Spread spectrum fiber LANs.

Abstract

In this thesis, a hybrid scheme is proposed for the purpose of suppressing the effect of external modulation and/or laser nonlinearities in Subcarrier Multiplexing (SCM) fiber optic communications systems. Hybrid CDMA/FDMA combines two schemes in such a way that the resulting hybrid network is robust against interference and is much more spectrally efficient than a CDMA system. Several possible architectures for the hybrid CDMA/FDMA subcarrier fiber optic Local Area Network (LAN) are introduced. These networks utilize CDMA and SCM, an asynchronous multiple access scheme with no waiting time. Direct Sequence Spread Spectrum Multiple Access (DS/SSMA), the most common form of CDMA in which each user is assigned a particular code sequence which modulates the carrier along with digital data, is employed. It is shown that by using the code sequence sets for which the shift-and-add property holds, the Intermodulation Products (IMPs) and harmonics have a similar interference-like effect as the non-matching sequences. Owing to the fact that shift-and-add property holds for conventional spreading sequences, i.e., Gold, Kasami and maximal-length sequences, the suppression of nonlinearity distortion is evaluated. An average error probability performance evalution of the selected configuration for a transceiver pair is presented. In analysis of the system, we assume the interference term arising from other users is gaussian distributed. The results are compared to that obtained from exact evaluation of interference distribution using the Gauss Quadrature Rule integration (GQR) method. We compare the performance of this scheme for two different code sequence sets (N = 127 Gold and N = 255 Kasami codes) and determine that there is a significant advantage in deploying the N = 255 Kasami codes. We also present some preliminary experimental results on the proposed LAN implementation as well as the transmission performance. The results show great promise.