When amplitude-modulated vestigial sideband (AM-VSB) video channels are transmitted through a nonlinear device such as a laser transmitter, second-order distortions (f1 ± f2, where f1 and f2 are two AM channel frequencies), and third-order distortions (f1 ± f2 ± f3) are generated. Due to the temporal properties of modulated AM-VSB signals, the second- and third-order distortions are not constant, but instead are time-varying distortions. The effect of the time-varying distortions is to generate burst errors in a hybrid subcarrier-multiplexed (SCM) AM/QAM lightwave system. As expected, the burst errors degrade the bit-error-rate (BER) performance of the transmitted QAM channels in such lightwave links. Often, cable operators and/or equipment vendors are using continuous-wave (CW) tones from a multitone generator instead of the actual modulated video carriers to characterize and set the operating point of a given video transport system. The result is that the impact of the bursty nonlinear distortions and clipping noise is often underestimated. The following article first explains the bursty behavior of the nonlinear distortions in a real-world cable TV system, and then proposes a novel solution to overcome this potential problem using an interleaver in a set-top box with a QAM channel frequency offset method.
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The effect of a convolutional interleaver with variable depth in a QAM
modem to combat burst-errors generated by time-varying nonlinear distortions in
multichannel AM-VSB/256-QAM video lightwave transmission systems has been
analyzed. By offsetting the QAM channel-center frequency relative to the
dominant CSO distortions with an I=204, J=1 interleaver, the 256-QAM coded BER
was reduced by more than 500 times, even in the presence of large CSO distortion
levels (> 60 dBc), compared with the no-interleaver case. These results
should help in the selection of the proper interleaver depth as well as the QAM
channel frequency plan, depending on the relative magnitudes of the CSO/CTB
distortions in the cable TV network.