Hybrid Precoding in mmWave Multiuser MIMO Systems with Delay Alignment Modulation (DAM)

This paper presents delay alignment modulation (DAM) toward utilizing the high spatial gain and multipath sparsity of millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) systems for cancelling the inter-symbol interference (ISI), which eliminates the need for traditional schemes such as multi-carrier transmission or channel equalization. We design a partially-connected transmit precoder (PCTPC) at the base station (BS), where each RF chain is connected to a subset of the BS antennas, followed by developing a novel DAM-based hybrid precoding scheme that introduces the delay pre-compensation and per-tap beamforming. The analog domain RF TPC is designed to maximize the array gain and the baseband TPC employs the minimum mean-squared error (MMSE) beamformer to mitigate ISI as well as inter-user interference (IUI). An advantage of the procedure is that the MMSE beamformer does not suffer from noise enhancement, which is otherwise prevalent in zero-forcing (ZF) beamforming. Simulation results depict the superiority of the proposed MU DAM system in comparison to a conventional multi-carrier MU OFDM system. We also demonstrate that the proposed precoding scheme is able to perform accurately both in perfect and imperfect channel state information (CSI) scenarios.