WILEY Digital radio system design User Manual
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DIGITAL RADIO SYSTEM
DESIGN
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DIGITAL RADIO
SYSTEM DESIGN
Grigorios Kalivas
University of Patras, Greece
A John Wiley and Sons, Ltd, Publication
This edition first published 2009
© 2009 John Wiley & Sons Ltd.,
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Library of Congress Cataloging-in-Publication Data
Kalivas, Grigorios.
Digital radio system design / Grigorios Kalivas.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-470-84709-1 (cloth)
1. Radio—Transmitter-receivers—Design and construction. 2. Digital communications—Equipment and
supplies—Design and construction. 3. Radio circuits—Design and construction. 4. Signal processing—Digital
techniques. 5. Wireless communication systems—Equipment and supplies—Design and construction. I. Title.
TK6553.K262 2009
621.384
131—dc22 2009015936
A catalogue record for this book is available from the British Library.
ISBN 9780470847091 (H/B)
Set in 10/12 Times Roman by Macmillan Typesetting
Printed in Singapore by Markono
To Stella, Maria and Dimitra and to the memory of my father
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Contents
Preface xiii
1 Radio Communications: System Concepts, Propagation and Noise 1
1.1 Digital Radio Systems and Wireless Applications 2
1.1.1 Cellular Radio Systems 2
1.1.2 Short- and Medium-range Wireless Systems 3
1.1.3 Broadband Wireless Access 6
1.1.4 Satellite Communications 6
1.2 Physical Layer of Digital Radio Systems 7
1.2.1 Radio Platform 7
1.2.2 Baseband Platform 9
1.2.3 Implementation Challenges 10
1.3 Linear Systems and Random Processes 11
1.3.1 Linear Systems and Expansion of Signals in Orthogonal Basis Functions 11
1.3.2 Random Processes 12
1.3.3 White Gaussian Noise and Equivalent Noise Bandwidth 15
1.3.4 Deterministic and Random Signals of Bandpass Nature 16
1.4 Radio Channel Characterization 19
1.4.1 Large-scale Path Loss 19
1.4.2 Shadow Fading 22
1.4.3 Multipath Fading in Wideband Radio Channels 22
1.5 Nonlinearity and Noise in Radio Frequency Circuits and Systems 32
1.5.1 Nonlinearity 32
1.5.2 Noise 38
1.6 Sensitivity and Dynamic Range in Radio Receivers 44
1.6.1 Sensitivity and Dynamic Range 44
1.6.2 Link Budget and its Effect on the Receiver Design 44
1.7 Phase-locked Loops 46
1.7.1 Introduction 46
1.7.2 Basic Operation of Linear Phase-locked Loops 46
1.7.3 The Loop Filter 48
1.7.4 Equations and Dynamic Behaviour of the Linearized PLL 50
1.7.5 Stability of Phase-locked Loops 53
1.7.6 Phase Detectors 55
1.7.7 PLL Performance in the Presence of Noise 59
1.7.8 Applications of Phase-locked Loops 60
References 62
viii Contents
2 Digital Communication Principles 65
2.1 Digital Transmission in AWGN Channels 65
2.1.1 Demodulation by Correlation 65
2.1.2 Demodulation by Matched Filtering 67
2.1.3 The Optimum Detector in the Maximum Likelihood Sense 69
2.1.4 Techniques for Calculation of Average Probabilities of Error 72
2.1.5 M -ary Pulse Amplitude Modulation (PAM) 73
2.1.6 Bandpass Signalling 75
2.1.7 M -ary Phase Modulation 82
2.1.8 Offset QPSK 89
2.1.9 Quadrature Amplitude Modulation 90
2.1.10 Coherent Detection for Nonideal Carrier Synchronization 93
2.1.11 M -ary Frequency Shift Keying 96
2.1.12 Continuous Phase FSK 98
2.1.13 Minimum Shift Keying 103
2.1.14 Noncoherent Detection 106
2.1.15 Differentially Coherent Detection (M -DPSK) 107
2.2 Digital Transmission in Fading Channels 112
2.2.1 Quadrature Amplitude Modulation 112
2.2.2 M -PSK Modulation 113
2.2.3 M -FSK Modulation 113
2.2.4 Coherent Reception with Nonideal Carrier Synchronization 114
2.2.5 Noncoherent M -FSK Detection 116
2.3 Transmission Through Band-limited Channels 117
2.3.1 Introduction 117
2.3.2 Baseband Transmission Through Bandlimited Channels 120
2.3.3 Bandlimited Signals for Zero ISI 122
2.3.4 System Design in Band-limited Channels of Predetermined Frequency Response 125
2.4 Equalization 128
2.4.1 Introduction 128
2.4.2 Sampled-time Channel Model with ISI and Whitening Filter 131
2.4.3 Linear Equalizers 134
2.4.4 Minimum Mean Square Error Equalizer 136
2.4.5 Detection by Maximum Likelihood Sequence Estimation 137
2.4.6 Decision Feedback Equalizer 138
2.4.7 Practical Considerations 139
2.4.8 Adaptive Equalization 140
2.5 Coding Techniques for Reliable Communication 141
2.5.1 Introduction 141
2.5.2 Benefits of Coded Systems 143
2.5.3 Linear Block Codes 143
2.5.4 Cyclic Codes 145
2.6 Decoding and Probability of Error 147
2.6.1 Introduction 147
2.6.2 Convolutional Codes 151
2.6.3 Maximum Likelihood Decoding 154
2.6.4 The Viterbi Algorithm for Decoding 156
2.6.5 Transfer Function for Convolutional Codes 157
2.6.6 Error Performance in Convolutional Codes 158
Contents ix
2.6.7 Turbo Codes 159
2.6.8 Coded Modulation 162
2.6.9 Coding and Error Correction in Fading Channels 164
References 168
3 RF Transceiver Design 173
3.1 Useful and Harmful Signals at the Receiver Front-End 173
3.2 Frequency Downconversion and Image Reject Subsystems 175
3.2.1 Hartley Image Reject Receiver 177
3.2.2 Weaver Image Reject Receiver 180
3.3 The Heterodyne Receiver 183
3.4 The Direct Conversion Receiver 185
3.4.1 DC Offset 186
3.4.2 I–Q Mismatch 188
3.4.3 Even-Order Distortion 189
3.4.4 1/f Noise 189
3.5 Current Receiver Technology 190
3.5.1 Image Reject Architectures 190
3.5.2 The Direct Conversion Architecture 206
3.6 Transmitter Architectures 208
3.6.1 Information Modulation and Baseband Signal Conditioning 209
3.6.2 Two-stage Up-conversion Transmitters 210
3.6.3 Direct Upconversion Transmitters 211
References 211
4 Radio Frequency Circuits and Subsystems 215
4.1 Role of RF Circuits 216
4.2 Low-noise Amplifiers 219
4.2.1 Main Design Parameters of Low-noise Amplifiers 219
4.2.2 LNA Configurations and Design Trade-offs 222
4.3 RF Receiver Mixers 227
4.3.1 Design Considerations for RF Receiver Mixers 227
4.3.2 Types of Mixers 228
4.3.3 Noise Figure 232
4.3.4 Linearity and Isolation 235
4.4 Oscillators 235
4.4.1 Basic Theory 235
4.4.2 High-frequency Oscillators 239
4.4.3 Signal Quality in Oscillators 241
4.5 Frequency Synthesizers 243
4.5.1 Introduction 243
4.5.2 Main Design Aspects of Frequency Synthesizers 244
4.5.3 Synthesizer Architectures 247
4.5.4 Critical Synthesizer Components and their Impact on the System Performance 253
4.5.5 Phase Noise 256
4.6 Downconverter Design in Radio Receivers 258
4.6.1 Interfaces of the LNA and the Mixer 258
4.6.2 Local Oscillator Frequency Band and Impact of Spurious Frequencies 261
x Contents
4.6.3 Matching at the Receiver Front-end 261
4.7 RF Power Amplifiers 263
4.7.1 General Concepts and System Aspects 263
4.7.2 Power Amplifier Configurations 264
4.7.3 Impedance Matching Techniques for Power Amplifiers 271
4.7.4 Power Amplifier Subsystems for Linearization 273
References 273
5 Synchronization, Diversity and Advanced Transmission Techniques 277
5.1 TFR Timing and Frequency Synchronization in Digital Receivers 277
5.1.1 Introduction 277
5.1.2 ML Estimation (for Feedback and Feed-forward) Synchronizers 280
5.1.3 Feedback Frequency/Phase Estimation Algorithms 282
5.1.4 Feed-forward Frequency/Phase Estimation Algorithms 286
5.1.5 Feedback Timing Estimation Algorithms 291
5.1.6 Feed-forward Timing Estimation Algorithms 293
5.2 Diversity 295
5.2.1 Diversity Techniques 295
5.2.2 System Model 296
5.2.3 Diversity in the Receiver 297
5.2.4 Implementation Issues 302
5.2.5 Transmitter Diversity 304
5.3 OFDM Transmission 306
5.3.1 Introduction 306
5.3.2 Transceiver Model 309
5.3.3 OFDM Distinct Characteristics 312
5.3.4 OFDM Demodulation 313
5.3.5 Windowing and Transmitted Signal 314
5.3.6 Sensitivities and Shortcomings of OFDM 315
5.3.7 Channel Estimation in OFDM Systems 339
5.4 Spread Spectrum Systems 342
5.4.1 Introduction and Basic Properties 342
5.4.2 Direct Sequence Spread Spectrum Transmission and Reception 348
5.4.3 Frequency Hopping SS Transmission and Reception 350
5.4.4 Spread Spectrum for Multiple Access Applications 352
5.4.5 Spreading Sequences for Single-user and Multiple Access DSSS 358
5.4.6 Code Synchronization for Spread Spectrum Systems 363
5.4.7 The RAKE Receiver 365
References 368
6 System Design Examples 371
6.1 The DECT Receiver 371
6.1.1 The DECT Standard and Technology 371
6.1.2 Modulation and Detection Techniques for DECT 372
6.1.3 A DECT Modem for a Direct Conversion Receiver Architecture 375
6.2 QAM Receiver for 61 Mb/s Digital Microwave Radio Link 394
6.2.1 System Description 394
6.2.2 Transmitter Design 396
Contents xi
6.2.3 Receiver Design 397
6.2.4 Simulation Results 403
6.2.5 Digital Modem Implementation 406
6.3 OFDM Transceiver System Design 416
6.3.1 Introduction 416
6.3.2 Channel Estimation in Hiperlan/2 418
6.3.3 Timing Recovery 423
6.3.4 Frequency Offset Correction 424
6.3.5 Implementation and Simulation 435
References 438
Index 441
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Preface
Radio communications is a field touching upon various scientific and engineering disciplines. From cellular radio, wireless networking and broadband indoor and outdoor radio to electronic surveillance, deep
space communications and electronic warfare. All these applications are based on radio electronic systems designed to meet a variety of requirements concerning reliable communication of information such
as voice, data and multimedia. Furthermore, the continuous demand for quality of communication and
increased efficiency imposes the use of digital modulation techniques in radio transmission systems
and has made it the dominant approach in system design. Consequently, the complete system consists of
a radio transmitter and receiver (front-end) and a digital modulator and demodulator (modem).
This book aims to introduce the reader to the basic principles of radio systems by elaborating on the
design of front-end subsystems and circuits as well as digital transmitter and receiver sections.
To be able to handle the complete transceiver, the electronics engineer must be familiar with diverse
electrical engineering fields like digital communications and RF electronics. The main feature of this
book is that it tries to accomplish such a demanding task by introducing the reader to both digital modem
principles and RF front-end subsystem and circuit design. Furthermore, for effective system design it is
necessary to understand concepts and factors that mainly characterize and impact radio transmission and
reception such as the radio channel, noise and distortion. Although the book tackles such diverse fields,
it treats them in sufficient depth to allow the designer to have a solid understanding and make use of
related issues for design purposes.
Recent advancements in digital processing technology made the application of advanced schemes (like
turbo coding) and transmission techniques like diversity, orthogonal frequency division multiplexing and
spread spectrum very attractive to apply in modern receiver systems.
Apart from understanding the areas of digital communications and radio electronics, the designer must
also be able to evaluate the impact of the characteristics and limitations of the specific radio circuits and
subsystems on the overall RF front-end system performance. In addition, the designer must match a link
budget analysis to specific digital modulation/transmission techniques and RF front-end performance
while at the same time taking into account aspects that interrelate the performance of the digital modem
with the characteristics of the RF front-end. Such aspects include implementation losses imposed by
transmitter–receiver nonidealities (like phase noise, power amplifier nonlinearities, quadrature mixer
imbalances) and the requirements and restrictions on receiver synchronization subsystems.
This book is intended for engineers working on radio system design who must account for every factor
in system and circuit design to producea detailed high-level design of the requiredsystem. Forthis reason,
the designer must have an overall and in-depth understanding of a variety of concepts from radio channel
characteristics and digital modem principles to silicon technology and RF circuit configuration for low
noise and low distortion design. In addition, the book is well suited for graduate students who study
transmitter/receiver system design as it presents much information involving the complete transceiver
chain in adequate depth that can be very useful to connect the diverse fields of digital communications
and RF electronics in a unified system concept.
To complete this book several people have helped in various ways. First of all I am indebted to
my colleagues Dimitrios Toumpakaris and Konstantinos Efstathiou for reading in detail parts of the
manuscript and providing me with valuable suggestions which helped me improve it on various levels.
xiv Preface
Further valuable help came from my graduate and ex-graduate students Athanasios Doukas, Christos
Thomos and Dr Fotis Plessas, who helped me greatly with the figures. Special thanks belong to Christos
Thomos, who has helped me substantially during the last crucial months on many levels (proof-reading,
figure corrections, table of contents, index preparation etc.).
1
Radio Communications: System Concepts, Propagation and Noise
A critical point for the development of radio communications and related applications was
the invention of the ‘super-heterodyne’ receiver by Armstrong in 1917. This system was used
to receive and demodulate radio signals by down-converting them in a lower intermediate
frequency (IF). The demodulator followed the IF amplification and filtering stages and was
used to extract the transmitted voice signal from a weak signal impaired by additive noise.
The super-heterodyne receiver was quickly improved to demodulate satisfactorily very weak
signals buried in noise (high sensitivity) and, at the same time, to be able to distinguish the
useful signals from others residing in neighbouring frequencies (good selectivity). These two
properties made possible the development of low-cost radio transceivers for a variety of applications. AM and FM radio were among the first popular applications of radio communications.
In a few decades packet radios and networks targeting militarycommunications gained increasing interest. Satellite and deep-space communications gave the opportunity to develop very
sophisticated radio equipment during the 1960s and 1970s. In the early 1990s, cellular communications and wireless networking motivated a very rapid development of low-cost, low-power
radios which initiated the enormous growth of wireless communications.
The biggest development effort was the cellular telephone network. Since the early 1960s
there had been a considerable research effort by the AT&T Bell Laboratories to develop a
cellular communication system. By the end of the 1970s the system had been tested in the field
and at the beginning ofthe 1980s the first commercial cellular systems appeared. Theincreasing
demand for higher capacity, low cost, performance and efficiency led to the second generation
of cellular communication systems in the 1990s. To fulfill the need for high-quality bandwidthdemanding applications like data transmission, Internet, web browsing and video transmission,
2.5G and 3G systems appeared 10 years later.
Along with digital cellular systems, wireless networking and wireless local area networks
(WLAN) technology emerged. The need to achieve improved performance in a harsh propagation environment like the radio channel led to improved transmission technologies like spread
spectrum and orthogonal frequency division multiplexing (OFDM). These technologies were
Digital Radio System Design Grigorios Kalivas
© 2009 John Wiley & Sons, Ltd
2 Digital Radio System Design
put to practice in 3G systems like wideband code-division multiple access (WCDMA) as well
as in high-speed WLAN like IEEE 802.11a/b/g.
Different types of digital radio system have been developed during the last decade that are
finding application in wireless personal area networks (WPANs). These are Bluetooth and
Zigbee, which are usedto realize wireless connectivity of personal devicesand home appliances
like cellular devices and PCs. Additionally, they are also suitable for implementing wireless
sensor networks (WSNs) that organize in an ad-hoc fashion. In all these, the emphasis is mainly
on short ranges, low transmission rates and low power consumption.
Finally, satellite systems are being constantly developed to deliver high-quality digital video
and audio to subscribers all over the world.
The aims of this chapter are twofold. The first is to introduce the variety of digital radio
systems and their applications along with fundamental concepts and challenges of the basic
radio transceiver blocks (the radio frequency, RF, front-end and baseband parts). The second is
to introduce the reader to the technical background necessary to address the main objective of
the book, which is the design of RF and baseband transmitters and receivers. For this purpose
we present the basic concepts of linear systems, stochastic processes, radio propagation and
channel models. Along with these we present in some detail the basic limitations of radio
electronic systems and circuits, noise and nonlinearities. Finally, we introduce one of the most
frequently used blocks of radio systems, the phase-locked loop (PLL), which finds applications
in a variety of subsystems in a transmitter/receiver chain, such as the local oscillator, the carrier
recovery and synchronization, and coherent detection.
1.1 Digital Radio Systems and Wireless Applications
The existence of a large number of wireless systems for multiple applications considerably
complicates the allocation of frequency bands to specific standards and applications across
the electromagnetic spectrum. In addition, a number of radio systems (WLAN, WPAN, etc.)
operating in unlicensed portions of the spectrum demand careful assignment of frequency
bands and permitted levels of transmitted power in order to minimize interference and permit
the coexistence of more than one radio system in overlapping or neighbouring frequency bands
in the same geographical area.
Below we present briefly most of the existing radio communication systems, giving some
information on the architectures, frequency bands, main characteristics and applications of
each one of them.
1.1.1 Cellular Radio Systems
A cellular system is organized in hexagonal cells in order to provide sufficient radio coverage
to mobile users moving across the cell. A base station (BS) is usually placed at the centre of the
cell for that purpose. Depending on theenvironment (rural or urban), the areas of thecells differ.
Base stations are interconnected through a high-speed wired communications infrastructure.
Mobile users can have an uninterrupted session while moving through different cells. This is
achieved by the MTSOs acting as network controllers of allocated radio resources (physical
channels and bandwidth) to mobile users through the BS. In addition, MTSOs are responsible
for routing all calls associated with mobile users in their area.
Second-generation (2G) mobile communications employed digital technology to reduce
cost and increase performance. Global system for mobile communications (GSM) is a very
Radio Communications: System Concepts, Propagation and Noise 3
successful 2G system that was developed and deployed in Europe. It employs Gaussian minimum shift keying (MSK) modulation, which is a form of continuous-phase phase shift keying
(PSK). The access technique is based on time-division multiple access (TDMA) combined
with slow frequency hopping (FH). The channel bandwidth is 200 kHz to allow for voice and
data transmission.
IS-95 (Interim standard-95) is a popular digital cellular standard deployed in the USA using
CDMA access technology and binary phase-shift keying (BPSK) modulation with 1.25 MHz
channel bandwidth. In addition, IS-136 (North American Digital Cellular, NADC) is another
standard deployed in North America. It utilizes 30 kHz channels and TDMAaccess technology.
2.5G cellular communication emerged from 2G because of the need for higher transmission
rates to support Internet applications, e-mail and web browsing. General Packet Radio Service
(GPRS) and Enhanced Data Rates for GSM Evolution (EGDE) are the two standards designed
as upgrades to 2G GSM. GPRS is designed to implement packet-oriented communication
and can perform network sharing for multiple users, assigning time slots and radio channels
[Rappaport02]. In doing so, GPRS can support data transmission of 21.4 kb/s for each of the
eight GSM time slots. One user can use all of the time slots to achieve a gross bit rate of
21.4 ×8 =171.2 kb/s.
EDGE is another upgrade of the GSM standard. It is superior to GPRS in that it can operate
using nine different formats in air interface [Rappaport02]. This allows the system to choose
the type and quality of error control. EDGE uses 8-PSK modulation and can achieve a maximum throughput of 547.2 kb/s when all eight time slots are assigned to a single user and no
redundancy is reserved for error protection. 3G cellular systems are envisaged to offer highspeed wireless connectivity to implement fast Internet access, Voice-over-Internet Protocol,
interactive web connections and high-quality, real-time data transfer (for example music).
UMTS (Universal Mobile Telecommunications System) is an air interface specified in
the late 1990s by ETSI (European Telecommunications Standards Institute) and employs
WCDMA, considered one of the more advanced radio access technologies. Because of the
nature of CDMA, the radio channel resources are not divided, but they are shared by all users.
For that reason, CDMA is superior to TDMA in terms of capacity. Furthermore, each user
employs a unique spreading code which is multiplied by the useful signal in order to distinguish
the users and prevent interference among them. WCDMA has 5 MHz radio channels carrying
data rates up to 2 Mb/s. Each 5 MHz channel can offer up to 350 voice channels [Rappaport02].
1.1.2 Short- and Medium-range Wireless Systems
The common characteristic of these systems is the range of operation, which is on the order
of 100 m for indoor coverage and 150–250 m for outdoor communications. These systems are
mostly consumer products and therefore the main objectives are low prices and low energy
consumption.
1.1.2.1 Wireless Local Area Networks
Wireless LANs were designed toprovide high-data-rate, high-performance wireless connectivity within a short range in theform of a network controlled by a number of central points (called
access points or base stations). Access points are used to implement communication between
two users by serving as up-link receivers and down-link transmitters. The geographical area
4 Digital Radio System Design
of operation is usually confined to a few square kilometres. For example, a WLAN can be
deployed in a university campus, a hospital or an airport.
The second and third generation WLANs proved to be the most successful technologies.
IEEE 802.11b (second generation) operates in the 2.4 GHz ISM (Industral, Scientific and
Medical) band within a spectrum of 80MHz. It uses direct sequence spread spectrum (DSSS)
transmission technology with gross bit rates of 1, 2, 5 and 11 Mb/s. The 11 Mb/s data rate
was adopted in late 1998 and modulates data by using complementary code keying (CCK)
to increase the previous transmission rates. The network can be formulated as a centralized
network using a number of access points. However, it can also accommodate peer-to-peer
connections.
The IEEE 802.11a standard was developed as the third-generation WLAN and was designed
to provide even higher bit rates (up to 54 Mb/s). It uses OFDM transmission technology and
operates in the 5 GHz ISM band. In the USA, the Federal Communications Commission (FCC)
allocated two bands each 100 MHz wide (5.15–5.25 and 5.25–5.35 GHz), and a third one at
5.725–5.825 GHz for operation of 802.11a. In Europe, HIPERLAN 2 was specified as the
standard for 2G WLAN. Its physical layer is very similar to that of IEEE 802.11a. However,
it uses TDMA for radio access instead of the CSMA/CA used in 802.11a.
The next step was to introduce the 802.11g, which mostly consisted of a physical layer specification at 2.4 GHz with data rates matching those of 802.11a (up to 54 Mb/s). To achieve that,
OFDM transmission was set as a compulsory requirement. 802.11g is backward-compatible
to 802.11b and has an extended coverage range compared with 802.11a. To cope with issues
of quality of service, 802.11e was introduced, which specifies advanced MAC techniques to
achieve this.
1.1.2.2 WPANs and WSNs
In contrast to wireless LANs, WPAN standardization efforts focused primarily on lower transmission rates with shorter coverage and emphasis on low power consumption. Bluetooth (IEEE
802.15.1), ZigBee (IEEE 802.15.4)and UWB (IEEE802.15.3) represent standardsdesigned for
personal area networking. Bluetooth is an open standard designed for wireless data transfer
for devices located a few metres apart. Consequently, the dominant application is the wireless
interconnection of personal devices like cellular phones, PCs and their peripherals. Bluetooth
operates in the 2.4 GHz ISM band andsupports data and voice traffic withdata rates of 780 kb/s.
It uses FH as an access technique. It hops in a pseudorandom fashion, changing frequency carrier 1600 times per second (1600 hops/s). It can hop to 80 different frequency carriers located
1 MHz apart. Bluetooth devices are organized in groups of two to eight devices (one of which is
a master) constituting a piconet. Each device of a piconet has an identity (device address) that
must be known to all members of the piconet. The standard specifies two modes of operation:
asynchronous connectionless (ACL) in one channel (used for data transfer at 723 kb/s) and
synchronous connection-oriented (SCO) for voice communication (employing three channels
at 64 kb/s each).
A scaled-down version of Bluetooth is ZigBee, operating on the same ISM band. Moreover, the 868/900 MHz band is used for ZigBee in Europe and North America. It supports
transmission rates of up to 250 kb/s covering a range of 30 m.
During the last decade, WSNs have emerged as a new field for applications of low-power
radio technology. In WSN, radio modules are interconnected, formulating ad-hoc networks.
Radio Communications: System Concepts, Propagation and Noise 5
WSN find many applications in the commercial, military and security sectors. Such applications concern home and factory automation, monitoring, surveillance, etc. In this case,
emphasis is given to implementing a complete stack for ad hoc networking. An important
feature in such networks is multihop routing, according to which information travels through
the network by using intermediate nodes between the transmitter and the receiver to facilitate reliable communication. Both Bluetooth and ZigBee platforms are suitable for WSN
implementation [Zhang05], [Wheeler07] as they combine low-power operation with network
formation capability.
1.1.2.3 Cordless Telephony
Cordless telephony was developed to satisfy the needs for wireless connectivity to the public
telephone network (PTN). It consists of one or more base stations communicating with one or
more wireless handsets. The base stations are connected to the PTN through wireline and are
able to provide coverage of approximately 100 m in their communication with the handsets.
CT-2 isa second-generationcordless phone system developed in the 1990s with extended range
of operation beyond the home or office premises.
On the other hand, DECT (Digital European Cordless Telecommunications) was developed
such that it can support local mobility in an office building through a private branch exchange
(PBX) system. In this way, hand-off is supported between the different areas covered by the
base stations. The DECT standard operates in the 1900 MHz frequency band. Personal handyphone system (PHS) is a more advanced cordless phone system developed in Japan which can
support both voice and data transmission.
1.1.2.4 Ultra-wideband Communications
A few years ago, a spectrum of 7.5 GHz (3.1–10.6 GHz) was given for operation of ultrawideband (UWB) radio systems. The FCC permitted very low transmitted power, because
the wide area of operation of UWB would produce interference to most commercial and even
military wireless systems. There are two technology directions for UWB development. Pulsed
ultra-wideband systems (P-UWB) convey information by transmitting very short pulses (of
duration in the order of 1 ns). On the other hand, multiband-OFDM UWB (MB-OFDM)
transmits information using the OFDM transmission technique.
P-UWB uses BPSK, pulse position modulation (PPM) and amplitude-shift keying (ASK)
modulation and it needs a RAKE receiver (a special type of receiver used in Spread Spectrum
systems) to combine energy from multipath in order to achieve satisfactory performance. For
very high bit rates (on the order of 500Mb/s) sophisticated RAKE receivers must be employed,
increasing the complexity of the system. On the other hand, MB-UWB uses OFDM technology
to eliminate intersymbol interference (ISI) created by high transmission rates and the frequency
selectivity of the radio channel.
Ultra-wideband technology can cover a variety of applications ranging from low-bit-rate,
low-power sensor networks to very high transmission rate (over 100 Mb/s) systems designed
to wirelessly interconnect home appliances (TV, PCs and consumer electronic appliances). The
low bit rate systems are suitable for WSN applications.
P-UWB is supported by the UWB Forum, which has more than 200 members and focuses
on applications related to wireless video transfer within the home (multimedia, set-top boxes,
6 Digital Radio System Design
DVD players). MB-UWB is supported by WiMediaAlliance, alsowith more than 200 members.
WiMedia targets applications related to consumer electronics networking (PCs TV, cellular
phones). UWB Forum will offer operation at maximum data rates of 1.35 Gb/s covering distances of 3 m [Geer06]. On the otherhand, WiMediaAlliance willprovide 480 Mb/s at distances
of 10 m.
1.1.3 Broadband Wireless Access
Broadband wireless can deliver high-data-rate wireless access (on the order of hundreds of
Mb/s) to fixed access points which in turn distribute it in a local premises. Business and residential premises are served by a backbone switch connected at the fixed access point and
receive broadband services in the form of local area networking and video broadcasting.
LMDS (local multipoint distribution system) and MMDS (multichannel multipoint distribution services) are two systems deployed in the USA operating in the 28 and 2.65 GHz bands.
LMDS occupies 1300 MHz bandwidth in three different bands around 28, 29 and 321 GHz and
aims to provide high-speed data services, whereas MMDS mostly provides telecommunications services [Goldsmith05] (hundreds of digital television channels and digital telephony).
HIPERACCESS is the European standard corresponding to MMDS.
On the other hand, 802.16 standard is being developedto specify fixed and mobile broadband
wireless access with high data rates and range of a few kilometres. It is specified to offer
40 Mb/s for fixed and 15 Mb/s for mobile users. Known as WiMAX, it aims to deliver multiple
services in long ranges by providing communication robustness, quality of service (QoS) and
high capacity, serving as the ‘last mile’ wireless communications. In that capacity, it can
complement WLAN and cellular access. In the physical layer it is specified to operate in bands
within the 2–11 GHz frequency range and uses OFDM transmission technology combined
with adaptive modulation. In addition, it can integrate multiple antenna and smart antenna
techniques.
1.1.4 Satellite Communications
Satellite systems are mostly used to implement broadcasting services with emphasis on highquality digital video and audio applications (DVB, DAB). The Digital Video Broadcasting
(DVB) project specified the first DVB-satellite standard (DVB-S) in 1994 and developed the
second-generation standard (DVB-S2) for broadband services in 2003. DVB-S3 is specified to
deliver high-quality video operating in the 10.7–12.75 GHz band. The high data rates specified
by the standard can accommodate up to eight standard TV channels per transponder. In addition
to standard TV, DVB-S provides HDTV services and is specified for high-speed Internet
services over satellite.
In addition to DVB, new-generation broadband satellite communications have been
developed to support high-data-rate applications and multimedia in the framework of
fourth-generation mobile communication systems [Ibnkahla04].
Direct-to-Home (DTH) satellite systems are used in North America and constitute two
branches: the Broadcasting Satellite Service (BSS) and the Fixed Satellite Service (FSS). BSS
operates at 17.3–17.8 GHz (uplink) and 12.2–12.7 GHz (downlink), whereas the bands for FSS
are 14–14.5 and 10.7–11.2 GHz, respectively.
Radio Communications: System Concepts, Propagation and Noise 7
Finally, GPS (global positioning satellite) is an ever increasing market for providing localization services (location finding, navigation) and operates using DSSS in the
1500 MHz band.
1.2 Physical Layer of Digital Radio Systems
Radio receivers consist of an RF front-end, a possible IF stage and the baseband platform
which is responsible for the detection of the received signal after its conversion from analogue
to digital through an A/D converter. Similarly, on the transmitter side, the information signal is
digitally modulated and up-converted to a radio frequency band for subsequent transmission.
In the next section we use the term ‘radio platform’to loosely identify allthe RF and analogue
sections of the transmitter and the receiver.
1.2.1 Radio Platform
Considering the radio receiver, the main architectures are the super-heterodyne (SHR) and the
direct conversion receiver (DCR). These architectures are examined in detail in Chapter 3, but
here we give some distinguishing characteristics as well as theirmain advantages and disadvantages in the context of some popular applications of radio system design. Figure 1.1 illustrates
the general structure of a radio transceiver. The SHR architecture involves a mixing stage just
after the low-noise amplifier (LNA) at the receiver or prior to the transmitting medium-power
and high-power amplifiers (HPA). Following this stage, there is quadrature mixing bringing the
received signal down to the baseband. Following mixers, there is variable gain amplification
and filtering to increase the dynamic range (DR) and at the same time improve selectivity.
When the local oscillator (LO) frequency is set equal to the RF input frequency, the received
signal is translated directly down to the baseband. The receiver designed following this
approach is called Direct conversion Receiver or zero-IF receiver. Such an architecture eliminates the IF and the corresponding IF stage at the receiver, resulting in less hardware but, as
we will see in Chapter 3, it introduces several shortcomings that can be eliminated with careful
design.
Comparing the two architectures, SHR is advantageous when a very high dynamic range
is required (as for example in GSM). In this case, by using more than one mixing stage,
amplifiers with variable gain are inserted between stages to increase DR. At the same time,
filtering inserted between two mixing stages becomes narrower, resulting in better selectivity
[Schreir02].
Furthermore, super-heterodyne can be advantageous compared with DCR when large
in-band blocking signals have to be eliminated. In DCR, direct conversion (DC) offset would
change between bursts, requiring its dynamic control [Tolson99].
Regarding amplitude and phase imbalances of the two branches, In-phase (I-phase) and
Q-phase considerably reduce the image rejection in SHR. In applications where there can be
no limit to the power of the neighbouring channels (like the ISM band), it is necessary to have
an image rejection (IR) on the order of 60 dB. SHR can cope with the problem by suitable
choice of IF frequencies [Copani05]. At the same time, more than one down-converting stage
relaxes the corresponding IR requirements. On the other hand, there is no image band in
DCR and hence no problem associated with it. However, in DCR, imbalances at the I–Q
A/D
RF Down Converter
Transmit/
8
Receive
Switch
RF Up Converter
RF Synthesized
Local Oscillator
IF Synthesized
Local Oscillator
0
90
A/D
Digital
Baseband
Processor
D/A
0
90
D/A
Figure 1.1 General structure of a radio transmitter and receiver
Radio Communications: System Concepts, Propagation and Noise 9
mixer create problems from the self-image and slightly deteriorate the receiver signal-to-noise
ratio (SNR) [Razavi97]. This becomes more profound in high-order modulation constellations
(64-QAM, 256-QAM, etc.)
On the other hand, DCR is preferred when implementation cost and high integration are
the most important factors. For example, 3G terminals and multimode transceivers frequently
employ the direct conversion architecture. DC offset and 1/f noise close to the carrier are
the most frequent deficiencies of homodyne receivers, as presented in detail in Chapter 3.
Furthermore, second-order nonlinearities can also create a problem at DC. However, digital
and analogue processing techniques can be used to eliminate these problems.
Considering all the above and from modern transceiver design experience, SHR is favoured
in GSM, satellite and millimetre wave receivers, etc. On the other hand, DCR is favoured in
3G terminals, Bluetooth and wideband systems like WCDMA, 802.11a/b/g, 802.16 and UWB.
1.2.2 Baseband Platform
The advent of digital signal processors (DSP) and field-programmable gate arrays (FPGAs),
dramatically facilitated the design and implementation of very sophisticated digital demodulators and detectors for narrowband and wideband wireless systems. 2G cellular radio
uses GMSK, a special form of continuous-phase frequency-shift keying (CPFSK). Gaussian
minimum-shift keying (GMSK)modem (modulator–demodulator) implementationcan be fully
digital and can be based on simple processing blocks like accumulators, correlators and lookup tables (LUTs) [Wu00], [Zervas01]. FIR (Finite Impulse Response) filters are always used
to implement various forms of matched filters. Coherent demodulation in modulations with
memory could use more complex sequential receivers implementing the Viterbi algorithm.
3G cellular radios and modern WLAN transceivers employ advanced transmission techniques using either spread spectrum or OFDM to increase performance. Spread spectrum
entails multiplication of the information sequence by a high-bit-rate pseudorandom noise (PN)
sequence operating at speeds which are multiples of the information rate. The multiple bandwidth of the PN sequence spreads information and narrowband interference to a band with
a width equal to that of the PN sequence. Suitable synchronization at the receiver restores
information at its original narrow bandwidth, but interference remains spread due to lack of
synchronization. Consequently, passing the received signal plus spread interference through a
narrow band filter corresponding to the information bandwidth reduces interference considerably. In a similar fashion, this technique provides multipath diversity at the receiver, permitting
the collection and subsequent constructive combining of the main and the reflected signal components arriving at the receiver. This corresponds to the RAKE receiver principle, resembling
a garden rake that is used to collect leaves. As an example, RAKE receivers were used to cope
with moderate delay spread and moderate bit rates (60 ns at the rate of 11 Mb/s [VanNee99].
To face large delay spreads at higher transmission rates, the RAKE receiver was combined
with equalization. On the other hand, OFDM divides the transmission bandwidth into many
subchannels, each one occupying a narrow bandwidth. In this way, owing to the increase in
symbol duration, the effect of dispersion in time of the reflected signal on the receiver is minimized. The effect of ISI is completely eliminated by inserting a guard band in the resulting
composite OFDM symbol. Fast Fourier transform (FFT) is an efficient way to produce
(in the digital domain) the required subcarriers over which the information will be embedded.
In practice, OFDM is used in third-generation WLANs, WiMAX and DVB to eliminate ISI.
10 Digital Radio System Design
From the above discussion it is understood that, in modern 3G and WLAN radios, advanced
digital processing is required to implement the modem functions which incorporate transmission techniques like spread spectrum and OFDM. This can be performed using DSPs [Jo04],
FPGAs [Chugh05], application-specific integrated circuits (ASICs) or a combination of them
all [Jo04].
1.2.3 Implementation Challenges
Many challenges to the design and development of digital radio systems come from the necessity to utilize the latest process technologies (like deep submicron complementary metal-oxide
semiconductor, CMOS, processes) in order to save on chip area and power consumption.
Another equally important factor has to do with the necessity to develop multistandard and
multimode radios capable of implementing two or more standards (or more than one mode
of the same standard) in one system. For example, very frequently a single radio includes
GSM/GPRS and Bluetooth. In this case, the focus is on reconfigurable radio systems targeting
small, low-power-consumption solutions.
Regarding the radio front-end and related to the advances in process technology, some
technical challenges include:
•
reduction of the supply voltage while dynamic range is kept high [Muhammad05];
•
elimination of problems associated with integration-efficient architectures like the direct
conversion receiver; such problems include DC offset, 1/f noise and second order
nonlinearities;
•
low-phase-noise local oscillators to accommodate for broadband and multistandard system
applications;
•
wideband passive and active components (filters and low-noise amplifiers) just after the
antenna to accommodate for multistandard and multimode systems as well as for emerging
ultrawideband receivers;
For all the above RF front-end-related issues a common target is to minimize energy
dissipation.
Regarding the baseband section of the receiver, reconfigurability poses considerable challenges as it requires implementation of multiple computationally intensive functions (like FFT,
spreading, despreading and synchronization and decoding) in order to:
•
perform hardware/software partition that results in the best possible use of platform
resources;
•
define the architecture based on the nature of processing; for example, parallel and
computationally intensive processing vs algorithmic symbol-level processing [Hawwar06];
•
implement the multiple functionalities of the physical layer, which can include several
kinds of physical channels (like dedicated channels or synchronization channels), power
control and parameter monitoring by measurement (e.g. BER, SNR, signal-to-interference
ratio, SIR).
The common aspect of all the above baseband-related problems is to design the digital
platform such that partition of the functionalities in DSP, FPGAs and ASICs is implemented
in the most efficient way.
Radio Communications: System Concepts, Propagation and Noise 11
1.3 Linear Systems and Random Processes
1.3.1 Linear Systems and Expansion of Signals in Orthogonal
Basis Functions
A periodic signal s(t) of bandwidth BScan be fully reproduced by N samples per period T,
spaced 1/(2B
of dimension N =2B
time waveforms like s(t). Hence, we can define the inner product of two signals s(t) and y(t)
in an interval [c
Using this, a group of signals ψ
) seconds apart (Nyquist’s theorem). Hence, s(t) can be represented by a vector
S
T. Consequently, most of the properties of vector spaces are true for
S
, c2] as:
1
c
s(t), y(t)=
(t) is defined as orthonormal basis if the following is satisfied:
n
ψ
(t), ψm(t)= δmn=
n
2
s(t)y∗(t)dt (1.1)
c
1
1, m = n
0, m = n
(1.2)
This is used to expand all signals {s
In this case, ψ
(t) is defined as a complete basis for {sm(t), m = 1, 2, ...M } and we have:
n
s
(t) =
m
(t), m = 1, 2, ...M } in terms of the functions ψn(t).
m
smkψk(t), smk=
k
T
sm(t)ψk(t)dt (1.3)
0
This kind of expansion is of great importance in digital communications because the group
{s
(t), m =1, 2,...M }represents allpossible transmitted waveforms in atransmission system.
m
Furthermore, if y
(t) is the output of a linear system with input sm(t) which performs
m
operation H[·], then we have:
y
(t) = H [sm(t)] =
m
smkH[ψk(t)] (1.4)
k
The above expression provides an easy way to find the response of a system by determining the
response of it, when the basis functions ψ
(t) are used as inputs.
n
In our case the system is the composite transmitter–receiver system with an overall impulse
response h(t) constituting, in most cases, the cascading three filters, the transmitter filter, the
channel response and the receiver filter. Hence the received signal will be expressed as
the following convolution:
(t) = sm(t) ∗ h(t) (1.5)
y
m
For example, as shown in Section 2.1, in an ideal system where the transmitted signal is only
corrupted by noise we have:
T
r(t) = s
(t) + n(t), rk=
m
r(t)ψk(t)dt = smk+ n
0
k
(1.6)
Based on the orthonormal expansion
s
(t) =
m
smkψk(t)
k
12 Digital Radio System Design
of signal sm(t) as presented above, it can be shown [Proakis02] that the power content of a
periodic signal can be determined by the summation of the power of its constituent harmonics.
This is known as the Parseval relation and is mathematically expressed as follows:
tC+T
t
C
0
|sm(t)|2dt =
1
T
0
+∞
k=−∞
|smk|
2
(1.7)
1.3.2 Random Processes
Figure 1.2 shows an example for a random process X (t) consisting of sample functions xEi(t).
Since, as explained above, the random process at a specific time instant t
a random variable, the mean (or expectation function) and autocorrelation function can be
defined as follows:
∞
, t2) = E[X (t1)X (t2)] =
R
XX(t1
E{X (t
)}=mX(tC) =
C
−∞
+∞
+∞
−∞
−∞
xp
(x)dx (1.8)
X (tC)
x1x2p
X (t1)X (t2)
(x1, x2)dx1dx
Wide-sense stationary (WSS) is a process for which the mean is independent of t and its
autocorrelation is a function of the time difference t
and t2[RX(t1−t2) =RX(τ)].
t
1
=τ and not of the specific values of
1−t2
A random process is stationary if its statistical properties do not depend on time. Stationarity
is a stronger property compared with wide-sense stationarity.
Two important properties of the autocorrelation function of stationary processes are:
corresponds to
C
2
(1.9)
(−τ) =RX(τ), which means that it is an even function;
(1) R
X
(2) R
(τ) has a maximum absolute value at τ =0, i.e. |RX(τ)|≤RX(0).
X
Ergodicity is a very useful concept in random signal analysis. A stationary process is ergodic
if, for all outcomes E
and for all functions f (x), the statistical averages are equal to time
i
averages:
T/2
E{f [X (t)]}= lim
T→∞
1
T
f [xEi(t)]dt (1.10)
−T/2
1.3.2.1 Power Spectral Density of Random Processes
It is not possible to define a Fourier transform for random signals. Thus, the concept of power
spectral density (PSD) is introduced for random processes. To do that the following steps are
taken:
(1) Truncate the sample functions of a random process to be nonzero for t < T :
x
(t), 0 ≤ t ≤ T
x
Ei
(t; T) =
Ei
0, otherwise
(1.11)
Radio Communications: System Concepts, Propagation and Noise 13
x
(t)
E
1
t1t
2
0
(t)
x
E
2
t
0
(t)
x
E
N
0
) X(t2)
X(t
1
t
t
Figure 1.2 Sample functions of random process X (t)
(2) Determine |XTi( f )|2from the Fourier transform XTi( f ) of the truncated random process
(t; T). The power spectral density S
x
Ei
( f ) for xEi(t; T) is calculated by averaging over a
x
Ei
large period of time T:
T
= lim
T→∞
2
(1.12)
(t; T) [Proakis02]:
Ei
|
X
E
2
|
( f )
Ti
T
(1.13)
(3) Calculate the average E|X
( f ) =
S
X
Ti
E
i
S
= lim
( f )|
xEi( f )
2
lim
T→∞
T→∞
over all sample functions x
|
2
|
X
( f )
Ti
T
|XTi( f )|
14 Digital Radio System Design
The above procedure converts the power-type signals to energy-type signals by setting them
to zero for t > T . In this way, power spectral density for random processes defined as above
corresponds directly to that of deterministic signals [Proakis02].
In practical terms, S
( f ) represents the average power that would be measured at frequency
X
f in a bandwidth of 1 Hz.
Extending the definitions of energy and power of deterministic signals to random processes,
we have for each sample function x
=x
E
i
(t):
Ei
2
(t)dt, Pi= lim
Ei
T→∞
1
2
(t)dt (1.14)
x
Ei
T
Since these quantities are random variables the energy and power of the random process X(t)
corresponding to sample functions x
E
X
(t) are defined as:
Ei
= E
X2(t)dt=RX(t, t)dt (1.15)
= Elim
P
X
T→∞
T/2
1
T
X2(t)dt=
−T/2
T/2
1
T
RX(t, t)dt (1.16)
−T/2
For stationary processes, the energy and power are:
= RX(0)
P
X
+∞
E
=
X
RX(0)dt (1.17)
−∞
1.3.2.2 Random Processes Through Linear Systems
If Y (t) is the output of alinear system with input thestationary random process X (t) and impulse
response h(t), the following relations are true for the means and correlation (crosscorrelation
and autocorrelation) functions:
+∞
= m
m
Y
X
(τ) = RX(τ) ∗h(−τ) (1.19)
R
XY
R
(τ) = RX(τ) ∗h(τ) ∗ h(−τ) (1.20)
Y
h(t)dt (1.18)
−∞
Furthermore, translation of these expressions in the frequency domain [Equations (1.21)–
(1.23)] provides powerful tools to determine spectral densities along the receiver chain in the
presence of noise.
m
= mXH(0) (1.21)
Y
S
( f ) = SX|H( f )|
Y
S
( f ) = SX( f )H∗( f ) (1.23)
YX
2
(1.22)
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