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Multiple-input multiple-output (MIMO) millimeter wave (mmWave) systems arevulnerable to hardware impairments due to operating at high frequencies andemploying a large number of radio- frequency (RF) hardware components. Inparticular, nonlinear power amplifiers (PAs) employed at the transmitterdistort the signal when operated close to saturation due to energy efficiencyconsiderations. In this paper, we study the performance of a MIMO mmWave hybridbeamforming scheme in the presence of nonlinear PAs. First, we develop astatistical model for the transmitted signal in such systems and show that thespatial direction of the inband distortion is shaped by the beamforming filter.This suggests that even in the large antenna regime, where narrow beams can besteered toward the receiver, the impact of nonlinear PAs should not be ignored.Then, by employing a realistic power consumption model for the PAs, weinvestigate the trade-off between spectral and energy efficiency in suchsystems. Our results show that increasing the transmit power level when thenumber of transmit antennas grows large can be counter-effective in terms ofenergy efficiency. Furthermore, using numerical simulation, we show that whenthe transmit power is large, analog beamforming leads to higher spectral andenergy efficiency compared to digital and hybrid beamforming schemes.

On the Energy Efficiency of MIMO Hybrid Beamforming for Millimeter Wave Systems with Nonlinear Power Amplifiers