NOMA

NOMA, or non-orthogonal multiple access, is a wireless communication technique that allows multiple users to share the same time and frequency resources. It does so by exploiting differences among users' channels or codes rather than separating users with orthogonal resources. The core idea is to transmit superposed signals and separate them at receivers using successive interference cancellation, aiming to improve spectral efficiency and user fairness.

In power-domain NOMA, users are superposed with different power levels and served on the same resource block. Receivers use successive interference cancellation, decoding the stronger users first and subtracting their signals to reveal weaker-user signals. The choice of user pairing and power allocation schedules the interference and determines performance. Code-domain NOMA, such as sparse code multiple access (SCMA), pattern division multiple access (PDMA), and multi-user shared access (MUSA), uses distinct codebooks or signaling structures to multiplex users in the same resource blocks; detection relies on multi-user detectors at the receiver.

Deployment considerations: Downlink transmission is the most common setting for NOMA; uplink is more complex due to SIC requirements at the base station and in mobile devices. Implementing effective SIC requires accurate channel state information and synchronization. NOMA introduces extra receiver complexity and can suffer from error propagation; its gains depend on user channel disparities and network load. When combined with multiple-input multiple-output (MIMO) and beamforming, NOMA can enhance capacity in dense networks.

Status: NOMA has been widely studied in academia and standardization forums as a potential enhancement for 5G and future networks (6G). It is not yet a dominant commercial feature of current 5G deployments, which largely rely on orthogonal multiple access in combination with MIMO. Ongoing research explores optimization, practical signaling, cross-layer design, and integration with other access schemes.