Various coding schemes and their effects on intersymbol interference in pulse amplitude modulation systems are discussed. First, the relation between imperfections in the baseband-equivalent channel and intersymbol interference is clarified and applied to explain the effect of correlative level coding and Gorog's frequency concept codes in reducing intersymbol interference. Another coding scheme is then introduced: construction of codes in the time domain with intersymbol interference directly in mind. A decimal code of length 4 and an alphanumeric code of length 6 are proposed as practical codes and their properties are discussed. Simulation results are presented to give quantitative comparison of these coding techniques. Curves of the vertical eye-opening vs transmission rate have been produced and we show that codes designed in the time domain achieve better performance than both the frequency concept codes and the conventional codes for a wide class of channel characteristics.
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