Friday, February 6, 2009

Strategies for co-locating radios

When more than one antenna is located at a service access point, it may require considerable skill to get maximum performance out of the combined system. This article is intended to point to some of the factors involved.

Selecting an Antenna Site

The best way to deal with co-location problems is to avoid them! When reviewing possible locations for a wireless network access point, you may find some sites that appear to be perfect: They have good visibility of the area, they already have antennas on them (so there will not be issues of land use zoning), and the owner is willing - even eager - to lease you space on an existing antenna tower and equipment shelter. Such an "antenna farm" is likely to be a problem site, and if you can find a well situated building with no other antennas on it, it will probably be a better site.

The fundamental problem of co-location is RF interference. While you may be able to get a good, strong signal from the access point to your subscribers, the response from the subscribers to the access point may not be received correctly even if it comes in at a signal level well above the receiver's sensitivity threshold if there is another signal from some other system that reaches your receiver on the same or a nearby frequency channel. When there are competing signals in the same band, your received signal has to be heard above the radio noise.

Resolving Problems

There are several avenues to explore in resolving interference problems.

Analyze the problem:

The first step should always be to survey the problem. If your radio is a UC Wireless LongRanger or WinRouter system, you should do a spectrum scan to determine the frequencies and signal levels of existing RF systems. Once you review the spectrum chart, you can hope to find one or more channels that are free of interference. Be sure to run the spectrum scan several times with a dwell time of 500 ms per channel in order to catch brief occupancies by frequency hopping modems.

When you observe a competing signal, try to identify the source of it. Is it on the site where you are located, or is it a signal being sent to the site by a directional antenna at another site?

Calculate your incoming signal strength

The system that you are trying to establish will have one end on the shared site, and one or more peers distant from the site. For each remote peer, compute the expected strength of their signal when it reaches the shared site. Verify that with the antenna you were planning to use, the desired signal is above the other signals that you see at that frequency. If not, change frequency until you find one that has low enough noise that the signal can get in.

If there is no frequency quiet enough, you must look into:

  • Can you increase the remote transmit power to get above the noise?
  • Can you increase the remote antenna gain?
  • Can you increase the local antenna gain? (This is likely if your new system is a point-to-point link, but not if it is an access point for a multipoint service.)
  • Can you improve the situation by changing polarization?

Can you find a better spot on the tower?

If the interference is from other antennas on the same tower, you may be able to reduce it or eliminate it by moving to a different spot on the tower. If the interference is coming in through the backside of a directive antenna, this helps two ways:
  • Increasing the distance to the other antenna from 3 feet to 20 feet will reduce its signal strength by about 15 dB
  • If the interference is coming in through a "sidelobe", the new location may move it outside the sidelobe and into a null zone
Can you find a better antenna pattern?

Where the number of links to be served from a location is limited, it helps to use the most restrictive antenna pattern that will still cover the subscribers. If there are only two links, you may be able to fit your antenna port with a splitter and use two directional antennas, so long as their beams don't overlap. Likewise, changing a multipoint hub from an omni antenna to a sector or panel antenna improves gain and reduces paths for noise ingress. Note in particular, that flat panel antennas often have very good backside rejection.

Make sure YOU don't create interference for someone else.

Once your own system is working, you need to confirm with the owners of other systems at the site that their systems are still working. It is possible that your transmission is on the frequency that they are receiving on.


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