Light-emitting diode (LED)-based visible light communication (VLC), combining illumination and communication, is a promising technique for providing high-speed, low-cost indoor wireless services. In indoor environments, multiple LEDs routinely used as lighting sources may also be concomitantly invoked to support wireless services for multiple users, thus forming a multiuser multiple-input-single-output (MU-MISO) system. Since the user terminals detect all the light rays impinging from multiple LEDs, inter-user interference may severely degrade the attainable system performance. Hence, we conceive a transceiver design for indoor VLC MU-MISO systems to suppress the multiuser interference (MUI). In contrast to classic radio-frequency (RF) communication, in VLC, the signals transmitted from the LEDs are restricted by optical constraints, such as the real-valued nonnegativity of the optical signal, the maximum permissible optical intensity, and the constant brightness requirements of the LEDs. Given these practical constraints, we design the optimal transceiver relying on the objective function of minimizing the maximum mean square error (MMSE) between the legitimate transmitted and received signals of the users and show that it can be readily found by solving a convex second-order cone program. Then, we also propose a simplified transceiver design by incorporating zero-forcing (ZF) transmit precoding (TPC) and show that the TPC coefficients can be efficiently found by solving a linear program. The performance of both the optimal and the simplified transceiver is characterized by comprehensive numerical results under diverse practical VLC system setups.

The user-centric (UC) association in optical wireless communication (OWC) forms amorphous cells (A-Cells) by considering the dynamic distribution and load demand of user equipments (UEs). This philosophy offers advantages over the conventional network-centric (NC) association that purely relies on a pre-defined and fixed network configuration, in terms of alleviating undesired inter-cell interference (ICI) and achieving superior system performance. However, constructing the optimal A-Cells for a given OWC network, including determining the appropriate number of A-Cells associated to their contained UEs, is deeply integrated with the UEs’ distribution and transmission conditions. To address the intractable issue, in this paper, we conceive an adaptive UC-OWC network that relies on a feedback-guided iterative framework, which is capable of jointly optimizing A-Cells formation, modulation-mode assignment and power allocation strategies. For the sake of attaining the optimized throughput of this adaptive network, we initialize the UC association by the designed k-means based genetic algorithm (KGA), which can then be iteratively adjusted based on the throughput feedback obtained via our proposed multi-user cross Q-learning (MUCQ) resource allocation algorithm. Simulation results indicate that, compared to conventional counterparts, our adaptive UC-OWC network is able to significantly improve throughput performance and reduce outage probability.

Hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) improves the spectral efficiency compared to asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) modulation, while retaining the advantage of high power efficiency. HACO-OFDM has found favor in numerous applications, but its peak-to-average-power ratio (PAPR) has remained a concern in optical wireless communications, since the family of existing PAPR reduction methods cannot be directly invoked for the superimposed HACO-OFDM signals. Hence, we analyze the characteristics of HACO-OFDM signals and develop a specific PAPR reduction technique relying on tone injection. Our numerical results show that the proposed method achieves significantly improved PAPR statistics compared to the conventional methods, leading to a significant bit error ratio (BER) reduction.

Capable of supporting the dual functionalities of dimming control and high-speed data transmission, dimmable optical orthogonal frequency division multiplexing (O-OFDM) has attracted increasing attentions in visible light communication (VLC). A major design challenge in the dimmable O-OFDM is to realize spectrally efficient VLC link compatible with dimming control while conforming to the low-complexity philosophy of the intensity modulation/direct detection (IM/DD) system. To cope with this challenge, a novel scheme is conceived, named as adaptively biased layered O-OFDM (ABLO-OFDM), which can be well integrated with the industry-preferred pulse width modulation (PWM) dimming technique. Similar to the conventional layered asymmetrically clipped O-OFDM (LACO-OFDM), multiple layers of subcarriers are utilized to transmit information data in ABLO-OFDM, resulting in high spectral efficiency. Only one inverse fast Fourier transform (IFFT) block is required in ABLO-OFDM to generate the layered O-OFDM signal compared to multiple IFFT blocks in LACO-OFDM. Moreover, the demodulation of the dimmable ABLO-OFDM (DABLO-OFDM) signal can straightforwardly rely on standard OFDM receiver, which is immune to the dimming process. Therefore, the proposed scheme achieves high spectral efficiency whilst possessing low-complexity transceiver compared to conventional O-OFDM. It is shown that DABLO-OFDM is superior to its counterparts in terms of the achievable spectral efficiency for intermediate dimming levels.

In direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM), a constant DC bias is introduced to render the bipolar signal non-negative. However, due to the inherent disadvantage of high peak-to-average power ratio (PAPR), the DC bias should be set to a relatively high level to suppress the clipping distortion, resulting in power inefficiency. Therefore, we conceive a novel adaptively biased optical OFDM (ABO-OFDM) for optical wireless communication (OWC), where the bias is dynamically optimized according to the signal magnitude to enhance the power efficiency. Meanwhile, the interference induced by the adaptive bias is placed on the reserved subcarriers, which avoids contaminating the legitimately transmitted information data. The implementation complexity of the ABO-OFDM transceiver is almost the same as asymmetrically clipped optical OFDM (ACO-OFDM), which is notably lower than the superimposed OFDM schemes. Numerical results have demonstrated that the proposed ABO-OFDM outperforms the conventional DCO-OFDM and ACO-OFDM in terms of power efficiency.