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The evolution of mobile communication networks has always been accompanied bythe advancement of ISI mitigation techniques, from equalization in 2G, spreadspectrum and RAKE receiver in 3G, to OFDM in 4G and 5G. Looking forward towards6G, by exploiting the high spatial resolution brought by large antenna arraysand the multi-path sparsity of mmWave and Terahertz channels, a novel ISImitigation technique termed delay alignment modulation (DAM) was recentlyproposed. However, existing works only consider the single-carrier perfect DAM,which is feasible only when the number of BS antennas is no smaller than thatof channel paths, so that all multi-path signal components arrive at thereceiver simultaneously and constructively. This imposes stringent requirementson the number of BS antennas and multi-path sparsity. In this paper, we proposea generic DAM technique to manipulate the channel delay spread viaspatial-delay processing, thus providing a flexible framework to combat channeltime dispersion for efficient single- or multi-carrier transmissions. We firstshow that when the number of BS antennas is much larger than that of channelpaths, perfect delay alignment can be achieved to transform the time-dispersivechannel to time non-dispersive channel with the simple delay pre-compensationand path-based MRT beamforming. When perfect DAM is infeasible or undesirable,the proposed generic DAM technique can be applied to significantly reduce thechannel delay spread. We further propose the novel DAM-OFDM technique, which isable to save the CP overhead or mitigate the PAPR issue suffered byconventional OFDM. We show that the proposed DAM-OFDM involves joint frequency-and time-domain beamforming optimization, for which a closed-form solution isderived. Simulation results show that the proposed DAM-OFDM outperforms theconventional OFDM in terms of spectral efficiency, BER and PAPR.

Delay Alignment Modulation: Manipulating Channel Delay Spread for Efficient Single- and Multi-Carrier Communication