Table of Contents
Advertisement
CDM-750 Advanced High-Speed Trunking Modem
Adaptive Coding and Modulation (ACM) Option
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 7-1. Table 7-2 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 it's worst case condition at 23.20 Mbps.
Condition
Clear Sky
Faded
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 desireable, for the majority of the year the DR will
exceed the customer contracted DR and justify the lower throughput for a larger
percentage of the year. This is essentially a tradeoff between minimum DR contracted
and effective DR averaged over the course of a year.
7.2.1
Example of ACM Operational Link
Most links are designed to deliver a level of link availability based on worst case conditions.
Since worst case conditions occur only a very small part of the time, the link has margin above
the minimum during the majority of the time. ACM shatters this limitation and operates at higher
data rates when conditions are good.
For example, consider a link with a 25 Mbps service level agreement (SLA) between Nigeria and
Germany that requires 99.8% availability. Once designed, the link will provide this level of SLA
Table 7-2. ModCod Comparison: ACM vs. CCM
ACM
Es/No (dB)
ModCod
6.7
QPSK 8/9
3.7
QPSK 3/5
ACM
Data Rate
34.49
23.20
7–3
Revision 1
MN-CDM750
CCM
CCM
ModCod
Data Rate
QPSK 3/5
23.20
QPSK 3/5
23.20
Advertisement
Table of Contents
