| Free space optical(FSO)communications have attracted widely interests of the researchers due to its advantage of large capacity,high security,strong anti-interference,and no authorization requirements.In the studies of the FSO communications,the researchers have been pursuing a communication terminal with high transmission rate,small size and low power consumption.With the developments of the FSO communications,there are increasing needs for a terminal which can meet above requirements,such as the applications of micro nanosatellite and unmanned aircraft.Those applications need less size,less weight and lower power consumption in one terminal.However,due to the small laser divergence angle of the FSO communication,traditional FSO communication systems are equipped with the pointing,acquisition and tracking systems which increase the power consumption and the size of the system.This hinders their applications to the above communications which require lower power consumption,less size and less weight.To satisfy above requirements,the researchers propose a modulating retro-reflector(MRR)based FSO communication system.Recently,the studies on the MRR FSO communication mainly include the experimental studies on the feasibility of the MRR FSO communication and the studies on the performance of the MRR FSO communication,where the performance studies are conducted from theoretical studies and experimental studies.Considering that the theoretical research is significant for the design and optimization of the practical applications,we will study the performance of the MRR FSO communication system by theoretical means.Our works include the following four parts.1.The existing researches on the performance of the MRR FSO communication system mainly consider the weak turbulence condition,and few studies have investigated on the performance under the atmospheric turbulence from weak to strong.However,the performance analysis under weak to strong turbulence condition is important for the design and optimization of the practical applications,therefore,we will investigate the performance of the MRR FSO communication under the weak to strong turbulence.In order to do this analysis,we first establish the system and channel model for the MRR FSO communication system.In the channel model,in order to descript the weak to strong turbulence,we descript the forward and the backward link atmospheric turbulence fading by the Gamma-Gamma model which is an efficient mathematical description for either weak or strong atmospheric turbulence fading.And in our analysis,the forward and the backward link atmospheric turbulence fading are considered to be independent.The probability density function(PDF)of the MRR link is essential for the performance analysis of the MRR FSO communication,then based on the above established channel model,the closed form expression for the PDF of the MRR link is derived.This obtained expression is further used to derive the closed form expressions for the average capacity,the average outage probability and the average bit error rate(BER).Finally,based on those obtained expressions,we analyze the system performance.Our obtained expressions are the basis of the performance analysis for the MRR FSO communications,and the performance analyses in our work play a guiding role in the system design.2.Above study does not consider the correlation between the forward and the backward links.Considering that there may be a correlation between the forward and the backward link turbulence fading,therefore,it is necessary to analyze the MRR FSO communication system performance under the correlated forward and backward link fading over weak to strong turbulence fading.In this paper,this problem will be investigated deeply.In order to do this analysis,we first establish the system and channel model for the MRR FSO communication system.In the channel model,the forward and the backward link atmospheric turbulence fading are considered to be correlated and descripted by the Gamma-Gamma distribution.Based on the above established channel model,two closed form expressions for the PDF of the MRR link are derived under the following two situations: the parameter of the forward and the backward link atmospheric turbulence fading are the same and different.Those obtained expressions are further used to derive the closed form expressions for the average capacity,the average outage probability and the average bit error rate(BER)under above two situations.Finally,based on those obtained expressions,we analyze the system performance.Our obtained expressions are the basis of the performance analysis for the MRR FSO communications,and the performance analyses in our work play a guiding role in the system design.3.Above studies and existing studies analyze the system performance with known instantaneous channel state information(CSI).Those analyses have practical significance since they are very useful for the evaluations of the practical system performance.However,the instantaneous CSI is unavailable in practice.In order to achieve communications under unknown instantaneous CSI,in this paper,a MRR FSO communication system with differential signaling is proposed and the performance of this system is analyzed.In order to do this analysis,we first establish the system and channel model for our proposed system.Then the instantaneous BER of the system with differential signaling is derived.Based on this result,we analyze the system BER performance under the Lognormal and Gamma-Gamma turbulence fading models.The Lognormal model can be used to analyze the system performance under weak turbulence.To do the analysis under this model,first,two closed form expressions for the joint PDFs under the following two conditions are derived: the atmospheric turbulence fading are independent or partially correlated,and fully correlated.Then by using above obtained expressions of the joint PDFs and the instantaneous BER,the closed form expressions for the average BER under above two conditions are derived.Finally,based on those obtained expressions,we analyze the system performance.Besides,the Gamma-Gamma model can extend above BER performance analysis to the weak to strong atmospheric turbulence fading condition.Our analyses under this model consider the situation that the extinction ratio is 0.First,the closed form expression for the instantaneous BER with extinction ratio of 0 is obtained.Then based on this expression,the closed form expression for the average BER with consideration of the atmospheric turbulence fading correlation is calculated.Finally,based on those obtained expressions,we analyze the system performance.4.Above MRR FSO communication system using differential signaling possesses the advantage of no prior information required.However,this scheme suffers from high system complexity.To solve this problem,this paper proposes an improved scheme,which is the MRR FSO communication system with an adaptive threshold.This scheme can achieve communication with simple system configuration and without the prior information.In order to do this analysis,we first establish the system and channel model for our proposed system.Then the construction for the optimal adaptive threshold is provided.Based on the above constructed adaptive threshold,the instantaneous BER is further derived.By using the PDFs derived in the part 1 and part 2,the closed form expressions for the average BER under the following three situations are calculated: 1)the forward and the backward link atmospheric turbulence fading are independent;2)the forward and the backward link atmospheric turbulence fading are correlated with the same parameters;3)the forward and the backward link atmospheric turbulence fading are correlated with different parameters.Finally,based on those obtained expressions,we analyze the system performance. |
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