Performance evaluation of optical CDMA systems using non-coherent detection and bipolar codes

The efficiency of an asynchronous optical CDMA (code-division multiple access) system is addressed. Two types of data modulations are considered: ASK and large-deviation CPFSK. Each user´s narrow-band information is spread by the use of an electrooptic phase modulator. The sequences considered are bipolar Kasami and Gold codes. The receiver consists of an optical filter of bandwidth ≈1/T, to filter the interfering signals, followed by simple, noncoherent detection. We thereby avoid all the difficulties usually met with coherent detection. Moreover, this type of demodulation is particularly suitable for a hybrid WDMA/CDMA (wavelength-division multiple access/code division multiple access) system where the spread-spectrum technique allows multiple users to share the same wavelength, thereby increasing the capacity of the network. Results are given in terms of the number of interfering signals that can be tolerated for different code periods N, and for an acceptable P_e. Comparison is made relative to the Gaussian approximation on the distribution of the interfering signals