Multi-cell Visible Light Communication Research Articles

Coherence Programming for Efficient Linearly Polarized Perovskite Light-Emitting Diodes.

Although quasi-two-dimensional (quasi-2D) perovskites are ideal material platforms for highly efficient linearly polarized electroluminescence owing to their anisotropic crystal structures, so far, there has been no practical implementation of these materials for the demonstration of linearly polarized perovskite light-emitting diodes (LP-PeLEDs). This scarcity is due to difficulty in orientation and phase distribution control of the quasi-2D perovskites while minimizing the defects, all of which are required to manifest aligned transition dipole moments (TDMs). To achieve this multifaceted goal, herein, we introduce a synergistic strategy to quasi-2D perovskites by incorporating both a trimethylolpropane triacrylate anchoring layer and 18-Crown-6 molecular passivator into the film fabrication process. It is found that the interfacial anchoring layer guides the oriented growth of perovskites along the (110) plane, whereas the molecular passivator reduces the number of defects and homogenizes the crystal phase. As a result, a quasi-2D perovskite film with macroscopically aligned TDM that renders high radiative recombination and the degree of linear polarization (DoLP) is constructed. This "coherence-programmed emission layer" demonstrates highly efficient LP-PeLEDs, not only achieving a maximum external quantum efficiency of ∼23.7%, a brightness of ∼36,142 cd/m2, and a DoLP of ∼38%, but also significantly improving the signal-to-interference-and-noise ratio in a multi-cell visible light communication system.

Impact of different receiver geometries on a reconfigurable intelligent surface assisted multi-cell VLC system in the presence of light path blockage.

We report the effect of integrating metasurface-aided reconfigurable intelligent surfaces (RISs) on the signal-to-interference-plus-noise ratio (SINR) and data rate of a multi-cell visible light communication (VLC) system. RIS has been deployed in the channel between transmitter and receiver to redirect the reflected light in the desired directions, even in the absence of line-of-sight (LoS) links. Results show that the introduction of RIS has improved average SINR but reduced average illumination level compared to a no-RIS system. As the quantity of RIS increases, a discernible improvement in the maximum SINR value is observed. Here, three different receiver geometries, namely, a photodiode (PD), freeform diversity receiver (FDR), and modified FDR (MFDR), have been adopted. The impact of individual receivers has been reported in the presence of light path blockage. MFDR geometry is found to be most suitable with more coverage probability compared to the other two receivers. With (40c m×24c m) RIS area, during blockage, MFDR maintains an average SINR of 21.95dB, which is 97.29% and 14.24% greater than PD and FDR, respectively.

Interference Management Strategies for Multiuser Multicell MIMO VLC Systems

This paper investigates different precoding strategies for rate splitting multiple access (RSMA) in the downlink of multi-cell visible light communication (VLC) networks. Since classical Shannon capacity formula does not hold for VLC, we first provide a lower bound on the channel capacity for RSMA in such interfering networks. Then, we formulate a spectral efficiency maximization problem to jointly find the optimal rate-splitting and transmit precoding. Beside that, since cell-edge users suffer from additional inter-cell interference, we propose to design the precoders of different RSMA signals utilizing coordinated beamforming (CB). Subsequently, aiming to improve the performance of the CB design for RSMA, while maintain a reduced complexity, we introduce two enhanced precoding strategies for RSMA. To the best of the authors’ knowledge such a contribution has not been considered before in the open literature. It is shown in the paper that the formulated optimization problem is non-convex and a sub-optimal, yet, a low complexity solution can be obtained efficiently using semi-definite relaxation combined with successive convex approximation. Through analytical results, we illustrate the flexibility and superiority of the proposed precoding strategies for RSMA over conventional coordinated space division multiple access and non-orthogonal multiple access for different scenarios and network loads.

Coordinated Beamforming Design for Multi-User Multi-Cell MIMO VLC Networks

Inter-cell interference (ICI) and inter-user interference (IUI) constitute a major issue towards achieving the optimum spectral efficiency (SE) and energy efficiency (EE) performance in multi-cell visible light communication (VLC) networks. Hence, advanced multiple access techniques need to be leveraged in order to improve the provided service to the users of such interfering networks. To this end, the present contribution proposes the integration of coordinated beamforming (CB) with rate-splitting multiple access (RSMA) in multi-cell VLC systems. Specifically, we consider the design of beamformers for the common and private streams in a coordinated manner between different attocells, which is shown to provide efficient mitigation of the incurred interference. Additionally, the formulated optimization problem aims to minimize the sum of the mean squared error across all attocells in order to jointly determine the optimum receive filters and coordinated transmit beamformers for RSMA streams. In this context, we illustrate through extensive computer simulations, which are carried out in a realistic setup that assumes noisy channel state information acquisition, the distinct flexibility and robustness of CB-based RSMA in mitigating the incurred interference. Finally, the offered results demonstrate the superiority of CB-based RSMA in terms of achievable SE and EE performance in multi-cell VLC networks compared to the conventional CB-based space division multiple access counterpart.

Design of an off-axis freeform diversity receiver to improve SINR performance of a multi-cell VLC system

This paper introduces a novel and promising design of a freeform diversity receiver (FDR) to reduce the inter-channel interference (ICI) of multi-cell visible light communication (VLC) system. Based on the potential of rotationally non-symmetric freeform optics, we have established the design of an off-axis VLC receiver front-end. In this study, we have performed the detailed design of the freeform surface profiles and have analysed the corresponding fabrication tolerances. Nonetheless, the communication performance of the VLC system using the proposed design has been evaluated with the help of Monte-Carlo based ray-tracing technique. Results show that the system with the proposed FDR significantly outperforms the existing state-of-the-art multi-cell VLC receivers in terms of signal-to-interference-plus-noise ratio (SINR). The spatial SINR ranges from 98 dB to 137 dB while 120.5 dB average SINR is attained over the communication floor. This compact size receiver (15.12 mm × 15.12 mm × 6.76 mm) with substantial wireless reception reliability has prospective future applications in the emerging field of visible light communication and its counterpart Li-Fi.

Optimal Design of Aperture Array Receivers for Indoor Multicell Visible Light Communication System

Optimal Design of Aperture Array Receivers for Indoor Multicell Visible Light Communication System

Performance analysis of user association strategy based on power-domain non-orthogonal multiple access in visible light communication multi-cell networks

A multi-cell visible light communication (VLC) network with non-orthogonal multiple access (NOMA) introduces an inter-cell interference (ICI), which causes poor experience of overlap-area user association. In this study, we focus on the strategic analysis of user association in multi-cell NOMA-VLC networks and reveal the relationship between the performance of user association and visible-light cell deployment. First, we establish the model of power-domain NOMA in multiple cells of VLC. Second, when the principle of proximity access (PPA) considers channel attenuation, we present the violation of the principle of proximity access (VPPA) based on signal-to-interference-plus-noise ratio (SINR) and derive a closed-form sufficient condition for overlap-area users based on the visible light channels. Third, based on the sufficient condition, we perform the strategic analysis of user association in two cases, i.e., the scenario probability with different numbers of users and the probability of the ratio of channel gains with an edge user are deduced. In addition, we evaluate the performance of user association by the deployment of lighting angles and inter-cell distance. Finally, the simulation results indicate that the VPPA achieves better data rate performance than the PPA based on the sufficient condition. Furthermore, the probability distributions in numerical results show that the situations with the VPPA reach a certain scale of occurrence probability.

Enhanced Performance of Asynchronous Multi-Cell VLC System Using OQAM/OFDM-NOMA

In this paper, an offset QAM/OFDM combined with non-orthogonal multiple access (OQAM/OFDM-NOMA) modulation technique is proposed and experimentally demonstrated in a multi-user and asynchronous multi-cell visible light communication (VLC) system. OQAM/OFDM-NOMA modulation can effectively improve system capacity and flexibility owing to non-requirement of cyclic prefix (CP) and flexible power allocation strategies. We evaluate the performance comparison between the proposed OQAM/OFDM-NOMA modulation and conventional OFDM-NOMA modulation in the multi-cell VLC system. Each cell consists of cell-center users with high signal-to-noise ratios (SNRs) and cell-edge users with low SNRs. According to the measured bit error rates (BERs) for the cell-center users and the cell-edge users, better user fairness can be achieved when the power ratio is 8 dB. Moreover, in the presence of asynchronous time offset for multi-cell VLC system, the experimental results show that the asynchronous time offset has a negligible impact on the cell-edge users using OQAM/OFDM-NOMA modulation. However, the asynchronous time offset will severely degrade the system performance in conventional OFDM-NOMA based VLC system. Therefore, for the user fairness and robustness against asynchronous time-offset, the proposed OQAM/OFDM-NOMA modulation can significantly outperform the conventional OFDM-NOMA modulation in the multi-user asynchronous multi-cell VLC system.

Reduction of SINR Fluctuation in Indoor Multi-Cell VLC Systems Using Optimized Angle Diversity Receiver

Due to severe inter-cell interference (ICI), the signal-to- interference-and-noise ratio (SINR) suffers from very significant fluctuation in indoor multi-cell visible light communication (VLC) systems, which greatly limits the overall system performance. In this paper, we propose and evaluate a generalized angle diversity receiver (ADR) structure to reduce SINR fluctuation in indoor multi-cell VLC systems. The generalized ADR consists of one top detector and multiple inclined side detectors. In an indoor multi-cell VLC system using the generalized ADR, the inclination angle of the side detectors is optimized in order to minimize the SINR fluctuation over the receiving plane, where the impact of receiver random rotation is considered. An optimized ADR with different numbers of detectors is analyzed with different LED layouts and diversity combining techniques in a typical indoor environment. Analytical results show that the SINR fluctuation is gradually reduced when more detectors are equipped in the optimized ADR. In an indoor two-cell VLC system, much more significant SINR fluctuation reduction is achieved by using select-best combining (SBC). However, nearly the same SINR fluctuations are obtained in an indoor four-cell VLC system when using SBC and maximal-ratio combining (MRC). By applying the optimized ADR, up to 15.9 and 32.4-dB SINR fluctuation reductions can be achieved in comparison to a conventional single-element receiver (SER) in indoor two-cell and four-cell VLC systems, respectively.