SRS

SRS is the uplink's equivalent of CSI-RS — a reference signal the UE sends so the gNB can sound the uplink channel. The network uses it for uplink scheduling and link adaptation, for uplink beam management, and for timing. But its most important role shows up in TDD massive MIMO, where channel reciprocity applies: because uplink and downlink share the same frequency, the gNB can measure the channel from one SRS transmission and infer the downlink channel from it.

That reciprocity trick is a big deal. Instead of relying on the UE to feed back bulky downlink CSI, the gNB derives the precoding it needs for downlink beamforming directly from SRS, which scales far better when you're serving many users across a 64-port array. SRS is configurable in periodic, semi-persistent, and aperiodic forms, and antenna switching lets a UE with more receive than transmit chains sound all its antennas in turn. The cost is uplink overhead and UE power, so the network tunes SRS density and periodicity to what scheduling actually needs.