Comtech EF Data CDM-625A - Manual (Page 682)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–2
M.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.