  In this paper, a channel estimator for wideband millimeter wave (mmWave)
massive multiple-input multiple-output (MIMO) systems with hybrid architectures
and low-resolution analog-to-digital converters (ADCs) is proposed. To account
for the propagation delay across the antenna array, which cannot be neglected
in wideband mmWave massive MIMO systems, the discrete time channel that models
the spatial wideband effect is developed. Also, the training signal design that
addresses inter-frame, inter-user, and inter-symbol interferences is
investigated when the spatial wideband effect is not negligible. To estimate
the channel parameters over the continuum based on the maximum a posteriori
(MAP) criterion, the Newtonized fully corrective forward greedy selection-cross
validation-based (NFCFGS-CV-based) channel estimator is proposed. NFCFGS-CV is
a gridless compressed sensing (CS) algorithm, whose termination condition is
determined by the CV technique. The CV-based termination condition is proved to
achieve the minimum squared error (SE). The simulation results show that
NFCFGS-CV outperforms state-of-the-art on-grid CS-based channel estimators.
