Sparse Array Beamformer Design for Massive MIMO Beamforming under Mutual Coupling

Fully digital beamforming based on Multiple-Input Multiple-Output (MIMO) array offers superior spatial degrees of freedom and multiplexing gain. However, in practical deployments with compact and high-density antenna arrays, the performance of massive MIMO systems becomes highly sensitive to mutual coupling effects among antenna elements. Ignoring these coupling effects often leads to severe performance degradation and misleading system performance assessment. To this end, this paper investigates sparse MIMO array optimization for fully digital beamforming under mutual coupling. A generalized convex optimization framework is proposed to design the sparse array configuration considering different mutual coupling models. Numerical simulations validate the effectiveness of the proposed algorithm under strong mutual coupling and show that optimal sparse MIMO array with mutual coupling accounted outperforms those designed without considering it in terms of beampattern. The results further indicate that the banded Toeplitz coupling model provides a good approximation of the theoretical impedance-based mutual coupling model.