Introduction

So far we have studied deterministic signals that are used to carry information. As far as the receiver is concerned, the stream of transmitted digital waveforms that carries information symbols is a random signal because the information is random. Also, as the transmitted signal travels through the channel, it is modified by noise, which is a random signal and is often referred to as a random process. Therefore, the receiver receives the transmitted signal plus noise. Such a channel is called an additive noise channel. Furthermore, wireless signals such as cellular signals and wireless LAN and MAN signals always travel through a time-varying multipath fading channel, which causes the signal envelopes to vary randomly. The time-varying phenomenon arises from Doppler shift, which is the result of the motion of the transmitters and/or receivers. The multipath fading is the result of the destructive interference of signal rays that travel via randomly delayed and attenuated paths. Therefore, the received signal itself becomes a random signal. To analyze random signals in communication receivers we need to know their statistics. In this section we explore some tools necessary for such a task. We divide the discussion into reviews of probability theory and random variables, and study of random processes and their applicability to communication theory.

Introduction

In this chapter, we provide the foundation for analyzing a wireless communication link. The purpose is to evaluate the signal-to-noise ratio at the receiver to assess the link performance. Evaluation of the signal power and noise power requires the path loss and receiver system noise temperature, respectively. The receiver consists of an antenna, a low-noise amplifier, a downconverter, and a demodulator. The concept of the thermal noise source and its noise temperature, as well as the antenna noise temperature, is discussed. We also introduce the effective noise temperature and noise figure of a two-port network such as the low-noise amplifier, downconverter, or demodulator. The effective noise temperature and noise figure of a cascade or series connection of two-port networks are derived. This leads to the evaluation of the receiver system noise temperature.

For free space links, such as satellite communications links, we introduce the Friis equation to calculate the path loss. For cellular communications links, we present the well-known Hata model. Many important aspects of cellular systems are also discussed, such as the frequency spectrum, standards, and the co-channel interference.

Basic wireless communication link

In wireless communications, the point-to-point link is the simplest connection between a transmitter and a receiver. In this basic link, the transmitted signal travels the line-of-sight path to the receiver and the channel is the free space. A typical wireless communication system is shown in Figure 5.1.

The most commonly used type of block transceiver is the so-called DFT-based transceiver, in which the polyphase matrices of the transnritter and receiver are related to low-cost DFT matrices in a simple way. The DFT-based transceiver has found applications in a wide range of transmission channels. wired [12, 154] or wireless [27]. It is typically called a DMT (discrete multitone) system for wired DSL (digital subscriber line) applications [7, 8] and an OFDM (orthogonal frequency division multiplexing) system for wireless local area networks [54] and broadcasting applications, e.g. digital audio broadcasting [39] and digital video broadcasting [40]. In an OFDM or DMT transceiver, the transmitter and receiver perform, respectively, IDFT and DFT computations. Another type of DFT-based transceiver, called a singlecarrier system with cyclic prefix (SC-CP), has also been of great importance in wireless transmission [55, 128, 130]. The SC-CP system transmits a block of synrbols directly after inserting a cyclic prefix while the receiver performs both DFT and IDFT computations.

For wireless transmission, the channel state information is usually not available to the transmitter. The transmitter is typically channel-independent and there is no bit or power allocation. Having a channel-independent transmitter is also a very useful feature for broadcasting applications, where there are many receivers with different transmission paths. In OFDM or SC-CP systems for wireless applications, there is usually no bit and power allocation. The transmitters have the desirable channel-independence property. The channel-dependent part of the transceiver is a set of M scalars at the receiver, where M is the number of subchannels. In DMT systems for wired DSL applications, signals are transmitted over copper lines. The channel does not vary rapidly. This allows tinre for the receiver to send back to the transmitter the channel state information, based on which bit and power allocation can be optimized. Using bit allocation, the disparity among the subchannel noise variances is exploited in the DMT system for bit rate maximization. The DMT system has been shown to be very efficient for high-speed transmission.

Introduction

Intersymbol interference (ISI) is a phenomenon in which the energy of a symbol spills over into succeeding symbols causing interference. Tight filtering at the transmitter and receiver and/or channel distortion can cause the waveform that represents a symbol to spread out into succeeding symbol periods. In a terrestrial wireless channel, a signal can travel from the transmitter to the receiver via multiple delayed paths (echoes). If the delays between paths are large compared to a symbol period, the energy carried by these echoes can cause significant ISI to the succeeding symbols. In this chapter, we study the Nyquist criterion for zero ISI and provide the design of an optimum demodulator that can achieve both maximum output signal-to-noise ratio and zero ISI. We then study linear equalizers, such as the zero-forcing linear equalizer (ZF-LE), and the mean-square error linear equalizer (MSE-LE), and nonlinear equalizers, such as the zero-forcing decision-feedback equalizer (ZF-DFE) and the mean-square error decision-feedback equalizer (MSE-DFE). Both nonlinear equalizers perform better than their linear counterparts. We then study the maximum likelihood sequence detection (MLSD), which is the optimum nonlinear equalization technique. Finally, the fractionally spaced equalizer is introduced to solve the timing error problem in the previous equalizers.

In autonomous ad hoc networks, nodes usually belong to different authorities and pursue different goals. In order to maximize their own performance, nodes in such networks tend to be selfish, and are not willing to forward packets for the benefit of other nodes. Meanwhile, some nodes might behave maliciously and try to disrupt the network and waste other nodes' resources. In this chapter, we present an attack-resilient cooperation-stimulation (ARCS) system for autonomous ad hoc networks to stimulate cooperation among selfish nodes and defend against malicious attacks. In the ARCS system, the damage that can be caused by malicious nodes can be bounded, cooperation among selfish nodes can be enforced, and fairness among nodes can also be achieved. Both theoretical analysis and simulation results are presented to demonstrate the effectiveness of the ARCS system. Another key property of the ARCS system is that it is completely self-organizing and fully distributed, and does not require any tamper-proof hardware or central management points.

Introduction

In emergency or military situations, nodes in an ad hoc network usually belong to the same authority and have a common goal. To maximize the overall system performance, nodes usually work in a fully cooperative way, and will unconditionally forward packets for each other. Emerging applications of ad hoc networks are now being envisioned also for civilian usage.

In this chapter we present a game-theoretic analysis of securing cooperative ad hoc networks against insider attacks in the presence of noise and imperfect monitoring. By focusing on the most basic networking function, namely routing and packet forwarding, we model the interactions between good nodes and insider attackers as secure-routing and packet-forwarding games. The worst-case scenarios in which initially good nodes do not know which the attackers are while the insider attackers know which nodes are good are studied. The optimal defense strategies have been devised in the sense that no other strategies can further increase the good nodes' payoff under attacks. Meanwhile, the optimal attacking strategies and the maximum possible damage that can be caused by attackers are discussed. Extensive simulation studies have also been conducted to evaluate the effectiveness of the strategies.

Introduction

Many important issues about security in ad hoc networks have not yet been fully addressed. One is the optimality measure of defense mechanisms. For example, what metrics should be used to measure the optimality of the defense mechanism? Under certain optimality metrics, what are the optimal defending strategies, especially when the environment is noisy and the monitoring is not perfect? What strategies should the attackers use to maximize the damage to the network, and consequently what is the maximum possible damage that the attackers can cause?

The performance of distributed networks depends on collaboration among distributed entities. To enhance security in distributed networks, such as ad hoc networks, it is important to evaluate the trustworthiness of participating entities since trust is the major driving force for collaboration. In this chapter, we present a framework to quantitatively measure trust, model trust propagation, and defend trust-evaluation systems against malicious attacks. In particular, we address the fundamental understanding of trust, quantitative trust metrics, mathematical properties of trust, dynamic properties of trust, and trust models. The attacks against trust evaluation are identified and defense techniques are developed.

Introduction

The fields of computing and communications are progressively heading toward systems of distributed entities. In the migration from traditional architectures to more distributed architectures, one of the most important challenges is security.

Currently, the networking community is working on introducing traditional security services, such as confidentiality and authentication, into distributed networks including ad hoc networks and sensor networks. However, it has also recently been recognized that new tools, beyond conventional security services, need to be developed in order to defend these distributed networks from misbehavior and attacks that may be launched by selfish and malicious entities. In fact, the very challenge of securing distributed networks comes from the distributed nature of these networks – there is an inherent reliance on collaboration between network participants in order to achieve the planned functionalities.

In dynamic spectrum access, users who are competing with each other for spectrum may have no incentive to cooperate, and they may even exchange false private information about their channel conditions in order to gain more access to the spectrum. In this chapter, we present a repeated spectrum sharing game with cheat-proof strategies. In a punishment-based repeated game, users have an incentive to share the spectrum in a cooperative way; and, through mechanism-design-based and statistics-based approaches, user honesty is further enforced. Specific cooperation rules have been developed on the basis of maximum-total-throughput and proportional-fairness criteria. Simulation results show that the scheme presented here can greatly improve the spectrum efficiency by alleviating mutual interference.

Introduction

In order to achieve more flexible spectrum access in long-run scenarios, we need to address the following challenges. First, in self-organized spectrum sharing, there is no central authority to coordinate the spectrum access of different users. Thus, the spectrum access scheme should be able to adapt distributively to the spectrum dynamics, e.g., channel variations, with only local observations. Moreover, users competing for the open spectrum may have no incentive to cooperate with each other, and they may even exchange false private information about their channel conditions in order to gain more access to the spectrum. Therefore, cheat-proof spectrum sharing schemes should be developed in order to maintain the efficiency of the spectrum usage.

This book was written with two goals in mind: to provide the underlying principles of digital communication and to study design techniques integrated with real world systems. The ultimate aim of a communication system is to provide reliable transmission of information to the user(s). This fundamental foundation was established in 1948 by Claude Shannon, the founding father of information theory, and led eventually to the development of modern digital communication. Analog communication is near extinction or at the very gate of it. The full spectrum dominance of digital communication has arrived and new frontiers are being established every decade; from cellular systems to wireless LAN and MAN, the bit rates are being pushed ever higher for ubiquitous mobile applications.

Knowing the limit of digital transmission is vital to the design of future communication systems, particularly mobile wireless systems, where both spectrum and power are precious resources, and design techniques can be used to manipulate these two main resources to fit real world applications. No single technique can cover all the requirements of a modern communication system, which makes it necessary for students to understand the intricate web between subsystems, each designed to support others to achieve the common goal of reliable communication.

Brief overview

This book provides the principles of digital communication and studies techniques to design and analyze digital communication systems for point-to-point and point-to-multipoint transmission and reception. Other than for radio broadcasting, modern communication systems are going digital, and in the USA the conversion of analog TV broadcasting into digital HDTV broadcasting at the beginning of 2009 signified the coming end of analog communications. Communications between living beings began with the voice, and the three biggest voice systems in the world are the telephone, and the cellular and radio broadcasting systems.

The dissemination of visual activities then propelled the development of TV broadcasting systems. The pioneer telephone network and radio broadcasting systems employed analog communication techniques, such as AM and FM, for transmission of analog voice, as did the analog TV broadcasting systems, which employed VSB-AM for picture transmission. The quality of the message, such as voice and images, at the analog receiver depends on how well the waveform that carries the message over the physical channel (twisted-pair telephone wires, coaxial and fiber-optic cables, space, and water) can be reproduced. In addition, the fidelity of the received message depends on the signal-to-noise ratio at the receiver input. For good analog communications, the signal-to-noise ratio must be large, and this requires high-power transmitters, such as are used in AM radio and TV broadcasting.

In this chapter, opportunistic multiple access to the under-utilized channel resources is investigated. By exploiting source burstiness, secondary cognitive nodes utilize primary nodes' periods of silence to access the channel and transmit their packets. Cognitive relays could also make use of these silent periods to offer spatial diversity without incurring losses of bandwidth efficiency. First, we consider the cognitive cooperation protocol and discuss two different relay-assignment schemes. A comparison of the two schemes is carried out through a maximum stable throughput analysis of the network. Then, secondary nodes' access to the remaining idle channel resources is considered. Queueing-theoretic analysis and numerical results reveal that, despite the fact that relays occupy part of the idle resources to provide cooperation, secondary nodes surprisingly achieve higher throughput in the presence of relays. The rationale is that relays help primary nodes empty their queues at faster rates; therefore, secondary nodes observe increased channel access opportunities. Moreover, the scenario where secondary nodes present themselves as relays to the primary network is also presented. By working as relays, secondary nodes further help primary nodes empty their queues, hence increasing their channel access opportunities and achieving higher throughput, as is revealed by our analytical and numerical results.

Introduction

The scarcity of energy and bandwidth, the two fundamental resources for communications, imposes severe limitations on the development of communications networks in terms of capacity and performance.