Facilitating spectral-efficiency study of multirate optical CDMA

Two multirate techniques, so-called the multiple-code (MC) and multicode-keying (MK) schemes, were recently proposed to support multimedia services with different bit rates in asynchronous optical code-division multiple-access (O-CDMA) systems. In the MC scheme, a user maps multiple data bit 1s to multiple code matrices and transmits them at the same time. In the MK scheme, a user only transmits one of its assigned codewords at a time, and each codeword represents a symbol of multiple data bits. In terms of error probability, the performances of these multirate schemes were also analyzed. Spectral efficiency (SE) is another figure-of-merit useful for comparing O-CDMA schemes. In this paper, the performances and SEs of the multirate MC and MK schemes are formulated and the conditions of which scheme offers a better SE are investigated with formal proofs. Numerical examples are given to validate the analyses.