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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Provides a clear, coherent review of all major wireless broadband standards with an emphasis on managing the explosive growth in mobile video 802.11ac/ad, 802.16m, 802.22, and LTE-Advanced are the emerging broadband wireless standards that offer many powerful wireless features. This book gives an accessible overview of the various standards and practical information on 802.11 link adaptation, 4G smartphone antenna design, wireless video streaming, and smart grids. Broadband Wireless Multimedia Networks distills the many complex wireless features in a clean and concise manner so that the reader can understand the key principles. Topics covered include adaptive modulation and coding, orthogonal frequency-division multiple access, single-carrier frequency-division multiple access, multiple antenna systems, medium access control time and frequency-division duplex, transmission, and the frame formats. With wireless operators now carrying a much greater amount of video traffic than data and voice traffic, the book also covers adaptive bit rate streaming and bandwidth management for 3D and HD video delivery to multi-screen personal devices. Featured chapters in the book are: * Overview of Broadband Wireless Networks * IEEE 802.11 Standard * IEEE 802.16 Standard * Long-Term Evolution * ATSC Digital TV and IEEE 802.22 Standards * Mesh, Relay, and Interworking Networks * Wireless Video Streaming * Green Communications in Wireless Home Area Networks Including over 180 chapter-end exercises and 200 illustrative figures; and accessible recorded tutorials, Broadband Wireless Multimedia Networks is ideal for industry professionals and practitioners, graduate students, and researchers.
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Veröffentlichungsjahr: 2012
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Table of Contents
Cover
WILEY SERIES ON INFORMATION AND COMMUNICATION TECHNOLOGY
TITLE PAGE
COPYRIGHT PAGE
PREFACE
CHAPTER 1 OVERVIEW OF BROADBAND WIRELESS NETWORKS
1.1 INTRODUCTION
1.2 RADIO SPECTRUM
1.3 SIGNAL COVERAGE
1.4 MODULATION
1.5 MULTIPATH MITIGATION METHODS
1.6 MULTIPLE ANTENNA SYSTEMS
1.7 INTERFERENCE
1.8 MOBILITY AND HANDOFF
1.9 CHANNEL ASSIGNMENT STRATEGIES
1.10 PERFORMANCE EVALUATION OF WIRELESS NETWORKS
1.11 OUTDOOR DEPLOYMENT CONSIDERATIONS
CHAPTER 2 IEEE 802.11 STANDARD
2.1 802.11 DEPLOYMENTS AND APPLICATIONS
2.2 802.11 TODAY
2.3 IEEE 802.11 STANDARD
2.4 IEEE 802.11 NETWORK ARCHITECTURE
2.5 IEEE 802.11 BASIC REFERENCE MODEL
2.6 IEEE 802.11 SECURITY
2.7 IEEE 802.11N AMENDMENT
2.8 NEW IEEE 802.11 MULTIGIGABIT TASK GROUPS
2.9 IEEE 802.11AC AMENDMENT
2.10 IEEE 802.11AD AMENDMENT
CHAPTER 3 IEEE 802.16 STANDARD
3.1 OVERVIEW OF IEEE 802.16
3.2 BASIC IEEE 802.16 OPERATION
3.3 IEEE 802.16-2004 STANDARD
3.4 IEEE 802.16E AMENDMENT
3.5 IEEE 802.16 MEDIUM ACCESS CONTROL
3.6 IEEE 802.16M AMENDMENT
3.7 WIMAX FORUM
3.8 WIRELESS ACCESS USING WIMAX
CHAPTER 4 LONG TERM EVOLUTION
4.1 HIGH SPEED PACKET ACCESS
4.2 LONG TERM EVOLUTION
4.3 LTE-ADVANCED
4.4 FEMTOCELLS
4.5 ANTENNA DESIGN CHALLENGES FOR 4G SMARTPHONES
CHAPTER 5 ATSC DIGITAL TV AND IEEE 802.22 STANDARDS
5.1 DIGITAL TV FREQUENCY CHANNELS
5.2 DIGITAL TV STANDARDS
5.3 MOBILE TV
5.4 THE IEEE 802.22 STANDARD
5.5 WHITESPACE ALLIANCE
CHAPTER 6 MESH, RELAY, AND INTERWORKING NETWORKS
6.1 INTRODUCTION
6.2 802.11 MESH NETWORKS
6.3 HYBRID WIRELESS MESH PROTOCOL
6.4 802.16 RELAY NETWORKS
6.5 802.11 INTERWORKING WITH EXTERNAL NETWORKS
CHAPTER 7 WIRELESS VIDEO STREAMING
7.1 HIGH-DEFINITION AND 3D VIDEOS
7.2 VIDEO COMPRESSION
7.3 VIDEO STREAMING INTERFACES AND STANDARDS
7.4 ADAPTIVE VIDEO STREAMING
7.5 3D VIDEO TRANSMISSION
7.6 MEDIA-ACTIVATED WIRELESS COMMUNICATIONS
CHAPTER 8 GREEN COMMUNICATIONS IN WIRELESS HOME AREA NETWORKS
8.1 ZIGBEE OVERVIEW
8.2 SMART GRID CHALLENGES
8.3 HOME AREA NETWORKS
8.4 FUTURE CHALLENGES
GLOSSARY
INDEX
WILEY SERIES ON INFORMATION AND COMMUNICATION TECHNOLOGY
Series Editors: T. Russell Hsing and Vincent K. N. Lau
The Information and Communication Technology (ICT) book series focuses on creating useful connections between advanced communication theories, practical designs, and end-user applications in various next generation networks and broadband access systems, including fiber, cable, satellite, and wireless. The ICT book series examines the difficulties of applying various advanced communication technologies to practical systems such as WiFi, WiMax, B3G, etc., and considers how technologies are designed in conjunction with standards, theories, and applications.
The ICT book series also addresses application-oriented topics such as service management and creation and end-user devices, as well as the coupling between end devices and infrastructure.
T. Russell Hsing, PhD, is the Executive Director of Emerging Technologies and Services Research at Telcordia Technologies. He manages and leads the applied research and development of information and wireless sensor networking solutions for numerous applications and systems. Email: [email protected]
Vincent K.N. Lau, PhD, is Associate Professor in the Department of Electrical Engineering at the Hong Kong University of Science and Technology. His current research interest is on delay-sensitive cross-layer optimization with imperfect system state information. Email: [email protected]
Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
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Library of Congress Cataloging-in-Publication Data is available.
ISBN 978-0-470-92354-2
PREFACE
Wireless access has evolved rapidly over the last two decades and has become the dominant medium for network connectivity due to the recent proliferation of sleek personal devices, such as smartphones and tablets. These devices primarily target high-resolution video consumption but have been employed in diverse applications. Ultra-thin touchscreen tablets may well replace flip-to-open laptops in the future, and this will eradicate the need for bulky wired network interfaces. It started with the cellular revolution (1990), followed by Wi-Fi (2000), and many personal devices are now demanding ever-increasing wireless bandwidth to enable high-quality video service. When the cellphone became the first wireless innovation to be available for widespread public use, it helped consumers in many ways, ranging from the convenience of casual conversations to mobile entertainment to emergency calls to preventing burglars from “cutting” the wire phone line before entering a house. Since then, impressive improvements in wireless data rates and coverage have directly benefited enterprise, home, and public access networks. This book covers a broad range of key wireless technologies with a comparative assessment of the strengths and weaknesses. It also discusses the role of high-speed indoor networks and broadband 4G systems in driving the utility of new personal devices with mobile video support. A specific emphasis will be placed on challenging deployments and implementation, such as dealing with the antenna constraints for handheld devices and overcoming the rate mismatch and variation in multiuser video transmission.
Despite recent progress, fundamental challenges in wireless deployment remain. For example, the communication link remains unpredictable, connections can be intermittent, and the bandwidth can be varying due to mobility and possible interference from on-site and non-network entities. A fixed network topology based on the star topology (with a wireless router or base station for relaying user traffic) is now the norm. Self-organizing ad hoc wireless networks are rare but direct point-to-point wireless connections between two user devices are occasionally used. Because there are many competing standards, selecting the appropriate wireless technology can be a difficult task. While the attributes and application of the technology play important roles, affordable pricing and value to consumers are the common denominators that determine success. Thus, a significant percentage of wireless users still prefers to use Wi-Fi even though smartphones and tablets have become 4G-ready. Besides the cost consideration, the connectivity of these personal devices does not work well without Wi-Fi. Wi-Fi has become the bedrock of many consumer electronics devices and the dominant technology for high-speed in-building and in-home networking. Some wireless carriers are even bundling Wi-Fi services to their cellular and Internet service offerings, allowing subscribers free Wi-Fi access in stores and coffeeshops all over the United States. Currently, LTE, WiMAX, and HSPA+ are forming a cellular ecosystem, providing broadband wireless Internet access in urban cities as well as underserved and unserved remote areas, and enabling new applications for health care, education, and public safety communications.
Spectrum availability is a key consideration to improving wireless data rates. LTE is unlikely to provide 100 Mbit/s on the downlink and 50 Mbit/s on the uplink anytime soon until more spectrum becomes available. According to the Federal Communications Commission (FCC), by 2014 mobile data traffic is expected to be 35 times greater than 2009 levels. The FCC’s national broadband plan calls for releasing 300 MHz of spectrum within the next 5 years and 500 MHz within the next 10 years to meet rising mobile data demands. In January 2010, the FCC opened a proceeding to explore expanded terrestrial mobile use in the mobile satellite service (MSS) bands (up to 90 MHz more). In May 2010, the commission approved an order that changes the rules governing 25 MHz of the Wireless Communications Services (WCS) spectrum in the 2.3 GHz band, which is now available for mobile broadband use. The rules are put in place to avoid interference issues. In September 2010, it approved the use of unlicensed white space spectrum, clearing the way for new classes of devices that take advantage of “super Wi-Fi.” In November 2010, the FCC approved a notice of proposed rulemaking that lays the groundwork for reallocating 120 MHz of broadcast TV spectrum for wireless, via incentive auctions with broadcasters. The rules will create a licensing framework for spectrum in the UHF/VHF bands and will allow for voluntary channel sharing. The TV broadcast spectrum rulemaking allows all spectrum in the TV bands to be opened to fixed, mobile, and broadcast services. The rulemaking also proposes rules that permit two or more TV broadcasters to share a single 6 MHz channel, although stations will retain rights to mandatory broadcast carriage. In April 2012, the FCC released an order that details how TV stations can share the same channel. All stations utilizing a shared channel retain spectrum usage rights sufficient to ensure enough capacity to operate one standard-definition TV program stream at all times. The order may motivate TV stations that agree to share channels to turn over broadcast TV spectrum to the FCC, which will in turn auction off the spectrum to support the bandwidth demand from smartphones, tablets, and other mobile data products. TV stations that contribute spectrum to such an auction may share the proceeds. The move will allow FCC to recover up to 120 MHz or 40% of all TV spectrum.
Although the data rate is an important metric to gauge the effectiveness of any new wireless technology, the availability of connected devices and applications are key drivers for adopting the technology. Mobile entertainment has become a key application due to the widespread adoption of video-capable smartphones and tablets. These devices have become ubiquitous with greatly expanded computing power and memory, improved displays, and wireless connectivity. The iPhone was the first mobile device to combine voice and Web applications successfully. Fifty million iPhones are expected to be sold in the final quarter of 2012. The iPad tablet shipped over 28 million units in 2011. These figures can be compared with iPod (released in October 2001 with 220 million units sold by September 2009) and Blackberry (160,000 units in 2001 with 36 million in use by 2009). 4G smartphones (e.g. Sprint’s Evo and Apple’s iPhone 5) and tablets (e.g. Motorola’s Xoom, Apple’s new iPad, and Samsung’s Galaxy Tab 10.1) have now become mainstream. The latest iPad tablet not only supports LTE, but also works with a number of carriers, such as Verizon, AT&T, Rogers, Bell, and Telus. It provides 10 hours of battery life on active usage and 9 hours on LTE usage. It also supports Wi-Fi personal hotspots and can share a network connection with up to 5 devices.
In the third quarter of 2010, the United States accounted for nearly 50% of smartphone sales worldwide. During the same quarter, Android emerged as the dominant mobile operating system in the United States with a market share of just over 40%. The utility of Android lies in the large community of developers writing apps that extend the functionality of the devices. There are currently over 100,000 Android apps, which control the device via Google-developed Java libraries. Apple iPhone apps are equally popular and are proliferating as well (over 10 billion downloads to date with 300,000 apps). They range from multiuser games and social apps to creative ones that allow the user to watch four camera views of the World Series on the iPhone. TV remains the single most important source of information and entertainment. In 2010, U.S. teenagers spent more than three times the amount of their spare time watching TV than they spent on social media. According to online video, which includes online TV, represents one of the key traffic types to be carried over personal smartphones and tablets with high-resolution displays. The year 2012 could be the first year that U.S. consumers watch more movies online than on DVDs, Blu-ray discs, and other physical video formats. However, supporting high-quality video presents a significant challenge to wireless carriers due to the higher bandwidth demands compared with data and voice traffic. Thus, many carriers switch from unlimited data plans to usage-based plans, enforcing caps in bandwidth usage. For example, AT&T currently charges 3G subscribers $15 for 200 MB of data and $25 for 2 GB. Verizon Wireless currently charges LTE subscribers $50 for 5 GB and $80 for 10 GB, with $10 per GB for over the limit usage. These figures should be compared with the monthly data usage caps currently imposed by AT&T for wireline access networks: 150 GB for DSL and 250 GB for U-Verse. Subscribers will be charged an additional $10 for every 50 GB beyond those limits. Clearly, there is a big gap between wireline and wireless caps, but note that there is no major difference between the DSL and LTE data rates. Streaming just two 2-hour movies can max out a 5 GB/month data plan. If music is streamed at a bit rate of 160 Kbit/s for an hour a day, this equates to about 2.2 GB of data per month. Adaptive streaming is a key technology to achieve reliable video delivery over heterogeneous wireless networks and multiscreen consumer devices. It provides autonomous bandwidth management and maintains quality of service even as wireless link conditions fluctuate and network congestion on the Internet vary.
So what’s next for wireless? Although there are some encouraging breakthroughs, improving the capacity and range of wireless networks remain paramount, and this must be properly balanced with implementation, deployment, and power consumption considerations. I have the privilege of reviewing many interesting research articles focusing on cognitive radio, vehicular, millimeter radio, sensor, nano, and bio networks, as well as the current buzz—network science involving identifying synergies in different network types. Unfortunately, many real-world issues are often brushed aside in these research articles. Allow me to provide some illustrations. Wireless sensor networks have received a lot of research attention. These networks aim to remove the wiring for both data connectivity and power supply. As an example, ultra-low power Zigbee switches are able to help turn on lights without wire cabling to connect the switch to the light. The use of Wi-Fi or Bluetooth switches will require replacement of hundreds or thousands of sensor batteries in a large building on a regular basis, which makes these sensors challenging to install and maintain. There are enormous benefits in sensors collecting time-critical information. Environmental sensor applications (e.g., factory/home/office automation and oil leak/earthquake detection), biosensors tracking human conditions and collecting vital signs of patients, and networked sensors (i.e., sensors that collect data from various locations and then independently upload this information to a network for human analysis) are just a few examples. However, I am somewhat skeptical about the practicality of self-autonomous sensor networks, which is the major thrust of many research articles. Here, I am referring to sensors directly managing other sensors via an ad hoc multihop network. Sensors are simple devices that are designed with limited functionality to reduce power consumption and cost requirements. For example, a typical temperature sensor sends only 200 bytes of data every 5 minutes. These sensors cannot be relied upon to control other sensors. For instance, there is a light sensor in my office that tends to put me to sleep whenever there are dark clouds outside. I am aware of other light sensors that automatically turn off the lights when the room is well lit with sunlight, but you can figure out how such sensors will fail in other ways. As another example, the temperature sensor in my home defies logic when it decides to turn on the air conditioner less frequently for a long duration of time, and at other times, turn it on more frequently for a shorter duration of time. In addition to the reliability issue, self-configuring and self-healing wireless sensor networks may trigger frequent network topology reconfiguration when channel conditions change and sensor nodes randomly run out of battery. This will in turn require more rerouting messages to be exchanged, draining battery power and causing more node failures. Thus, setting up a fail-safe, self-organizing wireless sensor network involving concurrent link and route establishments of hundreds of sensors (as in some military deployments and first responder operations) may be unrealistic. More practical validation work has to be conducted on such large-scale sensor networks using cheap and disposable life-size sensors.
The usefulness of cognitive radio is another area that deserves further validation. Like multiple antenna systems, cognitive radio addresses the issue of limited radio spectrum. The technology has been employed for wireless systems operating in the TV bands recently. However, one-way broadcast operations may be more suited for these bands due to the signal propagation delay. Sensing ongoing transmissions complicates two-way system deployment, as many spurious signals have to be filtered; and not all interfering signals can be detected, since the sensing range (usually based on detecting signal power levels) can be longer than the RF reception range (usually based on decoding the desired signals). Simply setting a predefined signal threshold may potentially lead to many false alarms, especially in wide-area deployments. Wi-Fi technology can be considered a form of cognitive radio since the medium must be sensed idle before a packet is transmitted. Because Wi-Fi is designed for use in short-range networks (typically spanning between 100 and 200 ft), faster signal propagation allows carrier sensing to work well at these distances. Its performance degrades dramatically in an exhibit hall when many transmitters try to coexist in the same frequency band. Cognitive radio networks suffer from a similar problem, which must be solved before efficient management of limited spectrum resources can be achieved. There are other practical issues that need to be overcome: cost-effective wideband antennas to deal with collocated interference, multiband interactions, insertion loss; low-noise wideband power amplifiers to process RF and IF processing; high-performance analog-to-digital and digital-to-analog conversion; and high-performance DSPs based on FPGA and ASIC designs. Finally, there is a nontrivial task of dynamically switching and adapting the modulation and coding schemes according to time-varying interference and fading conditions.
It is common for wireless communications researchers to associate technical depth with mathematical dexterity. More often than not, this clouds the motivation and usefulness of the research. In addition, many simplifying (and often unrealistic) assumptions arise when solving complex mathematical problems. Technical depth should be related to real-world engineering impact. Technically profound papers, such as the pioneering TCP/IP paper authored by Vince Cerf and Bob Kahn, would probably be rejected today as lack of technical depth (no equations). Fortunately, this IEEE transactions paper was published in 1974 and continues to drive the most important engineering invention for the last four decades or so—the Internet. This is the only paper that has become an interoperable data networking standard, and I am amazed how it has become even more relevant today, forming the basis to support high-quality video streaming on the Internet as well as mobile wireless networks, whereas many newer protocols that were designed for real-time multimedia streaming have failed when it comes to practical implementation. Good engineering research can be compared with good movies—you need a good storyline (motivation) with a supporting cast of good actors/actresses (researchers) and audio/visual effects (practical tools). It is not possible to break new grounds in nascent engineering research without real-world considerations. For example, many prominent scientists have been predicting pervasive wireless personal communications for over two decades but it was only the recent availability of touchscreen devices that really drives the demand for high-speed wireless networks. Similarly, the improved spectral efficiency promised by high-order multiple input multiple output (MIMO) systems has been severely challenged. It is like expecting voice recognition technology to work well when several people near the source are talking loudly at the same time. In addition, battery-operated user devices pose space and power constraints when building multiantenna transmitters. LTE handsets currently support single-antenna transmission. Although 802.11n access points and home gateways have just started using three spatial antenna streams, they will switch to a lower number of streams if end-user devices that support one or two streams are detected. Since the vast majority of smartphones and tablets only support a single stream, the higher capacity of three streams (or even two streams) cannot be attained for these devices. Judging from the 2012 CES Show, it appears many vendors are already moving toward single-antenna 802.11ac user devices and adapters with broader channel bandwidths and higher-order modulation instead of developing the complex option of four transmit spatial streams that is available in 802.11n. Thus, future wireless communications may well be dictated by single-transmitter user devices operating on wideband channels.
Transformative engineering research should lead to physical discoveries. Consider the following quote from Michael Faraday in 1827: “I could not imagine much progress by reading only, without experimental facts and trials … I was never able to make a fact my own without seeing it.” Even more remarkable is the quote from Hermann von Helmholtz in 1881: “Faraday performed in his brain the work of a great mathematician without using a single mathematical formula.” James Clerk Maxwell’s unifying electromagnetic equations may not hold much significance if they were formulated prior to the physical discoveries made by Faraday. To this end, I am glad that many wireless standard committees have taken the lead in commercializing many breakthrough wireless technologies. Although OFDM was tested in the labs in the 1980s, the first real-world demonstration of the technology actually began with 802.11a in 1999. The 802.11 and 802.16 Working Groups were the first to commercialize multiantenna MIMO technology. The 802.11 Working Group is now leading pioneering efforts to improve network throughput to gigabits per second using multiuser MIMO and 60 GHz technologies. The 802.16 and LTE standard groups should be credited for demonstrating the practicality of orthogonal frequency division multiple access (OFDMA) and frequency domain equalization, respectively. These wireless standards undergo numerous revisions from various vendors (many with working prototypes) before the standard is ratified. This is the benchmark for successful technology innovation because it helps validate new concepts, drive cost-effective implementation, ensure interoperability, and improve performance. An important wireless frontier for the future could be the removal of the power cord and batteries. Wireless electric power will solve many RF design constraints associated with many mobile devices. It will eliminate chargers and eventually, batteries. Since the human body is affected by electric fields and not magnetic fields, the magnetic field can be used to transport electric power wirelessly in a safe and efficient manner. Several promising experiments have been conducted to demonstrate the feasibility of wireless electricity. For example, Intel’s magnetic energy resonant wireless electric power prototype is able to light up a 60 W bulb that uses more power than a typical laptop.
I would like to thank Dr. Simone Taylor of John Wiley & Sons for her encouragement and patience in overseeing this book project. The excellent layout of the book is due to the professional efforts of Janet Hronek of Toppan Best-set Premedia Ltd. and Diana Gialo. I would also like to acknowledge my past collaborators and students who have been unselfish in sharing many useful comments. I wish to thank Eric Levine of the IEEE Communications Society for his tireless efforts in securing industry sponsors for eight of my online tutorials. The valuable feedback received from the tutorial participants helped to shape the current contents of the book. The advantage of writing a book as opposed to a research paper is that you can obtain critical but constructive reviews from leading experts whom you know. To this end, I am indebted to the following reviewers. They are Professors Tim Brown, Andrew Paplinski, Jiang Xie, Admela Jukan, and Michael Fang and Dr. Zhensheng Zhang. I am also grateful to Dr. Richard Van Nee, Dr. Christopher Hansen, Dr. Bjorn Bjerke, Dr. Chirag Patel, Naftali Chayat, Frank Gonzalez, and John Civiletto for their generous insights on various wireless topics. Finally, I wish to thank Dr. Frank Caimi for his contribution on 4G antenna design and Dr. Bob Heile for his chapter on ZigBee and green communications.
I hope this book will serve as a useful resource to many practicing engineers, as well as researchers and students. I have attempted to cover only the more popular wireless technologies and standards, which should equip the reader with a strong foundation to understand emerging technologies. Unsolved real-world issues in wireless networking are emphasized. I have included over 180 homework problems to help the reader master the concepts described in each chapter and discover new insights and utility of wireless technologies. The majority of these problems do not require math but are designed to stimulate critical thinking. Hopefully, this will inspire new breakthroughs. The solutions to these homework problems are available to instructors. I hope the concise writing style resonates with the reader, and I have attempted to spice that up with many interesting technology snippets. I wish I could include a collection of cartoons related to wireless that would make the book lighthearted, engaging, and hard to put down. I guess I will have to reserve them for the courses that I teach. I have an uneasy intuition that certain cartoon creators have greater technical vision and sharper minds than any of us. As a substitute, I have included 200 illustrative figures to aid comprehension and reinforce concepts. These figures are again available to instructors. Please feel free to send your comments, corrections, and questions to [email protected]. They will be gratefully received.
Benny Bing
CHAPTER 1
OVERVIEW OF BROADBAND WIRELESS NETWORKS
Mobility and flexibility make wireless networks effective extensions and attractive alternatives to wired networks. Wireless networks provide all the functionality of wired networks, but without the physical constraints of the wire itself. However, the wireless link possesses some unique obstacles that need to be solved. For example, the medium is a scarce resource that must be shared among network users. It can be noisy and unreliable where transmissions from mobile users interfere with each other to varying degrees. The transmitted signal power dissipates in space rapidly and becomes attenuated. Physical obstructions may block or generate multiple copies of the transmitted signal. The received signal strength normally changes slowly with time because of path loss, more quickly with shadow fading and very quickly because of multipath fading. The most distinguishing issues in wireless network design are the constraints placed on bandwidth and power efficiency.
The broadcast nature of wireless transmission offers ubiquity and immediate access for both fixed and mobile users, clearly a vital element of quad-play (voice, video, data, and mobile) services. Moving from one location to another does not lead to disruptive reconnections at the new site. Wireless technology overcomes the need to lay cable, which is difficult, expensive, and time consuming to install, maintain, and especially, modify. Providing wireline connectivity in rural or remote areas runs the risk of someone pulling the cable (and accessories such as amplifiers) out of the ground to sell! A wireless network avoids underutilizing the access infrastructure. Unlike wired access (copper, coax, and fiber), a large portion of wireless deployment costs is incurred only when a customer signs up for service. The Fiber-to-the-Home (FTTH) Council reported that in September 23, 2008, there were 13.8 million FTTH networks in North America but the adoption rate is only 3.76 million (about 27%) even though many of these homes are located in strategic neighborhoods. The take up rate improved marginally to 34% (7.1 million connected homes) with 20.9 million homes passed on March 30, 2011. The cable industry’s capital expenditure over the last 15 years is estimated at $172 billion. Broadband usage for cable services fared better but still fall below 50%. According to the National Cable and Telecommunications Association (NCTA), there were 129.3 million homes passed by cable video service in June 2011 (which translates to over 96% of U.S. households passed), but the take up rate is 45.5%. These numbers are unlikely to increase significantly in future with high-speed wireless and free broadcast services becoming widely available.
Terrestrial wireless access may offer portable and mobile service without the need for a proprietary customer premise equipment (CPE), such as a set-top box. This facilitates voice, TV, and Internet connectivity inside and outside the residential home. For instance, such connectivity can be made available on virtually any open space (e.g., on a fishing boat!), on fast moving vehicles and trains, and even when the subscriber moves to a foreign location. The ability to connect disparate end-user devices quickly and inexpensively remains one of the key strengths of wireless. New smartphones and tablets all come with two or more wireless network interfaces but no wired interfaces, thus making wireless connectivity indispensable. These devices demand higher wireless rates to support multimedia applications, including high-quality video streaming, which is in contrast to low bit rate voice applications supported by legacy cellular systems.
Because cellular systems cover long distances, they involve costly infrastructures, such as base stations (BSs) and require users to pay for bandwidth on a time or usage basis. Each BS may potentially serve a large number of mobile handsets. Coordination between BSs as users move across wireless coverage boundaries is achieved using a mobile backhaul, which also carries a variety of user traffic. The BS may prioritize near and far handsets. For example, the BS can reduce interference by transmitting at a lower power to closer handsets. In contrast, on-premise and geographically limited wireless local area networks (wireless LANs) require no usage fees, employ lower transmit power, and provide higher data rates than cellular systems. Wireless LANs are built around cheaper access points (APs) that connect a smaller number of stationary user devices, such as laptops or tablets to a wired network. However, achieving reliable high-speed wireless transmission is a challenging task. Besides the need to overcome traditional issues, such as multipath fading and interference from known and unknown sources, broadband wireless transmission also demands new methods to support highly efficient use of limited radio spectrum and handset battery power. This chapter discusses several fundamental topics related to broadband wireless networks. These include environmental factors, frequency bands, multicarrier operation, multiple antenna systems, medium access control, duplexing, and deployment considerations.
1.1 INTRODUCTION
Mobile broadband represents a multibillion dollar market. Service providers, including incumbent cable/telephone wireline providers, can increase the number of subscribers significantly by leveraging on broadband wireless solutions (e.g., in areas not currently served or served by competitors). The performance of a broadband wireless network is heavily dependent on the characteristics of the wireless channel, such as signal fading, multipath distortion, limited bandwidth, high error rates, rapidly changing propagation conditions, mutual interference of signals, and the vulnerability to eavesdrop and unauthorized access. Moreover, the performance observed by each individual user in the network is different and is a function of its location as well as the location of other interacting users. In order to improve spectral efficiency and hence, the overall network capacity, wireless access techniques need to be closely integrated with various interference mitigation techniques including the use of smart antennas, multi-user detection, power control, channel state tracking, and coding. Broadband wireless networks must also adequately address the combined requirements of wireless and multimedia communications. On one hand, the network must allow users to share the limited bandwidth resource efficiently to achieve higher rates. This implies two criteria: maximizing the utilization of the radio frequency spectrum and minimizing the delay experienced by the users. On the other hand, because the network supports multimedia traffic, it is expected to handle a wide range of bit rates together with various types of real-time and non–real-time traffic attributes and quality of service (QoS) guarantees.
More than 60% of Americans are using a wireless device to talk, send email, take pictures, watch video, listen to music, and play online games. Compressed video is a key traffic type that needs to be accommodated due to the emergence of many personal smartphones and tablet computers. Despite the smaller displays, many of these devices can support high-definition (HD) video with 720p (1280 × 720 pixels) picture resolutions. Highly efficient video coding standards, such as H.264/MPEG-4 Advanced Video Coding (AVC), are normally used to compress these videos for wireless delivery. This enables efficient use of radio spectrum, but the higher compression efficiency may also result in higher bit rate variability. In addition, compressed video is very sensitive to packet loss, with very limited time for packet retransmission, and wireless channels tend to be more error-prone than wired networks. Although wireless rates are typically lower than its wired counterparts, serving bandwidth-intensive applications, such as HD videos, may not always be an issue since such videos can be downloaded in the background. Users tend to watch videos in their own time, rather than according to broadcast schedules. However, real-time video applications (e.g., Skype video chat) may pose a problem depending on bandwidth availability.
Figure 1.1 shows the evolution of wireless access standards. Since the late 1990s, there were numerous digital cellular standards supporting second-generation (2G), 2.5 generation (2.5G), and third-generation (3G) services. These standards can be broadly categorized under code division multiple access (CDMA) or time division multiple access (TDMA), and there was much debate on the individual merits and capacity of these systems. However, high-speed fourth-generation (4G) wireless standards are converging towards multicarrier transmission as the defacto method and currently there are only two 4G standards. Unlike legacy digital cellular services that primarily support voice and low rate data, the demand for 4G wireless is driven by millions of personal devices that require high-speed Internet connectivity. These sleek devices exude mass appeal due to their usability, and many devices employ an open software platform for users to program their own applications or download other applications. 4G wireless is the missing link that allows multimedia applications running on these devices to become portable, thus enabling on-the-go entertainment.
Figure 1.1 Evolution of wireless access standards.
1.2 RADIO SPECTRUM
To encourage pervasive use of a wireless technology, the operating radio frequency (RF) band should be widely available. Locating a harmonized band is a difficult task because spectrum allocation is strictly controlled by multiple regulatory bodies in different countries. These include the Federal Communications Commission (FCC) in the United States, the European Committee of Post and Telecommunications Administrations (CEPT), Ofcom in the United Kingdom, the Radio Equipment Inspection and Certification Institute (MKK) in Japan, the Australian Communications and Media Authority (ACMA), and others.
1.2.1 Unlicensed Frequency Bands
Many wireless networks operate on unlicensed frequency bands, as illustrated in Figure 1.2. Many of these bands are available worldwide. Since the allocated spectrum is not licensed, large-scale frequency planning is avoided and ad hoc deployments are possible. Perhaps the most popular band is the 2.4 GHz industrial, scientific and medical (ISM) band, which has been adopted by the IEEE 802.11 wireless LAN standard. The rules for operating in this band were first released by the FCC in 1985, which also includes the 900 MHz and the 5.8 GHz bands. Another band that has become popular is the 5 GHz Unlicensed National Information Infrastructure (U-NII) frequency band. The large amount of radio spectrum in this band enables the provision of high-speed Internet and multimedia services. The rules for operating in the 5 GHz U-NII band were released by the FCC in 1997. The band is subdivided into three blocks of 100 MHz each, corresponding to the lower, middle, and upper U-NII bands. The FCC subsequently expanded the middle U-NII band when 255 MHz of bandwidth was added in 2003. Thus, the 5 GHz U-NII band offers substantially more bandwidth than the 2.4 GHz band (580 MHz vs. 83.5 MHz). Recently, the 60 GHz unlicensed band has emerged, providing about 7–8 GHz of bandwidth, which is significantly higher than the 2.4 and 5 GHz bands.
Figure 1.2 Popular unlicensed frequency bands at 2.4 GHz, 5 GHz, and 60 GHz.
1.2.2 The 2.4 GHz Unlicensed Band
The 2.4 GHz channel sets and center frequencies are shown in Table 1.1. The operating power for the 2.4 GHz band is limited to 100 mW (U.S.), 100 mW (Europe), and 10 mW/MHz (Japan). If transmit power control is employed, up to 1 W operation is permissible in the United States. A radiated power of 30 mW is normally used in 802.11 wireless LANs. In France, the output power for outdoor operation between 2454 and 2483.5 MHz is restricted to 10 mW. The total available bandwidth is 72 MHz in the United States, but generally higher in Europe and other parts of the world (83.5 MHz total bandwidth). The 2.4 GHz channels are defined on a 5 MHz channel grid (i.e., each channel has a bandwidth of 5 MHz). This results in 11-13 nonoverlapping channels. However, some wireless systems such as 802.11 wireless LANs require a higher channel bandwidth of 20 MHz. This gives rise to 11-13 overlapped 20 MHz channels or only 3 nonoverlapping 20 MHz channels in the 2.4 GHz band, as illustrated in Figure 1.3. These nonoverlapping channels need not be assigned sequentially and not all channels need to be assigned.
TABLE 1.1 2.4 GHz Channel Sets and Center Frequencies
Channel ID
North America (GHz)
Europe, Japan, and Australia (GHz)
1
2.412
2.412
2
2.417
2.417
3
2.422
2.422
4
2.427
2.427
5
2.432
2.432
6
2.437
2.437
7
2.442
2.442
8
2.447
2.447
9
2.452
2.452
10
2.457
2.457
11
2.462
2.462
12
2.467
13
2.472
Figure 1.3 2.4 GHz channel spacing in the United States and Europe.
Interference must be carefully evaluated in the 2.4 GHz band especially when deploying large-scale 802.11 networks, such as in conference or exhibit halls and airport terminals. Besides 802.11 devices, many other devices operate in the same GHz band. They include 802.15.1 (Bluetooth) and 802.15.4 (ZigBee) devices, as well as digital cordless phones and microwave ovens that do not have in-built interference avoidance mechanisms. Microwave ovens remain one of the detrimental sources of interference in the 2.4 GHz band due to the high operating power (typically 750 to 1000 W). These ovens are present in almost every home and office. Due to the difficulty in controlling interference, devices must observe etiquette during transmission so that incompatible systems may co-exist. Basic elements of etiquette are to listen before transmit (to detect ongoing transmissions), and to limit transmit time and transmit power. The carrier (activity)-sensing mechanism used in 802.11 provides natural etiquette support.
1.2.3 The 5 GHz Unlicensed Band
Unlike the 2.4 GHz band, 5 GHz channels are defined on a 20 MHz grid. The channel sets, center frequencies, and operating power for the 5 GHz band are shown in Table 1.2. The 5 GHz channel assignment and bandwidth are shown in Figure 1.4. The U-NII lower and upper bands (four channels each) are normally employed in the United States, whereas the U-NII middle band is normally supported in Europe. Note that the center frequencies of the 20 MHz channels start (and end) at different points in each band. The highest number of contiguous channels of 11 is available in the 5.470–5.725 GHz middle band. A significant part of the 5.8 GHz ISM band (ranging from 5.725 to 5.850 GHz) has been absorbed in the upper U-NII band. However, ISM channel 165 with a center frequency of 5.825 GHz falls outside of the U-NII band. Thus, the total available bandwidth at 5 GHz (ISM + U-NII) is 580 MHz, giving 24 nonoverlapping channels. The upper U-NII band holds the most promise, as it allows the possibility of longer operational range without the need for a range extender.
TABLE 1.2 5 GHZ U-NII Channel Sets, Center Frequencies, and Operating Power
Figure 1.4 5 GHz channel assignment and bandwidth.
Some 5 GHz wireless channels overlap with radar frequencies. Unlike the 2.4 GHz band, however, most interference sources at 5 GHz are located outdoors and may be attenuated sufficiently if penetrated indoors. Nevertheless, dynamic frequency selection (DFS) and transmit power control (TPC) are interference mitigation techniques recommended for unlicensed 5 GHz operation. With DFS, a 5 GHz device can automatically detect radar transmissions and change to a different channel. With TPC, the transmit power can be reduced by several dB below the maximum permitted level.
1.2.4 The 60 GHz Unlicensed Band
A higher frequency band generally implies a higher amount of available bandwidth. At high frequencies, oxygen absorption in the atmosphere leads to rapid fall off in signal strength. Although the operating range becomes limited, this facilitates frequency reuse (i.e., bandwidth reclamation) in high-capacity, picocell (very small cell) wireless systems. There are two oxygen absorption bands ranging from 51.4 to 66 GHz (band A) and 105 to 134 GHz (band B). There are also peaks in water vapor absorption at 22 and 200 GHz. The oxygen absorption is lower in band B than band A, while the water vapor attenuation is higher. These observations suggest that band A is more suitable for communications than band B. The Millimeter Wave Communications Working Group first recommended the use of the 60 GHz band in a report released by the FCC in 1996 [1]. The 60 GHz frequency band is also available worldwide (Figure 1.5). It is uniquely suited for carrying extremely high data rates (multi-Gbit/s) over short distances. Transmit power is typically limited to 10 mW, and the band is subdivided into four nonoverlapping channels of 2.16 GHz each. Nearly 7 GHz of unlicensed spectrum is available in the United States and Japan, whereas 8 GHz of bandwidth is available in Europe. Currently, there are no coexistence issues.
Figure 1.5 60 GHz frequency allocation.
1.2.5 Licensed Frequency Bands
The popular 3GPP Evolved Universal Terrestrial Radio Access (E-UTRA) licensed frequency bands are listed in Table 1.3. The downlink (DL) band is employed in the transmission from the BS to the handset. Conversely, the uplink (UL) band is employed in the transmission from the handset to the BS. High-speed packet access (HSPA) systems are deployed in the major E-UTRA cellular bands. For example, there are over 400 tri-band 850/1900/2100 MHz HSPA devices that support global roaming. Many HSPA devices also support legacy global system for mobile communications (GSM), general packet radio service (GPRS), and Enhanced Data rates for GSM Evolution (EDGE), giving rise to quad-band 850/900/1800/1900 MHz devices. A combination of higher spectrum (e.g., 1800/1900/2100 MHz) for improved capacity and sub-1 GHz spectrum (e.g., 700/850/900 MHz) for improved coverage in rural areas and urban in-buildings, is highly desirable. However, with 4G wireless standards employing larger bandwidths (such as 40 MHz), it is important to be able to use bands that offer wider bandwidths. Thus, the International Telecommunication Union (ITU) identified the 2.6 GHz band for supporting mobile broadband services. This extension band is large enough to allow operators to deploy wideband channels to achieve faster data speeds. In addition, some 700 MHz spectrum (also known as digital dividend spectrum) is released for 4G wireless services as analog TV broadcasters migrate to more efficient digital TV platforms. In the 2007 ITU World Radio Conference, the allocation of 700 MHz spectrum for mobile service has been harmonized in the following regions:
698–806 MHz for the Americas
790–862 MHz for Europe, Middle East, and Africa
698–862 MHz or 790–862 MHz for Asia.
TABLE 1.3 Major E-UTRA Frequency Bands
Source: 3GPP TS.104 V10.2.0.
The United States is currently the only country in the world that can build ubiquitous wireless Internet access and communications using the 700 MHz and 2.5 GHz (2.496 to 2.69 GHz) Educational Broadband Service (EBS) spectrum.
1.3 SIGNAL COVERAGE
Wireless networks employ either radio or infrared electromagnetic waves to transfer information from one point to another. The use of a wireless link introduces new restrictions not found in conventional wired networks. The quality of the wireless link varies over space and time. Objects in a building (e.g., structures, equipment, and people) can block, reflect, and scatter transmitted signals. In addition, problems of noise and interference from both intended and unintended users must also be solved. While wired networks are implicitly distinct, there is no easy way to physically separate different wireless networks. Well-defined network boundaries or coverage areas do not exist since users may move and transmissions can occur in various locations of the network. Wireless networks lack full connectivity and are significantly less reliable than the wired physical layer (PHY). Thus, one of the most important aspects of wireless system design is to ensure that sufficient signal levels are accessible from most of the intended service areas. To support mobility, separate wireless coverage areas or cells must be properly overlapped to ensure service continuity. Estimating signal coverage requires a good understanding of the communication channel, which comprises the antennas and the propagation medium. Usually, additional signal power is needed to maintain the desired channel quality and to offset the amount of received signal power variation about its average level. These power variations can be broadly classified under small-scale or large-scale fading effects. Small-scale fades are dominated by multipath propagation (caused by RF signal reflections), Doppler spread (caused by relative motion between transmitter and receiver), and movement of surrounding objects. Large-scale fades are characterized by attenuation in the propagation medium and shadowing caused by obstructing objects. These effects are explained in the following sections.
1.3.1 Propagation Mechanisms
Signal propagation patterns are unpredictable and changes rapidly with time. Consequently, signal coverage is not uniform, even at equal distances from the transmitter. A transmitted RF signal diffuses as it travels across the wireless medium. As a result, a portion of the transmitted signal power arrives directly at the receiver, while other portions arrive via reflection, diffraction, and scattering. Reflection occurs when the propagating signal impinges on an object that is large compared to the wavelength of the signal (e.g., buildings, walls, and surface of the earth). When the path between the transmitter and receiver is obstructed by sharp, irregular objects, the propagating wave diffracts and bends around the obstacle even when a direct line-of-sight (LOS) path does not exist. Finally, scattering takes place when obstructing objects are smaller than the wavelength of the propagating signal (e.g., people, foliage).
1.3.2 Multipath
Among the various forms of radio signal degradations, multipath fading assumes a high degree of importance. Multipath is a form of self-interference that occurs when the transmitted signal is reflected by objects in the environment such as walls, trees, buildings, people, and moving vehicles. When a signal takes multiple paths to reach the receiver, the received signal becomes a superposition of different components (Figure 1.6), each with a different delay, amplitude, and phase. These components form different clusters, and, depending on the phase of each component, interfere constructively and destructively at the receiving antenna, thereby producing a phenomenon called multipath fading. Such fading produces a variable bit error rate that can lead to intermittent network connectivity and significant delay variation (jitter).
Figure 1.6 Multipath propagation and signal scattering.
Multipath fading represents the quick fluctuations in received power and is therefore commonly known as fast (or Rayleigh) fading. In addition, it is often classified as small-scale fading because the rapid changes in signal strength only occur over a small area or time interval. Multipath fading is affected by the location of the transmitter and receiver, as well as the movement around them. Such fading tends to be frequency selective or frequency dependent. Of considerable importance to wireless network designers is not only the depth but also the duration of the fades. Fortunately, it has been observed that the deeper the fade, the less frequently it occurs and the shorter the duration when it occurs. The severity of the fades tends to increase as the distance between the transmitter and receiver, and the number of reflective surfaces in the environment, increase. Multipath fading can be countered effectively using diversity techniques, in which two or more independent channels are somehow combined. The motivation here is that only one of the channels is likely to suffer a fade at any instant of time.
Since multipath propagation results in varying travel times, signal pulses are broadened as they travel through the wireless medium. This limits the speed at which adjacent data pulses can be sent without overlap, and hence, the maximum information rate a wireless system can operate. Thus, in addition to frequency-selective fading, a multipath channel also exhibits time dispersion. Time dispersion leads to intersymbol interference (ISI) while fading induces periods of low signal-to-noise ratio (SNR), both effects causing burst errors in wireless digital transmission. Figure 1.7 shows the impact of ISI. In this case, the same delay spread is assumed. Thus, while multipath propagation causes fast fading at low data rates, at high data rates (i.e., when the delay spread becomes comparable with the symbol interval), the received signals become indistinguishable, giving rise to ISI. Lowering the data symbol rate and/or introducing a guard time interval (also known as a cyclic prefix [CP]) between symbols can help mitigate the impact of time dispersion.
Figure 1.7 Intersymbol interference.
The performance metric for a wireless system operating over a multipath channel is either the average probability of error or the probability of outage. The average probability of error is the average error rate for all possible locations in the cell. The probability of outage represents the error probability below a predefined signal threshold for all possible locations in the cell.
1.3.3 Delay Spread and Time Dispersion
Delay spread is caused by differences in the arrival time of a signal from the various paths when it propagates through a time-dispersive multipath channel. The net effect of the arrival time difference is to spread the signal in time. The delay spread is proportional to the length of the path, which is in turn affected by the span of the propagating environment, as well as the location of the objects around the transmitter and receiver. The delay spread decreases at higher frequencies due to greater signal attenuation and absorption. A negative effect of delay spread is that it results in ISI. This causes data symbols (each representing one or more bits) to overlap in varying degrees at the receiver. Such overlap results in bit errors that increase as the symbol period approaches the delay spread. The effect becomes worse at higher data rates and cannot be solved simply by increasing the power of the transmitted signal. To avoid ISI, the duration of the delay spread should not exceed the duration of a data symbol, which carries a set of information bits.
The root mean square (rms) delay spread is often used as a convenient measure to estimate the amount of ISI caused by a multipath wireless channel. The maximum achievable data rate depends primarily on the rms delay spread and not the shape of the delay spread function. The rms delay spread in an indoor environment can vary significantly from 30 ns in a small room to 250 ns in a large hall. In outdoor environments, a delay spread of 10 μs or less is common (for a range of 1000 ft or less), although for non-LOS cases, a delay spread in the order of 100 µs is possible. If the product of the rms delay spread and the signal bandwidth is much less than 1, then the fading is called flat fading. If the product is greater than 1, then the fading is classified as frequency selective.
1.3.4 Coherence Bandwidth
A direct consequence of multipath propagation is that the received power of the composite signal varies according to the characteristics of the wireless channel in which the signal has traveled (Figure 1.8). More importantly, multipath propagation often leads to frequency-selective fading, which refers to nonuniform fading over the bandwidth occupied by the transmitted signal. The fades (notches) are usually correlated at adjacent frequencies and are decorrelated after a few megahertz. The severity of such fading depends on how rapidly the fading occurs relative to the round-trip propagation time on the wireless link. The bandwidth of the fade (i.e., the range of frequencies that fade together) is called the coherence bandwidth. This bandwidth is inversely proportional to the rms delay spread. Thus, ISI occurs when the coherence bandwidth of the channel is smaller than the modulation bandwidth. If the coherence bandwidth is small compared with the bandwidth of the transmitted signal, then the wireless link is frequency selective, and different frequency components are subject to different amplitude gains and phase shifts. Conversely, a wireless link is nonfrequency selective if all frequency components are subject to the same attenuation and phase shift. Frequency-selective fading is a more serious problem since matched filters that are structured to match the undistorted part of the spectrum will suffer a loss in detection performance when the attenuated portion of the spectrum is encountered. Either the data rate must be restricted so that the signal bandwidth falls within the coherence bandwidth of the link or other techniques such as spread spectrum must be used to suppress the distortion. The delay spread caused by multipath is typically greater outdoors than indoors due to the wider coverage area. This gives rise to a higher coherence bandwidth in indoor environments. For example, an indoor channel with a delay spread of 250 ns corresponds to a coherence bandwidth of 4 MHz. An outdoor channel with a larger delay spread of 1 µs implies a smaller coherence bandwidth of 1 MHz.
Figure 1.8 Signal fading characteristics (2.4 GHz band).
Signals with bandwidth larger than the coherence bandwidth of the channel may make effective use of multipath by resolving (isolating) many independent signal propagation paths to provide better SNR at the receiver. This is exploited by some multiple antenna systems. On the other hand, multipath interference can be avoided by keeping the symbol rate low, thereby reducing the signal bandwidth below the coherence bandwidth. Although a wideband receiver can resolve more paths than another receiver with a narrower bandwidth, this may be done at the expense of receiving less energy and more noise per resolvable path.
1.3.5 Doppler Spread
Doppler spread is primarily caused by the relative motion between the transmitter and receiver. It introduces random frequency or phase shifts at the receiver that can result in loss of synchronization but affects LOS and reflected signals independently. Reflected signals affected by Doppler shifts are perceived as noise contributing to intercarrier interference (ICI) in multicarrier transmission. The Doppler effect may also be due to the movement of reflecting objects (e.g., vehicles, humans) that causes multipath fading. In an indoor environment for instance, the movement of people is the main cause of Doppler spread. A person moving at 10 km/h can induce a Doppler spread of ±22 Hz at 2.4 GHz. The Doppler spread for indoor channels is highly dependent on the local environment, providing different shapes for different physical layouts. On the other hand, outdoor Doppler spreads consistently exhibit peaks at the limits of the maximum Doppler frequency. Typical values for Doppler spread are 10–250 Hz (suburban areas), 10–20 Hz (urban areas), and 10–100 Hz (office areas).
Just as coherence bandwidth is inversely related to the delay spread, coherence time is defined as the inverse of the Doppler spread. The coherence time determines the rate at which fading occurs. Fast fading occurs when the fading rate is higher than the data symbol rate. The coherence time is a key parameter that affects channel feedback mechanisms in high-speed mobile systems. For example, the delay in sending channel feedback information from the handset to the BS may exceed the coherence time. This renders the feedback outdated by the time the BS processes the information. The relationship between coherence time and coherence bandwidth is shown in Figure 1.9. This relationship forms the basis for fading channel classification.
Figure 1.9 Coherence time and coherence bandwidth.
1.3.6 Shadow Fading
Besides multipath fading, large physical obstructions (e.g., walls in indoor environments, buildings in outdoor environments) can cause large-scale shadow fading. In this case, the transmitted signal power is blocked and hence severely attenuated by the obstruction. The severity of shadow fading is dependent on the relative positions of the transmitter and receiver with respect to the large obstacles in the propagation environment, as well as the number of obstructing objects and the dielectric properties of the objects. Unlike multipath fading (which is usually represented by a Rician or Rayleigh distribution), shadow fading is generally characterized by the probability density function of a log-normal or Gaussian distribution. Increasing the transmit power can help to mitigate the effects of shadow fading although this places additional burden on the handset battery and can cause interference for other users.
1.3.7 Radio Propagation Modeling
The transmitted signal power normally radiate (spread out) in all directions and hence attenuates quickly with distance. Thus, very little signal energy reaches the receiver, giving rise an inverse relationship between distance and path loss. Depending on the severity, the decay in signal strength can make the signal become unintelligible at the receiver. Radio propagation analysis allows the appropriate power or link budget to be determined between the RF transmitter and receiver. It can be very complex when the shortest direct path between the transmitter and receiver is blocked by fixed or moving objects, and the received signal arrives by several reflected paths. The degree of attenuation depends largely on the frequency of transmission. For example, lower frequencies tend to penetrate objects better, while high frequency signals encounter greater attenuation.
For clear LOS paths in the vicinity of the receiving antenna, signal attenuation is close to free space. This is the simplest signal loss model where the received signal power decreases with the square of the distance between the transmitter and receiver. For instance, the signal strength at 2 m is a quarter of that at 1 m. At longer distances away from the receiving antenna, an increase in the attenuation exponent is common (Figure 1.10). In this case, the signal attenuation is dependent not only on distance and transmit power but also on reflecting objects, physical obstructions, and the amount of mutual interference from other transmitting users. Small changes in position or direction of the antenna, shadowing caused by blocked signals and moving obstacles (e.g., people and doors) in the environment may also lead to drastic fluctuations in signal strength. Similar effects occur regardless of whether a user is stationary or mobile. Hence, while the free-space exponent may be relevant for short distance transmission (e.g., up to 10 m), the path loss is usually modeled with a higher-valued exponent of 3 to 5 for longer distances. For indoor environments, where objects move very slowly, fading is primarily due to the receiver. Thus, the path loss (a distance-related phenomenon) is independent of fast fading (a time-related phenomenon).
Figure 1.10 Signal attenuation for omnidirectional antenna with spherical (isotropic) radiation pattern.
An accurate characterization of the propagation mechanism is difficult since this is greatly influenced by a number of factors, such as antenna height, terrain, and topology. Radiowave propagation modeling is usually based on the statistics of the measured channel profiles (time and frequency domain modeling) or on the direct solution of electromagnetic propagation equations based on Maxwell’s equations. The most popular models for indoor radio propagation are the time domain statistical models. In this case, the statistics of the channel parameters are collected from measurements in the propagation environment of interest at various locations between the transmitter and receiver. Another popular method involves ray tracing, which assumes that all objects of interest within the propagation environment are large compared with the wavelength of propagation, thus removing the need to solve Maxwell’s equations. Its usefulness is ultimately dependent on the accuracy of the site-specific representation of the propagation environment.
Modeling the channel characteristics of narrowband and wideband signals is different. For narrowband signals, the emphasis is on the received power whereas for wideband communications, both the received signal power and multipath characteristics are equally important. A further distinction exists between models that describe signal strength as a function of distance as opposed to a function of time. The former is used to determine coverage areas and intercell interference while the latter is used to determine bit error rates and outage probabilities.
