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

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Optical Networks A Practical Perspective

Measuring the quality of optical signals is one of the most important tasks in optical communications. A variety of metrics are available, namely the general shape of the eye diagram, the optical signal-to-noise power ratio (OSNR), the Q-factor as a measure of the eye opening, the error vector magnitude (EVM) that is especially suited for quadrature amplitude modulation (QAM) formats, and the bit error ratio (BER). While the BER is the most conclusive quality determinant, it is sometimes difficult to quantify, especially for simulations and off-line processing. We compare various metrics analytically, by simulation, and through experiments. We further discuss BER estimates derived from OSNR, Q-factor and EVM data and compare them to measurements employing six modulation formats at symbol rates of 20 GBd and 25 GBd, which were generated by a software-defined transmitter. We conclude that for optical channels with additive Gaussian noise the EVM metric is a reliable quality measure. For nondata-aided reception, BER below 0.01 can be estimated from measured EVM.

Quality metrics for optical signals: Eye diagram, Q-factor, OSNR, EVM and BER

In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power and low mass requirement. FSO communication uses optical carrier in the near infrared (IR) and visible band to establish either terrestrial links within the Earths atmosphere or inter-satellite or deep space links or ground to satellite or satellite to ground links. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate (BER) performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for both terrestrial and space links. It will provide details of various performance mitigation techniques in order to have high link availability and reliability of FSO system. The first part of the paper will focus on various types of impairments that poses a serious challenge to the performance of FSO system for both terrestrial and space links. The latter part of the paper will provide the reader with an exhaustive review of various techniques used in FSO system both at physical layer as well as at the upper layers (transport, network or link layer) to combat the adverse effects of the atmosphere. Further, this survey uniquely offers the current literature on FSO coding and modulation schemes using various channel models and detection techniques. It also presents a recently developed technique in FSO system using orbital angular momentum to combat the effect of atmospheric turbulence.

Free Space Optical Communication: Challenges and Mitigation Techniques

A wavelength division multiplexing (WDM) access network using high-speed free-space optical (FSO) communication for the distribution link is proposed. Combining FSO communication with optical fiber can reduce the system cost and provide high-bandwidth access in regions where optical fiber installation is problematic. The WDM channels suffer from interchannel crosstalk, while the FSO communication performance in a clear atmosphere is limited by atmospherically induced scintillation. These impairments, plus the amplified spontaneous emission noise from optical amplification, combine in a potentially problematic way, particularly in the upstream direction, which is investigated here. This turbulence-accentuated crosstalk effect is considered for the cases of 1) signal turbulent but crosstalk not and 2) crosstalk turbulent but signal not. Error floors are obtained in each case. The FSO link length that can be supported in the general case of the hybrid network is investigated.

WDM FSO network with turbulence-accentuated interchannel crosstalk

The use of an optical pre-amplifier at the receiver of free-space optical (FSO) communication systems necessitates the consideration of both turbulence and amplified spontaneous emission (ASE) noise in performance modelling. Until now, this problem has been typically approached by the use of a Gaussian approximation (GA) for the conditional probability of error with averaging then performed using the received irradiance probability density function (PDF) (governed by scintillation). However, the GA is deficient as it only uses the first two moments. Moment generating function (MGF) techniques, notably the Chernoff bound (CB) and modified Chernoff bound (MCB), are used to evaluate the bit error rate performance of an optically pre-amplified FSO system in the presence of both atmospheric turbulence and ASE noise. The MGF-based methods incorporate a fuller statistical description of the signal and noise processes encountered in the optically pre-amplified case when compared to the GA. The lognormal, gamma-gamma, K and negative exponential distributions have been used to characterise the weak, moderate, strong and saturated turbulence regimes. The MCB gives the tightest bound upon the BER compared to the CB, particularly at lower gains, and, as it also can be exceeded by the GA at higher gains, it is a logical method to use in general.

Improved bit error rate evaluation for optically pre-amplified free-space optical communication systems in turbulent atmosphere

Optical preamplification introduces non-Gaussian electrical domain noises (beat noises) to digital pulse position modulation (DPPM) free-space optical (FSO) communication systems impaired by atmospheric turbulence, which do not appear for a simple PIN photodiode-based FSO receivers and thus necessitate a more sophisticated approach to bit error rate (BER) modelling. To explore this BER performance analyses, notably the Gaussian approximation, Chernoff bound and modified Chernoff bound (MCB), for such a system are presented and the MCB is seen to be an appropriate technique to use. Furthermore, the effectiveness of spatial diversity techniques, specifically aperture averaging, in mitigating the turbulence effect is investigated. The gamma-gamma distribution model is used to characterise the whole range of turbulence conditions. The results reveal the superiority of DPPM with improved receiver sensitivity (at a binary data rate 2.5-Gb/s and at typical FSO BER of 10 -9 ) of about 7-9-dB [for coding level of 5 and optical link length (for turbulent interaction) of 1500-m] more than an equivalent optically preamplified on-off keying non-return-to-zero approach, depending on the level of turbulence.

Performance evaluation of optically preamplified digital pulse position modulation turbulent free-space optical communication systems

Massive deployment of free space optical (FSO) communication systems in future optical access networks will use wavelength division multiplexing (WDM) technology to leverage extended reach and enhanced user capacity. An analytical framework for the estimation of outage probability in a turbulence-affected WDM FSO system is presented in this study. The authors derive simplified expressions of the probability of outage in the presence of turbulence-accentuated interchannel crosstalk, and show how design parameters such as the FSO link length, demultiplexer adjacent channel rejection and receiver aperture diameter affect the performance of the system. Numerical results show that the outage probability due to irradiance fluctuation is worsened in WDM FSO systems by the effects of turbulence-affected interchannel crosstalk signals. Specifically, the average crosstalk power and the degree of irradiance fluctuations on the crosstalk set a limit on the outage probability performance of the system which appears in the form of outage floors.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-opt.2016.0057

Outage probability of WDM free-space optical systems affected by turbulence-accentuated interchannel crosstalk

Wavelength division multiplexing (WDM) has been proposed for fibre, intersatellite, free space and indoor optical communication systems. Digital pulse position modulation (DPPM) is a more power efficient modulation format than on-off keying (OOK) and a strong contender for the modulation of free-space systems. Although DPPM obtains this advantage in exchange for a bandwidth expansion, WDM systems using it are still potentially attractive, particularly for moderate coding levels. However, WDM systems are susceptible to interchannel crosstalk and modelling this in a WDM DPPM system is necessary. Models of varying complexity, based on simplifying assumptions, are presented and evaluated for the case of a single crosstalk wavelength. For a single crosstalk, results can be straightforwardly obtained by artificially imposing the computationally convenient constraint that frames (and thus slots also) align. Multiple crosstalk effects are additionally investigated, for the most practically relevant cases of modest coding level, and using both simulation and analytical methods. In general, DPPM maintains its sensitivity advantage over OOK even in the presence of crosstalk while predicting lower power penalty at low coding level in WDM systems.

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Andrew J. Phillips

Papers

WDM FSO network with turbulence-accentuated interchannel crosstalk

Improved bit error rate evaluation for optically pre-amplified free-space optical communication systems in turbulent atmosphere

Performance evaluation of optically preamplified digital pulse position modulation turbulent free-space optical communication systems

Outage probability of WDM free-space optical systems affected by turbulence-accentuated interchannel crosstalk

Performance evaluation of digital pulse position modulation for wavelength division multiplexing FSO systems impaired by interchannel crosstalk