Capacity of Holographic MIMO Systems with Mutual Coupling

With a massive number of antennas densely deployed in a compact area, holographic multiple-input multiple-output (HMIMO) systems are envisioned to be a key enabling technology for improving the data rate and coverage of 6 G networks. Unfortunately, the reduced spacing between radiation elements, which enables HMIMO to better exploit the channel propagation characteristics, also causes increased mutual coupling (MC) and reduced radiation efficiency. It is thus critical to understand the effect of MC on the capacity of HMIMO systems, which is not yet available in the literature. In this paper, we investigate the ergodic mutual information (EMI) and associated capacity-achieving transmit covariance design for HMIMO systems with MC. To this end, we first derive the closed-form expression for the EMI of HMIMO systems with MC, by leveraging random matrix theory (RMT). Then, based on the derived results, we propose an MC-aware algorithm to maximize the EMI by optimizing the transmit covariance matrix. Numerical simulations validate the accuracy of the theoretical analysis and the effectiveness of the proposed MC-aware algorithm. It is observed that the halfwavelength antenna spacing is not optimal especially with low signal-to-noise ratio.