Research On Hybrid Automatic Repeat Request Scheme For Hybrid FSO/RF Links

As a research hotspot for wireless backhaul network,Free Space Optical(FSO)communication has advantages of high data rate,free spectrum,easy deployment,and high confidentiality.However,FSO communication system is sensitive to the atmospheric environment,which limit the system reliability.Combining the FSO link with radio frequency(RF)link in mutual backup or parallel transmission,hybrid FSO/RF communication system can be an efficient way to get good availability and reliability in practical applications.The research of hybrid FSO/RF system mainly include:swithing strategies between FSO and RF link,link adaptive mechanism,and hybrid channel coding method.This thesis mainly investigate the HARQ transmission scheme for hybrid FSO/RF communication system,together with their outage probability and throughput under different transmission mode and channel conditions.The main work is as follows:(1)Based on the influence of different weather conditions on FSO and RF links,choose the channel models of FSO link and RF link as Gamma-Gamma distribution model and Rayleigh distribution model respectively.Then introduce the basic prinpciples of forward error correction,ARQ technology and three types of HARQ protocols,together with their characterics.(2)Investigate the HARQ transmission protocols for hybrid FSO/RF communication links.Then the system outage probability,throughput,and average transmission times were derived for three different types of HARQ schemes.By using theoretical analysis and numerical computation methods,the performance of HARQ system for hybrid FSO/RF communication links was analyzed under different transmission conditions.(3)The performance of HARQ schemes for hybrid FSO/RF links was simulated and analyzed by using Monte Carlo method with LDPC coding as FEC under different channel conditions and modulation types.The research results show that compared with the non-HARQ scheme,the HARQ scheme for hybrid FSO/RF links studied in this paper significantly improves the system performance.When one link has worse channel condition,the reliability of the hybrid system can be ensured by adjusting the transmission mode and rate appropriately.When the channel quality of the RF link is bad,the data transmission ratio of the FSO link can be appropriately increased to improve the system frame error rate performance,and vice versa.Therefore,by selecting suitable transmission strategy and HARQ scheme parameters according to the state of the channel,hybrid system can have good availability and reliability performance.