Power Leakage Elimination for Wideband mmWave Massive MIMO-OFDM Systems: An Energy-Focusing Window Approach

When the large antenna array and the large bandwidth are deployed in a massive multiple-input multiple-output (MIMO) system, the spatial-wideband effect and the related beam squint effect make the energy of each multipath component distributed mainly within a square area in angle-delay domain rather than distributed like an impulse. With the finite numbers of antennas and subcarriers in practical massive MIMO-OFDM systems, the power leakage of each path outside the corresponding square area is non-negligible. In this paper, we first derive the millimeter-wave (mmWave) massive MIMO channel model for OFDM transceivers and then design an optimal energy-focusing window, which shapes the channel in spatial-frequency domain to maximally concentrate the energy of each multipath component inside the corresponding square area in angle-delay domain. With the help of this window, the power leakage can be significantly reduced. As a result, the sparsity of mmWave channels can be primely preserved and the inter-user interference can be significantly alleviated for angle- and delay-based channel estimation and precoding. Simulation results demonstrate the effectiveness of the proposed window design.