7 Mile 802.11b in the Snow

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.

Adam

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

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.

Craig discusses some of the totgraphical issues, and David Taylor has an
interesting story. If the 7 mile link doesn't work, maybe you could do a
hop in the middle.

http://www.craig-bartell.com/ documents a 7KM link.
Oops, KM, not M... still, the process works.

http://www.nodomainname.co.uk/Equati..._broadband.htm

--
---
Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5

In the Usenet newsgroup alt.internet.wireless, in article
<4ddcf1lc09kids2dp0ai73vg0vofc77tdb@4ax.com>, Jeff Liebermann wrote:

>I'm not sure how much snow will affect the path.

See if you can find an old copy of CCIR XIII Plenary Assembly in Geneva,
1974, Volume V, Report 233-3. This one is often referenced in stuff
on propagation loss studies. See the Electromagnetic-Wave Propagation
chapter in the ITT Reference Data for Engineers (from Sams) as an
example. On the _path loss_ alone, snow is not a factor below 6 GHz.

From a paper produced for NASA Ames in 1984:

-----------
One might ask if the half inch per hour rainfall rate is realistic. In some
regions of the country, this would be an overkill, while in some other areas,
it is not nearly enough. Other forms of precipitation cause different
effects. Snowfall at a rate of 15 centimeters (5.9 inches) per hour with a
liquid equivalent of 15 millimeters per hour has little effect up to 10 GHz.
At 18 GHz, attenuation is approximately 0.01 decibels/kilometer. Hail has
an attenuation effect dependent on the state. Dry hail is generally small,
compared to raindrops of the same liquid content. As the hailstone (or
snowflake) begins to melt, the attenuation increases to the point (with a
diameter of a tenth wavelength, and 10 to 20 percent melted) it may be
double that of the equivalent liquid content. Fog (or clouds) produce
attenuation based on visibility and frequency. Table 3 lists typical
attenuation rates for various visibilities.

Visibility in meters
Frequency 30 meters 90 meters 300 meters
3.0 GHz 0.02 dB/KM 0.004 dB/KM 0.001 dB/KM
9.4 GHz 0.20 dB/KM 0.04 dB/KM 0.007 dB/KM
24.0 GHz 1.25 dB/KM 0.25 dB/KM 0.04 dB/KM
-----------

>Probably not much except where it detunes the antenna.

or gets into the connectors, cable, what-not.

>You might consider an overpriced radome to fit over the dish.

Depends on the weather, but MAY be a good idea. Don't forget to include
a slight increase in the path loss due to the radome, and the change in
wind loading on the supporting structure..

Old guy