What are the advantages and challenges of using non-orthogonal multiple access (NOMA) for IoT?

1 How NOMA works

NOMA is based on the principle of superposition coding and successive interference cancellation (SIC). Superposition coding means that the transmitter encodes and combines the signals of different users with different power coefficients, creating a single composite signal. SIC means that the receiver decodes and subtracts the signals of different users in a certain order, based on their power levels. The user with the highest power level is decoded first, and its signal is removed from the composite signal. Then, the user with the second highest power level is decoded, and so on, until the user with the lowest power level is decoded. This way, NOMA can achieve higher spectral efficiency and user fairness than orthogonal multiple access (OMA), which allocates different frequency and time resources to different users.

3 Challenges of NOMA for IoT

NOMA also has several challenges for IoT, mainly related to the complexity and security of the system. First, NOMA requires accurate channel state information (CSI) at the transmitter and the receiver, to optimize the power allocation and the decoding order of the users. However, obtaining and updating CSI for massive and dynamic IoT devices can be costly and impractical, especially in fading and noisy environments. Moreover, CSI errors can degrade the performance and fairness of NOMA, leading to inter-user interference and decoding errors. Second, NOMA requires sophisticated signal processing and computation at the receiver, to perform SIC and decode multiple signals. This can increase the energy consumption and hardware requirements of IoT devices, which are usually constrained by battery life and processing capacity. Moreover, SIC can introduce error propagation and decoding delay, affecting the reliability and latency of IoT communications. Third, NOMA poses security risks for IoT devices, due to the exposure of their signals and power levels to other users. For example, malicious users can eavesdrop or jam the signals of legitimate users, or exploit the power allocation to launch denial-of-service or impersonation attacks. Therefore, NOMA needs to adopt effective encryption and authentication mechanisms to protect the privacy and integrity of IoT data.

4 Here’s what else to consider

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