In general, visible light communication (VLC) systems, which utilise white light-emitting diodes (LEDs), only offer a bandwidth limited to the lower MHz region. Therefore, providing VLC-based high data rate communications systems becomes a challenging task. To address this challenge, we propose a solution based on multiplexing in both the frequency and space domains. We experimentally demonstrate a 4 × 4 imaging multiple-input multiple-output (MIMO) VLC system (i.e., space multiplexing) utilising multi-band carrier-less amplitude and phase (m-CAP) modulation (i.e., frequency multiplexing). Independently, both MIMO and m-CAP have shown the remarkable ability to improve transmission speeds in VLC systems, and hence, here we combine them to further improve the net data rate. We investigate link performance by varying the number of subcarriers m, link distance L, and signal bandwidth Bsig. From all the values tested, we show a data rate of ~249 Mb/s can be maximally achieved for for m = 20, Bsig = 20 MHz, and L = 1 m, at a bit error rate of 3.2×10-3 using LEDs with ~4 MHz bandwidth. Index Terms-Modulation bandwidth, multi-band carrier-less amplitude and phase modulation, multiple-input multiple-output, visible light communications.
Underwater wireless optical communications is an emerging solution to the expanding demand for broadband links in oceans and seas. In this paper, a cellular underwater wireless optical code division multiple-access (UW-OCDMA) network is proposed to provide broadband links for commercial and military applications. The optical orthogonal codes (OOC) are employed as signature codes of underwater mobile users. Fundamental key aspects of the network such as its backhaul architecture, its potential applications and its design challenges are presented. In particular, the proposed network is used as infrastructure of centralized, decentralized and relay-assisted underwater sensor networks for high-speed real-time monitoring. Furthermore, a promising underwater localization and positioning scheme based on this cellular network is presented. Finally, probable design challenges such as cell edge coverage, blockage avoidance, power control and increasing the network capacity are addressed. Index Terms-Underwater wireless optical communications, optical CDMA networks, underwater sensor networks, relayassisted transmission, MIMO, localization and positioning, power control.Among many multiple-access schemes, optical code division multiple access (OCDMA) is receiving much attention as a promising access technique to share common resources among asynchronous users without any central controller, which is highly desirable in underwater environment. Amongst the first generation OCDMA-based systems, using optical orthogonal codes (OOC) in fiber-optic communications was introduced by Salehi in 1989 [4], while capability of this scheme to free space, infrared indoors and visible light communication have been recently studied [5]. Furthermore, performance analysis of an underwater wireless optical CDMA (UW-OCDMA) network is presented in [6]. In this article, we elaborate possible challenges and potential applications of cellular UW-OCDMA network based on OOCs. In a typical UW-OCDMA network, mobile and fixed users communicate to an optical base transceiver station (OBTS). Each active user transmits its data using a unique OOC code.In particular, this article describes the proposed UW-OCDMA network architecture and discusses its potential application in local sensor networks and underwater localization. Furthermore, possible challenges regarding blockage avoidance, cell edge coverage, power control algorithms, and increasing the number of active users are discussed.
Optical wireless communications (OWC) are emerging as cost-effective and practical solutions to the congested radio frequency-based wireless technologies. As part of OWC, optical camera communications (OCC) have become very attractive, considering recent developments in cameras and the use of fitted cameras in smart devices. OCC together with visible light communications (VLC) is considered within the framework of the IEEE 802.15.7m standardization. OCCs based on both organic and inorganic light sources as well as cameras are being considered for low-rate transmissions and localization in indoor as well as outdoor short-range applications and within the framework of the IEEE 802.15.7m standardization together with VLC. This paper introduces the underlying principles of OCC and gives a comprehensive overview of this emerging technology with recent standardization activities in OCC. It also outlines the key technical issues such as mobility, coverage, interference, performance enhancement, etc. Future research directions and open issues are also presented.
In this paper, a non-line-of-sight multiple-input multiple-output space and time division multiple access optical camera communications system is proposed for an indoor environment. Mask matching and equal-gain combining (EGC) schemes as well as differential modulation and frame subtraction are used. We propose a unique packet structure to label the transmitters and a new detection method for data extraction from the captured video streams. We outline a comprehensive theoretical model and have developed an experimental testbed to evaluate the performance of the proposed system. The results highlight that zooming and defocusing of the camera does not have a significant impact on the system performance, therefore the aperture can be set to its maximum value. The system performs well over a link span of 10 m with a low transmit power of 12 mW and in the presence of ambient light due to the non-linear conversion of RAW to JPEG. Using mask matching and EGC improves the tolerance of the system to the noise.
In optical camera communication (OCC) systems leverage on the use of commercial off-the-shelf image sensors to perceive the spatial and temporal variation of light intensity to enable data transmission. However, the transmission data rate is mainly limited by the exposure time and the frame rate of the camera. In addition, the camera's sampling will introduce intersymbol interference (ISI), which will degrade the system performance. In this paper, an artificial neural network (ANN)-based equaliser with the adaptive algorithm is employed for the first time in the field of OCC to mitigate ISI and therefore increase the data rate. Unlike other communication systems, training of the ANN network in OCC is done only once in a lifetime for a range of different exposure time and the network can be stored with a look-up table. The proposed system is theoretically investigated and experimentally evaluated. The results record the highest bit rate for OCC using a single LED source and the Manchester line code (MLC) non-return to zero (NRZ) encoded signal. It also demonstrates 2 to 9 times improved bandwidth depending on the exposure times where the system's bit error rate is below the forward error correction limit. INDEX TERMS Optical camera communication, ANN equaliser, visible light communications, rolling shutter.
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