Multipair two-way massive MIMO AF relaying with ZFR/ZFT and hardware impairments over high-altitude platforms

HAPs (High-Altitude Platforms) are one of the most promising alternative infrastructures for providing wireless communications to overcome the shortcomings of terrestrial-tower-based and satellite systems. This paper investigates a multipair two-way MM-AF (Massive MIMO Amplify-and-Forward) relay system over HAPs, where multiple user pairs exchange information within a pair through a relay with a very large number of antennas, and a HAP channel is modeled to follow a Rician fading distribution because of the presence of a line-of-sight path. First, the effect of hardware impairments on a multipair two-way MM-AF system is taken into consideration and is modeled using transmit and receive distortion noises. Then, ZFR/ZFT (Zero-Forcing Reception/Zero-Forcing Transmission) processing matrices of HDR (Half-Duplex Relaying) and FDR (Full-Duplex Relaying) with imperfect channel state information are presented. Finally, the asymptotic expressions of an end-to-end SINR are derived. Theoretical analyses and simulation results show that when the number of relay antennas grows very large, the SE (Spectral Efficiency) of an MM-AF relay system is limited by hardware impairments at the users instead of those at the relay, or by other types of interference. Thus, the quality of the hardware at a relay can be decreased without significantly degrading performance.