  The evolution of mobile communication networks has always been accompanied by
the advancement of ISI mitigation techniques, from equalization in 2G, spread
spectrum and RAKE receiver in 3G, to OFDM in 4G and 5G. Looking forward towards
6G, by exploiting the high spatial resolution brought by large antenna arrays
and the multi-path sparsity of mmWave and Terahertz channels, a novel ISI
mitigation technique termed delay alignment modulation (DAM) was recently
proposed. However, existing works only consider the single-carrier perfect DAM,
which is feasible only when the number of BS antennas is no smaller than that
of channel paths, so that all multi-path signal components arrive at the
receiver simultaneously and constructively. This imposes stringent requirements
on the number of BS antennas and multi-path sparsity. In this paper, we propose
a generic DAM technique to manipulate the channel delay spread via
spatial-delay processing, thus providing a flexible framework to combat channel
time dispersion for efficient single- or multi-carrier transmissions. We first
show that when the number of BS antennas is much larger than that of channel
paths, perfect delay alignment can be achieved to transform the time-dispersive
channel to time non-dispersive channel with the simple delay pre-compensation
and path-based MRT beamforming. When perfect DAM is infeasible or undesirable,
the proposed generic DAM technique can be applied to significantly reduce the
channel delay spread. We further propose the novel DAM-OFDM technique, which is
able to save the CP overhead or mitigate the PAPR issue suffered by
conventional OFDM. We show that the proposed DAM-OFDM involves joint frequency-
and time-domain beamforming optimization, for which a closed-form solution is
derived. Simulation results show that the proposed DAM-OFDM outperforms the
conventional OFDM in terms of spectral efficiency, BER and PAPR.
