terization and parameter estimation. The number of antennas on the UE will not be as large as the corresponding number at the BS; however, the number of UE antennas can be much larger than what is used today in mmWave frequencies. The stud- ies suggest that the performance improves as the array aperture increases, but the impact of the aperture is mainly visible when the users are closely grouped (i.e., they have high corre- lation). Furthermore, there is in general a good channel resolvability in the sense that the larger the aperture, the greater the resolvability. One important aspect to remember is that, for physically large arrays, there can be large differences in received power levels over the array, which affects user resolvability. For beamforming solutions, the antenna elements in an array must be placed close together. All the analog components (phase shifters, LNAs, PAs, etc.) should be tightly packed behind the antenna elements (active antenna systems). The dense packing of antenna elements creates two main effects: (1) spatial correlation and (2) mutual coupling [21]. The NR operation in mmWave bands necessitates the use of active antenna systems and large antenna arrays in the base stations and the devices. While this goal is enabled by rela- tively small-sized antenna arrays in mmWave frequencies, it significantly increases the implementation complexity and pushes the limits of antenna design and semiconductor tech- nologies. The fully integrated mmWave transceivers and antennas require sophisticated design and consideration of power efficiency, component placement and routing, mutual couplings, and thermal constraints both at the component level and system level within a small area or volume. These considerations directly affect the achievable performance and the RF requirements and are applicable to the NR base stations and the devices, given that in mmWave frequencies, the BS/UE transceiver design and development will have less dif- ferences compared to frequency bands below 6 GHz. The extremel