Speaker
Description
This work describes new techniques for low carrier-to-noise ratio (CNR) FM demodulation
and their implementation in GNU Radio. FM radio was developed nearly 90
years ago and is still in common use for commercial broadcasting, emergency services,
and amateur radio. They key advantage of FM over other analog modulations (i.e. AM
and SSB) is the threshold effect where received signals with a CNR above a threshold
level will be demodulated to audio signals with a signal-to-noise ratio (SNR) larger than
the CNR. However, demodulated FM signals below the threshold are subject click noise,
which rapidly degrades audio quality and intelligibility. With modern signal processing
approaches, it is possible to build practical demodulators that extend the FM threshold
to a lower CNR, thus improving communication networks that employ FM.
The presentation begins with a review of the classical theory of noise in FM receivers
based on the work of S.O. Rice[1]. Next, it covers a number of techniques for demodulation
that leverage the natural structure of FM signals in noise, including the treatment
of large noise events as erasures. Once identified, these erasures can be removed using
band-limited signal interpolation with the discrete Papoulis-Gershberg algorithm[2]. An
example result for a 1 KHz audio sine wave modulated with a 5 KHz deviation is shown
in the plot below.
Last, an example implementation of the modern FM demodulator in GNU Radio is
demonstrated and compared with commercial receivers for amateur radio communications.
Keywords: Frequency modulation, Demodulation, Threshold extension, Amateur radio, Signal
processing, Machine learning.
References
[1] S. O. Rice. Noise in FM Receivers. In Murray Rosenblatt, editor, Time Series Analysis,
chapter 25. John Wiley and Sons, New York, 1962.
[2] Paulo Jorge S. G. Ferreira. Interpolation and the Discrete Papoulis-Gerchberg Algorithm. IEEE
Transactions on Signal Processing, 42(10):2596–2606, 1994.
| Talk Length | 30 Minutes |
|---|---|
| Acknowledge | Acknowledge In-Person |