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Radio Interference - Causes and Cures

Interference is Everywhere

Today’s public safety radio communications are marvels of technology. Where in the past radio users were grouped onto discreet radio channels with limited interoperability across agencies or jurisdictions, many communities, counties, and states have embraced trunked radio solutions that have the potential to process many millions of push-to-talk transmissions a month, serving tens of thousands of radio users. By sharing a base set of radio channel resources within a trunked radio architecture, far more users can be communications-supported as compared to prior conventional means.

While most radio transactions are conducted smoothly, to where the radio infrastructure blends into the background virtually unnoticed, the menace of interference and its potential for communications disruption exists. Interference can be unintentional, such as when atmospheric conditions arise whereby signals emanating from very distant locations exhibit enhanced propagation characteristics. This sort of interference potential has existed since the beginning of radio communications. If you’re an engineer or amateur radio hobbyist, such events are welcome with open arms. Yet, if your mission requires no-break communications, these events are unwanted and disrupted.

Those operating single-channel conventional radio systems simply had to suffer with unintentional interference as the radio spectrum is intrinsically a shared, but regulated resource. Trunked radio systems, where groups of radio channels are used, include adaptive interference mitigation schemes to minimize impact to system users. For example, radio channel base stations are equipped with interference “hit” monitors. If these devices sense an abnormal level of interference, that channel can be automatically removed from the available working channel pool until better conditions prevail. Thus, users would be unaware of an interference condition unless, of course, a sizable number of channels were impacted by simultaneous interference.

Should multiple channels be interference-impacted in a trunked radio system, the net effect would be potentially delayed calls (queuing), but not necessarily blocked calls. In fact, a properly designed trunked radio system would continue to process calls even down to two actual channels (i.e., one control and one working/voice channel). So, what about the trunked system’s control channel…what happens if that key channel is the interfered channel? Here’s the ingenious part: the control channel would automatically move to the next available channel in the trunking pool thereby triggering the fleet of user radios to quickly find it and re-affiliate with the radio system.

Some manufacturers will support up to four potential control channels in their trunking scheme whereas others can support as many as twenty-four. In any case, a properly engineered and configured trunked radio system would automatically strive to maintain control channel operations in the face of interference, unintentional or intentional.

Noise Floor and Unintentional Interference

To understand the issue of radio interference and how to work around it, one must first understand the concept of noise floor and its impact on radio communications. The radio signal that emanates from a tall tower or a small hand-carried radio has the same “hill to climb”. It must traverse a given distance and be strong enough at the distant end to be reliably detected/heard for communications to occur. This is true for analog voice or digital communications.

If the detected signal is received some degree above the normal electrical noise level, then meaningful communications is possible. At first blush, the noise floor of a typical 800MHz radio system can be extremely low…less than a tenth of a millionth of a volt. No big deal, right?

Well, not exactly since typical incoming radio signals from distant users could range from 1 to 10 microvolts…very strong ones being in the order of 25 microvolts or greater. So, we aren’t talking about anything big in terms of level…these are still just small fractions of a volt. Now, what happens if a source of interference was to suddenly cause the noise floor at a receiver site to rise to 15 or 20 microvolts and the receiver provides voice audio at good clarity when the signal is 10 times greater than the noise? What it means is that only a radio unit located very close to a tower site would be heard and those further away would likely be buried in the noise floor and undetected.

Intentional Interference

The noise floor interference situation described above is the unintentional type. Now let’s talk about the other.

The World is comprised of good people, bad people, and idiots. Fortunately, most people have good intentions, but the few in the latter categories have been known to intentionally interfere with radio systems. Doing so, however, requires a significant level of electronic prowess, which further dilutes those capable of doing harm to communication systems. Worse, if public safety radio systems are so attacked, the result could be the harmful delay in dispatching lifesaving services. To the person needing immediate Police, Fire or EMS assistance, intentional interference can become an unprecedented disaster.

In general, these characters will often deploy small battery-powered transmitters to jam vital communications. To be most effective, those sorts of interference generators must be placed relatively close to radio tower sites.

A radio shop equipped with appropriate radio direction-finding equipment can chase down these jamming devices in relative short order. Some jammers resort to more sophisticated means which we won’t go into for obvious reasons except to note that trunked radio systems are more able to mitigate this sort of interference than fixed-frequency systems. Ideally, if an agency uses fixed conventional systems, it is important to have backup systems available on different frequency bands since jammers tend to concentrate on one band and not the full spectrum available to public safety.

Other Interference Considerations

In the configuration of radio systems, it is advantageous for the infrastructure design to be comprised of multiple tower sites using receiver voting and simulcast transmitters. By so doing, the process of jamming the host radio system is made far more difficult due to site diversity…meaning the jammer would have to adversely impact multiple tower sites and not merely one.

The above discussion has been toward channelized jamming whereby the interference is specific to frequencies assigned to a public safety entity. Cellular and FirstNet operations are not channelized into narrow spectrum segments but occupy a broader range of frequencies that does not lend itself to narrow-channel jamming techniques.

Now, don’t get overconfident that we’ll be interference free by embracing broadband LTE, folks. The bad guys have that covered, too. Cellular signal jammers, which by the way are illegal in the United States but have been known to pop up in some restaurants desiring a cell-free environment, are intentional noise floor generators. These devices emit high-energy broadband white noise throughout the normal cellular frequency bands, thereby making it difficult to impossible for nearby cellphone to hear incoming calls from distant cell sites.

The more expensive jammers are band-limited to restrict jamming to just cellular operations. Those of more “questionable design” and cost-saving measures emit signals that are disruptive to both cellular and public safety 700/800MHz operations – equal opportunity interferers. All things being equal, the wider the occupied bandwidth the less signal power per kilohertz. Thus, the location of a broadband interference device must be in very close proximity to the device/system being interfered with. So, from a detection standpoint, location of a broadband signal jammer is significantly easier than for narrowband system.

Discussion Takeaways

In summary, effective countermeasures to radio communication interference include:

· Having reliable radio direction-finding and spectrum monitoring equipment available.

· Use of trunked radio technology, where appropriate.

· Use of multiple transmit/receive sites to spread interference susceptibility.

· Maintenance of alternate-frequency backup systems.

· Integrate both LMR and LTE technology

into your radio solution basket.

· Do not “advertise” the location of critical infrastructure tower sites.

If unintentional interference as caused by atmospheric ducting or if ray bending occurs so often that required communications are hampered, the best advice is a frequency change. Frequency management firms have unique software tools that have proven highly effective In locating good spectrum - VHF through 800MHz - in areas where others (including radio manufactures and resellers) claim no viable channels exist. Help may be just a phone call away.



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