Quadrature NOMA: A Low-Complexity Multiple Access Technique With Coordinate Interleaving

In this letter, we offer a low-complexity multiple access method based on coordinate interleaving coined as quadrature non-orthogonal multiple access (NOMA). It is shown that by properly utilizing two quadrature carriers, both error performance enhancement and reduction in the number of successive interference cancellation (SIC) operations can be attained as compared to the standard power-domain NOMA approach. Specifically, in a <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>-user network with single-antenna nodes, the reduction in the number of SIC operations is more than 50% as compared to the original NOMA scheme. When <inline-formula> <tex-math notation="LaTeX">$K=2$ </tex-math></inline-formula>, the power gain obtained by the inspected technique can be as large as 2.5 dB without resorting to any SIC process at all. Further, as compared to the original NOMA idea, the considered approach requires fewer number of SIC stability conditions on power allocation and functions for any power allocation strategy (without an error floor) when <inline-formula> <tex-math notation="LaTeX">$K=2$ </tex-math></inline-formula>. Some similar gains are also applicable for <inline-formula> <tex-math notation="LaTeX">$K>2$ </tex-math></inline-formula>. The decrease in the number of SIC operations manifests itself by boosting the immunity of the system against possible failures during the SIC process.