SRS is Sounding Reference Signal is a reference signal transmitted by the UE in the uplink direction which is used by the eNodeB to estimate the uplink channel quality over a wider bandwidth. SRS is a UL reference signal which is transmitted by UE to the base station. SRS gives information about the combined effect of multipath fading, scattering, Doppler, and power loss of the transmitted signal. Sounding reference signals are uplink physical signals employed by user equipment (UE) for uplink channel sounding, including channel quality estimation and synchronization. Unlike Demodulation reference signals (DM-RS), SRS is not associated with any physical uplink channels and they support uplink channel-dependent scheduling and link adaptation. SRS assist in:
- Codebook-based closed-loop spatial multiplexing
- Control uplink transmit timing
- Reciprocity-based downlink precoding in multi-user MIMO setups
- Quasi co-location of physical channels and reference signals
In 5G NR, the SRS is transmitted by the UE for uplink channel sounding, which includes channel estimation and synchronization. An NR-SRS is an uplink orthogonal frequency division multiplexing (OFDM) signal filled with a Zadoff-Chu sequence on different subcarriers. For the purposes of communications, the SRS is used for closed-loop spatial multiplexing, uplink transmitting timing control, and reciprocity multi-user downlink precoding. To utilize the channel sounding function, the SRS must be known by both the UE and the gNB. UE act as a mobile transmitter and gNB act as a base station receiver.
Base station estimates the channel quality using this reference signal and manages further resource scheduling, Beam management, and power control of the signal. So SRS provides information to gNB about the channel over the full bandwidth and using this information, gNB takes decisions for resource allocation which has better channel quality as compared to other Bandwidth regions.
SRS of 5G-NR important points
- Max SRS Symbols can be 1,2 or 4.
- In the time domain, SRS Resources spans over 1,2, or 4 consecutive symbols.
- SRS will always be transmitted in the last six symbols of the slot.
- Always configured as UE Specific only
- SRS transmission is flexible
- 5G NR has additional flexibility of SRS transmission.
- Max 12 UEs can transmit SRS simultaneously using one antenna port.