Differential Modulation Based Non-Coherent Grant-Free NOMA Under Timing Offsets

In this paper, we propose a new transmission scheme for grant-free non-orthogonal multiple access (GF-NOMA) based on orthogonal frequency division multiplexing (OFDM) in the presence of timing offsets. The proposed method utilizes differential phase-shift keying (DPSK) between subcarriers, eliminating the need for channel estimation and mitigating the degradation of data detection caused by timing offsets. Furthermore, the proposed method conducts active user detection based on the coordinate descent method, which offers a lower computational complexity compared to the conventional method based on the structured generalized approximate message passing (S-GAMP). Computer simulations demonstrate that the proposed method achieves a lower miss detection probability compared to S-GAMP and a lower bit error rate than the method using PSK.