Joint In-phase and Quadrature Non-orthogonal Multiple Access for Multi-User VLC

Non-orthogonal multiple access (NOMA) is a promising modulation scheme for enhancing the spectral efficiency in visible light communication (VLC). Unlike radio frequency (RF) systems, the channel conditions for different users in VLC heavily depend on their locations. This introduces a severe user unfairness problem that could limit the use of NOMA in many practical VLC scenarios, particularly those that need to support a large number of users. To address this issue, this paper proposes and demonstrates a new NOMA scheme named joint in-phase and quadrature NOMA (JIQ-NOMA) for a multi-user VLC system. In contrast to the conventional NOMA scheme, JIQ-NOMA independently employs NOMA in both the in-phase (I) and quadrature (Q) domains, utilizing pulse amplitude modulation (PAM). Therefore, an additional degree of freedom for power allocation is introduced, enabling a more efficient solution to address the user unfairness issue in NOMA. Moreover, to optimize the system performance, we also introduces a two-dimensional (2D) power allocation method, which includes I domain/Q domain (I/Q) power allocation to ensure fairness among users within each user pair and joint I domain and Q domain (JIQ) power allocation to guarantee fairness among different user pairs. Simulation and experimental evaluations are both conducted under various channel conditions to validate the benefits of the proposed JIQ-NOMA scheme over conventional orthogonal multiple access (OMA) and NOMA schemes. Experimental results show that, in a four-user VLC system, when applying the proposed JIQ-NOMA scheme compared to the conventional NOMA scheme, the required peak-to-peak voltage (Vpp) used to drive the light-emitting diode (LED) transmitter can be reduced by $21.5\%$ , and the transmission distance can be extended by $16.7\%$ at a target bit error rate (BER) of $3.8 \times 10^{-3}$ .