Alpha Omega Wireless Blog

Wireless Interference - The Effect on Unlicensed Wireless Backhaul

Posted by Joe Wargo on Fri, Apr 16, 2010 @ 06:06 PM

When evaluating wireless backhaul technology, whether point to point wireless, point to multipoint wireless, or wireless mesh, the possibility of radio frequency interference disrupting a wireless link poses a concern. Interference can degrade a radio system's performance and in some cases even prevent the system from functioning at all.

For the purpose of this article "Wireless Interference" is defined as a wireless signal that alters, modifies, or disrupts the desired wireless signal as it travels from the transmitting source antenna to the receiving antenna.

Typical wireless interference is a result caused by the introduction of two or more radio waves being received into the receiving antenna from unwanted radio frequency (RF) signals disrupting the system's communication. Typically these signals are at or near the same frequency as the receive frequency of an established wireless backhaul system. The source of interference is usually from other transmitters operating very close in frequency to the impacted system or caused by "multipath" which is a result of a wireless signal reaching the receiving antenna from two or more paths.

Wireless interference can cause fading or noise on the receiving wireless antenna lowering the quality of signal. Noise is often measured by SNR ("signal to noise ratio") or a relationship of the desired signal quality to the level of undesired or corrupting signal (background noise). This can make it difficult for a wireless system to clearly understand the signal (communications) from the desired transmitting wireless radio. Interference can come into the receiving antenna either in or out of phase. The wireless backhaul system has to differentiate from the signal it should be receiving from its partnered outdoor wireless bridge from the wireless signal it is hearing from the wireless interference source.

This can be thought of in terms much like that of listening to music. Even if the music you are trying to listen to is at a desired volume but there is a lot of background noise, it can be difficult to listen to the music. The background noise can drown out the signal of the desired source causing missed bits of information. In an outdoor wireless backhaul system we see this as errors caused by dropped packets and/or multiple resends having to occur. A wireless link will have a certain threshold of wireless interference it can overcome before experiencing issues. This is often referred to a wireless system's CIR or carrier to interference ratio.

For a wireless backhaul system to operate properly it must maintain a quality receive signal level ("RSL"). Wireless bridges are designed to operate with a certain level of "Fade Margin" that allows the system to operate at a predictable reliability (for most point to point wireless systems 20 to 25dB of Fade Margin is recommended, but many point to multipoint wireless and wireless mesh systems can meet the manufactures requirements at a lower amount of Fade Margin). This means if a system has an RSL of -50dBm and it has a receiver threshold of -72dBm, you'll have 22dB of Fade Margin or the amount of dB signal strength a system can lose before you will experience errors (referred to as BER - Bite Error Ratio) or loss of connectivity.

Even if a wireless backhaul radio receiver has a good RSL the quality of the signal can be distorted by interference. A wireless bridge must have enough Fade Margin to overcome the interfering signal or have good enough CIR to function properly.

Wireless interference in regards to outdoor wireless backhaul often occurs with unlicensed wireless bridges ("license-exempt") operating in the 902-928MHz (spread spectrum), 2.4GHz, 5.3GHz, 5.4GHz, and 5.8GHz frequency bands. Note: 60GHz millimeter wave, often used in gigabit wireless backhaul, is unlicensed but is extremely immune to interference due to its inherent features of narrow beam widths and the occurrence of oxygen absorption over fairly short relative distance.

With unlicensed wireless bridges it can never be guaranteed that the wireless link will operate interference free and with any predictable reliability. Many manufactured wireless backhaul systems can help overcome interference by having a good carrier to interference ratio inherent with the hardware and by proper RF path design and wireless installation. Prior to deploying an outdoor unlicensed wireless backhaul a wireless spectrum analysis should be performed. A wireless installation company should use a proper Spectrum Analyzer to evaluate the amount of potential interfering signals in the desired frequency that is to be used (e.g. if you are deploying a 5.8GHz wireless backhaul system, as defined by the U-NII radio band of 5.725 to 5.825GHz, you would want to capture the amount of potential interference and the source of any particular wireless interference on any given channel in the 5.8GHz spectrum that will be used). A spectrum analysis will only show the amount of potential interference on the date it was taken. Other unlicensed wireless systems may be deployed at a later date that can introduce newer wireless interference.

Wireless Spectrum Analysis

Safe guards to help avoid or overcome unwanted wireless interference can be taken by choosing the appropriate wireless backhaul hardware. Using directional antennas can narrow the wireless signal's beam width, which in turn also narrows the amount listening area, while providing overall higher system gain. Using different polarizations (e.g. vertical vs. horizontal). Many of today's wireless radio systems have the ability to utilize iDFS (intelligent Dynamic Frequency Selection) that allows the wireless radio system to optimize by choosing the best wireless backhaul channel to operate on. Newer wireless bridge systems can also take advantage of using OFDM and MIMO that can help overcome interference.

Best practices should always be used by consulting an experienced outdoor wireless integrator who can perform proper spectrum analysis, path engineering and path calculations, consult on which manufacturer's equipment would be best for the solution, and perform proper wireless installation.


Tags: General, Un-lincesed Wireless, Wireless 101