THEORETICAL AND EXPERIMENTAL-ANALYSIS OF CLIPPING-INDUCED IMPULSIVE NOISE IN AM-VSB SUBCARRIER MULTIPLEXED LIGHTWAVE SYSTEMS

A new theoretical and experimental analysis of clipping-induced impuls ive noise in directly modulated AM-VSB/digital hybrid lightwave system s is presented. The theoretical model is based on asymptotic clipping and shot noise theories, and the probability density of combined Gauss ian and impulsive noise is obtained by performing a numerical inverse Fourier transform on the closed-form characteristic function. Similar analyzes in previously published work [3], [4] have resulted in closed -form expressions for the BER in hybrid AM-VSB/QAM systems, but these models, which do not use the numerical inverse Fourier transform, have either required the use of an experimentally measured clipping rate u sing a spectrum analyzer [3] or have had limited BER prediction accura cy [4]. Application of our theory to experimental results for the BER in a hybrid AM-VSB/64 QAM system demonstrates that the performance of the model is better than the models of previously published works. The oretical analysis of a similar hybrid system employing the N-VSB HDTV digital modulation format rather than QAM indicates that an improvemen t in BER is achieved if 8-VSB at double the symbol rate is used instea d of 64 QAM. We also present the first experimental characterization o f the amplitude and time distribution of 1) low-frequency bandpass and 2) down-converted impulsive noise and we show that the time interval between clipping events has a high probability of being 167 mu s, 1.5 mu s, or 4 ms in an NTSC system with unmodulated carriers. These resul ts have potential impact on forward error correcting codes used to ame liorate the degrading effects of clipping.