Research On Mimo Optical Wireless Communication System In Free Space

In order to improve the performance of communication system, the MIMO(Mutlti-Input Multi-output) technology compared with SISO (Single-InputSingle-Output) technology has been arised to utilize the spacial diversity gain.TheMIMO technology has been used wildy in RF wireless communication systems,such as the Wi-Fi system which has been combinded with OFDM technolgoy. Butthe scenario will be different in the domain of FSO(Free Space Optical) system, andthe expansion in this domain is slow, becasue of the weaknesses which is intrisic inFSO, such as IM/DD(Intensity Modulation/Direct Detection).So it is necessary toinvestigate the how the MIMO improve the performance in FSO system. This thesisdoes some research on how to improve the communication quality which isgenerated by misalignment errors.Firstly, this thesis mend the traditional channel model for FSO system,i.e,addmisalignment error interference to the weak turbulance interference.In the term ofmath expression, the logarithmic normal distribution has been mended withmisalignment error interference attenuation factor. Two scenarios has beendiscussed in channel modoling, one is the misalignment in X and Y two directions,the other one is only in X or Y unidirection. Then the ourage probabilty andapproximate expression in high SNR have been derived.Secondly, the important modulation method PPM(Pulse Position Modulation)and its derived form MPPM(Multi-Pulse Position Modulation) have beeninvestigated in order to find out the performance improvement compared withtraditional modulation type. The soft output in every time-slot expression has beenderived in PPM and MPPM modulations. The simulation results have showed that inthe same BER the maximum gap between MPPM and PPM is2.75dB, MPPM isbetter.It is necessary to investigate the constellation optimization problem in MPPM,because the cardinality of MPPM symbol set is always lareger than the actualnumber we needed. Three methods in discrete and continuos optimization has beendiscussed. Finally,the key structure in MIMO system space-time code has been researchedin this thesis.Through look into the basic theory of space-time code, the differencesbetween RF wireless system and FSO system has been listed and theOSTBC(Orthogonal Space Time Block Codes) has been migrated from RF domainto FSO domain. The simulation has been depolyed to compare the OSTBC andRepetition code and the results show that REP is better than OSTBC, specifically,atBER109, REP has gained3.35dB...