Comtech EF Data CDM-750 - Manual (Page 239)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix C MN-CDM750
C–3
margin is directly converted into a higher link Es/No: 3.7 dB + 3.0 dB margin = new Es/No of
6.7dB. This non faded Es/No of 6.7 dB allows for a much higher ModCod to be used, offering a
higher Spectral Efficiency. The corresponding Spectral Efficiencies in this example range from
1.1600 to 1.7244.
The Data Rate (DR) and Symbol Rate are related by the symbol rate as follows:
DR (Data Rate) = SR (Symbol Rate) x SE (Spectral Efficiency)
For a 20 Msps link, this corresponds to the data rates listed in Table C‐1. Table C‐2 compares the
ACM link with its CCM counterpart. In ACM mode, the link operates most of the time with clear
sky conditions at 34.49 Mbps and drops back to 23.20 Mbps only when the link is faded. In CCM
mode the link must remain in its worst case condition at 23.20 Mbps.
Table C-2. ModCod Comparison: ACM vs. CCM
Condition Es/No (dB)
ACM
ModCod
ACM
Data Rate
CCM
ModCod
CCM
Data Rate
Clear Sky 6.7 QPSK 8/9 34.49 QPSK 3/5 23.20
Faded 3.7 QPSK 3/5 23.20 QPSK 3/5 23.20
There are a few important factors to consider for ACM operation:
• The digital communication system must tolerate a change in data rate. This excludes
synchronous interfaces such as G.703 T3/E3 interfaces, which operate at a fixed data rates
(44.768 / 34.368 Mbps).
• Ethernet is a suitable data stream because the packet nature of this interface
accommodates changing data rates.
• The bit rate is not allowed to change arbitrarily. Link conditions determine the operational
ModCod, as measured by the far side demod Es/No reading. The ModCod chosen must
always operate at or above the minimum threshold for reliable communications for any
given ModCod.
• There are generally two types of scenarios where ACM operation is run:
o Minimum Guaranteed DR: A link budget is run using worst case fade conditions and a
fixed availability (ex. 99.8%). The ModCod selected in this link budget is considered to
be the lowest acceptable ModCod that meets the DR and availability of the link
budget. 99.8% of the year the data rate of the link will be at or greater than this
minimum data rate. For most of the year the DR will exceed the minimum guaranteed
figure, 0.2% of the year the link will be down or at a DR that does not meet the
minimum guaranteed figure.
o Relaxed Availability: A link availability below typical thresholds (ex: 97%) is used to
calculate a worst case fade condition at a desired data rate. It is understood that
although the availability is less than desirable, for the majority of the year the DR will