  Low-resolution analog-to-digital converters (ADCs) and hybrid beamforming
have emerged as efficient solutions to reduce power consumption with
satisfactory spectral efficiency (SE) in massive multiple-input multiple-output
(MIMO) systems. In this paper, we investigate the performance of a hybrid
receiver in uplink massive MIMO orthogonal frequency-division multiplexing
(OFDM) systems with low-resolution ADCs and oversampling. Considering both the
temporal and spatial correlation of the quantization distortion (QD), we derive
a closed-form approximation of the frequency-domain QD covariance matrix, which
facilitates the evaluation of the system SE. Then we jointly design the analog
and baseband combiners to maximize the SE. The formulated problem is
significantly challenging due to the constant-modulus constraint of the analog
combiner and its coupling with the digital one. To overcome the challenges, we
transform the objective function into an equivalent but more tractable form and
then iteratively update the analog and digital combiner. Numerical simulations
verify the superiority of the proposed algorithm compared to the considered
benchmarks and show the resilience of the hybrid receiver to beam squint for
low-resolution systems. Furthermore, the results show that the proposed hybrid
receiver design with oversampling can achieve significantly higher energy
efficiency compared to the digital one.
