HF data communication for extremely low SNR and high interference level

By using low data rates and both intelligent signal conception and digital signal processing (DSP) for estimation and detection, it is possible to realize new applications in the field of digital short wave communications (HF). For links with unknown interference level or low transmitter power, unmatched antennas and high path attenuation a special transmission principle was designed and realized. The authors see applications within emergency systems, for transmission of messages ensuring a reliable link establishment (e.g. frequency hopping) or for network control functions under severe interference conditions (intended or unintended). This low rate data modem is working in the baseband (300-3kHz) and can be operated with any HF SSB-transceiver. The use of a TMS 320 DSP architecture results in full real-time numerical signal processing and allows the implementation of detection algorithms based on FFT. The HF channel is investigated not only for detection of the desired signal but also for dealing with the interference situation, which is involved in the decoding process of the received data. By modifying parameters in the DSP software it is possible to obtain an adaptive mode where the signalling format tracks the quality of the channel. >