Doppler Spread Analysis for Reducing High-Mobility Massive MIMO V2V Channel Time-Variation

In this paper, we consider fast time-varying channels of high-mobility vehicle-to-vehicle (V2V) communications for massive multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Large-scale uniform linear arrays (ULA) are configured at the transmitter and receiver to separate multiple angle domain Doppler frequency offsets (DFOs) based on transmit and receive beamforming with high spatial resolution. Then each beamforming branch comprises only one dominant DFO. Next, we perform the conventional channel estimation method for each beamforming branch, and carry out maximum-ratio-combining (MRC) for data detection. Power spectrum density (PSD) and Doppler spread of the equivalent link between the transmitter and receiver are derived and regarded as the criterion for assessing the residual channel time-variation caused by limited antennas in practice. Interestingly, the asymptotic scaling law between Doppler spread and the number of transceiver antennas shows that Doppler spread is proportional to the maximum DFO and decreases at the rate of $\sqrt {\frac{1}{{N_T^2}} + \frac{1}{{N_R^2}}} $, where N T and N R are the number of transmit and receive antennas, respectively. Simulation results confirm the validity of the proposed Doppler suppression framework for high-mobility V2V communications.