Optical wireless communication systems

The emerging field of optical wireless communication (OWC) systems is seen as potential complementary technology to the radio frequency wireless communications in certain applications. It is deemed as a possible technology in the future 5th Generation communication networks to address the spectrum congestion and improve the system's capacity. More research and developments in OWC is still needed in order for it to be adopted in current and future communication systems. This special issue brings together research papers on OWC covering free space optic, visible communications and ultraviolet communications

Impact of Link Parameters and Channel Correlation on the Performance of FSO Systems With the Differential Signaling Technique

We investigate the effects of link parameters and the channel correlation coefficient on the detection threshold, Q-factor, and bit-error-rate (BER) of a free-space optical system employing a differential signaling scheme. In systems employing differential signaling schemes, the mean value of the signal is used as the detection threshold level, provided that differential links are identical or highly correlated. However, in reality, the underlying links are not essentially identical and have a low level of correlation. To show the significance of the link parameters as well as the correlation coefficient, we derive analytical relations describing the effect of weak turbulence and we determine the improvement of Q-factor with the channel correlation. Further, for the same signal-to-noise ratio, we demonstrate that a link with a higher extinction ratio offers improved performance. We also propose a closed-form expression of the system BER. We present experimental results showing improved Q-factor for the correlated channel case compared to the uncorrelated channel

MIMO Systems: Principles, Iterative Techniques, and advanced Polarization

International audienceThis chapter considers the principles of multiple-input multiple-output (MIMO) wireless communication systems as well as some recent accomplishments concerning their implementation. By employing multiple antennas at both transmitter and receiver, very high data rates can be achieved under the condition of deployment in a rich-scattering propagation medium. This interesting property of MIMO systems suggests their use in the future high-rate and high-quality wireless communication systems. Several concepts in MIMO systems are reviewed in this chapter. We first consider MIMO channel models and recall the basic principles of MIMO structures and channel modeling. We next study the MIMO channel capacity and present the early developments in these systems concerning the information theory aspect. Iterative signal detection is considered next; it considers iterative techniques for space-time decoding. As the capacity is inversely proportional to the spatial channel correlation, MIMO antennas should be sufficiently separated, usually by several wavelengths. In order to minimize antennas' deployment, we present advanced polarization diversity techniques for MIMO systems and explain how they can help to reduce the spatial correlation in order to achieve high transmission rates. We end the chapter by considering the application of MIMO systems in local area networks, as well as their potential in enhancing range, localization, and power efficiency of sensor networks

Contrasting space-time schemes for MIMO FSO systems with non-coherent modulation

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Multiple Access Techniques for VLC in Large Space Indoor Scenarios: A Comparative Study

The growing demand for high speed indoor wireless connectivity is among the driving forces for data transmission based on visible-light communications (VLC). For relatively large-space indoor scenarios, the development of appropriate spectrally-efficient multiple-access (MA) techniques enables efficient handling of multiple users, in particular, in dealing with the limited modulation bandwidth of the light-emitting diodes. In this paper, we present a comparative study between different MA techniques proposed in the recent literature for VLC networks. The most appropriate schemes for large-scale network deployments are further investigated in different scenarios to contrast their performance in terms of the achievable throughput

PAM- and CAP-Based Transmission Schemes for Visible-Light Communications

Visible light communications (VLCs) have received extensive attention in the research community thanks to their advantages of high bandwidth, low cost, robustness to electromagnetic interference, operation in an unregulated spectrum, and high degree of spatial confinement in indoor scenarios. One of the main limitations for high data-rate transmission in VLC systems is the limited modulation bandwidth of commercial light emitting diodes. To circumvent this limitation, spectrally efficient modulation schemes should be used. Optical orthogonal frequency-division multiplexing (O-OFDM)-based schemes have become very popular and several proofs of concept have shown their ability to attain over gigabits per second transmission rates. We consider here the use of pulse amplitude modulation and carrier-less amplitude and phase modulation schemes together with frequency-domain equalization (FDE) at the receiver as interesting alternatives to O-OFDM. We show the advantages of the former schemes in terms of the peak-to-average-power ratio, and demonstrate through numerical results the merits of FDE-based signaling in attaining high data rates

UAV Location Optimization in MISO ZF Pre-coded VLC Networks

Use of unmanned aerial vehicles (UAVs) to provide on-demand communications has been receiving growing interest, especially for use in remote and hard-to-reach areas. Also, the use of light-emitting diode -based lighting in UAVs has opened opportunities for data transmission through visible-light communications. To manage multi-user interference while avoiding complex handover procedures, we consider the use of zero forcing (ZF) pre-coding. Since the performance of ZF pre-coding depends on the correlation between channel gains of users, we propose in this paper to reduce it by means of location optimization of UAVs. More specifically, we use particle swarm optimization with the objective of maximizing the overall achievable network throughput. Furthermore, to relax the optimization requirements at UAVs, we investigate the case when the optimization is performed at a specific rate under different mobility conditions

Dimming-Aware Interference Mitigation for NOMA-Based Multi-Cell VLC Networks

This letter proposes inter-cell interference mitigation in a dimming-aware way for multi-cell visible-light communication networks, through efficient time-scheduling, scaling, and coordination of non-orthogonal multiple access (NOMA) transmissions at the access points. By this method, users are grouped and served in different time slots depending on whether they are at the center or at the edge of the cell. This reduces the number of NOMA users per time slot, which decreases the network computational complexity, by reducing the average number of successive interference cancellation steps. Comparison of the proposed scheme with the classical NOMA over different dimming constraints shows an improvement of up to 39% and 37% in the average sum-rate and fairness, respectively, for a 30% duty cycle transmission in a 4-cell scenario with 8 users