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Synchronization is a critical function in digital communications; its failures may have catastrophic effects on the transmission system performance. Furthermore, synchronization circuits comprehend such a large part of the receiver hardware that their implementation has a substantial impact on the overall costs. For these reasons design engineers are particularly concerned with the development of new and more efficient synchronization structures. Unfortunately, the advent of digital VLSI technology has radically affected modem design rules, to a point that most analog techniques employed so far have become totally obsolete. Although digital synchronization methods are well established by now in the literature, they only appear in the form of technical papers, often concentrating on specific performance or implementation issues. As a consequence they are hardly useful to give a unified view of an otherwise seemingly heterogeneous field. It is widely recognized that a fundamental understanding of digital synchronization can only be reached by providing the designer with a solid theoretical framework, or else he will not know where to adjust his methods when he attempts to apply them to new situations. The task of the present book is just to develop such a framework.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
In synchronous digital transmissions the information is conveyed by uniformly spaced pulses and the received signal is completely known except for the data symbols and a group of variables referred to as reference parameters. Although the ultimate task of the receiver is to produce an accurate replica of the symbol sequence with no regard to reference parameters, it is only by exploiting knowledge of the latter that the detection process can properly be performed. A few examples are sufficient to illustrate this point.
Umberto Mengali, Aldo N. D’Andrea

2. Principles, Methods and Performance Limits

Abstract
This chapter lays the groundwork for the material in the book and addresses three major themes. Section 2.2 describes synchronization functions in a digital receiver and indicates methods to pinpoint design limits on the synchronization errors. Section 2.3 is an overview of maximum likelihood parameter estimation theory, with emphasis on synchronization applications. A distinction is made between wanted and unwanted parameters, the former being those of interest in a given situation and with respect to which the maximum of a likelihood function is to be sought. The computation of likelihood functions for wanted parameters is investigated. Section 2.4 establishes limits to the performance of practical synchronizers. The most popular limit is the Cramer-Rao bound to the variance of unbiased estimators. It is argued that this limit is difficult to compute in most practical cases. A simpler limit is the modified Cramer-Rao bound, which is used as a benchmark in performance evaluations throughout the book.
Umberto Mengali, Aldo N. D’Andrea

3. Carrier Frequency Recovery with Linear Modulations

Abstract
A frequency recovery system accomplishes two basic functions: (i) it derives an estimate v̂ of the carrier frequency offset; (ii) it compensates for this offset by counter-rotating the received waveform r(t) at an angular speed 2πv̂. In the ensuing discussion we distinguish between two major cases [1]:
(i)
the offset is much smaller than 1/T,
 
(ii)
the offset is on the order of the symbol rate 1/T.
 
Umberto Mengali, Aldo N. D’Andrea

4. Carrier Frequency Recovery with CPM Modulations

Abstract
Continuous phase modulation (CPM) encompasses a class of signaling schemes that conserve and reduce signal energy and bandwidth at the same time. Furthermore, the signals in this class have a constant envelope and therefore are very attractive in radio channels employing low-cost non-linear power amplifiers. Notwithstanding these favorable features, current CPM applications are still limited to a few simple modulation schemes (basically, MSK and its generalizations) because of implementation complexity and synchronization problems [1]. Research efforts are under way and advances in these areas are expected in the near future.
Umberto Mengali, Aldo N. D’Andrea

5. Carrier Phase Recovery with Linear Modulations

Abstract
In this chapter we investigate algorithms for carrier phase estimation with linear modulations. As with frequency recovery, ML estimation methods will play a central role in our study. We shall see that various approximations to the ML formulation are possible, leading to different estimation methods. Thus, a rather disparate set of synchronization schemes is anticipated. This is also a consequence of the many scenarios that can be thought of, depending on the specific modulation format and the availability of data/clock information. In this regard the following categories may be envisaged:
(i)
Modulation format:
  • Modulation may be either offset or non-offset.
 
(ii)
Additional knowledge:
  • Clock information may be available or not.
  • Information symbols may be known or not. When they are, they may come either from a known preamble (data-aided schemes) or from the detector output (decision-directed schemes)
 
(iii)
Estimator topology:
  • Estimators may be either open loop or closed loop.
 
Umberto Mengali, Aldo N. D’Andrea

6. Carrier Phase Recovery with CPM Modulations

Abstract
In this chapter we consider carrier phase recovery for CPM signaling. By and large the presentation follows the same route taken with linear modulations. In particular, data-aided (DA) estimation is considered first, then we concentrate on decision-directed (DD) methods and, finally, on non-data-aided (NDA) techniques (either clock-aided or non-clock-aided).
Umberto Mengali, Aldo N. D’Andrea

7. Timing Recovery in Baseband Transmission

Abstract
In this chapter we investigate timing recovery in PAM baseband transmission. Additional features that arise with modulated signals will be explored in Chapters 8 and 9 for PAM and CPM modulations, respectively. The reason for first concentrating on baseband signals is motivated by the need of avoiding any distractions caused by modulation matters. As we shall see in the later chapters, such matters are easily taken into account when the basic concepts relevant to baseband systems are understood.
Umberto Mengali, Aldo N. D’Andrea

8. Timing Recovery with Linear Modulations

Abstract
In this chapter we address timing recovery with modulated PAM signals. For reasons that will soon be explained, the discussion is divided into several parts, depending on the specific modulation format and the operating conditions. For example, a distinction between non-offset and offset modulations is useful as different signal representations call for separate analyses and lead to different solutions. In the sequel we first discuss non-offset modulations and then consider offset QPSK modulation (OQPSK).
Umberto Mengali, Aldo N. D’Andrea

9. Timing Recovery with CPM Modulations

Abstract
The last topic in this book is timing recovery with CPM signals. The chapter has the same profile as the previous one, except that we shall only be concerned with transmissions over AWGN channels since CPM timing recovery with multipath channels has received scarce attention thus far. Although it is widely believed that conventional clock synchronizers can be used even with fading channels, a closer look at the question will be worthwhile as it is likely that better methods can be discovered by taking the fading channel statistics into account. This problem is left as a subject of further study.
Umberto Mengali, Aldo N. D’Andrea

Backmatter

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