About frequency spectrum from WLAN equipment
Page translated 030423When designing sites with multiple access points practical tests have shown that you get better total throughput with certain equipment than with others. We did a measurement of frequency spectrum for a number of popular WLAN radios to show how they differ. All measurements were made on IEEE 802.11b equipment.
What does the standard say?
The standard IEEE 802.11b states how a frequency spectrum from a radio may look like. 11 MHz from the center frequency (fc) the signal level shall be 30 dB less than at fc. At 22 MHz from the fc the signal level shall be 50 dB less. We can then define this "mask".
spectrum maskWhat does it look like?
We did this measurement on a number of radios. Please note that the measurement is not made according to the IEEE 802.11b standard with RBW 100 kHz and VBW 10 kHz. The relative difference between the measurements is relevant, though. The measurement was made with peak hold.
Most units use the Prism chipset. What differs is mainly the maximum output power.
Linksys WET11 The manufacturer states 15 dBm output power but it is obviously more.
airBridge Output power can be software adjusted 11-17 dBm.
BreezeCom DS.11-BU The manufacturer states 14 dBm output power, but there is a peak at 20 dBm. The side bands are noticably higher than on other radios.
Orinoco Well what can we say? This is something different.All plots were taken on channel 7, fc=2442 MHz, with a HP8563E spectrum analyser.
Conclusion
The Orininoco cards have a far more clean and narrow frequency spectrum than the other radios tested. This also applies to OEM versions like Avaya, HP, Compaq, Dell etc.The new cards with Ruby chipset have a similar frequency spectrum. The receive sensitivity is less though, about -82 dBm at 11 Mbps datarata which is according to the data sheets. From experience we know that most of the old cards have about -85 dBm sensitvity.The above measurements confirm something that we have suspected for a long time, Orinoco equipment can co-exist on the same site with better performance than many other brands.
Please send any questions to Niklas Gunhamn or Tonny Vilnersson.
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