A question has arisen concerning the similarity of a paper published by M.I. Celebiler and G.M. Coupe (ibid., vol. COM- 26, pp. 257-267, Feb. 1978.) and the above-named one published by the authors (ibid., vol. COM-29, pp. 285-297, Mar. 1981). The first paper considers the problems of cochannel and intersymbol interferences in CPSK systems. In 1981 paper the results of Celebiler and Coupe were extended to the case of quadrature modulated systems. Because the 1981 paper is an extension of Celebiler and Coupe, it naturally makes heavy use of their results. It was an unfortunate oversight on our part that the appropriate amount of credit was not given to Celebiler and Coupe. We did reference the workbut because the method used was very similar, it would have been appropriate to cite it at several points in our paper and perhaps draw more from their results, rather than repeating their procedure. We would like to apologize to Celebiler and Coupe for this oversight and acknowledge the usefulness of their results in interference analyses.

A method is presented for determining the performance of a quadrature-carrier modulation system in terms of probability of error in the presence of additive white Gaussian noise, intersymbol interference, and cochannel interference. This method has been applied to determine the error rate of several quadrature-carrier modulation systems using Butterworth receiving filters of different orders. It has been foand that among the different quadrature-carrier modulation systems studied, sinusoidal frequency-shift keying seems to exhibit the best overall performance in a cochannel and intersymbol interference environment. These results are shown through a number of performance curves that provide useful data for the systems designer.

The performance of a minimum-shift-keying (MSK) system has been studied in which the carrier and clock references are noisy and derived from the received waveform by means of narrowband tracking loops. It has been found that the probability of error is greatly affected by the characteristics of the synchronization structures and that the system is more sensitive to demodulation phase errors than the antipodal PSK system.