sible to meet the unwanted emission limits directly outside the channel bandwidth, if certain time-domain windowing or frequency-domain shaping are not used. Filtering is always used both in the form of time-domain digital filtering of the baseband signal or analog-domain filtering of the RF signal. The nonlinear characteristics of the PA must be taken into account, since it is the cause of intermodulation products outside the channel bandwidth. A power backoff can be used to operate in linear region of the PA at the cost of lower power efficiency, thus the amount of backoff should be minimized in practice. For this reason, additional linearization schemes can be employed [e.g., digital predistortion (DPD) for the PA]. These are especially impor- tant for the BS, where there are fewer restrictions on implementation complexity and use of advanced linearization schemes is an essential part of controlling unwanted emissions. The emission mask defines the permissible OOB spectrum emissions outside the transmis- sion bandwidth. Due to support of flexible channel bandwidths by NR, the determination of the frequency boundary between OOB emissions and spurious emissions is more involved for the NR BS and the devices. For the NR BS, the problem of implicit variation of the boundary between OOB and spurious domains with the varying channel bandwidth is addressed by not defining an explicit boundary. The 3GPP solution for this problem is the concept of OBUE for the NR base stations instead of the spectrum mask that is usually defined for OOB emissions. The OBUE requirement applies over the entire BS transmitter operating band, plus an additional 10-40 MHz on each side, as shown in Fig. 5.9. All requirements outside of that range are set by the regulatory spurious emission limits, based on the ITU-R recommendations. As shown in Fig. 5.9, a large part of the OBUE is defined over a frequency range that for smaller channel bandwidths can be both in spurious and OOB domains. This means that the limits for the frequency ranges th