Welcome to the CSIR Meraka Institute's "COIN" Blog

Sunday, August 01, 2004

Radio theory and link planning for Wireless LAN (WLAN) - good summary

Radio theory and link planning for Wireless LAN (WLAN)

Everyone should know the free space loss equation in their head

Loss [ dB] = 32.44 + 20(Log(distance[km]) + Log(freq[MHz]))

Useful cable losses

RG58 = 1 dB/m
RG213 = -.6 dB/m
RG174 = 2 dB/m (often used in pigtails)
LMR-400 = 0.22 dB/m

Typical WiFi sensitivity for orinoco cards

11Mbps = -82dBm
5.5Mbps = -87dBm
2Mbps = -92dBm
1Mbps = -94dBm

Typical allowed signal to noise ratios for orinoco cards

11Mbps = 16dB
5.5Mbps = 11dB
2Mbps = 7dB
1Mbps = 4dB

Typical Noise level at 2.4GHz = -100dBm. Compute S/N level eg. at 11Mbps = -84dBm but sensitivity is -82dBm so sensitivity is the limiting factor.

Just worked out that with our 2 8dBi omnis, 2dB loss in the RF cable each side of the link and the 200mW SANOA cards it is possible to acheive a theoretical distance of 5km with a 3dB margin (margin probably a bit tight), 4km will give you a 5dB margin - probably more realistic.

No comments: