High data rate radio communications through the ionospheric channel

An innovative system of digital transmission for long range applications in the HF band (3–30 MHz) is presented. The technical challenge is to increase significantly the data transfer rate when compared with standard modems (typically 4.8 kbits/s in a 3 kHz bandwidth). For this purpose, an original array of collocated receiving antennas is developed as support for the implementation of a blind spatio temporal equalization. Consequently, the effective bandwidth is extended up to 12 kHz. An experimental 780 km range radio link is tested with a data transfer rate attaining 40 kbits/s and a BER (bit error rate) remaining moderate. Still images are transmitted with a specific source-channel coding and the good quality of service is confirmed by the fidelity of the restored images.