Hybrid and full-digital beamforming in mmWave Massive MIMO systems: A comparison considering low-resolution ADCs

In a millimeter-wave (mmWave) Massive multiple-input multiple-output (MIMO) systems, full-digital beamforming (i.e., connecting each antenna with a specific radio-frequency (RF) chain) becomes inefficient due to the hardware cost and power consumption. Therefore, hybrid analog and digital transceiver where the number of RF chains are much smaller than that of the antennas has drawn great research interest. In this work, we investigate the use of low-resolution analog-to-digital converters (ADCs) in the uplink of multi-user hybrid and full-digital mmWave Massive MIMO systems. To be specific, we compare the performance of full-digital minimum mean square error (MMSE) and hybrid MMSE beamforming in both sum rates and energy efficiency. Accurate approximations of sum rates and energy efficiency are provided for both schemes, which captures the dominant factors. The analytical results show that full-digital beamforming outperforms hybrid beamforming in terms of sum rates and requires only a small portion (γ) of antennas used by hybrid beamforming to achieve the same sum rates. We given sufficient condition for full-digital beamforming to outperform hybrid beamforming in terms of energy efficiency. Moreover, an algorithm is proposed to search for the optimal ADC resolution bits. Numerical results demonstrate the correctness of the analysis.