Subject: Re: Bit errors, is this really an issue ?

From: [email-masked]
Date: Thu, 24 Sep 1998 23:43 -0700 (PDT)

In modern systems (i.e. Ka-band), forward error correction (FEC), such 
     as Reed-Soloman coding, is used to correct link errors including those 
     caused by rain fading. The nature of the code is that it has a rapid 
     "fall off" meaning that either the link is completely clean (BER 
     around 10E-12 or better) or no bits are getting through. 
     
     So, of course, the question is: what is the threshold where the link 
     dissapears in a rain fade. It is not possible to answer this question 
     in general since it depends on the parameters of the link coding 
     (FEC), data rate, transmit and receive antennas, transmitted power, 
     etc.
     
     My understanding (which is rather rudimentary) is that link outages 
     are on the time scale of rain storms (or rain cells, as the link 
     engineers call them). As the rain cell moves into the beam, the 
     received power drops and (correctable) errors are injected into the 
     bit stream. Once the errors reach a certain density the link FEC 
     cannot correct them and the link is down until the rain cell moves out 
     of the beam area and the power increases.
     
     So, to answer your question, I don't think you will find many 
     corrupted packets emerging from a link in rain fade. 
     
     However, these satellite systems are designed to minimize rain fade 
     (while also minimizing transmitted power which drives terminal cost). 
     The NASA ACTS program has supported considerable study of propagation 
     characteristics at Ka band and system developers are using this data 
     to engineer their links.
     
     Hope this helps,
     
     --aaron

______________________________ Reply Separator _________________________________
Subject: Bit errors, is this really an issue ?
Author:  [email-masked] at mime 
Date:    9/24/98 5:55 AM
     
Let us take the most common situation, i.e. rain fade. Can someone 
explain to me its quantitative impact on BER. Is the satellite link 
completely out of order when it rains or can we still find rescue in 
methods like ARQ or some TCP which is able to distinguish corruption 
from congestion ? Is there information available on the noise 
characteristics on modern satellite links ? (BER, distribution of bit 
errors, ...)

RFC-822-headers:
Date: Thu, 24 Sep 1998 14:55:58 +0200
From: Jordi Nelissen - Alcatel Bell Research <[email-masked]>
Subject: Bit errors, is this really an issue ?
Sender: [email-masked]
Organization: http://www.alcatel.com

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From: William D Ivancic <[email-masked]>
Date: Thu, 24 Sep 1998 17:02:22 -0400

At 11:43 PM 9/24/98 -0700, [email-masked] wrote:
>     caused by rain fading. ......Once the errors reach a certain density
the link FEC 
>     of the beam area and the power increases.

Correct.  Basically, the question for FEC systems using Block Codes such as
Reed-Solomon codes is what are the availability statistics rather than what
are the error statistics.

Will
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NASA Lewis Research Center
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USA
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From: Jordi Nelissen - Alcatel Bell Research <[email-masked]>
Date: Fri, 25 Sep 1998 14:39:36 +0200

Dear tcpsat,

thank you for your valuable answers on my question. It made me realize
that there is no such thing as 'the BER of "a" satellite link'. The
problem is indeed (as usual) much more complex. So back to reading !

Cheers, Jordi
-- 
_____________________________________________________________________
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Alcatel Research - Network Architecture - Traffic Technology
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