In the case of
Dynamic Scheduling, the UE can get scheduling assignments/grants in every
subframe. This gives the network full flexibility in assigning the resources to
the UE at the cost of transmission of resource allocation information on PDCCH
in every subframe. This also gives the flexibility of varying the resource
allocation based on the reported channel conditions
For services such as
VoIP, the packet size is small and the inter-arrival time of VoIP packets is
constant (i.e., AMR codec provides one packet every 20ms during active period and one silence indicator (SID) at 160ms). The control signaling overhead
(PDCCH) is too much for the E-UTRAN in order to support a large number of VoIP
users. So, the optimal solution is to allocate the resources at once and let the
UE use these resources instead of allocating the resources periodically.
How
does the eNB configure the UE with SPS?
The eNB can
configure the UE with SPS at any time but, typically this is done at the time
of dedicated bearer establishment for the VoIP service. SPS can be
configured/re-configured by RRC at any time using SPS-Config. This SPS-Config
includes the configuration for semiPersistSchedC-RNTI
(sps-CRNTI), sps-ConfigDL and sps-ConfigUL.
SPS can be configured
only in the uplink (sps-ConfigUL), or
in the downlink (sps-ConfigDL) or in
both directions. Configuration of SPS doesn’t mean that the UE can start using
SPS grants/assignments. The eNB has to explicitly activate SPS as explained below, in order for the UE to use SPS grants/assignments. So, SPS configuration and activation are two
different things, eNB first configures the UE with SPS and then activates the
same. The eNB can explicitly release SPS without release SPS RRC configuration
SPS
Activation/Release
When configuring SPS in
any direction either UL or DL, SPS C-RNTI
is mandatorily provided by the eNB. Soon after the UE is configured with SPS C-RNTI, the UE is configured by
higher layers to decode PDCCH with CRC scrambled by the SPS C-RNTI.
A UE shall monitor
PDCCH with CRC scrambled by the SPS
C-RNTI in every subframe as the eNB can activate/re-activate/release SPS at
any time using Downlink control information (DCI)
DCI Format 0 is used
to activate/release SPS in UL; DCI Format 1/1A/2/2A/2B/2C is used to activate SPS in
DL. In order to release SPS in DL, only DCI Format 1A can be used. The received DCI format on SPS C-RNTI
can be a grant/assignment for a retransmission or for activation/re-activation/release
of SPS. 3GPP TS 36213 has tabulated the validation procedure for activation/re-activation/release
of SPS. A UE shall validate a SPS assignment PDCCH only if all the following
conditions are met:
· the CRC parity bits obtained for the
PDCCH payload are scrambled with the SPS C-RNTI
· the new data indicator field is set to
‘0’. In case of DCI formats 2, 2A, 2B and 2C, the new data indicator field refers
to the one for the enabled transport block
Validation is achieved if all the
fields for the respective used DCI format are set according to Table 9.2-1 or
Table 9.2-1A. If validation is achieved, the UE shall consider the received DCI
information accordingly as a valid SPS activation or release. If validation is not achieved, the received
DCI format shall be considered by the UE as having been received with a non-matching
CRC
SPS Configuration
details
The following high level IEs are
provided by RRC.
SPS-ConfigDL has the following IEs
(for the FDD mode of operation)
SPS-ConfigUL has the following IEs
(for the FDD mode of operation)
SPS Operation in
Uplink direction
For UL SPS, SPS C-RNTI, semiPersistSchedIntervalUL,
implicitReleaseAfter and optionally p0-Persistent values are configured by
RRC. After configuring SPS in UL direction, the eNB can send DCI Format 0 on SPS C-RNTI to activate the SPS in UL
direction. After receiving activation command, the UE should consider that the uplink grant has
been configured and should consider sequentially that the Nth grant
occurs in the subframe for which the below condition satisfies:
(10 * SFN + subframe) = [(10 * SFNstart
time + subframestart time) + N * semiPersistSchedIntervalUL]
modulo 10240
Where SFNstart time and subframestart
time are the SFN and subframe, respectively, at the time the
configured uplink grant were (re-) initialized.
So, the UE shall use the grant
provided in the SPS activation DCI Format 0 once every semiPersistSchedIntervalUL subframes and shall transmit PUSCH using
this grant. If there is no data to be transmitted, the Multiplexing and
Assembly entity shall provide MAC PDU containing zero MAC SDU to be transmitted
on Semi-Persistent Scheduling resource. The UE shall clear the configured
uplink grant immediately after implicitReleaseAfter
number of consecutive new MAC PDUs each containing zero MAC SDUs, on the Semi-Persistent
Scheduling resource. This is called as implicit release of UL SPS configured
grant. After this the UE shall not use this configured grant for new
transmissions at the occasions satisfying the above equation. Note that, retransmissions
for Semi-Persistent Scheduling can continue after clearing the configured
uplink grant
The eNB can explicitly send DCI Format
0 to indicate SPS release. Upon receiving DCI Format 0 which indicates SPS
release, the UE should clear the configured uplink grant
Optionally, the eNB can configure the
UE with p0-Persistent which is used
to calculate the transmit power of PUSCH using SPS C-RNTI if the eNB wants the
UE to use different power as compared to PUSCH on C-RNTI. Basically, in the
PUSCH Tx power calculation, the UE should use p0-NominalPUSCH-Persistent
and p0-UE-PUSCH-Persistent in place
of p0-NominalPUSCH and p0-UE-PUSCH respectively. If the IE p0-Persistent absent, the UE shall use p0-NominalPUSCH and p0-UE-PUSCH in PUSCH Tx power calculation
SPS Operation in
Downlink direction
For DL SPS, SPS C-RNTI, semiPersistSchedIntervalDL,
numberOfConfSPS-Processes and n1PUCCH-AN-PersistentList values are
configured by RRC. After configuring SPS in DL direction, the eNB can send DCI
Format 1/1A/2/2A/2B/2C on SPS C-RNTI
to activate the SPS in DL direction. From this point on the UE should consider
that the downlink assignment has been configured and should consider sequentially
that the Nth assignment occurs in the subframe for which the below
condition satisfies:
(10 * SFN + subframe) = [(10 * SFNstart
time + subframestart time) + N * semiPersistSchedIntervalDL]
modulo 10240
Where SFNstart time and subframestart
time are the SFN and subframe, respectively, at the time the
configured downlink assignments were (re-) initialised.
The UE shall use this configured
assignment once in every semiPersistSchedIntervalDL
subframes and decode PDSCH (no corresponding PDCCH).
In order to transmit HARQ response
(Ack/Nack) on PUCCH, the UE would need to calculate frequency resources. In the dynamic scheduling case, a PDCCH
(DCI) is present, so the UE calculates Ack/Nack resources based on nCCE. For the SPS case, from the
DCI format indicating DL SPS activation, the TPC
command for PUCCH field shall be used as an index to one of the four PUCCH
resource values (n1PUCCH-AN-PersistentList)
configured by RRC.
If the eNB wants to release configured
assignment without releasing RRC configuration, it would explicitly send DCI
Format 1A to indicate SPS release. Upon receiving DCI Format 1A which indicates
SPS release, the UE should clear the configured downlink assignment
Another important aspect is the HARQ
process ID calculation. In case of dynamic scheduling the corresponding DCI
format indicates the HARQ process ID, whereas in the case of SPS, there is no PDCCH corresponding to PDSCH. The HARQ process ID needs to be calculated based
on the below equation:
HARQ Process ID = [floor (CURRENT_TTI/semiPersistSchedIntervalDL)]
modulo numberOfConfSPS-Processes
where CURRENT_TTI = [(SFN * 10) + subframe
number]. Current TTI is
the TTI in which PDSCH has been received on the configured assignment
Re-transmission
Handling
Re-transmission handling on SPS C-RNTI (UL or DL) is an important
aspect. When using dynamic scheduling, the eNB would use same NDI (New Data
Indicator) as was sent for initial (new) transmission to indicate the grant/assignment received is for a
re-transmission. When using SPS, the grant/assignment for new transmission is
not provided every time as it is configured already. It has been already explained
for SPS activation/release, that NDI is set to 0. If the eNB wants to send a
grant/assignment for a retransmission it would toggle NDI i.e., NDI is always
set to 1 for a re-transmission
Notes:
o When Semi-Persistent Scheduling for uplink or
downlink is disabled by RRC, the corresponding configured grant or configured
assignment shall be discarded
o
Semi-Persistent Scheduling is supported on the
PCell only
o
The UE should clear any configured downlink
assignments and uplink grants when timeAlignmentTimer
expires or during MAC reset or when UE has transmitted the scheduling request
for maximum number of times (dsr-TransMax)
o
Except for handover or releasing SPS, E-UTRAN
does not reconfigure sps-Config (RRC)
when there is a configured downlink assignment or a configured uplink grant
SPS Occasions can be calculated using the below tool!
SPS calculations