WiFi Spectrum Analysis: Uncovering RF Interference

One of the key steps that a WLAN engineer can take to improve WiFi performance is to conduct spectrum analysis, which is a method for visualizing the radio frequency in a surveyed area and determining the strength of a signal. In addition to seeing that strength, a spectrum analyzer can find interference that would negatively impact wireless performance.

WiFi spectrum analysis is performed throughout the WLAN lifecycle. Before deployment, spectrum analysis finds interference that may affect a wireless deployment. An engineer can find the interference and either remove it or design around it. After deployment, spectrum analysis determines whether interference exists and engineers often use it for troubleshooting when a WLAN is performing poorly. Spectrum analysis can help optimize a WiFi network by finding the interfering transmitter.

Various kinds of interference corrupt WiFi signals. Common interference sources are:

To find the source of interference, an engineer uses directional antennas in conjunction with the spectrum analysis application. The application will measure the area and plot the interfering signal pattern on the screen and along with its strength.

The process is not easy as the engineer has to distinguish the noise from other valid signals. This is why the directional antenna is used to help pinpoint the location of the interference. 

The closer an engineer gets to the interfering source, the amplitude increases on the application and is indicated, usually, in red.

Understanding WiFi spectrum analyzers

What exactly are we looking for on a spectrum analyzer? The analyzer will pick up all signals on the 2.4 GHz and 5 GHz frequencies. When searching for an interfering transmitter, we are looking for a number of characteristics.

Density is an easy identifier. When you are close to the source of interference, it will begin to show itself on the application. If it’s communicating often, the duty cycle will be recorded and the signal begins to appear red in color.

Other things to note are the RF characteristics. More specifically, you want to take note of its shape. Some signals have distinct shapes. For example, 802.11g ERP-OFDM appears very differently than 802.11b.

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In addition to the shape of a signal, there is also a pattern. Every transmitter has its own signature. Some transmitters hop on different frequencies while Bluetooth looks different from a cordless phone, which looks different from a video transmitter.

Built-in WiFi card vs. a dedicated analyzer

A common question regarding spectrum analysis is why can’t the built-in, or USB, WiFi adapter be used for spectrum analysis?

It boils down to the limitations of WiFi adapters; they’re unable to demodulate non-WiFi signals.

When a WiFi device wants to communicate, it will perform clear channel assessment (CCA) to determine whether the shared medium is busy -- basically, seeing if any RF energy is detected. The kicker is that WiFi adapters can only hear WiFi energy, meaning other WiFi devices such as laptops, tablets, mobile phones, and WiFi cameras. Built-in WiFi cards cannot understand microwave signals or cordless phones transmitting on 2.4 GHz.

Required equipment

Spectrum analysis will require the use of a special adapter capable of reading not only WiFi signals, but non-WiFi signals as well.

Keep in mind that some adapters can only work in 2.4 GHz or 5 GHz. There are also dual-radio adapters, which offer the best flexibility. Using more than one adapter will allow you to capture more data, often by placing one adapter on 2.4 GHz and the other on 5 GHz, for example.

The one I use most frequently is Metageek Chanalyzer with Wi-Spy DBx. Chanalyzer is the application which helps you identify interference and Wi-Spy DBx is the adapter. Other wireless spectrum analyzers include Fluke Networks AirMagnet Spectrum XT, Tektronix RSA306 USB Spectrum Analyzer, and RF Explorer.

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Learn more about wireless infrastructure and supporting the mobile enterprise in the Wireless & Mobility Track at Interop Las Vegas this spring. May 2-6. Don't miss out -- Register now!