Comtech EF Data CDM-625A Installation And Operation Manual page 644

CDM-625A Advanced Satellite Modem
Appendix M
M.2 Background
ACM is not a new concept. It has been used for some time in wireless communications, including
terrestrial microwave applications and, more recently, over satellite links. Its primary function is to
optimize throughput in a wireless data link, by adapting the modulation order used and the
Forward Error Correction code rate – both of which directly affect spectral efficiency (expressed in
bits per second per Hertz) according to the noise conditions (or other impairments) on the link.
Implicit in this concept is that the symbol rate (and power) of the wireless communication system
must remain constant. This ensures that the bandwidth allocated for a particular link is never
exceeded.
Given that the symbol rate does not change, if modulation and coding are changed, the data
rate must therefore be modified. This is expressed in the simple equation:
Symbol rate = bit rate / (modulation order * code rate)
For example, for Rate 3/4 QPSK (where modulation order = 2):
Symbol rate = bit rate * 0.666
Re-arranging:
So, in changing to a higher modulation order or code rate, the bit rate is increased, and in
changing to a lower modulation order or code rate, the bit rate is reduced.
However, there are a number of important factors to be considered, namely:
•
The digital communications system must be able to tolerate a change in bit rate.
Synchronous serial interfaces (such as G.703 E1, which operated at a fixed data rate of
2.048 Mbps) are totally unsuitable in a scheme where data rate is changing. The only
practical application for this scheme is a packet-based scheme that will tolerate a
change in data rate, and which has mechanisms within its protocols to recognize when
increased or reduced bandwidth is available. The best example of this is Ethernet, and
this discussion is limited to schemes that employ it.
•
The bit rate cannot be changed arbitrarily. The link noise conditions, described in terms
of Eb/No or SNR, must be able to support reliable communications for the given
modulation order and code rate. This is a key point, as in fact, the link SNR is the input
that drives the adaptation.
Bit rate = symbol rate * modulation order * code rate
M–2
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MN-CDM625A