LTE: Multiple Timing Advances for uplink Carrier Aggregation

The Timing Advance related concepts are discussed very much in detail in the post Timing Advance and Time Alignment Timer.

For a UE configured with multiple serving cells in Release-10, the same uplink transmission timing is applied in all serving cells, based on the timing advance on the PCell. This means that base station transceivers of different CCs should be at the same physical location (collocated) to avoid different propagation delays.

In a heterogeneous network (HetNet) for example (as shown below), PCell and a remote radio head (RRH) which are located at different locations (non-collocated) may experience different propagation delays. So, the use of single TA is not practical.

From Release-11 onwards, it is possible to handle CA with CCs requiring different timing advances, for example combining CC from eNodeB with CC from RRH (as shown above) to support non-collocated cells.

Also, support of different uplink transmission timings on different serving cells address the deployment scenario where the propagation delays are different on different serving cells due to e.g. frequency selective repeaters.

It is not practical to maintain TA for each serving cell; instead, it would make sense to group a set of collocated serving cells, so that, the same TA is maintained across all the serving cells belonging to that group. Also, it is very important to have a timing reference cell for the entire group.

In Release-11, Timing Advance Group (TAG) was introduced. A TAG consists of one or more serving cells with the same uplink TA and same downlink timing reference cell. Each TAG contains at least one serving cell with configured uplink, and the mapping of each serving cell to a TAG is configured by RRC.

The TAG containing PCell is called as pTAG (Primary Timing Advance Group). For the pTAG, the UE uses PCell as timing reference.

If a TAG contains only SCells(s), and no PCell, then it is called as sTAG (Secondary Timing Advance Group). In a sTAG, the UE may use any of the activated SCells of this TAG as a timing reference cell, but should not change it unless necessary.

The UE has a configurable timer called timeAlignmentTimer per TAG. This TAG specific timeAlignmentTimer is provided by RRC at the time of sTAG configuration.

E-UTRAN adds or releases sTAG with the help of stag-ToAddModList-r11 and stag-ToReleaseList-r11 respectively and are part of mac-MainConfig in Release-11. 

Configuration of an sTAG includes stag-Id which indicates the TAG of an SCell and a TAG specific timeAlignmentTimer (timeAlignmentTimerSTAG-r11) as shown below.

At the time of SCell addition, the E-UTRAN may optionally indicate the STAG identity for the corresponding SCell.  MAC-MainConfigSCell-r11 is introduced for this purpose and as shown below it only contains STAG-id.

If the field stag-Id is not configured for an SCell (e.g. absent in MACMainConfigSCell), then the SCell is considered to be part of the pTAG.

The number of TAGs that can be configured depends on the TAG capability of the UE.

The UE indicates its support of multiple timing advances using multipleTimingAdvance-r11 under BandCombinationParameters during UE capability transfer procedure.

Initial Uplink Timing Alignment for a sTAG

The initial timing alignment on PCell (or pTAG) can be obtained via UE or eNodeB initiated Random Access (RA) procedure. But the initial UL timing alignment of sTAG is obtained only by an eNodeB initiated RA procedure.

As shown below, the SCell in a sTAG can be configured with RACH resources at the time of SCell addition. These parameters are part of UL configuration under RadioResourceConfigCommonSCell-r10.

In order to establish timing advance for a sTAG, the eNodB may initiate a non-contention based random access (RA) procedure with a PDCCH order that is sent on a scheduling cell of an activated SCell of the sTAG. i.e., the PDCCH order can be received on the same SCell (non-cross carrier scheduling) or on the scheduling cell (cross carrier scheduling with CIF).

It is worth noting that for the pTAG, the PDCCH order reception is and PRACH preamble transmission are only supported on the PCell.

The RA procedure on an SCell shall only be initiated by a PDCCH order which means that UE MAC sublayer cannot initiate RA procedure on SCell in order to obtain TA or for the case of ‘UL data arrival’.

As of Release-11, contention based RA procedure is not supported on SCell.

Upon receiving the PDCCH order, the UE transmits PRACH preamble on the SCell for which the PDCCH order is intended.

The RAR reception takes place on PCell using RA-RNTI in common search space. The grant received in RAR is valid for the SCell on which PRACH preamble was transmitted.

When the UE receives RAR for an SCell, the UE applies the TA Command received in the RAR to the sTAG to which the SCell belongs. As usual, the UE starts or restarts the TimeAlignmentTimer associated with this sTAG.

It is very important to note that the RACH initiation by PDCCH order is only supported for an activated SCell.

When SCell is deactivated, the ongoing Random Access procedure on the SCell, if any, is aborted.

Another difference as compared to the RA procedure on PCell is that, the UE, after transmitting PRACH preamble for maximum number times, shall not indicate RA problem to upper layers and it just considers that the RA procedure was unsuccessful.

Timing Advance Command MAC CE

The timing advance for a TAG (pTAG or sTAG) can also be obtained by means of Timing Advance Command MAC CE. For this purpose, the existing Timing Advance Command MAC CE has been enhanced to signal different TA values for different TAGs.

As shown below, previously reserved values are now modified to indicate a new 2-bit Timing Advance Group Identity (TAG Id). The 6-bit Timing Advance Command field is unchanged compared to Release-8.

Since the TAG Id field is 2-bits, it can only indicate values from 0 to 3. The TAG containing the PCell has TAG Identity 0. So, at most three sTAGs can be configured.

Upon reception of a timing advance command (via RAR or MAC CE) for a TAG, the UE shall adjust uplink transmission timing for PUSCH/SRS for all the serving cells in that TAG. Additionally, if the TAG contains the PCell, then uplink transmission timing for PUCCH on the PCell shall also be adjusted.

Maintenance of Uplink Time Alignment

As explained above, the UE maintains TimeAlignmentTimer per TAG. The TimeAlignmentTimer is used to control how long the UE considers the Serving Cells belonging to the associated TAG to be uplink time aligned.

The UE shall start or restart the TAG associated timeAlignmentTimer when a Timing Advance Command is received in a RAR or MAC CE for the corresponding TAG.

The synchronization status of the UE follows the synchronization status of the pTAG. When the timer associated with pTAG is not running, the timer associated with a sTAG shall not be running.

When the timeAlignmentTimer associated with the pTAG is expired, the UE shall:

-      flush all HARQ buffers for all serving cells belonging to pTAG as well as sTAG;
-      notify RRC to release PUCCH/SRS for all serving cells
-      clear any configured downlink assignments and uplink grants (applicable for PCell only);
-      consider that all the running timeAlignmentTimers (timers for sTAG as well) as expired.

When the TimeAlignmentTimer associated with the sTAG is expired, the UE shall:

-      flush all HARQ buffers for all the serving cells belonging to this sTAG;
-      notify RRC to release SRS for all the serving cells belonging to this sTAG.

The UE shall not perform any uplink transmission on a Serving Cell except the RA Preamble transmission when the TimeAlignmentTimer associated with the TAG to which this Serving Cell belongs is not running.

When the timeAlignmentTimer associated with the pTAG is not running, the UE shall not perform any uplink transmission on any Serving Cell except the RA Preamble transmission (only) on the PCell.

Reference: 3GPP TS 36.300, 36.321, 36.213 and 36.331