HF radio offers a unique combination of cost effectiveness and versatility for long-haul communications. In recent years, computer technology and high-speed digital signal processing have enhanced the performance and reliability of HF communications systems, resulting in reduced operator involve-ment in establishing HF communications circuits. At the same time, new technology has dramatically reduced the size and weight of HF radio equipment. Diverse capabilities, which formerly required separate pieces of equipment, are now combined and embedded into the radio transceiver itself.
Examples of HF Communications Systems Harris Corporation, RF Communications Division, designs, manufactures, and installs turnkey radio communications
systems for worldwide government, military, and commercial markets. Here are some examples of how these HF systems come together in a modern communications network to meet complex communications needs.

Secure Data System

Figure 8-1 shows a typical secure HF data transmission system, which can be used whenever it is necessary to transfer data securely between two points. The serial modem, which uses FEC coding, also provides real-time channel equalization and data interleaving for protection against fading, and automatic excision filters remove interference from up to four sources. The transmit modem data rate adjusts to the terminal data rate and is selected
on the basis of an LQA (estimate of channel quality). The amount of coding (redundancy) used in the FEC varies as a function of the selected modem data rate. Thus, if poor channel quality is predicted, a relatively low data rate and a more powerful FEC code will be designated.

Country-Wide HF Data Communications System

A country-wide HF data communications system, which provides economical, long-range communications, is shown in Figure 8-2. The HF data communications system links a fixed central communications center and 12 subordinate stations located throughout the country. The system incorporates an ALE capability that offers fully automatic operation with unattended processing of incoming messages. Each subordinate station has additional HF and VHF radios that provide voice and data communications to mobile stations in its vicinity. In the data communications mode, an ARQ message protocol is used for error detection and correction. The central
station is a fixed installation with separate transmit and receive control sites. Intersite communications and control are via microwave or a landline link.

HF Telephone System

An HF radio link can extend the reach of a telephone network, as shown in Figure 8-3. The system operates much like the cord-less telephone widely used in homes today, but covers hundreds of thousands of miles using HF radio. The HF telephone system enables users to place calls to and from mobile radio transceivers into the commercial switched telephone network or private subscriber telephone lines. Calls from the field can be placed over HF, VHF, or UHF to anywhere in the world through the base station telephone switch or exchange. To initiate a call, the user enters a telephone number just as if the Remote Access Unit (RAU ) were a telephone set connected directly to the base station telephone exchange. At this point, the number dialed is transmitted through the RAU to the Telephone Interface Unit (TIU). As the TIU dials the digits and the telephone rings, call progress tones are heard by the mobile operator. In order to contact anyone in the field, a telephone user dials a telephone number (or the extension) to
which the TIU is connected — from anywhere in the world. The call is automatically answered by the TIU and the user is connected directly with the field radio.

Ground-to-Air Communications System

Figure 8-4 is a block diagram of a ground-to-air communica-tions system with a split-site ground station capable of simulta-neous data, facsimile, or voice communications with up to four airborne platforms. This system dedicates one ground-based receiver-transmitter pair and an associated controller to ALE. Once a ground-air link is established, the station controller hands off the traffic channel to another receiver-transmitter pair. This system also incorporates the cordless telephone capability
described above. Thus, an airborne platform has access to the telephone network. Each aircraft incorporates an HF transceiver with built-in ALE controller and data modem, plus a 400-watt solid-state power amplifier and antenna coupler. Intersite communications between receiver and transmitter sites are via radio or landline.

HF Digital Video Imaging Communications System

This system captures, digitizes, and transmits video images in near real time from a mobile unit to a base station via an HF data link. Figure 8-5 shows a scenario in which an unattended still-video camera sends images to an imaging terminal via a fiber-optic link. The terminal captures and digitizes the image and sends the data to a modem in the transceiver, which relays the data to the base. Communications may be via a two-way link
that uses an ARQ protocol to obtain error-free transmission of the image, or a one-way link in which FEC coding reduces the probability of error in the received message.

Broadband Transmitter System

The biggest HF communication problem that must be solved on board large naval ships is how to run multiple HF transmit and receive circuits simultaneously without interfering with each other, and that all circuits operate through a very few number of antennas (due to size and space limitations). Harris has developed the optimum solution to these problems with its ultra-linear broadband transmitting system. Harris’ system also
supports rapid frequency changes through use of ALE and frequency hopping. Figure 8-6 is a simplified block diagram of a solid-state transmitter system capable of delivering up to 4 kW in the 2- to 30-MHz frequency range into an antenna. Signals from up to eight independent audio sources modulate HF exciters. The outputs of the exciter route through a signal distribution unit into a bank of linear solid-state power amplifiers, each capable of delivering 500 watts. The signal distribution unit allows various combinations of exciter signals to be applied to the power ampli-
fiers, so that, for example, the signal from a single exciter may be
applied to all eight amplifiers. The amplifier outputs are added in a passive power combiner and supplied to an antenna.

HF Tactical Communications Network

Figure 8-7 shows a portion of a tactical communications network that provides coverage over distances ranging from less than 50 miles to more than 1,500 miles. In a network of this type, the individual elements include frequency hopping, encryption, and ALE capabilities. Network requirements dictate that links are provided between the fixed headquarters site and fixed installations for quasi-permanent military regions and zones. Provisions are made for communications between headquarters and task forces at fixed, non-permanent installations. Lower echelon communications have a combination of fixed, mobile, and man-portable equipment. Frequency management of the network is a headquarters responsibility.

HF E-Mail and Inter-Networking

Electronic mail and other inter-networking technologies are becoming increasingly important for interoffice communications. However, many users find that communications between remote stations are difficult and/or expensive, due to costly telephone or satellite charges. Harris’ HF radios and systems are an excellent alternative for providing these services to distant users or stations. Typical applications include:

• Naval ship-to-shore and ship-to-ship communications.
• Embassy Ministry of Foreign Affairs communications.
• Oil/Gas/Mining operations.

In the following discussion, we will focus on naval applica-tions; similar configurations support other HF E-mail and inter-networking communications system requirements. An HF E-mail system for naval ships and deployed forces that supports naval communications, including administrative, logistic, and engineering order-wire traffic, is shown in Figure 8-8.
A typical shipboard HF E-mail system consists of a Harris RF-6750 Wireless Gateway, an RF-7210 ALE Controller, an RF-5710 High-Speed HF Modem, and connection to a Harris HF radio system (the RF-590A Receiver and RF-1140 Transmitter). The modem and radio system are remotely controlled and
managed by the Wireless Gateway computer. The RF-6750 Wireless Gateway provides seamless data trans-fers between common networked applications, such as E-mail and FTP file transfer, running on geographically separated Local Area Networks (LANs). This system also supports the applica-tion
of sending mobile HF data messages over the Internet. The data transfers are accomplished automatically using HF radio. Unlike conventional network routers and gateways, the RF-6750 is designed specifically to operate over HF radio circuits.

System Design Considerations

Harris’ RF Communications Division has a communications systems engineering department staffed by engineers who are specialists in the design of custom equipment for the “one-of-a-kind” type of application. The following are some of the factors that we consider in designing a modern HF radio system.

System definition

• Who are the users?
• What is their location?
• Are communications one-way or two-way?
• What are the interfaces with other communications media?
• What is the operating environment (hostile or friendly, rural or urban)?

Transfer of information

• What type of traffic is there (voice, data, images)?
• Do the priority levels differ, depending on the message source and/or content?
• What are the security levels for safeguarding the information?

Message protection and security

• What is the correct type of error detection and correction for data?
• What type of COMSEC is needed?
• Will spread-spectrum or frequency-hopping techniques be used to avoid interception or jamming?
• Is excision filtering needed to remove interfering signals?

System availability
• What is the probability of transferring messages in real time?
• Can alternate routing be used to enhance message availability?
• Can lower priority traffic use store-and-forward techniques?
• Are there any operational restrictions due to propagation, transmitter power, or other constraints?

Traffic analysis
• What are the typical message lengths?
• What is the average number of messages per unit of time?
• What are the message priorities?
• When is the peak traffic?
• What are the types of traffic?

Projected growth for each category of traffic
• What impact do higher traffic levels have on system implementation?
• Are additional nodes and/or relays necessary?

Impact on message structure
• Is the format for data message compatible with traffic requirements?
• Include security classification, priority, source, and destination address.

Site
• Is this a fixed or mobile site?

Fixed site
• Are the receiving, transmitting, and control functions collocated or separate?
• Is this a permanent or temporary installation?
• Are there any frequency restraints for collocated receivers and transmitters?
• What are the staffing requirements?
• What are the environmental considerations?
• What type of power is available?
• Is uninterrupted power a requirement?

Mobile site

• Is the equipment designed for a vehicle, ship, shelter, or aircraft?
• Are manpacks required?
• What are the antenna limitations and constraints?
• What are the physical size constraints?
• What are the bandwidth and primary power requirements?
• What are the environmental considerations?

Communications protocol

• Is there a return channel for ARQ?
• Is ALE being used?
• What are the data protocols?

Equipment selection

• Transmitter requirements: Power output, solid state versus tube, broadband or narrowband, allowable distortion, frequency range, tuning speed, remote control?
• Receiver requirements: Selectivity, dynamic range, distortion, remote control?
• Antenna requirements: Gain, bandwidth, polarization, radiation pattern, available terrain, remote control?

Data communications systems

• What is the data rate?
• How is data being protected (interleaving, encryption)?
• What is the modulation scheme?
• Is the modem serial or parallel tone?

Interface to other equipment and systems

• What other equipment is required (fax, data terminal, imaging systems)?
• What other types of systems are involved?
• Is there an interface with VHF/UHF radio systems, satellite, or switched telephone networks?

Command and control

• Will operation be attended or unattended?
• Is self-test required?
• Are the transmitter, receiver, and control sites at different places (split site)?

SUMMARY

• Modern HF radio is small and lightweight. Features and capa-bilities,
which formerly required additional equipment, are now embedded into the radio transceiver.
• HF radio plays a key role in modern long-range telecommuni-cations systems, often working in conjunction with other media, such as satellites, cellular networks, and telephone landlines.
• A systems approach is needed to obtain the best results in designing a modern HF radio communications network.