QAMA: Hierarchical QAM based Downlink Multiple Access with a Simple Receiver

In this paper, we propose a nonorthogonal downlink multiple access scheme, where the base station assigns the bit streams to different users from an adjustable hierarchical quadrature-amplitude-modulation (QAM) constellation and all users deploy a simple non-iterative receiver with the same complexity as in an orthogonal multiple access (OMA) system. We analyze the achievable information rates of the proposed scheme, termed as QAMA, and show that it possesses a similar rate region as the hybrid OMA and MUST approach in the 5G standard, while the latter relies on successive interference cancellation (SIC) receivers and dynamic switching of two distinct modules. We propose a design procedure to determine a few transmission modes that closely approximate the rate region via a simple polygon. Thanks to the receiver simplicity, easy scalability to more than two users, and the near-optimal rate region, QAMA is an appealing candidate for 6G cellular and other advanced wireless systems.