Spectral Efficiency Study of Two Multirate Schemes for Asynchronous Optical CDMA

Multirate transmission techniques, such as multiple-code (MC) and multicode-keying (MK) schemes, have been proposed for asynchronous optical code-division multiple-access (O-CDMA) systems supporting multimedia services. A user in the MC scheme transmits several codes at the same time to represent the transmission of several data bit ones. The number of parallel codes transmitted at a time is proportional to the user's bit rate and the number of data bit ones. While the MK scheme only transmits one code at a time, each code represents a symbol and each transmitted symbol represents multiple data bits. Data rate is varied by changing the symbol size, which, in turn, changes the number of data bits per baud, similar to MODEM communications. In this paper, the spectral efficiencies (SEs) of these two asynchronous schemes are derived for the comparison of their performances and efficiencies as a whole. The SE 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 a better SE than the single-rate OOK scheme, while the MK scheme can have a better SE than the single-rate OOK scheme under certain conditions.