Approximate Gram-Matrix Interpolation for Wideband Massive MU-MIMO

A broad range of linear and non-linear equalization and precoding algorithms for wideband massive multi-user (MU) multiple-input multiple-output (MIMO) wireless systems that rely on orthogonal frequencydivision multiplexing (OFDM) or single-carrier frequency-division multiple access (SC-FDMA) requires the computation of the Gram matrix for each active subcarrier, which results in excessively high computational complexity. In this paper, we propose novel, approximate algorithms that reduce the complexity of Grammatrix computation for linear equalization and precoding by exploiting correlation across subcarriers. We analytically show that a small fraction of Gram-matrix computations in combination with approximate interpolation schemes are sufficient to achieve near-optimal error-rate performance at low computational complexity in wideband massive MU-MIMO systems. We furthermore demonstrate that the proposed methods exhibit improved robustness against channel-estimation errors compared to exact Gram-matrix interpolation algorithms that typically require high computational complexity.