Base station uses SRS for following purposes:
· Channel aware packet scheduling and link adaptation
· When channel reciprocity exists, SRS can be used for downlink channel estimation.
· When channel reciprocity does not exists, SRS can be used for uplink beam management.
· Selection of uplink precoding weights for both non-codebook based PUSCH transmission and codebook based PUSCH transmissions.
· Uplink measurements.
UE transmits SRS to base station. Base station measures uplink propagation channel using SRS. These uplink measurements are used for the following:
· Base station will measure uplink propagation channel for each resource block within an active bandwidth part. Packet scheduler will identify the best set of resource blocks to allocate. Uplink SINR can be measured using SRS, this data is used as an input to link adaptation to select the appropriate modulation and coding scheme.
· In case of TDD deployments if there is a channel reciprocity, these measurements can be used to estimate downlink propagation channel. To measure downlink channel sounding there are two cases:
- Number of transmit paths equal to the number of receive antennas: In this case UE will transmit SRS from each receive antenna. Base station can deduce downlink propagation channel towards each antenna.
- Number of transmit paths at UE less than the number of receive antennas: UE should be able to do antenna switching to send SRS from each receive antenna. UE should inform its antenna switching capability to base station.
· In case of non-codebook based transmissions, UE transmits a set of SRS where each SRS is pre-coded with different set of UE generated weights. Base station evaluates each SRS and provides feedback to UE about which weights to be applied in PUSCH transmissions. In non-codebook transmissions UE assumes the channel reciprocity where the UE generates the uplink precoding weights based on downlink measurements.
· In case of codebook based transmissions, UE transmits a set of non-precoded SRS. Base station uses this SRS to select the appropriate antenna port for PUSCH, selects the appropriate rank and precoding weights. Base station provides feedback to UE in terms of SRS resource indicator (SRI), Rank indicator (RI) and transmit precoding matrix indicator (TPMI). Codebook based transmissions will not assume channel reciprocity.
· SRS is used to identify best UE transmit beam and best base station receive beam.
When using SRS with channel reciprocity for downlink propagation channel, SRS does not indicate downlink interference conditions. UE has to send additional information to indicate downlink interference conditions. eg. CQI reports.
- UE can be configured to send SRS on 1/2/4 antenna ports. Antenna ports can be identified as 1000, 1001,1002,1003.
- SRS can be configured to use 1/2/4 symbols in time domain, these symbols are located in last 6 symbols of the slot.
- SRS transmissions can use upto 272 resource blocks (RBs). UE uses a transmission comb of sizes 2 and 4 to select a specific set of subcarriers.
- SRS uses specific cyclic shift. When using transmission comb of size 4, available cyclic shifts are 12. When using transmission comb of size 2, available cyclic shifts are 8. When UE transmits SRS on multiple antenna ports, each antenna port is allocated with different cyclic shift.
SRS resource sets
SRS is configured using one or more SRS resource sets where each SRS resource set contains one or more up to 16 SRS resources. SRS triggering mechanism and SRS use case can be configured at SRS resource set level.
SRS resource set IE is defined in 3GPP 38.331:
- resourceType indicates SRS triggering mechanisms. This can be of 3 types: periodic, aperiodic and semi-persistent.
- alpha, p0, pathlossReferenceRS, srs-PowerControlAdjustmentStates used for SRS power control calculations.
An SRS resource is configured by SRS-Resource IE. SRS-Resource IE is defined in 3GPP 38.331:
3GPP 38.211 184.108.40.206 explains the SRS and section 220.127.116.11.1 explains the SRS resource.