tenna isolation and (2) transmit diversity gain at the cell edge. 12.2.4.3 Impact of Sounding Reference Signal (SRS) Switching as Enhancement to Downlink MIMO In FR1, the UE is mandated to support 4 receive (RX) antennas in high-frequency bands (n7, n38, n41, n77, n78, and n79). In the TDD bands, the 4x4 DL MIMO system per- formance is suboptimal as channel reciprocity can only be accurately exploited using the port on which SRS symbols are transmitted, that is, on the primary antenna port. At best, for UL MIMO-capable UEs, SRS sounding can be performed on two out of four antennas, thereby partially improving the 4x4 matrix channel estimation. To further enhance channel estimation and to better exploit channel reciprocity, 5G supports "SRS antenna switching," which requires the UE to perform uplink sounding on all RX physical antenna ports. This feature complicates the RF front-end multiplexing /de-multiplexing architecture as several additional RF switches are needed to connect each RX antenna port to the PA module(s). This feature is defined for one transmitter (1T) two receivers (1T/2R), 1T/4R, 2T/4R, and an equal number of Tx/Rx. The concept is illustrated in Figure 12.9 for 1T/4R (left) and 2T/4R (right). To account for additional insertion losses due to new RF switches and due to the large routing distance separating the PA from some of the remote Rx diversity antennas, a 3 dB transmit power reduction is allowed for SRS antenna switching in the Pcmax equations. One of the challenges for UL MIMO-capable UEs is to provide an RF-FE (RF-front end) architecture which allows the UE to reconfigure from 2T4R to 1T4R at little or no additional insertion losses. At the time of writing, there are still many of those aspects that need completion, such as support for intra-band EN-DC operation, or support for FR2, where SRS switching could be performed across panels, or between the two polar- izations of a given panel when transmissions occur on a single polarization. 12.2.5 RAN4 Technical Specification (TS) Survival Gu