LTE: Semi-Persistent Scheduling

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 N^th grant occurs in the subframe for which the below condition satisfies:

(10 * SFN + subframe) = [(10 * SFN_{start time} + subframe_{start time}) + N * semiPersistSchedIntervalUL] modulo 10240

Where SFN_{start time} and subframe_{start 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 N^th assignment occurs in the subframe for which the below condition satisfies:

(10 * SFN + subframe) = [(10 * SFN_{start time} + subframe_{start time}) + N * semiPersistSchedIntervalDL] modulo 10240

Where SFN_{start time} and subframe_{start 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 n_CCE. 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-Interval           activation-SFN           activation-Subframe          UL-DL Config
10 20 32 40 64 80 128 160 320 640                                                         0 1 2 3 4 5 6

uplink SPS?            downlink SPS?             Is TDD?             twoIntervalConfig?

SPS Occasions will be displayed here