September 4, 2015
One of the biggest challenges associated with finding interferences is that it can come from so many sources. The cause can be from the inside the base station, due to poor connections, or external sources such as a rusty fence or jammer.
Engineers and field technicians equip themselves with a variety of test tools to help in this daily quest to find the cause of interference that can lower network performance. Today’s handheld spectrum analyzers have features such as Spectrograms to accurately analyze signals. A variety of antennas are available that can be easily attached to the analyzer to help track and locate interference sources. Recently, handheld interference hunters (figure 1) equipped with a GPS receiver, antenna, electronic compass, and a user-selectable preamplifier have been developed to further improve the process.
The most invaluable tool, however, is knowledge. Before starting any hunt, you should ask some important questions. The answers will probably help you locate the interference source and correct it before it causes dropped calls, slow loading and buffering issues that will frustrate operators and consumers alike.
1. Is It On-Channel Interference?
Operators often assign frequency or codes in a tiling pattern to help control cell overlaps. Factors that can cause excessive overlap and subsequent interference include:
- Antenna tilt
- Antennas mounted on high buildings
- Better than expected signal propagation over water
- Errors in frequency settings
- Excessive multi-path
The first three factors all have a common cause – the antenna is transmitting further than intended because it is aimed too high. Water allows radio waves to propagate better than over land.
Frequency setting errors are more prevalent than you might imagine. If a cellular operator has 500 cell sites and each cell has nine radios – not uncommon – there are 4,500 radios to maintain. If all of them are not set to the right frequency at all times, you can have interference.
Typically, phones can handle four or five signal paths and still get signal gain. If there are more paths, however, they are seen as interference.
2. Can In-band Interference Be My Problem?
If the interfering signal is within the pass band of the receiver’s receive filter, receiver desense can occur. These signals can be generated from other carriers, IMD products, or harmonics. The key takeaway is that a signal only needs to make it through any receive filter to the front end of the radio receiver to cause interference.
3. Are External Sources Causing Impulse Noise?
Impulse noise is created whenever a flow of electricity is abruptly started or stopped. A surprising variety of items can create impulse noise:
- Lighting suppression devices
- Electrical motors from elevators or the like
- Electric fences
- Power lines, which may arc and spark
- Light dimmers
Most of these impulse noise sources affect the lower frequencies, generally below 500 MHz. Micro-arcing or fritting is the exception, since it is generated by the RF signal and can affect reception at any frequency. It is typically very broadband, measuring more than 1 GHz wide.
4. Can Harmonics Throw My Signal Off?
Governing bodies normally regulate the power level of harmonics, which are multiples of an RF carrier. Equipment, however, does fail or fall out of specification. Many of those failure mechanisms create high harmonic levels. Also, if the original broadcast is at a high power level, even legal harmonics can be powerful enough to create problems.
Another cause can be a damaged transmitter. If only half of the specified sine wave is transmitted, high power harmonics will be created across much of the RF spectrum. This type of harmonic display is called a comb or “picket fence” signal.
5. Is Interference Due To Passive Intermodulation (PIM)?
Also called the Rusty Bolt Effect, PIM is caused when two or more strong RF signals combine in a non-linear device, such as a transistor, diode, or even the crystals found in corrosion or rust. It can be caused by a rusty fence, rusty bolts, corroded rooftop air conditioners, or even a rusty barn roof. Of course, it’s also possible that loose connectors in an antenna feed line or poorly configured transmitters can be the cause.
PIM has become so prominent that we have established a separate blog, The PIM Source. You can learn much more about the topic by reading those posts.
6. How Close Am I To Finding A Near-Far Problem?
In the case where a wide area RF coverage is overlaid with a smaller area coverage, and the two operating frequencies are close enough to give receivers a problem, the nearby, in-band-but-off-frequency signal can overload a receiver trying to listen to the weaker signal. The near-far problem can also happen between cell towers, as long as the mobile device can’t make a handover. This may occur when a device from one operator is broadcasting a strong signal to reach a distant cell tower. If a cell tower operated by another operator is near the mobile device, that second carrier’s receiver may be temporarily desensed by the mobile device.
7. Is Someone Causing Intentional Interference?
Intentional interference sources can be caused by jammers. This is a common issue in shopping malls, where employers want to ensure employee productivity, as well as in cars or at military bases. Recognizing this fact early on can help when conducting a hunt in these environments.
Once you have answers to these questions, you can select and use the test solution(s) most effective to locate interference sources in your particular scenario. This can be a handheld spectrum analyzer or a mobile interference hunting system such as the one shown in figure 2.
To learn more about these test solutions, as well as read more in-depth answers to the questions posed in this post, download a free application note.