Abstract-The effect of the companding process on QAM signals

has been under investigation for the past several years. The

compander, included in the PCM telephone network to improve

voice performance, has an unusual affect on digital QAM data

signals which are transmitted over the same channel. The quantization

noise, generated by the companding process which is multiplicative

(and asymmetric), degrades the detectability performance

of the outermost points of the QAM constellation more

than that of the inner points.

The combined effect of the companding noise and the inherent

white gaussian noise of the system, leads us to a re-examination of

signal constellation design.

In this paper we investigate the detectability performance of a

number of candidates for signal constellations including, a typical

rectangular QAM constellation, the same constellation with the

addition of a smear-desmear operation, and two new improved

QAM constellation designs with two-dimensional warpi