Research On The Channel Influence And Antijamming Methods Of Wireless OCDMA Systems

Wireless Optical Code Division Multiple Access (WOCDMA) not only can reserve the advantage of CDMA technology in RF communication, but also can use huge bandwidth and have simple network protocol, random access and other characteristics. It's the hotspot of researching optical communication. But the environment wireless channel is worse, and address codes in WOCDMA system aren't totally orthogonal with each other, that cause multi-access interference(MAI). So this thesis does some research on how to reduce channel noise and the MAI to improve the system's performance.Firstly, this thesis analyzes the factors of affecting WOCDMA system in free space and atmosphere channels and then does simulation experiments. According to simulation experiment results, the free space channel is ideal and the system'BER becomes higher with more users. In atmosphere channel, when atmospheric scintillation is stronger, WOCDMA system becomes worse. And more users can also make the system's BER higher. So MAI is the major factor of affecting system's performance in free space channel. Atmospheric scintillation and MAI is the two important factors of affecting system's performance in atmosphere channels.Secondly, for restraining MAI in free space WOCDMA system, this thesis researches systems based on channel codes and differential detection technology, reduces the BER formulas and do simulation experiments. According to simulation experiment results, WOCDMA system based on channel coded has better performance than the uncoded system in the same chip rate. For WOCDMA system based on differential detection technology, the BER is 4dB lower than the conventional system when the received optical power is -30dBm. The performance of systems based on both channel code and differential detection is better than the system only based on differential detection. So using channel code and differential detection can restrain the MAI effectively and improve the performance for the WOCDMA system in free space.Finally for reducing atmospheric scintillation and the MAI in atmospheric WOCDMA system, this thesis researches the systems based on multi-beam propagating technology and differential detection technology, deduces the BER formulas and then do simulation experiments. According to simulation experiment results, multi-beam propagating can improve the system performance very well compared with single-beam propagating. Moreover, more beams we use, better performance we get both in weak scintillation and strong scintillation. And for atmospheric WOCDMA system based on differential detection, the BER is 4dB lower than the conventional system when the received optical power is -30dBm. So using multi-beam propagating technology and differential detection technology can improve the performance of atmospheric WOCDMA system.From the above, this thesis analyzes the channel noise influence on WOCDMA systems and employs effective antijamming methods to improving the system performance.