A chirp modem incorporating interference excision

The authors describe an experimental 75-baud chirp modem designed to operate at high frequency (HF), which is able to reject interference from other HF users by adaptive filtering at the receiver. It also has inherently good performance in multipath propagation and in white noise, and is thus capable of robust digital communication. The system has been tested over a 170 km point-to-point HF skywave link, in which short chirp messages (approximately 30 s duration) were alternated in time with frequency-exchange keyed (FEK) messages. The total duration of the tests was 72 hours, and included day and night operation. In these tests, the chirp system had an overall performance which was superior to that of FEK. FEK occasionally suffered from long periods (e.g. 1 hour) of severe failure, essentially due to interference, whereas the chirp modem achieved more continuous useful operation.<>