Synchronization: Carrier and Timing Recovery

Abstract

The theme of this chapter might well be “…timing is everything.” In the course of our discussion in Chapter 4, we saw that the detection of a baseband digital data sequence presumed proper timing at the receiver. (See Section 4.10, particularly.) The same requirement for timing is present in the detection of passband signals; however, as we saw in Section 5.2, carrier phase coherency is also necessary. The roles of each of these synchronization subsystems are shown in Figure 6.1a. The carrier tracking system provides an estimate of the received carrier phase θ^, while the timing recovery system provides an estimate of the proper sampling epoch to the receiver sampling system Ä^. The effect of a poorly designed carrier loop will be to increase the dispersion of the received symbols about their nominal values, bringing the received points considerably closer to the decision boundaries and decreasing the margin against an error (caused say by a noise burst); of course, large phase perturbations can cause errors without any noise. In Figure 6.1b we show how the transmitted symbol s ₁ is rotated by phase jitter to the point u, and then further distorted by noise to the point z; note that the received point is within the decision region associated with s ₂ so that an error will be made. Similarly, timing phase errors will cause the receiver to sample away from the maximum eye opening, and reduce the margin for error. It is the purpose of this chapter to present the various aspects of synchronization.