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.

翻译：移动通信网络的演进始终伴随着消除符号间干扰（ISI）技术的发展，从2G的均衡技术、3G的扩频与RAKE接收机，到4G和5G的正交频分复用（OFDM）。面向6G时代，通过利用大规模天线阵列带来的高空间分辨率以及毫米波/太赫兹信道的多径稀疏性，近期提出了一种新型ISI抑制技术——延迟对齐调制（DAM）。然而，现有工作仅考虑了单载波完美DAM，该方案仅在基站天线数不小于信道路径数时可行，此时所有多径信号分量能同时且相长地到达接收端。这会对基站天线数与多径稀疏性施加严格限制。本文提出一种通用型DAM技术，通过空域-时延联合处理操控信道时延扩展，从而为抑制信道时间色散提供灵活框架，进而实现高效的单载波或多载波传输。首先证明，当基站天线数远大于信道路径数时，通过简单的时延预补偿与基于路径的最大比传输（MRT）波束赋形，可实现完美延迟对齐，将时间色散信道转换为非时间色散信道。当完美DAM不可行或不适用时，所提通用DAM技术可显著缩减信道时延扩展。进一步提出新型DAM-OFDM技术，能够节省传统OFDM的循环前缀（CP）开销或缓解其峰均功率比（PAPR）问题。研究表明，所提DAM-OFDM涉及频域与时域的联合波束赋形优化，并推导出其闭式解。仿真结果表明，所提DAM-OFDM在频谱效率、误码率（BER）和峰均比（PAPR）方面均优于传统OFDM。