Comtech EF Data CDM-710, Инструкция по установке и эксплуатации

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CDM-710 Broadcast Satellite Modem Revision 11
Forward Error Correction Options MN/CDM710.IOM
7–2
LDPC also functions differently than Viterbi decoding by using iterative decoding. In this process
the data initially corrected by the LDPC decoder is re-encoded and run through the decoder again
to correct additional errors. Key to this is the soft decision output from the LDPC decoder and a
high-speed processor operating at a rate much higher than the data rate. The LDPC decoder runs
the iterative process as many times as possible before corrected data is finally outputted to make
way for a new block of data entering the decoder. LDPC also uses interleaving to spread the
errors. In contrast, Viterbi error correction operates by passing data through the convolutional
error correction process a single time.
The error correcting capability of LDPC is improved by using large block sizes. This also
increases latency. However, in one-way broadcast applications this is not a drawback. Links with
LDPC normally operate at multi-megabit data rates where latency effects are reduced. The
standard block size for LDPC is 64,800 bits, and for lower data rate applications there is a short
frame block at 16,800 bits that suffers only a small error correcting loss (0.2 to 0.3 dB) compared
to the standard block.
7.3.1 Range of Data Rates
For the range of Data Rates, refer to Sect. 1.5 SUMMARY OF SPECIFICATIONS .
7.3.2 Eb/No, Es/No Spectral Efficiency, and Occupied Bandwidth
Depending upon the operating mode DVB standard uses different modes of specifying performance
with a modem in IF Loop and Additive White Gaussian Noise (AWGN):
•
DVB-S (QPSK with Viterbi and Reed Solomon): BER = 2x10
-4
after Viterbi (before
Reed Solomon) and QEF after Reed Solomon at the specified Eb/No and includes a modem
implementation loss of 0.8 dB and the noise bandwidth increase due to the outer code (10
log 188/204 = 0.36 dB).
Quasi-Error-Free ( QEF ) corresponds to less than one uncorrected error event per hour, or
BER = 10
-10
to 10
-11
at the input of an MPEG-2 demultiplexer. This is the error rate most
commonly used.
•
DVB-DSNG (8-PSK and 16-QAM with Viterbi and Reed Solomon): Similar to DVB-S.
The modem implementation ranges from 1.0 dB (8-PSK 2/3) to 2.1 dB (16-QAM 7/8).
•
DVB-S2 (QPSK, 8-PSK, 16-APSK and 32-APSK with LDPC and BCH): PER (packet
error rate) = 10-7 after LDPC and BCH at the specified Es/No. This is a theoretical value
with perfect carrier recovery and symbol synchronization, and no modem oscillator phase
noise. The manufacturer decides the implementation margin and specifies performance.
The other difference is the use of PER (packet error rate) based upon a 188 or 204 byte
MPEG frame size instead of BER (bit error rate).
Also, note the use of Es/No instead of Eb/No. When links operate at constant symbol rate,
this is a good method for comparing the performance of different modulation types and
code rates.
The relation between the two quantities is given by:
Eb/No = Es/No - 10xLog(Spectral Efficiency)