Physical Layer Security of Partial-NOMA and NOMA in Poisson Networks

Security is an issue in non-orthogonal multiple access (NOMA) and partial-NOMA because a user may decode the message of its paired-user with which it shares a resource element (RE). Three scenarios are studied where, of the paired-users, the eavesdropper is: 1) an actively malicious strong-user, 2) a passive strong-user, 3) an actively malicious weak-user. We define the event of secure-communication in each scenario and derive the corresponding secrecy probabilities for partial-NOMA and NOMA. Our results highlight that with careful selection of the RE's overlap α, partial-NOMA can significantly outperform NOMA in terms of secrecy probability. Further, careless selection of α can cause partial-NOMA to perform worse than NOMA. We show the non-trivial impact of incorporating the impact of intercell interference on secrecy. Our results shed light on parameter-selection if knowledge of the eavesdropper type is available highlighting that security can be improved without traditional techniques such as jamming that increase power consumption and interference. While NOMA decoding uses successive-interference-cancellation (SIC), partial-NOMA decoding employs receive-filtering followed by flexible-SIC (FSIC). We show that not employing receive-filtering or using SIC instead of FSIC can have a drastic negative impact on secrecy, highlighting the role of the partial-NOMA decoding approach in enhancing secure-communication.