Are PLLs dead? A tutorial on kalman filter-based techniques for digital carrier synchronization

Carrier synchronization is a fundamental stage in the receiver side of any communication or positioning system. Traditional carrier phase tracking techniques are based on well-known phase-locked loop (PLL) closed-loop architectures, which are still the methods of choice in modern receivers. Those techniques are well understood, easy to tune, and perform well under benign propagation conditions, but their applicability is seriously compromised in harsh propagation environments, where the signal may be affected by high dynamics, shadowing, strong fadings, multipath effects, or ionospheric scintillation. From an optimal filtering standpoint, the Kalman filter (KF) is clearly a powerful alternative, but the synchronization community seems still reluctant to exploit all the potential it has to offer. The purpose of this article is twofold: i) to review the basics and state of the art on both PLL and KF-based tracking techniques and ii) to present and justify the reasoning behind the systematic use of KF-based tracking approaches instead of the well-established PLL-based architectures from both theoretical and practical points of view. To support the discussion, two specific scenarios of interest to the aerospace community are numerically evaluated: robust carrier tracking of global navigation satellite systems' signals and synchronization in a deep space communications system.