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IEEE 802.16

WiMAX

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Introduction WiMAX stands for Worldwide Interoperability for

Microwave Access.

It is a telecommunications technology that provides

.

Considered as a Metropolitan Area Network (MAN).

Providing mobile broadband connectivity across citiesand a wireless alternative to wired broadband access

such as aDSL and cable.

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IEEE 802.16

WiMAX is standardized under the reference IEEE802.16.

Fixed WiMAX is standardized in 2004 under the. .

Mobile WiMAX is standardized in 2005 under thereference IEEE 802.16e. It is an amendment to802.16d.

WiMAX release 2 is currently under research and willbe standardized under the reference IEEE 802.16m.

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Physical Layer

Fixed WiMAX uses Orthogonal Frequency DivisionMultiplexing (SOFDMA)

Mobile WiMAX uses Scalable Orthogonal Frequency

Division Multiplexing SOFDMA.

256 sub-carriers are used.

The maximum range of operation is 50 km (31 miles).

The typical values of channel bandwidth are 1.25 MHz,

5 MHz, 10 MHz or 20 MHz).

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Scalable Frequency Division Multiplexing

Scaling of the Fast Fourier transform (FFT) to thechannel bandwidth.

s eeps e carr er spac ng cons an across erenchannel bandwidths.

Constant carrier spacing results in a higher spectrumefficiency in wide channels, and a cost reduction innarrow channels.

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Spectrum

WiMAX operates in the 2 to 11 GHz range of the

spectrum.

WiMAX Forum has published three licensed spectrum

profiles: 2.3 GHz, 2.5 GHz and 3.5 GHz.

2.5 GHz USA

2.3 GHz Asia 3.5 GHz Jordan

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Fixed vs. Mobile WiMAX IEEE 802.16e advances IEEE 802.16d by :

Mobility.

.

Advanced antenna diversity schemes.

MIMO technology.

Denser sub-channelization.

Turbo Coding.

Low-Density Parity Check.

Extra QoS class.

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Medium Access Control

(MAC)

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MAC Layer The MAC layer of WiMAX is divided into three distinct

components:

The service-specific convergence sublayer (CS).

The common-part sublayer.

The security sublayer.

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MAC Layer Some of the important functions of the MAC layer in

WiMAX are : Segment or concatenate the service data units

(SDUs) received from higher layers into the MAC.

Select the appropriate burst profile and power level tobe used for transmission.

Retransmission of MAC PDUs that were received

erroneously by the receiver. Provide QoS control and priority handling of MAC

PDUs belonging to different services.

Provide security and key management Provide power-saving mode and idle-mode operation

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Service-Specific Convergence Sublayer

The CS, which is the interface between the MAC layer andlayer 3 of the network, receives data packets from thehigher layer.

These higher-layer packets are also known as MAC

service data units (SDU).

The CS is responsible for performing all operations thatare dependent on the nature of the higher-layer protocol,

such as header compression.

The CS can be viewed as an adaptation layer that masksthe higher-layer protocol and its requirements from therest of the MAC and PHY layers of a WiMAX network.

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Common-Part & Security Sublayers

The common-part sublayer of the MAC layer performs allthe packet operations that are independent of the higherlayers, such as

Fragmentation and concatenation of SDUs into MACPDUs.

Transmission of MAC PDUs.

QoS control.

ARQ.

The security sublayer is responsible for Encryption.

Authorization.

Proper exchange of encryption keys between the BSand the MS.

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Packet Header Suppression

One of the key tasks of the CS is to perform packet headersuppression (PHS).

At the transmitter, this involves removing the repetitive part of theheader of each SDU.

For exam le, if the SDUs delivered to the CS are IP ackets,

the source and destination IP addresses contained in theheader of each IP packet do not change from one packet tothe next and thus can be removed before being transmittedover the air.

At the receiver, the repetitive part of the header can be reinsertedinto the SDU before being delivered to the higher layers.

The PHS protocol establishes and maintains the required degree

of synchronization between the CSs at the transmitter and thereceiver.

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MAC PDU

Each MAC PDU consists of a header followed by a payloadand a cyclic redundancy check (CRC).

The CRC is based on IEEE 802.3 and is calculated on theentire MAC PDU; the header and the payload.

as wo ypes o s : The generic MAC PDU , used for carrying data and MAC-

layer signaling messages. A generic MAC PDU starts witha generic header and followed by a payload and a CRC.

The bandwidth request PDU , used by the MS to indicateto the BS that more bandwidth is required in the UL, dueto pending data transmission. A bandwidth request PDU

consists only of a bandwidth-request header, with nopayload or CRC.

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Generic MAC Header Fields

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Bandwidth Request MAC Header Fields

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Additional Header Fields

WiMAX also defines five subheaders that can be used in ageneric MAC PDU :

1. Mesh subheader. Follows generic header when meshnetworking is used.

2. Fragmentation subheader. Follows the generic MACheader and indicates that the SDU is fragmented overmultiple MAC PDUs.

3. Packing subheader. Indicates that multiple SDUs or SDUfragments are packed into a single MAC PDU and areplaced at the beginning of each SDU or SDU fragment.

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4. Fast-feedback allocation subheader. Indicates that thePDU contains feedback from the MS about the DL

channel state information. This subheader provides thefunctionality for channel state information feedback forMIMO and non-MIMO implementations.

5. Grant-management subheader. Used by the MS, conveysvarious messages related to bandwidth management,such as polling request and additional-bandwidth request.Using this subheader is more efficient than thebandwidth-request PDU for additional bandwidth duringan ongoing session, since it is more compact and doesnot require the transmission of a new PDU. The

bandwidth-request PDU is generally used for the initialbandwidth request.

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Bandwidth Request & Allocation

In the downlink, all decisions related to the allocation of

bandwidth to various MSs are made by the BS on a per CIDbasis, which does not require the involvement of the MS.

As MAC PDUs arrive for each CID, the BS schedules themfor the PHY resources, based on their QoS requirements.

Once dedicated PHY resources have been allocated for thetransmission of the MAC PDU, the BS indicates thisallocation to the MS, using the DL-MAP message.

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In the uplink, the MS requests resources by either using astand-alone bandwidth-request MAC PDU or piggybackingbandwidth requests on a generic MAC PDU.

Since the burst profile associated with a CID can changedynamically, all resource requests are made in terms ofbytes of information, rather than PHY-layer resources, such

as number of subchannels and/or number of OFDMsymbols.

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Bandwidth requests in the UL can be incremental oraggregate requests.

When it receives an incremental bandwidth request for apart cu ar , t e a s t e quant ty o an w trequested to its current perception of the bandwidth need.

Similarly, when it receives an aggregate bandwidth requestfor a particular CID, the BS replaces its perception of thebandwidth needs of the connection with the amount ofbandwidth requested.

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The Type field in the bandwidth request header indicateswhether the request is incremental or aggregate.

Bandwidth requested by piggybacking on a MAC PDU canbe only incremental.

When multiple CIDs are associated with a particular MS, theBS-allocated UL aggregate resources for the MS rather thanindividual CIDs.

When the resource granted by the BS is less than theaggregate resources requested by the MS, the UL schedulerat the MS determines that allocation and distribution of thegranted resource among the various CIDs, based on the

amount of pending traffic and their QoS requirements.

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Polling Mechanism

In WiMAX, polling refers to the process whereby dedicatedor shared UL resources are provided to the MS to makebandwidth requests.

These allocations can be for an individual MS or a group ofMSs.

When an MS is polled individually, the polling is calledunicast, and the dedicated resources in the UL are allocatedfor the MS to send a bandwidth-request PDU.

The BS indicates to the MS the UL allocations for unicastpolling opportunities by the UL MAP message of the DLsubframe.

Since the resources are allocated on a per MS basis, the UL

MAP uses the primary CID of the MS to indicate theallocation.

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Polling Mechanism

The primary CID is allocated to the MS during the network

entry and initialization stage and is used to transport allMAC-level signaling messages.

An MS can also d namicall re uest additional CIDs, known

as secondary CIDs, which it can use only for transportingdata.

MSs that have an active unsolicited grant service (UGS)

connection are not polled, since the bandwidth request canbe sent on the UGS allocation either in the form of abandwidth request PDU or by piggybacking on generic MACPDUs.

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Polling Mechanism

If the MS does not have additional bandwidth requirements,it sends a dummy MAC PDU during the unicast poll, and theType field of the header indicates that it is a dummy MACPDU.

poll.

If sufficient bandwidth is not available to poll each MSindividually, multicast or broadcast polling is used to poll a

group of users or all the users at a time.

All MSs belonging to the polled group can request bandwidthduring the multicast/broadcast polling opportunity.

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Polling Mechanism

In order to reduce the likelihood of collision, only MSs withbandwidth requirements respond.

WiMAX uses a truncated binary exponential backoff algorithmfor contention-resolution during a multicast/broadcast poll.

When it needs to send a bandwidth request over amulticast/broadcast poll, the MS first enters a contentionresolution phase, if selecting a uniformly distributed randomnumber between 0 and BACKOFF WINDOW.

This random value indicates the number of transmissionopportunities—allocated resources for multicast/broadcastpoll—the MS will wait before sending its bandwidth request.

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Polling Mechanism

BACKOFF WINDOW is the maximum number of transmissionopportunities an MS can wait before sending the pending

bandwidth request.

If it does not receive a bandwidth allocation based on the ULMAP messa e within a time window, the MS assumes that its

bandwidth request message was lost, owing to collision withanother MS, in which case MS increases is backoff window bya factor of 2—as long as it is less than a maximum backoffwindow—and repeats the process.

If bandwidth is still not allocated after a maximum number ofretries, the MAC PDU is discarded.

The maximum number of retries for the bandwidth request is

a tunable parameter and can be adjusted by either the serviceprovider or the equipment manufacturer, as needed.

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Quality of Service

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Quality of Service (QoS)

One of the key functions of the WiMAX MAC layer is toensure that QoS requirements for MAC PDUs belonging to

different service flows are met as reliably as possible giventhe loading conditions of the system.

that are tied to the overall QoS, such as latency, jitter, datarate, packet error rate, and system availability must be metfor each connection.

The WiMAX MAC layer uses a scheduling service to deliverand handle SDUs and MAC PDUs with different QoSrequirements.

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Scheduling Services

A scheduling service uniquely determines the mechanismthe network uses to allocate UL and DL transmission

opportunities for the PDUs.

WiMAX defines five scheduling services.

1. The unsolicited grant service (UGS) is designed tosupport real-time service flows that generate fixed-sizedata packets on a periodic basis, such as T1/E1 andVoIP. UGS offers fixed-size grants on a real-time

periodic basis and does not need the SS to explicitlyrequest bandwidth, thus eliminating the overhead andlatency associated with bandwidth request.

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Scheduling Services

2. The real-time polling services (rtPS) is designed to

support real-time services that generate variable-sizedata packets on a periodic basis, such as MPEG(Motion Pictures Experts Group) video. In this service

,

the MS to request bandwidth. The unicast pollingopportunities are frequent enough to ensure that latencyrequirements of real-time services are met. This servicerequires more request overhead than UGS does but ismore efficient for service that generates variable-sizedata packets or has a duty cycle less than 100 percent.

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Scheduling Services

3. The non-real-time polling services (nrtPS) is very similar to

rtPS except that the MS can also use contention-basedpolling in the uplink to request bandwidth. In nrtPS, it isallowable to have unicast polling opportunities, but the

order of few seconds, which is large compared to rtPS. Allthe MSs belonging to the group can also requestresources during the contention-based polling opportunity,which can often result in collisions and additionalattempts.

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Scheduling Services

4. The best-effort service (BE) provides very little QoS

support and is applicable only for services that do nothave strict QoS requirements. Data is sent wheneverresources are available and not required by any other

- .

contention-based polling opportunity to request bandwidth.

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Scheduling Services

5. The extended real-time polling service (ertPS), a new

scheduling service introduced with the IEEE 802.16estandard, builds on the efficiencies of UGS and rtPS. Inthis case, periodic UL allocations provided for a particular

requesting additional bandwidth. This features allowsertPS to accommodate data services whose bandwidthrequirements change with time. In the case of UGS, unlikeertPS, the MS is allowed to request additional bandwidthduring the UL allocation for only non-UGS-relatedconnections.

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THE END

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