Timing Advance (TA) in LTE

In LTE, when a UE wants to establish RRC connection with eNB, it transmits a Random Access Preamble, eNB estimates the transmission timing of the terminal based on this. Now eNB transmits a Random Access Response which consists of timing advance command, based on that UE adjusts the terminal transmit timing.

The timing advance is initiated from E-UTRAN with MAC message that implies and adjustment of the timing advance.

3GPP TA Requirements

• Timing Advance adjustment delay
UE shall adjust the timing of its uplink transmission timing at sub-frame n+6 for a timing advancement command received in sub-frame n.
• Timing Advance adjustment accuracy 
The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal to ±4* TS seconds to the signalled timing advance value compared to the timing of preceding uplink transmission. The timing advance command is expressed in multiples of 16* TS and is relative to the current uplink timing.

Maintenance of Uplink Time Alignment

The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink time aligned

- if the Random Access Preamble was not selected by UE MAC then UE applies the Timing Advance Command and starts or restarts timeAlignmentTimer.

- else if the timeAlignmentTimer is not running then UE applies the Timing Advance Command starts timeAlignmentTimer; when the contention resolution is considered not successful  then UE stops timeAlignmentTimer.

- else ignore the received Timing Advance Command.

Timing Advance Command MAC Control Element

The Timing Advance Command MAC control element is identified by MAC PDU subheader with LCID value =  11101 (Timing Advance Command) .

It has a fixed size and it consists of a single octet as show below.

Timing Advance Command MAC control element has following fields.

How to calculate TA in LTE?

The eNB measures the required timing advance based on the received UE signal arrival time. It commands the UE to adjust the transmission time. This is performed on a per-need basis.

It is signaled by means of a special MAC control element; LCID = 11101. The signaled granularity is 16 Ts. The value range for adjustment is 8 bit and related to the current UL timing.

As Ts. = 1 / (2048 x 15000) sec = 1 / 30720000 sec the granularity is given by 0.52 μsec (corresponding to 78 m).

If UE is in-sync the timing advance is valid. Otherwise the RACH-procedure establishes a valid timing advance. An 11 bit value (range 0,1..1282) is signaled to establish the initial offset.

Source: 3GPP specifications (36.133 & 36.321)