Abstract
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.
© 2009 IEEE
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