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lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 edit

Rain fading charts for USA/Canada

Click for full size
ITU rain regions

rain outage by region
Click for full size
rain attenuation table
A very critical 60Ghz link failed this week due to moderate rain even though vendor techie had "guaranteed" this would never happen based on their in-house calculator.

Vendors use 1970s ITU region charts for predicting how many minutes per year a link could be down in a particular region. El-Nino, global warming, etc has changed the picture a bit.

Needless to say I told the IT guy "I told you" a few times as I predicted this would happen on many rainy days based on this simple chart by ITU/IEEE.

But it is quite easy to do your own due dilligence before spending $$$$$ on a link.

1. Ask vendor to provide realistic SNR margin requirement for highest throughput totally ignoring rain fade assuming you will deploy in the Sahara desert.

2. Ask if radio automatically adjusts speed as bit-error rates increase or decrease, and the SNR required for each of the lower speeds - again assuming Sahara deployment.

3. The numbers could be like 6dB for QPSK, 12dB for QAM16, 18dB for QAM64 and 24dB for QAM256, etc. You can be more conservative by adding 3dB.

4. Calculate "dry" link margins using as accurate radio and antenna specs as you can get from vendor.

5. Now look up the dB attenuation in various types of rain for your radio's operating frequency from the bottom chart. Multiply by link distance and subtract from "dry" link margin to calculate "wet" link margin - hopefully it is a positive number.

Hahausuck
Premium
join:2003-12-14
kudos:2

I was taught to always factor for somewhere around 35dB of fade margin.

Even with todays equipment and how it can operate down in the muddy areas of the spectrum, I still do not trust it. Critical links (like backhauls) always get factored with plenty of fade margin.
--
"No job is so important, and no service is so urgent that we cannot take the time to do it safely."
-- AT&T



John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:5

said by Hahausuck:

Critical links (like backhauls) always get factored with plenty of fade margin.
At what point are you over-driving the receiver though?

When is too much..."too much"?
--
A is A

LLigetfa

join:2006-05-15
Fort Frances, ON
kudos:1
reply to Hahausuck

said by Hahausuck:

I was taught to always factor for somewhere around 35dB of fade margin.
35dB from what reference? 35dB above what would lose the signal entirely or 35dB above an acceptable speed/latency/jitter threshhold?
Are we still talking about 60GHz?

Hahausuck
Premium
join:2003-12-14
kudos:2

Too much is when you are breaking EIRP limits



Or if your radio does not have enough bandwidth in the front end, and it over-loads the hell out of it. If this is the case then you probably should switch manufacturers.

If you are very close, naturally in those instances one should use proper judgement.

30-35 above your receive sensitivity. In my experience this has been around -60dB, but this is all relative as it depends on the equipment you are using. You can, it seems bend the rules depending on the radio system involved.

I should also add that one might consider other environmental factors such as dust storms. It is fairly dry where we are, but we had two dust storms last summer that knocked an old microwave system off the air. I do not know of when the system was last PM'd, so it may be that the antenna system needs checking.

--
"No job is so important, and no service is so urgent that we cannot take the time to do it safely."
-- AT&T


cmaenginsb1
Premium
join:2001-03-19
Palmdale, CA
reply to lutful

For backhaul links in 5 Ghz we would use a 20 db minimum difference between SNR or recieve threshold which ever is higher. (For example recieve threshold might be -90 but noise is -85 and RSSI was -65). Design minimum was 30 db from reciever threshold.

We prefer to get the reciever threshold and tx power from the manufacturer and not rely on their charts or other information. From this you can calculate fade margin and apply climate margins as well.
--
CCNA, Comtrain Certified Tower Climber


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to Hahausuck

said by Hahausuck:

I should also add that one might consider other environmental factors such as dust storms.
While rain and snow attenuation has been researched a lot, dust and smog has not been covered as well.

BTW most vendors provide a table of RX sensitivity that often take into account the higher SNR requirements I mentioned for different modulation.

If they provide a single number only (often around -85dBm) it is for their lowest complexity modulation (maybe QPSK) so adding 30dB above that for QAM64 is good practice.