Delay Alignment Modulation: Manipulating Channel Delay Spread for Efficient Single- and Multi-Carrier Communication
Haiquan Lu, Yong Zeng
Abstract
The evolution of mobile communication networks has always been accompanied by the advancement of inter-symbol interference (ISI) mitigation techniques, from equalization in the second-generation (2G), spread spectrum and RAKE receiver in the third generation (3G), to orthogonal frequency-division multiplexing (OFDM) in the fourth-generation (4G) and fifth-generation (5G). Looking forward towards the sixth-generation (6G), by exploiting the high spatial resolution brought by large antenna arrays and the multi-path sparsity of millimeter wave (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 base station (BS) antennas is no smaller than that of channel paths, so that all multi-path signal components can be aligned for arriving 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. To gain some insights, 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 maximal-ratio transmission (MRT) beamforming. When perfect DAM is infeasible or undesirable, the proposed generic DAM technique can be applied to significantly reduce the channel delay spread. Based on such results, we further propose the novel DAM-OFDM technique, which is able to save the cyclic prefix (CP) overhead or mitigate the peak-to-average-power ratio (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 achieves significant performance gains over the conventional OFDM, in terms of spectral efficiency, bit error rate (BER) and PAPR.