-FIXED BROADBAND WIRELESS ACCESS
An update of TTG-3 Report [1]
Ferdo Ivanek
MTT-S Representative to CCIP
Ivanek@stanford.edu
1.
Introduction
The objective of this update of the TTG-3 Report [1] is to evaluate the wireless
alternatives to wireline broadband access provided over cable TV networks, local
telephone networks (e.g. DSL, T1), and direct fiber connections to the end user.
Developments in the U.S. are only covered in accordance with the IEEE-USA
framework.
The introduction starts with terminology and background. Section 1.1 covers the basic
broadband wireless access (BWA) implementation options, and Section 1.2 describes
the BWA framework of the IEEE 802 LAN/MAN Standards Committee. Section 1.3
draws attention to the “fixed wireless continuum” that includes MAN (metropolitan
access network), LAN (local area network), PAN (personal area network) and optical
wireless access. Sections 2 and 3 cover fixed BWA in frequency bands above 20 GHz
and below 6 GHz, respectively, which account for the predominant applications in the
U.S. Section 4 addresses free-space optical access, and Section 5 presents the
conclusions and comments.
Terminology - Broadband wireless access, or BWA, is the most widely used generic
term for subscribers’ “first-mile”/”last-mile” terrestrial fixed broadband wireless access to
the telecommunications network. More specific terms are used in regulatory and
standardization contexts. The FCC established BWA rules under different names for
different frequency bands, such as, Multipoint Distribution Service (MDS), Local
Multipoint
Distribution
Service
(LMDS),
and
Unlicensed
National
Information
Infrastructure (U-NII). Within the IEEE 802 LAN/MAN Standards Committee,
2
WirelessMANTM designates the family of standards for BWA systems, where “MAN”
stands for “metropolitan area network”.
Background - Large scale fixed BWA deployment in the U.S. was stimulated by the
Telecommunications Act of 1996 and expanded rapidly until 2001, when it suffered a
serious setback under the impact of a combination of adverse conditions. Nevertheless,
the post-crash landscape shows achievements of lasting value and a promising level of
continuing activities, for example:

the best established BWA service, deployed in the 39 GHz band, emerged from
Chapter 11 protection under new ownership and continues regular operation;

the downsized systems supplier community is intensifying work on the next
generation(s) of BWA systems with greatly improved capabilities at lower cost;

FCC proceedings are in progress for new BWA applications in the 70 GHz, 80 GHz
and 90 GHz frequency bands;

BWA standardization within the IEEE Standards Association is in progress with
widespread industry participation.
1.1 BWA implementation options
The BWA implementation options include:

licensed and unlicensed operation in a number of frequency bands allocated to the
fixed and mobile services, as well as free-space optical operation;

access link lengths ranging from hundreds of meters to over 20 kilometers,
depending on the propagation characteristics that are determined by the choice of
frequency band and path, and by the local meteorological conditions;
3

point-to-point (P-P) systems applications in star/cellular and ring topologies, point-tomultipoint (P-MP) systems in cellulatr topology, and multipoint-to-multipoint (MP-MP)
systems in mesh topology;

the predominant transmission formats and protocols: SONET/SDH, ATM, IP and
Ethernet;

frequency division multiple access (FDMA) and time division multiple access (TDMA)
with frequency division duplex (FDD) or time division duplex (TDD) operation;

the use of advanced signal processing techniques for efficient spectrum utilization,
countermeasures for electromagnetic wave propagation impairments, and privacy;

quality of service (QoS) ranging from ATM equivalent to best effort.
The multitude of options make it possible to implement a variety of BWA systems
optimized for specific applications.
1.2 The IEEE standardization framework
Recognizing the BWA potential, the IEEE 802 LAN/MAN Standards Committee
established the IEEE 802.16 Working Group on Broadband Wireless Access. Work is in
progress on a family of standards intended to enable multiple vendors to produce
interoperable P-MP and MP-MP mesh BWA systems for a number of frequency bands in
the 2 - 66 GHz frequency range. Significantly, this is the first U.S. standardization effort
in fixed terrestrial wireless access systems, complementing the work of IEEE 802.11
Working Group on Wireless Local Area Networks. IEEE 802.16 activities are highly
productive, they are transnational, greatly influence international standardization efforts,
and substantially improve the overall BWA outlook.
IEEE 802.16 developed a common medium access control layer (MAC) and separate
physical layers (PHY) for the 10 - 66 GHz and 2 - 11 GHz frequency ranges,
4
respectively. This differentiation takes into account the facts that the higher frequency
range encompasses an order of magnitude more spectrum available for BWA
applications but is usable only for line-of-sight (LOS) operation, whereas near-line-ofsight or non-line-of-sight (NLOS) operation is feasible only in the lower frequency range.
Sections 2 and 3 reflect this fundamental distinction and focus on the frequency ranges
above 20 GHz and below 6 GHz, respectively, which encompass the major BWA bands.
1.3
The fixed BWA panorama: MAN, LAN, PAN and optical wireless access
This draft adopts the IEEE 802.16 definition of BWA, which covers only MAN. However,
optimal MAN development requires taking into account related developments within the
“fixed wireless continuum” which encompasses MAN, LAN and PAN, as well as optical
wireless access. IEEE 802 addresses LAN in 802.11 and PAN in 802.15. In this broader
context, BWA complements and competes not only with wireline alternatives but also
with mobile broadband wireless alternatives. The latter case is best exemplified with the
fast growing wireless LAN deployments that already satisfy widespread “nomadic” fixed
access needs targeted by third generation (3G) or fourth generation (4G) mobile
systems (e.g. 2 Mb/s) at some future time. But 3G/NG will only be able to accomplish
such needs by making inefficient use of their capabilities optimized for mobile operation.
2
BWA in frequency bands above 20 GHz
Since propagation impairments generally worsen with increasing frequency and thereby
shorten the usable path lengths, frequency bands above 20 GHz are particularly well
suited for “first-mile/last-mile” subscriber access connections. Only line-of-sight (LOS)
paths are usable in this frequency range.
The acronym LMDS, which stands for Local Multipoint Distribution Service, commonly
applies to BWA applications in these frequency bands.Originally, LMDS was planned to
be a one-way wireless substitute for cable TV in the 28 GHz band. It has not
materialized as planned, and this band is now targeted for BWA applications. However,
5
substantial deployment of this kind of service started first in the 39 GHz band, and then
in the 24 GHz band, both by CLEC’s providing LOS access to office buildings in urban
areas. Initial deployment used P-P links in cellular network configurations. P-MP
systems were introduced more recently. Subscriber densities currently range up to about
100 stations per square kilometer, and provide substantial room for further growth. Data
rates ranged initially from T1 to T3, and more recently to OC-3 for P-P systems and 200
Mb/s for P-MP systems. These BWA networks are typically designed to match the bit
error rate performance of fiber access and to achieve 99.999% availability. P-P systems
carrying OC-12 have been introduced recently, as well as a Gigabit Ethernet system for
the unlicensed 60 GHz band.
More widespread deployment of such BWA systems is facilitated by a WRC-2000
decision and by IEEE 802 LAN MAN standardization. WRC-2000 made available several
bands in the 30 - 66 GHz frequency range for “high density applications in the fixed
service”, whereas the IEEE 802.16 Working Group developed the IEEE 802.16
WirelessMANTM air interface standard for P-MP systems in the 10 - 66 GHz frequency
range. Maturing technology for ever higher frequencies continues to shift the limit of
cost-competitive BWA applications upwards. As a consequence, FCC proceedings are
in progress for new applications in the 70 GHz, 80 GHz and 90 GHz frequency bands,
and industry is already aiming at even higher frequency bands. These developments
open the prospect of large scale BWA deployment at ever higher data rates that are
increasingly more competitive with both fiber cable and free-space optical access up to
multi-Gb/s data rates. As a near-term example, a 10 Gigabit Ethernet system operating
in the license-exempt 57 - 64 GHz frequency range is announced to become available in
2003.
3.
BWA in frequency bands below 6 GHz
The general trend of propagation conditions, pointed out in the first paragraph of Section
2, makes the bands below 6 GHz particularly desirable for BWA applications requiring
longer LOS subscriber access links and/or denser service penetration through the use of
NLOS links. However, the frequency spectrum available for BWA applications in this
6
frequency range is more than an order of magnitude smaller than above 20 GHz, and is
partially targeted for mobile usage.
The major frequency bands below 6 GHz that are available for BWA in the U.S. are the
2.5 GHz band licensed for the Multichannel Multipoint Distribution Service (MMDS), and
the 5 GHz bands for the Unlicensed National Information Infrastructure (U-NII). Both
types of applications are covered by the IEEE 802.16a WirelessMAN standard under
development.
3.1 MMDS
BWA is a more recent application in the 2.5 GHz band originally intended for “wireless
cable TV” sharing with the closed circuit Instructional Television Fixed Service (ITFS).
ILECs lacking wireline local access facilities were the principal BWA users, so far,
providing wireless alternatives to DSL and Cable TV modems. The initial deployment
pattern using first-generation MMDS systems was similar to that of LMDS, but the more
favorable propagation conditions in the 2.5 GHz band made it possible to deploy
substantially larger cells in LOS operation, as well as to operate near-line-of-sight links.
Non-line-of-sight (NLOS) operation is being planned using substantially improved and
more versatile second-generation MMDS systems, which promise to substantially
increase service penetration in comparison with that of LOS operation. NLOS operation
is also expected to facilitate self installation by the subscriber, particularly with indoor
mounted antennas. However, NLOS imposes a trade-off between increased service
penetration and lower achievable availability in comparison with LOS operation.
Nevertheless, the typical 99.9% availability for NLOS operation is considered to be
acceptable to a sizeable subscriber population if pricing is competitive.
First generation MMDS systems turned out to be too costly in competition with the cable
TV and DSL alternatives. This brought MMDS deployment to a temporary halt, awaiting
the second generation P-MP and MP-MP mesh systems which promise a number of
substantial improvements in deployment, operation and service profitability. These new
systems are being evaluated by interested service providers. However, major new
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MMDS deployment is unlikely before the completion of the IEEE 802.16a standard for
the air interface in the licensed bands in the frequency range 2 - 11 GHz. The current
draft includes three options: single-carrier, orthogonal frequency multiplexing (OFDM)
and orthogonal frequency division multiple access (OFDMA).
In addition to this short-term uncertainty, there is the long-term uncertainty about the
continued suitability of the 2.5 GHz band for BWA applications. WRC-2000 identified this
band for IMT-2000 use, and the FCC opened it up for shared mobile use but without
apparent intent to restrict MMDS and ITFS usage. Market forces may well determine the
future usage of this band in the U.S. Much will depend on the preference of the major
service providers who are in both the fixed BWA and cellular mobile markets.
Significantly, IEEE 802.16 is already considering limited mobility as a near-term
objective, and has recently been approached with a proposal to also consider mobile
BWA in licensed bands “supporting mobility at vehicular speeds.”
There are also concerns about whether the 200 MHz wide 2.5 GHz band is wide enough
for extensive MMDS deployment. One recent proposal is to extend MMDS usage into
the adjacent license-exempt 2.4 GHz industrial, scientific and medical (ISM) band.
However, the ISM band is already crowded with a number of different users. Wireless
LANs based on the IEEE standard 802.11b account for the fastest growing usage of this
band. Relief is expected from the new LAN version based on the IEEE standard 802.11a
for applications in the U-NII bands.
3.2
U-NNI
The FCC Report and Order on the Unlicensed National Information Infrastructure
produced a uniquely versatile solution for an attractive set of multimedia service needs
of a diverse group of wireless users requiring substantially higher data rates than
available from MMDS. A total of 300 MHz of spectrum was made available for this
purpose in the 5 GHz frequency range, subdivided into three 100 MHz segments
differentiated by technical requirements restricting potential interference to licensed and
license-free systems sharing the same frequency band. The FCC differentiation includes
8
identification of preferential usage of each of the three bands: 5.150 – 5.250 GHz for
indoor LAN, 5.250 – 5.350 GHz for campus-type LAN, and 5.725 – 5.825 GHz for
“community networks with a typical range of several kilometers.” This allows to satisfy a
great variety of needs with a small variety of systems operating within adjacent or closely
spaced frequency bands. These favorable conditions would be enhanced by making the
5.470 – 5.725 GHz band also available for U-NII applications, as has been proposed
recently.
The FCC differentiation makes the band 5.725 – 5.825 GHz the most suitable one for
BWA applications of conventional P-P systems, or P-MP systems being standardized in
the IEEE 802.16a project. However, the license-exempt status of all three bands tends
to relax the intended application differentiation. As an example, some 802.11a products
cover all three bands. And there are more indications of the tendency to use the licenseexempt U-NII spectrum more liberally than differentiated by the FCC. This is facilitated
by the lack of rules for efficient spectrum use and coordination among services deployed
in the same area. The risk of interference will increase with increasing band usage, and
it is in the service providers’ best interest to address this potential problem in some
cooperative way. A precedent exists for BWA applications in the licensed bands above
10 GHz, for which IEEE 802.16 has developed IEEE Standard 802.16.2 entitled “IEEE
Recommended Practice for Local and Metropolitan Area Networks – Coexistence of
Fixed Broadband Wireless Systems”.
3.3
2.4 GHz ISM band: the significance of the IEEE 802.11b LAN developments
The rapid growth of LAN deployment in the 2.4 GHz license-exempt band, based on the
IEEE standard 802.11b, offers useful lessons not only for the LAN prospects in the U-NII
bands, based on the IEEE standard 802.11a, but also for MAN prospects in a number of
bands, based on the IEEE 802.16 standard. The single most important lesson seems to
be that license-exempt operation and product consumerization are the most effective
catalysts for large scale fixed BWA applications, especially when both apply. But this is
just another demonstration of the benefits of improving user friendliness in the
development of telecommunications services, which are accelerating the “do it yourself”
9
trend among end users, enterprises and communities. Multi-tenant buildings, smalloffice/home-office (SOHO), and residential wireless MAN applications in either licensed
or license-exempt band would similarly benefit from product consumerization and ease
of self-installation. Subscribing to a BWA service has the potential of becoming as
simple as purchasing a TV set and installing an indoor or outdoor antenna.
4.
Free-space optical access
Free-space optical access systems naturally surpass the data rate capability of
millimeter-wave BWA systems, and are therefore the most promising competitor to fiber
cable access for link lengths that make it possible to achieve 99.999% availability under
the local visibility conditions in the presence of fog, smog and mist. However, this
availability requirement limits optical link lengths to a greater extent than for millimeterwave links, except in localities with highest rainfall rates. Some suppliers of free-space
optical systems consider lower availabilities, e.g. 99.9%, acceptable to users without a
more favorable alternative, and they propose millimeter-wave back-up for “stretched”
links requiring 99.999% availability. Both possibilities would improve the market outlook.
5.
Conclusions and comments
Prerequisite: cost-competitive technology - The progress of wireless technology,
spectrum availability and systems implementations effectively support the deployment of
a variety of BWA services for which market demands provide a sound business case.
Cost-competitiveness in providing a needed service is the ultimate prerequisite that
systems suppliers and service providers are jointly striving to satisfy and improve under
persistent market pressures.
Impact of business conditions - The future of fixed BWA depends to a large extent
on factors other than technological capabilities to satisfy subscribers’ needs for
broadband access. The recent BWA setback provides valuable indications on the
negative impact of lacking cost-effectiveness, unintentional negative consequences of
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regulatory measures, persistent strong competitive pressures from dominant incumbent
telecommunications service providers, bursting of the “telecom bubble” and the drying
up of necessary funding for BWA service providers and their systems suppliers.
Continuing progress - Nevertheless, fixed terrestrial BWA systems and services will
continue making substantial contributions to the build-up of broadband subscriber
access by complementing and competing with the wireline alternatives. The multiple
implementation options listed in Section 1 provide the basis for these developments, and
advantageous trade-offs lead to implementations that receive best market acceptance.
The inherent wireless deployment flexibility makes possible “modular” service build-out
that shortens time to market and makes possible gradual investment. These features
facilitate end-to-end infrastructure ownership which enhances the service provider’s
competitive position.
Commonalities and trade-offs - Existing and emerging fixed BWA implementations
borrow from both conventional P-P microwave link technology and wireless mobile
technology, but also introduce novel solutions. BWA operates in a number of frequency
bands within a wide frequency range that overlaps with mobile service bands at the
lower end, and extends to millimeter wave bands at the upper end. The basic
commonalities of fixed BWA and mobile cellular wireless are in the usage of P-MP cells
and in NLOS operation. However, different trade-offs apply because BWA services
require substantially higher data rates, performance and availability, all of which reduce
the potential NLOS service penetration in comparison with mobile cellular systems.
Innovation - BWA innovation is evident in several ways. For example, emerging MPMP mesh networks are being introduced to substantially improve BWA service
penetration and quality by establishing tandem LOS subscriber connections around
obstacles. This makes it possible to serve more subscribers with better availability than
in a P-MP cell using NLOS subscriber connections. In MP-MP mesh networks, each
subscriber station simultaneously functions as a P-MP hub. Built-in alternative traffic
routing, a distinctive capability of MP-MP systems, greatly improves subscriber access
availability. This is a significant advantage over both P-MP and P-P systems which
would require duplicating equipment to achieve a similar availability improvement. The
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increased hardware complexity of subscriber stations needed to achieve the greater
MP-MP mesh network functionality is apparently realizable with readily available costeffective technology.
Satisfying the different market demands - In both P-MP and MP-MP mesh networks
the subscribers share the hub station capacity, which is advantageous where dynamic
data rate allocation is used. However, subscribers requiring BWA service at higher data
rates than can be provided by an optimized P-MP or MP-MP mesh network, will need to
be served with millimeter-wave and/or optical P-P links. A redundant combination of mmwave and optical links on the same path allows to substantially stretch the link length
beyond the capability of either one to achieve the required availability. This is the most
promising BWA approach to providing multi-Gb/s transmission rates to subscribers
whose only other alternative would be a more costly fiber optic cable connection.
Outlook - The BWA outlook depends on technological and business prerequisites. This
update leads to the conclusion that prerequisite cost-effective BWA technology is
available and is satisfactorily advancing. The recent bursting of the “telecom bubble”
clouded the business outlook. It thus remains to be seen to what extent BWA can be
expected to satisfy the growing demand for broadband access at prices that end users
will find worth paying for.
Acknowledgement
Harold Sobol, principal author of the TTG-3 Report [1] and Roger B. Marks, Chair, IEEE
802.16 Working Group on Broadband Wireless Access, kindly reviewed the successive
drafts of this update and provided valuable suggestions for improvement. Additional
editing help and was kindly provided by George Heiter, Past Chair of Committee MTT16, Michael Green, Manager of Global Product Compliance, Atheros Communications,
and Peter A. Soltesz, President, PAS-COM. Thanks to all!
Reference:
1. Harold Sobol, Ferdo Ivanek, Sastri Kota, Amir Zaghloul, Bernard Bennington,
Clara Manders, David Weinreich and Eric J. Schimmel, Wireless Technologies
and Infrastructures for Subscriber Access, Technology Task Group 3, info,
Vol. 2, No. 2, April 2000, pp. 131-146.