In GSM system MS sends its data three time slots after it received the data from the BTS.

In GSM system MS sends its data three time slots after it received the data from the BTS.  This is ok as long as MS-BTS distance is small but increasing distance requires consideration of propagation delay as well. To handle it Timing advance (TA) is conveyed by network to MS and current value is sent to the MS within the layer 1 header of each SACCH. BTS calculates the first TA when it receives RACH and reports it to the BSC and BSC/BTS passes it to UE during Immediate Assignment. 

In UMTS Timing Advance parameter was not used but in LTE Timing Advance is back. 

In LTE, when UE wish 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

• when a Timing Advance Command MAC control element is received  then UE applies the Timing Advance Command and start or restart timeAlignmentTimer.
• when a Timing Advance Command is received in a Random Access Response message then one of following action is performed by UE

– 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.

• when timeAlignmentTimer expires UE flushes all HARQ buffers, notifies RRC to release PUCCH/SRS and clears any configured downlink assignments and uplink grants.

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.

• R: reserved bit, set to “0”
• Timing Advance Command: This field indicates the index value TA (0, 1, 2… 63) used to control the amount of timing adjustment that UE has to. The length of the field is 6 bits.

Source: 3GPP specifications (36.133 & 36.321)