Interferencia do zigbee

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MICAz-Based ZigBee
and WiFi Coexistence

Avoiding RF Interference Between WiFi and Zigbee
This application note discusses how to avoid RF interference when deploying WiFi and IEEE
802.15.4 / Zigbee radios simultaneously or in close proximity. The testing and deployments
conducted for this application note used Crossbow’s MICAz Zigbee-ready wireless Smart-Dust
sensors and a CrossbowStargate Gateway running both high-power and low-power WiFi cards.
When properly configured, the issue of RF interference and lost data can be avoided. However,
without proper care and software configuration serious interference issues can occur.
Disclaimer: Crossbow does not represent this as a complete study, but it is merely practical tips
for deployment based on our experience in the field. Wealso can’t confirm whether these test
would be relevant for other manufacturers hardware.
WiFi and IEEE802.15.4 / ZigBee Sprectrum
To understand the potential for problems, a review of the RF spectrums and available channels
for WiFi (802.11b/g) and Zigbee (802.15.4) is shown below. Since the RF channels in ZigBee
and WiFi overlap there is a cause for concern. This concern has also been shownto be an issue in
field testing, as discussed later in this application note.

Table 1: WiFi RF Channels

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MICAz-Based ZigBee
and WiFi Coexistence
Figure 1 shows the RF channel spectrum of IEEE801.15.4 / ZigBee against IEEE802.11b / WiFi.

Figure 1: IEEE802.15.4 and IEEE802.11 Channels
What is an RF Spectrum ?
To users who are new to RF and Wireless technologies, an RF spectrumplot as shown in Figure 1 is a
graphical way of showing how much energy and at what frequencies a radio operates. The Y-axis is the RF
energy or power, and the X-axis is the frequency. Think of the frequency as the “radio station”, that the
device transmits on. Because both ZigBee and WiFi are “spread spectrum”, they do not operate on a single
radio channel, they actually occupy bands offrequencies. This results in the hump shape shown above.
The radio channel assignment is made by reference to the center frequency of the band (i.e., the center of
the hump). As you can see WiFi channels are wider in frequency than ZigBee. This means that WiFi
occupies more RF spectrum per channel than ZigBee.

Field Trials with MICAz and Stargate using Low-power and High-power WiFi
A verycommon field deployment combines a MICAz (ZigBee-ready radio) based sensor network
and a WiFi network using Stargate. A number of field trials were conducted to examine the RF
interference patterns while running this combination. The results presented here are based on a 6
node MICAz network and Stargate. Each MICAz node was running our Xmesh (surge_reliable
configuration) networking layer on basestation and remote nodes. In the results, we varied the
RF Channel selections and compared the packet delivery success rate and percentage of duplicate
In addition the first test was run with no WiFi card card attached to the Stargate. Another group
of tests were run with a standard power 802.11b WiFi card – Netgear MA701. A final group of
tests were run with a high power (up to 23dBM) WiFi card – SMC Networks SMC2532W-B.
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MICAz-Based ZigBee
and WiFi Coexistence
During the tests with WiFi enabled, there was continuous traffic on the WiFi channel including a
circular retransmission of an 8 MByte file across the WiFi network. The WiFi channel 3 – i.e.
2.422 GHz central frequency was used to connect to the access point. The output power of the
MICAz was atmaximum RF power.

Figure 2: Test Topology

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MICAz-Based ZigBee
and WiFi Coexistence
The packet delivery rates show that when the MICAz’s ZigBee and the Stargate’s WiFi channel
overlap the packet delivery rate is reduced from 100%. When the channels are separated further
in frequency, the packet rate is in the normal 99-100% rate. As expected, the degradation is more...