Multi-Domain Conversions of High Dimensional Channel Characteristics for Massive MIMO-OFDM

In this paper, we first define a high dimensional (HiDi) channel characteristics, i.e., space-frequency covariance, for wideband MIMO-OFDM systems. We then design the conversion of the HiDi covariance in frequency domain from one carrier frequency to another, e.g., for FDD systems. Specifically, we apply the projection method in a Hilbert space to estimate the power angle delay spectrum and form the frequency domain conversion of the space-frequency covariance. We also obtain the asymptotic solutions when considering the infinite delay spread, which significantly reduces the complexity. Moreover, we generalize the conversions of space-frequency covariance in both spatial and frequency domains with two exemplary multi-panel scenarios. We then apply the general solutions to a specific antenna array configuration, i.e., uniform linear array (ULA), and obtain the explicit expressions of conversions. Numerical simulations demonstrate the efficiency of the designed conversions.