Compressive sensing based time-frequency joint non-orthogonal multiple access

Non-Orthogonal Multiple Access (NOMA) has been considered to be one of the promising key technologies for future wireless communications and broadcasting return channels due to its high spectral efficiency under massive connectivity. The major challenge of NOMA is to realize interference cancellation and detect the optimal transmitted signals among multiple users. The message passing algorithm (MPA) based multi-user detection (MUD) has been developed to approximate optimal signals from multiple users. However, the conventional MPA always suppose that the user-activity is exactly known at the receiver, which is impractical in real systems. Thus, precise user-activity detection is significant in realizing MPA based NOMA system. In this paper, we propose a compressive sensing based time-frequency joint NOMA scheme in the uplink grant-free low density signature orthogonal frequency division multiplexing (LDS-OFDM) systems, where the priori information obtained from the time-domain m-sequence and the frequency-domain training sequence are utilized for user-activity detection under the framework of CS, while the MPA is performed for the successive user-data detection. The proposed method has a superior performance and less complexity compared to the conventional MPA detector in numerical stimulation.