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What Is Non-Orthogonal Multiple Access?

What Is NOMA?

Non-Orthogonal Multiple Access (NOMA) is a multiple-access technique that allows two or more users to share the same frequency, time, and coding resources simultaneously. Unlike conventional multiple-access methods such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Orthogonal Frequency Division Multiple Access (OFDMA), which separate users into orthogonal communication resources, NOMA intentionally allows users to overlap. The users are then separated at the receiver using advanced signal-processing techniques.

The most widely studied form of NOMA is power-domain NOMA. In this approach, multiple users are transmitted simultaneously on the same radio channel but at different power levels. Users with weaker radio channels are allocated higher transmission power, while users with stronger channels receive lower power. At the receiver, a technique known as Successive Interference Cancellation (SIC) is used to separate the overlapping signals by decoding and removing the stronger signals before recovering the weaker ones.

A useful analogy is listening to two people speaking simultaneously, one loudly and one quietly. By first concentrating on the louder speaker and mentally ignoring that conversation, it becomes easier to hear the quieter speaker afterwards. Successive interference cancellation performs a similar process mathematically by decoding one signal and subtracting it from the combined received waveform before decoding the next.

One of the principal advantages of NOMA is improved spectral efficiency. Since multiple users share the same communication resources simultaneously, the available spectrum can support more users and higher overall throughput than traditional orthogonal multiple-access techniques. NOMA can also improve fairness by allowing users with poorer radio channels to receive greater transmission power without requiring dedicated frequency or time resources.

NOMA has attracted considerable interest for dense wireless networks, Internet of Things (IoT) systems, and future 6G communication systems, where very large numbers of devices may need to communicate simultaneously. It has also been investigated for satellite communications and integrated terrestrial-satellite networks as a means of improving overall system capacity.

The principal disadvantage of NOMA is increased receiver complexity. Successive interference cancellation requires accurate channel estimation and additional signal processing, and any errors in decoding one user's signal may affect the recovery of subsequent users. As the number of simultaneous users increases, both the computational complexity and the sensitivity to decoding errors also increase.

It is important to distinguish NOMA from OFDMA. In OFDMA, users are assigned different groups of orthogonal subcarriers, ensuring that their transmissions do not interfere with one another. In NOMA, users intentionally share the same communication resources, with the receiver relying on signal-processing techniques to separate them. Consequently, OFDMA generally offers lower receiver complexity, while NOMA has the potential to achieve higher spectrum utilization under suitable operating conditions.

Today, NOMA remains an active area of communications research. Although commercial 4G and 5G systems rely primarily on OFDMA, NOMA continues to be investigated for future wireless networks because of its potential to increase network capacity, improve spectrum efficiency, and support the enormous number of devices expected in next-generation communication systems. While its widespread deployment is still evolving, NOMA represents one of the most promising candidates for future multiple-access technologies.