Robust Hybrid Beamforming in Millimeter Wave Relay Networks With Imperfect CSI

Millimeter-wave (mmWave) communication offers rich spectrum resources and acts as a key enabling technology for future wireless communication systems. Hybrid (digital and analog) beamforming and relay techniques are important for mmWave implementations considering the characteristics of mmWave signals, as well as the practical limitations of equipment size, power consumption, and hardware cost. In this paper, a robust hybrid beamforming scheme is presented for mmWave multiple-input multiple-output relay networks adopting amplify-and-forward strategy at the relays. Unlike most existing designs that are based on the perfect channel-state information (CSI), CSI imperfectness is considered in the proposed robust beamforming scheme. An accurate approximation of the average received signal-to-noise ratio is derived and used as the design criterion for the developed iterative beamforming optimization at different nodes. An orthogonal matching pursuit-based algorithm is then utilized to design the hybrid beamforming schemes. Simulation results show that the proposed robust beamforming scheme with affordable computational complexities provides substantial performance gains compared with the existing non-robust designs.