HIGH FREQUENCY RADIO AUTOMATIC LINK …

1 HIGH FREQUENCY RADIO . AUTOMATIC link establishment (ALE). APPLICATION handbook . PREPARED FOR. NATIONAL COMMUNICATIONS SYSTEM. OFFICE OF TECHNOLOGY AND STANDARDS. PREPARED BY. NATIONAL TELECOMMUNICATIONS AND INFORMATION. ADMINISTRATION (NTIA). INSTITUTE FOR TELECOMMUNICATION SCIENCES (ITS). BOULDER, CO. September 1998. HIGH FREQUENCY RADIO . AUTOMATIC link establishment (ALE). APPLICATION handbook . CONTENTS. Chapter No. Page Part I. User's Guide Preface . i 1 Introduction . 1. 2 Requirements for AUTOMATIC and Adaptive HF RADIO 4. 3 ALE .. 20. 4 Third Generation HF Messaging Protocol .. 27. Part II. System Engineer's Guide 5 System Design Criteria 58. Part III. Network Manager's Guide 6 Establishing Network Parameters 108. Annexes Annex 1 Communications Media .. 129. Annex 2 Propagation Prediction . 161. Annex 3 HF Transmitting and Receiving Equipment 209. Annex 4 Examples of HF ALE RADIO Networks .. 217. Annex 5 Linking Protection.

2 223. Annex 6 HF E-mail 229. Annex 7 HF Internet Access .. 243. Annex 8 Terminology 265. Annex 9 References/Bibliography 285. - i - HF RADIO ALE APPLICATION handbook . PREFACE. High FREQUENCY (HF) communications has been an essential part of worldwide information transmission since the advent of RADIO and has advanced nearly in step with information technology. Today, advanced thinkers in the field envision HF RADIO technology as embracing and supporting such technological advances as HF e-mail and compressed multimedia voice and data services. A guide has long been needed that brings together into one concise user's handbook all of the working knowledge of HF AUTOMATIC link establishment (ALE) RADIO technology. Many publications exist that deal with one or more aspects of this technology . sometimes in great detail. This guide, which is general in scope, provides a broad approach. It is a tutorial for explaining the basics of this HF ALE technology and for passing along working knowledge for hands-on operation of HF ALE systems.

3 This guide is the product of many authors who have combined their considerable expertise. This document was edited by the Institute for Telecommunication Sciences (ITS), Boulder, Colorado. Technical contributions were received from Dr. John Goodman of TCI/BR Communications, from Harris Corporation, from Dr. Eric Johnson of New Mexico State University, and from Rockwell International. The editors secured written permission from the holders of the copyright when any copyrighted material was used herein. In the United States, both the federal and military communities have produced standards to define the details necessary for interoperation among radios of different manufacture. Such interoperability is especially important in order (a) to fulfill specialized or cooperative missions, often one branch of the military service must communicate with a second branch, and (b). government agencies have a tradition of purchasing from only one manufacturer for reasons of familiarity.

4 This standardization has been very important in the growth of HF ALE RADIO technologies: without standardization, inter-service communications over long distances might not be possible. But this standardization is burdened by the proliferation of special features which, when incorporated into the standard, result in bulky, unwieldy documents that are awkward to use because of their size. To address this problem of size, the authors of current standards are making a conscious effort to remove tutorial text from their documents. Hence the need for a handbook with tutorial material that is easy for the layman to read and that contains sufficient details for a basic understanding of HF ALE RADIO technology. This HF RADIO ALE. User's Guide addresses that need. This guide begins with a brief introduction to the propagation properties that make HF. RADIO work. That introduction sets the stage for later discussion about the propagation and physical media of HF RADIO .

5 The introductory chapter identifies the ways in which HF ALE. RADIO technology relieves the operator of much of the work associated with maintaining information on propagation conditions, tracking the stations with which communications are possible, and noting the frequencies that can be used for those communications. The introductory chapter also discusses the terms AUTOMATIC and adaptive, which are an integral part of any discussion of HF ALE RADIO technology. The scope and definition of adaptive systems are identified to define the user's requirements that must be met despite the difficult propagation environment. The introduction also presents a brief historical perspective of HF RADIO to date, outlining its impact on the HF communication systems engineer. The introduction also identifies the automation and adaptive concepts described in detail later in the handbook . Finally, the introduction introduces the organization of the handbook .

6 - 1 . PART I. USER'S GUIDE. CHAPTER 1. INTRODUCTION. Purpose RADIO FREQUENCY (rf) transmission between 3 and 30 MHz by ITU convention is called high FREQUENCY (HF) or "shortwave" RADIO . HF is the widely used communication band for long distances. The use of HF RADIO transmissions has been experiencing a renewal of interest and investment because of the realization that satellite and terrestrial communication modes are vulnerable to electronic countermeasures and physical destruction. HF RADIO transmissions, although of much lower overall reliability and limited data bandwidth than other communication bands, allow communication over long distances and can be expected to function throughout a global conflict, and even to recover more rapidly from the effects of nuclear detonations [Bennett et al., 1987]. This level of survival and recovery may not be the case for satellite and terrestrial communication modes. The ionosphere is a key element of HF skywave communications.

7 Transmitted HF RADIO waves hitting the ionosphere are bent or refracted. When they are bent sufficiently, the waves are returned to earth at a distant location. Often at the distant location they are reflected to the sky again, only to be returned to earth yet again, even farther from the transmitter. This HF skywave hopping or skipping ( , transmitter-to-ionosphere-and back to receiver on the ground) can increase communication to very long distances (1 hop: <4000 km, 2 hops: 4000 to 7000 km, 3 hops: 7000. to 12000 km). This propagation phenomenon is such that many amateur RADIO operators ( hams ). at certain times carry out satisfactory communication at distances greater than halfway around the world with 1 W to 2 W of radiated power. In fact, if the medium were noiseless and there were no interference, the required power could even be less [Freeman, 1997]. Unfortunately the HF environment is not noiseless and interference does exist; additionally, other details of the HF propagation environment are also constantly changing.

8 Optimum HF. propagation can vary by location, FREQUENCY , season, time of day; can have cyclic variations; and can be affected by unexpected ionospheric disturbances. This handbook 's Annex 1, The Communications Media, describes in detail the HF propagation medium and how these variations and disturbances will affect it. The amount of bending or refraction of the HF signal is FREQUENCY dependent. During certain periods of the day or night, one FREQUENCY might propagate well but perhaps during other periods of day or night propagation might be poor or non-existent. A. generalization might be: the high-end frequencies are best during the day, low-end frequencies are best during night, but even this generalization is not an accurate descriptor all of the time. - 2 . Scope and definition of adaptive systems How, then, can we use this medium for reliable communication? A few years ago, communication in the HF band relied on the tracking of propagation variations by using computer propagation modeling programs to interpolate variations, and by relying on the skills of the operators to listen to noisy channels for communication links.

9 The characteristics of this communication mode might include: 1. the mode requires very labor-intensive operator duties, 2. the propagation is variable in nature, 3. the mode is vulnerable to jamming, and 4. blackouts are possible in an ionized atmosphere such as a nuclear explosion. To rectify at least some of these problems, technology has now provided an improved adaptive RADIO and other improvement schemes to simplify the lives of HF communicators. A new class of RADIO , under microprocessor control, has a more robust modulation/demodulation scheme, includes error coding, and includes rapidly switching antenna tuners and couplers. The new class of RADIO has also added automation features such as FREQUENCY selection/management, link establishment , link maintenance, and networking protocols to relieve the operator of these duties. Historical perspectives and background To insure that this new technology would not develop completely unchecked with a number of manufacturers all producing incompatible RADIO systems, the development cycle also included the development of HF RADIO standards.

10 The United States has produced Federal Standards and military Standards defining all the protocols that comprise AUTOMATIC link establishment (ALE) RADIO operations [Young et al., 1994]. Corresponding international standards are in the process of being produced. The cooperation in the standards process between government agencies and industry manufacturers has worked so well that the HF communications systems designer can concentrate on the system-level details of the design, using a document agreed upon by industry leaders. Organization of this handbook This handbook describes the requirements and attributes of AUTOMATIC and adaptive HF RADIO systems. Within the chapters, the HF ionospheric channel and communications propagation medium are discussed, as are the details of how AUTOMATIC and adaptive equipment can be used to track the variations in the HF communication medium. AUTOMATIC link establishment is discussed, as are advanced HF modems and networks.