  Delay alignment modulation (DAM) is a promising technology to achieve
ISI-free wideband communication, by leveraging delay compensation and
path-based beamforming, rather than the conventional channel equalization or
multi-carrier transmission. In particular, when there exist a few strong
time-dispersive channel paths, DAM can effectively align different propagation
delays and achieve their constructive superposition, thus especially appealing
for intelligent reflecting surfaces (IRSs)-aided communications with
controllable multi-paths. In this paper, we apply DAM to multi-IRS aided
wideband communication and study its practical design and achievable
performance. We first provide an asymptotic analysis showing that when the
number of base station (BS) antennas is much larger than that of IRSs, an
ISI-free channel can be established with appropriate delay pre-compensation and
the simple path-based MRT beamforming. We then consider the general system
setup and study the problem of joint path-based beamforming and phase shifts
design for DAM transmission, by considering the three classical beamforming
techniques on a per-path basis, namely the low-complexity path-based MRT
beamforming, the path-based ZF beamforming for ISI-free DAM communication, and
the optimal path-based MMSE beamforming. As a comparison, OFDM-based multi-IRS
aided communication is considered. Simulation results demonstrate that DAM
outperforms OFDM in terms of spectral efficiency, BER, and PAPR.
