PRACTICAL CODING FOR QAM TRANSMISSION OF HDTV

This paper describes a practical approach to digital transmission of compressed HDTV. We demonstrate how modulation schemes based on QPSK modulation can be directly incorporated into QAM-based modulation systems. We shall argue that this leads directly to an easily implementable structure that is both efficient in bandwidth and data reliability. The use of a concatenated code is known to provide an effective and practical approach to achieving low BER, high data rate, and modest implementation complexity. It is our contention that the correct solution to the concatenated coding problem for HDTV transmission is to simply extend the modulation codes developed for QPSK - to - QAM modulation. In nonconcatenated situations, a trellis code based on a binary code at rate 2/3 is usually best; this fact follows from the study of the asymptotic coding gain of a trellis code. However, this is not the case for higher error rates at the output of the trellis decoder (e.g., when a symbol error correcting decoder follows as in a concatenated code). The reason for this follows from an analysis of the effect of the number of ''nearest neighbors'' on the error rate. A four-way partition of QAM is a natural extension of QPSK modulation; it is a simple matter to incorporate any good QPSK code into a trellis coding scheme for QAM modulation. We propose a concatenated coding scheme based on QPSK trellis codes and symbol error correcting coding. A specific example is presented which shows the advantages of this approach.