Abstract
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<sub>s</sub> 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.
© 2007 IEEE
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