WIMAX

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Outline

• I. Introduction
• II. 802.16 Physical Layer
• III. 802.16 MAC Layer
• IV. QoS Support in IEEE 802.16
• V. Handover Procedure

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I. Introduction
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• The development of high-performance backbone
  networks was immediately followed by the rapid
  dissemination of broadband wired access
  technologies.
• Such as leased lines based on fiber-optic links,
  cable modems using coaxial systems, and digital
  subscriber line (xDSL) access networks.

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• Many new services are based on multimedia
  applications
• voice over IP (VoIP)
• video conferencing
• video on demand (VoD)
• massive online gaming
• peer-to-peer.

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• Unlike traditional TCP/IP services, multimedia
  applications usually require strict network
  guarantees
• reserved bandwidth
• bounded delays

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Broadband wireless Access

• The International Telecommunication Union (ITU),
  which reported that Broadband Wireless Access
  (BWA), although still in the early stage of its
  growth, is one of the most promising solutions
  for broadband access.

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• Standards for BWA are being developed within IEEE
  802.16.
• To promote 802.16-compliant technologies, the
  Worldwide Interoperability for Microwave Access
  (WiMAX) Forum was founded, with more than 300
  member companies.

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• It is envisaged that the first 802.16-compliant
  products to be deployed will very likely be aimed
  at providing last-mile Internet access for
  residential users mainly high-speed Internet
  access and small and medium-sized enterprises
  (SMEs).

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• For the SME market, 802.16 will provide a
  cost-effective alternative to existing solutions
  based on very expensive leased-line services.

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802.16 family
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Deployment

• 802.16 systems can be deployed effectively where
• Users are dissatisfied with the current packet
  and/or network interface.
• Network operators need to reach customers
  cost-effectively.
• New service offerings are available for 2.5G/3G
  augmentation.

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Salient Advantages

• Some of the more salient advantages of a wireless
  system for broadband access based on the 802.16
  standard are as follows
• Bandwidth on demand (BOD)
• Higher throughput
• Scalable system capacity
• Coverage
• Quality (CBR or UBR)
• Cost (investment risk and end-user fee)

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II. 802.16 Physical Layer
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• Physical Layer
• The physical layer of the IEEE 802.16 air
  interface operates
• 1066 GHz band for IEEE 802.16 (LOS)
• 211 GHz band for IEEE 802.16a (NLOS)
• The physical layer supports data rates in the
  range of 32130 Mb/s depending on the
  transmission bandwidth (e.g., 20, 25, or 28 MHz)
  as well as the modulation and coding schemes
  used.

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• In the 1066 GHz, the air interface used for this
  band is Wireless-SC (single carrier).
• In the 211 GHz band three different air
  interfaces can be used as follows
• WirelessMAN-SCa for single-carrier modulation
• WirelessMAN-OFDM. The MAC scheme for the
  subscriber stations is TDMA.
• WirelessMAN-OFDMA for OFDM-based transmission
  using2048 subcarriers.

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• To enhance data transmission rate, an adaptive
  modulation and coding (AMC) technique is
  supported in the IEEE 802.16 standard.

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• Since the quality of the wireless link between a
  BS and a subscriber station depends on the
  channel fading and interference conditions,
  through AMC the radio transceiver is able to
  adjust the transmission rate according to the
  channel quality (i.e., signal-to-noise ratio
  SNR at the receiver).
• Reed-Solomon (RS) code concatenated with an inner
  convolution code is used for error correction.

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III. 802.16 MAC Layer
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• Medium Access Control Layer
• The 802.16 standard specifies two modes for
  sharing the wireless medium point-to-multipoint
  (PMP) and mesh (optional).

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• IEEE 802.16/WiMAX uses a connection-oriented MAC
  protocol that provides a mechanism for the
  subscriber stations to request bandwidth from the
  BS.
• A 16-bit connection identifier (CID) is used
  primarily to identify each connection to the BS.

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• 802.16 MAC layer
• Sublayer
• Service Specific Convergence Sublayer (CS)
• MAC-Common Part Sublayer (CPS)
• Privacy Sublayer (PS)

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• On the downlink, the BS broadcasts data to all
  subscriber stations in its coverage area. Each
  subscriber station processes only the MAC
  protocol data units (PDUs) containing its own CID
  and discards the other PDUs.

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• For TDD-based access, a MAC frame (i.e.,
  transmission period) is divided into uplink and
  downlink subframes.
• The lengths of these subframes are determined
  dynamically by the BS and broadcast to the
  subscriber stations through downlink and uplink
  MAP messages (DL-MAP and UL-MAP) at the beginning
  of each frame.
• The MAC protocol in the standard supports dynamic
  bandwidth allocation.

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• PMP mode
• The BS serves a set of SSs within the same
  antenna sector in a broadcast manner.

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• Mesh Operation Mode
• In addition to the single-hop PMP operation
  scenario, the WiMAX standard (e.g., IEEE 802.16a)
  also defines the multihop mesh networking
  scenario among the subscriber stations (i.e.,
  client meshing).
• Meshing among the BSs (i.e., infrastructure
  meshing) has not been standardized yet.

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QoS Support in IEEE 802.16

• The mesh mode traffic can be routed through
  other SSs and can occur directly among SSs.

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• Task Group 802.16j established by the IEEE 802.16
  mobile multihop relay (MMR) study group is
  working on the standardization of relay-based
  infrastructure meshing.

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IV. QoS Support in IEEE 802.16
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• QoS in wireless networks is usually managed at
  the medium access control (MAC) layer.

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• In PMP mode, uplink (from SS to BS) and downlink
  (from BS to SS) data transmissions occur in
  separate time frames.
• In the downlink subframe, the BS transmits a
  burst of MAC protocol data units (PDUs).
• Since the transmission is broadcast, all SSs
  listen to the data transmitted by the BS.
• In the uplink subframe, any SS transmits a burst
  of MAC PDUs to the BS in a time-division multiple
  access (TDMA) manner.

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• Based on measurements at the physical layer, any
  SS adapts over time the interval usage code (IUC)
  in use.
• That is, modulation, rate, and forward error
  correction (FEC) scheme, for both downlink
  (downlink IUC, DIUC) and uplink (uplink IUC,
  UIUC) transmissions.

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Duplex

• Downlink and uplink subframes are duplexed using
  one of the following techniques
• Frequency-division duplex (FDD)
• Time-division duplex (TDD)

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Frame structure with FDD and TDD
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• The MAC protocol is connection-oriented all data
  communications, for both transport and control,
  are in the context of a unidirectional
  connection.
• At the start of each frame, the BS schedules the
  uplink and downlink grants in order to meet the
  QoS requirements.

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• Each SS learns the boundaries of its allocation
  within the current uplink subframe by decoding
  the UL-MAP message.

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• The DL-MAP message contains the timetable of the
  downlink grants in the forthcoming downlink
  subframe. Downlink grants directed to SSs with
  the same DIUC are advertised by the DL-MAP as a
  single burst.
• Both maps are transmitted by the BS at the
  beginning of each downlink subframe for both FDD
  and TDD modes.

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QoS functions within the BS and SSs
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• Since the BS controls the access to the medium in
  the uplink direction, bandwidth is granted to SSs
  on demand.

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• Bandwidth-request mechanisms
• Unsolicited granting
• Unicast poll
• Broadcast polls

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• Unsolicited granting
• A fixed amount of bandwidth on a periodic basis
  is requested during the setup phase of an uplink
  connection. After that phase, bandwidth is never
  explicitly requested.

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• Unicast poll
• A unicast poll consists of allocating to a polled
  uplink connection the bandwidth needed to
  transmit a bandwidth request.
• If the polled connection has no data awaiting
  transmission (backlog, for short), or if it has
  already requested bandwidth for all of its
  backlog, it will not reply to the unicast poll,
  which is thus wasted.

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• Broadcast polls
• A collision occurs whenever two or more uplink
  connections send a bandwidth request by
  responding to the same poll, in which case a
  binary exponential backoff algorithm is employed.

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• Bandwidth requests can be piggybacked on a PDU.
• Bandwidth requests are used on the BS for
  estimating the residual backlog of uplink
  connections.

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• Based on the amount of bandwidth requested (and
  granted) so far, the BS uplink scheduler
  estimates the residual backlog at each uplink
  connection and allocates future uplink grants
  according to the respective set of QoS parameters
  and the virtual status of the queues.

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• Although bandwidth requests are per connection,
  the BS nevertheless grants uplink capacity to
  each SS as a whole.
• When an SS receives an uplink grant, it cannot
  deduce from the grant which of connections it
  was intended for by the BS.

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• An SS scheduler must also be implemented within
  each SS MAC, in order to redistribute the granted
  capacity to all of its own connections.

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• The 802.16 MAC specifies four different
  scheduling services in order to meet the QoS
  requirements of multimedia applications UGS,
  rtPS, nrtPS, and BE.

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• Unsolicited Grant Service (UGS)
• UGS is designed to support real-time applications
  (with strict delay requirements) that generate
  fixed-size data packets at periodic intervals
• Ex T1/E1 and VoIP without silence suppression.

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• The guaranteed service is defined so as to
  closely follow the packet arrival pattern, with
  the base period equal to the unsolicited grant
  interval and the offset upper bounded by the
  tolerated jitter.
• The grant size is computed by the BS based on the
  minimum reserved traffic rate.

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• real-time Polling Service (rtPS)
• rtPS is designed to support real-time
  applications (with less stringent delay
  requirements) that generate variable-size data
  packets at periodic intervals
• Ex MPEG video and VoIP with silence suppression.

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• The key QoS parameters for rtPS connections are
  the minimum reserved traffic rate and the maximum
  latency.
• The BS periodically grants unicast polls to rtPS
  connections.

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QoS Support in IEEE 802.16

• non-real-time Polling Service (rtPS) and Best
  Effort (BE)
• nrtPS and BE are designed for applications that
  do not have any specific delay requirement.
• The main difference between the two is that nrtPS
  connections are reserved a minimum amount of
  bandwidth, which can boost performance of
  bandwidth-intensive applications, Ex FTP.

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QoS Support in IEEE 802.16

• nrtPS and BE uplink connections request bandwidth
  by either responding to broadcast polls from the
  BS or piggybacking a bandwidth request on an
  outgoing PDU.
• Ex
• For nrtPS FTP
• For best-effort HTTP, SMTP

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V. Handover Procedure
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• The handover process may be used in the following
  examples
• When the MS moves and needs to change the BS to
  which it is connected in order to provide a
  higher signal quality.
• When the MS can be serviced with higher QoS at
  another BS.

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• Network topology advertisement
• A BS shall broadcast information about the
  network topology using the MOB_NBR-ADV message.
• The message provides channel information for
  neighboring base station normally provided by
  each BSs own DCD/UCD message transmissions.

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• MS scanning of neighbors BSs
• An MS may request an allocation of a group of
  scanning intervals with interleaving intervals of
  normal operation using the MOB_SCN-REQ message.
• Scanning interval and interleaving interval
  repeat with the number of Scan iteration.
• The MS indicates in this message the estimated
  duration of time it requires for the scan.

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• In the MOB_SCN-REQ message the MS shall indicate
  group of neighbor BSs of which only Scanning or
  Scanning with Association are requested by MS
  (recommended by BS).
• Upon reception of the MOB_SCN-REQ message, the BS
  shall respond with a MOB_SCN-RSP message.

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• The MOB_SCN-RSP message shall either grant the
  requesting MS a scanning interval that is at
  least as long as requested by that MS, or deny
  the request.
• Following reception of a MOB_SCN-RSP message
  granting the request, MS may scan for one or more
  MS during the time interval allocated in the
  message.

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• When a BS is identified through scanning, the MS
  may attempt to synchronize with its downlink
  transmissions, and estimate the quality of the
  PHY channel.
• The serving BS may buffer incoming data addressed
  to the MS during the scanning interval and
  transmit that data after the scanning interval
  during any interleaving interval of after exit of
  the Scanning mode.

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• Association procedure
• Association Level 0
• Scan/Association without coordination
• The serving BS and the MS negotiate about the
  association duration and intervals.
• Association Level 1
• Association with coordination
• The Serving BS provides association parameters to
  the MS and coordinates association between the MS
  and neighboring BSs.

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• Association Level 2
• Network assisted association reporting
• The Serving BS will then coordinate the
  association procedure with the requested
  neighboring BSs in a fashion similar to
  association Level 1.
• The MS is required only to transmit the CDMA
  ranging code at the neighbor BS. Then The MS does
  not have to wait for RNG-RSP from the neighbor
  BS.
• The RNG-RSP information on PHY offsets will be
  sent by each neighbor BS to the Serving BS. The
  Serving BS may aggregate all ranging related
  information into a single MOB_ASC_REPORT message.

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• Handover Process
• Cell reselection
• Handover Decision and Initiation
• Synchronization to Target BS downlink.
• Ranging
• Termination of MS Context.

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• Cell reselection
• MS may use Neighbor BS information acquired from
  a decoded MOB_NBR-ADV message, or may make a
  request to schedule scanning intervals to scan.
• Neighbor BS for the purpose of evaluating MS
  interest in handover to potential target BS.

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Cell selection with ranging
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• HO Decision and Initiation
• The decision may originate either at the MS or
  the serving BS.
• The HO Decision consummates with a notification
  of MS intent to handover through MOB_MSHO-REQ or
  MOB_MSHO-REQ.

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• Synchronization to Target BS downlink
• MS shall synchronize to downlink transmissions of
  Target BS and obtain DL and UL transmission
  parameters.
• If MS had previously received a MOB_NBR-ADV
  message including
• Target BSID
• Physical Frequency
• DCD/UCD
• this process may be shortened.

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• Ranging
• MS and target BS shall conduct Initial Ranging or
  Handover Ranging.
• If MS RNG-REQ includes serving BSID, then target
  BS may make a request to serving BS for
  information on the MS over the backbone network
  and serving BS may respond.
• Network re-entry proceeds may be shortened by
  target BS possession of MS information obtained
  from serving BS over the backbone network.

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• Termination of MS Context
• The final step in handover.
• Termination of MS Context is defined as serving
  BS termination of context of all connections
  belonging to the MS and context associated with
  them.
• Information of queues
• ARQ state-machine
• Counters
• Timers
• Header suppression information

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Handover and initial network entry
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Thank you!