Channel hardening, favorable equalization and propagation in wideband massive MIMO

This paper analyzes the channel hardening and favorable propagation behavior of frequency-selective massive MIMO channels. To this purpose the concept of favorable equalization is introduced to characterize the property of the channel to become frequency flat as the number of antennas grows when proper pre-filtering is adopted. It is shown that classic OFDM-based massive MIMO and time-reversal schemes, usually considered and analyzed as different technologies, are particular cases of the same framework. Their generalization leads to the concept of massive waveforming, which allows the creation of parallel wideband AWGN-like links between the base station and the users.