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WiMAX Fundamentals
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Chapter 1: Introduction to Broadband Wireless
Broadband wireless sits at the confluence of two of the most remarkable growth stories of the telecommunications industry in recent years. Both wireless and broadband have on their own enjoyed rapid mass-market adoption. Wireless mobile services grew from 11 million subscribers worldwide in 1990 to more than 2 billion in 2005 [1]. During the same period, the Internet grew from being a curious academic tool to having about a billion users. This staggering growth of the Internet is driving demand for higher-speed Internet-access services, leading to a parallel growth in broadband adoption. In less than a decade, broadband subscription worldwide has grown from virtually zero to over 200 million [2]. Will combining the convenience of wireless with the rich performance of broadband be the next frontier for growth in the industry? Can such a combination be technically and commercially viable? Can wireless deliver broadband applications and services that are of interest to the endusers? Many industry observers believe so.
Before we delve into broadband wireless, let us review the state of broadband access today. Digital subscriber line (DSL) technology, which delivers broadband over twisted-pair telephone wires, and cable modem technology, which delivers over coaxial cable TV plant, are the predominant mass-market broadband access technologies today. Both of these technologies typically provide up to a few megabits per second of data to each user, and continuing advances are making several tens of megabits per second possible. Since their initial deployment in the late 1990s, these services have enjoyed considerable growth. The United States has more than 50 million broadband subscribers, including more than half of home Internet users. Worldwide, this number is more than 200 million today and is projected to grow to more than 400 million by 2010 [2]. The availability of a wireless solution for broadband could potentially accelerate this growth.
Figure 1.1
Worldwide subscriber growth 1990–2006 for mobile telephony, Internet usage, and broadband access [1, 2, 3]
Chapter 1: Introduction to Broadband Wireless (Cont)
What are the applications that drive this growth? Broadband users worldwide are finding that it dramatically changes how we share information, conduct business, and seek entertainment. Broadband access not only provides faster Web surfing and quicker file downloads but also enables several multimedia applications, such as real-time audio and video streaming, multimedia conferencing, and interactive gaming. Broadband connections are also being used for voice telephony using voice-over-Internet Protocol (VoIP) technology. More advanced broadband access systems, such as fiber-to-the-home (FTTH) and very high data rate digital subscriber loop (VDSL), enable such applications as entertainment-quality video, including high-definition TV (HDTV) and video on demand (VoD). As the broadband market continues to grow, several new applications are likely to emerge, and it is difficult to predict which ones will succeed in the future.
So what is broadband wireless? Broadband wireless is about bringing the broadband experience to a wireless context, which offers users certain unique benefits and convenience. There are two fundamentally different types of broadband wireless services. The first type attempts to provide a set of services similar to that of the traditional fixed-line broadband but using wireless as the medium of transmission. This type, called fixed wireless broadband, can be thought of as a competitive alternative to DSL or cable modem. The second type of broadband wireless, called mobile broadband, offers the additional functionality of portability, nomadicity[1] and mobility. Mobile broadband attempts to bring broadband applications to new user experience scenarios and hence can offer the end user a very different value proposition. WiMAX (worldwide interoperability for microwave access) technology, the subject of this book, is designed to accommodate both fixed and mobile broadband applications.
In this chapter, we provide a brief overview of broadband wireless.
The objective is to present the the background and context necessary
for understanding WiMAX. We review the history of broadband wireless,
enumerate its applications, and discuss the business drivers and
challenges. In Section 1.7, we also survey the technical challenges
that need to be addressed while developing and deploying broadband
wireless systems.
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Chapter 1.1 Evolution of Broadband Wireless
The history of broadband wireless as it relates to WiMAX can be traced back to the desire to find a competitive alternative to traditional wireline-access technologies. Spurred by the deregulation of the telecom industry and the rapid growth of the Internet, several competitive carriers were motivated to find a wireless solution to bypass incumbent service providers. During the past decade or so, a number of wireless access systems have been developed, mostly by start-up companies motivated by the disruptive potential of wireless. These systems varied widely in their performance capabilities, protocols, frequency spectrum used, applications supported, and a host of other parameters. Some systems were commercially deployed only to be decommissioned later. Successful deployments have so far been limited to a few niche applications and markets. Clearly, broadband wireless has until now had a checkered record, in part because of the fragmentation of the industry due to the lack of a common standard. The emergence of WiMAX as an industry standard is expected to change this situation.
Given the wide variety of solutions developed and deployed for broadband wireless in the past, a full historical survey of these is beyond the scope of this section. Instead, we provide a brief review of some of the broader patterns in this development. A chronological listing of some of the notable events related to broadband wireless development is given in Table 1.1.
WiMAX technology has evolved through four stages, albeit not fully distinct or clearly sequential: (1) narrowband wireless local-loop systems, (2) first-generation line-of-sight (LOS) broadband systems, (3) second-generation non-line-of-sight (NLOS) broadband systems, and (4) standards-based broadband wireless systems.
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1.1.1 Narrowband Wireless Local-Loop Systems
Naturally, the first application for which a wireless alternative was developed and deployed was voice telephony. These systems, called wireless local-loop (WLL), were quite successful in developing countries such as China, India, Indonesia, Brazil, and Russia, whose high demand for basic telephone services could not be served using existing infrastructure. In fact, WLL systems based on the digital-enhanced cordless telephony (DECT) and code division multiple access (CDMA) standards continue to be deployed in these markets.
In markets in which a robust local-loop infrastructure already existed for voice telephony, WLL systems had to offer additional value to be competitive. Following the commercialization of the Internet in 1993, the demand for Internet-access services began to surge, and many saw providing high-speed Internet-access as a way for wireless systems to differentiate themselves. For example, in February 1997, AT&T announced that it had developed a wireless access system for the 1,900MHz PCS (personal communications services) band that could deliver two voice lines and a 128kbps data connection to subscribers. This system, developed under the code name "Project Angel," also had the distinction of being one of the first commercial wireless systems to use adaptive antenna technology. After field trials for a few years and a brief commercial offering, AT&T discontinued the service in December 2001, citing cost run-ups and poor take-rate as reasons.
During the same time, several small start-up companies focused solely on providing Internet-access services using wireless. These wireless Internet service provider (WISP) companies typically deployed systems in the license-exempt 900MHz and 2.4GHz bands. Most of these systems required antennas to be installed at the customer premises, either on rooftops or under the eaves of their buildings. Deployments were limited mostly to select neighborhoods and small towns. These early systems typically offered speeds up to a few hundred kilobits per second. Later evolutions of license-exempt systems were able to provide higher speeds.
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1.1.2 First-Generation Broadband Systems
As DSL and cable modems began to be deployed, wireless systems had to evolve to support much higher speeds to be competitive. Systems began to be developed for higher frequencies, such as the 2.5GHz and 3.5GHz bands. Very high speed systems, called local multipoint distribution systems (LMDS), supporting up to several hundreds of megabits per second, were also developed in millimeter wave frequency bands, such as the 24GHz and 39GHz bands. LMDS-based services were targeted at business users and in the late 1990s enjoyed rapid but short-lived success. Problems obtaining access to rooftops for installing antennas, coupled with its shorter-range capabilities, squashed its growth.
Table 1.1 Important Dates in the Development of Broadband Wireless
|
Date |
Event |
|
February 1997 |
AT&T announces development of fixed wireless technology code named "Project Angel" |
|
February 1997 |
FCC auctions 30MHz spectrum in 2.3GHz band for wireless communications services (WCS) |
|
September 1997 |
American Telecasting (acquired later by Sprint) announces wireless Internet access services in the MMDS band offering 750kbps downstream with telephone dial-up modem upstream |
|
September 1998 |
FCC relaxes rules for MMDS band to allow two-way communications |
|
April 1999 |
MCI and Sprint acquire several wireless cable operators to get access to MMDS spectrum |
|
July 1999 |
First working group meeting of IEEE 802.16 group |
|
March 2000 |
AT&T launches first commercial high-speed fixed wireless service after years of trial |
|
May 2000 |
Sprint launches first MMDS deployment in Phoenix, Arizona, using first-generation LOS technology |
|
June 2001 |
WiMAX Forum established |
|
October 2001 |
Sprint halts MMDS deployments |
|
December 2001 |
AT&T discontinues fixed wireless services |
|
December 2001 |
IEEE 802.16 standards completed for > 11GHz. |
|
February 2002 |
Korea allocates spectrum in the 2.3GHz band for wireless broadband (WiBro) |
|
January 2003 |
IEEE 802.16a standard completed |
|
June 2004 |
IEEE 802.16-2004 standard completed and approved |
|
September 2004 |
Intel begins shipping the first WiMAX chipset, called Rosedale |
|
December 2005 |
IEEE 802.16e standard completed and approved |
|
January 2006 |
First WiMAX Forum–certified product announced for fixed applications |
|
June 2006 |
WiBro commercial services launched in Korea |
|
August 2006 |
Sprint Nextel announces plans to deploy mobile WiMAX in the United States |
In the late 1990s, one of the more important deployments of wireless broadband happened in the so-called multichannel multipoint distribution services
(MMDS) band at 2.5GHz. The MMDS band was historically used to provide
wireless cable broadcast video services, especially in rural areas
where cable TV services were not available. The advent of satellite TV
ruined the wireless cable business, and operators were looking for
alternative ways to use this spectrum. A few operators began to offer
one-way wireless Internet-access service, using telephone line as the
return path. In September 1998, the Federal Communications Commission
(FCC) relaxed the rules of the MMDS band in the United States to allow
two-way communication services, sparking greater industry interest in
the MMDS band. MCI WorldCom and Sprint each paid approximately $1
billion to purchase licenses to use the MMDS spectrum, and several
companies started developing high-speed fixed wireless solutions for
this band.
The first generation of these fixed broadband wireless solutions
were deployed using the same towers that served wireless cable
subscribers. These towers were typically several hundred feet tall and
enabled LOS coverage to distances up to 35 miles, using high-power
transmitters. First-generation MMDS systems required that subscribers
install at their premises outdoor antennas high enough and pointed
toward the tower for a clear LOS transmission path. Sprint and MCI
launched two-way wireless broadband services using first-generation
MMDS systems in a few markets in early 2000. The outdoor antenna and
LOS requirements proved to be significant impediments. Besides, since a
fairly large area was being served by a single tower, the capacity of
these systems was fairly limited. Similar first-generation LOS systems
were deployed internationally in the 3.5GHz band.
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1.1.3 Second-Generation Broadband Systems
Second-generation broadband wireless systems were able to overcome the LOS issue and to provide more capacity. This was done through the use of a cellular architecture and implementation of advanced-signal processing techniques to improve the link and system performance under multipath conditions. Several start-up companies developed advanced proprietary solutions that provided significant performance gains over first-generation systems. Most of these new systems could perform well under non-line-of-sight conditions, with customer-premise antennas typically mounted under the eaves or lower. Many solved the NLOS problem by using such techniques as orthogonal frequency division multiplexing (OFDM), code division multiple access (CDMA), and multiantenna processing. Some systems, such as those developed by SOMA Networks and Navini Networks, demonstrated satisfactory link performance over a few miles to desktop subscriber terminals without the need for an antenna mounted outside. A few megabits per second throughput over cell ranges of a few miles had become possible with second-gener_ation fixed wireless broadband systems.
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1.1.4 Emergence of Standards-Based Technology
In 1998, the Institute of Electrical and Electronics Engineers (IEEE) formed a group called 802.16 to develop a standard for what was called a wireless metropolitan area network, or wireless MAN. Originally, this group focused on developing solutions in the 10GHz to 66GHz band, with the primary application being delivering high-speed connections to businesses that could not obtain fiber. These systems, like LMDS, were conceived as being able to tap into fiber rings and to distribute that bandwidth through a point-to-multipoint configuration to LOS businesses. The IEEE 802.16 group produced a standard that was approved in December 2001. This standard, Wireless MAN-SC, specified a physical layer that used single-carrier modulation techniques and a media access control (MAC) layer with a burst time division multiplexing (TDM) structure that supported both frequency division duplexing (FDD) and time division duplexing (TDD).
After completing this standard, the group started work on extending and modifying it to work in both licensed and license-exempt frequencies in the 2GHz to 11GHz range, which would enable NLOS deployments. This amendment, IEEE 802.16a, was completed in 2003, with OFDM schemes added as part of the physical layer for supporting deployment in multipath environments. By this time, OFDM had established itself as a method of choice for dealing with multipath for broadband and was already part of the revised IEEE 802.11 standards. Besides the OFDM physical layers, 802.16a also specified additional MAC-layer options, including support for orthogonal frequency division multiple access (OFDMA).
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Sidebar 1.1 A Brief History of OFDM
Although OFDM has become widely used only recently, the concept
dates back some 40 years. This brief history of OFDM cites some
landmark dates.
1966: Chang shows that multicarrier modulation can solve the multipath problem without reducing data rate [4]. This is generally considered the first official publication on multicarrier modulation. Some earlier work was Holsinger's 1964 MIT dissertation [5] and some of Gallager's early work on waterfilling [6].
1971: Weinstein and Ebert show that multicarrier modulation can be ac_complished using a DFT [7].
1985: Cimini at Bell Labs identifies many of the key issues in OFDM tra_nsmission and does a proof-of-concept design [8].
1993:
DSL adopts OFDM, also called discrete multitone, following succe_ssful
field trials/competitions at Bellcore versus equalizer-based systems.
1999: The IEEE 802.11 committee on wireless LANs releases the 802.11a standard for OFDM operation in 5GHz UNI band.
2002:
The IEEE 802.16 committee releases an OFDM-based standard for wireless
broadband access for metropolitan area networks under revision 802.16a.
2003: The IEEE 802.11 committee releases the 802.11g standard for operation in the 2.4GHz band.
2003: The multiband OFDM standard for ultrawideband is developed, showing OFDM's usefulness in low-SNR systems.
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Sidebar 1.1 A Brief History of OFDM (Continued)
Further revisions to 802.16a were made and completed in 2004. This revised standard, IEEE 802.16-2004, replaces 802.16, 802.16a, and 802.16c with a single standard, which has also been adopted as the basis for HIPERMAN (high-performance metropolitan area network) by ETSI (European Telecommunications Standards Institute). In 2003, the 802.16 group began work on enhancements to the specifications to allow vehicular mobility applications. That revision, 802.16e, was completed in December 2005 and was published formally as IEEE 802.16e-2005. It specifies scalable OFDM for the physical layer and makes further modifications to the MAC layer to accommodate high-speed mobility.
As it turns out, the IEEE 802.16 specifications are a collection of standards with a very broad scope. In order to accommodate the diverse needs of the industry, the standard incorporated a wide variety of options. In order to develop interoperable solutions using the 802.16 family of standards, the scope of the standard had to be reduced by establishing consensus on what options of the standard to implement and test for interoperability. The IEEE developed the specifications but left to the industry the task of converting them into an interoperable standard that can be certified. The WiMAX Forum was formed to solve this problem and to promote solutions based on the IEEE 802.16 standards. The WiMAX Forum was modeled along the lines of the Wi-Fi Alliance, which has had remarkable success in promoting and providing interoperability testing for products based on the IEEE 802.11 family of standards.
The WiMAX Forum enjoys broad participation from the entire cross-section of the industry, including semiconductor companies, equipment manufacturers, system integraters, and service providers. The forum has begun interoperability testing and announced its first certified product based on IEEE 802.16-2004 for fixed applications in January 2006. Products based on IEEE 802.18e-2005 are expected to be certified in early 2007. Many of the vendors that previously developed proprietary solutions have announced plans to migrate to fixed and/or mobile WiMAX. The arrival of WiMAX-certified products is a significant milestone in the history of broadband wireless.
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1.2 Fixed Broadband Wireless: Market Drivers and Applications
Applications using a fixed wireless solution can be classified as point-to-point or point-to-multipoint. Point-to-point applications include interbuilding connectivity within a campus and microwave backhaul. Point-to-multipoint applications include (1) broadband for residential, small office/home office (SOHO), and small- to medium-enterprise (SME) markets, (2) T1 or fractional T1-like services to businesses, and (3) wireless backhaul for Wi-Fi hotspots. Figure 1.2 illustrates the various point-to-multipoint applications.
Consumer and small-business broadband: Clearly, one of the largest applications of WiMAX in the near future is likely to be broadband access for residential, SOHO, and SME markets. Broadband services provided using fixed WiMAX could include high-speed Internet access, telephony services using voice over IP, and a host of other Internet-based applications. Fixed wireless offers several advantages over traditional wired solutions. These advantages include lower entry and deployment costs; faster and easier deployment and revenue realization; ability to build out the network as needed; lower operational costs for network maintenance, management, and operation; and independence from the incumbent carriers.
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