Baseband Line Codes Via Spectral Factorization.

We describe a method of designing baseband line codes with prescribed spectral nulls in the transmitted spectrum. These codes have the important property that the transmitted power is adjustable (with a concomitant change in spectral shape) and can be made arbitrarily close to the innovations power, while keeping the signal separation constant. The method is a significant generalization of the approach introduced by Forney and Calderbank [7] that provides a spectral null at d.c. We apply results from linear prediction theory and the essential design step requires the spectral factorization of a certain trigonometric polynomial. The line code that results can easily be used in conjunction with a large class of trellis coded modulation schemes (just as in [7]). We discuss specific constructions, emphasizing the design of line codes with a double null at d.c.