Building the interface

The following is an article that was written for the RSGB's RadCom magazine. This gives background to the Gateway project and discusses the technical issues.

AN HF GATEWAY FOR EMERGENCY COMMUNICATIONS

In October 2006, the RSGB helped Steve Richards, G4HPE, to obtain a Special Research Permit from the UK regulator Ofcom allowing experimentation with a frequency-agile Gateway that links HF amateur radio with the Internet. After two fascinating years, Steve describes the concepts, hardware, the on-air testing and the lessons learned to date.

Figure 1: Hurricane Ike, Texas, September 2008

The start of my interest in this subject evolved from my membership of the Raynet HF Team [1] and IRESC, the International Radio Emergency Support Coalition [2]. Both provide traditional amateur radio links to assist with the flow of emergency traffic between disaster zones and sources of control and assistance. But what if that help is at some distance from the disaster? As that distance becomes greater, the vulnerability of the radio path increases, through requiring complex chains of repeaters or through being at the mercy of propagation effects. IRESC, an international organisation, aims to provide such paths right across the globe. To achieve this reliably on demand asks a lot of radio alone. The Internet Radio Linking Project [3] has shown that using the Voice Over Internet Protocol (VoIP) to provide part of the path can enhance the effectiveness of a communications circuit.

WHAT IS A GATEWAY?

A Gateway bridges the technological gap between two different communications environments. In this application, the Gateway has a radio frequency port and an Internet port and permits communication to pass, in both directions, between the two. Physically, the Gateway will comprise of several pieces of hardware; the radio transceiver, a computer connected to the Internet, and an interface which connects between them to control audio and switching.

Figure 2: The shack at G4HPE. The Gateway interface sits on top of the HF transceiver with the VoIP computer to the right

A Gateway is different to a Repeater. A Repeater listens on an input channel and reproduces what it receives on an output channel – all users operate with their transmitter and receiver frequencies split across the two. A Gateway with two ports can both transmit and receive on either and the switching action is automated, reacting to the traffic flow. Effectively, the Gateway flip-flops to order. It listens on both ports until a signal is heard on one of them, which causes the unused port to go into transmit. When the signal ends, a reply may be received which causes the Gateway to switch the communications path into the opposite direction.

It is the switching, of course, that is critical to correct operation! Detecting a busy channel on the Internet side is usually no problem because the computer can change the voltage on one of its connectors when a signal is received. An FM receiver is also helpful, because a busy channel can be detected through a change in state of the receiver’s squelch circuit.

With both of these systems, it is also possible to detect the difference between silence and active signals to facilitate the changeover required. But with an HF receiver working in SSB mode, it is not usually appropriate to use a noise-detecting squelch. Instead, the channel must be continually monitored for human speech, which gives rise to the tricky issue of how to discern between the intended message and the multitude of shortwave noises and unwanted signals. This is what makes setting up an HF Gateway such a challenge, so more about this later!

WHAT CAN AN HF GATEWAY BE USED FOR IN EMERGENCY COMMS?

In a simple form, an HF Gateway could be used by a radio amateur to monitor their transmitted signal at a distance. For example, a net controller would be able to monitor their HF transmissions as received by a distant Gateway then peeled back to their headphones via VoIP. This would not only allow them to identify any transmission defects, but also gauge propagation variations. They might also be able to hear weak outstations via the distant receiver that they could not discern via their own station directly, a sort of widely spaced diversity antenna!

HF Gateways create the possibility of using a transceiver geographically near to the zone of interest while the user can be located anywhere on the globe, courtesy of the instantaneous shrinking of the world permitted by VoIP linking. For example, it would be possible for a radio amateur to take a VoIP-capable laptop to the offices of a relief agency and, via the Gateway, put them in direct touch with their teams in the field even if they are half a world apart. Furthermore, once in the electronic domain, the received audio can be safely and securely streamed to a website where interested consumers can monitor, if appropriate, without any danger of return intrusion and without loading the available bandwidth of the VoIP network itself.

Figure 3: Hurricane Hanna, September 2008, wrecks official communications in the Turks & Caicos Islands - amateur radio is often the first communications to be re-established

THE SPECIAL RESEARCH PERMIT

To explore the practicality of an HF Gateway, an application for a Notice of Variation to the normal amateur radio licence was made. The RSGB gave the project its support. The then Spectrum Manager, Colin Thomas G3PSM, and HF Manager, John Gould G3WKL, were instrumental in helping to write up the application for presentation to OfCom and for seeing the application process through. Mark Gregory, G4LCH, the Internet Specialist of the Data Communications Committee [4], also provided much support.

An application of this nature had never been successful before. The emphasis would be on how useful such a facility could be to emergency communications where significant range within the UK was required, so the application concerned 40, 80 and 160m. It was important that the Gateway could be frequency agile, to allow for a choice of the best frequency to serve the zone of interest under the prevailing propagation conditions. It was also important that a realistic RF power level could be used. Interference-free traffic was necessary, and it was also agreed that the licensee should attend the Gateway during operation.

In October 2006, it was a great pleasure to be granted the Special Research Permit and experimentation could begin in earnest. At OfCom, Steve Roper G8MXZ and Rod Wilkinson G3TXA gave every possible support to get the project under way and have since granted extensions to the original NOV period to allow further tests.

Figure 4: The prototype Gateway interface- not very pretty but it works!

PRINCIPAL AREAS OF THE RESEARCH

1. Squelch and switching

When operating an SSB port on HF, the situation is complex. Unwanted artefacts, such as data transmissions, ‘tuning up’ carriers, off-frequency speech, man-made noises and lightning static can all contribute to holding the Gateway in a single direction. Fortunately, most modern HF transceivers provide a signal-level squelch that goes some way to muting the weaker unwanted signals, but this is not a very controllable solution because it reacts in the same way to both the wanted and unwanted signals. What is needed is an ‘intelligent squelch’, one that can discern between human speech and everything else.

Detection of the characteristics of human speech is complicated. Successful recognition has to take into account the spectral make-up of the signal, from low bass through to high treble sections. The syllabic or rhythmic nature of the spoken word is a factor, as is the pattern of percussive and sibilant elements of language.

In isolating what is human speech and what is not, variations in the technical quality of received audio and the differences in patterns of speech between different languages, dialects and nationality must not hinder effective detection.

Military developers and a few civilian circuit designers have produced highly complex squelch circuits for use in such trying radio environments. Some will be familiar with the Icom VSC ‘Voice Squelch Control’ facility that is provided with some of its products. This ‘opens the squelch only when a modulated signal is detected and ignores unmodulated, beat noise signals’ is all they will say! There is a very affordable amateur construction project offered by AH6LE [5] for this purpose. Such circuits, effectively FM discriminators, attempt to ignore non-human audio signals.

For the HF Gateway in this research, the interface has been fitted with the VOS-4 intelligent squelch circuit available from Naval Communications in the USA [6]. This appears to work very well. It is generally successful in opening on speech and ignores noise, tuning transmitters and data transmissions. Settings have to be quite carefully adjusted to achieve optimum performance.

It is notable that the solutions found so far all appear to require the construction of electrical circuits, with fixed components on a circuit board. It would seem so much more flexible to create a speech-recognition system in software terms; that is, a computer program that can carry out this task using DSP (digital signal processing). Such a facility might allow 'fine tuning' of its operation by adjusting key parameters and observing the change in performance that results. Bear in mind that the task is not to understand the words being said, simply that the signal is confirmable as human conversation. If you are someone who has an interest in developing suitable software, or you have come across such an item in the public domain, many would be very pleased to hear from you!

Another squelch option that could be explored is the use of audio tone access codes, known as ‘CTCSS’ (Continuous Tone-Coded Squelch System), where the squelch will only open after receiving the correct sequence of tones. It’s also worth noting here that some of the digital speech modes for HF, such as WinDRM [7], may again make the use of a simple noise squelch all that is necessary.

2. Optimum transceiver settings

Modern transceivers offer the operator a comprehensive number of parameters,

many of which can be adjusted to optimise the transmitted and received signals. In terms of audio treatment, the settings of these parameters are likely to be different when the transceiver is being used for Gateway operation as compared to direct microphone-to-air, particularly as signals are passing between the radio equipment and a computer. Furthermore, there is a sub-layer of control signals (Push-to-Talk ‘PTT’ and squelch status lines at least) that are also subject to the setting of various parameters (switching sense, delay aspects, hang-time). On the Internet control side, the software similarly requires the careful setting of such controls.

3. Radio Frequency induction

To be effective in longer-range emergency communications, an HF Gateway will

need to employ practical output power levels. The Gateway is licensed up to 21dB(W) which is deemed to be a realistic upper limit. The use of higher power levels allows for understanding the issues that arise when significant radio frequency energy is being generated close to domestic (that is, unbalanced) audio installations – this probably includes the Gateway interface equipment itself! It is important to be able to recognise the symptoms of RF induction and to understand techniques for minimising such problems, which can lead to substandard radio transmissions and audio distortion.

4. Audio Quality issues

Communication via a radio transceiver requires different audio tailoring

compared to Internet-based speech circuits. When communicating via radio, clarity and ‘punch’ are important whereas when speaking from computer to computer a more natural (and fuller bandwidth) quality is sought. Thus, treatment is necessary in order to optimise audio quality when bringing one domain into the other. This project will hopefully lead to good-practice guidelines on level, equalisation and compression settings, as well as hints on operating and even microphone technique. In terms of the physical interface between computer and transceiver, the issues explored include safety isolation, signal levels, noise and distortion.

5. Operator skills

There is no doubt that the operation of an HF Gateway requires skills on behalf of

the supervising operator that can only be acquired through practice and experiencing the problems that arise in real situations. Many amateurs involved with ‘talk-through’ operations for emergency communications will know how technically demanding this role can be. The design of the interface must allow the operator to listen and talk to both sides of the Gateway either singly or simultaneously, understanding that there are potentially two nets of operators that remain unaware of each other unless the Gateway is invoked. The operator must have the appropriate switching facilities to instantly control the Gateway manually if necessary, particularly in the case of interference. Once the Gateway is operating, skill in identification and control is required in order to avoid confusion.

LEGAL ISSUES

It should firstly be noted that several fixed-frequency 29MHz FM Gateways are in

use in the UK. Therefore, it would not be unusual for non-UK amateurs to use these Gateways, speaking over the Internet from their home country but effectively being transmitted from a station in the UK and working any other amateurs in range of the RF link. The potential licensing issues surrounding this operation have thus already been considered. In effect, such stations are identifying themselves by giving their normal home callsigns but are being radiated from within a different country. It is not unlike inviting a visiting radio ham friend to operate your station under your direct supervision.

In this research, it has been very beneficial to be able to allow non-UK amateurs to operate without legal restriction. This showed most powerfully how a control station located on the far side of the earth can be entirely effective in controlling a net of European HF stations. Conversely, it has also been shown that a radio operator on HF can satisfactorily control a net of VoIP participants.

VoIP programs usually allow for all activity across the Gateway to be recorded as audio files on the local computer. This would be used at all times to provide a legal record of Gateway activity as well as for useful examination during test debriefing.

Figure 5: A temporary station set up by Chinese radio amateurs of the ‘875’ Club, at Lingyan Guanyin Mountain in Dujianyan, near Chengdu city. It was established to support local emergency communications in the aftermath of the 2008 earthquake. The HF antennas were clamped to street signs.

INTERFERENCE AND SECURITY

In this project, it has been possible to eliminate deliberate interference through

strictly controlled access to the VoIP port of the Gateway, coupled with controlled net operation and, when necessary, manual intervention of the link on the RF side.

The Echolink VoIP program [8], whilst not the only choice, has been used for the Internet side of the Gateway for several reasons. Only licensed radio amateurs are able to use Echolink. On downloading the software, it is disabled until the network managers have received a copy of the licence of the individual. I have personally never experienced any form of malicious interference in all the years that I have been active on Echolink. Echolink has powerful facilities that completely control the ability to access the Gateway and, additionally, whether connected stations are enabled to transmit or only monitor. This means that the VoIP side has its ‘house fully in order’ before any connection to the radio side of the Gateway is made.

REMOTE PUSH-TO-TALK (PTT)

One of the problems with interfacing VoIP systems with radio equipment is that they are 'simplex' in operation. Both the VoIP and radio circuits occupy a single channel, therefore they can only be in either transmit or receive mode, but never both. The Gateway must switch the direction of audio flow through it if two-way communication is to be successful. If one side of the Gateway is permanently busy with traffic, the Gateway can become locked in a single direction. In practice, the VoIP side of the Gateway is not a problem: when a station finishes their transmission, the channel falls silent and the Gateway is free to switch. It is not quite so clear-cut on the radio side. Even with the use of both noise-based and speech recognition squelch systems, the receiver can still be excited by unwanted audio. An example of this is when strong radio stations are operating close to the Gateway frequency, within the receiver pass-band, which can fool even the intelligent squelch into thinking that wanted speech is present.

To get round this problem, a method of remotely forcing the Gateway's HF radio into transmit is required. Fortunately, the radio amateur world enjoys the skills of Len Stefanelli, N8AD [9], who has devised many ingenious software solutions for controlling radio transceivers. He was both patient and kind in assisting me and he has developed a small and robust program called TCP Gateway for allowing a remote VoIP user to directly control the radio transmit/receive (PTT) line via the Internet.

The way it works is that the Gateway computer runs a simple TCP server program. The remote VoIP user runs the corresponding TCP client program. The remote user firstly issues a connect command and, via the Internet, the Gateway operator receives an incoming connect request. The Gateway operator can allow or deny access depending on the credentials provided - if there is any confusion the two can use a simple text box to exchange questions and answers. Once the remote user has been granted access, they see a small control sub-screen containing large red and green buttons. These two buttons are the transmit and receive buttons respectively. When the user clicks on the red button, the remote TCP command forces the Gateway radio into transmit. The user can then speak over the VoIP circuit knowing that they are being relayed onto the radio channel. When they have finished speaking, they click on the green button and the radio drops back into receive.

This system is simple, yet secure, and always remains entirely under the supervisory control of the Gateway operator. To make best use of the remote switching, the Gateway operator removes all squelch controls from the radio receiver. The receiver produces permanent audio, whether noise or wanted speech. The Gateway is locked in one direction, from radio channel through to VoIP channel. VoIP users are now effectively monitoring the radio channel continuously. If a call is received on the radio channel, the VoIP Control Station can force the transceiver into transmit in order to reply. This way of working is very solid and positive in action.

BUILDING AND TESTING

The initial work was to design and construct the interface between the HF transceiver and the Internet computer, to create a single unit that would allow the Gateway operator to control all aspects of the system. Cable looms were made up to provide the various connections necessary; transmit and receive audio, squelch circuits and switching on both sides of the interface, plus external microphone and headphone monitoring for the operator. The construction included audio transformers to isolate the external devices and took measures to minimise the danger of RF induction. The intelligent squelch circuitry was also installed within the interface unit.

Figure 6: The basic signal paths that comprise the HF Gateway

Once the interface was up and running, on-air tests commenced. Initially, simple activities were undertaken, such as relaying traffic in one direction or another through the system so that audio could be tailored and the most suitable settings of the multitude of parameters could be found. Members of the Raynet HF Team (on the HF radio side) and IRESC (using their Echolink conference facility) gave much of their time in providing themselves and their stations as guinea pigs! A lot was learned about operating practice on behalf of the Gateway supervisor and the participants alike.

A user-group of interested radio amateurs soon built up and the tests were announced so that as many people as possible could take part. As confidence in the system grew, the tests became more expansive and for the past year several major exercises have been undertaken with a complete set of simulated scenarios and mock traffic messages being emailed out to the worldwide participants in advance. For example, Dennis Baumgarte, AE2EE, near New York, activated the local FEMA [10] emergency communications centre at Batavia Airport and worked into his own Echolink Gateway via the Genesee County ARES group’s UHF/VHF repeater system. He used the Internet connection to work on 80m in the UK via the HF Gateway, controlling the net very effectively for a considerable time.

Perhaps the most complicated use of the Gateway to date was during the IARU ‘GlobalSET’ simulated emergency test in May 2008 [11]. This is an international emergency communications exercise across all IARU regions, designed to improve working between different organisations across the world. The Gateway was heavily used with many participants taking advantage of the system to pass messages over great distances. An example of this is shown in Figure 7, which shows Ray Abela, 9H1RA, operating his 70cm handheld in the Civil Protection Centre on the island of Malta, working via his own Internet Gateway (back at his home nearby) to emerge on HF in the UK where he was able to work many European stations while the appalling propagation conditions in his own region meant that no contacts would otherwise have been made.

Figure 7: Ray Abela, 9H1RA, briefs Malta Amateur Radio League colleagues in the message format for GlobalSET 2008 and (inset) passes the message through the HF Gateway in the UK, 1600 miles away, directly from his own 70cm portable.

About forty contacts were made over the four-hour exercise period, with some exotic callsigns (such as VU2RBI, HP1/OE5CEN, VE7XAT and ZS6BUU) appearing on the lower HF bands in the UK! It is worth noting that many messages were successfully passed across paths that would have been impossible by radio alone, and all this with the Gateway performing reliably even though a European contest was in full flood on the bands at the same time.

There is a website [12] for those interested in the HF Gateway and every test has been carefully documented there. The reports can be freely downloaded as PDF files. The details also contain many audio clips, recorded both on the VoIP and the radio side of the Gateway, so that the performance of the system can be assessed.

WHERE TO FROM HERE?

The Gateway as a basic tool has been seen to work effectively. It is hoped that OfCom may consider granting further research periods, or even perhaps a continuing NoV that could be applied for by interested experimenters, which would open up even more possibilities. For example, a network of several HF Gateways could be maintained at the ready, so that should a major problem hit a large area of the UK it would be possible to invoke a predetermined plan to provide radio coverage into the area that had lost all other connectivity and to provide two-way links with a central command facility located outside the disaster zone.

There are several areas where further development can be made. As mentioned, the idea of using software to achieve speech recognition must surely be a better solution than fixed circuitry. It is probable that such work has been undertaken somewhere in the world but it has not been possible to track this down so far!

Operationally, working via an HF Gateway brings special demands on communications skills and the production of a set of guidelines would be helpful.

Another area that requires attention is how to optimise the audio of VoIP users. The Echolink program, for example, provides only very basic audio tailoring options. In particular, there is no facility for compression or limiting. At one extreme, the audio can be very distorted and this produces a badly degraded signal when transmitted via the HF transceiver. At the other end of the scale, audio that is under-modulated is subject to noise and can fail to fully modulate the transmitter, effectively generating a weaker signal to the distant radio user. It has been found that many VoIP users have not paid much attention to the quality or level of their audio with the result that their signal as transmitted is difficult to understand on the HF radio frequency, particularly if the frequency is not all that clear. It seems a forlorn hope to expect all VoIP users to optimise their audio and therefore a much more comprehensive set of filters and compressor/limiters should be developed for future inclusion in VoIP software.

Figure 8: Fortunato Bonnici, 9H1ES, and members of the MARL GlobalSET team set up a portable station in the car park of the Civil Protection Centre

REFERENCES

[1] RAYNET HF TEAM: A subsidiary of Raynet in the UK, the HF Team are

interested in exploring the benefits of HF to emergency communications.

www.raynet-hf.net

[2] IRESC: IRESC is an international organisation that tries to enable

communications between disaster areas and sources of support, using a mix of

traditional amateur radio and Internet speech networks.

www.iresc.org

[3] THE INTERNET RADIO LINKING PROJECT: This well-established system

uses custom software to link repeaters and nodes over the Internet, expanding the

radio horizon for the amateur radio hobby.

http://www.irlp.net/

[4] RSGB DATA COMMUNICATIONS COMMITTEE: A focal point for

information about the Data Communications work of the RSGB Emerging

Technologies Co-ordination Committee.

www.dcc.rsgb.org

[5] SYLLABIC SQUELCH HOME CONSTRUCTION PROJECT: A circuit

developed by Kenneth Arck AH6LE and others, designed to differentiate between

human speech and the other signals encountered on HF frequencies.

http://www.ah6le.net/hf_squelch.html

[6] NAVAL COMMUNICATIONS INTELLIGENT SQUELCH VOS-4: This

system is available for purchase. The website also contains interesting

information about speech recognition techniques.

http://www.naval.com/vos/

[7] WINDRM: This program encodes and decodes speech for high quality digital

speech over HF radio.

http://n1su.com/windrm/

[8] ECHOLINK: Information and downloads available from the comprehensive

website.

http://www.echolink.org/

[9] LEN STEFANELLI, N8AD: Len maintains a website bursting with interesting

ideas and his own software developments which can be downloaded. The

TCP_GATEWAY program, described in the text, is available here.

http://www.hfremote.us/

[10] FEMA: The Federal Emergency Management Agency that has strong ties with

amateur radio in the USA.

http://www.fema.gov/

[11] GlobalSET: A report by Greg Mossop, G0DUB, on the May 2008 GlobalSET

exercise can be viewed at the following link.

http://www.iaru-r1.org/GlobalSETReportMay08.pdf

[12] G4HPE-L HF GATEWAY PROJECT WEBSITE: This site contains reports on all of the tests and exercises undertaken for the special research permit and includes many audio clips.

http://hfgateway.website.orange.co.uk/