Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

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Survey on Free Space Optical Communication: A Communication Theory Perspective

Course Description This is an advanced course in which we explore the field of Statistical Optics. Topics covered include such subjects as the statistical properties of natural (thermal) and laser light, spatial and temporal coherence, effects of partial coherence on optical imaging instruments, effects on imaging due to randomly inhomogeneous media, and a statistical treatment of the detection of light. Development of this more comprehensive model of the behavior of light draws upon the use of tools traditionally available to the electrical engineer, such as linear system theory and the theory of stochastic processes.

http://ieeexplore.ieee.org/iel5/3/23094/01073023.pdf

Statistical optics

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.

/pdf/ber-performance-of-free-space-optical-transmission-with-249sp2pvzf.pdf

BER Performance of Free-Space Optical Transmission with Spatial Diversity

In this paper, we compare the performance of multiple-input-multiple-output (MIMO) techniques applied to indoor optical wireless communications (OWC) assuming line-ofsight (LOS) channel conditions Specifically, several 4 × 4 setups with different transmitter spacings and different positions of the receiver array are considered The following MIMO algorithms are considered: Repetition Coding (RC), Spatial Multiplexing (SMP) and Spatial Modulation (SM) Particularly, we develop a framework to analytically approximate the bit error ratios (BERs) of these schemes and verify the theoretical bounds by simulations The results show that due to diversity gains, RC is robust to various transmitter-receiver alignments However, as RC does not provide spatial multiplexing gains, it requires large signal constellation sizes to enable high spectral efficiencies In contrast, SMP enables high data rates by exploiting multiplexing gains In order to provide these gains, SMP needs sufficiently low channel correlation SM is a combined MIMO and digital modulation technique We show that SM is more robust to high channel correlation compared to SMP, while enabling larger spectral efficiency compared to RC Moreover, we investigate the effect of induced power imbalance between the multiple transmitters It is found that power imbalance can substantially improve the performance of both SMP and SM as it reduces channel correlation In this context, we also show that blocking some of the links is an acceptable method to reduce channel correlation Even though the blocking diminishes the received energy, it outweighs this degradation by providing improved channel conditions for SMP and SM For example, blocking 4 of the 16 links of the 4 × 4 setup improves the BER performance of SMP by more than 20 dB, while the effective signal to noise ratio (SNR) is reduced by about 2 dB due to the blocking Therefore, MIMO techniques can provide gains even under LOS conditions which provide only little channel differences

http://www.homepages.ed.ac.uk/hxh/Li-Fi_PAPERS/13_fath_1.pdf

Performance Comparison of MIMO Techniques for Optical Wireless Communications in Indoor Environments

In this paper, we present relay-assisted transmission as a powerful fading mitigation tool for free-space optical systems operating in atmospheric turbulence channels. We study both serial (i.e., multi-hop transmission) and parallel (i.e., cooperative diversity) relaying encoupled with amplify-and-forward and decode-and-forward modes. We consider an aggregated channel model which takes into account both path-loss and turbulence-induced log-normal fading. Since fading variance is distance-dependent in free-space optical systems, relay-assisted transmission takes advantage of the resulting shorter hops and yields significant performance improvements. We derive outage probability of the relaying schemes under consideration which are further confirmed through Monte-Carlo simulations. Our outage probability analysis demonstrates that an impressive performance improvement of 18.5 dB is possible with the use of a single relay at a target outage probability of 10-6.

/pdf/relay-assisted-free-space-optical-communication-8tvjw2m9mc.pdf

Relay-Assisted Free-Space Optical Communication

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. We present error rate performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, which are modeled as a correlated K distribution under strong turbulence conditions. We derive an upper bound on the pairwise error probability (PEP) and then apply the union-bound technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to verify the analytical results.

/pdf/error-rate-performance-of-coded-free-space-optical-links-375zsiz1ie.pdf

Error rate performance of coded free-space optical links over strong turbulence channels

Broadband communications for indoor power-line networks with impulsive noise using orthogonal frequency division multiplexing (OFDM) is considered. From earlier investigations, it is known that this channel suffers from multipath fading and frequency selectivity along with man-made impulsive bursty noise. Nevertheless, the calculated channel capacity limit promises very high data rates over this channel. Enhancement techniques, such as coding can help an OFDM system to tackle impulsive noise burst. In this paper, the bit error rate (BER) performance of the OFDM system under impulsive noise and frequency fading is theoretically analyzed and closed form expression for the performance is derived. Two different impulsive noise environment models are studied and it is confirmed that both models represent the same environment. Furthermore, a theoretical upper bound on the performance of coded OFDM system is obtained, given perfect interleaving. The effect of the interleaver length on coding performance is also studied. Simulations show that the upper bound is quite tight for the case of employing a longer interleaver. The effect of interleaver size on the system performance is studied, as well

Performance analysis of uncoded and coded OFDM broadband transmission over low voltage power-line channels with impulsive noise

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. Spatial diversity, which involves the use of multiple laser transmitters/receivers, can be used over FSO links to mitigate turbulence-induced fading, resulting in error-rate-performance improvement. In this paper, we investigate BER (bit error rate) performance of MIMO (multiple-input multiple-output) FSO links over lognormal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our derivations clearly demonstrate the effect of diversity in a lognormal channel, which turns out to be much different from a Rayleigh fading channel, which is frequently used to model radio-frequency (RF) wireless fading channels.

http://ieeexplore.ieee.org/iel5/9623/30414/01404690.pdf

BER performance of MIMO free-space optical links

Broadband communications for indoor power-line networks with impulsive noise using Orthogonal Frequency Division Multiplexing (OFDM) is considered in this paper. From earlier investigations, it is known that this channel suffers from multipath fading and frequency selectivity along with man-made impulsive bursty noise. Nevertheless, the calculated channel capacity limit promises very high data rates over this channel. Enhancement techniques, such as concatenated coding can help an OFDM system to tackle impulsive noise burst. In this paper we employ Digital Fountain codes, which are a new class of erasure detecting codes. These codes are considered the state-of-the-art discovery in coding theory due to their simplicity and performance.

S.M. Navidpour

Papers

BER Performance of Free-Space Optical Transmission with Spatial Diversity

Error rate performance of coded free-space optical links over strong turbulence channels

Performance analysis of uncoded and coded OFDM broadband transmission over low voltage power-line channels with impulsive noise

BER performance of MIMO free-space optical links

Fountain codes for impulsive noise correction in low-voltage indoor power-line broadband communications