Index assignment in redundancy-coded data communication systems

Abstract

A modem which represents the transmitted primary data stream using signal points selected via a trellis code is provided with the further capability of transmitting secondary data. In particular, a standard constellation of signal points is expanded to include a number of signal points which is greater than that needed to transmit the primary data. Particular primary data word values are representable by either of a predetermined pair of signal points associated with that value--one signal point of the pair being a standard signal point and the other being an expanded signal point. The specific one of the two signal points of the pair that is transmitted at any given time is chosen as a function of whether the value of the next bit in the secondary data stream waiting to be transmitted is a '0' or a '1'. The location of each expanded signal point in the constellation is selected such that its distance from the nearest signal point in the trellis code subset of its associated standard signal point is substantially at least as great as the minimum distance between any two signal points in that subset. Index values are assigned to the signal points of each subset in such a way that those index values change monotonically with increasing power of the signal points. Illustratively, in particular, the index values increase monotonically with increasing power of the signal points. Thus the signal point having the lowest power within a subset will have the lowest index value; the signal point having the second lowest power within a subset will have the second lowest index value; and so forth.