Double-side delay alignment modulation for multi-user millimeter wave and terahertz communications

Delay Alignment Modulation (DAM) is an innovative broadband modulation technique well-suited for millimeter Wave (mmWave) and Terahertz (THz) massive Multiple-Input Multiple-Output (MIMO) communication systems. Leveraging the high spatial resolution and sparsity of multi-path channels, DAM effectively mitigates Inter-Symbol Interference (ISI) by aligning all multi-path components through a combination of delay pre-compensation (or post-compensation) and path-based beamforming. As such, ISI is eliminated while preserving multi-path power gains. In this paper, we investigate multi-user double-side DAM, which incorporates both delay pre-compensation at the transmitter and post-compensation at the receiver, in contrast to prior works that primarily focus on single-side DAM with only delay pre-compensation. Firstly, we derive the constraint on the number of introduced delays and formulate the corresponding delay pre/post-compensation vectors tailored for multi-user double-side DAM, given a specific number of delay compensations. Furthermore, we demonstrate that when the number of Base Stations (BSs)/User Equipment (UE) antennas is sufficiently large, single-side DAM—where delay compensation is performed only at the BS/UE—is preferable to double-side DAM, since the former results in less ISI to be spatially eliminated. Next, we propose two low-complexity path-based beamforming strategies based on the eigen-beamforming transmission and ISI-Zero Forcing (ZF), respectively. On this basis, we further analyze the achievable sum rates. Simulation results verify that with a sufficiently large number of BS/UE antennas, single-side DAM is adequate for ISI elimination. Moreover, compared to the benchmarking scheme of Orthogonal Frequency Division Multiplexing (OFDM), multi-user BS-side DAM achieves higher spectral efficiency and lower Peak-to-Average Power Ratio (PAPR).