Spectral efficiency comparison of SAC-OCDMA systems using unipolar and bipolar encoding techniques

We compare the spectral efficiency of unipolar and bipolar encoding for spectral-amplitude coding optical code-division multiple access (SAC-OCDMA) systems operating with incoherent sources considering the intensity noise as the primary noise source. For both encoding techniques, the spectral efficiency of four SAC codes such as Hadamard, modified quadratic congruence (MQC), balanced incomplete block design (BIBD) and zero cross-correlation (ZCC) is evaluated and compared as a function of the number of users and code length for a BER= 10^-10. Analytical derived equations and results verified that ZCC code can support high number of users and leads to the best achievable spectral efficiency than other SAC codes considered. Furthermore, bipolar encoding improves the spectral efficiency as compared to unipolar technique, e.g., bipolar ZCC has 7.7 dB spectral efficiency advantage than unipolar ZCC.