Subband Complements and Performance Aspects

This paper examines the performance issues relating to the quadrature amplitude modulation (QAM) and vestigial sideband (VSB) transmultiplexers ynthesized in [l]. First, an analysis of the limitations of the configured systems regarding intersymbol interference and crosstalk suppression arising from the use of practical filters is made. Based on these observations, a new design technique for an FIR low-pass prototype that takes the practical degradations into account is formulated. The procedure involves the unconstrained optimization of an error function. A performance evaluation reveals that for four of the five systems, the new method is superior to a minimax approach in that lower intersymbol interference and crosstalk distortions are achieved with a smaller number of filter taps. For the other transmultiplexer, the advantage of the optimized design over the minimax design is in the added flexibility of taking crosstalk into account thereby diminishing the crosstalk distortion. The five transmultiplexers can be converted into new subband systems. We show how the optimized design approach formulated for the transmultiplexers carries over to the new subband systems.