Optical orthogonal frequency-division multiplexing (OFDM) and probabilistic constellation shaping (PCS) have emerged as powerful techniques to enhance the performance of optical wireless communications (OWC) systems. While PCS improves spectral efficiency and adaptability, we show that its integration with optical OFDM can inadvertently increase the peak-to-average power ratio (PAPR) of the signal, exacerbating clipping distortion due to signal clipping. This letter investigates the impact of PCS on the PAPR of direct current-biased optical OFDM (DCO-OFDM) waveforms and proposes an optimization of PCS that maximizes channel capacity, considering clipping distortion. The optimization problem is shown to be complex and non-convex. We thus present a suboptimal yet efficient solving approach based on projected gradient descent to solve the problem. Simulation results demonstrate the superiority of the proposed approach over the conventional uniform signaling, particularly under severe clipping distortion conditions.

Non-Orthogonal Multiple Access (NOMA) has been widely recognized as a promising multiple access technique to enhance spectral efficiency and user fairness in next-generation wireless networks. However, when applied to Visible Light Communication (VLC) systems, its performance can be limited due to high correlation among users' channels. To address this limitation, this letter proposes an adaptive hybrid NOMA-Time Division Multiple Access (TDMA) scheme for multi-user VLC systems, aiming to improve users' sum-rate performance while maintaining low complexity. In the proposed scheme, users are divided into groups where each group is served in a different time slot using TDMA. Within each group, up to two users can be served simultaneously using NOMA. A central challenge lies in determining which users should be paired together for NOMA, as the effectiveness of Successive Interference Cancellation (SIC) employed by NOMA depends on the difference between users' channel gains. To address this, for a pair of users, we determine the range of their channel gain ratio within which the pair benefits more from NOMA or TDMA. Identifying the lower and upper bounds of this range is formulated as two optimization problems which are solved efficiently using the Successive Convex Approximation (SCA) method. Simulation results demonstrate that the proposed scheme outperforms the conventional hybrid NOMA-TDMA method under different numbers of users and transmit Light-Emitting Diode (LED) powers.