ments. For New Radio Access RF and Transceiver Design Considerations TX_OUT (by pass) Switches Impedance matching Antenna TX_IN circuit RFFE bias RX_OUT RX_IN (by pass) Figure 5.28 Example of 5G NR n77/n78 RF front-end block diagram [26]. example, one can derive a simple filter response, if an ideal environment with a separate high-band antenna and no coexistence requirements are assumed. On the other hand, if we consider a more complex radio environment, such as a multi-radio platform with simulta- neous Wi-Fi transmission, the filter requirements become more stringent. Thus it is impor- tant to take note of the radio environment, antenna topology, and in-device coexistence requirements in order to specify the optimum filter. In other words, the filter design and antenna topology into which the front-end module will be subsequently integrated must be customized to the specific use case or application (see Fig. 5.28). Antenna configurations play a significant role in the development and deployment of 5G products. While market requirements are slowly becoming clearer, there is already some uncertainty as to which of the optional features will be supported. One feature is the fast hopping sounding reference signal, which uses the transmitter to send a series of known symbols across all of the downlink receive antennas in the UE in order to better calibrate the MIMO channel and improve the downlink SNR. This process is key to enhanced MIMO and beamforming operations. The SRS carrier switching was recently introduced in LTE Rel-14 to assist the eNB in obtaining the channel state information of secondary TDD cells in LTE carrier aggregation scenarios. The SRS transmit switching allows the UE to route its SRS transmissions to all other available antenna ports. Assuming the channel reciprocity assumption holds, which should be the case for TDD operation, this feature enables gNB to estimate CSI on secondary downlink-only cells. By applying this concept to multi-user MIMO, the network performance is further enhanced, whi