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
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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 protected]
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