Practical performance and system capacity of lightwave AM SCM video systems

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Virginia Tech

Subcarrier multiplexing (SCM) on lightwave systems is an important technique for the near term implementation of broadband services by both telecommunication and cable TV companies. With advance in opto-electronics technology in the mid-1980s, lightwave SCM systems can now be practically implemented to provide a carrier platform for both digital and analog signals and (for short distance applications) are presently more cost effective than time division multiplexed systems.

AM SCM systems are particularly attractive for multichannel video signal transmission due to their compatibility with the National Television Systems Committee Amplitude Modulated Vestigial Sideband (NTSC AM-VSB) TV format. However, AM SCM systems are generally recognized to have a limited system capacity due to large carrier-to-noise ratio (CNR) and low nonlinear distortion requirements. This research dissertation contains a comprehensive study of the system capacity of such systems based on theoretical analysis, experiment, and simulation.

The practical performance of lightwave AM SCM systems are limited by the laser diode threshold-nonlinearity and laser relative intensity noise, the photodiode shot noise, and the receiver thermal noise. The practical system performance is evaluated and compared with that of the theoretical performance limit. The analysis indicates that AM SCM systems have sufficient system capacity for typical CATV loop distribution and supertrunking systems. It is shown that previous reported limits are generally overly conservative. A sensitivity analysis identifies the critical performance limiting parameters and provides system designers with achievable system performance as device characteristics improve.

For 1550 nm AM SCM systems employing erbium-doped fiber amplifiers (EDFA) operating on the 1300 nm single-mode-fiber of telephone networks, there are additional sources of nonlinear distortion and noise to be considered. The nonlinear distortion produced by laser-chirp and fiber-induced-dispersion and the noise produced by the EDFA can significantly degrade the system performance. The analysis shows that the maximum link distance is limited by the composite-second-order (CSO) distortion, and the maximum number of subscribers is limited by the EDFA excess fluctuation noise.