  In this paper, we study the impact of frequency-selectivity on the gap
between the required per-user transmit power in the residual CFO scenario (i.e.
after CFO estimation/compensation at the base-station (BS) from [6]) and that
in the ideal/zero CFO scenario, for a fixed per-user information rate, in
single-carrier massive MU-MIMO uplink systems with the TR-MRC receiver.
Information theoretic analysis reveals that this gap decreases with increasing
frequency-selectivity of the channel. Also, in the residual CFO scenario, an
$\mathcal{O}(\sqrt{M})$ array gain is still achievable ($M$ is the number of BS
antennas) in frequency-selective channels with imperfect channel estimates.
