Abstract
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.
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