What Is NOMA?

Non-orthogonal multiple access (NOMA) is a radio access technique in which multiple users share the same time-frequency resource block simultaneously, differentiated from one another through power levels or code domains rather than through orthogonal frequency or time allocations. Conventional access schemes such as OFDMA assign each user a separate subcarrier or time slot, which limits the number of simultaneous connections to the number of orthogonal resources available. NOMA removes this constraint, allowing the system to serve more users per unit of spectrum at the cost of requiring more sophisticated receivers to separate overlapping transmissions.

The technique draws on superposition coding theory, originally developed in information theory by Cover and Bergmans in the 1970s, combined with successive interference cancellation (SIC) at the receiver. Power-domain NOMA, the form most widely studied for cellular systems, and code-domain NOMA, which distinguishes users through sparse spreading codes, represent the two primary branches of the field.

Power-Domain NOMA and Successive Interference Cancellation

In power-domain NOMA, the base station transmits the superimposed signals of all served users simultaneously on the same subcarrier, assigning higher transmit power to users with weaker channel conditions and lower power to users with stronger channels. At the receiver, SIC decodes the signal in order of power level: the strongest signal is decoded first and subtracted from the received composite before the next signal is decoded. This ordered cancellation process allows each user to recover its intended data despite the intentional co-channel interference. The IEEE Communications Surveys and Tutorials overview of power-domain NOMA in 5G systems examines the capacity analysis, power allocation strategies, user pairing, and the practical trade-offs between system throughput and user fairness that arise in heterogeneous deployment scenarios.

Integration with MIMO and OFDM

NOMA gains additional performance when combined with multiple-input multiple-output (MIMO) antenna systems and orthogonal frequency division multiplexing (OFDM). MIMO-NOMA uses spatial beamforming to cluster users by their channel directions, applying NOMA power-domain multiplexing within each beam cluster and spatial separation between clusters. This combination requires joint design of beamforming vectors and power allocation levels to balance intra-cluster interference against inter-cluster separation. Research on joint power allocation and beamforming for NOMA in 5G millimeter-wave systems shows that carefully optimized MIMO-NOMA outperforms both conventional MIMO-OMA and single-user beamforming in terms of sum rate under typical deployment conditions. OFDM-NOMA extends the power-domain principle across multiple subcarriers, with user grouping and subcarrier assignment added as additional optimization variables.

NOMA in Free-Space Optical and Emerging Networks

NOMA principles extend beyond radio frequency to free-space optical (FSO) communication links, where multiple optical beams or power levels carry simultaneous data streams between transceivers without physical fiber. In FSO-NOMA systems, atmospheric turbulence introduces random fading analogous to multipath fading in radio channels, and SIC-based receivers must account for this channel variability. Code-domain NOMA variants such as sparse code multiple access (SCMA) use multi-dimensional codebooks to spread user signals across time-frequency resources with controlled collision patterns that lower-complexity iterative receivers can resolve. The arXiv survey on NOMA for 5G and beyond covers both power-domain and code-domain variants and their integration with massive MIMO, millimeter-wave bands, and heterogeneous network architectures.

Applications

NOMA has applications in a range of fields and system types, including:

• Massive machine-type communications and IoT networks requiring high connection density
• Cellular radio networks seeking improved spectral efficiency in congested urban deployments
• Free-space optical communication links between satellites and ground stations
• Downlink broadcast channels in heterogeneous networks with mixed user channel qualities
• Cooperative relay networks where NOMA power allocation aids the relay selection process