 3300-4200 MHz and 4400-5000 MHz (the exact availability depends on the regional regulations). 5 gNB: NR base station. 13.4 Multi-RAT Coexistence and Modem Architecture PDCCH#1 PDCCH#2 subframe PUSCH#1 PUSCH#2 via NUL via SUL Figure 13.8 Example of switching between NUL and SUL. is to configure an SUL in a wide area coverage band such as 9006 or 1800 MHz7, while the downlink and the NUL remain at 3.5 GHz.8 It is up to network scheduling decision whether to allow uplink transmission in the SUL or in NUL - the corresponding decision is informed to the UE via DCI. The associated design challenge for the UE is therefore to switch the uplink transmission path on the fly by DCI. Figure 13.8 shows one example in which the UE alternates between SUL and NUL in consecutive subframes. 13.4.2.3 Carrier Aggregation Asymmetric CA configuration (e.g. 2DL 1UL) is a popular setting in LTE, reflecting the typical downlink bias of data traffic. This concept can be exploited to address the uplink coverage issue. The basic idea is that the original NR carrier is reconfigured as an SCell9 (i.e. it is used for downlink-only transmission), while the PCell¹0 is reconfigured to a lower wide area coverage band (e.g. 900 MHz), as shown in Figure 13.9. From a UE implementation viewpoint, it is analogous to an intra-gNB handover pro- cedure with the additional challenge of supporting mixed-numerology HARQ timing. The low band is likely to operate with a 15kHz SCS to coexist with the incumbent 2DL/1UL CA configuration UE at cell center UE at cell edge NR PCell NR SCell (downlink only) (LTE FDD band) (NR TDD band) (LTE FDD band) (NR TDD band) Uplink Downlink Figure 13.9 Illustration of CA with spectrum sharing to address the uplink coverage issue. 6 LTE Band 8 (B8). 7 LTE Band 3 (B3). 8 NR Band 77 (n77). 9 SCell: secondary cell in a CA configuration. 10 PCell: primary cell in a CA configuration (~equivalent to the serving cell in a non-CA configuration). 13 5G Modem Design Challenges LTE UL 234567890123 LTE DL 0123456789 Figure 13.10 Illustrati