Wi-Fi network interference, analysis, and
optimization
Kenneth Wyatt - February 15, 2015
Wi-Fi and Interference
One aspect related to EMC today is that, with the proliferation of embedded wireless systems in all
imaginable products, comes the risk of increasing interference and resulting slowdown in data
transfer. As we transition to a more mobile society, the number of Wi-Fi (home appliances and
control) and Bluetooth-enabled devices (speakers, headset, watches, etc.) is rapidly increasing. Add
to that the increasing number of residential control (lighting, thermostats, and security systems)
utilizing wireless technology. But, more importantly, if you live or work in the city, it’s not unusual to
have ten, or more, separate 2.4 GHz Wi-Fi access points (APs) or “hot spots” within range of your
own system. Fortunately, I live out in the country, but my system can still “see” a half-dozen APs
nearby. If two, or more, nearby APs are using the same channel, the result can be a slowdown in
data transfer for both systems. Add to this many wireless phones, baby monitors, and microwave
ovens, all operating in this same 2.4 GHz Industrial, Scientific, and Medical (ISM) band and you can
imagine the resulting interference issues possible.
The above are only the most common sources of interference in the 2.4 GHz ISM band. Here is a
more complete list:
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Wi-Fi (IEEE 802.11b, 802.11g, and 802-11n
Bluetooth (IEEE 802.15.1)
ZigBee (IEEE 802.15.4)
Microwave ovens (2.450 GHz center frequency)
Cordless telephones and baby monitors
Custom FCC Part 15 devices (Internet of Things)
Microwave lighting
Licensed users (Amateur radio at 2.39 to 2.45 GHz)
Wi-Fi
While Wi-Fi is allocated to two different bands (2.4 and 5 GHz), predominantly most APs, computers,
and mobile devices, still operate in the 2.4 GHz band. Here’s the worldwide channel allocations for
Wi-Fi in this band (“ETSI” is the European channel plan).
Figure 1 - Wi-Fi channel worldwide allocations. Figure, courtesy Robert Morrow (www.wirelessseminars.com).
Wi-Fi uses Orthogonal Frequency Division Multiplexing (OFDM) for 802-11a, g and n, and variations
of Direct Sequence Spread Spectrum (DSSS) for 802-11b. Wi-Fi also uses a “listen-before-talk”
scheme called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to avoid collisions
with other APs on the same channel. Of course, the more APs there are on a given channel, the more
likely the data throughput will be slowed due to the forced CSMA/CA activity and repeated data
packets.
As you can see, the number of channels is rather limited. But, the news gets worse from a practical
standpoint, because these channels are overlapping in order to fit within the limited ISM band. In
the U.S., there are 11 channels on 5 MHz spacings with just three non-overlapping channels; 1, 6,
and 11 (in Europe; channels 1, 7, and 13). When two APs operate on the same channel, collisions are
reduced because they both use CSMA/CA and each can sense a transmission from the other. Rather
than jam each other, they instead coordinate their transmissions so both can support a reasonable
throughput. Unfortunately, most AP manufacturers set the default channel to one of these channels,
so the chances are pretty good your own AP will be clashing with your neighbor’s. However, if any
nearby APs are set to one of these overlapping “rogue” channels they will create havoc to the
“standardized” channels on either side, because adjacent channels are not recognized as valid APs.
Therefore, it’s best to stick with channels 1, 6, or 11.
Figure 2 - There are only three non-overlapping Wi-Fi channels in the U.S. 2.4 GHz ISM band.
Figure, courtesy Robert Morrow (www.wireless-seminars.com). Bluetooth and Wi-Fi Coexistance
Bluetooth
As you may be aware, Bluetooth (BT) devices operate in the same 2.4 GHz ISM band as Wi-Fi, with
channels starting at 2402 and ending at 2480 MHz, with 1 MHz channel spacings. BT uses a
Frequency Hopping Spread Spectrum (FHSS) protocol and has a moderate data rate of 1, 2, and 3
Mb/s. The range is generally limited to about 10m.
Figure 3 - Example of a few hop-channels for Bluetooth. Figure, courtesy Robert Morrow
(www.wireless-seminars.com).
Wi-Fi and Bluetooth Coexistence
You may wonder how BT avoids interfering with Wi-Fi, and visa versa. The earliest BT devices
indeed had an issue with collisions, but later 802.15.1 standards included the concept of Adaptive
Frequency Hopping (AFH). Basically, BT devices evaluate interference on a particular hop channel
and if it senses a Wi-Fi channel in use, will automatically avoid that channel. Recently, BT Low
Energy has been developed and uses a similar AFH protocol and channel scheme, except the data
rate is nominally 1 Mb/s.
Figure 4 - Graphical example of Wi-Fi channels with Bluetooth adaptive frequency hopping (AFH)
used to avoid collisions. Figure, courtesy Robert Morrow (www.wireless-seminars.com).
Here’s an example of coexistence using the Thereby Thandar PSA2702T swept frequency spectrum
analyzer. Note the Wi-Fi signal on channel 6, with the BT frequency hopping occurring on both sides
of the Wi-Fi channel.
Figure 5 - Spectral plot of Wi-Fi channel 6 with Bluetooth frequency hopping while avoiding the WiFi signal.
By using a real-time spectrum analyzer, we can resolve even more information. Using the Tektronix
RSA306 USB-based real-time spectrum analyzer, it’s possible to see three different APs on the same
channel, along with several BT hop frequencies. I have persistence turned on, so the latent image of
past BT hops and Wi-Fi bursts may be observed.
Figure 6 - A real-time spectral plot with spectrogram (waterfall) display showing a Wi-Fi channel
with Bluetooth hopping while avoiding the Wi-Fi signal. Note that because of the extremely fast
capture rate, you can observe three separate Wi-Fi APs on the same channel. This would be
impossible to see on a normal swept frequency spectrum analyzer.
Interference Issues
Interference Issues
As mentioned previously, multiple Wi-Fi APs can degrade the data throughput of systems set up on
the same, or adjacent, channels. What happens when we add other interfering devices? For example,
many wireless telephones operate using FHSS in the same 2.4 GHz ISM band. These phones do not
have anti-collision protocols and they’ll transmit over the top of existing Wi-Fi and BT devices,
thereby causing multiple “resends” of the packetized data streams. Obviously, placing your wireless
phone near your AP would compromise the data throughput whenever the phone was in use.
Figure 7 - An example of wireless telephone interference to Wi-Fi channel 6.
How about microwave ovens? The magnetron operates at a (nominal) center frequency of 2.450
GHz. However, observing the actual signal using “max hold” on the PSA2702T spectrum analyzer,
we see that the oven wipes out a large portion of the 2.4 GHz ISM band. Certainly, channel 6 would
be the wrong channel to use in the proximity of this particular oven.
Figure 8 - An example of microwave oven interference to Wi-Fi channel 6.
Here’s another example captured with the Tektronix RSA306 real-time spectrum analyzer. You can
see that the analyzer is fast enough to have captured a couple of the microwave pulses, which occur
every 16 ms. These pulses are derived from the half-wave rectifier in the magnetron power supply.
This view includes the maximum (40 MHz) real-time bandwidth of the RSA306, which is less than
half the total bandwidth of the 2.4 GHz band. Note, however, nearly the entire 40 MHz is unusable
due to the level of interference. The only saving grace is that microwave ovens are only used briefly.
Figure 9 - A real-time spectral plot showing the signal from a microwave oven. The blue persistence
zone is a picture of the actual interference, while the orange line is an instantaneous capture of the
magnetron signal. Note the pulsed 16 ms periodicity, due to the half-wave rectifier in the magnetron
power supply. Solutions, Conclusions and References
Solutions
Obviously, we should avoid deploying APs near interfering devices and appliances when depending
on Wi-Fi systems for fast throughput, like streaming video. As for multiple Wi-Fi systems located
close together, the best route is channel management and possibly moving your APs to the 5 GHz
ISM band (802-11ac or 802.11n), where there are far fewer interference sources. Another advantage
is that the 5 GHz ISM band has 23 non-overlapping channels from which to select.
There are a number of Wi-Fi network evaluation software packages for both Windows and Mac
operating systems that can help you evaluate both 2.4 and 5 GHz wireless networks. I use the “Wi-Fi
Explorer” from Adrian Grenados (pictured) for the Mac in the example below. The advantage is that
the software can use\utilize the built-in Wi-Fi 2.4 and 5 GHz receivers built into the laptop to capture
all the nearby APs, including signal strength, network names, and Service Set Identifiers (SSIDs).
Equivalent Windows software would include Kismet (www.kismetwireless.net) or inSSIDer
(www.inssider.com).
Figure 10 - “Wi-Fi Explorer” screen capture showing how the software can identify all the nearby
access points, along with system names, SSIDs and signal strength. to that most APs are located in
one of the three non-overlapping channels. Notice one AP is set to a "rogue" channel 10 and so will
cause extreme data slowing to those APs on channel 6 and 11.
Also useful in managing your Wi-Fi network is “heat-mapping” software, such as the Mac-based “WiFi Scanner” from www.netspotapp.com. Equivalent Windows software might include Ekahau
(www.ekahau.com). By creating a simple floorpan of your home or facility using the built-in drawing
tool and then walking around with your laptop in hand, you can map out the signal strength of a
particular Wi-Fi system. By observing “dead zones”, you can relocate the AP to a more suitable
location or add “slave” APs to fill in the gaps.
In addition, for best system performance when designing a large system of multiple APs, be sure to
set their channels to 1, 6, or 11 in an alternating selection of channels and in patterns of slightly
overlapping reception ranges. For example, the first set of four APs could be set to channel 1, 6, 11,
then back to 6 or 1, etc. In other words, be sure there are no two adjacent APs set to the same
channel by maximizing the distance between any two APs on the same channel.
Figure 11 - An example of a Wi-Fi “heat map” of my own system. Mapping out the signal strength is
useful in managing and optimizing Wi-Fi networks.
Conclusion
With the proliferation of wireless devices and networking systems, the risk of decreased data
throughput will only increase. By using low cost spectrum analysis tools or software-based Wi-Fi
network analyzers and mapping tools, interference sources may be identified and wireless networks
managed and optimized.
Acknowledgement
I’d like to thank my long-time friend and colleague, Dr. Robert Morrow, for the kind use of several of
his graphics. He specializes in wireless and Bluetooth networking and provides public and in-house
seminars on these topics. He may be reached via his web site, www.wireless-seminars.com.
For More Information:
Real-time spectrum analyzers
Tektronix RSA306 Review (Part 1 and Part 2)
Another good real-time spectrum analyzer would include the Signal Hound BB60C. See the review
here (Part 1 and Part 2).
Swept frequency spectrum analyzer mentioned: Thurlby Thandar PSA2702T spectrum analyzer
review.
References
How You and Your Neighbor Are Making Each Other’s W-Fi Worse, How-To Geek.
Dr. Robert Morrow, public and in-house wireless and Bluetooth seminars.
www.wireless-seminars.com