otherwise be lost. Furthermore, orthogonality of the propagation vectors implies that independent and identically distributed complex-Gaussian inputs are optimal SO that the achievable rates are in fact the true channel capacities [61]. The studies on massive MIMO have been mainly focused on frequencies below 6 GHz, where the transceiver hardware technology is very mature. The same concept can be applied in mmWave bands, where many antennas might be required since the effective aperture of the antenna is much smaller. However, the hardware implementation will be more challeng- ing. The support of mobility will be more difficult because the coherence time will be an order of magnitude shorter due to higher Doppler spread, which reduces the spatial multi- plexing capability. The channel impulse response between a user terminal and a base station can be represented by an Nantennas dimensional vector. Since the Nuser channel vectors are mutually non- orthogonal in general, advanced interference cancellation receivers are needed to suppress New Radio Access Physical Layer Aspects (Part 2) 585 interference and achieve the sum capacity of the multi-user channel. As we mentioned ear- lier, the favorable propagation is an environment where the Nuser users' channel vectors are mutually orthogonal (i.e., their inner products are zero). The favorable propagation channels are ideal for multi-user transmission since the interference is removed by simple linear pro- cessing (i.e., MRC and ZF) that utilizes the channel orthogonality. The question is whether there are any favorable propagation channels in practice. An approximate form of favorable propagation is achieved in non-LOS environments with rich scattering, where each channel vector has independent stochastic entries with zero mean and identical distribution. Under these conditions, the inner products (normalized by Nantennas) approach to zero as the num- ber of antennas increases; meaning that the channel vectors tend to be orthogonal as Nantennas increases. The suffici