Research On The Multipath Channel Influence And Multipath Countermeasures Of WOCDMA Systems

In wireless optical code division multiple access （WOCDMA） systems, theindoor channel and atmospheric channel contain multi-path effects. Multi-pathcauses the pulse attenuation and broadening. This has a serious impact on theperformance of WOCDMA systems. Therefore, this thesis studied the impacts ofmultipath channels on WOCDMA systems and the multi-path countermeasures.First, the characteristics and the models of the channels in WOCDMA systemswere studied. According to the problem that the traditional theoretical model ofindoor multipath channel impulse response function can not describe the multi-peakchannel impulse response, utilized the model of multi-exponential decay function tomodify the model of the indoor channel. The experimental results show that themodified model can describe the multi-peak characteristic of indoor multipathchannel properly. The scattering properties of atmospheric multipath channels werestudied. The simulation of atmospheric multipath channel impulse response showthat in atmospheric multipath channel, the delay spread of the channel increaseswith the atmospheric optical thickness increases.Secondly, the WOCDMA system performance under multipath channel isanalyzed. This thesis obtained the probability distribution model of the interferencein WOCDMA system under multipath channel, which is determined by the averagepower of a single user and the characteristics of the code chosen in WOCDMAsystems and analyzed and compared the BER performance of one-dimensionaladdress codes and two-dimensional address codes of the WOCDMA system inmultipath channel. The experimental results show that the two-dimensional addresscodes performs better than the one-dimensional address codes in multipath channel.At low ratio of signal to interference, the performance of two-dimensional addresscodes becomes worse. With the improvement of the signal to interference ratio, BERgradually decreased. The BER is lower than10^-4, when the system usingtwo-dimensional address codes of the code length49, and the signal to interferenceratio reaches to-9dB.Finally, multipath countermeasures for WOCDMA systems have been studied.This thesis designed a multi-optical hard-limiting decoder and utilized thedeconvolution channel equalization technology to achieve joint suppression ofmultipath effects on the system. Compared with the non-hard-limiting structure, theBER of the multi-optical hard-limiting structure decreases about103times to104times, when the system using two-dimensional address codes of the code length49.The longer the code is, the more the BER decreases. The deconvolution channel equalization technology allow the systems using shorter codes to contain moreinstantaneous users for certain BER requirements.