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

From: Luis Miguel Brugarolas <[email-masked]>
Date: Wed, 30 Sep 1998 08:27:42 +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]
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RFC-822-headers:
Date: Tue, 29 Sep 1998 08:11:04 +0200
From: Luis Miguel Brugarolas <[email-masked]>
Subject: Re: Bit errors, is this really an issue ?
Sender: [email-masked]
Organization: SIRE, Sistemas y Redes =?UNKNOWN?Q?Telem=E1ticas?=