Wireless Networks

Lecture 42
IEEE 802.16
Dr. Ghalib A. Shah

1


WiMAX Basics
 WiMAX or 802.16 is an effort by the IEEE to develop a
standards based air interfaces for the licensed and
unlicensed radio frequencies from 2 to 66 GHz
 The approach they have taken is to develop a common
MAC – Media Access Control sub layer of the data link
layer
 Then to offer differing physical layers to accommodate
the needs of the different frequencies and regulatory
environments
 The IEEE believes that the existing approaches to
delivering wireless data services do have the potential
for long term growth when used outside of the local
area network
2
2


IEEE 802.16 Overview
 Family of standards for wireless metropolitan
area networks (WMAN)
 Provide broadband (i.e., voice, data, video)

connectivity
 Specifies the air interface, including the
medium access control (MAC) layer and
multiple physical layer specifications
 802.16e is an amendment to 802.16d (fixed or
nomadic wireless broadband) to support
mobility
► Vehicular speeds up to 75 mph
3


WiMAX Forum
 Formed in J une 2001 to promote conformance
and interoperability of the 802.16 standard
 Develops “system profiles” that define
mandatory and optional features of standard
IEEE®802.16e Mobile
Broadband Wireless
Amendment

Mandatory and
Optional Features

Mobile WiMAX
System Profile
Release-1

IEEE®802.16-2004
Fixed Broadband
Wireless Standard


[1]

4


802.16 Evo lutio n

802.16
Dec 2001

• Original fixed wireless broadband air Interface 
for 10 – 66 GHz
•  Line­of­sight only
•  Point­to­Multi­Point applications

802.16a
Jan 2003

• Extension for 2­11 GHz
•  Non­line­of­sight
•  Point­to­Multi­Point applications

802.16d
Oct 2004

• Revised and replaced previous versions
• WiMAX System Profiles

802.16e

Dec 2005
[2]

• MAC/Physical layer enhancements to support 
subscribers moving at vehicular speeds

5


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 are
10.5, 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
6


IEEE 802.16 Standard
802.16

802.16d/Hipe rMAN

802.16e

Co mple te d

De c e mbe r 2001

June  2004 (802.16d)

Es timate  2005

S pe c trum

10 ­ 66 GHz

< 11 GHz

< 6 GHz

Channe l 
Co nditio ns


Line  o f S ig ht Only

No n Line  o f S ig ht

No n Line  o f S ig ht

Bit Rate

32 – 134 Mbps  in 
28MHz c hanne l 
bandwidth

Up to  75 Mbps  in 20MHz  Up to  15 Mbps  in 
c hanne l bandwidth
5MHz c hanne l 
bandwidth

Mo dulatio n

QPS K, 16QAM and 
64QAM

OFDM 256 FFT
QPS K, 16QAM, 64QAM

S c alable  OFDMA
128 to  2048 FFT

Mo bility


Fixe d

Fixe d

Po rtable

Channe l 
Bandwidth
s

20, 25 and 28 MHz 

1.75 to  20 MHz

1.75 to  20 MHz

7


Why do we need broadband wireless access?
 Fill the gap between high data rate wireless
LAN and very mobile cellular networks.
 Wireless alternative to cable and DSL for lastmile broadband access
► Developing countries
► Rural areas

 Provide high-speed mobile data and
telecommunications services


8


802.11 v 802.16
► 802.11’s media access control protocol is optimized for shorterrange topologies
► It also was not designed to serve a large number of users
► Wireless MAN, on the other hand, was designed to solve the
problems of delivering wireless broadband networks over longer
distances and through more difficult environments, such as
heavily wooded areas

9

9


Comparison 802.11 and 802.16
Technology 802.11

802.16

Range

< 300 feet

< 30 Mile ( typical 3~4)

Coverage

Optimized for 

indoor short range
2.7 bps/Hz peak. 
 <= 54Mbps in 20MHz 

Outdoor LOS & NLOS

Data rate

5bps/Hz peak, 
<100Mbps in 20 MHz

Scalability 1­10 CPE CSMA/CA

1­ hundreds CPE TDMA

QOS

On demand BW  voice
Video, data10

No QOS


Network Architecture

Source: WiMAX Nuts and Bolts – Steve Hilton [3]

11



Physical Layer


Five physical layer modes

802.16d

802.16e

Designation

Applicability

WirelessMAN-SC

10 -66 GHz

WirelessMAN-SCa

Below 11GHz
Licensed bands

WirelessMAN-OFDM

Below 11GHz
Licensed bands

WirelessMAN-OFDMA

Below 11GHz

Licensed bands

WirelessHUMAN

Below 11GHz
Licensed-exempt bands
12


Orthogonal Frequency Division Multiplexing
(OFDM)
 Multiplexing technique that divides the channel
into multiple orthogonal sub channels
 Input data stream is divided into several
substreams of a lower data rate (increased
symbol duration) and each substream is
modulated and simultaneously transmitted on a
separate sub channel
 High spectral efficiency, resilient to
interference, and low multi-path distortion

13


Conventional FDM and OFDM

Source: Broadband Wireless Access (W-PAN, W-LAN, WiMAX,
14 Wi-Mob) (including
OFDM concepts) - A. K. Seth [4]



Orthogonal Frequency Division Multiple Access
(OFDMA)
 Multiple-access/multiplexing scheme
► a multiple-access/multiplexing scheme that provides
multiplexing operation of data streams from multiple
users onto the downlink sub-channels and uplink
multiple access by means of uplink sub-channels.
► Dynamically assign a subset of subchannels to
individual users

 WirelessMAN-OFDMA based on scalable
OFDMA (SOFDMA)
► Support scalable channel bandwidths from 1.25 to
20 MHz
15


Other Physical Layer Features
 Hybrid automatic repeat request (HARQ)
► Adjusts automatically to channel conditions
► Receiver saves failed transmission attempts to help future
decoding
•

Every transmission helps increase probability of success

 Multiple-in Multiple-out (MIMO)
► Multiple antennas on sender and receivers
► Takes advantage of multi-path

► Increased spectral efficiency

16


TDD
 The 802.16e PHY supports TDD and Full and HalfDuplex FDD operation;
 To counter interference issues, TDD does require
system-wide synchronization;
 TDD is the preferred duplexing mode for the following
reasons:
► TDD enables adjustment of the downlink/uplink ratio to
efficiently support asymmetric downlink/uplink traffic,
► Unlike FDD, which requires a pair of channels, TDD only
requires a single channel for both downlink and uplink
providing greater flexibility
► Transceiver designs for TDD implementations are less
complex and therefore less expensive.
17


MAC Layer
 Connection-oriented
 A fundamental premise of the MAC architecture
is quality of service (QoS)
 QoS provided via service flows

18



19


MAC Layer

Service Specific Convergence
Sublayer

Interface to higher layer
protocols, classifies
incoming data, etc.

MAC Common Part Sublayer

Core MAC functions (i.e.,
scheduling, connection
maintenance,fragmentation),
QoS control

Privacy Sublayer
Encryption, authentication,
secure key exchange
20