Sunday, 4 October 2015

LTE FDD Features-The Understanding of Timing Advance(TA)

 Incident Description (Incident Phenomena)
Timing Advance(TA) is related with the UE distance from the eNB. This FAQ provide the understanding of the timing advance.
 Solution
1. Timing Advance Command
A device receives a timing advance command from the network and adjusts uplink transmission timing for PUCCH / PUSCH / SRS
 
2.  Frame structure

Basic time unit : Ts = 1 / (15000 x 2048) seconds
Radio frame duration : Tf = (150 x 2048) x Ts = 307200 x Ts = 10 ms
NTA = timing offset between uplink and downlink radio frames at the UE, expressed in units of Ts

3.  MAC Timing Advance Command

Distance between a device and the network antenna :
A timing advance of one Ts is equivalent to a distance of (3 x 108 x 1 / (15000 x 2048)) / 2 = 4.89 m
Distance = propagation speed (light speed) x one Ts delay / 2 (downlink path + uplink path)

4.  Transmission timing adjustments

Initial absolute Random Access Response MAC payload (11 bits) TA range = 0, 1, 2,..., 1282
//
that is RAR massage
Random Access Response NTA = TA x 16. Timing adjustment is always positive
Example : TA = 1 then NTA = 1 x 16 Ts. Distance equals 16 x 4.89 m = 78.12 m
Distance maximum between a device and the network antenna = 1282 x 78.12 m = 100.156 km (62.23 mi)  
There are similar TA in GSM, CDMAand WCDMA
GSM_TA: 550m
WCDMA_PD: 234m;
CDMA_chip: 244m

Periodic relative Timing Advance Command MAC Control Element (6 bits) TA range = 0, 1, 2,..., 63

Timing Advance Command NTA,new = NTA,old + (TA - 31) x 16. Timing adjustment can be negative, null or positive
//included in SIB2
Example : TA = 30 then NTA,new = NTA,old + (30 - 31) x 16 Ts = NTA,old - 1 x 16 Ts. Distance variation equals -1 x 16 x 4.89 m = - 78.12 m
Distance variation maximum = - 31 x 16 x 4.89 m = - 2.42 km or + 32 x 16 x 4.89 m = + 2.5 km (1.55 mi)
//
TA value range from 0 to 63, so distance variation range from -31TA ~ 32TA, that is -2.42~2.5 km

Budi Prasetyo

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