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

From: Heidi E Anderson <[email-masked]>
Date: Thu, 24 Sep 1998 11:55:36 -0500

Jordi,

While there is much information on BER and FEC's as they pertain to 
satellite links, I thought it might be helpful to explain the basic usage 
of BER in a satellite network.  BER is simply one measurand of satellite 
link performance and is typically the parameter chosen to represent this 
contractually.  Therefore, it is also usually provided as a user available 
statistic in "some form".

In designing a satellite link, the coding, power, receiver sensitivity, and 
expected link noise are all accounted for in configuring the channel for a 
certain availability.  The receiver sensitivity is a function of absolute 
power and gain of the signal over the noise floor referred to as 'energy 
per bit per noise energy normalized over 1 Hz' (Eb/No).  Of course, classic 
text book digital communication theory calculates error performance as a 
function of Eb/No and there are numerous sources on expected performance 
curves depending on modulation and coding.  At an earth station user level, 
the performance of the link is often represented as Eb/No or 'Signal 
Qualilty'.  This is useful as a referential measure but as an absolute 
measure, the calculation varies among satellite equipment providers.  Some 
calculate the actual BER of the data stream and then back out the Eb/No via 
calculation while others directly measure the energy in the signal to some 
given accuracy.

Either way, satellite channels are designed with a margin to overcome 
interference from a number of sources including rain to a certain level of 
required availability.  As the Eb/No decreases at the receiver, the BER 
will continue to increase until eventually accurate decoding is not 
possible.  Depending on the protocol, TCP or otherwise, this will typically 
result in slowed response time as the signal degrades.  Specifically in the 
case of rain fade, there are are rain rate models, such as Crane, that 
specify an experimentally derived expected rain rate and rain drop size for 
a particular geographic region.  These parameters are used to calculate an 
interference level to overcome for a region.  In addition, the rain affects 
the noise temperature of the sky as seen by the receiving station.  These 
two factors are considered in the total link budget to increase the 
probability of error-free transmission to a level as stated by the 
availability figure.  Even when transmission becomes impossible to decode, 
protocol can be configured maintaining the session link to resume 
transmission upon channel recovery.

Therefore, yes, BER can improve with the application of different coding 
and error-correction techniques.  However, the BER as it relates to 
availability, is part of a set of variables in the link budget equation 
that are adjusted according to need of the user and equipment provider. 
 For example, a better FEC may be implemented to reduce antenna size, 
reduce transmission power, or improve spectral efficiency by implementing 
higher-order modulation all while keeping the BER constant.  Of course, 
let's never forget the engineering vs. business game of design cost.

Due to the cost of satellite capacity and the desire to make end-user 
equipment more estecically pleasing to improve wide-spread usage, the above 
options are usually chosen and BER is maintained at 1 x 10^-6 or 1x 10^-7. 
 In a pure R&D environment, however, we can certainly consider BER as the 
variable keeping many other parameters constant and see where that leads. 
 Hopefully, this helps explain the current usage of these parameters in 
production digital satellite networks.

Heidi E. Anderson			Enterprise Rent-A-Car
WAN Engineering				600 Corporate Park Drive
314.512.2057				Clayton, MO 63105
314.512.4057 FAX
[email-masked]

On Thursday, September 24, 1998 7:56 AM, Jordi Nelissen - Alcatel Bell 
Research [SMTP:[email-masked]] wrote:
> Dear tcpsat,
>
> I have a question with respect to the effect of BER on TCP performance
> on satellite links. From draft-ietf-tcpsat-stand-mech-06.txt, I read :
>
> 'Typical bit error rates (BER) for a satellite link today are on the
> order of 1 error per 10 million bits (1 x 10^-7) or less frequent.
> Advanced error control coding (e.g., Reed Solomon) can be added to
> existing satellite services and is currently being used by many
> services.  Satellite error performance approaching fiber will become
> more common as advanced error control coding is used in new systems.'
>
> As I understand this, in optimal operation, a modern satellite link can
> approach fiber BER, provided adequate FEC is used. Further on I also
> read :
>
> 'FEC should not be expected to fix all problems associated with noisy
> satellite links.  There are some situations where FEC cannot be expected
> to solve the noise problem (such as military jamming, deep space
> missions, noise caused by rain fade, etc.).'
>
> 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, ...)
>
> thanks, Jordi
> --
> _____________________________________________________________________
> Jordi Nelissen - [email-masked]
>
> Alcatel Research - Network Architecture - Traffic Technology
> Francis Wellesplein 1, 2018 Antwerp, Belgium
> Phone: external: +32(0)3240 7192, alcanet: +6057192
> Fax:   external: +32(0)3240 9932, alcanet: +6059932
>
> Visit our webpage :
> http://www.rc.bel.alcatel.be/projects/networkarchitecture/traffic
> _____________________________________________________________________

---

From: "Marc Dumon" <[email-masked]>
Date: Thu, 24 Sep 1998 22:20:22 +0200

Dear Jordi,
I'll try to give you an answer to your question:

For a satellite link the BER is a specified requirement. For a specific BER
one can determine the Eb/No (Energy per bit / Noise spectrum density) from
the formula
" Probability of making an error=1/2 * erfc(Eb/No)^(1/2). (erfc is the
complementary error function)."

Example : if the satellite link requires a BER of 10^-5, the above formula
gives us a Eb/No of +/- 9dB.
Conclusion : The BER can be reduced by increasing the [Eb/No](dB). This can
be achieved by increasing the transmitted power (Eb)
and/or reducing the noise (No).
In a satellite link the Eb/No can be increased by using bigger antenna's,
more power, and/or better equipment (with lower system noise).
It's logic that the cost of the equipment increases aswell.

Another technique to reduce the BER (for a fixed Eb/No) is by using error
control coding.
There are two important types of error control coding :
Error detecting code (ARQ) and forward error correcting (FEC).
Error detecting code can detect that one bit in a code word is wrong but it
cannot know which bit it is. The complete word must be retransmitted (ARQ).
With forward error correction (FEC) a the wrong bit is corrected without
retransmission.

The "coding gain"is the difference in Eb/No with and without coding.
FI : Coding gain for BER = 10^-6 with FEC (7/8) = 3,7dB; for FEC(1/2) =
6,4dB; and for FEC(1/2 + Reed Solomon) = 7.9dB.
Example : for a certain system with a given Eb/No of 3dB the BER increases
from +/- 5*10^-1 (!) without error coding
to +/- 10^-7 if FEC (1/2) with ReedSolomon is used.
To achieve the same quality (BER = 10^-7) without error correction you need
an Eb/No of +/-12dB.

So your conclusion is correct : a satellite link can approach fiber BER,
provided adequate FEC is used and/or enough power is available.

Now the rain :
This is more complex. The rain attenuation increases with increasing
frequency (Ku-band is more sensitive to rain than C-band),
and depends also on the polarization (attenuation for horizontal
polarization is much bigger that for vertical polarization)
and the geographical location, the time of the year and the elevation.
The ITU publishes statistical tables in which the world is divided in
different rain zone's. (Belgium is in rain zone E)
This tables show the fraction of time that a given attenuation will be
exceeded.

Example : for Belgium at 12 GHz and H-polarization :
An average of 99.999% of the year the rain attenuation does NOT exceed
7,38dB (0,088h/year it exceeds 7,38dB)
99,991% of the year the rain attenuation does NOT exceed 3,42dB (0,789h/year
it does)
and 99,000% of the year the rain attenuation does NOT exceed 0,41dB.

If you have a system with an Eb/No of +/-8dB without error correction you
will have a BER of +/- 10^-3 under clear sky condition.
An average of 0,088h/year the rain attenuation exceeds 7,38dB resulting in a
broken link.
An average of 0,789h/year the  rain attenuation exceeds 3,42dB resulting in
an BER of > 10^-2
and 99,000% of the year the BER will be +/- 10^-3.

Now if yo use exactly the same system but with FEC + RS you will have a BER
of <<10^-7 with a Eb/No of 8dB.
An average of 0,088h/year the rain attenuation exceeds 7,38dB still
resulting in a broken link.
But average of 0,789h/year the  rain attenuation exceeds 3,42dB resulting in
an BER of <10^-7.

I hope I gave you an answer on you question, and if you need additional
information,please do not hesitate to contact me.

Marc Dumon

 _____________________________________________________________________
Marc Dumon
ORBIT satellite telecom nv.
Lange Leemstraat 144A
B-2018 Antwerp
Belgium
Tel : +32-95-23.86.94
Fax : +32-3-239.86.95
e-mail : [email-masked]

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On
> Behalf Of Jordi Nelissen - Alcatel Bell Research
> Sent: Thursday, September 24, 1998 2:56 PM
> To: tcpsat
> Cc: Jordi Nelissen@RC
> Subject: Bit errors, is this really an issue ?
>
>
> Dear tcpsat,
>

---

From: "Marc Dumon" <[email-masked]>
Date: Thu, 24 Sep 1998 22:29:47 +0200

Dear Jordi,

Now the rain :

Marc Dumon
____________________________________________________________________
Marc Dumon
ORBIT satellite telecom nv.
Lange Leemstraat 144A
B-2018 Antwerp
Belgium
Tel : +32-95-23.86.94
Fax : +32-3-239.86.95
e-mail : [email-masked]

---

From: Luis Miguel Brugarolas <[email-masked]>
Date: Mon, 28 Sep 1998 14:57:54 +0200

Dear tcpsat,

I am a newcomer to the list, and I have just seen the BER discussion.

I do have may years experience in the design of satellite modems, and there 
is an undisclosed effect at low Signal To Noise Ratio. When symbol rate is 
low or demodulator chain local oscillator chain has not pure enough 
oscillators (poor phase noise), the demodulator performance degrades 
sharply, much worse than the thermal noise erfc function.

Poor phase noise produces frequent cycle slips: the carrier tracker looses 
a cycle and delays or advances itself PI/2 (for QPSK). In an uncoded 
transmission, this procudes few errors, but the effect is terrible for FEC, 
so that it is needed to restart synchronization, thus producing burst 
errors of thousands of bits (somehow user configurable), thus uncorrectable 
by RS or other mechanisms. These bursts of errors ruins the long term BER 
and makes the link virtually unusable for very low SNR.

As previously stated, this effect can be compensated by proper hardware 
design, but for low cost commercial systems, this cannot be given for 
granted.

Best regards

Luis Miguel Brugarolas

-------------------------------------------------------------
Luis Miguel Brugarolas     SIRE, Sistemas y Redes Telematicas
Parque Tecnologico de Madrid                 c/Isaac Newton 1
28760 TRES CANTOS (madrid)                              SPAIN
Tf: +34-1-8043580   Fx: +34-1-8043717  email: [email-masked]
-------------------------------------------------------------

---

From: Luis Miguel Brugarolas <[email-masked]>
Date: Tue, 29 Sep 1998 08:11:04 +0200

Dear tcpsat,

Best regards

Luis Miguel Brugarolas

---

From: nghia PHAM <[email-masked]>
Date: Wed, 30 Sep 1998 15:45:22 -0400

On 28-Sept, you wrote

>transmission, this procudes few errors, but the effect is terrible for
FEC, 
........

The phase noise problem is critical only for low symbol rate transmissions
(3 Msymbols/s or under). For Direct-To-Home transmission, the symbol
rate used is within 20-30 Msymbols/s.
Even with the poor consumer front-end (phase noise of -70dBc at
10 kHz offset), we have never seen cycle slip with our DVB services (27.5
Msymbols/s)

Nghia PHAM
Digital System Engineer
EUTELSAT- Paris
Fax:33 1 53 98 47 98
Tel: 33 1 53 98 47 21