Comtech EF Data CTOG-250 - Manual (Page 223)

E–1
Appendix E. OUTBOUND ACM
(ADAPTIVE CODING AND
MODULATION)
E.1 Introduction
This appendix focuses on the physical layer implementation/mechanization of Advanced VSAT
Outbound Adaptive Coding and Modulation (referred to hereafter as Outbound ACM).
Outbound ACM is a technique that allows for automatic change in modulation and FEC Code
Rate in response to changing link conditions for individual remotes sites in a ‘shared TDM (Time-
division multiplexing) Carrier’.
The primary objective of Outbound ACM is to capture historically unused satellite system link
margin and convert this margin into additional data throughput for each remote site. An equally
critical objective is to maintain optimum data transmission efficiency for all remotes sharing the
TDM, independent of individual remote site link conditions (i.e., disadvantaged or weather-
impaired remotes should have minimal impact on non-disadvantaged and clear-sky remote
sites).
In a simple Point-to-Point (non-TDM) ACM system, the physical layer symbol rate is constant
(and occupied bandwidth is therefore constant), and power is also held constant. However, the
utilized waveform varies with an assortment of modulation and coding combinations called
ModCods. The ModCods are selected to span a range of Es/No (Energy per Symbol-to-Noise
density ratio) so that, if a system detects a change in link margin (e.g., fading), it can use a
different ModCod to preserve the link, albeit at a different user throughput rate. Therefore, as
Es/No increases or decreases in an ACM system, so does the user data rate.
Similarly, in a Point-to-Multipoint (shared TDM) ACM system, the physical layer symbol rate is
constant; therefore, occupied bandwidth is constant and power is also held constant. However,
the nature of the shared-TDM can make use of statistical multiplexing across a small or large
number of remotes in which the ‘individual bandwidth requirements’ are, generally,
simultaneously not at peak demand. Accordingly, not all remotes are equal in Point-to-
Multipoint architectures – this holds true from both the physical configuration (Antenna, beam
coverage contour, etc.) and Service Lease Agreement (SLA) perspectives. Because of this, a good
Point-to-Multipoint ACM configuration must be a combination of a) optimal physical layer