Fresnel transform orthogonal chirp modulation of non-orthogonal multiple access optical communication systems

The integration of power-domain non-orthogonal multiple access (PD-NOMA) with visible light communications (VLC) effectively improves spectrum utilization. However, in NOMA-VLC systems, orthogonal frequency division multiplexing (OFDM) is susceptible to indoor multipath effects, which degrades system reliability and communication efficiency. To address this issue, this paper proposed a chirp modulation technique based on the fresnel transformation to enhances data transmission reliability by suppressing multipath effects. The PD-NOMA-VLC system model under the visible light channel was analyzed, and a PD-NOMA-VLC system based on orthogonal chirp division multiplexing (OCDM) was designed. A fixed power allocation method was employed according to the users' channel gains to achieve NOMA multi-user multiplexing in the power domain.The discrete fresnel transform was utilized to perform OCDM modulation, effectively mitigating multipath effects. Theoretical analysis and experimental results demonstrated that the PD-NOMA-VLC system modulated by the proposed method met the forward error correction bit error rate threshold. The communication performance for user 1 and user 2 improves by 3.3 dB and 3.2 dB, respectively, with an average improvement of 3.25 dB for both users, effectively suppressing multipath effects. Furthermore, at modulation orders of 16, 32, 64, 128, and 256, the system achieves bit error rate (BER) performance improvements of 2.3 dB, 2.4 dB, 2.8 dB, 6.1 dB, and 4.2 dB, respectively, thereby enhancing communication efficiency while ensuring reliability.