Robust slow and medium rate data transmission

The authors describe the evolution of a chirp modem which includes adaptive excision of interference at the detector. The modem also has good performance in multipath propagation, and in noise, and is thus capable of robust HF data transmission. The first version of the modem operated at 75 bps. The principle was then extended to data rates up to 600 bps. Also, to achieve operation in extremely severe interference, a very robust mode of operation at 22 bps has been developed. Initial implementations used VLSI analogue transversal filters, but digital implementation has now been achieved at the lower data rates, using the Texas Instruments TMS320C25 digital signal processor