Physical Layer Security for NOMA-OFDM Using Power Hopping

This article presents a physical layer security (PLS) technique for non-orthogonal multiple access (NOMA), which can provide joint secrecy, fairness, and bit error rate (BER) improvement. The approach employs power hopping (PH), where users’ power coefficients are dynamically varied according to the instantaneous channel conditions. By randomizing both the detection order and the effective power allocation, PH obscures the underlying NOMA structure from a passive eavesdropper (Eve), thereby severely degrading the data detection capability. To enable secure coordination, a precoding-based mechanism is developed for sharing the PH patterns among legitimate users, leveraging the random and independent nature of wireless channels. The BER and secrecy outage probability (SOP) of the proposed framework are analytically characterized and compared to conventional NOMA under various channel and eavesdropping configurations. The obtained analytical results, corroborated by Monte Carlo simulations, confirm the effectiveness of the proposed scheme, where the SOP of legitimate users is never compromised regardless of Eve’s channel conditions, while incurring only negligible additional complexity compared to conventional NOMA. Furthermore, the fairness and BER performance of stronger users are significantly improved through the combined use of PH and optimal power allocation.