TIMING ADVANCE
* Timing advance is used to control the uplink transmission timing of individual UE. It is applicable to the PUSCH, PUCCH andSounding Reference Signal. The PRACH does not use timing advance because the UE transmits it before the network is able to provide any timing instructions
* Timing advance helps to ensure that transmissions from all UE are synchronised when received by the eN ode B
* The general concept of timing advance is shown in Figure 209. This figure illustrates 2 UE. The UE furthest from the eNode B requires a larger timing advance to compensate for the larger propagation delay.
* Figure 210 illustrates the radio frame timing for the 2 UE shown in Figure 209. The downlink radio frame arrives at UE 1 relatively late as a result of the larger propagation delay. This figure illustrates that the timing advance equals x propagation delay.
* UE are first provided with timing advance information during the Random Access procedure used to make the transition from RRC Idle mode to RRC Connected mode. An 11 bit timing advance command is included within the Random Access Response
* These 11 bits are used to signal a value between 0 and 1282. The variable NrA =signalled value x 16, which leads to a maximum value of 1282 x 16 = 20512
* The actual timing advance applied by the UE is given by:
o FDD: timing advance= NrA x Ts
o TDD: timing advance= CNrA + 624) x Ts
where the value ofTs is given by 1 I 30720 ms
* The TDD case includes a fixed additional624 Ts which is equivalent to 20 ~J.S . This additional offset allows time for the eNode B to switch from receiving to transmitting. This additional guard period is illustrated in Figure 17 within section 3.2.2. The timing advance instructed by the eN ode B corresponds to NrA so the UE adds this additional offset of 624 T s when defining its transmission timing
* The maximum value ofNrA corresponds to 0.6677 ms when multiplied by Ts. Based upon the speed of light (3 x 108 ms.1), this corresponds to a round trip distance of 200 km, and a cell range of 100 km.
* After the Random Access procedure, timing advance commands are provided using the Timing Advance MAC Control Element which can be included as part ofthe MAC header. The Timing Advance MAC Control Element includes a 6 bit timing advance command which provides a range from 0 to 63. This control element is illustrated in Figure 211.
* The signalled value of the timing advance command within the MAC Control Element corresponds to TA within the equation:
NTAnew=NTAold+(TA-31)x 16
Subtracting 31 from the value ofT A allows the eN ode B to shift the timing advance in both positive and negative directions, i.e. thetiming advance command provided during the Random Access procedure is an absolute timing advance, whereas the subsequent timing advance commands provided within the MAC Control Elements are relative and defme changes to the existing timing advance
* The value of NT Anew is applied within the FDD and TDD timing advance equations shown earlier in this section
* Timing advance commands received during downlink subframe 'n' are applied to uplink subframe 'n+6'
* When a timing advance command causes subframe 'm+ 1' to overlap with subframe 'm' the UE transmits all of subframe 'm' but does
not transmit the overlapping part of subframe 'm+ 1'
* 3GPP References: TS 36.211, TS 36.213, TS 36.321
* Timing advance is used to control the uplink transmission timing of individual UE. It is applicable to the PUSCH, PUCCH andSounding Reference Signal. The PRACH does not use timing advance because the UE transmits it before the network is able to provide any timing instructions
* Timing advance helps to ensure that transmissions from all UE are synchronised when received by the eN ode B
* The general concept of timing advance is shown in Figure 209. This figure illustrates 2 UE. The UE furthest from the eNode B requires a larger timing advance to compensate for the larger propagation delay.
* Figure 210 illustrates the radio frame timing for the 2 UE shown in Figure 209. The downlink radio frame arrives at UE 1 relatively late as a result of the larger propagation delay. This figure illustrates that the timing advance equals x propagation delay.
* UE are first provided with timing advance information during the Random Access procedure used to make the transition from RRC Idle mode to RRC Connected mode. An 11 bit timing advance command is included within the Random Access Response
* These 11 bits are used to signal a value between 0 and 1282. The variable NrA =signalled value x 16, which leads to a maximum value of 1282 x 16 = 20512
* The actual timing advance applied by the UE is given by:
o FDD: timing advance= NrA x Ts
o TDD: timing advance= CNrA + 624) x Ts
where the value ofTs is given by 1 I 30720 ms
* The TDD case includes a fixed additional624 Ts which is equivalent to 20 ~J.S . This additional offset allows time for the eNode B to switch from receiving to transmitting. This additional guard period is illustrated in Figure 17 within section 3.2.2. The timing advance instructed by the eN ode B corresponds to NrA so the UE adds this additional offset of 624 T s when defining its transmission timing
* The maximum value ofNrA corresponds to 0.6677 ms when multiplied by Ts. Based upon the speed of light (3 x 108 ms.1), this corresponds to a round trip distance of 200 km, and a cell range of 100 km.
* After the Random Access procedure, timing advance commands are provided using the Timing Advance MAC Control Element which can be included as part ofthe MAC header. The Timing Advance MAC Control Element includes a 6 bit timing advance command which provides a range from 0 to 63. This control element is illustrated in Figure 211.
* The signalled value of the timing advance command within the MAC Control Element corresponds to TA within the equation:
NTAnew=NTAold+(TA-31)x 16
Subtracting 31 from the value ofT A allows the eN ode B to shift the timing advance in both positive and negative directions, i.e. thetiming advance command provided during the Random Access procedure is an absolute timing advance, whereas the subsequent timing advance commands provided within the MAC Control Elements are relative and defme changes to the existing timing advance
* The value of NT Anew is applied within the FDD and TDD timing advance equations shown earlier in this section
* Timing advance commands received during downlink subframe 'n' are applied to uplink subframe 'n+6'
* When a timing advance command causes subframe 'm+ 1' to overlap with subframe 'm' the UE transmits all of subframe 'm' but does
not transmit the overlapping part of subframe 'm+ 1'
* 3GPP References: TS 36.211, TS 36.213, TS 36.321
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