Study On The Transmission Performance Of Wireless Optical Communication System Based On Lognormal Distribution

Underwater wireless optical communication(UWOC)is a communication technology which employs laser as the carrier wave to transmit information through ocean or lake.Compared with the traditional communication technology employing sound as the carrier wave,UWOC has the advantages of high bandwidth,high speed,low delay,strong anti-interference ability,good confidentiality and low power consumption.In recent years,with the increase of human exploration and development of the ocean and the development of related technologies,the UWOC has gradually become a hot research topic for researchers in the world.However,the optical signal transmitted through seawater is affected by the scattering absorption of seawater and the ocean turbulence,which limits the transmission of optical signals.Therefore,how to reduce the influence of these attenuation factors and extend the effective communication distance is the key to improve the performance of the UWOC system.Recent study indicates that the aperture averaging technique is also applicable to UWOC,and the large aperture receiving device can suppress the light intensity flicker effect caused by ocean turbulence,thereby improving system performance.In addition,relay assist technology can be used to extend communication distance and multi-user diversity(MD)technology can exploit channel time-varying and fading independence between multiple users to optimize overall system performance by increasing channel capacity.Therefore,combining the three technologies will inevitably have a considerable effect on improving the performance of the UWOC system.In this paper we employ the lognormal distribution model(LN)and introduce the LDPC coding technique and aperture averaging technique to the UWOC system.Then the effects of parallel multi-hop relay assist and multi-user diversity on system performance are researched.The main contents are as follows:1? Based on the LN distribution model,the maximum and minimum criteria is employed as the optimal path selection method to study the parallel multi-hop architecture with direct link.The theoretical expression of the average bit error rate(ABER)of the system is derived based on the independent equivalence distribution link and the independent non-equivalent distribution link distribution.The ABER trend is analyzed in detail when the link length,temperature variance dissipation rate,receiving aperture,and relay structure change.The Monte Carlo(MC)method is employed to simulate and verify the correctness of the theoretical model and results.Finally,the LDPC channel coding technology is introduced to the communication system,and the joint effects of different code rates,different maximum iterations on relay cooperative LDPC coding performance are studied by numerical simulation.2? In the UWOC multi-user diversity system,the maximum signal-to-noise ratio scheduling strategy,proportional fair scheduling strategy,selective scheduling strategy and improved proportional fair scheduling strategy are introduced based on the LN channel fading model.Firstly,a brief description of the working mechanisms of these scheduling algorithms is given.Then,Binary phase shift keying(BPSK)method is used to derive the average channel capacity and average bit error rate expression of the UWOC system under the condition of maximum SNR algorithm and selective scheduling algorithm,and the MC simulation is employed to compare and analyze the average error rate of the above algorithms.Finally,the effects of the receiving aperture size and various ocean parameters on the throughput of the UWOC system are researched.The results show that with the increase of the number of users,the maximum SNR algorithm always has the best bit error rate(BER)performance,and the BER performance of the selection scheduling algorithm is improved rapidly.Moreover,the aperture size and ocean parameters have a certain impact on the system throughput,and the aperture average effect may suppress the diversity gain.The research in this paper can provide theoretical guidance for designing underwater wireless optical communication systems and improving their performance.