Energy-Efficient Transmission Technologies For Underwater Visible Light Communications With Single Photon Detection

Underwater visible light communication(OWC)system is a wireless communication technology for data transmission in unguided water environment through light.Compared with underwater radio-frequency wireless communication system and acoustic wireless communication,VLC can provide a higher transmission bandwith and data rate,which is regarded as a promising candidate to realize underwater high data rate communication.However,when propogates through the underwater channel,light signal surfers a severe deterioration due to absorption,scattering and turbulence.Therefore,current VLC systems can be only applied for short-range communication,which limits the role of VLC plaied in the future underwater wireless networks.For VLC systems,a method to extend the communication range is replacing the conventional photondoide(PD)by single photon avalanche diode(SPAD)which has a higher sensitivity.Different from commonly encountered PD VLC system with Gaussian noise,the output of SPAD VLC system is Poisson distributed due to the photon-counting nature of SPADs.Thus,it is necessary to research the high efficient transmission technologies for Possion channel.Catering to the need of long-range transmission for underwater VLC,this article is focused on receiver design,energy-efficeint constellation design and low-complexity signal detection method for Poisson channel,which are summarized as follows.1.In view of the problem that for Poisson channels the error proformance of maximum likelihood(ML)receiver is difficult to form a tractable one and there is no currently avalaible constellation design criterion,Anscombe root(AR)receiver is estanblished based on AR transform.Guided by the well established theory in Gaussian channel,we realize signal estimation and provided signal design criterion.By AR transform,the Poisson distributed received signal can be transformed into Gaussian distributed,then transmitted signal can be estimated by comparing Euclidean distance in transform domain.By analyzing the performance of AR receiver,the Hellinger distance based constellation design criterion is devised.The simulations of the comparison between ML and AR receiver are carried out,whose results indicate that AR receiver has a performance approaching ML receiver and the performance of AR receiver is inversely proportional to system rate.2.Catering to the need of high efficient signal transmission for the SPAD-VLC systems,an energy-efficeint 1-D constellation is designed for ideal Poisson channel is formulated based on the proposed Hellinger distance criterion.This constellation is designed by minimizing the average transmitted optical power for a fixed minimum Hellinger distance and shown to be the squared version of unipolar PAM constellations.For Poisson channels,the proposed constellation is more energy-efficient than PAM constellation and is the best 1-D constellation currently available for the considered system.3.Aiming at the single photon avalanche diode(SPAD)underwater multi-input multi-output optical wireless communication(MIMO-OWC)systems,the energy-efficient space code is deviced for log-normal Poisson channel.Based on the Hellinger distance,the constellation design problem is proposed.Hellinger space code(HSC)is obtained by maximizing the worst-case minimum Hellinger distance between any two distinct signal vectors with a fixed normalized average transmitted optical power budget.Actually,HSC is the spatial-repetitional transmission of the scalar SPAM constellation,which is proved to the most energy-efficient space constellation currently avalible for considered system by simulations.4.For multi-dimensional space,we collaboratively design the signal from each dimension under ideal Poisson channels.The energy-efficeint multi-dimensional constellation design problem is formulated based on Hellinger distance,which is described as: devise a multi-dimensional constellation in nonnegative integer set such that the average transmitted optical power is minimized subject to a fixed minimum Hellinger distance between two distinct signal vectors.An optimal solution for this problem is proved to be formed by arranging the answer of multivariable quadratic Diophantine equation in the increasing order of average optical power.Simulations indicate that the proposed constellation outperforms SPAM and PAM.5.Aiming at dealing with the problem of high complexity of ML receiver and combining the feature of fast detection method for Gaussian channel,the low-complexity receiver is established by approximating the original detection thresholds by an equal spaced version.This revision reduces receiver complexity at the cost of a slight performance lost.To investigate the performance loss caused by the low-complexity receiver,simulations are carried out to compare the performance of the proposed receiver and ML receiver,whose results indicate that the performance loss is lower than 0.25 dB with high optical power.