8.10.3 Non-Orthogonal Multiple Access (NOMA)

Non-orthogonal multiple access represents a different departure from classical partitioning. Instead of separating users strictly in frequency, time, code, or space, NOMA allows multiple users to share identical time–frequency resources through power-domain superposition combined with advanced receiver processing.

In a typical NOMA configuration, signals intended for different users are superimposed at the transmitter with distinct power levels. Users with weaker channel conditions are generally assigned higher transmit power, while users with stronger channel conditions are assigned lower power. At the receiver, successive interference cancellation (SIC) is employed to separate the composite signal. The receiver first decodes the higher-power signal, subtracts it from the received waveform, and then decodes the remaining lower-power signal.

Under favorable channel conditions and with accurate channel-state information, NOMA can improve spectral efficiency and enhance fairness among users with disparate link qualities. It has been studied extensively in the context of 5G and beyond, including extensions to multi-antenna systems, cooperative relaying, and coordinated multi-point transmission.

However, NOMA introduces significant practical challenges. Successive interference cancellation increases receiver complexity and power consumption. Performance depends critically on accurate channel-state information and precise power control. In scenarios with large power disparities, mobility, or long propagation delays, maintaining stable SIC performance can be difficult. For these reasons, orthogonal multiple-access schemes remain the baseline in most standardized broadband systems, with NOMA largely confined to research studies and limited experimental deployments.

NOMA has also been investigated for satellite and non-terrestrial networks, where power-domain multiplexing may offer theoretical spectral-efficiency gains. Nevertheless, practical deployment remains limited, as receiver complexity, synchronization constraints, and link-budget disparities present substantial challenges.