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WiMAX Basics
WiMAX or 802.16 is an effort by the IEEE to develop astandards based air interfaces for the licensed andunlicensed radio frequencies from 2 to 66 GHz
The approach they have taken is to develop a commonMAC – Media Access Control sub layer of the data linklayer
Then to offer differing physical layers to accommodatethe needs of the different frequencies and regulatoryenvironments
The IEEE believes that the existing approaches todelivering wireless data services do have the potentialfor long term growth when used outside of the localarea network
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IEEE 802.16 Overview
Family of standards for wireless metropolitanarea networks (WMAN)
Provide broadband (i.e., voice, data, video)connectivity
Specifies the air interface, including themedium access control (MAC) layer andmultiple physical layer specifications
802.16e is an amendment to 802.16d (fixed ornomadic wireless broadband) to supportmobility
Vehicular speeds up to 75 mph
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802.16 Evolution
Extension for 2-11 GHz
Non-line-of-sight
Point-to-Multi-Point applications
Original fixed wireless broadband air Interfacefor 10 – 66 GHz
Line-of-sight only
Point-to-Multi-Point applications
Revised and replaced previous versions
WiMAX System Profiles
MAC/Physical layer enhancements to supportsubscribers moving at vehicular speeds
Dec 2001
802.16a Jan 2003
802.16d Oct 2004
802.16e Dec 2005
[2]
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Characteristics of 802.16 Frequency Ranges
10 - 66 GHz
Short wavelength
Line-of-sight (LOS) required
Negligible multipath
The commonly used frequencies in this range are10.5, 25, 26, 31, 38, and 39 GHz
2 – 11 GHz
Longer wavelength
LOS not required
Improved range and in-building penetration
Multipath effects may be significant
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Why do we need broadband wireless access?
Fill the gap between high data rate wirelessLAN and very mobile cellular networks.
Wireless alternative to cable and DSL for last-mile broadband access
Developing countries
Rural areas
Provide high-speed mobile data andtelecommunications services
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802.11 v 802.
►
802.11’s media access control protocol is optimized for shorter-range topologies
►
It also was not designed to serve a large number of users
►
Wireless MAN, on the other hand, was designed to solve theproblems of delivering wireless broadband networks over longerdistances and through more difficult environments, such asheavily wooded areas
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Network Architecture
Source: WiMAX Nuts and Bolts – Steve Hilton [3]
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Physical Layer
Five physical layer modes
802.16d
802.16e
Designation
Applicability
WirelessMAN-SC
10 -66 GHz
WirelessMAN-SCa
Below 11GHzLicensed bands
WirelessMAN-OFDM
Below 11GHzLicensed bands
WirelessMAN-OFDMA
Below 11GHzLicensed bands
WirelessHUMAN
Below 11GHzLicensed-exempt bands
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Conventional FDM and OFDM
Source: Broadband Wireless Access (W-PAN, W-LAN, WiMAX, Wi-Mob) (includingOFDM concepts) - A. K. Seth [4]
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Orthogonal Frequency Division Multiple Access
(OFDMA)
Multiple-access/multiplexing scheme
a multiple-access/multiplexing scheme that providesmultiplexing operation of data streams from multipleusers onto the downlink sub-channels and uplinkmultiple access by means of uplink sub-channels.
Dynamically assign a subset of subchannels toindividual users
WirelessMAN-OFDMA based on scalableOFDMA (SOFDMA)
Support scalable channel bandwidths from 1.25 to20 MHz
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TDD
The 802.16e PHY supports TDD and Full and Half-Duplex FDD operation;
To counter interference issues, TDD does requiresystem-wide synchronization;
TDD is the preferred duplexing mode for the followingreasons: ►
TDD enables adjustment of the downlink/uplink ratio toefficiently support asymmetric downlink/uplink traffic,
►
Unlike FDD, which requires a pair of channels, TDD onlyrequires a single channel for both downlink and uplinkproviding greater flexibility
►
Transceiver designs for TDD implementations are less complexand therefore less expensive.
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MAC Layer
Connection-oriented
A fundamental premise of the MAC architectureis quality of service (QoS)
QoS provided via service flows
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MAC Layer
Service Specific Convergence
Sublayer
MAC Common Part Sublayer
Privacy Sublayer
Interface to higher layerprotocols, classifiesincoming data, etc.Core MAC functions (i.e.,scheduling, connectionmaintenance,fragmentation),QoS controlEncryption, authentication,secure key exchange
