Cache-Aided MIMO Communications: DoF Analysis and Transmitter Optimization

Cache-aided MIMO communications aims to jointly exploit both coded caching~(CC) and spatial multiplexing gains to enhance communication efficiency. In this paper, we first analyze the achievable degrees of freedom~(DoF) in a MIMO-CC system with CC gain (t), where a server with (L) transmit antennas communicates with (K) users, each equipped with (G) receive antennas. We demonstrate that the enhanced achievable DoF is (\max_{\beta, \Omega} \Omega \beta), where the number of users (\Omega) served in each transmission is fine-tuned to maximize DoF, and (\beta \le \min\big(G, \nicefrac{L \binom{\Omega-1}{t}}{1 + (\Omega - t - 1)\binom{\Omega-1}{t}}\big)) represents the number of parallel streams decoded by each user. Second, we introduce an effective transmit covariance matrix design aimed at maximizing the symmetric rate, solved iteratively via successive convex approximation. Third, we propose a new class of MIMO-CC schemes using a novel scheduling mechanism leveraging maximal multicasting opportunities to maximize delivery rates at given SNR levels while adhering to linear processing constraints. Lastly, we devise linear multicast beamforming strategies tailored for the flexible scheduling schemes in MIMO-CC systems and present an iterative solution for the efficient design of beamformers. Extensive numerical simulations are used to verify the results of the paper.