Adjustable phase shift pilots for sparse massive MIMO-OFDM channels

We propose adjustable phase shift pilots (APSPs) for wideband massive multiple-input multiple-output (MIMO) systems employing orthogonal frequency division multiplexing (OFDM) to reduce the pilot overhead. Based on a physically motivated channel model, we first establish a relationship between channel space-frequency correlations and the channel power angle-delay (AD) spectrum in the massive antenna array regime, which reveals the channel sparsity in massive MIMO-OFDM. With this channel model, we then investigate channel estimation (CE) for massive MIMO-OFDM with APSPs. We show that CE performance in terms of sum mean square error can be minimized if the user terminals' channel power distributions in the AD domain are non-overlapping with proper phase shift scheduling. The Simulation results demonstrate that the proposed APSP approach can provide substantial performance gains in terms of achievable spectral efficiency over the conventional phase shift orthogonal pilot approach in typical high mobility scenarios.