SUMMARY

HF Radio transmission is subject to a wide range of variations. Much analysis has been devoted to Rayleigh Fading and other variations in the signal over periods of a few seconds. Modem and waveform design have been focused on addressing these short-term variations. There are also longer-term variations, which were initially modelled in “The Walnut Street Model of Ionospheric HF Radio Propagation” [1]. This modelling was refined by a group at Harris Corporation, which broke down the variation into two basic elements: Intermediate Term Variation (ITV) and Long Term Variation [2]. Isode, in collaboration with Rockwell Collins, made measurements of ITV in 2014, which fitted well with the ITV model and were reported on at the HF Industries Association in September 2014 [3] and in a subsequent white paper [4].

The work cited shows that ITV has a significant effect on the error patterns seen above the modem level. Errors at the modem level that are not detected and passed upwards will need to be addressed at the link level or application level. Choice of HF parameters and, in particular, transmission speed is key to optimizing throughput or latency. The STANAG 5066 link layer is widely used with HF, and performance has usually been optimized for throughput. Transmissions are made for periods of up to two minutes, and speed may be changed for each transmission.

The classic approach to the choice of speed has been to look at the frame error rate of the last transmission and choose a speed based on this. Anecdotal evidence suggests that this approach does not work very well, as there is a tendency to oscillate between speeds without stable convergence on an optimal speed. The Isode 2014 measurements suggest that ITV leads to error pattern variation such that two minutes is insufficient time to determine an optimal transmission rate for the next period, based on error pattern, and suggest that determining speed for the next two minutes of transmission based on performance of the previous two minutes is sub-optimal.

When considering ITV, transmission quality may be thought of as a wave of varying quality with periodic variation in the range of 10 seconds to a few minutes. The Isode 2014 measurements suggest that SNR values are a good indication of transmission quality and can be used to examine the “quality wave”. The paper will look at two application strategies to operate in light of the characteristics of this wave. The first strategy is to pick a single setting that will be used for several minutes to optimize throughput, which might be thought of as “cutting through the wave”. The second approach considered is to change parameters much more frequently to deal with varying conditions and to analyse how predictable variations are short-term and if it would be viable to adapt quickly to conditions and “ride the wave”. The goal of this analysis is to help inform a decision as to whether this approach would gain sufficient performance to justify the additional overhead and complexity of rapid parameter change.

SNR data from multiple measurements on three links is analysed to look at both of these approaches. Many modern applications, such as real-time chat, have relatively low volumes of data and a desire to optimize latency, and these need to be used in conjunction with “bulk” applications, where it is important to optimize throughput. The paper concludes by examining how the results can be used in support of this mix of applications.