Wireless Optical communication [WOC] is recently emerging field, which gives solution for the drawbacks of Radio and Microwave communication issues. Optical fiber communication has already developed and accepted by its high performance such as high data capacity, speed etc. But WOC experiences atmospheric turbulence problems and distant dependent path loss. And scintillation and aerosol particles (Rain, haze, fog and drizzle) bring this major challenge for the WOC system by inducing the atmospheric attenuation and scattering. The optimization of WOC system is required for atmospheric effects, fog attenuation, rain attenuation, physical obstructions, scintillations, building sway etc. Relaying techniques are the best solution for minimizing the distant dependent loss. The idea of this optical system is so promising that this system can become the required solution to real time communication system. And this has been signified by the increasing sophistication of optical (and infrared) components and techniques, along with rapidly expanding communication requirements. Here we have made a review on various factors related to WOC and analyzed for improving its performance.
Keywords: Wireless Optical communication, scattering, atmospheric turbulence, outage probability, scintillation, attenuation, Bit Error Rate
Wireless optical communication or free space optics is a telecommunication technology that transmits data between two points by using light propagating in free space. This broadband technology works on the principle of line of sight where the optical pulse modulated signals is used to wirelessly transmit data and provide full duplex (bi-directional) capability. It ranges from hundreds of meters to a few kilometers. Attenuation and scattering resulted by Scintillation and aerosol particles (Rain, haze, fog and drizzle) are the major challenges of this system. Fixed wireless RF technologies have its advantages like low installation cost and rapid deployment but have a lower bandwidth which intern limits the data capacity of the system. WOC provides on the other hand a unique alternative technology options that can meet the growing and seemingly unquenchable demand of bandwidth for short distance applications. It transports full duplex data at Giga bit per second rates. This WOC technology is also used to reach the locations where fiber could not yet be laid economically. The advantages of WOC are large bandwidth, high immunity and electromagnetic interference, not required spectrum licensing, higher data rate and secure data communication. The optimization of WOC is required for atmospheric effects, fog attenuation, rain attenuation, physical obstructions, scintillations, building sway etc. The idea of this optical system is so promising that this system operating partly or completely inside the atmosphere can become the required solution to real time communication system. And this has been signified by the increasing sophistication of optical (and infrared) components and techniques, along with rapidly expanding communication requirements
2. System Design
The design of WOC systems will be strongly required to analyze and recommend the components and techniques such as wavelength, modulation, detection, channel modelling, types of mitigation techniques on atmospheric turbulence, single hope and Multi hop techniques, optimal relay placement etc.
2.1. Modulation and Detection
To transmit optical signal through atmospheric turbulence channels, we need to modulate it using different techniques like Phase Shift Keying (PSK), Subcarrier intensity modulation, On/Off Keying (OOK) etc. But the main motive behind doing this is to calculate the Bit Error Rate (BER) in association with it, as this is the factor which tells us whether its feasible to transmit signal or not. In an open-air long-distance WOC system, communication between optical transceivers is established via point-to-point line-of sight (LOS) WOC links.) PIN. To modulate the signal, intensity modulation and direct detection using OOK is widely used . An efficient coded modulation scheme with Q-ary pulse position modulation is also proposed for multiple input/output WOC systems . In the regime of strong atmospheric turbulence, the coding scheme provides excellent coding gain. The BER performance of a WOC link employing SIM in the gamma-gamma atmospheric turbulence has been presented. SNR of -29 dB is required in weak turbulence to obtain BER of 10-6, as characterized by log intensity variance Ga2 of 0.2. The SNR increases with the rise of turbulence to reduce the scintillation effect. Since, spatial diversity makes use of two photo detectors , up to 21 dB reduction can be achieved in SNR at BER of 10-6 in moderate turbulence. The Subcarrier PSK intensity modulation is helpful in effective mitigation of scintillation effects induced by atmospheric turbulence in optical communication. The calculation shows that in the presence of atmospheric turbulence the performance of subcarrier PSK intensity modulation is better than that of OOK intensity modulation. The upper bound figure calculated for WOC employing convolution coded Subcarrier BPSK is better than OOK. Therefore, to improve the performance and to reduce the transmission power of the optical system, subcarrier PSK intensity modulation and convolutional codes can be used  through atmospheric turbulence channels. And it is found that at BER of micro level using 4 independent PIN-photo detectors, a diversity gain of more than 38dB can be achieved [5-7].
2.2. Wavelength Selection
The atmospheric attenuation can be considered as the only major factor instead of bandwidth capabilities of available components, eye safety limits, and cost for selecting a transmission wavelength for a WOC system. The atmospheric attenuation in haze conditions and in fog conditions are wavelength dependent. There is no advantage of 1550 nm over 785 nm as the visibility is less than 500 m . From the result analysis, WOC wavelength with 1550nm produces less effect in scattering coefficient and atmospheric attenuation .Optimization of the WOC system transmission can be done by short distance range between the transmitter and receiver. Based on the analysis, it is recommended to install WOC system with link range up to 2 km and 1550nm wavelength.
2.3. Mitigation Techniques
The techniques considered here are based on the statistical properties of turbulence-induced signal intensity fading, as functions of both temporal and spatial coordinates[9-11]. In this paper detection techniques for free-space optical communication systems in which the correlation length of intensity is greater than receiver aperture and turbulence correlation time is the receiver observation interval. These techniques help in mitigation of the log-amplitude fluctuations induced by turbulence. The statistical distributions of turbulence-induced fading form the bases of the study of Maximum Likely hood detection schemes over turbulence channels. This states that if the statistics of marginal fading is known, we can apply ML symbol-by-symbol to detect the unknown state of instantaneous fading and thereby improve the detection efficiency. And we can apply Maximum Likely hood Sequence Detection, which can further improve the detection performance only if we know the temporal correlation of fading, i.e., the joint temporal distribution of turbulence-induced fading. The statistical distributions of turbulence-induced fading form the bases of the study of ML detection schemes over turbulence channels. It can be considered that these techniques can help in mitigating the effects of turbulence-induced log-amplitude fluctuations because this scheme is successfully established for a distance of 8 km [9-11]. Linear Cyclic Code has been implemented using the same channel model, which shows that at the cost of additional bandwidth, considerable improvements in the BER can be obtained. Using intensity modulation with direct detection (IM/DD) the performance of long range links has been degraded by scintillation noise, particularly over ranges of several hundred meters or longer. The bit interval is much shorter than the intensity correlation time in many high-bit-rate WOC links, i.e., hence, for many consecutive bit intervals the instantaneous state of the intensity fluctuations does not change much. As compared to symbol-by-symbol detection, it is experimentally observed that PSAD gives an SNR gain of about 1.9 dB, while MLSD yields an SNR gain of about 2.4 dB (both at 10 error probability). In WOC links signal fading can be mitigated by error-control codes. To study its efficiency, error performance bound analysis is done to give us approximate bit-error probability. First the upper bound has been derived on the pair-wise codeword-error probability for correlated atmospheric turbulence channel. Then an approximation for this upper bound for weak turbulence to avoid the complex multidimensional integration has been derived . Based on a temporally correlated K channel model error coded WOC systems are operating over atmospheric turbulence channels. This channel model describes strong turbulence conditions, unlike the classically used lognormal assumption. In Kchannel model, an upper bound on the PEP has been derived and then the union-bound technique in conjunction with the derived PEP bound is applied . Spatial diversity due to multiple antennas on 2D arrays can increase net link bandwidth. On the other hand, simultaneous transmission between the elements on array can cause inter-channel interference, reducing effective channel bandwidth. For good divergence properties of the transceivers 2D array gives excellent bandwidth performance over short range WOC. In a LAN environment multiple hops of WOC channels can be easily implemented. By time multiplexing and coding techniques the interference of the system can be reduced, which will also improve the performance of the system. Multiple wavelengths and filtering can also reduce interference .Multiple small apertures can be formed by ideal photon counting reception, together with an optical MIMO system having multi pulse PPM across laser sources. Assuming independent fading on source/detector pairs, both lognormal and non-fading, and Rayleigh-fading models have been treated. Further, when peak laser power is constrained, multi pulse transmission shows clear superiority over classic PPM . The atmospheric turbulence on the propagation of electromagnetic waves at optical wavelengths scatters light. In WOC link the signal degradation is also caused by scintillations of the received beam. A large receiver can collect all the transmitted data and can check the atmospheric-turbulence-induced intensity variance. But since making a large receiver is expensive, an optimized receiver aperture size is required to reduce intensity scintillations. A ratio of normalized intensity variance of a receiver of some size to that of a point detector is called the aperture-averaging factor (F) and it is a measure of aperture averaging . The penalty due to the extraction of optical carrier prior to downlink-signal detection is overcome by the sensitivity enhancement arising from the improved modulation depth as a result of the suppression of optical carrier. The schemes are simple, potentially low cost, and applicable to a variety of modulation formats and over a wide range of frequencies . If the constituent beams overlap at the detector and are properly separated at transmitter the scintillation index can be significantly reduced. This is shown by the scintillations produced by a multi emitter beam in the presence of turbulence . These results demonstrate that similar SISO systems with appropriate scaling in the channel variance can represent the WOC links with transmit and receive diversity. In other words, the effect of spatial diversity manifests itself as a decrease of the channel variance. It can be observed that the performance loss due to spatial correlation might be severe, demonstrating that efficient separation between apertures and strict co-alignment is crucial to achieve the promised diversity gains from multiple transmitters/receivers . Two new methods were proposed for measuring visibility effect for fog and haze for WOC links . The performance of the system as a function of the principal parameters of such a link are the length of the link, the aperture diameter of the receiver and the atmospheric turbulence conditions between the transmitter and the receiver. Furthermore, it is investigated as particular influence on a typical WOC system and proposed ways to increase the practical link efficiency. The analysis discussed in this paper  can be further applied to the study of the capacity in multiple-input multiple-output (MIMO) WOC systems or to other modulation schemes such as pulse position modulation (PPM).In this paper  to understand the effects of scintillation on light propagation of laser beam, a laser range experiment was performed. Results shown the moderate turbulence conditionsyet, by a lognormal distribution the irradiance fluctuations found to be well modeled. It was observed that the fluctuations were varying slowly with time, on milliseconds scale (similar to theoretical results). On the base of experimental observations, spatial correlation between apertures is negligible. In terms of un-coded BER and ergodic capacity, a WOC MIMO QPPM channel was analyzed. It was shown that using optimal detection for un-coded transmission at high SNR, knowledge of the fading coefficients is required than the SNR. This upper bound was further simplified by approximating the distribution of the sum of lognormal random variables as a lognormal distribution. The loss in performance was shown to be negligible when EGC is used in terms of un-coded BER. The conditional mutual information over the fading distribution and the ergodic channel capacity is computed via Monte Carlo simulations. The results demonstrated the importance of using both strong coding techniques and MIMO to mitigate the detrimental effects of scintillation . To establish optical link, a transceiver system is proposed for dynamic routing and automatic tracking for WOC. The proposed architecture has M receivers in M X M configuration that can dynamically orient themselves at different angles to establish a link between two transceivers. The condition for this is that, the link will only be setup if there is a free wavelength available at receiver or transmitter or nodes along the route. This system will reduce the blocking probability and increase the recovery percentage of link failures. This feature has been tested on OPNET software simulation [24, 25].Temperature gradient around the buildings can also lead to atmospheric turbulence which in-turn affect the efficiency of the system. It is caused by the fluctuation in the refractive index around the building and leads to bending or widening of transmitted light beams. The performance of this system is greatly affected by the time-variant properties of transmitting medium. Temperature gradient in the transmitting medium results in beam widening and bending. Different dependencies of structure of refractive index parameters are found in urban areas and areas full of buildings [26, 27].
2.3.1 Channel Modelling
To mitigate turbulence-induced fading of signal, we can make use of error control coding over WOC links. For high turbulence channel (modeled as K-distribution), error performance bounds are derived for coded WOC operating over atmospheric turbulence channel. Then upper bounds on BER performance can be derived on the PEP for the closed form of K channel and then apply the transfer function technique in conjunction with the derived PEP bound [26, 27].
An efficient lognormal channel model for WOC communications based on IMDD links is presented, considering the signal intensity fluctuations in addition to the adverse effect induced by the channel coherence. In Reference , considering a second-order Gaussian statistical model assuming an unbounded plane wave operating under weak irradiance variation conditions. Assuming a gamma-gamma division for rough atmosphere, the error performance of a WOC link modeled as a PPM Poisson channel which has been analyzed in closed form. Since at smaller SNR values, the solution of the related integral suffers from numerical instability, it is tells that a Gauss-Laguerre quadrature successfully replaces the latter, which provides stable conditions in the low SNR range too. Verified by simulation results, the gamma-gamma model approaches for heavy turbulences and in low turbulent atmosphere lognormal scheme is suitable .For K-distributed turbulence, a closed-form expression of the moment generating function is derived and this function is used to obtain the error rate for BPSK, BPSK with spatial diversity, and DPSK. Asymptotic error rates in large signal-to-noise ratio regions are obtained based on this analytical moment generating function. And all this is done to study the error performance of a coherent free space optical system with K-distributed turbulence [30-32].Transmit Laser Selection (TLS) diversity scheme have been recently proposed for WOC system and its BER performance has been investigated over K-distributed turbulence channels. For a limiting case of extremely rough conditions modelled by negative exponential distribution, a closed-form expression for the average BER has been presented in open technical text. The combined analytical results can be used efficiently in strong turbulence conditions to analyze the performance of system as well as to investigate the effects of design parameters on system performance. TLS not only increase the system performance, but also the diversity order.
2.3.2. Diversity gain Analysis
In this paper , MIMO transmission with diversity gain WOC systems operating in lognormal turbulence-fading channels is considered. Parallel DF [Decode Forward] relaying, and serial DF relaying schemes are also discussed. Based on the outage probability for WOC diversity systems, relative diversity order has been derived.
3. Relay based WOC
In this paper , they have investigated parallel and serial relaying techniques assuming Decode Forward scheme. The fact that in WOC systems, fading variance depends upon the distance, it constitutes a major difference between wireless optical and wireless RF systems. This enables multi-hop WOC transmission work excellently over shorter distance in the resulting hops and brings significant improvements against the degrading effects of fading induced by turbulence. It has also been found that parallel relaying as a possible replacement for serial relaying, where the use of multiple transmitter apertures, which are directed to relay nodes, gives us an artificial broadcasting. The performance improvements for both serial and parallel relaying schemes are analyzed through the derivation of Monte-Carlo simulations and also through outage probability.WOC seems amazing, but it has few practical challenges; major one is scintillation. To overcome this, cooperative techniques and Multiple Input Multiple Output (MIMO) have been proposed. When transmitter, receiver and relay have same statistics, the decode-and-forward (DF) strategy does not exploit the spatial diversity, similar to RF communication. However, amplify-and-forward (AF) strategy provides the spatial diversity, which its order is high at weak fading . By properly choosing the location of relay, we can achieve higher order of diversity by cooperative strategies compared to the two-transmitter case. The assumption of perfect Channel Side Information (CSI) at the receiver has solved the problem of outage probability minimization and power control law has been presented under the peak and average sum power constraints. The outage probability has been remarkably improved by the relay-assisted transmission in the atmospheric Poisson and the increment in the relay count also enhances this improvement. Also, CSI at the transmitter is not useful for a single-hop Poisson fading channel with the short-term constraint . The outage probability over the single-hop Poisson fading channel also get improved by this information with the long-term constraint as well as that over the multi-hop Poisson fading channel under both the short and long term power constraints.
3.1. Outage Analysis
With the fading of the optical link, all the real time purposes of the system are considered not to be in service. This makes the outage probability an important metric for the communication system designer. The reduction in outage probability and power gain over non-diversity systems can be achieved by using the diversity in atmospheric optical communication systems. We have to do so because the duration of outage and the outage probability are important performance parameters that can strongly affect data networking. For example, end-to-end network delay may get affected by frequent and long outages and may also trigger the adverse responses by the transport layer protocol (such as session termination in TCP), which can deeply affect the network performance. Since this can provide such noticeable gain in the performance of the system, we believe that for clear atmospheric optical channel, diversity should be considered as one of the system techniques, particularly the context of data networking. After getting the description of statistical models of fading channels, which are frequently used in design and analysis of communication systems, we can bring our focus on the information theory of fading channels, by highlighting the capacity as most important performance measure and examining both multi and single user transmission. The information-theoretic arguments made it clear which coding or decoding method to be used in achieving the ultimate performance in a fading time-varying environment and code design description and techniques of equalization. The outage probability and average error probability can be studied, evaluated and analyzed for the special case of a dual-hop system with the numerical examples provided for illustration purposes in the paper. We can easily extend this analysis to other turbulent channel models by which we may contribute in many open issues for future investigation such as how it affects the overall performance of the system and error rate analysis of WOC multi-hop systems -. For the same end-to-end link range and power, the error performance of the serial relaying WOC communication is betterthan that of parallel relaying. Also for both clear and rough weather conditions, the mean error rate in the case of multi-hop is smaller than that of the single hop. More importantly, the variance of the error rate is significantly smaller for multi-hop operation. This narrow error variance helps in designing an effective multi-hop network. Since the range of the desired error rates is smaller as compared to single hop operations, this approach is more energy efficient to improve the WOC link reliability. So, with increase in the number of hops, the error behavior of the system improves but it leads delay, which increases at each hop. Now, the future problem we see here is to optimize the tradeoff between end-to-end delay in a multi-hop scenario and errors. And yet another future problem we have found out is the overall system's cost in multi-hop scenario resulted by simultaneously optimizing the system reliability -. Apart from multi-hop WOC system, the outage performance of wireless optical communication systems implemented by parallel relay nodes over strong turbulence channel is another area of study. For the evaluation of outage probability, an aggregated channel model covering the summed-up effects of pointing errors and atmospheric turbulence, atmospheric attenuation are to be considered. An exact analytical expression for end-to-end outage probability is needed for given weather and misalignment conditions. Conclusively, the parallel relay transmission is a reliable measure for the WOC's outage probability [37-40].
4. Evolutionary Techniques
This paper compared the performance of the ABC [Artificial Bee Colony]with that of PSO[Particle Swarm Optimization and GA[Genetic Algorithm]. From the simulation results we can conclude that the proposed algorithm has the ability to move over the local minimum and can be effectively used for multivariable, multimodal function optimization. Investigating the effects of control parameters on the performance of ABC algorithm and the convergence speed of algorithm still has their scope in future studies. WOC is in great demand for both short and very short path applications. But there are few problems like aerosol scattering, air turbulence, hydrometeors and suspended water particle that induce extra losses due to scattering. Simulation found that the rain attenuation affect for marshal model is less than for Charbonneau model because in marshal model used empirical expression based on fitting data while Charbonneau model used physical method based on measurement data. This difference lies for 1000m, while for 300m or so, the received power, data rate, link margin, and signal-to-noise ratio decrease with increasing rain rate for two studied models [43-44].
Real-world optimization problems are very difficult and full of uncertainty. Few of the algorithms like meta-heuristic outperformed when the complexity level increased. Inspired from the intelligent foraging behavior of honeybee, the ABC algorithm is a recently introduced population-based meta-heuristic optimization technique .Modification in ABC algorithm based on genetic algorithm crossover and mutation operations will create new candidate solution with improved performance of the algorithm. Further, changing the search process by amplifying exploitation in the later stage when good solutions are already found by introducing guided onlookers for strong exploitation. This improves ABC algorithm by representing new approach to balance between exploitation and exploration . ABC algorithm is formed to solve clustering problems and revealed promising results in processing time and solution quality although, noresearch has yet considered employing this algorithm for deviation detection. Deviation detection is a data-mining task with many application domains including the detection of fraud, e-commerce criminal activities, outbreak detection, and network intrusion. Apart from clustering problem solving at good speed, it can be reformulated to detect clustering-based deviations. The performance of deviation detection method is measured with the detection rate (DR) and false alarm rate (FR) measurement .Relay-assisted WOC makes use of the fact that atmospheric turbulence fading variance depends on distance and yields significant performance gains by taking the advantage of short hops. Now, to determine the optimal position of the relays in series and parallel WOC relaying to minimize the outage probability and qualify performance improvements, we need to study both of them. In series relaying, the outage probability is minimized when the consecutive nodes are placed at equal distance on the path from source to destination. And for parallel, all the relays should be placed at the same place closer to the source, and the exact position depends upon the system and channel parameters.
4.1 Optimal Relay Placement
Relays are used when direct line-of-site is not there between source and receiver in WOC link, especially for long distance. Instead of all available relays transmitting simultaneously, only one relay can be selected to participate in the communication link. This selection will be based on channel state information obtained from either all or last used WOC link. Thus, the need for synchronized relay transmission is avoided and the slowly varying nature of atmospheric channel is considered. To activate only one relay at a time in each transmission slot, alternative protocols to the all-active relaying scheme were proposed. This gives considerable benefits in terms of implementation complexity as it avoids the need to synchronize the transmissions of multiple relays. There are two types of relay section protocols: DSS and select-max. Select-max relaying at the cost of all CSI link requirements, provides excellent performance gains as compared to the all-active scheme. While, DSS relaying has same performance as select-max for N=2 but does not offer performance gain beyond that, because it exploits only two relays with max-equivalent distance. However, DSS is less complex alternative due to its limited CSI requirements. Now, addressing to power allocating problem in WOC links, in order to account for distance-dependent nature of atmospheric effects, optimal and more computationally attractive solutions were derived for each protocol [44-45]. Comparison is done between basic Artificial Bee Colony, Harmony Search and Bees algorithm. As a result, it was found  that Artificial Bee Colony algorithm is superior over other two algorithms, as it preserves producing reasonable results even for high dimensions. Also, as compared to Bees algorithm and Harmony Search, ABC algorithm employs less control parameter to be tuned. Modification in ABC algorithm based on genetic algorithm crossover and mutation operations will create new candidate solution with improved performance of the algorithm. Further, changing the search process by amplifying exploitation in the later stage when good solutions are already found by introducing guided onlookers for strong exploitation. This improves ABC algorithm by representing new approach to balance between exploitation and exploration [42-43].
5. Results And Comparison
Fig2. Outage probability vs power margin for2 relays
Fig3.Outage probability vs power margin for 6 relays.
Fig. 4outage probability with number of iterations
Other evolutionary techniques such as shuffled frog leaping ,Invasive weed optimization ,Particle Swarm optimization are also used for optimal relay placement.The algorithms are studied as flow-charts, then the respective programs are coded. All the algorithms use the fitness function to calculate the cost value to arrive into a conclusion. The primary aim is to minimize the fitness function to obtain optimized relay position. The outrage probability versus power margin is plotted for all the algorithms and the results are compared [48-50].
The preferred modulation schemes, channel modelling, detection techniques for WOC links are discussed and analyzed. The signal fading is distant dependent and has path loss with respect to atmospheric turbulence effects. Cooperative WOC can mitigate the scintillation effects. Evolutionary algorithms can be used for optimal relay placement. In this paper, we have investigated the optimum relay placement for serial and parallel.WOC relaying. Relay locations have been optimized for serial and parallel relaying.For serial relaying, the outage probability is minimized when the consecutive nodes are placed equi-distant along path from the source to destination while in case of parallel relaying, the outage probability is minimized when all the relays are placed at same location between source and destination along the direct link between source and destination and are closer to source. The exact location varies with change in channel parameters, optimization algorithm parameters.
By analyzing and implementing with various algorithms, we can obtain optimal relay placement and that can be recommended for real time scenario which is the optimum need of this era.
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