How to Combat the Effect of Bursty Nonlinear Distortions in Multichannel AM-VSB/256-QAM Video Lightwave Transmission Systems
Robust transmission of 64/256-QAM channels over current hybrid fiber/coax cable-TV networks is achieved with the use of a forward-error-correction (FEC) scheme in the QAM modems. For some digital video broadcast (DVB) applications, the FEC scheme consists of a Reed-Solomon (R-S) T=8 (204,188) code, convolutional interleaver, and a randomizer.1 Interleaving the RS symbols before transmission and deinterleaving after reception evenly disperses the burst errors in time, and thus they can be corrected by the FEC in the QAM receiver as if they were randomly distributed.2 A convolutional interleaver is typically characterized by the number of its shift registers, which is also called the "depth" I (symbols), and by the symbol delay increment J per register.3 Due to memory cost and end-to-end delay for the transmitted symbols, it is advantageous to limit the interleaver (I, J) values in certain applications.
Recently, there have been many studies on the performance of 64/256-QAM channels in the presence of nonlinear distortions in hybrid AM/QAM video lightwave transmission systems. Most of these studies have analyzed the effect of clipping distortion on the bit-error-rate (BER) of the QAM channel using CW carriers from a multitone generator.4,5 However, the effect of bursty Composite-Second-Order (CSO) and Composite-Triple-Beat (CTB) distortions on the BER degradation of the QAM channel using modulated video carriers has mostly been ignored. Without an interleaver, the R-S code in the QAM modem cannot correct the generated burst errors.
In this study, the effect of the interleaver depth in the QAM modem to combat burst-errors generated by time-varying CSO distortions in hybrid multichannel AM-VSB/256-QAM video lightwave transmission system has been analyzed. The bursty behaviors of the CSO distortions, which are produced by the modulated video signals at the laser transmitter, are responsible for the bit-error-rate (BER) degradation of the 256-QAM channel. Using an I=204, J=1 interleaver along with QAM channel-frequency offset with respect to the dominant CSO distortions, 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 and QAM frequency offset case.