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Radio theory and link planning for Wireless LAN

Radio theory and link planning for Wireless LAN

Contains several forms for calculation of Power in terms of dBm, Loss in a co-axial cable, theoretical antenna gain, radiated power from antenna, free space loss, receiver sensitivity, signal-to-noise ratio, Fresnel ellipsoid, and a lot more.
Signal to Noise Ratio Example

Receiver sensitivity is not the only parameter for the receiver, we have also to take into account the signal to noise power ratio. It’s the minimum power difference to achieve between the wanted received signal and the noise (thermal noise, industrial noise due for example to microwave ovens, interering noise due to other WLAN on the same frequency band). It is defined as:

Signal/Noise Ratio [dB] = 10 * Log10 (Signal Power [W] / Noise Power [W])

If the signal is more powerful than the noise, signal/noise ratio (also called S/N ratio) will be positive. If the signal is buried in the noise, the ratio will be negative. In order to be able to work at a certain data rate the system needs a minimum S/N ratio:

  • Orinoco PCMCIA Silver/Gold: 11Mbps => 16 dB ; 5.5 Mbps => 11 dB ; 2 Mbps => 7 dB ; 1 Mbps => 4 dB.

If the noise level is very low then the system will be limited more by the receiver sensitivity than by the S/N ratio. If the noise level is high then it will be the Signal/Noise ratio that will count to achieve a given data rate. If the noise level is high we will need more received power. In normal conditions whithout any other WLAN on the frequency and whithout industrial noise the noise level will be around -100dBm. For example to achieve a 11 Mbps data rate with an Orinoco 802.11b card we would need a received power 16dB higher (S/N ratio) so a level of -100+16=-84 dBm but in fact the minimum receiver sensitivity is at -82 dBm…higher than -84. It means in that case the mimimum receiver sensitivity is the limiting factor for the system.

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