  Nonlinear distortion stemming from low-cost power amplifiers may severely
affect wireless communication performance through out-of-band (OOB) radiation
and in-band distortion. The distortion is correlated between different transmit
antennas in an antenna array, which results in a beamforming gain at the
receiver side that grows with the number of antennas. In this paper, we
investigate how the strength of the distortion is affected by the frequency
selectivity of the channel. A closed-form expression for the received
distortion power is derived as a function of the number of multipath components
(MPCs) and the delay spread, which highlight their impact. The performed
analysis, which is verified via numerical simulations, reveals that as the
number of MPCs increases, distortion exhibits distinct characteristics for
in-band and OOB frequencies. It is shown that the received in-band and OOB
distortion power is inversely proportional to the number of MPCs, and it is
reported that as the delay spread gets narrower, the in-band distortion power
is beamformed towards the intended user, which yields higher received in-band
distortion compared to the OOB distortion.
