Comtech EF Data CDM-840 - Manual (Page 182)

CDM-840 Remote Router Revision 2
Appendix B MN-CDM840
B–2
LDPC also uses interleaving to spread the errors. In contrast, Viterbi error correction operates by
passing data through the convolutional error correction process using a single error correction
pass.
The error correcting capability of LDPC is enhanced by use of large block sizes. Although large
block sizes can increase latency in low bitrate applications (typically less than 2Mbps), this is not
a drawback in one-way broadcast applications. Links with LDPC normally operate at multi-
megabit data rates where latency effects are minimal. The standard block size for LDPC is 64,800
bits and, for lower data rate applications, a short frame block at 16,800 bits suffers only a small
error correcting loss (0.2 to 0.5 dB) compared to the standard block.
B.2.1 Range of Data Rates
See Sect. 1.4 SUMMARY OF SPECIFICATIONS for the range of available data rates.
B.2.2 BER, QEF, Eb/No, Es/No Spectral Efficiency, and Occupied Bandwidth
Depending on the operating mode, the DVB standard uses different modes of specifying
performance with a unit in IF Loop and Additive White Gaussian Noise (AWGN):
• DVB-S2 standard: "Quasi Error Free" (QEF) is defined as "less than one uncorrected
error-event per transmission hour at the level of a 5 Mbits/s single TV service decoder",
approximately corresponding to a Transport Stream Packet Error Ratio equal to a
PER<10 -7 before demultiplexer. A packet is defined as block of 188-byte MPEG frame
size data.
• Es/No vs. Eb/No: The DVB-S2 standard commonly refers to the use of Es/No instead of
Eb/No. When links operate at a 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 – 10
log
(Spectral Efficiency)
• Occupied Bandwidth: Occupied bandwidth is defined as the bandwidth between -10 dB
points of the power spectral density, which are approximately:
Occupied Bandwidth = 1.19
x
Symbol Rate, for 35% Rolloff
= 1.15
x
Symbol Rate, for 25% Rolloff
= 1.12 x Symbol Rate, for 20% Rolloff
* Taken at the -10 dB points on the plot of power spectral density, the occupied bandwidth is 1.19 x Symbol Rate for
35%, and 1.15 x Symbol Rate for 25%.