Physical Layer Security for Weak User in MISO NOMA Using Directional Modulation (NOMAD)

This letter investigates the physical layer security of the data from the weak user (WU) against interception by the strong user (SU) in non-orthogonal multiple access (NOMA) systems. The proposed NOMA using directional modulation (NOMAD) allows access of the intended symbols at the WU while giving access to a different but valid lower order symbol alphabet to the SU for performing successive interference cancellation. The optimal lower order symbol alphabet and directional modulation vectors are designed in a multiple-input single-output framework. The sum and secrecy rate results show that NOMAD provides non-zero secrecy rates against the zero secrecy rate of NOMA and also delivers spectral efficiency benefit of NOMA. Further, NOMAD performs better than an orthogonal multiple access, such as a zero-forcing beamformer. A combined NOMA/NOMAD can present trade-offs between sum and secrecy rates. Bit error results are also presented confirming the advantages of the proposed NOMAD.