The signal hunters

We profile radio monitoring, a techinque used to by network operators locating sources of interference, regulators countering pirate radio, and the armed forces hunting out signals from terrorists.

The signal hunters

Pirate radio stations account for the majority of illegal broadcasts. They tend to operate within cities, generally on the FM radio frequencies of 87.5 to 108MHz. But interference also arises from other causes: poorly installed wireless LANS, older CB equipment and amateur radio, badly suppressed electrical equipment, or even faulty lightbulbs or thermostats.

Regulators (such as Ofcom) police radio spectrum by pinpointing sources of interfering radio signals. While interference takes many forms, regulatory authorities have a duty to act when it is caused with intent, particularly if it causes interference with the safety critical air traffic and marine bands.

Meanwhile, network operators are waging their own battle with radio interference. In response to problems such as poor voice quality, dropped calls or low data rates, network operators employ field engineers to track down and eliminate the interference. Faulty network equipment is a major source of the problem.

Interference is also more prevalent nowadays because network operators continually add voice and data services, so the licensed bands become more susceptible to it. The trend to install multiple basestations on each site has also increased interference potential.

With spectrum scarcity increasingly being felt, modern digital communications such as GSM and TETRA employ modulation techniques such as burst structures and frequency hopping. These technologies make more efficient use of the spectrum, and maximise signal speed and integrity.

However, the transmission techniques of clandestine eavesdropping devices closely follow the trend of commercial communications systems. Sophisticated encryption is becoming common. Detecting and intercepting sources of enemy communication falls to radio experts within the armed services, who are routinely required to characterise, locate and demodulate rogue signals.

Portability

While a fully-featured spectrum analyser might be used for spectrum preplanning and verification when a new radio system is installed, hunting interference or rogue signals demands more portable and ergonomic solutions. The signals may not be picked up at ground level, so the signal hunter has to find high ground or connect to a basestation antenna with the interference monitoring equipment.

For military personnel, intercepting a rogue signal coming from a satellite phone, for example, may need to be done on foot and surreptitiously. Bulky, heavy equipment is out of the question. Conventional spectrum analysers once dominated the radio monitoring market, but laptop-based and handheld formats are supplementing these, with small and lightweight, rugged options now available that fit easily inside a rucksack.

Aside from the physical issues, whether someone is pinpointing a signal from a terrorist cell or a source of interference for a TETRA network, many of the system requirements are the same. There is a growing trend for communications system to move higher up the frequency spectrum. For example, 802.11b/g operates at 2.4GHz and 802.11a at 5GHz. Hence, a wide frequency band with a high upper frequency will help to future-proof any investment, and a device with a range up to 7.5GHz is suitable for many radio-monitoring applications.

Snapshot

Frequency hopping and short duration bursts require a portable monitoring receiver to have a wide instantaneous bandwidth, or they will not be detected. Meanwhile, on-board memory is also a benefit, enabling data to be collected on site and saved for later analysis.

Such a snapshot provides a field engineer with evidence for any further systems investigation and proves useful if the problem needs to be brought to the attention of another network operator, for example.

Any radio-monitoring receiver will need good performance in its own right. If the receiver has poor linearity, it risks generating its own interference products, particularly in the presence of strong signals.

In addition to good linearity, a good noise figure is important - for example, it needs to be sensitive enough to detect the same type of signals that a basestation is receiving. Other useful features include an adequately sized display so that users can view suspect signals in spectrum and waterfall diagrams, for example, and the ability to record data such as a demodulated audio signal.

It is unlikely a radio-monitoring device will be able to offer all the necessary attributes for any given situation. For example, a one-box solution might be capable of detecting rf interference and cable testing, but it won't necessarily be sensitive nor fast. High sensitivity and a high scan speed will be needed in many situations - whether regulatory, commercial, or military - to capture weak or short-term signals.

The future

The expectation is that tomorrow's commercial networks will need to provide total connectivity and interoperability - a challenge, when existing spectrum scarcity is borne in mind. This will mean that frequency management, and thus radio monitoring, simply becomes a more important issue.

Although manual interference detection methods are labour-intensive, at present there aren't many alternatives.

A network of interconnected, static passive systems that could be deployed across the UK to provide continuous spectrum monitoring is in development by QinetiQ, but it could be years before this comes to fruition.

In the meantime, cheaper, more portable and durable radio monitoring equipment is being applied in some new situations. One of these is in human disaster zones where, in combination with a directional antenna, a radio monitoring device can be used to home-in on emitters such as mobile and cordless phones, and emergency beacons, to locate and save buried persons after earthquakes. Portable radio monitoring certainly has untapped potential.

John Andrews is a product specialist for radio monitoring equipment at Rohde & Schwarz

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