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Table K-2. VersaFEC Implementation of ACM – 100 ksymbols/sec Example Case
ModCod
Modulation
00
01
QPSK
02
QPSK
03
QPSK
04
QPSK
05
8-QAM
06
8-QAM
07
8-QAM
08
16-QAM
09
16-QAM
10
16-QAM
11
16-QAM
Latency = 34 milliseconds, NOT INCLUDING WAN BUFFER OR SATELLITE PATH
By way of comparison, consider the same 100 ksymbols/second rate, but this time using DVB-S2. It
becomes clear that there is an unintended penalty (besides demodulator complexity) to having a
constant number of bits per block. Each time the ModCod is lowered and the throughput is reduced,
the latency grows accordingly due to the block size being related to data rate, not symbol rate.
Remembering that, for the ACM case, the system latency is equal to the latency of the worst-case
ModCod, DVB-S2 shows a severe penalty. For 16k short blocks, this calculates to be 329
milliseconds (+ WAN Buffer delay) versus 34 milliseconds (+ WAN Buffer delay) for VersaFEC
ACM. For 64k block DVB-S2, the core latency is 4 times higher. Assuming a WAN Buffer of 20
milliseconds:
•
Latency for 64k block DVB-S2 ACM at 100 ksps = 1336 milliseconds
•
Latency for 16k block DVB-S2 ACM at 100 ksps = 349 milliseconds
•
Latency for VersaFEC ACM at 100 ksps = 54 milliseconds
For the example shown, the latency for a 16k block DVB-S2 ACM scheme is
approximately 7 times higher than VersaFEC ACM.
The latency for a 64k block DVB-S2 ACM scheme is approximately 25 times
higher than VersaFEC ACM.
Appendix K
Spectral efficiency,
Code Rate
BPSK
0.488
0.533
0.631
0.706
0.803
0.642
0.711
0.780
0.731
0.780
0.829
0.853
OVERALL SYSTEM LATENCY = Worst-case ModCod (ModCod0)
CDM-425 Advanced Satellite Modem
Bit rate (throughput)
bps/Hz
0.49
49 kbps
1.07
107 kbps
1.26
126 kbps
1.41
141 kbps
1.61
161 kbps
1.93
193 kbps
2.13
213 kbps
2.34
234 kbps
2.93
293 kbps
3.12
312 kbps
3.32
332 kbps
3.41
341 kbps
K–11
Revision 0
Minimum Latency,
In milliseconds
34
32
30
28
26
30
28
27
27
26
25
25
MN-CDM-425
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