Sounding reference signal (SRS) triggered in a random access channel (RACH) procedure
Certain aspects of the present disclosure provide techniques for transmitting sounding reference signals (SRSs) triggered by a downlink (DL) random access channel (RACH) message (e.g., msg2). A method that may be performed by a user equipment (UE) generally includes receiving a DL RACH message that indicates the UE is to transmit an SRS, transmitting the SRS, in accordance with the indication, and transmitting an uplink (UL) RACH message.
1. A method for wireless communications performed by a user equipment (UE), comprising:
receiving a downlink (DL) random access channel (RACH) message that indicates the UE is to transmit a sounding reference signal (SRS), wherein the DL RACH message comprises a flag in the DL RACH message that activates transmission of the SRS by the UE, and wherein parameters for transmitting the SRS are determined implicitly based on the DL RACH message;
transmitting the SRS in accordance with the DL RACH message; and
transmitting an uplink (UL) RACH message after transmitting the SRS.
2. The method of claim 1 , wherein the DL RACH message comprises a random access response (RAR) message or a physical downlink control channel (PDCCH) for the RAR message.
3. The method of claim 1 , wherein the parameters comprise time and frequency resources for the SRS.
4. The method of claim 3 , wherein the time and frequency resources for the SRS are determined based on at least one of:
time and frequency resources of the DL RACH message or time and frequency resources of the UL RACH message; or
one or more rules.
5. The method of claim 1 , wherein at least some of the parameters are:
predefined,
configured via system information, or
predefined and configured via the system information.
6. The method of claim 1 , wherein the UE transmits the SRS using a same transmit port over multiple orthogonal frequency division multiplexed (OFDM) symbols.
7. The method of claim 1 , wherein the UE transmits the SRS using a different transmit port for each of multiple orthogonal frequency division multiplexed (OFDM) symbols.
8. The method of claim 1 , wherein the UE is configured to monitor for the DL RACH message that indicates the UE is to transmit the SRS only when the UE is operating in certain frequency ranges or with certain subcarrier spacing (SCS).
9. The method of claim 1 , wherein the UE is configured to monitor for the DL RACH message that indicates the UE is to transmit the SRS only when the UE is associated with certain synchronization signal block (SSB) beams.
10. The method of claim 1 , wherein the UE is configured to monitor for the DL RACH message that indicates the UE is to transmit the SRS, based on a certain type, method, or format of a physical random access channel (PRACH) transmission by the UE.
11. The method of claim 1 , wherein the UE is configured to monitor for the DL RACH message that indicates the UE is to transmit the SRS when the UE has transmitted repeated PRACHs or a PRACH preamble from a specific preamble subset.
12. An apparatus for wireless communication by a user equipment (UE), comprising:
memory; and
one or more processors coupled to the memory, the one or more processors being configured, individually or collectively, to:
receive a downlink (DL) random access channel (RACH) message that indicates the UE is to transmit a sounding reference signal (SRS), wherein the DL RACH message comprises a flag in the DL RACH message that activates transmission of the SRS by the UE, and wherein parameters for transmitting the SRS are determined implicitly based on the DL RACH message;
transmit the SRS in accordance with the DL RACH message; and
transmit an uplink (UL) RACH message after transmission of the SRS.
13. A method for wireless communications performed by a network entity, comprising:
transmitting, to a user equipment (UE), a downlink (DL) random access channel (RACH) message that indicates the UE is to transmit a sounding reference signal (SRS), wherein the DL RACH message comprises a flag in the DL RACH message that activates transmission of the SRS by the UE, and wherein parameters for transmitting the SRS are determined by the UE implicitly based on the DL RACH message;
monitoring for the SRS, in accordance with the DL RACH message; and
monitoring for an uplink (UL) RACH message after monitoring for the SRS.
14. The method of claim 13 , wherein the DL RACH message comprises a random access response (RAR) message or a physical downlink control channel (PDCCH) for the RAR message.
15. The method of claim 13 , wherein the parameters comprise time and frequency resources for the SRS.
16. The method of claim 15 , wherein the time and frequency resources for the SRS are determined by the UE based on at least one of:
time and frequency resources of the DL RACH message or time and frequency resources of the UL RACH message; or
one or more rules.
17. The method of claim 13 , wherein at least some of the parameters are:
predefined,
configured via system information, or
predefined and configured via the system information.
18. The method of claim 13 , further comprising:
receiving the SRS;
performing measurements on the SRS; and
selecting, based on the measurements, a precoder for transmitting a second DL RACH message.
19. The method of claim 13 , further comprising:
applying beam sweeping whereby different beams are used for reception of the SRS over multiple OFDM symbols;
selecting a refined beam based on a reference signal received power (RSRP) of the SRS received using the different beams; and
using the selected refined beam for reception of the UL RACH message.
20. The method of claim 13 , wherein the network entity is configured to transmit the DL RACH message that indicates the UE is to transmit the SRS only when operating in certain frequency ranges or with certain subcarrier spacing (SCS).
21. The method of claim 13 , wherein the network entity is configured to transmit the DL RACH message that indicates the UE is to transmit the SRS only when the UE is associated with certain synchronization signal block (SSB) beams.
22. The method of claim 13 , wherein the network entity is configured to transmit the DL RACH message that indicates the UE is to transmit the SRS, based on a certain type, method, or format of a physical random access channel (PRACH) transmission by the UE.
23. The method of claim 13 , wherein the network entity is configured to transmit the DL RACH message that indicates the UE is to transmit SRS when the UE has transmitted repeated PRACHs or a PRACH preamble from a specific preamble subset.
24. An apparatus for wireless communication by a network entity, comprising:
memory; and
one or more processors coupled to the memory, the one or more processors being configured, individually or collectively, to:
transmit, to a user equipment (UE), a downlink (DL) random access channel (RACH) message that indicates the UE is to transmit a sounding reference signal (SRS), wherein the DL RACH message comprises a flag in the DL RACH message that activates transmission of the SRS by the UE, and wherein parameters for transmitting the SRS are determined by the UE implicitly based on the DL RACH message;
monitor for the SRS, in accordance with the DL RACH message; and
monitor for an uplink (UL) RACH message after the SRS is monitored for.