. The new timing advance values shall be sent via a downlink signalling message (TA-message) on PTCCH/D. Network can send timing advance information also in Packet Timing Advance/Power Control and Packet Uplink Ack/Nack messages on PACCH.

6.5.7.2.1 Mapping on the multiframe structure

Figure 19 shows the mapping of the uplink access bursts and downlink TA-messages on groups of eight 52-multiframes:

– the TAI value shows the position where a slot is reserved for a MS to send an access burst (e.g. T1 means 52-multiframe number n and idle slot number 2). TAI value defines the used PTCCH sub-channel.

– every second PDCH multiframe starts a downlink TA-message.

52-multiframe number n:

uplink TAI=0 TAI=1

B0

B1

B2

0

B3

B4

B5

1

B6

B7

B8

2

B9

B10

B11

3

downlink TA_message 1 TA message 1

52-multiframe number n + 1:

uplink TAI=2 TAI=3

B0

B1

B2

4

B3

B4

B5

5

B6

B7

B8

6

B9

B10

B11

7

downlink TA message 1 TA message 1

52-multiframe number n + 2:

uplink TAI=4 TAI=5

B0

B1

B2

8

B3

B4

B5

9

B6

B7

B8

10

B9

B10

B11

11

downlink TA message 2 TA message 2

52-multiframe number n + 3:

uplink TAI=6 TAI=7

B0

B1

B2

12

B3

B4

B5

13

B6

B7

B8

14

B9

B10

B11

15

downlink TA message 2 TA message 2

52-multiframe number n + 4:

uplink TAI=8 TAI=9

B0

B1

B2

16

B3

B4

B5

17

B6

B7

B8

18

B9

B10

B11

19

downlink TA message 3 TA message 3

52-multiframe number n + 5:

uplink TAI=10 TAI=11

B0

B1

B2

20

B3

B4

B5

21

B6

B7

B8

22

B9

B10

B11

23

downlink TA message 3 TA message 3

52-multiframe number n + 6:

uplink TAI=12 TAI=13

B0

B1

B2

24

B3

B4

B5

25

B6

B7

B8

26

B9

B10

B11

27

downlink TA message 4 TA message 4

52-multiframe number n + 7:

uplink TAI=14 TAI=15

B0

B1

B2

28

B3

B4

B5

29

B6

B7

B8

30

B9

B10

B11

31

downlink TA message 4 TA message 4

B0 – B11 = Radio blocks

Idle frames are numbered from 1 to 31 [odd numbers]

PTCCH frames are numbered from 0 to 30 [even numbers]

Figure 19: Mapping of the uplink access bursts and downlink timing advance signalling messages

The BTS shall update the timing advance values in the next TA-message following the access burst. To illustrate this, an MS that transmits an access burst in frames numbered 0, 2, 4, or 6 receives its updated timing advance value in TA message 2. This MS can also find this updated timing advance value in subsequent TA messages 3, 4, and 1, but only has to read these if TA message 2 was not received correctly.

An MS entering the Transfer state shall ignore the TA-messages until the MS has sent its first access burst. This is to avoid the use of timing advance values, derived from access bursts sent by the MS that previously used the same TAI.