Design and modelling of electronic processing circuits for optical code division multiple access communication networks

摘要

Code Division Multiple Access (CDMA) has been proposed for optical fibre networks to a achieve high-speed connectivity, asynchronous operation and network control simplifications. Traditionally, all-optical devices have been proposed to encode, decode and process CDMA signals because the bandwidth in the optical medium is higher than electronic processing techniques. Today's advances in integrated circuit technologies coupled with bandwidth efficient circuit topologies, may provide robust alternatives for CDMA over fibre applications. This thesis overs two areas of work. First, a new circuit for encoding and decoding incoherent CDMA signals in the electrical domain is proposed. This structure makes use of the high bandwidth of distributed amplifier-like topologies to achieve very wideband operation. As proof of concept, a distributed transversal filter was designed with a commercial available GaAs MMIC process; modelling results show feasibility of CDMA encoding/decoding at 40 GChip/s employing transistors with cut-off frequencies of 60 GHz. Limitations and potential practical applications of these design ideas are discussed. The second part of the thesis focuses on the impact of the time skewing due to optical-fibre Group Velocity Difference in Wavelength-Hopping Time-Spreading (WHTS) Optical CDMA and the compensation of this impairment through electronic techniques. A new network model was created to assess the impact of such impairment in these systems. Conclusions about the time skewing impact on the auto and cross-correlation properties of the system are extracted. This model was employed to assess the use of an electronic distributed transversal filters as time skewing compensator in Optical CDMA networks. Finally, an experimental setup of a WHTS Optical CDMA network was built to analyze the effect of time skewing and to assess the practical feasibility of its compensation with a distributed transversal filter. The Fiber Bragg Grating based system comprises 2.5 GBit/s transmission (20 GChip/s) with five wavelengths. Conclusions about the practical limitations are derived and presented.