Link maintenance and channel evaluation techniques for HF radiocommunication links

Methods for estimating the prevailing channel characteristics for use in an automatic modem waveform selection management system for high frequency (HF) radio circuits are addressed in this thesis. Current techniques for channel evaluation and management are reviewed in detail and their limitations noted. Various channel probe signals are considered including Barker sequences, long sequences found by means of neural network methods, modified Legendre sequences and a sequence which forms part of the STANAG-4285 waveform. Experiments have been undertaken in which the new methods of channel probing are evaluated and compared with those currently in use. A prototype system that automatically changes the modem data rate based on measured frame error rate is demonstrated. This system can significantly improve data throughputs compared with fixed data rate systems. The use of in-built or add-on channel probing methods that do not require breaks in the data transmission are promoted over dedicated probing methods which require a break in the normal data communication signal. The concept of reducing the relative amplitude of an add-on probe (and hence improve the SNIR) by using a signal cancelling technique at the receiver is introduced. Finally, recommendations for additional work necessary to realise an automated modem waveform management system to complement existing frequency management systems are made.