On 7 Aug 2005 09:13:17 -0700, adamstoffel@gmail.com wrote:

>I need to make a 7 mile wifi link that will stand up to interference
>like snow. Speed is not an issue. I am fine with only 1Mbps. I was
>thinking about using 24dbi dishes on each end, and radios with TX
>18dbm, and a sensitivity of -80dbm. I will mount the radios in water
>tight enclosures next to the dishes.
>
>Will this work??
>
>Any suggestions? Under $500.

Well, let's do the math using your numbers. Well, maybe not. They're
wrong and need work. The typical receiver sensitivity at 1Mbit/sec is
-89dBm, not -80dBm. That's a huge difference.

Most bridges manage to squeeze out +15dBm, not +18dBm (despite what
the data sheet claims). Just to add some confusion, the power output
is usually slightly higher for 802.11g than for 802.11b.

You also don't wanna run 1Mbit/sec because it hogs way too much
airtime. Methinks the best for a bridge is the slowest ODFM speed of
6Mbit/sec. The sensitivity is roughly the same as 1Mbits/sec. It
should give you 3Mbits/sec thruput, which methinks is good enough.

The object of this exercise is to get a fade margin of greater than
20dB. Any less will result in way too much dropouts and errors.

Receiver Sensitivity for DI-624 (and most other similar radios):
* 54Mbps OFDM, 10% PER, -68dBm)
* 48Mbps OFDM, 10% PER, -68dBm)
* 36Mbps OFDM, 10% PER, -75dBm)
* 24Mbps OFDM, 10% PER, -79dBm)
* 18Mbps OFDM, 10% PER, -82dBm)
* 12Mbps OFDM, 10% PER, -84dBm)
* 11Mbps CCK, 8% PER, -82dBm)
* 9Mbps OFDM, 10% PER, -87dBm)
* 6Mbps OFDM, 10% PER, -88dBm)
* 5.5Mbps CCK, 8% PER, -85dBm)
* 2Mbps QPSK, 8% PER, -86dBm)
* 1Mbps BPSK, 8% PER, -89dBm)

Running the numbers through:
http://www.terabeam.com/support/calculations/som.php

TX Power : +15dBm
TX Coax Loss : -4dBm
TX Antenna gain : +24dBm
Path Loss : (7 miles)
RX Antenna gain : +24dBm
RX Coax Loss : -4dBm
RX sensitivity : -88dbm

yields a fade margin of 21.9dB. Yep, that should work. I would like
to see more than 20dB fade margin, but I don't think that's going to
happen without more tx power.

I'm not sure how much snow will affect the path. Probably not much
except where it detunes the antenna. You might consider an overpriced
radome to fit over the dish.


--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
AE6KS 831-336-2558