| Summary: | 博士 === 國立中興大學 === 電機工程學系所 === 95 === Two-dimensional (2-D) wavelength-hopping time-spreading (or so-called wavelength-time) coding schemes have been studied recently for supporting greater numbers of subscribers and simultaneous users than conventional one-dimensional approaches in optical code-division multiple-access (O-CDMA) systems. In this dissertation, several new families of wavelength-time codes are constructed and analyzed.
First, a new family of 2-D wavelength-time codes, so-called quadratic-congruence carrier-hopping prime codes (QC-CHPC) with expanded code cardinality and the maximum cross-correlation function of two (i.e., λc=2) is constructed and analyzed. One application of the large code cardinality of our λc=2 codes is multicode-keying wavelength-time O-CDMA, in which each user is allocated with multiple code matrices, instead of just one code matrix in the conventional on-off keying (OOK) O-CDMA. System throughput is increased because a lower baud rate O-CDMA system can be used to support higher bit-rate transmission since each code matrix is used to represent a "symbol" of several data bits. User code confidentiality is improved because of symbol transmission. The theoretical results show that there is a trade-off between the performance and the number of code matrices per user.
It is expected that future communication systems support a variety of services (e.g., data, voice, and video). Users with different bit-rate and quality-of service (QoS) requirements will be accommodated simultaneously. To support multimedia services with different discrete bit-rate requirements, "multiple-length" QC-CHPCs are constructed algebraically in the dissertation. In contrary to conventional single-length codes, our analysis shows that the performance of these multiple-length codes improves as the code length decreases, thus supporting services prioritization in O-CDMA. Moreover, the relationship of the normalized spectral efficiency (NSE) and code lengths of the multiple-length QC-CHPCs is studied. Our results show that the NSE improves as the number of simultaneous users with short code matrices increases, which, however, decreases the total number of simultaneous users in the systems. Thus, the choice of which code-length distribution to use depends on whether system efficiency or total number of the simultaneous users is important.
Furthermore, QoS is another important issue to be considered in multimedia applications. One way to adjust QoS in O-CDMA is through weight variations in code matrices, besides changing length. In the dissertation, the constructions of the variable-weight, multiple-length CHPC, Extended CHPC (ECHPC), and mutli-wavelength optical orthogonal code (MWOOC) are proposed and analyzed. Our results show that the performance of these variable- weight, multilength codes could be controlled by matrix length, weight, but bit rate is only controlled by matrix length. Our study also shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Moreover, we also show that code weight is the dominating factor in controlling code performance and, thus, is important in guaranteeing the QoS of those media (e.g., video) that require high bit-rate and real support.
Finally, a new family of 2-D wavelength-time codes, so-called bipolar-bipolar codes, is proposed for supporting substantially more subscribers and simultaneous users than conventional 1-D O-CDMA codes in the dissertation. The new codes use bipolar codes for both wavelength hopping and time spreading. As a result, the performance of the bipolar-bipolar coding scheme is better than the bipolar-unipolar coding scheme which uses bipolar codes for wavelength hopping, but unipolar codes for time spreading, while the code cardinality is as good as the latter.
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