Improved FM demodulation in a fading environment

Analog frequency modulation remains an important format for wireless communications, such as in the AMPS cellular telephone standard. Frequency modulation codes the signal as the instantaneous frequency of the transmitted waveform, so FM demodulation corresponds to the problem of instantaneous frequency (IF) estimation. Time-frequency methods have been shown to provide better IF estimates at low SNR, which makes them attractive for FM demodulation in low-SNR scenarios. However, fading is a common problem in mobile wireless communications. We demonstrate here that a high-performance IF estimation method based on an adaptive short-time Fourier transform works well under fading conditions and offers much better performance than traditional methods such as a phase lock loop (PLL). While more expensive than PLLs, basic versions of this approach should be implementable in real time on a single DSP microprocessor.