  Terahertz (THz) communication is widely considered as a key enabler for
future 6G wireless systems. However, THz links are subject to high propagation
losses and inter-symbol interference due to the frequency selectivity of the
channel. Massive multiple-input multiple-output (MIMO) along with orthogonal
frequency division multiplexing (OFDM) can be used to deal with these problems.
Nevertheless, when the propagation delay across the base station (BS) antenna
array exceeds the symbol period, the spatial response of the BS array varies
across the OFDM subcarriers. This phenomenon, known as beam squint, renders
narrowband combining approaches ineffective. Additionally, channel estimation
becomes challenging in the absence of combining gain during the training stage.
In this work, we address the channel estimation and hybrid combining problems
in wideband THz massive MIMO with uniform planar arrays. Specifically, we first
introduce a low-complexity beam squint mitigation scheme based on
true-time-delay. Next, we propose a novel variant of the popular orthogonal
matching pursuit (OMP) algorithm to accurately estimate the channel with low
training overhead. Our channel estimation and hybrid combining schemes are
analyzed both theoretically and numerically. Moreover, the proposed schemes are
extended to the multi-antenna user case. Simulation results are provided
showcasing the performance gains offered by our design compared to standard
narrowband combining and OMP-based channel estimation.
