Reducing atmospheric turbulence effects in FSO/CDMA systems by using multi-wavelength PPM signaling

In this paper, we present a comprehensive study of the effects of atmospheric turbulence, including intensity fluctuation and pulse broadening, on the performance of FSO/CDMA systems. A realistic model of Gaussian pulse propagation is used for analyzing the bit error rate (BER). The numerical results show that pulse position modulation (PPM) with high modulation level should not be used for FSO/CDMA systems as it requires to send high chip rate (i.e., short pulse), which is significantly affected by pulse broadening effect. To overcome the limitation of PPM, we propose to use multi-wavelength PPM (MWPPM), which achieves high modulation level by combining PPM and wavelength shift keying. By using MWPPM, the effects of both intensity fluctuation and pulse broadening are mitigated thus the BER is reduced. In addition, we found that the performance of FSO/CDMA systems using MWPPM can be further improved and lower BER can be achieved by using avalanche photodiode with the gain around 80 to 100.