Like any other segment of the information technology industry, broadband wireless access (BWA) thrives on interoperability standards. In the case of data communications networks, the IEEE-802 LAN/MAN Standards Committee has become the world leader in local-area network and metropolitan-area network standards.
The success of the IEEE-802.3 Ethernet standard and the IEEE-802.11 wireless LAN standard are stunning examples of the power of 802's open industry-consensus process. The BWA industry is following a similar path through the IEEE-802.16 Working Group on Broadband Wireless Access, which is developing the IEEE-802.16 wireless MAN standard for wireless metropolitan-area networks. This standard, covering licensed and license-exempt bands from 2 to 66 GHz worldwide, is creating a firm foundation for the development of the industry.
After some initial organization efforts, Working Group 802.16 began its work in July 1999. Hundreds of people have participated in the bimonthly, week-long meetings. The group currently has 200 members and official observers from over 100 companies.
IEEE 802.16 addresses "first-mile" applications of wireless technology to link commercial and residential buildings to high-rate core networks and thereby provide access to those networks. The 802.16 group's work has primarily aimed at a point-to-multipoint topology with a cellular deployment of basestations, each tied to core networks and in contact with fixed-wireless subscriber stations.
The subscriber stations typically include rooftop-mounted antenna/radio units connected to indoor network interface units, although in some cases both units could be indoors or both outdoors. Initial work has aimed at businesses, with much of the market focus on small- to medium-size enterprises. Attention has increasingly turned toward residential opportunities, particularly as the lower frequencies have become available for two-way service.
Working Group 802.16 is now completing a draft of the IEEE-802.16 Standard Air Interface for Fixed Broadband Wireless Access Systems. The document includes a flexible media-access control (MAC) layer. The accompanying physical layer (PHY) is designed for 10 to 66 GHz, informally known as the local multipoint distribution service (LMDS) spectrum. The standard is not yet final, but the draft is stable and has passed the Working Group's letter ballot, pending resolution of comments proposed to improve it. Publication is planned for late this year.
Simultaneously, the Working Group is developing amendments to the base 802.16 standard to accommodate lower frequencies. Amendment 802.16a will address licensed bands from 2 to 11 GHz; the primary target in the United States is the multichannel multipoint distribution service (MMDS) bands. Amendment 802.16b aims at the needs of license-exempt applications around 5 to 6 GHz, sometimes known as U-NII bands.
Prior to the publication of its air interface, the Working Group will publish the IEEE 802.16.2 Recommended Practice for Coexistence of Fixed Broadband Wireless Access Systems, which concentrates on 23.5 GHz to 43.5 GHz. The document has been balloted and is scheduled for final approval in June 2001. Extensions to the lower frequencies are under consideration.
IEEE 802.16 maintains a close working relationship with standards bodies in the International Telecommunications Union and ETSI, particularly with the Hiperaccess and HiperMAN programs of ETSI's Broadband Radio Access Networks (Bran) project and with ETSI Working Group TM4.
The 802.16 Working Group follows the traditional 802 approach of developing multiple PHY options supported by a common MAC. Although the service requirements of the three air interface projects differ, 802.16 MAC is flexible enough to support all of them, with extensions.
In general, the point-to-multipoint architecture assumes a time-division multiplexed downlink from the basestation, with subscriber stations in a given cell and sector sharing the uplink, typically by time-division multiple access. Uplink access is controlled by the basestation, which has a set of scheduling mechanisms at its disposal for optimizing system performance.
The MAC draws from the data-over-cable (Docsis) standard that has been successfully deployed in hybrid-fiber coaxial cable systems, which have a similar point-to-multipoint architecture. However, the MAC protocol engine is a new design. It is a connection-oriented MAC able to tunnel any protocol across the air interface with full quality-of-service (QoS) support. Asynchronous transfer mode and packet-based convergence layers provide the interface to higher protocols. While extensive bandwidth allocation and QoS mechanisms are provided, the details of scheduling and reservation management are left unstandardized and provide an important mechanism for vendors to differentiate their equipment. A privacy sublayer provides both encryption and authentication.
An important MAC feature is the option of granting bandwidth to a subscriber station rather than to the individual connections it supports. That provides the option of allowing a smart subscriber station to manage its bandwidth allocation among its users, which can make for more efficient allocation in multitenant commercial or residential buildings. Efficiency is also enhanced by the provision for header suppression, concatenation, fragmentation and packing.
The 10- to 66-GHz physical layer assumes line-of-sight propagation with no significant concern about multipath propagation. Two basic modes are provided. The continuous mode uses frequency-division duplexing (FDD), with simultaneous uplink and downlink on separate frequencies. A continuous time-division multiplexed downstream allows a powerful concatenated coding scheme with interleaving.
The burst mode allows time-division duplexing (TDD), with the uplink and downlink sharing a channel but not transmitting simultaneously. This allows dynamic reassignment of the uplink and downlink capacity. This mode also allows "burst FDD," which supports half-duplex FDD subscriber stations that do not simultaneously transmit and receive and may therefore be less expensive. Both TDD and burst FDD support adaptive-burst profiles in which modulation (QPSK, 16-QAM or 64-QAM) and coding may be dynamically assigned on a burst-by-burst basis.
The 802.16 group has been developing a standard for 2- to 11-GHz BWA. In the United States, the primary targeted frequencies are in the MMDS bands, mostly from 2.5 to 2.7 GHz. Worldwide, 3.5 GHz and 10.5 GHz are likely applications. Because nonline-of-sight operation is practical and because of the lower component costs, those bands are seen as good prospects for residential and small-business services. The spectrum availability is suitable to those uses.
The 802.16 group has decided to support both single-carrier and multicarrier PHY options. The single-carrier proposal, submitted by staff from 16 companies, uses frequency-domain equalization. The multicarrier proposal, submitted by staff from 17 companies and an industry consortium, uses orthogonal frequency-division multiplexing (OFDM) and orthogonal frequency-division multiple access (OFDMA). Both support TDD and FDD; the detailed proposals are on the Web (http:// ieee802.org/16/).
MAC enhancements are also under development. Final approval of this amendment is planned for March 2002, but a stable draft is expected months earlier.
In the case of license-exempt bands, 802.16 is tasked to develop a physical layer based on the 802.11a OFDM and/or HiperLAN/2 PHYs. Coordination of basestations under independent operators in unlicensed spectrum is an important issue facing this group. MAC enhancements under development include an optional mesh architecture in addition to the point-to-multipoint topology-testimony to the flexibility of the 802.16 MAC. Final approval is scheduled for March 2002.
IEEE 802.16 represents a broad industry commitment to develop interoperable equipment for fixed-broadband wireless access. The challenging effort of creating high-quality standards began in 1999. Good standards take time, but 802.16 has demonstrated its ability to move quickly. The time spent in refining IEEE standards is not wasted because it is applied to solving technical problems and consolidating innovations. The result is the crystallization of standards that define the next generation of equipment, not the past generation.
With the consensus participation of the industry, these standards are set to define a major new alternative method of broadband access.
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