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Über dieses Buch

This book provides a concise but lucid explanation of the fundamentals of spread-spectrum systems with an emphasis on theoretical principles. Throughout the book, learning is facilitated by many new or streamlined derivations of the classical theory. Problems at the end of each chapter are intended to assist readers in consolidating their knowledge and to provide practice in analytical techniques. The choice of specific topics is tempered by the author’s judgment of their practical significance and interest to both researchers and system designers.

The evolution of spread spectrum communication systems and the prominence of new mathematical methods in their design provided the motivation to undertake this new edition of the book. This edition is intended to enable readers to understand the current state-of-the-art in this field. More than 20 percent of the material in this edition is new, including a chapter on systems with iterative channel estimation, and the remainder of the material has been thoroughly revised.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Channel Codes and Modulation

Abstract
This chapter reviews fundamental results of coding and modulation theory that are essential to a full understanding of spread-spectrum systems. Channel codes, which are also called error-correction or error-control codes, are vital in fully exploiting the potential capabilities of spread-spectrum communication systems. Although direct-sequence systems greatly suppress interference, practical systems require channel codes to deal with the residual interference and channel impairments such as fading. Frequency-hopping systems are designed to avoid interference, but the possibility of hopping into an unfavorable spectral region usually requires a channel code to maintain adequate performance. In this chapter, coding and modulation theory [1–5] are used to derive the required receiver computations and the error probabilities of the decoded information bits. The emphasis is on the types of codes and modulation that have proved most useful in spread-spectrum systems.
Don Torrieri

Chapter 2. Direct-Sequence Systems

Abstract
A spread-spectrum signal is one with an extra modulation that expands the signal bandwidth greatly beyond what is required by the underlying coded-data modulation. Spread-spectrum communication systems are useful for suppressing interference, making secure communications difficult to detect and process, accommodating fading and multipath channels, and providing a multiple-access capability. Spread-spectrum signals cause relatively minor interference to other systems operating in the same spectral band. The most practical and dominant spread-spectrum systems are direct-sequence and frequency hopping systems.
Don Torrieri

Chapter 3. Frequency-Hopping Systems

Abstract
Frequency hopping is the periodic changing of the carrier frequency of a transmitted signal. This time-varying characteristic potentially endows a communication system with great strength against interference. Whereas a direct-sequence system relies on spectral spreading, spectral despreading, and filtering to suppress interference. the basic mechanism of interference suppression in a frequency-hopping system is that of avoidance. When the avoidance fails, it is only temporary because of the periodic changing of the carrier frequency. The impact of the interference is further mitigated by the pervasive use of channel codes, which are more essential for frequency-hopping than for direct-sequence systems. The basic concepts, spectral and performance aspects, and coding and modulation issues are presented in the first five sections of this chapter. The effects of partial-band interference and jamming are examined, whereas the impact of multiple-access interference is presented in Chap. 6. The most important issues in the design of frequency synthesizers are described in the final section.
Don Torrieri

Chapter 4. Code Synchronization

Abstract
A spread-spectrum receiver must generate a spreading sequence or frequency-hopping pattern that is synchronized with the received sequence or pattern; that is, the corresponding chips or dwell intervals must precisely or nearly coincide. Any misalignment causes the signal amplitude at the demodulator output to fall in accordance with the autocorrelation or partial autocorrelation function. Although the use of precision clocks in both the transmitter and the receiver limit the timing uncertainty in the receiver, clock drifts, range uncertainty, and the Doppler shift may cause synchronization problems. Code synchronization, which is either sequence or pattern synchronization, might be obtained from separately transmitted pilot or timing signals. It may be aided or enabled by feedback signals from the receiver to the transmitter. However, to reduce the cost in power and overhead, most spread-spectrum receivers achieve code synchronization by processing the received signal.
Don Torrieri

Chapter 5. Fading and Diversity

Abstract
Fading is the variation in received signal strength due to a time-varying communications channel. It is primarily caused by the interaction of multipath components of the transmitted signal that are generated and altered by changing physical characteristics of the propagation medium. The principal means of counteracting fading are diversitymethods, which are based on the exploitation of the latent redundancy in two or more independently fading copies of the same signal. This chapter provides a general description of the most important aspects of fading and diversity methods. The rake demodulator, which is of central importance in most direct-sequence systems, is shown to be capable of exploiting undesired multipath signals rather than simply attempting to reject them. The multicarrier direct-sequence system, which is described in the final section, is an alternative method of exploiting multipath signals that has practical advantages.
Don Torrieri

Chapter 6. Code-Division Multiple Access

Abstract
Multiple access is the ability of many users to communicate with each other while sharing a common transmission medium. Wireless multiple-access communications are facilitated if the transmitted signals are orthogonal or separable in some sense. Signals may be separated in time (time-division multiple access or TDMA), frequency ( frequency-division multiple access or FDMA), or code (code-division multiple access or CDMA).CDMA is realized by using spread-spectrum modulation while transmitting signals from multiple users in the same frequency band at the same time. All signals use the entire allocated spectrum, but the spreading sequences or frequency-hopping patterns differ. Information theory indicates that in an isolated cell, CDMA systems achieve the same spectral efficiency as TDMA or FDMA systems only if optimal multiuser detection is used. However, even with single-user detection, CDMA is advantageous for cellular networks because it eliminates the need for frequency and time-slot coordination among cells, allows carrier-frequency reuse in adjacent cells, and imposes no sharp upper bound on the number of users.
Don Torrieri

Chapter 7. Detection of Spread-Spectrum Signals

Abstract
The ability to detect the presence of spread-spectrum signals is often required by cognitive radio, ultra-wideband, and military systems. This chapter presents an analysis of the detection of spread-spectrum signals when the spreading sequence or the frequency-hopping pattern is unknown and cannot be accurately estimated by the detector. Thus, the detector cannot mimic the intended receiver, and alternative procedures are required. The goal is limited in that only detection is sought, not demodulation or decoding. Nevertheless, detection theory leads to impractical devices for the detection of spread-spectrum signals. An alternative procedure is to use a radiometer or energy detector, which relies solely on energy measurements to determine the presence of unknown signals. The radiometer has applications not only as a detector of spread-spectrum signals, but also as a sensing method in cognitive radio and ultra-wideband systems.
Don Torrieri

Chapter 8. Systems with Iterative Channel Estimation

Abstract
The estimation of channel parameters, such as the fading amplitude and the power spectral density of the interference plus noise, is essential to the effective use of soft-decision decoding. Channel estimation may be implemented by the transmission of pilot signals that are processed by the receiver, but pilot signals entail overhead costs, such as the loss of data throughput. Deriving maximum-likelihood channel estimates directly from the received data symbols is often prohibitively difficult. There is an effective alternative when turbo or low-density parity-check codes are used. The expectation-maximization algorithm provides an iterative approximate solution to the maximum-likelihood equations and is inherently compatible with iterative demodulation and decoding. Two examples of advanced spread-spectrum systems that apply the expectation-maximization algorithm for channel estimation are described and analyzed in this chapter. These systems provide good illustrations of the calculations required in the design of advanced systems.
Don Torrieri

Backmatter

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