| Summary: | 博士 === 國立中興大學 === 電機工程學系所 === 97 === Multirate transmission techniques, such as multiple-code (MC) and multicode-keying (MK) schemes, have been proposed for asynchronous optical code-division multipleaccess
(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 dissertation, 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.
Besides the MC and MK schemes, a “multilength transmission scheme has also been proposed for O-CDMA system supporting multirate, multimedia services. In this scheme, data rate is dynamically matched to the users’ needs through the assignment of codes of different lengths, such that longer-length codes are used for slower-rate transmission and shorter-length codes are for a faster-rate transmission. Also in this dissertation, the SE of the multilength scheme is derived and compared to the conventional singlelength
OOK O-CDMA scheme. The multilength scheme is found to support larger SE and total number of simultaneous users than the single-length scheme under certain conditions. Our study also shows, for the first time, that the multilength scheme gets more benefit, in terms of both SE and performance, from supporting more number of high-bit-rate (also higher priority) services than low-bit-rate ones.
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