Improved techniques for monitoring the HF spectrum

Abstract

A critical review of contemporary papers on modulation recognition, signal separation, and Single Station Location (SSL) is described in the context of High-Frequency (HF) radio-communications. High-frequency communications is undergoing resurgence despite advances in long-range satellite communication systems. Defense agencies are using the HF spectrum for backup communications as well as for spectrum surveillance applications. Spectrum management organizations are monitoring the HF spectrum to control and enforce licensing. This type of activity usually requires a system that is able to determine the location of a source of transmissions, separate valid signals from interferers and noise, and characterize signals-of-interest (SOI). The immediate aim is to show that commercial-of-the-shelf (COTS) equipment can be used to locate HF transmission sources, enhance SOIs and reject interference, and recognize signal types. The described work on single-station-location (SSL), signal separation, and modulation recognition is contributing to these goals. This paper describes the overall objectives and some of the disadvantages and benefits of various schemes for single-station-location (SSL), signal separation, and modulation recognition. It also proposes new approaches that may relieve shortcomings of existing methods -including selection of benchmarks or modulations for various transmission scenarios and propagation modes, and use of multiple digital receivers or compression techniques to improve modulation recognition, signal separation, and location of HF emitters.

Modulation recognition for HF signals

“…This paper continues the work presented earlier by Giesbrecht, Clarke, and Abbott. 2,3 In the previous work we discussed the preliminary application of coherence 4 and entropy 5 to real HF signals. This work discusses in more detail the fundamentals of coherence and entropy and how they might be used in modulation recognition.…”

Parameters for automatic modulation recognition of HF signals

“…Our previous work [6,7,8,9,10] discusses coherence and entropic distance in detail, however we briefly review these parameters here. We also re-introduce a parameter, first proposed by Aisbett [1], which estimates the true power of a signal contaminated by noise.…”

Modulation recognition for real HF signals

“…However, this shall be deferred to later in the discussion for reasons that will soon become clear. The only review now necessary is that of previous work [3][4][5][6] on coherence and Benedetto's entropy 7 measure. Coherence is unlikely to yield good separation between signal types, but Benedetto's entropy parameter is a feasible indicator of signal type.…”

“…This paper continues the work presented earlier by Giesbrecht, Clarke, and Abbott. [3][4][5][6] In the context of the previous work, it briefly reviews the performance of the coherence function and discusses the outcome of applying Benedetto's entropy 7 measure. This paper also discusses the non-Gaussian nature of electro-magnetic noise in the HF spectrum, and describes a way forward in the development of a robust modulation recognition system.…”