Performance of the FASTNET Narrowband VHF Frequency Hopping Tactical Radio in Challenging Multipath Environments

FASTNET is the latest tactical VHF radio from THALES. It offers multiple waveforms in frequency hopping, including a new (time multiplex based) waveform called GEOMUX. Multipath situations had been identified and synchronization multipath related changes had been designed for the previous radio generation for Switzerland. Realistic propagation channels are required for the development and testing of a robust communication systems. Thus, Channel sounding in the 30-88 MHz band has been performed in Switzerland (in Jura and the Alps) and has confirmed the existence of challenging multipath propagation with delay spread up to 80 μs. This results in a dual problem : Inter Symbol Interference (ISI) up to 3 modulated symbols in 25 kHz channel bandwidth and potential problems in hop synchronization strategy. In this article, a highly sensitive multipath detection channel sounder is first introduced. Then, an analysis of the measured channel impulse responses is provided. A method to extract propagation parameters from previous measurements is also described. A realistic and reproducible channel model has been developed and injected into a waveform test environment. FASTNET signal processing has been successfully tested on this test environment. The results were confirmed with dedicated field trials in Switzerland, both for the legacy Combat Net Radio (CNR) mode and the multi-services GEOMUX modes. It shows significant range improvements due to the equalization and synchronization design.