Algorithms for Communications Systems and their Applications, 2nd Edition

Chapter 14Synchronization

In quadrature amplitude modulation (QAM) systems, coherent detection requires the generation of a carrier signal at the frequency of the transmitted modulated signal. Although the nominal frequency of the transmitter and the receiver are the same, imperfections of the components or Doppler shift effects determine an unknown (significant) difference between the receiver oscillator frequency and the carrier frequency of the received signal.

In this chapter, we will discuss methods for carrier phase and frequency recovery, as well as algorithms to estimate the timing phase. To avoid ambiguity, we refer to the latter as timing recovery algorithms, dropping the term phase. These algorithms are developed for application in the pulse amplitude modulation (PAM) and QAM transmission systems of Chapter 7, and the spread spectrum systems of Chapter 10. The problem of carrier frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems will be addressed in Section 14.9.

Carrier recovery must be performed using one of the following two strategies.

1. The first consists in multiplexing, usually in the frequency domain, a special signal, called pilot signal, at the transmitter. This allows extracting the carrier at the receiver and therefore synchronizing the receive oscillator in phase and frequency with the transmit oscillator. If the pilot signal consists of a non‐modulated carrier, carrier recovery is obtained by the phase‐locked loop ...