Sorry if some of you received this message twice.
This message is to contribute to the discussions about error
characteristics on satellite links.
It supports the statement that, if properly engineered, satellite
links do exhibit an on-off characteristic similar to fibers.
1) The DVB standard (www.dvb.org) for Direct Video Broadcasting
uses concatenated FEC to sensure quasi-error-free satellite link
transmission.
Basically, the FEC is composed of two codes:
- the outer code is a block code of Reed-Solomon type (204,188,t=8)
meaning that it can correct 8 erroneous bytes in a block of 204 bytes
(188 useful + 16 parity)
- the inner code is convolutional code of constraint length 7
(state-of-the-art).
The most commonly used ratio for the inner code is 3/4, meaning one
parity every 3 useful bits.
Schematically (simplified):
User bytes->RS outer code->Conv. inner code->satellite link (add noise)
-->Conv. inner decode ->RS outer decode->reconstructed user bytes.
The US DSS standard uses a similar coding scheme.
2) Define Raw BER as BER before decoding by inner FEC (BER after
demodulation).
It can be shown that:
Raw BER= 2E-2,
==> BER =2E-4 after inner FEC (with soft decision decoding)
==> BER<1E-11 after outer FEC decoding
This is less than one error event per hour, at 40Mbit/s user bit rate.
(I have checked that in my lab).
3) Furthermore, it can be shown that:
Raw BER =1.6E-2
==> BER=7E-5 after inner FEC decoding
==> BER<1E-15 after outer FEC decoding
That is less than one error event per year.
4) Furthermore, it can be shown that when the RS cannot
decode it produces very large strings of errors. Usually, the block
code synchronisation fails as well. This occurs
at raw BER=5E-2 approximately.
This is why we prefer to talk about error events rather than
BER at the user bits level.
5) For digital TV broadcasting in the Ku-band (satellite frequencies
in 10.7-12.75 GHz) we at EUTELSAT dimension the satellite link so that the
raw
BER of 2E-2 is not exceeded more than 99.7% of the worst month
( the month of the year when it rains more than any others),
and for the worst-case interference scenario.
Current statistics show that, for a properly installed consumer
dish (60cm), there are no more than 5 signal
outages in the year, each lasting about 5 to 20 minutes. Higher dish
size offer less outages. Increasing by 40% the dish size gives an
additional
3 dB margin, resulting in 99.9% availability in the worst-month.
(the trade-off is between the aesthetic of your balcony/garden against
the last minutes of signal availability).
Again, when the signal gets below threshold, even by a tiny 1 dB, the
link is completely corrupted. When the signal is 0.5 dB above threshold,
the link is error-free. There is no more soft failure in digital satellite
transmissions!
6) The chips which implement the FEC decoding include as well
the demodulator (QPSK). They can handle user data rate of up to 60 Mbit/s.
They offer several inner FEC ratios: 1/2, 2/3,3/4,5/6,6/7,7/8,
automatically
detected.
They sell for less than 15$ thanks to the economy of scale of the consumer
market. Therefore there is no reason why, today, satellite links
cannot be designed to offer error free. Even in situations of jamming by
an intentional enemy, the satellite link when properly designed, exhibit
an on-off pattern exactly like fibers.
7) Still some issues:
In my opinion, the real trouble comes when a satellite link (or a long
fiber) is
concatenated with an error prone network (such as a congested network with
paquet drop).
That's where you get the combined effect of long delay and unreliable
links.
If satellite gateways could check the BER of its incoming traffic (user TCP
checksum), it could deny corrupted packets to get into the long delay link.
There remain the case where the error-prone network is downstream the
satellite link.....May be a good rule of practice is to design the
inter-networking
so that the satellite link is always closest to the end point of use, if
that's ever
possible.....
Nghia PHAM
Digital System Engineer
EUTELSAT - European Telecommunications Satellite Organisation
70 rue Balard - PARIS, FRANCE
http:www.eutelsat.org
ph:33-1 53 98 47 21, fax: 33-1 53 98 47 98
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:38 EST