aming. The LTE and NR coexistence in the same spectrum, which is required for non-standalone and early deployments of 5G systems, makes it possible to deploy NR in the existing frequency allocations. Since the co-channel NR and LTE carriers need to be aligned at sub- carrier level, some restrictions are imposed on the NR channel raster in order to align the position of the NR and LTE carriers. The NR further supports multiple numerologies with 658 Chapter 5 subcarrier spacing (SCS) ranging from 15 to 120 kHz, with direct implications on the time and frequency structures. The subcarrier spacing has certain implications on the RF front- end in terms of the roll-off of the transmitted signal, which impacts the guard bands that are allocated between the transmitted resource blocks and the edge of the frequency band. The NR also supports mixed numerologies on the same carrier, which has further RF implica- tions since the guard bands may need to be different at the two edges of the band. In some cases, there are certain limitations concerning the site where the gNB equipment is deployed. The candidate sites are often shared between operators or an operator deploys multiple technologies at one site, which creates additional requirements for the gNB trans- mitters and receivers to operate in close proximity of other BS transceivers (e.g., blocking effects). The coexistence between operators of TDD systems in the same band is in general provided by inter-operator synchronization in order to avoid interference between downlink and uplink transmissions of different operators. This means that all operators need to have the same uplink/downlink configurations and frame synchronization, which is not by itself an RF requirement, but it is implicitly assumed in the 3GPP specifications. The RF require- ments for unsynchronized systems are inevitably much stricter. The frequency bands are regionally defined, and new bands are added continuously for each generation of mobile systems, which means each new release of 3GPP specificat