Digital Communications with Emphasis on Data Modems

10 CARRIER ACQUISITION AND TRACKING

10.1 INTRODUCTION

This chapter focuses on the application of phaselock techniques to acquire and track the carrier frequency of a modulated waveform. Typically, the received carrier frequency error relative to the receiver and demodulator local oscillators far exceed the phaselock loop (PLL) frequency pull‐in range requiring frequency estimation and correction. In these cases, the unknown frequency can be resolved by slowly sweeping the PLL over the range of the frequency uncertainty or by estimating the frequency error using a waveform preamble; the choice is one of expediency, however, in most applications a preamble or in some cases a midamble is provided to assist in the received waveform acquisition as discussed in Chapter 11. Phaselock techniques are used in a variety of applications involving the synchronization of various continuous wave (CW) oscillators for coherent heterodyning in a receiver or demodulator; however, although these applications are addressed in this chapter, the main focus is on the acquisition and tracking of data‐modulated waveforms involving the baseband or Costas implementation of the PLL. Acquisition and tracking of the carrier phase and frequency of waveform‐modulated carrier signals is essential for optimum coherent detection and examples are given for multiphase shift keying (MPSK), quadrature phase shift keying (QPSK), offset QPSK (OQPSK), and minimum shift keying (MSK) modulations. The material in ...