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3452 results in Wireless Communications

Page 37 of 173

  • 2 - Models and Preliminaries

  • Thomas L. Marzetta, Erik G. Larsson, Linköpings Universitet, Sweden, Hong Yang, Hien Quoc Ngo, Queen's University Belfast
  • Book:
    Fundamentals of Massive MIMO
    Published online:
    03 November 2016
    Print publication:
    17 November 2016, pp 19-44

  • Summary

    This chapter introduces the basic signal and channel models to be used throughout the book. We use standard complex baseband representations of all signals and noise, with the implicit assumption that all signals are eventually modulated onto a carrier with frequency fc and wavelength λ = c/fc, where c is the speed of light. Also, unless stated explicitly, all Gaussian random variables are complex-valued and circularly symmetric; see Appendix A for a treatment of such variables.

    Single-Antenna Transmitter and Single-Antenna Receiver

    The wireless channel takes an input signal x(t), emitted by a transmit antenna, and yields an output signal y(t), observed at a receive antenna. The relation between x(t) and y(t) is linear, owing to the linearity of Maxwell's equations. However, this relation generally is timevarying, since the transmitter, receiver, and other objects in the propagation environment may move relative to one another.

    Coherence Time

    The time during which the channel can be reasonably well viewed as time-invariant is called the coherence time and denoted by Tc (measured in seconds). To relate Tc to the characteristics of the physical propagation environment, we consider a simple two-path propagation model where a transmit antenna emits a signal x(t) that reaches the receiver both directly via a LoS path, and via a single specular reflection; see Figure 2.1(a). If both paths have unit strength, and the bandwidth of x(t) is small enough that time-delays can be

    approximated as phase shifts, then by the superposition principle the received signal is

    where d1 and d2 are the propagation path lengths defined in Figure 2.1(a).

    Suppose, for the sake of argument, that when the receiver is located as shown in Figure 2.1(a), d1/λ and d2/λ are integers. Then the two paths add up constructively and y(t) = 2x(t). Next, if the receiver is displaced d meters to the right, so that we have the situation in Figure 2.1(b), the received signal will instead be

    The two paths add up destructively if the cosine in (2.2) is equal to zero. As shown in Figure 2.2(a), this occurs periodically for displacements d that are spaced λ/2 meters apart. The channel may be considered time-invariant as long as the receiver does not move farther than this distance, λ/2.

  • 7 - Dedicated Wireless Energy Harvesting in Cellular Networks: Performance Modeling and Analysis

  • from Part II - Architectures, Protocols, and Performance Analysis
  • Edited by Dusit Niyato, Nanyang Technological University, Singapore, Ekram Hossain, University of Manitoba, Canada, Dong In Kim, Vijay Bhargava, University of British Columbia, Vancouver, Lotfollah Shafai, University of Manitoba, Canada
  • Book:
    Wireless-Powered Communication Networks
    Published online:
    01 December 2016
    Print publication:
    17 November 2016, pp 246-264

  • Summary

    Introduction

    Energy harvesting in wireless cellular networks is a cornerstone of emerging 5G and beyond 5G (B5G) cellular networks as it aims to “cut the last wires” of the existing wireless devices [1]. In particular, energy harvesting has a significant potential to attract subscribers since it promotes mobility and connectivity anywhere and anytime, which is one of the key visions of next-generation wireless networks. In general, wireless energy harvesting can be classified according to the following two categories.

    1. • Ambient energy harvesting (EH). This refers to energy harvested from renewable energy sources (such as thermal, solar, wind, etc.) as well as energy harvested from radio signals of different frequencies in the environment that can be sensed by EH receivers (e.g., co-channel interference, TV or radio broadcasting, etc.).

    2. • Dedicated EH. This enables the intentional transmission of energy from dedicated energy sources to energy harvesting devices.

    To satisfy the power demands of delay-constrained wireless applications, it is of utmost importance to ensure the availability of sufficient energy at the user terminals whenever required. This fact has motivated researchers toward the development of dedicated wireless-powered cellular networks (WPCNs) where dedicated energy sources or hybrid access points (HAPs) take care of both energy transfer and information transmission to and from the subscribers.

    In this chapter, we focus on dedicated EH techniques. We first highlight the associated challenges. Next, we theoretically characterize and comparatively analyze a number of different network architectures for centralized and distributed dedicated wireless EH. Numerical results are provided to validate the analytical results.

    Major Challenges in Dedicated Wireless Energy Harvesting

    In this section, we will discuss a number of major challenges related to dedicated wireless energy harvesting (WEH) from the perspective of network architecture and modeling and resource allocation.

    Network Architectures for Wireless Energy Harvesting

    Different network architectures have been studied for WEH. However, most of the studies have been limited to a two- or three-node network model, a central base station (BS) that takes care of both the wireless information transmission and energy transfer, and follows a specific configuration of energy harvesting; i.e., a user harvests energy from a centralized half-duplex BS or full-duplex BS or through randomly deployed power beacons (PBs), etc.

  • 6 - Backscattering Wireless-Powered Communications

  • from Part II - Architectures, Protocols, and Performance Analysis
  • Edited by Dusit Niyato, Nanyang Technological University, Singapore, Ekram Hossain, University of Manitoba, Canada, Dong In Kim, Vijay Bhargava, University of British Columbia, Vancouver, Lotfollah Shafai, University of Manitoba, Canada
  • Book:
    Wireless-Powered Communication Networks
    Published online:
    01 December 2016
    Print publication:
    17 November 2016, pp 217-245

  • Summary

    Introduction

    The concept of modulating backscatter for communication was first introduced by Stockman in 1948 [1] and promptly received a lot of attention from researchers and developers owing to its potential advantages. Basically, backscatter communication is a technique that allows wireless nodes to communicate without requiring any active radiofrequency (RF) components on the tag [2]. In a conventional backscatter communication system (CBCS), there are two main components, called the wireless tag reader device (WTRD) and the wireless tag device (WTD), as illustrated in Figure 6.1. The WTD in the CBCS is able not only to harvest energy from the received signals, but also to modulate and reflect the signals back to the WTRD. The signal reflection is caused by the intentional mismatch between the antenna and the load impedance at the WTD. Theoretically, when the load impedance is varied, it will generate the complex scatter coefficient which can be used to modulate the reflected signal with information bits. The WTRD then uses the receive antenna to receive reflected signals from the WTD and demodulate these signals to obtain the useful information.

    In conventional backscattering communication systems, there are two special features that differ from traditional communication systems. First, in conventional backscattering communication systems, the receivers (i.e., WTRDs) have to be equipped with a power source to transmit RF signals to the transmitter (i.e., WTDs). Second, the transmitters do not need to be equipped with a power source to transmit data because they will reflect signals received by the receivers instead of generating their own signals. The second feature is the most important characteristic and also the main objective for the development of conventional backscattering communication systems. This special communication feature of CBCSs has received a great deal of attention, mainly because of the successful implementation of RFID systems and the potential use in sensor devices that are small in size and have a low power supply.

    Typically, backscattering communication systems operate using RF signals and require the WTRD to be able to transmit RF signals to the WTD. However, a new solution, called ambient backscatter communication, which utilizes RF signals from ambient sources, e.g., TV signals [3] and Wi-Fi signals [4], to help the WTRD to obtain data from the WTD without generating RF signals has recently been introduced.

Wireless-Powered Communication Networks
  • Architectures, Protocols, and Applications
  • Edited by Dusit Niyato, Ekram Hossain, Dong In Kim, Vijay Bhargava, Lotfollah Shafai
  • Published online:
    01 December 2016
    Print publication:
    17 November 2016
  • Learn the fundamentals of architecture design, protocol optimization, and application development for wireless-powered communication networks with this authoritative guide. Readers will gain a detailed understanding of the issues surrounding architecture and protocol design, with key topics covered including relay-based energy harvesting systems, multiple-antenna systems for simultaneous wireless information and power transfer (SWIPT), performance modeling and analysis, and ambient wireless energy harvesting based cellular systems. Current applications of energy harvesting and transfer in different wireless networking scenarios are discussed, aiding the understanding of practical system development and implementation issues from an engineering perspective. The first book to provide a unified view of energy harvesting and wireless power transfer networks from a communications perspective, this is an essential text for researchers working on wireless communication networks and wireless systems, RF engineers, and wireless application developers.

  • Preface

  • Thomas L. Marzetta, Erik G. Larsson, Linköpings Universitet, Sweden, Hong Yang, Hien Quoc Ngo, Queen's University Belfast
  • Book:
    Fundamentals of Massive MIMO
    Published online:
    03 November 2016
    Print publication:
    17 November 2016, pp 1-4
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  • Summary

    Simplification ofmodes of proof is notmerely an indication of advance in our knowledge of a subject, but is also the surest guarantee of readiness for further progress.

    – W. Thomson [1st Baron Kelvin] and P. G. Tait, Elements of Natural Philosophy, 1873

    There are three timeless truths in the field of wireless communications:

  • • Demand for wireless throughput, both mobile and fixed, will always increase.

  • • The quantity of available electromagnetic spectrum will never increase, and the most desirable frequency bands that can propagate into buildings and around obstacles and that are unaffected by weather constitute only a small fraction of the entire spectrum.

  • • Communication theorists and engineers will always be pressured to invent or to discover breakthrough technologies that provide higher spectral efficiency.

    • Given the history of more than a century of wireless innovation, we must look to the physical layer for breakthrough technologies. The central assertion of this book is that Massive MIMO constitutes a breakthrough technology. It is a scalable technology whereby large numbers of terminals simultaneously communicate through the entire allocated frequency spectrum. What enables this aggressive multiplexing is, first, an excess number of service antennas compared with terminals, and, second, performing the multiplexing and de-multiplexing based on measured propagation characteristics rather than on assumed propagation characteristics. A disproportionate number of service antennas compared with active terminals makes it likely that the propagation channels are conducive to successful multiplexing, and basing the multiplexing on direct channel measurements makes the antenna array tolerances independent of the number of antennas. The activity of growing the number of antennas relative to the number of active terminals renders the simplest types of multiplexing and de-multiplexing signal processing exceedingly effective, and it permits the same quality of service with reduced radiated power. Low radiated power is conducive to frequency reuse. The combined action ofmany antennas eliminates frequency-dependent fading and simplifies power control. By virtue of the time-division duplexing operation, the propagation channel characteristics aremeasured on the uplink and used both for uplink data detection and downlink beamforming. This facilitates operation in high-mobility scenarios. Also, by performing appropriate power control, Massive MIMO yields uniformly good service to all terminals, as measured in terms of 95%-likely throughputs on both the uplink and downlink.

  • 4 - Cooperative Networks with Wireless Energy Harvesting

  • from Part II - Architectures, Protocols, and Performance Analysis
  • Edited by Dusit Niyato, Nanyang Technological University, Singapore, Ekram Hossain, University of Manitoba, Canada, Dong In Kim, Vijay Bhargava, University of British Columbia, Vancouver, Lotfollah Shafai, University of Manitoba, Canada
  • Book:
    Wireless-Powered Communication Networks
    Published online:
    01 December 2016
    Print publication:
    17 November 2016, pp 135-169

  • Summary

    Introduction

    Energy harvesting has emerged as one of the enabling technologies for Green Communication. As the name suggests, energy harvesting is a technique by which energy of ambient sources, for example, solar, wind, and thermal, is converted into electrical energy and used to power network equipment or mobile devices [1, 2]. This technology makes use of perpetual renewable energy sources, which reduces the consumption of high-cost constant energy sources. Lately, wireless power transfer has gained much attention in the field of energy harvesting since wireless signal is also an ambient source of energy in itself [3, 4]. Owing to the lower range of harvested energy, RF energy harvesting has potential for powering smaller network nodes.

    Wireless energy harvesting technology has been studied in two different paradigms of wireless communication networks. The first is simultaneous wireless information and power transfer (SWIPT) in downlink [5–7]. The SWIPT technique is used to transmit wireless energy to user equipments (UEs) whilst carrying out downlink information transmission to them as shown in Figure 4.1. The technological requirements necessary to enable this simultaneous information and energy transmission at the receiver circuit will be discussed later in this chapter. The SWIPT technique increases the energy efficiency of the network since the energy cost decreases for UEs capable of harvesting energy from the received information signal [8]. In other words, energy spent at the information transmitter is partially reused at the receiver. The reuse of energy is partial mainly because of the path-loss and non-ideal energy harvesting efficiency.

    On the other hand, when the UEs are carrying out uplink transmission, it is not possible to transmit energy to them using the SWIPT technique. Hence, another important paradigm of wireless energy harvesting networks is wireless-powered communication (WPC) in the uplink [9, 10]. In this technique, some fraction of the uplink transmission duration is dedicated to downlink energy transmission (DET) from the access point (AP) to the UEs. The energy harvested during this time is utilized by the UEs for uplink information transmission (UIT) to the AP in the remaining fraction of time as shown in Figure 4.2.

  • Preface

  • Edited by Dusit Niyato, Nanyang Technological University, Singapore, Ekram Hossain, University of Manitoba, Canada, Dong In Kim, Vijay Bhargava, University of British Columbia, Vancouver, Lotfollah Shafai, University of Manitoba, Canada
  • Book:
    Wireless-Powered Communication Networks
    Published online:
    01 December 2016
    Print publication:
    17 November 2016, pp xi-xiv

  • Summary

    Recently, there has been an upsurge of research interest in wireless-powered communication networks. These networks are based on energy harvesting and/or energy transfer technology, for mobile devices using wireless propagation media. This technology offers the capability of using different types of wireless medium, such as radio frequency and magnetic induction, to carry energy from dedicated sources to wireless nodes or to harvest energy from ambient sources. Therefore, this has become a promising solution to power energy-constrained wireless networks. Conventionally, energy-constrained wireless networks such as wireless sensor networks have a limited lifetime, which leads to significant deterioration in network performance and usability. By contrast, a network with wireless energy harvesting and transfer capability can be powered without using a fixed power supply. For example, it can harvest energy from environmental sources such as solar and wind energy or from other dedicated or non-dedicated sources which are tetherless. Hence, there is no need to charge or replace the batteries physically, which can improve the flexibility and availability of the network substantially. Wireless energy has many advantages over other energy sources, including indoor support and stable and more predictable supply.

    There are three major types of wireless energy harvesting and transfer technique, namely, radio frequency (RF), inductive coupling, and magnetic resonance coupling techniques. In RF energy harvesting, radio signals with frequencies in the range from 3 kHz to 300 GHz are used as a medium to carry energy in the form of electromagnetic radiation. Inductive coupling is based on magnetic coupling that delivers electrical energy between two coils tuned to resonate at the same frequency. The electric power is carried through the magnetic field between two coils. Magnetic resonance coupling utilizes evanescent-wave coupling to generate and transfer electrical energy between two resonators. The resonator is formed by adding a capacitance on an induction coil. Inductive coupling and magnetic resonance coupling are near-field wireless transmission techniques featuring high power density and conversion efficiency. By contrast, RF energy transfer can be regarded as a far-field energy transfer technique. It is suitable for powering a larger number of devices distributed over a wide area. Wireless energy harvesting and transfer have found many applications and have recently been implemented in many devices, including mobile phones, healthcare devices, sensors, and RFID tags.

  • 10 - Cognitive Radio Networks with Wireless Energy Harvesting

  • from Part III - Applications of Wireless Energy Harvesting and Transfer
  • Edited by Dusit Niyato, Nanyang Technological University, Singapore, Ekram Hossain, University of Manitoba, Canada, Dong In Kim, Vijay Bhargava, University of British Columbia, Vancouver, Lotfollah Shafai, University of Manitoba, Canada
  • Book:
    Wireless-Powered Communication Networks
    Published online:
    01 December 2016
    Print publication:
    17 November 2016, pp 338-382

  • Summary

    Introduction

    A cognitive radio network (CRN) is an intelligent radio network in which unlicensed users (i.e., secondary users) can opportunistically access idle channels when such channels are not occupied by licensed channels (i.e., primary users). The main purpose of CRNs is to utilize available spectrum efficiently, since spectrum is becoming more and more scarce due to the boom in wireless communication systems. Basically, we can define a cognitive radio as a radio that can change its transmitter parameters as a result of interaction with the environment in which it operates [1]. From this definition, there are two main characteristics of cognitive radio that are different from traditional communication systems, namely cognitive capability and reconifigurability [2].

    1. • Cognitive capability. This characteristic enables cognitive users to obtain necessary information from their environment.

    2. • Reconfigurability. After gathering information from the environment, the cognitive users can dynamically adjust their operating mode to environment variations in order to achieve optimal performance.

    To support these characteristics of cognitive radio, there are four main functions that need to be implemented for cognitive users.

    1. • Spectrum sensing. The goal of spectrum sensing is to determine the channel status and the activity of the licensed users by periodically sensing the target frequency band.

    2. • Spectrum analysis. The information obtained from spectrum sensing is used to schedule and plan for spectrum access.

    3. • Spectrum access. After a decision has been made on the basis of spectrum analysis, unlicensed users will be allocated access to the spectrum holes.

    4. • Spectrum mobility. Spectrum mobility is a function related to the change of operating frequency band of cognitive users. The spectrum change must ensure that data transmission by cognitive users can continue in the new spectrum band.

    Under these functions, cognitive users can utilize the limited spectrum resource in a more efficient and flexible way.

    Recently, the development of wireless power transfer technologies has brought a new research direction, called wireless-powered cognitive radio networks (CRNs), which would seem to provide a promising solution for energy conservation for CRNs. In wireless-powered CRNs, secondary users are able to harvest energy wirelessly and then use the harvested energy to transmit data to the primary channels opportunistically.

Fundamentals of Massive MIMO
  • Thomas L. Marzetta, Erik G. Larsson, Hong Yang, Hien Quoc Ngo
  • Published online:
    03 November 2016
    Print publication:
    17 November 2016
  • Written by pioneers of the concept, this is the first complete guide to the physical and engineering principles of Massive MIMO. Assuming only a basic background in communications and statistical signal processing, it will guide readers through key topics in multi-cell systems such as propagation modeling, multiplexing and de-multiplexing, channel estimation, power control, and performance evaluation. The authors' unique capacity-bounding approach will enable readers to carry out effective system performance analyses and develop advanced Massive MIMO techniques and algorithms. Numerous case studies, as well as problem sets and solutions accompanying the book online, will help readers put knowledge into practice and acquire the skill set needed to design and analyze complex wireless communication systems. Whether you are a graduate student, researcher, or industry professional working in the field of wireless communications, this will be an indispensable guide for years to come.

  • 12 - Antenna Integration

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 363-396

  • Summary

    INTRODUCTION

    Chapter 2 provided an introduction to antenna fundamentals in which antennas were described as transducers of guided electrical signals to unguided electromagnetic waves, and vice versa. In Chapter 4, receive antennas were viewed as receptors of environmental noise, resulting in sensitivity-limiting electrical noise characterized in terms of antenna temperature. Intervening chapters have introduced methodology for analysis and design of radios which accounts for effective radiated power and sensitivity (Chapter 7), impedance matching (Chapter 9), and noise within a receiver (Chapter 11). We are now in a position to confront some antenna issues that were bypassed in Chapter 2, namely: How to most effectively integrate an antenna with radio electronics so as to achieve system-level design goals. In this chapter we address the topic of antenna integration so as to close this gap in our review of radio electronics, before moving on to the topics of receiver and transmitter design (Chapters 15 and 17, respectively).

    The organization of this chapter is as follows. In Section 12.2 we consider the manner in which an antenna contributes to the SNR achieved by a receiver, thereby limiting sensitivity. In Section 12.3 we consider VSWR and efficiency of power transfer in transmit operation. These considerations are closely related to the issue of impedance matching between antenna and transceiver, which is addressed in Section 12.4. The trend toward increasingly small antennas is considered, first in terms of the impedance-matching challenge, and then (in Section 12.5) in terms of practical limits on an antenna size. Section 12.6 gives a brief introduction to antenna tuners, a resurgent technology that facilitates improvements in the performance of radios that must cover large tuning ranges using antennas which are narrowband or electrically-small. This chapter concludes with an introduction to baluns (Section 12.7), which are needed to accomplish the common tasks of interfacing single-ended antennas to differential transceivers, and differential antennas to single-ended transceivers.

    RECEIVE PERFORMANCE

    Let us now quantify the role an antenna plays in the sensitivity of a radio receiver. This issue was previously addressed in Sections 7.4 and 7.5, and a review of those sections is recommended before engaging with this section. We begin by elaborating on the concepts presented in those sections and consider the implications for the design of antennas and the transmission lines and matching networks used to interface them to receivers.

  • 9 - Impedance Matching

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 259-284

  • Summary

    INTRODUCTION

    In Chapter 8 we noted the central role two-port theory plays in RF circuit design, and especially the role of reflection coefficients in determining the stability and gain of two-ports. Reflection coefficients were introduced as surrogates for source and load impedance (recall Equations (8.32) and (8.37)), facilitating the use of scattering parameters. In this chapter we consider impedance matching, which is the process of converting impedances (equivalently, embedded reflection coefficients) from a given value to some other value.

    Applications for impedance matching include (1) maximizing power transfer between twoports, (2) eliminating reflections (not the same thing!), and (3) converting from a given impedance to another impedance necessary for setting the stability and/or gain of a two-port. In each of these cases, the problem is essentially as shown in Figure 9.1: We wish to design a passive two-port that converts the output impedance of the source, ZS = RS + jXS, into the desired input impedance for the load, ZL = RL + jXL. Usually we would like to achieve this with a TPG as close to 1 as possible, and often there is a frequency span over which a minimum TPG which must be achieved. At the design frequency, the nominal embedded input impedance Zin of the matching two-port is either for maximum power transfer or ZS for reflectionless matching. Similarly, the nominal embedded output impedance Zout of the matching two-port is either or ZL at the design frequency.

    The organization of this chapter is as follows. In Section 9.2 we consider some rudimentary approaches to impedance matching, including resistive matching, transformer matching, and reactance canceling. Section 9.3 introduces narrowband matching using two discrete reactances in an “L” configuration, which is useful both as a standalone technique as well as a building block for more sophisticated techniques. Section 9.4 considers the issue of bandwidth, and introduces the concept of quality factor to provide some insight into how the bandwidth of matching circuits is determined and can be manipulated. In Section 9.5 we combine the concepts of previous sections to develop impedance matching circuits having bandwidth which can be wider or narrower than that obtained by using discrete two-reactance matching. Section 9.6 explains how to obtain differential versions of the single-ended circuits developed in earlier sections.

  • 16 - Frequency Synthesis

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 486-515

  • Summary

    INTRODUCTION

    Frequency synthesis is the generation of a periodic waveform having a specified frequency. Here we concern ourselves primarily with the generation of sinusoids, which are commonly required as LO signals in frequency conversion, and as carriers in analog modulations such as AM and FM. In this chapter we consider the synthesis of sinusoids using feedback oscillators (Sections 16.2–16.6), phase-locked loop synthesizers (Section 16.8), and direct digital synthesis (Section 16.9). The problem of phase noise and its effect on radio system performance is introduced in Section 16.4. Also, Section 16.10 summarizes additional considerations that arise in IC implementation of frequency synthesis devices.

    LC FEEDBACK OSCILLATORS

    In Section 8.5.1, it was noted that two-ports that are active and include internal feedback are prone to instability, and that one particular consequence of instability could be oscillation. In that context, oscillation was undesirable; now, however, let us consider how we might exploit this type of mechanism for frequency synthesis.

    The LC Resonator

    Let us begin with a well-known passive circuit that exhibits oscillatory behavior: The LC resonator, which is an inductor (the “L”) and a capacitor (the “C”) in parallel as shown in Figure 16.1. One first establishes a steady-state condition (Figure 16.1(a)) by applying a DC voltage source set to some non-zero value v0> 0, which charges the capacitor to a voltage of v0. At this point no current flows through the capacitor; subsequently, the voltage across the inductor is zero.

    Oscillation begins when the applied voltage is removed, as indicated in Figure 16.1(b). No longer constrained by the source voltage, the capacitor begins to equalize the charge between its plates by sending current through the inductor. The resulting current through the inductor creates a magnetic field which grows stronger the longer the current flows. The voltage v across the capacitor decreases because the net difference in charge across the plates is diminishing.

    Once the charge is evenly distributed between the plates of the capacitor (i.e., the capacitor is fully discharged), v = 0 (Figure 16.2(a)). However, the magnetic field that has built up in the inductor keeps the current flowing, and so the redistribution of charge across the capacitor plates continues. A short time later we have the situation shown in Figure 16.2(b): The voltage across the capacitor is negative and decreasing, the flow of current is decreasing, and the magnetic field of the inductor is decreasing.

  • 10 - Amplifiers

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 285-337

  • Summary

    INTRODUCTION

    The primary purpose of most amplifiers used in radio frequency systems is to increase signal magnitude. This is necessary in receivers because propagation path loss results in a signal arriving with voltage or current which is far too small to be digitized or directly demodulated. Amplification appears in transmitters when it is convenient to synthesize a signal at low power and amplify it, as opposed to synthesizing signals directly at the intended transmit power; the latter is difficult for all but very simple analog modulations.

    This chapter is organized as follows. Section 10.2 introduces the transistor, its use as a gain device, and provides a brief summary of transistor types and device technologies. The design of narrowband single-transistor amplifiers is introduced in four parts: biasing (Section 10.3), small-signal design to meet gain requirements (Section 10.4), designing for noise figure (Section 10.5), and designing for voltage standing wave ratio (Section 10.6). Section 10.7 presents a design example intended to demonstrate the principles of amplifier design as laid out in the previous sections. Section 10.8 introduces the concepts of multistage, differential, and broadband transistor amplifiers. Section 10.9 describes the implementation of transistor amplifiers as integrated circuits.

    TRANSISTORS AS AMPLIFIERS

    The fundamental building block of modern radio frequency amplifiers is the transistor. A transistor is a three-terminal semiconductor device in which the current between two terminals can be manipulated using the voltage or current at a third terminal. The application to RF design is most easily explained in the context of a particular device – for our purposes, the silicon (Si) bipolar junction transistor (BJT) is a good place to start.

    Bipolar Transistors

    A BJT is depicted in Figure 10.1. The three terminals of a BJT are known as the base, the collector, and the emitter. The associated terminal currents and voltages are as defined in Figure 10.1. The Si BJT comes in two varieties, known as “NPN” and “PNP,” referring to the geometry of semiconductor materials within the device. The NPN variant is somewhat more common in RF applications and is specifically considered here (and shown in Figure 10.1); the PNP variant is relatively easily understood once the NPN variant is mastered.

  • 6 - Digital Modulation

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 139-202

  • Summary

    INTRODUCTION

    In Section 5.1 we defined modulation as the representation of information in a form suitable for transmission. Digital modulation might be defined as the representation of information as a sequence of symbols drawn from a finite alphabet of possible symbols. This definition is not completely satisfactory, since it would seem to include a variety of techniques that many practitioners would consider to be analog modulations; these include continuous wave (CW) signaling (the use of an interruptible carrier to send Morse code). In Section 5.1 we settled on the notion that digital modulation also requires the information to be transmitted to be represented in digital (not merely discrete-time) form at some point in the modulation process, whereas this is not required in analog modulation.

    Overview of a Digital Communications Link and Organization of this Chapter

    With this definition in mind, Figure 6.1 shows an overview of a digital communications link. Here is the usual signal flow.

  • • The starting point of the link is source coding, which is the processing of data and/or digitized analog message signals – e.g., voice – in order to render the data in a form better-suited for digital modulation. A common goal in source coding is compression; that is, reduction of data rate so as to reduce the required bandwidth. Source coding is addressed in Section 6.2.

  • • Data are often encrypted for security. The principles of encryption are not specific to radio communications and so are not addressed further in this book.

  • • Channel coding is the processing of data with the goal of making them more robust to propagation channel impairments, including noise, multipath, and interference. Channel coding is addressed in Sections 6.13 (error control coding) and 6.14 (interleaving).

  • • At this point the data are processed by what is traditionally considered “modulation.” The result is an analog signal representing a discrete-time finite-alphabet message, and which is suitable for frequency conversion and amplification. It is this form of the signal that is ultimately transmitted to the other end of the link. Modulation is addressed in Sections 6.3, 6.4, and portions of Sections 6.10–6.12, 6.18 and 6.19.

  • Preface

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp xxxix-xlii

  • Summary

    The topic of this book is the analysis and design of radio systems and their constituent subsytems and devices. Chapters 1–6 cover the basics of radio: antennas, propagation, noise, and modulation. Chapters 8–18 cover radio hardware: circuits, device technology where relevant, and the basics of digital signal processing. The intervening Chapter 7 (“Radio Link Analysis”) serves as the bridge between these two parts of the book by demonstrating how system-level characteristics lead to particular hardware characteristics, and vice versa. Appendix B serves as a summary of commonly-encountered radio systems in this context. Appendix A addresses modeling of path loss in terrestrial channels, building on the fundamentals presented in Chapter 3 (“Propagation”).

    This book presumes the reader is interested primarily in communications applications, however, most of the material is relevant to other applications of radio including navigation, radar, radio astronomy, geophysical remote sensing, and radio science.

    The use of the term “systems engineering” in the title of this book bears some explanation. In the parlance of communications engineers, the scope of this book is limited to considerations at the physical layer and below, with little coverage of issues at higher layers of the protocol stack. Further, some readers may judge that only Chapters 1, 7, and Appendix B (or maybe just Chapter 7) address systems engineering, in the sense of complete “end-to-end” system analysis and design. Nevertheless, the title “Radio Systems Engineering” seems appropriate because each topic – especially those in Chapters 8–18 – is presented in the context of system-level design, with extensive cross-referencing to associated topics in the first half of the book and to Chapter 7.

    Intended uses of this book This book is has been written with three possible uses in mind.

  • • Primary textbook in senior- and beginning graduate-level courses in radio systems engineering. This book can serve as a single textbook for a two–three semester course sequence. Associated courses typically have titles such as “radio communications systems,” “wireless engineering,” “wireless systems,” etc.

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