In the transmission of multiple phase-shift keyed (MPSK) signals, phase jitter caused by the transmission medium is a common problem. To reduce the performance loss caused by phase jitter, differential MPSK is often used, wherein the difference between the phases of two successive symbols, rather than the absolute phases of the symbols themselves, represents the information. This differential technique also avoids the necessity of establishing and maintaining a phase reference, a task which is often difficult in MPSK systems due to "node slips" in the reference.
In receiving and decoding MPSK signals, two approaches are commonly employed. The simplest is to take the difference between successive received phases and then compare this phase difference with a set of threshold values to decide upon the transmitted symbols. This technique results in a loss of 3dB in the signal to noise ratio relative to a coherent receiving system, since noise perturburances in both received phases add to perturb the difference. A second approach is to establish a coherent phase reference and make individual phase node decisions based on the reference, and then logically take the difference between these decisions to establish which symbols were actually transmitted. In many cases, this latter approach is preferable, since only one phase error is associated with each nodal decision.
Now, if the second approach is used on signals wherein phase jitter is present, it is conventional to use a phase-locked loop to attempt to track out the jitter, so that the effect of the jitter on the nodal decisions will be minimized. Since phase error measurements are normally available in MPSK receivers even when data is being transmitted, it is not difficult to establish such a loop. A problem arises, however, when the jitter frequency and amplitude are such that a substantial phase can occur in a few symbol times. If the loop attempts to average the phase measurements to reduce the effects on the noise, it is unable to track the phase jitter close enough.