Physical Layer Secure Transmission for NOMA-Aided Cell-Free Massive MIMO Networks

To address the challenges of limited network access and reduced scalability in cell-free massive multiple-input multiple-output (MIMO) networks due to users sharing the same time-frequency resource blocks, secure transmission using non-orthogonal multiple access (NOMA) was studied. Physical layer security issues like pilot spoofing and meeting varying security requirements by categorizing user devices as secure or regular were tackled. The research considered the use of a conjugate beamforming precoder under short-term power constraints and implemented successive interference cancellation (SIC) in the downlink. Closed-form expressions for the achievable ergodic rate of users and the upper bound of the ergodic eavesdropping capacity were derived. The network security performance was evaluated in detail by the achievable secrecy rate and sum achievable secrecy rate. Simulation results confirmed that NOMA technology significantly boosts user access capacity. However, the SIC in NOMA systems has a cumulative effect, emphasizing the need for precise SIC implementation to achieve secure transmission in NOMA-aided Cell-Free Massive MIMO networks.