Tuesday 17 December 2013

All About Timing Advance (TA)

What is Timing Advance?

- UL Transmission in LTE is Not Synchronized.

- Used to control the Uplink Timing of Individual UE.
- Ensure that transmissions from All UE are Synchronized when received by the eNodeB.
- UE furthest from the eNodeB requires a larger Timing Advance to compensate for the Larger Propagation Delay.
- The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink
time aligned
timeAlignmentTimerCommon(Common For All UEs In a Cell) included in SIB2.
timeAlignmentTimerDedicated (UE specific value for Time Alignment Timer) is included in the RRC Connection Reconfiguration Message.

Timing Advance = 2 * Propagation Delay.

Timing Advance = N-TA * TS


0 < N-TA <=20152
TS = 1/30720 ms

So Maximum Timing Advance = 20512 * 1/30720 = 0.6677 ms.

Based on the speed of light this allows a maximum propagation distance of 100 km.

Timing Advance is initialized in RAR command using 11 bit TA command. 

Timing Advance in RAR takes a value from 0 - 1282

According to Spec 36.321:

N-TA = Signaled Value (TA Command) * 16

            (0 - 1282)

Once TA is initialized in the Random Access Response UE gets TA command from eNodeB using TA MAC Control Element.

TA command in MAC Control Element is of 6 bit length. Takes a value of 0 - 63.

According to Spec 36.321:

Timing Advance calculated from TA value received from TA MAC Control Element :

N-TA-New = N-TA-OLD + (TA -31) * 16

Subtracting 31 from the TA command received in MAC Control Element Allows eNodeB to move Timing Advance in Both in Positive and Negative direction.

Timing Advance Command Received in the Nth Subframe Applied to (N+6)th subframe.

The UE shall not perform any uplink transmission except the Random Access Preamble transmission when TATimer is not running.

When due to timing advance (X+1) subframe overlaps with subframe X, UE should transmit all subframes till X subframe and do not transmit overlapping part of subframe (X+1)

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