3
rd
Generation Rack-
Mount RDMS™
185
Quasonix, Inc.
Figure 304: Phase Trajectory Never Forgets
9.1.1
Trellis Demodulation Summary
The basic premise of trellis demodulation is that the signal from the transmitter follows a known path through the
phase tree. When the demodulator knows this, it can use a sequence of several symbols to help make better decisions
about each individual bit. This process improves BER performance by about 3.5 to 5 dB over conventional FM
detection. However, this assumes that the transmitter is really following the "known" and "correct" phase tree, and
this assumption is NOT always true.
High phase noise can reduce the trellis detection gain because phase noise corrupts the tree. The following figures
illustrate the differences in trellis detection gain depending on the amount of phase noise introduced.
Figure 305: Trellis Detection Gain with Zero to Minimum Phase Noise
