Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix K K–2 MN-CDM-425
K.2 Background
ACM 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:
Bit rate = symbol rate * modulation order * code rate
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, some
important factors must be considered:
•
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.