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TETRA the Noise Source: Preventing InterferenceTim JarvisManufacturers of short-range, license-exempt radio communications devices should take steps to prevent TETRA signals from disrupting or blocking operation of their products.
What Is TETRA? TETRA is a European standard for professional digital mobile radio communications. It was developed by the European Telecommunications Standardization Institute (ETSI)1 as a harmonized digital replacement for the many conflicting analog mobile radio standards currently in use in Europe. TETRA systems are not currently operable in the United States. TETRA has been aimed at two target markets:
Since TETRA's inception, its use (handsets per square kilometer) was always intended to be lower than GSM, which has mass-market consumer appeal. For this reason, frequency allocations were sought lower down the spectrum where cell sizes could be much larger and, therefore, network rollout costs would be significantly lower than for higher frequencies. After much debate, harmonized allocations around 400 MHz were finally secured. Currently, further allocations near 450 and 900 MHz are being considered.2 TETRA was designed to be interleaved with existing analog PMR and PAMR systems operating in the same frequency band, allowing for a gradual replacement of the current systems with TETRA.3 The TETRA standard achieves this goal by employing a 25-KHz channel separation compatible with analog systems and placing severe limitations on adjacent channel power. These requirements dictate a narrowband modulation technique such as differential quadrature phase-shift keying (DQPSK) combined with highly linear transmitter circuitry. In practice, manufacturers use Cartesian loop feedback to combine linear performance with low-current operation. The narrow bandwidth dictated the need for a slow frame rate for maximum data payload efficiency. However, the rate of 17 frames per second creates worse interference in some electronics equipment than equivalent GSM radiation.4 More troublesome are TETRA's frequency allocations, which are adjacent to the popular and crowded license-exempt ISM bands.5 Table I presents the ISM power levels for various telemetry products.
Spectrum Allocations All TETRA installations in the United Kingdom currently use the 380–385 and 390–395 MHz frequency pairs. The European Conference of Postal and Telecommunications Administrations (CEPT) has allocated the entire spectrum from 380 to 400 MHz for TETRA, but the current spectrum holder (the UK Ministry of Defence) has yet to release the upper 5 MHz. Recommended TETRA radio spectrum allocations are given in Figure 1.
 Figure 1. TETRA spectrum allocations. In Europe, TETRA is currently occupying the 410–420 and 420–430 MHz bands, but the remaining bands earmarked for TETRA have yet to be released. At this point, only a few TETRA OEMs have developed 900-MHz equipment. Unfortunately, this part of the spectrum is already crowded. Worst yet, all TETRA allocations seem to either overlap with or appear adjacent to both UK and European ISM bands. TETRA Interference TETRA operates on frequencies adjacent to consumer license-exempt telemetry bands. Traditionally, these telemetry products are poor at rejecting near-frequency interference and, therefore, their signals are easily blocked. TETRA is a noise source for two main reasons:
Who Should Care. TETRA affects many different manufacturers and users, including consumers who may not be able to get into their cars after driving into a TETRA cell because it can interfere with an automobile's electronic locking signal. Designers and manufacturers of license-exempt telemetry products should also take additional steps to prevent blocking. Designers and manufacturers of safety-critical and life-support equipment must take particular note because a TETRA handset can induce RF fields in excess of those fields used to test such products.6 Electronic Equipment TETRA's long frames induce low-frequency components that are hard to filter out in a wide variety of electronic equipment. In a recent study on the effects of TETRA signals on safety-critical medical equipment, a simulated TETRA signal was shown to produce more damaging interference effects than an equivalent GSM signal. Such effects could pose serious problems in hospital environments where GSM phones are banned near intensive care wards. Ambulance staff and paramedics will increasingly be carrying TETRA handsets in these areas, which will cause an even greater threat to the safe operation of life-critical equipment. The radiated power levels of both TETRA and GSM handsets can create near-field RF levels greatly in excess of the current European Class A limit of 10 V/m, which forms the basis of immunity requirements for industrial and safety-critical equipment. Manufacturers must take additional EMC design steps to provide immunity from TETRA handsets operated near the equipment. This is particularly true of measuring equipment, which usually contains sensitive analog electronics operating at low signal levels. Much has been written on how to design for RF immunity, including one article written by this author dealing specifically with sensitive analog electronics .7,8,9 An explanation of these techniques is beyond the scope of this article. Low-Power Radio Equipment TETRA and the crowding of the radio spectrum have effectively ended the days of cheap and cheerful wideband, license-exempt telemetry systems such as the key fob–controlled central locking immobilizer mechanisms in many cars. The UK Department of Trade and Industry Radiocommunications Agency (DTI-RA) will withdraw the old wideband MPT1340 specification10 used for such devices at 417.900 to 418.100 MHz. This allocation sits inside a band now allocated and used throughout Europe by TETRA (see Table I). The agency is currently offering these telemetry devices bands at 433.050–434.790 and 868–870 MHz. Wideband receiver apparatus is being discouraged in favor of narrowband channelized operation in either 12.5- or 25-KHz channels. This change will force manufacturers to use crystals with tighter temperature tolerances, which will drive up the costs of transmitter fobs. Nevertheless, crystals with coefficients as low as 10 ppm can be obtained in volume at prices as low as 33.5p ($0.22). Volume cost is calculated at 0.25 million parts per annum. The primary impact of this bandwidth change, however, is on receiver design. Traditionally, receiver characteristics have not been examined before a license-exempt radio product is granted type approval. The Radio and Telecommunication Terminal Equipment (R&TTE) Directive11 further deregulates type examination requirements for these products. In response to these factors, the Low Power Radio Association (LPRA)12 and others have formed the RadioMark scheme,13 which will develop standards for receiver performance. To prevent TETRA signals from blocking low-power radio receivers, designers should employ the following techniques:
Conclusion TETRA is likely to be deployed at a fairly fast pace throughout Europe. Manufacturers and users of all electronic equipment should be aware of TETRA as a potentially troublesome noise source. Short-range, license-exempt radio communications devices, in particular, will need special attention to prevent TETRA signals from disrupting or even completely blocking their operation. Serious interference with legacy systems seems almost inevitable in the short to medium term. References 01. ETS 300 392, "TETRA standard," ETSI TC-RES F-06921, ETSI, Sophia Antipolis, France. 02. Current and Future Spectrum for TETRA—A Consultative Document from the DTI and the Radiocommunications Agency, December 1997, http://www.radio.gov.uk/document/misc/traneuro/traneuro.htm. 03. ETR 300-1, Part 1, "Overview, Technical Description and Radio Aspects," ETSI EP-TETRA F-06921, European Telecommunications Standardization Institute, Sophia Antipolis, France. 04. "Electromagnetic Compatibility Aspects of Radio-Based Mobile Telecommunications Systems, Final Report," LINK Personal Communications Programme, DTI & EPSRC. University of York, University of Hull, ERA Technology Ltd., Cambridge Consultants Ltd., 1999. 05. Short Range Devices—Information Sheet [on-line] (Radiocommunications Agency, 1999); available from Internet: http://www.radio.gov.uk/document/ra_info/ra114/ra114.htm. 06. IEC 61000-4-3 (EN 55024:1998), "Radiated Electric Field Susceptibility," International Electrotechnical Commission, Brussels. 07. EMC for Product Designers, Newnes, Oxford: 1992. 08. K Armstrong, "Design Techniques for EMC, Parts 1–6," UK EMC Journal nos. 21–26, Nutwood UK Ltd. 09. TP Jarvis and IR Marriott, "Improving the RF Immunity of Sensitive Analog Electronics," UK EMC Journal no. 9. Nutwood UK Ltd. 10. "MPT 1340 Performance Specification, Transmitters and Receivers for Use in Telemetry, Telecommand and In-Building Security Equipment Operating in the Frequency Band 417.900 MHz to 418.100 MHz and In-Vehicle Equipment Including Radio Keys in the Frequency Band 433.720 MHz to 434.120 MHz," December 1997. Radiocommunications Agency [London]. 11. European Directive 1999/5/EC. Tim Jarvis is senior design consultant with KTL Hull Development Group (Hull, UK). Photo by WILLIAM WESTHEIMER/THE STOCK MARKET |
The deployment of TETRA (TErrestrial Trunked RAdio) systems is gathering speed. Dolphin Telecommunications (Basingstoke, Hants, UK) has deployed a pan-European public access mobile radio (PAMR) network for professional users, and most European public safety organizations are deploying their own private mobile radio (PMR) networks. Two features of TETRA make it a particularly annoying source of radio interference: its relatively slow frame rate of 17 frames per second, and its spectrum allocation adjacent to all major European industrial, scientific, and medical (ISM) bands. This article investigates TETRA as a noise source, identifies those who should be concerned with its effects, and discusses how to eliminate TETRA interference.