Spectral efficiency comparison of two multirate O-CDMA schemes

Various multirate transmission techniques, such as the multiple-code (MC) and multicode-keying (MK) schemes, have been proposed for optical code division multiple access (O-CDMA) systems with multimedia services. In this paper, a metric of comparison, so-called the normalized spectral efficiency (NSE), is derived to compare, as a whole, the performance and system efficiency of these two schemes. The NSE of the conventional single-rate on-off-keying (OOK) O-CDMA scheme is also derived for a baseline comparison. Our numerical results show that the MC scheme always has larger NSE than the single-rate OOK scheme, while the MK scheme has the largest NSE under certain conditions.