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Dealing with interference in a crowded RF spectrum

Posted: 29 Apr 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Interference? wireless equipment? GSM? Passive Intermodulation? LTE?

800MHz LTE transmissions can also interfere with digital cable TV (DVB-C) reception if the receiver, or the cables connecting to the receiver, are improperly shielded. This can impair not only TV reception, but also telephone and internet service carried on the same cable.

A number of techniques are used today to try to prevent interference with TV broadcasts, with varying levels of success. One of the most common is to install a low-pass/band-pass filter between the antenna and the receiver; if this helps DVB-T interference, however, it will not fix DVB-C problems.

Unfortunately, commercially available and affordable filters that block 800MHz LTE transmissions also attenuate the DVB-T channels that are close to the LTE bands. While this is not a problem in locations with a good DVB-T signal, it can actually disrupt the service for a location with a weak DVB-T signal.

Effective methods for dealing with interference
The first requirement in dealing with interference problems is to locate and identify the source of the interference, whether it be CCI, PIM, a jammer, or (in the case of digital video broadcast) LTE transmissions. Many types of RF test instrument can potentially be used for this. Dedicated interference detection equipment, however, will save the user a great deal of time, because of the precision with which it locates and analyses the source of interference.

For PIM problems, the Anritsu MW8219A PIM Master is ideal: it can generate two tones of up to 40W into the transmission system, and measure precisely the occurrences of passive intermodulation. Its 'Distance to PIM' feature lets the user exactly locate the source of the interference. The MA2700A Interference Hunter is equally helpful for other kinds of interference.

Once located, the interference can normally be eliminated easily. In the case of DVB-C broadcasts, filtering is ineffective because the technology uses overlapping frequency bands. The correct approach here is to specify the receiver, cables, modem and other components with high resistance to interferers.

As for DVB-T, it can be helpful to replace an omni-directional antenna with a uni-directional antenna (the smaller the beam width, the better). This is not a guaranteed fix: it will make no difference if the LTE base station is located in the same direction as the DVB-T transmitter.

And in general, the best cure for interference is prevention: using excellent components and installing them properly eliminates almost all risk of interference. But since no real-world operation is ever perfect, it is useful to know that precise and accurate instruments are available that will quickly find the source of an interference problem when it occurs.

About the authors
Eder Eiras is Business Development Manager at Anritsu.

Mathias Hofer is Regional Account Manager at Anritsu.

To download the PDF version of this article, click here.


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