OFDM-Based Private-Public Communications Relying on RIS: A Simple Framework

A novel orthogonal frequency division multiplexing (OFDM)-based private-public communication framework is proposed, which harnesses reconfigurable intelligent surface (RIS) to improve the sum private and public rates. A two-user RIS-enabled OFDM system is considered, where the total subcarriers are partitioned into three segments, with two exclusively assigned to users for private/secure communications, while the third segment is reserved for public messages. The active beamformer at the transmitter and the passive reflecting beamformer at the RIS are jointly optimized to maximize the summation of the private/secure rate and the public rate. A novel joint design algorithm is proposed by capitalizing on the mathematical structure of the optimal active beamformer, eliminating the need for iterative updates between the transmit beamformer and the RIS-reflecting beamformer. Numerical results underscore the superior computational efficiency of our proposed algorithms compared to state-of-the-art methods based on alternating optimization, all while achieving nearly identical sum-rate performance.