| In recent years,with the increasing demand for ultra-high bandwidth,economic security and high-speed wireless data transmission,free space optical(FSO)communication has shown increasing scientific research and commercial potential with the advantages of high transmission rate,wide available frequency band,low power loss and good confidentiality.However,the optical signal propagating in free space is majorly affected by the atmospheric turbulence,fog,path loss and pointing errors,resulting in signal attenuation and degradation of communication system performance.In order to alleviate the deterioration of FSO link performance,relay technology can effectively guarantee the reliability of system communication.Therefore,this paper combines the relay technology with the coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology,which has high receiving sensitivity,strong anti-polarization film dispersion ability and good spectrum efficiency,and make the following research on the FSO communication system.Firstly,a theoretical model of decode and forward(DF)multi-hop CO-OFDM FSO system is proposed based on M-distributed atmospheric turbulence channel.The probability density function(PDF)of the joint attenuation channel gain of multi-hop CO-OFDM FSO system is derived.Then on this foundation,we derive the closed form mathematical expressions for estimating the system outage probability and symbol error rate(SER).Furthermore,the effects of key factors such as relay link length,number of relay nodes and subcarriers on the outage and SER performance of the proposed system are analyzed.Simulation results show that the outage probability of multi-hop system reaches 100%when the normalized threshold is greater than-8 d B.When the SER is 10-5,the average signal-to-noise ratio(SNR)of the multi-hop system under moderate turbulence with the link length between relays d=1000 m is reduced by about 14 d B and 19 d B compared to 2000 m and 3000 m,respectively.Besides,increasing the number of relay hops,subcarriers,length of the link between adjacent relays and higher atmosphere turbulence intensity will decrease the performance of the system.Properly reducing the normalized threshold and increasing the average SNR can improve system performance.Secondly,a new theoretical model of DF serial-parallel combined relay CO-OFDM FSO system based on M-distributed atmospheric turbulence channel is proposed.Take advantage of the integral properties of Meijer G function,we derive the accurate mathematical expressions of outage probability and SER of the system under the condition of joint attenuation channel.Furthermore,the changing relationship between the performance of CO-OFDM FSO system and the key factors such as weather conditions,combined structure parameters and mapping order is studied.Under certain conditions,compared with serial relay or parallel relay,the serial-parallel combined relay can further expand the communication coverage and bring greater advantages to the performance of CO-OFDM FSO system.It is concluded that the outage and SER performance of the system are more significantly affected as the weather conditions change from very clear to drizzle,haze,and light fog.Especially,in light fog weather condition,the SNR required for the system to maintain normal communication is over 98 d B.Finally,from the perspective of reducing the complexity of the system structure and saving cost,we design a serial-parallel combined relay device of amplify-and-forward(AF)in FSO communication,and give the concrete realization project.This device greatly expands the communication coverage,improves the system's ability to resist atmospheric turbulence and increases the system's channel capacity.In summary,this paper deeply studies the effects of some key parameters on the outage and SER performance of CO-OFDM FSO multi-hop and serial-parallel combined relay systems,and lays a theoretical foundation for the practical application of relay technology in FSO communication. |
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