No LTE No NR (a) Single uplink operation - like TDM sharing (b) Dynamic power sharing Figure 13.5 LTE/NR Power sharing schemes: (a) SUO-like TDM sharing and (b) dynamic power sharing. exceeds the power class of the UE. However, as a UE moves toward the edge of the cell, the SUO-based scheme falls back to single-uplink mode; that is, a TDM pattern is applied for EN-DC uplink transmission. On the other hand, the DPS scheme can use dual uplink transmissions as long as residual power is available. As a result, to maximize the overall system throughput, the DPS scheme is a better option. From a modem implementation viewpoint, DPS requires the modem to coordinate the power allocation between the LTE and NR links. Coordinating the power alloca- tion for different timings is challenging. In LTE, there is a fixed 4 ms timing relationship between an uplink grant and the corresponding uplink transmission. However, in NR, the timing relationship is dynamic and configured by the network (see Section 13.3.2). Figure 13.6 shows one example of DPS operation. In this example, NR uplink is granted two slots ahead of the NR transmission, while LTE uplink is granted four subframes ahead of LTE transmission. Both transmissions happen to occur at subframe N + 5. The internal processing time for power allocation between the two links becomes stringent, LTE power control LTE DL subframe LTE UL subframe NR power control Power adjustment for LTE/NR uplink NR DL slot NR UL slot Figure 13.6 Illustrative example of DPS operation. 13 5G Modem Design Challenges baseband baseband Signaling Front-end & Front-end & RF system RF system baseband baseband Mediator circuit Figure 13.7 Illustrative examples of shared RF system. compared to the LTE-only design. In the example, the 5G NR module is unaware of NR power control information until subframe N + 3 arrives. Therefore, the available time to calculate the appropriate power allocation between the two transmissions is effectively two subframes less the timing advance and the decoding of NR's power 