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
