System and method of low power demodulation of continuous phase modulated waveforms
View Patent ↗A radio system demodulates a continuous phase modulation waveform with a pulse count demodulator. The use of a pulse count demodulator reduces the receiver complexity and reduces power consumption, which makes such a system well suited for hand held or mobile units. The pulse count demodulator delays the CPM signal by a time increment t 0 , and subtracts the delayed signal from the original signal creating a difference signal. The difference signal is rectified and low pass filtered resulting in an average signal representing the phase change. In non-square wave CPM signals, the signal is first clipped removing the amplitude information.
1. A hand held radio system with a limited power supply for receiving information from a continuous phase modulation (CPM) waveform signal comprising:
a delay circuit delaying the CPM signal by a time increment t 0 ;
a subtractor creating a difference from the CPM signal and the delayed CPM signal;
a half-wave rectifier for rectifying the difference; and
a low pass filter integrating the rectified difference to obtain an average signal,
wherein the amplitude of the average signal represents a phase change.
2. The hand held radio system according to claim 1 , further comprising a pulse forming circuit for forming square wave signals, wherein said pulse forming circuit is upstream of the delay circuit.
3. The hand held radio system according to claim 1 , further comprising a calibration circuit for adjusting the impulse response of the low pass filter based on a training sequence.
4. The hand held radio system according to claim 1 , further comprising a calibration circuit for adjusting the impulse response of the low pass filter based on the expected amplitude of the average signal.
5. The hand held radio system according to claim 1 , wherein the radio system is a coherent FSK system.
6. The hand held radio system according to claim 1 , further comprising a calibration circuit for adjusting the impulse response of the low pass filter with a phase locked loop.
7. A method of low power demodulation of a constant envelope waveform signal comprising the steps of:
providing a constant envelop waveform signal containing information symbols;
clipping the constant envelope waveform to create a square wave signal;
delaying the square wave signal in a delay circuit to obtain a time delayed square signal;
subtracting the time delayed square signal from the square wave signal to create a difference signal;
rectifying the difference signal to create a half wave rectified signal;
integrating the rectified signal to create an average signal; and,
extracting the information symbols from the constant envelope waveform based on the amplitude of the average signal.
8. The method of claim 7 , where the step of integrating the rectified signal further comprises the step of calibrating the average signal.
9. The method of claim 8 , wherein the step of calibrating the average signal comprises the step of selecting a calibration value based on an average signal value from a known training signal.
10. The method of claim 7 , wherein the constant envelope waveform is a Continuous Phase Modulation waveform.
11. The method of claim 7 , wherein the constant envelope waveform has a SNR greater than 0 dB.
12. The method of claim 8 wherein the step of calibrating is accomplished with a phase locked loop.