DSP Compensation for Distortion in RF Filters

There is a growing demand for the high quality TV programs such as High Defini-tion TV (HDTV). The CATV network is often a suitable solution to address thisdemand using a CATV modem delivering high data rate digital signals in a cost ef-fective manner, thereby, utilizing a complex digital modulation scheme is inevitable.Exploiting complex modulation schemes, entails a more sophisticated modulator anddistribution system with much tighter tolerances. However, there are always distor-tions introduced to the modulated signal in the modulator degrading signal quality.In this research, the effect of distortions introduced by the RF band pass filterin the modulator will be considered which cause degradations on the quality of theoutput Quadrature Amplitude Modulated (QAM) signal. Since the RF filter’s am-plitude/group delay distortions are not symmetrical in the frequency domain, oncetranslated into the base band they have a complex effect on the QAM signal. UsingMatlab, the degradation effects of these distortions on the QAM signal such as BitError Rate (BER) is investigated.In order to compensate for the effects of the RF filter distortions, two differentmethods are proposed. In the first method, a complex base band compensation filteris placed after the pulse shaping filter (SRRC). The coefficients of this complex filterare determined using an optimization algorithm developed during this research. Thesecond approach, uses a pre-equalizer in the form of a Feed Forward FIR structureplaced before the pulse shaping filter (SRRC). The coefficients of this pre-equalizerare determined using the equalization algorithm employed in a test receiver, with itstap weights generating the inverse response of the RF filter. The compensation ofRF filter distortions in base band, in turn, improves the QAM signal parameters suchas Modulation Error Ratio (MER). Finally, the MER of the modulated QAM signalbefore and after the base band compensation is compared between the two methods,showing a significant enhancement in the RF modulator performance.