Downlink Visible Light Communication Research Articles

Mobile Phone Camera-Based Indoor Visible Light Communications With Rotation Compensation

Downlink visible light communication (VLC) using a mobile phone camera is presented. The proposed scheme consists of an 8 × 8 light-emitting diode (LED) array for data transmission and eight white LEDs for an adequate level of illumination. The white LEDs are also used to acquire camera focus and light metering. Moreover, an efficient rotation compensation feature is provided in the form of a special header frame called keyframe to allow multiple users to receive the data simultaneously at 90°, 180°, and 270° orientations. Unlike conventional LED allocation methods for the rotation compensation, this keyframe-based method improves the data rate significantly. The experimental results demonstrate that the data rate achieves up to 1280 bps, whereas the theoretical simulations exhibit a potential data rate of up to 5120 bps using the identical LED array. The proposed scheme can be considered to be an efficient short-range camera-based VLC in an indoor environment with advanced features such as illumination, relatively high data rate, and robust camera rotation compensation for multiuser access.

Visible light communications heterogeneous network (VLC-HetNet): new model and protocols for mobile scenario

In order to resolve the crowded radio spectrum of wireless communication systems, visible light communications (VLC), which uses a vast unregulated and free light spectrum, has emerged to be a viable solution. However, duplex communication, user mobility, multi-user access and transmission mechanisms are becoming challenging tasks in VLC network. In this article, we propose a new VLC heterogeneous network (VLC-HetNet) model which merges VLC and radio frequency (RF) network. VLC channel is only used for downlink transmission, while RF channels are served for uplinks in any situation, or for downlinks only without VLC hotspots coverage. New VLC frame, multi-user access mechanism, horizontal and vertical handover protocols are presented to support the VLC-HetNet model, especially for solving the problems at multi-user mobility scenario. Simulation results show improvements in capacity performance of the VLC-HetNet, when compared to RF system. Besides, the average vertical handover number and VLC downlink dwelling time ratio have been analysed.

Non-Orthogonal Multiple Access for Visible Light Communications

The main limitation of visible light communication (VLC) is the narrow modulation bandwidth, which reduces the achievable data rates. In this paper, we apply the non-orthogonal multiple access (NOMA) scheme to enhance the achievable throughput in high-rate VLC downlink networks. We first propose a novel gain ratio power allocation (GRPA) strategy that takes into account the users' channel conditions to ensure efficient and fair power allocation. Our results indicate that GRPA significantly enhances system performance compared to the static power allocation. We also study the effect of tuning the transmission angles of the light emitting diodes (LEDs) and the field of views (FOVs) of the receivers, and demonstrate that these parameters can offer new degrees of freedom to boost NOMA performance. Simulation results reveal that NOMA is a promising multiple access scheme for the downlink of VLC networks.

Users First: User-Centric Cluster Formation for Interference-Mitigation in Visible-Light Networks

Visible light communication (VLC) combined with advanced illumination may be expected to become an integral part of next-generation heterogeneous networks. In order to mitigate the performance degradation imposed by the intercell-interference (ICI), a user-centric (UC) cluster formation technique employing vectored transmission (VT) is proposed for the VLC down-link system, where multiple users may be simultaneously supported by multiple access points (APs). In contrast to the traditional network-centric (NC) design, the UC-VT cluster formation is dynamically constructed and adjusted, rather than remaining static. Furthermore, we consider the critical issue of multiuser scheduling (MUS) relying on maximizing the “sum utility” of this system, which leads to a joint cluster formation and MUS problem. In order to find a practical solution, the original problem is reformulated as a maximum weighted matching (MWM) problem relying on a user-AP distance-based weight and then a low-complexity greedy algorithm is proposed, which offers a suboptimal yet compelling solution operating close to the optimal value found by the potentially excessive-complexity exhaustive search. Our simulation results demonstrate that the proposed greedy MUS algorithm combined with the UC-VT cluster formation is capable of providing an average user throughput of about 90% of the optimal throughput, which is about three times the throughput provided by the traditional cellular design in some of the scenarios considered.

Multiuser MISO Transceiver Design for Indoor Downlink Visible Light Communication Under Per-LED Optical Power Constraints

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.