Research On Irradiance Scintillation And Mitigation Technology In Atmospheric Laser Communications

Atmospheric laser communications (ALC) uses laser beams as the modulated carrierto establish a wireless link between two outdoor sites through the atmospheric channel.Compared with conventional radio frequency (RF) systems, there are severalsignificant advantages offered by ALC systems that are the direct consequence of thehigher frequencies (shorter wavelengths) associated with optical waves. Among theseadvantages are higher data rates, higher security (very narrow beam), smaller size andweight of the components. Moreover, ALC is also no need for frequency licensing,immune to electromagnetic interference, and no radiation on the human body. Ascompared with fiber optics systems, ALC systems are quickly deployable and morecost-and time-efficient due to its wireless connection property. With these advantages,ALC can find wide applications in the field of broadband access, local area networkinterconnection, metropolitan area network extension, aerospace, militarycommunication, and emergent communication. Currently, ALC becomes a researchhotspot in the communication field.But all the time, the deleterious atmospheric effects on ALC links is difficult tomitigate, and thus limits the widespread deployment of ALC systems. For instance,inclement weather (e.g. heavy fog) imposes severe optical power losses. Even in theclear weather conditions, the atmosphere is not entirely benign. Refractive-indexrandom fluctuations, also known as optical turbulence, leads to intensity scintillation,beam wander and beam spread. Scintillation represents turbulence-induced irradiancefluctuations in temporal and spatial domain. This random fluctuation leads to fading ofthe received signal below a detectable threshold, and even causing communicationinterruption.Spatial diversity and aperture averaging are the two well accepted techniques formitigating scintillation. But it is still necessary to discuss and evaluate the performanceimprovement by these two techniques before their wide application in practicalsystems. Firstly, this paper introduces the research progress in this field. As for spatialdiversity, there have been obtained a number of theorectical results. However, most ofthe previous results were limited to Lognormal (weak turbulence), K distribution andNegative Exponential (strong turbulence) fading environments. In fact, althoughGamma-Gamma model is more mathematically complex, it can cover a wide range of turbulence conditions (from weak to strong). As for aperture averaging, recent researchindicated that although the Gamma-Gamma channel model matches well with theexperimental data for a pointer receiver, it deviates from the experimental results of thefinite aperture receiver. In this context, a new and accurate channel model for thereceiver-aperture-averaging scintillation, named Exponentiated Weibull model, wasproposed. However, to the best of our knowledge, the evaluation of the ALC systemperformance based on this model has not been documented. The channel parameters inLognormal and Gamma-Gamma model are related to scintillation index. Previousresults for scintillation index were obtained by using Kolmogorov power-lawsprectrum. But recent experimental results showed that upper atmosphere exhibitsnon-Kolmogorov properties. To this end, the main research works are listed as follows:1. Under Gamma-Gamma fading, the performance of ALC systems employing equalgain combining (EGC) diversity reception technique is investigated. For the intensitymodulation/direct detection (IM/DD) system, the exact closed-form expression for theaverage bit error rate (BER) of on-off keying (OOK) is firstly derived for independent,but not necessarily identically distributed (i.n.i.d) fading. The obtained expression hasno constraints on the values of the channel parameters. For the intensity modulationand optical preamplification reception system, the exact closed-form BER expressionfor binary pulse position modulation (BPPM) over i.n.i.d fading is firstly developedunder the assumption that the amplifier sponsetanous emission (ASE) noise dominatesthe receiver noise. To gain more insight, a closed-form asymptotic BER formula is alsoderived at high transmitted optical power. With the derived formula, the diversity gainof the considered system is analyzed. For the heterodyne coherent detection system,closed-form upper bounds for the outage probability (OP) and the average BER ofbinary phase shift keying (BPSK) are derived. The computer simulations verify thetightness of the bounds.2. The effects of aperture averaging on mitigating irradiance scintillation areinverstigated based on the Exponetiated Weibull fading model. A computationallyefficient and approximated expression is presented for evaluating the average BER ofOOK modulation by use of the Gauss–Laguerre quadrature rule. The computersimulation program is designed, and the theoretical results are verified by thesimulation results. The proposed numerical approach can be used for fastly evaluatingthe BER performance improvement by aperture averaging under different turbulenceconditions.3. By use of non-Kolmogorov power-law spectrum and the extended Rytov theory, irradiance scintillations for horizontally propagating optical waves and their effects onthe fade statistics of ALC systems are studied. Based on one of the non-Kolmogorovtype spectrums, the generalized von Kármán spectrum, analytical expressions aredeveloped for the scintillation index of a plane wave and spherical wave that are validunder moderate-to-strong irradiance fluctuations. With these general models, theimpacts of finite inner and outer scales on the scintillation index of an optical wave areexamined under various non-Kolmogorov fluctuations conditions. By use of thegeneralized modified atmospheric spectrum that features inner scale parameter andhigh wave number ‘‘bump’’ for non-Kolmogorov atmospheric turbulence, tractableexpressions are developed for the scintillation index of a plane wave and a sphericalwave that are valid under weak and moderate-to-strong irradiance fluctuations. Withthe derived scintillation index and the Gamma–Gamma distribution, fade statistics forALC systems are investigated. The presented numerical results reveal some newdetails about how the inner scale and spectrum exponent values influence thescintillation index and the associated fade statistics.4. Uplink laser satellite-communication system performance is studied based onnon-Kolmogorov spectrum. A newly proposed three-layer attitude spectrum model thatexhibits Kolmogorov properties in boundary layer and non-Kolmogorov properties inportions of the upper troposphere and stratosphere is adopted. Using this spectrum inweak turbulence, the scintillation index for an uplink collimated untracked Gaussianbeam that is subject to scintillation and beam wander impairments is analyzed. Basedon the obtained scintillation index and the Lognormal channel model, the outageprobability of the considered system is examined. It is shown that there exists anoptimum transmitter beam radius that can minimize the scintillation index, andtherefore minimizes the outage probability. The physical interpretion for the existenceof optimum transmitter beam radius is provided, and the results are compared with theconventional Kolmogorov results.