Standardization of an intermediate duration HF channel variation model

Abstract

For many years, the standard model of an ionospheric channel used for analysis and performance evaluation of HF communications techniques has been the model of Watterson, as embodied in the ITU-R Recommendation Rec. F.1487. However, for the modern adaptive techniques used for HF communications, including automatic link establishment and data link protocols employing data rate adaptation and/or type II hybrid-ARQ techniques, this model is importantly incomplete. While it simulates quite adequately the signal perturbations (Doppler and multipath spread) observable over periods of up to a few seconds, this model fails to represent the variations in channel quality (specifically, signal to noise ratio - SNR) that occur over time periods of a few seconds to tens of minutes, and which can have a crucial impact on the performance of systems using such techniques. In recent work, Harris Corporation has recorded and analyzed extensive time series of SNR measurements on a sample of ionospheric channels, both long-haul and Near Vertically Incident Skywave (NVIS); developed a model framework capable of modelling the longer-term channel quality variation observed in these time series; incorporated the model framework into a Watterson-model channel simulator; and used this enhanced channel simulator to measure and analyze the impact of this longer-term channel variation on the performance of two widely-used HF data link protocols. This paper begins with an overview of the developed model and provides a synopsis of the work completed so far to validate the model. The paper continues with a brief discussion of the implementation of this model in a DSP or PC based real time HF channel simulator. Finally the paper proposes how best to incorporate this additional model in to an international standard such as ITU ITU-R Recommendation Rec. F.1487.