Performance analysis of optical frequency-domain encoding CDMA enhancement of frequency division multiplexing

Optical frequency division multiplexing (FDM) technique has the advantage of fully orthogonal transmissions. However, FDM system permits only a small number of FDM channels despite of a great effort, such as frequency stabilization. On the other hand, frequency-domain encoding code-division multiple-access (FE-CDMA) has been widely studied as a type of optical CDMA. In this system, encoding is done in the frequency domain of an ultrashort light pulse spread by optically Fourier transform. However, FE-CDMA accommodates very limited number of simultaneous users, though this scheme uses a vast optical bandwidth. It is attractive to consider the combination of both advantages of FDM and FE-CDMA. We propose FE-CDMA enhancement of FDM (FDM/FE-CDMA). Since in FDM/FE-CDMA the total bandwidth is partitioned into M optical bands and each band is encoded by the code with code length of Nc, we expect nearly perfect orthogonal transmissions. In addition, since the creation of FDM bands is realized by a passive filter, the optical frequency is precisely controlled and the optical frequency allocation is flexible. We derive the bit error rate (BER) as a function of the number of simultaneous users, bit rate, and the utilization efficiency of total bandwidth. We compare the performance of FDM/FE-CDMA with that of the conventional FE-CDMA in terms of the number of simultaneous users on condition that each chip width is constant. As a result, we show that FDM/FE-CDMA can support the larger number of simultaneous users than the conventional FE-CDMA at a given bit error rate under the same total bandwidth. key words: FE-CDMA, FDM, spatial filler, phase mask, ultrashort light pulse, multi-access interference (MAI).