RSMA for Overloaded MIMO Networks: Low-Complexity Design for Max-Min Fairness

Rate-Splitting Multiple Access (RSMA) is a robust multiple access scheme for multi-antenna wireless networks. In this work, we study the performance of RSMA in downlink overloaded networks, where the number of transmit antennas is smaller than the number of users. RSMA has been investigated in overloaded networks in previous works by formulated optimization problems and their solutions by interior-point methods. This limits the practical use of such designs in practical systems. Our aim is to develop low-complexity precoding, rate, and power allocation techniques for RSMA for its use in practical overloaded networks. First, we provide analysis and closed-form expressions for optimal power and rate allocations considering max-min fairness when low-complexity precoders are employed. The derived closed-form solutions are used to propose a low-complexity RSMA system design for precoder selection and resource allocation for arbitrary number of users and antennas under perfect and imperfect Channel State Information at the Transmitter (CSIT). We compare the performance of the proposed design with benchmark designs based on Space Division Multiple Access (SDMA) with and without user scheduling. By numerical results, we show that the proposed low-complexity RSMA design achieves a significantly higher rate compared to the SDMA-based benchmark designs under perfect and imperfect CSIT.