2002-06-04
`
`IEEE C802.16-02/05
`
`Project
`
`Title
`
`IEEE 802.16 Broadband Wireless Access Working Group
`<http://ieee802.org/16>
`
`IEEE Standard 802.16: A Technical Overview of the WirelessMAN™
`Air Interface for Broadband Wireless Access
`
`Date Submitted
`
`2002-06-04
`
`Source(s)
`
`Re:
`
`Abstract
`
`Purpose
`
`Notice
`
`Patent Policy
`and Procedures
`
`Roger Marks
`NIST
`325 Broadway
`Boulder, CO 80305
`
`IEEE Std 802.16
`
`Voice: +1-303-497-3037
`Fax: +1-303-497-3037
`mailto:r.b.marks@ieee.org
`
`The broadband wireless access industry, which provides high-rate network
`connections to stationary sites, has matured to the point at which it now has
`a standard for second-generation wireless metropolitan area networks. IEEE
`Standard 802.16, with its WirelessMAN™ air interface, sets the stage for
`widespread and effective deployments worldwide. This article overviews the
`technical medium access control and physical layer features of this new
`standard.
`
`This article was written by Carl Eklund, Roger B. Marks, Kenneth L.
`Stanwood, and Stanley Wang. It was published in IEEE Communications
`Magazine, June 2002, pp. 98-107.” For more details, see:
`
`<http://www.comsoc.org/ci1/Public/2002/Jun/index.html>
`
`This document will improve the effectiveness of participants in IEEE Working Group
`802.16 by providing an overview of the published base standard upon which the Working
`Group is currently developing several amendments.
`
`This document has been prepared to assist IEEE 802.16. It is offered as a basis for discussion and is
`not binding on the contributing individual(s) or organization(s). The material in this document is
`subject to change in form and content after further study. The contributor(s) reserve(s) the right to add,
`amend or withdraw material contained herein.
`
`The contributor is familiar with the IEEE 802.16 Patent Policy and Procedures (Version 1.0)
`<http://ieee802.org/16/ipr/patents/policy.html>, including the statement “IEEE standards may include
`the known use of patent(s), including patent applications, if there is technical justification in the
`opinion of the standards-developing committee and provided the IEEE receives assurance from the
`patent holder that it will license applicants under reasonable terms and conditions for the purpose of
`implementing the standard.”
`
`Early disclosure to the Working Group of patent information that might be relevant to the standard is
`essential to reduce the possibility for delays in the development process and increase the likelihood
`that the draft publication will be approved for publication. Please notify the Chair
`<mailto:r.b.marks@ieee.org> as early as possible, in written or electronic form, of any patents
`(granted or under application) that may cover technology that is under consideration by or has been
`approved by IEEE 802.16. The Chair will disclose this notification via the IEEE 802.16 web site
`<http://ieee802.org/16/ipr/patents/notices>.
`
`1
`
`HONDA 1029
`
`

`

`
`
`
`
`
`2002-06-04
`
`Copyright
`Permission
`
`IEEE C802.16-02/05
`
`Copyright ©2002 Institute of Electrical and Electronics Engineers, Inc. Reprinted, with
`permission, from
`, June 2002, pp. 98-107. This material is
`IEEE Communications Magazine
`posted here with permission of the IEEE. Internal or personal use of this material is
`permitted. However, permission to reprint/republish this material for advertising or
`promotional purposes or for creating new collective works for resale or redistribution must
`be obtained from the IEEE (contact pubs-permissions@ieee.org).
`
`By choosing to view this document, you agree to all provisions of the copyright laws
`protecting it.
`
`2
`
`

`

`TOPICS IN BROADBAND ACCESS
`
`IEEE Standard 802.16:
`A Technical Overview of the
`WirelessMAN™ Air Interface for
`Broadband Wireless Access
`
`Carl Eklund, Nokia Research Center
`Roger B. Marks, National Institute of Standards and Technology
`Kenneth L. Stanwood and Stanley Wang, Ensemble Communications Inc.
`
`ABSTRACT
`
`The broadband wireless access industry, which
`provides high-rate network connections to sta-
`tionary sites, has matured to the point at which
`it now has a standard for second-generation
`wireless metropolitan area networks. IEEE Stan-
`dard 802.16, with its WirelessMAN™ air inter-
`face, sets the stage for widespread and effective
`deployments worldwide. This article overviews
`the technical medium access control and physical
`layer features of this new standard.
`
`INTRODUCTION AND
`MARKET OPPORTUNITIES
`IEEE Standard 802.16-2001 [1], completed in
`October 2001 and published on 8 April 2002,
`defines the WirelessMAN™ air interface specifi-
`cation for wireless metropolitan area networks
`(MANs). The completion of this standard her-
`alds the entry of broadband wireless access as a
`major new tool in the effort to link homes and
`businesses to core telecommunications networks
`worldwide.
`As currently defined through IEEE Stan-
`dard 802.16, a wireless MAN provides network
`access to buildings through exterior antennas
`communicating with central radio base stations
`(BSs). The wireless MAN offers an alternative
`to cabled access networks, such as fiber optic
`links, coaxial systems using cable modems, and
`digital subscriber line (DSL) links. Because
`wireless systems have the capacity to address
`broad geographic areas without the costly infra-
`structure development required in deploying
`cable links to individual sites, the technology
`may prove less expensive to deploy and may
`
`lead to more ubiquitous broadband access.
`Such systems have been in use for several years,
`but the development of the new standard marks
`the maturation of the industry and forms the
`basis of new industry success using second-gen-
`eration equipment.
`In this scenario, with WirelessMAN technolo-
`gy bringing the network to a building, users inside
`the building will connect to it with conventional
`in-building networks such as, for data, Ethernet
`(IEEE Standard 802.3) or wireless LANs (IEEE
`Standard 802.11). However, the fundamental
`design of the standard may eventually allow for
`the efficient extension of the WirelessMAN net-
`working protocols directly to the individual user.
`For instance, a central BS may someday exchange
`medium access control (MAC) protocol data with
`an individual laptop computer in a home. The
`links from the BS to the home receiver and from
`the home receiver to the laptop would likely use
`quite different physical layers, but design of the
`WirelessMAN MAC could accommodate such a
`connection with full quality of service (QoS).
`With the technology expanding in this direction, it
`is likely that the standard will evolve to support
`nomadic and increasingly mobile users. For exam-
`ple, it could be suitable for a stationary or slow-
`moving vehicle.
`IEEE Standard 802.16 was designed to
`evolve as a set of air interfaces based on a com-
`mon MAC protocol but with physical layer spec-
`ifications dependent on the spectrum of use and
`the associated regulations. The standard, as
`approved in 2001, addresses frequencies from
`10 to 66 GHz, where extensive spectrum is cur-
`rently available worldwide but at which the
`short wavelengths introduce significant deploy-
`ment challenges. A new project, currently in the
`balloting stage, expects to complete an amend-
`
`Portions are U.S. Govern-
`ment work, not subject to
`U.S. Copyright.
`
`98
`
`0163-6804/02/$17.00 © 2002 IEEE
`
`IEEE Communications Magazine • June 2002
`
`3
`
`

`

`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.
`
`ment denoted IEEE 802.16a [2] before the end
`of 2002. This document will extend the air inter-
`face support to lower frequencies in the 2–11
`GHz band, including both licensed and license-
`exempt spectra. Compared to the higher fre-
`quencies, such spectra offer the opportunity to
`reach many more customers less expensively,
`although at generally lower data rates. This sug-
`gests that such services will be oriented toward
`individual homes or small to medium-sized
`enterprises.
`THE 802.16 WORKING GROUP
`Development of IEEE Standard 802.16 and the
`included WirelessMAN™ air interface, along
`with associated standards and amendments, is
`the responsibility of IEEE Working Group
`802.16 on Broadband Wireless Access (BWA)
`Standards (http://WirelessMAN.org). The Work-
`ing Group’s initial interest was the 10–66 GHz
`range. The 2–11 GHz amendment project that
`led to IEEE 802.16a was approved in March
`2000. The 802.16a project primarily involves the
`development of new physical layer specifica-
`tions, with supporting enhancements to the
`basic MAC. In addition, the Working Group
`has completed IEEE Standard 802.16.2 [3]
`(“Recommended Practice for Coexistence of
`Fixed Broadband Wireless Access Systems”) to
`address 10–66 GHz coexistence and, through
`the amendment project 802.16.2a, is expanding
`its recommendations to include licensed bands
`from 2 to 11 GHz.
`Historically, the 802.16 activities were initiated
`at an August 1998 meeting called by the National
`Wireless Electronics Systems Testbed (N-WEST)
`of the U.S. National Institute of Standards and
`Technology. The effort was welcomed in IEEE
`802, which opened a Study Group. The 802.16
`Working Group has held weeklong meetings at
`least bimonthly since July 1999. Over 700 individ-
`uals have attended a session. Membership, which
`is granted to individuals based on their atten-
`dance and participation, currently stands at 130.
`The work has been closely followed; for example,
`the IEEE 802.16 Web site received over 2.8 mil-
`lion file requests in 2000.
`TECHNOLOGY DESIGN ISSUES
`MEDIUM ACCESS CONTROL
`The IEEE 802.16 MAC protocol was designed
`for point-to-multipoint broadband wireless
`access applications. It addresses the need for
`very high bit rates, both uplink (to the BS)
`and downlink (from the BS). Access and band-
`width allocation algorithms must accommo-
`date hundreds of terminals per channel, with
`terminals that may be shared by multiple end
`users. The services required by these end users
`are varied in their nature and include legacy
`time-division multiplex (TDM) voice and data,
`Internet Protocol (IP) connectivity, and packe-
`tized voice over IP (VoIP). To support this
`variety of services, the 802.16 MAC must
`accommodate both continuous and bursty traf-
`fic. Additionally, these services expect to be
`assigned QoS in keeping with the traffic types.
`The 802.16 MAC provides a wide range of ser-
`vice types analogous to the classic asyn-
`
`chronous transfer mode (ATM) service cate-
`gories as well as newer categories such as
`guaranteed frame rate (GFR).
`The 802.16 MAC protocol must also support
`a variety of backhaul requirements, including
`both asynchronous transfer mode (ATM) and
`packet-based protocols. Convergence sublayers
`are used to map the transport-layer-specific traf-
`fic to a MAC that is flexible enough to efficient-
`ly carry any traffic type. Through such features
`as payload header suppression, packing, and
`fragmentation, the convergence sublayers and
`MAC work together to carry traffic in a form
`that is often more efficient than the original
`transport mechanism.
`Issues of transport efficiency are also
`addressed at the interface between the MAC
`and the physical layer (PHY). For example, the
`modulation and coding schemes are specified in
`a burst profile that may be adjusted adaptively
`for each burst to each subscriber station. The
`MAC can make use of bandwidth-efficient burst
`profiles under favorable link conditions but shift
`to more reliable, although less efficient, alterna-
`tives as required to support the planned 99.999
`percent link availability.
`The request-grant mechanism is designed to
`be scalable, efficient, and self-correcting. The
`802.16 access system does not lose efficiency
`when presented with multiple connections per
`terminal, multiple QoS levels per terminal, and a
`large number of statistically multiplexed users. It
`takes advantage of a wide variety of request
`mechanisms, balancing the stability of con-
`tentionless access with the efficiency of con-
`tention-oriented access.
`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.
`Along with the fundamental task of allocating
`bandwidth and transporting data, the MAC
`includes a privacy sublayer that provides authen-
`tication of network access and connection estab-
`lishment to avoid theft of service, and it provides
`key exchange and encryption for data privacy.
`To accommodate the more demanding physi-
`cal environment and different service require-
`ments of the frequencies between 2 and 11 GHz,
`the 802.16a project is upgrading the MAC to
`provide automatic repeat request (ARQ) and
`support for mesh, rather than only point-to-mul-
`tipoint, network architectures.
`THE PHYSICAL LAYER
`10–66 GHz — In the design of the PHY speci-
`fication for 10–66 GHz, line-of-sight propaga-
`tion was deemed a practical necessity. With this
`condition assumed, single-carrier modulation
`was easily selected; the air interface is designat-
`ed “WirelessMAN-SC.” Many fundamental
`design challenges remained, however. Because
`of the point-to-multipoint architecture, the BS
`basically transmits a TDM signal, with individu-
`al subscriber stations allocated time slots serial-
`ly. Access in the uplink direction is by
`time-division multiple access (TDMA). Follow-
`ing extensive discussions regarding duplexing, a
`
`IEEE Communications Magazine • June 2002
`
`99
`
`4
`
`

`

`TDM portion
`
`TDM
`DIUC a
`
`TDM
`DIUC b
`
`TDM
`DIUC c
`
`TDMA portion
`
`Broadcast
`control
`DIUC = 0
`
`Preamble
`
`TDMA
`DIUC g
`
`Preamble
`
`TDMA
`DIUC f
`
`Preamble
`
`TDMA
`DIUC e
`
`Preamble
`
`TDMA
`DIUC d
`
`Preamble
`
`Burst start points
`
`• WirelessMAN-OFDMA: This uses orthogo-
`nal frequency-division multiple access with
`a 2048-point transform. In this system, mul-
`tiple access is provided by addressing a sub-
`set of the multiple carriers to individual
`receivers.
`Because of the propagation requirements, the
`use of advanced antenna systems is supported.
`It is premature to speculate on further
`specifics of the 802.16a amendment prior to its
`completion. While the draft seems to have
`reached a level of maturity, the contents could
`change significantly in balloting. Modes could
`even be deleted or added.
`
`PHYSICAL LAYER DETAILS
`The PHY specification defined for 10–66 GHz
`uses burst single-carrier modulation with adap-
`tive burst profiling in which transmission param-
`eters, including the modulation and coding
`schemes, may be adjusted individually to each
`subscriber station (SS) on a frame-by-frame
`basis. Both TDD and burst FDD variants are
`defined. Channel bandwidths of 20 or 25 MHz
`(typical U.S. allocation) or 28 MHz (typical
`European allocation) are specified, along with
`Nyquist square-root raised-cosine pulse shaping
`with a rolloff factor of 0.25. Randomization is
`performed for spectral shaping and to ensure bit
`transitions for clock recovery.
`The forward error correction (FEC) used is
`Reed-Solomon GF(256), with variable block size
`and error correction capabilities. This is paired
`with an inner block convolutional code to robust-
`ly transmit critical data, such as frame control
`and initial accesses. The FEC options are paired
`with quadrature phase shift keying (QPSK), 16-
`state quadrature amplitude modulation (16-
`QAM), and 64-state QAM (64-QAM) to form
`burst profiles of varying robustness and efficien-
`cy. If the last FEC block is not filled, that block
`may be shortened. Shortening in both the uplink
`
`DL-MAP UL-MAP
`
`Preamble
`
`■ Figure 1. The downlink subframe structure.
`
`burst design was selected that allows both time-
`division duplexing (TDD), in which the uplink
`and downlink share a channel but do not trans-
`mit simultaneously, and frequency-division
`duplexing (FDD), in which the uplink and down-
`link operate on separate channels, sometimes
`simultaneously. This burst design allows both
`TDD and FDD to be handled in a similar fash-
`ion. Support for half-duplex FDD subscriber
`stations, which may be less expensive since they
`do not simultaneously transmit and receive, was
`added at the expense of some slight complexity.
`Both TDD and FDD alternatives support adap-
`tive burst profiles in which modulation and cod-
`ing options may be dynamically assigned on a
`burst-by-burst basis.
`
`2 – 1 1 G H z — The 2–11 GHz bands, both
`licensed and license-exempt, are addressed in
`IEEE Project 802.16a. The standard is in bal-
`lot but is not yet complete. The draft current-
`ly specifies that compliant systems implement
`one of three air interface specifications, each
`of which provides for interoperability. Design
`of the 2–11 GHz physical layer is driven by
`the need for non-line-of-sight (NLOS) opera-
`tion. Because residential applications are
`expected, rooftops may be too low for a clear
`sight line to a BS antenna, possibly due to
`obstruction by trees. Therefore, significant
`multipath propagation must be expected. Fur-
`thermore, outdoor-mounted antennas are
`expensive due to both hardware and installa-
`tion costs.
`The three 2–11 GHz air interface specifica-
`tions in 802.16a Draft 3 are:
`• WirelessMAN-SC2: This uses a single-carri-
`er modulation format.
`• WirelessMAN-OFDM: This uses orthogonal
`frequency-division multiplexing with a 256-
`point transform. Access is by TDMA. This
`air interface is mandatory for license-
`exempt bands.
`
`The PHY
`specification
`defined for
`10–66 GHz uses
`burst
`single-carrier
`modulation with
`adaptive burst
`profiling in which
`transmission
`parameters,
`including the
`modulation and
`codling schemes,
`my be adjusted
`individually to
`each subscriber
`station on a
`frame-by-frame
`basis. Both TDD
`and burst FDD
`variants are
`defined.
`
`100
`
`IEEE Communications Magazine • June 2002
`
`5
`
`

`

`SS transition
`gap
`
`Tx/Rx transition
`gap (TDD)
`
`Initial
`maintenance
`opportunities
`(UIUC = 2)
`
`Request
`contention
`opps
`(UIUC = 1)
`
`SS 1
`scheduled
`data
`(UIUC = i)
`
`SS N
`scheduled
`data
`(UIUC = j)
`
`Access
`burst
`
`Collision
`
`Access
`burst
`
`Bandwidth
`request
`
`Collision
`
`Bandwidth
`request
`
`■ Figure 2. The uplink subframe structure.
`
`and downlink is controlled by the BS and is
`implicitly communicated in the uplink map (UL-
`MAP) and downlink map (DL-MAP).
`The system uses a frame of 0.5, 1, or 2 ms.
`This frame is divided into physical slots for the
`purpose of bandwidth allocation and identifica-
`tion of PHY transitions. A physical slot is
`defined to be 4 QAM symbols. In the TDD vari-
`ant of the PHY, the uplink subframe follows the
`downlink subframe on the same carrier frequen-
`cy. In the FDD variant, the uplink and downlink
`subframes are coincident in time but are carried
`on separate frequencies. The downlink subframe
`is shown in Fig. 1.
`The downlink subframe starts with a frame
`control section that contains the DL-MAP for
`the current downlink frame as well as the UL-
`MAP for a specified time in the future. The
`downlink map specifies when physical layer tran-
`sitions (modulation and FEC changes) occur
`within the downlink subframe. The downlink
`subframe typically contains a TDM portion
`immediately following the frame control section.
`Downlink data are transmitted to each SS using
`a negotiated burst profile. The data are transmit-
`ted in order of decreasing robustness to allow
`SSs to receive their data before being presented
`with a burst profile that could cause them to lose
`synchronization with the downlink.
`In FDD systems, the TDM portion may be fol-
`lowed by a TDMA segment that includes an extra
`preamble at the start of each new burst profile.
`This feature allows better support of half-duplex
`SSs. In an efficiently scheduled FDD system with
`many half-duplex SSs, some may need to transmit
`earlier in the frame than they receive. Due to
`their half-duplex nature, these SSs lose synchro-
`nization with the downlink. The TDMA preamble
`allows them to regain synchronization.
`Due to the dynamics of bandwidth demand
`for the variety of services that may be active, the
`mixture and duration of burst profiles and the
`presence or absence of a TDMA portion vary
`dynamically from frame to frame. Since the
`recipient SS is implicitly indicated in the MAC
`
`P MAC PDU which has started
`in previous TC PDU
`
`First MAC PDU,
`this TC PDU
`
`Second MAC PDU,
`this TC PDU
`
`Transmission convergence sublayer PDU
`
`■ Figure 3. TC PDU format.
`
`headers rather than in the DL-MAP, SSs listen
`to all portions of the downlink subframe they are
`capable of receiving. For full-duplex SSs, this
`means receiving all burst profiles of equal or
`greater robustness than they have negotiated
`with the BS.
`A typical uplink subframe for the 10–66 GHz
`PHY is shown in Fig. 2. Unlike the downlink,
`the UL-MAP grants bandwidth to specific SSs.
`The SSs transmit in their assigned allocation
`using the burst profile specified by the Uplink
`Interval Usage Code (UIUC) in the UL-MAP
`entry granting them bandwidth. The uplink sub-
`frame may also contain contention-based alloca-
`tions for initial system access and broadcast or
`multicast bandwidth requests. The access oppor-
`tunities for initial system access are sized to
`allow extra guard time for SSs that have not
`resolved the transmit time advance necessary to
`offset the round-trip delay to the BS.
`Between the PHY and MAC is a transmis-
`sion convergence (TC) sublayer. This layer per-
`forms the transformation of variable length
`MAC protocol data units (PDUs) into the fixed
`length FEC blocks (plus possibly a shortened
`block at the end) of each burst. The TC layer
`has a PDU sized to fit in the FEC block current-
`ly being filled. It starts with a pointer indicating
`where the next MAC PDU header starts within
`the FEC block. This is shown in Fig. 3.
`The TC PDU format allows resynchroniza-
`tion to the next MAC PDU in the event that the
`previous FEC block had irrecoverable errors.
`
`IEEE Communications Magazine • June 2002
`
`101
`
`6
`
`

`

`referenced with 16-bit connection identifiers
`(CIDs) and may require continuously granted
`bandwidth or bandwidth on demand. As will be
`described, both are accommodated.
`Each SS has a standard 48-bit MAC address,
`but this serves mainly as an equipment identifi-
`er, since the primary addresses used during
`operation are the CIDs. Upon entering the
`network, the SS is assigned three management
`connections in each direction. These three con-
`nections reflect the three different QoS
`requirements used by different management
`levels. The first of these is the basic connec-
`tion, which is used for the transfer of short,
`time-critical MAC and radio link control
`(RLC) messages. The primary management
`connection is used to transfer longer, more
`delay-tolerant messages such as those used for
`authentication and connection setup. The sec-
`ondary management connection is used for the
`transfer of standards-based management mes-
`sages such as Dynamic Host Configuration
`Protocol (DHCP), Trivial File Transfer Proto-
`col (TFTP), and Simple Network Management
`Protocol (SNMP). In addition to these man-
`agement connections, SSs are allocated trans-
`port connections for the contracted services.
`Transport connections are unidirectional to
`facilitate different uplink and downlink QoS
`and traffic parameters; they are typically
`assigned to services in pairs.
`The MAC reserves additional connections for
`other purposes. One connection is reserved for
`contention-based initial access. Another is
`reserved for broadcast transmissions in the
`downlink as well as for signaling broadcast con-
`tention-based polling of SS bandwidth needs.
`Additional connections are reserved for multi-
`cast, rather than broadcast, contention-based
`polling. SSs may be instructed to join multicast
`polling groups associated with these multicast
`polling connections.
`
`MAC PDU Formats — The MAC PDU is the
`data unit exchanged between the MAC layers of
`the BS and its SSs. A MAC PDU consists of a
`fixed-length MAC header, a variable-length pay-
`load, and an optional cyclic redundancy check
`(CRC). Two header formats, distinguished by
`the HT field, are defined: the generic header
`(Fig. 4) and the bandwidth request header.
`Except for bandwidth request MAC PDUs,
`which contain no payload, MAC PDUs contain
`either MAC management messages or conver-
`gence sublayer data.
`Three types of MAC subheader may be pre-
`sent. The grant management subheader is used
`by an SS to convey bandwidth management
`needs to its BS. The fragmentation subheader
`contains information that indicates the presence
`and orientation in the payload of any fragments
`of SDUs. The packing subheader is used to indi-
`cate the packing of multiple SDUs into a single
`PDU. The grant management and fragmentation
`subheaders may be inserted in MAC PDUs
`immediately following the generic header if so
`indicated by the Type field. The packing sub-
`header may be inserted before each MAC SDU
`if so indicated by the Type field. More details
`are provided below.
`
`LEN
`msb (3)
`
`Rsv (1)
`
`EKS
`(2)
`
`CI (1)
`
`Rsv (1)
`
`Type (6)
`
`EC (1)
`
`HT = 0 (1)
`
`LEN lsb (8)
`
`CID msb (8)
`
`CID Isb (8)
`
`HCS (8)
`
`■ Figure 4. Format of generic header for MAC PDU.
`
`Without the TC layer, a receiving SS or BS
`would potentially lose the entire remainder of a
`burst when an irrecoverable bit error occurred.
`
`MEDIUM ACCESS CONTROL DETAILS
`The MAC includes service-specific convergence
`sublayers that interface to higher layers, above
`the core MAC common part sublayer that car-
`ries out the key MAC functions. Below the com-
`mon part sublayer is the privacy sublayer.
`SERVICE-SPECIFIC CONVERGENCE SUBLAYERS
`IEEE Standard 802.16 defines two general ser-
`vice-specific convergence sublayers for map-
`ping services to and from 802.16 MAC
`connections. The ATM convergence sublayer
`is defined for ATM services, and the packet
`convergence sublayer is defined for mapping
`packet services such as IPv4, IPv6, Ethernet,
`and virtual local area network (VLAN). The
`primary task of the sublayer is to classify ser-
`vice data units (SDUs) to the proper MAC
`connection, preserve or enable QoS, and
`enable bandwidth allocation. The mapping
`takes various forms depending on the type of
`service. In addition to these basic functions,
`the convergence sublayers can also perform
`more sophisticated functions such as payload
`header suppression and reconstruction to
`enhance airlink efficiency.
`COMMON PART SUBLAYER
`Introduction and General Architecture — In
`general, the 802.16 MAC is designed to support
`a point-to-multipoint architecture with a central
`BS handling multiple independent sectors simul-
`taneously. On the downlink, data to SSs are mul-
`tiplexed in TDM fashion. The uplink is shared
`between SSs in TDMA fashion.
`The 802.16 MAC is connection-oriented. All
`services, including inherently connectionless ser-
`vices, are mapped to a connection. This provides
`a mechanism for requesting bandwidth, associat-
`ing QoS and traffic parameters, transporting and
`routing data to the appropriate convergence sub-
`layer, and all other actions associated with the
`contractual terms of the service. Connections are
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`

`Transmission of MAC PDUs — The IEEE
`802.16 MAC supports various higher-layer pro-
`tocols such as ATM or IP. Incoming MAC SDUs
`from corresponding convergence sublayers are
`formatted according to the MAC PDU format,
`possibly with fragmentation and/or packing,
`before being conveyed over one or more connec-
`tions in accordance with the MAC protocol.
`After traversing the airlink, MAC PDUs are
`reconstructed back into the original MAC SDUs
`so that the format modifications performed by
`the MAC layer protocol are transparent to the
`receiving entity.
`IEEE 802.16 takes advantage of incorporat-
`ing the packing and fragmentation processes
`with the bandwidth allocation process to maxi-
`mize the flexibility, efficiency, and effectiveness
`of both. Fragmentation is the process in which a
`MAC SDU is divided into one or more MAC
`SDU fragments. Packing is the process in which
`multiple MAC SDUs are packed into a single
`MAC PDU payload. Both processes may be ini-
`tiated by either a BS for a downlink connection
`or an SS for an uplink connection.
`IEEE 802.16 allows simultaneous fragmenta-
`tion and packing for efficient use of the band-
`width.
`
`PHY Support and Frame Structure — The
`IEEE 802.16 MAC supports both TDD and
`FDD. In FDD, both continuous and burst down-
`links are supported. Continuous downlinks allow
`for certain robustness enhancement techniques,
`such as interleaving. Burst downlinks (either
`FDD or TDD) allow the use of more advanced
`robustness and capacity enhancement tech-
`niques, such as subscriber-level adaptive burst
`profiling and advanced antenna systems.
`The MAC builds the downlink subframe start-
`ing with a frame control section containing the
`DL-MAP and UL-MAP messages. These indicate
`PHY transitions on the downlink as well as band-
`width allocations and burst profiles on the uplink.
`The DL-MAP is always applicable to the cur-
`rent frame and is always at least two FEC blocks
`long. The first PHY transition is expressed in the
`first FEC block, to allow adequate processing
`time. In both TDD and FDD systems, the UL-
`MAP provides allocations starting no later than
`the next downlink frame. The UL-MAP can,
`however, allocate starting in the current frame as
`long as processing times and round-trip delays
`are observed. The minimum time between
`receipt and applicability of the UL-MAP for an
`FDD system is shown in Fig. 5.
`
`Radio Link Control — The advanced technolo-
`gy of the 802.16 PHY requires equally advanced
`radio link control (RLC), particularly the capa-
`bility of the PHY to transition from one burst
`profile to another. The RLC must control this
`capability as well as the traditional RLC func-
`tions of power control and ranging.
`RLC begins with periodic BS broadcast of
`the burst profiles that have been chosen for the
`uplink and downlink. The particular burst pro-
`files used on a channel are chosen based on a
`number of factors, such as rain region and equip-
`ment capabilities. Burst profiles for the downlink
`are each tagged with a Downlink Interval Usage
`
`Frame n – 1
`DL-MAP n – 1
`UL-MAP n
`
`Frame n
`DL-MAP n
`UL-MAP n + 1
`
`Frame n + 1
`DL-MAP n + 1
`UL-MAP n + 2
`
`Frame
`control
`Downlink
`subframe
`
`Uplink
`subframe
`
`Round-trip delay + Tproc
`
`■ Figure 5. Minimum FDD map relevance.
`
`Code (DIUC). Those for the uplink are each
`tagged with an Uplink Interval Usage Code
`(UIUC).
`During initial access, the SS performs initial
`power leveling and ranging using ranging request
`(RNG-REQ) messages transmitted in initial
`maintenance windows. The adjustments to the
`SS’s transmit time advance, as well as power
`adjustments, are returned to the SS in ranging
`response (RNG-RSP) messages. For ongoing
`ranging and power adjustments, the BS may
`transmit unsolicited RNG-RSP messages com-
`manding the SS to adjust its power or timing.
`During initial ranging, the SS also requests to
`be served in the downlink via a particular burst
`profile by transmitting its choice of DIUC to the
`BS. The choice is based on received downlink
`signal quality measurements performed by the
`SS before and during initial ranging. The BS
`may confirm or reject the choice in the ranging
`response. Similarly, the BS monitors the quality
`of the uplink signal it receives from the SS. The
`BS commands the SS to use a particular uplink
`burst profile simply by including the appropriate
`burst profile UIUC with the SS’s grants in UL-
`MAP messages.
`After initial determination of uplink and
`downlink burst profiles between the BS and a
`particular SS, RLC continues to monitor and
`control the burst profiles. Harsher environmen-
`tal conditions, such as rain fades, can force the
`SS to request a more robust burst profile. Alter-
`natively, exceptionally good weather may allow
`an SS to temporarily operate with a more effi-
`cient burst profile. The RLC continues to adapt
`the SS’s current UL and DL burst profiles, ever
`striving to achieve a balance between robustness
`and efficiency. Because the BS is in control and
`directly monitors the uplink signal quality, the
`protocol for changing the uplink burst profile for
`an SS is simple: the BS merely specifies the pro-
`file’s associated UIUC whenever granting the SS
`bandwidth in a frame. This eliminates the need
`for an acknowledgment, since the SS will always
`receive either both the UIUC and the grant or
`neither. Hence, no chance of uplink burst profile
`mismatch between the BS and SS exists.
`In the downlink, the SS is the entity that
`monitors the quality of the receive signal and
`therefore knows when its downlink burst profile
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`

`BS
`
`SS
`
`DL data at DIUC n
`
`RNG-REQ or DBPC-REQ
`change to DIUC k
`
`Send DL data
`at DIUC k
`
`C/(N + I)
`too low
`for DIUC n
`
`Yes
`
`Continue
`monitoring
`DL data
`through
`DIUC n
`
`DL data at DIUC k
`RNG-RSP or DBPC-RSP
`
`No
`
`Monitor DL
`data
`only throu