Performance analysis of internally coded time-hopping coherent ultrashort light pulse CDMA scheme in fiber-optic communication systems

We consider an internally coded time-hopping coherent ultrashort light pulse code division multiple access scheme (TH-CULP CDMA) and analyze its performance in the fiber-optic communication systems. This system combines the TH and CULP CDMA techniques and exploits the advantages of both. In our method, each bit time interval is divided into N-s frames, and the spectral phase-coded pulse is transmitted in one of these frames. Two exclusive PN sequences are assigned to each user. One is added to a superorthogonal convolutional encoder output to select the transmission frame, and the other is used to encode the phase of the ultrashort pulse in the spectral domain. We evaluate the bit error rate (BER) of the system considering the effects of the multiple access interference (MAI) and thermal noise for both soft and hard decoders. The BER is evaluated using both the Chernoff bound and saddle point approximation. We compare the performance of our proposed system with that of the conventional CULP CDMA system. The numerical results indicate that for the same bit rate and processing gain, the performance of our proposed system (TH-CULP CDMA) is substantially better than that of the conventional CULP CDMA system. In addition, we observe that the hard decoder has a better performance at large number of users compared to the soft decoder.