A received continuous phase modulation (CPM) signal (which is formed with a set of pulse shaping functions) is approximated as a phase shift keying (PSK) modulated signal (which is formed with just the dominant pulse shaping function having the largest energy). Channel estimation and data detection are performed in accordance with the CPM-to-PSK approximation. A signal power estimate and a noise power estimate are obtained for the received CPM signal and have errors due to the CPM-to-PSK approximation. The difference .DELTA. between the energy of the dominant pulse shaping function and the energy of the remaining pulse shaping functions is determined. An approximation error is estimated based on the signal power estimate and the difference .DELTA.. A C/I estimate for the received CPM signal is computed based on the signal power estimate, the noise power estimate, and the approximation error estimate.