Key Performance Of The Underwater Wireless Optical Communication Over Turbulent Channels

As an important base for the sustainable development of human beings,oceans contain rich resources and play an increasingly prominent role in the economic,cultural and military fields.The increasingly frequent marine activities such as environmental observation,scientific research,resource exploitation and national defense construction,as well as the increasing popularity of autonomous underwater vehicles and underwater sensor networks,put forward higher demands to underwater wireless communication.Compared with traditional acoustic communication,underwater wireless optical communication(UWOC)has attracted significant attention due to its high data rate and low delay.However,absorption,scattering and turbulence effects in the underwater channel are some of the challenges that need to be overcome to exploit the full potential of UWOC.Absorption and scattering mainly cause signal attenuation and delay spread which have been analyzed comprehensively.Relatively speaking,more in-depth research on underwater optical turbulence is needed.Underwater turbulence results in beam wandering,beam spreading,spot jitter and intensity scintillation.The turbulence can exist in short-range UWOC,which greatly degrades communication performance.Therefore,to improve the effectiveness and reliability of UWOC,it is of great significance to investigate the fading characteristics of underwater turbulence and corresponding methods to mitigate fading.In this dissertation,spatial diversity is considered to alleviate the signal fading caused by the underwater turbulent channel.Using statistical models that characterize turbulence,the UWOC performance with spatial diversity and equal gain combination(EGC)over turbulent channels is studied and analyzed.The main contents are as follows:1.Aiming at the degradation on the performance of UWOC over weak turbulence channels,QAM and DCO-OFDM are adopted to improve spectrum efficiency and reduce inter-symbol interference(ISI);Spatial diversity is utilized to mitigate turbulence-induced signal fading and the performance of QAM DCO-OFDM spatial-diversity equal-gain combining UWOC in lognormal-distributed fading channels is presented.Firstly,assuming that signal attenuation due to absorption and scattering and turbulence-induced fading are independent from each other,the influence of the underwater channel on received light intensity is expressed as the product of the path loss(channel DC gain)and fading coefficients.The path loss coefficient is obtained by Monte Carlo(MC)simulations based on the photon tracing method,and the fading coefficient is expressed as a lognormaldistributed random variable(RV).Secondly,based on the approximation of the weighted sum of the lognormal-distributed fading coefficients of mutually independent branches,the analytical expressions of average bit error rate(BER)and outage probability of the system are derived.Finally,the accuracy of the expressions is verified by MC simulations for different turbulence scenarios and diversity orders,and meanwhile the performance of the system is analyzed in detail.The results have demonstrated that the spatial diversity is effective to reduce the average BER and outage probability.The effectiveness of spatial diversity is more obvious in strong turbulence than that in weak turbulence.With increasing the diversity order the spatial diversity gain increases and then saturates at a certain value.In the lognormal-distributed fading channels with scintillation indexes of 0.1,0.3,0.6 and0.9,the diversity gains of the 2×4 spatial diversity system are 2.4d B,6.2d B,9.4d B and11.5d B,respectively.2.CAP modulation is adopted to simplify the system implementation and ensure a high data rate,and moreover LDPC codes are combined with spatial diversity to further improve system resilience to the fading induced by weak-to-strong turbulence-induced fading and meanwhile keep simple implementation.The performance of LDPC-coded and CAPmodulated system is evaluated over the GG-distributed fading channels with spatial diversity and EGC receiver.Firstly,based on the CHFs of the GG-distributed fading coefficients of mutually independent branches,the analytical expression of average BER without coding is derived,which is expressed in the forms of Meijer's G or Fox's H functions.Secondly,to further improve the ability to mitigate fading and reduce the required signal-to-noise ratio(SNR),LDPC codes are combined with spatial diversity,and then the encoding/decoding algorithms of QC-LDPC codes as well as the initial log-likelihood ratio(LLR)calculation are introduced in detail.Finally,the theoretical calculation is carried out according to the expression and verified using MC simulations.In addition,the BER performance of the LDPC-coded system is simulated.By analysis and comparison,it is shown that the combination can improve the ability to mitigate fading at a small SNR making it suitable for the cases of strong turbulence,high-order modulation and power limitation.In the GGdistributed fading channel with a scintillation index of 1.02,the diversity gain of the LDPCcoded 2×2 spatial diversity system is about 10.3d B.3.Since DCO-OFDM and CAP are with relatively high power dissipation due to adding DC bias and their performance changes with DC bias,ACO-OFDM without DC bias is adopted to improve power efficiency.Different from the CHFs-based method used in chapter4,the approximation of the weighted sum of mutually independent GG-distributed coefficients is adopted to comprehensively estimate the performance of UWOC in chapter5.Firstly,based on the approximation of the weighted sum the analytical expressions of the average BER,outage probability and ergodic capacity of the system are derived.Furthermore,the analytical expressions of the average BER and ergodic capacity are simplified by Gauss-Laguerre quadrature integral formula and Fox's H function operations,respectively.Compared with the CHFs-based method,approximating the weighted sum of independent GG-distributed variables not only ensures correctness but also is helpful to the derivation of the outage probability and ergodic capacity as well as the calculation simplification.Then the accuracy of these expressions is verified by MC simulations for different fading environments,and meanwhile the results are analyzed and discussed.Furthermore,Comparisons of average BER for different modulation orders(4-QAM & 16-QAM)and different analytical methods(CHF-based and approximation of the weighted sum-based)are carried out.Meanwhile,diversity gains are obtained by MC simulations for different fading models(lognormal and GG distributions)but with the same scintillation index.And thus,the performance of the ACO-OFDM spatial diversity UWOC system is comprehensively and deeply evaluated.The results of this dissertation provide the theoretical basis for the design,prediction and evaluation of high-speed UWOC over fading channels,which has a wide application prospect and practical significance in various marine activities.