Adaptive data link protocols: design and performance over automated HF skywave links

HF today offers increased reliability on account of equipment improvements and the application of embedded processing, facilitating network frequency and link management. This has lead to a resurgence of interest in its use for data communications where the emphasis is on achieving high integrity without sacrificing throughput. Powerful techniques are needed to achieve these goals in the hostile HF medium with its inherent channel capacity variations and where spectral efficiency considerations preclude the use of highly redundant coding (and overheads) and limit the amount of repetition which can be tolerated. The key concept pervading modern HF systems is adaption, and this needs to be included in the design of data link protocols. In the protocol described throughput is varied in response to changes in channel condition (both interference and propagation) while maintaining consistent high integrity. Note that the protocol is more than just a means of providing an error free data stream; it is an integral part of the management of modern networks covering control, as well as message functions. The authors discuss the design aspects of new selective ARQ data link protocols and shows the results of extensive live trials over an HF test-bed facility.<>