WiMAX Bisic Theories

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WiMAX Basic Theories

Date Author Remarks

2008.5 WiMAX P&O Team Chines Version

2009.8 WiMAX P&O Team English Version


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Constants

Section1 Introduction to WiMAX ................................................................................................................5

1.1 Wireless Introduction ...........................................................................................................................5

1.1.1 Wireless Network Topologies.....................................................................................................5

1.1.2 Wireless Technologies................................................................................................................6

1.1.3 Kinds of Wireless Networks.......................................................................................................6

1.1.4 Wireless Broadband Access (WBA)...........................................................................................7

1.2 Related Organization............................................................................................................................7

1.2.1 IEEE ...........................................................................................................................................7

1.2.2 WiMAX Forum..........................................................................................................................7

1.3 What is WiMAX...................................................................................................................................8

1.3.1 WiMAX is:.................................................................................................................................8

1.3.2 What is 802.16d..........................................................................................................................9

1.3.3 What is 802.16e..........................................................................................................................9

1.3.4 WiMax Speed and Range .........................................................................................................10

1.3.5 Why WiMAX...........................................................................................................................11

1.3.6 WiMAX Goals..........................................................................................................................11

1.4 Salient Features of WiMAX...............................................................................................................12

1.4.1 OFDM-based physical layer.....................................................................................................12

1.4.2 Very High Peak Date Rate........................................................................................................12

1.4.3 Scalable bandwidth and rate support........................................................................................12

1.4.4 Adaptive modulation and coding (AMC).................................................................................12

1.4.5 Link-layer retransmissions .......................................................................................................13

1.4.6 Support for TDD and FDD.......................................................................................................13

1.4.7 Orthogonal frequency division multiple access (OFDMA)......................................................13

1.4.8 Flexible and dynamic per user resource allocation ..................................................................13

1.4.9 Support for advanced antenna techniques ................................................................................14

1.4.10 Qulity of service support ........................................................................................................14

1.4.11 Robust security.......................................................................................................................14

1.4.12 Support for mobility...............................................................................................................14


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1.4.13 IP-based architecture ............................................................................................................. 14

Section2 OFDM........................................................................................................................................... 16

2.1 OFDM System Description ............................................................................................................... 16

2.2 OFDM Orthogonality ........................................................................................................................ 18

2.3 How to Overcome Inter Symbol Interference (ISI) ........................................................................... 18

2.4 How to Overcome Inter Carrier Interference (ICI)............................................................................ 19

Section3 OFDMA........................................................................................................................................ 20

3.1 OFDMA Basics.................................................................................................................................. 20

3.2 OFDMA SymbolStructure and Sub-Channelization......................................................................... 21

3.3 Scalable OFDMA .............................................................................................................................. 23

3.4 TDD Frame Structure ........................................................................................................................ 24

3.5 Other Advanced PHY Layer Features................................................................................................ 25

3.6 Difference between OFDMA and OFDM.......................................................................................... 27

Section4 WiMAX MAC .............................................................................................................................. 29

4.1 Common MAC Concepts .................................................................................................................. 29

4.1.1 CS Sublayer ............................................................................................................................. 29

4.1.2 MAC CPS Sublayer................................................................................................................. 30

4.2 Quality of Service (QoS) Support...................................................................................................... 35

4.3 MAC Scheduling Service .................................................................................................................. 37

4.4 Mobility Management ....................................................................................................................... 39

4.4.1 Power Management ................................................................................................................. 39

4.4.2 Handoff.................................................................................................................................... 39

4.5 Security.............................................................................................................................................. 41

Section5 WiMAX Advanced Features ....................................................................................................... 42

5.1 Smart Antenna Technologies ............................................................................................................. 42

5.2 Fractional Frequency Reuse .............................................................................................................. 44

5.3 Multicast and Broadcast Service (MBS) ........................................................................................... 46

Section6 WiMAX Network Architecture................................................................................................... 48

Section7 WiMAX Channel Estimation...................................................................................................... 51

7.1 Introduction ....................................................................................................................................... 51


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7.2 Channel Estimation ............................................................................................................................52

7.2.1 Transmitter ...............................................................................................................................52

7.2.2 Channel ....................................................................................................................................53

7.2.3 Reciever....................................................................................................................................56

Section8 WiMAX Major Benefits...............................................................................................................59


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Section1 Introduction to WiMAX

Knowledge

What is WiMAX ------------------------------------------------------------------Level 1 2

WiMAX Benefits -----------------------------------------------------------------Level 1 2

Salient Features -------------------------------------------------------------------Level 1 2

1.1 Wireless Introduction

Wireless means transmitting signals using radio waves as the medium instead of wires.

Wireless technologies are used for tasks as simple as switching off the television or as

complex as supplying the sales force with information from an automated enterprise

application while in the field. Now cordless keyboards and mouse PDAs, pagers and digital

and cellular phones have become part of our daily life.

Some of the inherent characteristics of wireless communications systems which make it

attractive for users are given below.

Mobility: A wireless communications system allows users to access information beyondtheir desk and conduct business from anywhere without a cable connectivity.

Reachability: Wireless communications systems enable people to be better connected

and reachable without any limitation of any location.

Simplicity: Wireless communication system is easy and fast to deploy in comparision of

cabled network. Initial setup cost could be a bit high but other advantages overcome that high

cost.

Maintainability: Being a wireless system, you do no need to spend too much to maintain

a wireless network setup.

Roaming Services: Using a wireless network system you can provide service any where

any time including train, busses, airoplans etc.

New Services: Wireless communications systems provide new smart services like SMS

and MMS.

1.1.1 Wireless Network Topologies

There are basically three ways to setup a wireless network.

Point-to-point bridge: As you know a bridge is used to connect two networks. A


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point-to-point bridge interconnects two buildings having different networks. For example, a

wireless LAN bridge can interface with an Ethernet network directly to a particular access

point.

Point-to-multipoint bridge: This topology is used to connect three or more LANs that

may be located on different floors in a building or across buildings.

Mesh or ad hoc network: This network is an independent local area network that is not

connected to a wired infrastructure and in which all stations are connected directly to one

another.

1.1.2 Wireless Technologies

Wireless technologies can be classified in different ways depending on their range. Each

wireless technology is designed to serve a specific usage segment. The requirements for each

usage segment are based on a variety of variables, including Bandwidth needs, Distance

needs and Power.

1.1.3 Kinds of Wireless Networks

Wireless Wide Area Network (WWAN):

This network enables you to access the Internet via a wireless wide area network

(WWAN) access card and a PDA or laptop.

These networks provide a very fast data speed compared with the data rates of mobile

telecommunications technology, and their range is also extensive. Cellular and mobile

networks based on CDMA and GSM are good examples of WWAN.

Wireless Personal Area Network (WPAN):

These networks are very similar to WWAN except thier range is very limited.

Wireless Local Area Network (WLAN): This network enables you to access the Internetin localized hotspots via a wireless local area network (WLAN) access card and a PDA or

laptop.

It is a type of local area network that uses high-frequency radio waves rather than wires

to communicate between nodes.


telecommunications technology, and their range is very limited. Wi-Fi is the most widespread

and popular example of WLAN technology.


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Wireless Metropolitan Area Network (WMAN):

This network enables you to access the Internet and multimedia streaming services via a

wireless region area network (WRAN).


telecommunication technology as well as other wireless network, and their range is also

extensive.

1.1.4 Wireless Broadband Access (WBA)

Broadband wireless is a technology that promises high-speed connection over the air. It

uses radio waves to transmit and receive data directly to and from the potential users

whenever they want it. Technologies including 3G, Wi-Fi, WiMAX and UWB work together

to meet unique customer needs.

BWA is a point-to-multipoint system which is made up of base station and subscriber

equipment. Instead of using the physical connection between the base station and the

subscriber, the base station uses an outdoor antenna to send and receive high-speed data and

voice-to-subscriber equipment.

BWA offers an effective, complementary solution to wireline broadband, which has

become globally recognized by a high percentage of the population.

1.2 Related Organization

1.2.1 IEEE

IEEE802.16 is a broadband radio MAN technology intended to provide a fixed

broad radio access system with an efficient, applicable, and interoperable access

means. Its protocol is focused on the contents on the MAC layer and physical layer.

IEEE 802.16e defines the physical layer and MAC layer of the air interface for

the radio broadband access system that supports mobility, and at the same time,

includes the definition of PKMv2 encryption.

1.2.2 WiMAX Forum

WiMAX Forum (WMF) is a nonprofit production group founded on April 9, 2001 by

equipment and device suppliers that adopt the 802.16 standard, with an intention to coordinate

world-wide broadband radio technologies and promote development of the WiMAX industry

chain.


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With increasing concern for the WiMAX technologies in this industry, WiMAX Forum has

more and more members, and has set up in succession Certification Work Group (CWG),

Technology Work Group (TWG), Regulatory Work Group (RWG), Market Work Group (MWG),

Service Provider Work Group (SPWG), Network Work Group (NWG), and Application Work

Group (AWG). Accordingly, this organization is extending its objectives gradually. Apart from

certification, it is devoted to requirement analysis, application scenario exploration, and WiMAX

network architecture research with regard to the operable broadband radio access system, thus

promoting powerfully the development of the broadband radio access technologies and market.

WiMAX has become an alias of compliance with the 802.16 specification system.

1.3 What is WiMAX

WiMAX is one of the hottest broadband wireless technologies around today. WiMAX

systems are expected to deliver broadband access services to residential and enterprise customers

in an economical way.

Loosely, WiMax is a standardized wireless version of Ethernet intended primarily as an

alternative to wire technologies ( such as Cable Modems, DSL and T1/E1 links ) to provide

broadband access to customer premises.

More strictly, WiMAX is an industry trade organization formed by leading communications

component and equipment companies to promote and certify compatibility and interoperability of

broadband wireless access equipment that conforms to the IEEE 802.16 and ETSI HIPERMAN

standards.

WiMAX would operate similar to WiFi but at higher speeds, over greater distances and for a

greater number of users. WiMAX has the ability to provide service even in areas that are difficult

for wired infrastructure to reach and the ability to overcome the physical limitations of traditional

wired infrastructure.

WiMAX was formed in April 2001, in anticipation of the publication of the original 10-66

GHz IEEE 802.16 specifications. WiMAX is to 802.16 as the Wi-Fi Alliance is to 802.11.

1.3.1 What is WiMAX

Acronym for Worldwide Interoperability for Microwave Access

Based on Wireless MAN technology.


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A wireless technology optimized for the delivery of IP centric services over a wide

area.

A scaleable wireless platform for constructing alternative and complementary

broadband networks.

A certification that denotes interoperability of equipment built to the IEEE 802.16 or

compatible standard. The IEEE 802.16 Working Group develops standards that address

two types of usage models:

A fixed usage model (IEEE 802.16-2004).

A portable usage model (IEEE 802.16e).

1.3.2 What is 802.16d

This is targeted to provide a broadband internet connection to indoor users. The SS

operating on this standard use indoor antenna and a limited mobility (portable devices) is allowed.

802.16d uses orthogonal frequency division multiplexing (OFDM) as its physical layer

specification to enable NLOS communication below 11 GHz. Since OFDM is used, the receiver

is made simple by elimination of bulky equalizer. The other features have nearly been kept

similar in all the physical profiles of the standards Variable FFT size and symbol time is specified,

which could be fixed depending on type of environment and allocated bandwidth.. FEC includes

concatenated RS-CC followed by interleaving. Similar to 802.16a, AAS, STC schemes are

provided but are kept optional.

1.3.3 What is 802.16e

Specifications are provided such that mobility of the SS at 125 KMPH is allowed.

Orthogonal frequency division multiple access (OFDMA) is used as the physical layer

scheme. Channel check codes (LDPC). Data is randomized and interleaved to avoid loss

of carrier recovery and burst errors. In addition to AAS, STC, optional multi input multi

output (MIMO) scheme has been specified. Code division multiple access (CDMA)

codes are used along with the random window length based contention control algorithm

for initial ranging, periodic ranging, bandwidth request and handoff. The inter BS

communications have been defined, which will be used as a backbone network between

the BSs to aid the inter-cell mobile subscriber station (MSS) handoff. This ensures fast

and accurate synchronization at the cost of slightly increased complexity. Similar to


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802.16d, variable FFT size and symbol time is provided which could be set depending

on the environment and allocated bandwidth.

Put together, the 802.16 technology would enable the SS to get broadband wireless

access (BWA) at all times in all locations, either when stationary, or at pedestrian speed

or when traveling at 125 KMPH.

Few of the difference between 802.16d and 802.16e are presented here. In OFDM,

SS uses all the available subcarriers for the allocated time, but in OFDMA, user is

allocated region having definition in both time and frequency. The subcarrier mapping is

different in both the standards, resulting in channel estimation done in 802.16d being

complex, but done less number of times. In 802.16e the channel estimation is simple, butmore frequently done (because data considered, per iteration is less Channel is flat only

over limited subcarriers). Another difference is use of CDMA codes for ranging in

802.16e, the receiver performs correlation to detect the user, and hence more processing

is involved.

1.3.4 WiMax Speed and Range

WiMAX is expected to offer initially up to about 40 Mbps capacity per wireless

channel for both fixed and portable applications, depending on the particular technicalconfiguration chosen, enough to support hundreds of businesses with T-1 speed

connectivity and thousands of residences with DSL speed connectivity. WiMAX can

support voice and video as well as Internet data.

WiMax will be to provide wireless broadband access to buildings, either in

competition to existing wired networks or alone in currently unserved rural or thinly

populated areas. It can also be used to connect WLAN hotspots to the Internet. WiMAX

is also intended to provide broadband connectivity to mobile devices. It would not be as

fast as in these fixed applications, but expectations are for about 15 Mbps capacity in a 3km cell coverage area.

With WiMAX users could really cut free from today.s Internet access arrangements

and be able to go online at broadband speeds, almost wherever they like from within a

MetroZone.

WiMAX could potentially be deployed in a variety of spectrum bands: 2.3GHz,

2.5GHz, 3.5GHz, and 5.8GHz


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1.3.5 Why WiMAX

WiMAX can satisfy a variety of access needs. Potential applications include

extending broadband capabilities to bring them closer to subscribers, filling gaps

in cable, DSL and T1 services, Wi-Fi and cellular backhaul, providing last-100

meter access from fibre to the curb and giving service providers another

cost-effective option for supporting broadband services.

WiMAX can support very high bandwidth solutions where large spectrum

deployments (i.e. >10 MHz) are desired using existing infrastructure keeping

costs down while delivering the bandwidth needed to support a full range of

high-value, multimedia services.

WiMAX can help service providers meet many of the challenges they face due to

increasing customer demands without discarding their existing infrastructure

investments because it has the ability to seamlessly interoperate across various

network types.

WiMAX can provide wide area coverage and quality of service capabilities for

applications ranging from real-time delay-sensitive voice-over-IP (VoIP) to

real-time streaming video and non-real-time downloads, ensuring that subscribers

obtain the performance they expect for all types of communications.

WiMAX, which is an IP-based wireless broadband technology, can be integrated

into both wide-area third-generation (3G) mobile and wireless and wireline

networks, allowing it to become part of a seamless anytime, anywhere broadband

access solution.

Ultimately, WiMAX is intended to serve as the next step in the evolution of 3G

mobile phones, via a potential combination of WiMAX and CDMA standards called

4G.

1.3.6 WiMAX Goals

A standard by itself is not enough to enable mass adoption. WiMAX has stepped

forward to help solve barriers to adoption, such as interoperability and cost of

deployment. WiMAX will help ignite the wireless MAN industry, by defining and

conducting interoperability testing and labeling vendor systems with a "WiMAX

Certified" label once testing has been completed successfully.


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1.4 Salient Features of WiMAX

WiMAX is a wireless broadband solution that offers a rich set of features with a lot of

flexibility in term of development options and potential services offerings. Some of the

more salient features that deserve hightlighting are as follows:

1.4.1 OFDM-based physical layer

WiMAX physical layer (PHY) is based on the orthogonal frequency division multiplexing,

a schem that offers good resistance to multipath, and allows wimax to operate in NOL-Sight

conditions. OFDM is now widely recognized as the method for mitigating mutipath for

broadband wireless.

1.4.2 Very High Peak Date Rate

WiMAX is capable of supporting very high peak data rate. In fact, the peak data rate

can reach 74Mbps when operationg using 20MHz wide spectrum. More typically, using a 10MHz

spectrum operating using TDD schem with a 3:1 downlink-to-uplink ratio, the peak PHY data

rate is 25Mbps and 6.7Mbps for downlink and uplink, respectively. These peak data rate are

achieved when using 64 QAM moduration with rate 5/6 err-correcting coding. Under viry good

signal condition, even higher data rate may be achieved using multiple antennas and spatial

multiplexing.

1.4.3 Scalable bandwidth and rate support

WiMAX has a scalable physical layer architecture that allows for the date rate to scal easily

with available channel bandwidth. This scalability is supported in the OFDMA mode, where the

FFT(fast fourier transform) size may be scaled based on the available bandwidth. For example, a

WiMAX system may use 128-, 512-, 1,024bit FFTs based on whether the channel bandwidth is

1.25MHz, 5MHz, 10MHz, respectively. This scaling may be done dynamically to support user

roaming across different network that may have different bandwidth allocations.

1.4.4 Adaptive modulation and coding (AMC)

WiMAX supports a number of modulation and forward effort correction (FEC) coding

schemes and allows the schemes to change on per user and per frame bisis, based on the

channel conditions. AMC is an effective mechanism to maximize the throughput in a

time-varying channel. The additive algorithm typically calls for the use of the highest

modulation and coding scheme that can be supported by the signal-to-nosie and inteference


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ratio at the receiver such that each user is provided with the highest data rate that can be

suppotted in their respective links.

1.4.5 Link-layer retransmissions

For connections that require henced reliability, WiMAX supports automatic

retransmission requests (ARQ) at the link layer. ARQ-enabled connections require each

transmitted packet to be acknowleged by the receiver; unacknoeleged packets are assumed

to be lost and are retransmitted. WiMAX also optionally support hybrid-ARQ(HARQ),

which is an effective hybrid between ARQ and FEC.

1.4.6 Support for TDD and FDD

IEEE 802.16 2004 and IEEE 802.16 2005 both support time division duplexing and

frequency division duplexing as well as a half duplex FDD, which allows a low-cost system

implementation. TDD is favored by a majority of implementions because of its advantages:

(1) flexibility to choose downlink-to-uplink date rate rato.

(2) ability to exploit channel reciprocity.

(3) ability to implement nonpaired spectrum and less complex transceiver design.

1.4.7 Orthogonal frequency division multiple access (OFDMA)

Mobile WiMAX use OFDM as a multi-acess technique, whereby different users can be

allocated different subsets of the OFDM tones. OFDMA facilitates the exploitation of

frequency diversity and multiuser divisity to significantly improve the system capcity.

1.4.8 Flexible and dynamic per user resource allocation

Both uplink and downlink resource allocation are controlled by a scheduler in the base

station. Capicity is shared among multiple users on a demand basis, using a burst TDM

scheme. When using the OFDMA-PHY mode, multiplexing is additionally done in the

frequency dimension, by allocating different subsets of OFDM subcarriers to different users.

Resources may be allocated in the spatial domain as well when using the optional advanced

antenna system (AAS). The standard allows for bandwidth resources to be allocated in time,

frequency, and space and has a flexible mechanism to convey the resource allocation

information on a frame-by-frame basis.


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1.4.9 Support for advanced antenna techniques

The WiMAX solution has a number of hooks built into the physicl-layer design, which

allows for the use of multiple-antenna techniques, such as beamforming, space-time coding,

and spatial multiplexing. These shemes can be used to improve the overall system capacity

and spectral efficiency by deploying mulitiple anttenas at the transmitter and/or receiver

side.

1.4.10 Qulity of service support

The WiMAX MAC layer has a connection-oriented architechtrue that is design to

support a variety of applications, including voice and multimedia services. The system

offers support for constant bit rate, real-time, and non-real-time time traffic flows, in

addition best-effort data traffic. WiMAX MAC is designed to support a large number of

users, with multiple connections per terminal, each with its own Qos requirement.

1.4.11 Robust security

WiMAX support strong encryption, using Advanced Encryption Starded (AES), and

has a robust privacy and key-management protocol. The system also offers a very flexible

authentication architecture based on Extensible Authentication Protocol (EAP), which

allows for a variety of user credentials, including username/password, digital certificates,

and smart cards.

1.4.12 Support for mobility

The mobile WiMAX variant of the system has mechanisms to support secure seamless

handovers for deley-tolerant full-mobility applications, such as VoIP. The system also has

built-in support for power-saving mechanisms that extend the battery life of handheld

subscriber devices. Physical-layer enhancements, such as more frequent channel estimation,

uplink subchannelization, and power control, are also specified in support of moble

applications.

1.4.13 IP-based architecture

The WiMAX Forum has defined a reference network architecture that is based on an all-IP

platform. All end to end services are delivered over an IP architecture relying on IP-based

protocols for end-to-end transport, Qos, session management, security, and mobility. Reliance on

IP allows WiMAX to ride the declining costcurves of IP processing, facilitate easy convergence


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with other networks, and exploit the rich ecosystem for application development that exsits for

IP.


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Section2 OFDM

Knowledge

OFDM System Basics-------------------------------------------------------------Level 1 2

OFDM Orthogonality -------------------------------------------------------------Level 1 2

Overcome ISI-----------------------------------------------------------------------Level 1 2

Overcome ICI----------------------------------------------------------------------Level 1 2

2.1 OFDM System Description

OFDM = Orthogonal Frequency Division Multiplexing (Figure 3.1).

OFDM converts a high rate broadband signal into many parallel low rate

narrowband signals.

Low rate signals have large symbol periods, which make OFDM signal

resistant to multipath delay spread.

OFDM uses a Fast Fourier Transform (FFT) to allow overlap in frequency

of individual narrowband signals.

More efficient than conventional multi-carrier.

Figure 3.1 OFDM description in both time and frequency division

OFDM is a multi carrier transmission scheme where the information is

transmitted on multiple subcarriers, with a lower data rate, instead of one high


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data rate carrier (Figure 3.2) and moreover, the subcarriers are orthogonal to

each other, leading to saving of bandwidth (Figure 3.3).

Figure 3.2 Three orthogonal subcarriers shown separately (in practice a

sum of 3 is transmitted)

Figure 3.3 Comparing of FDMA and OFDM

The major disadvantage of an OFDM system is its requirement of perfect

synchronization in time and frequency. But the advantages of using OFDM are

far more and provide enough reasons for the popularity of the OFDM systems.

A typical channel fade will degrade only a few of the subcarriers, which in most

cases can be compensated by use of efficient interleaving and channel coding.

OFDM systems can be implemented very efficiently by using the Inverse Fast

Fourier transform (IFFT) at the transmitter and Fast Fourier transform (FFT) at


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the receiver. The overall complexity and its increase with data rate in OFDM

systems is far less than the single carrier systems, hence OFDM is becoming a

widely accepted technology and more prominent to be used in future mobile

wireless communication standards.

2.2 OFDM Orthogonality

For successful operation of OFDM system, it is required that the

subcarriers should never loose orthogonality between each other at any time.

The advantage of an OFDM system is lost when the subcarriers are no longer

orthogonal to each other. This puts forward quite stringent requirements to befulfilled by the transmitter and the receiver.

0dt(2f)tsin2ft2sinT

0

where T = f1

Ideally, to maintain orthogonality we need that the symbol duration be

exactly inverse of the subcarrier spacing and the FFT be considered over

symbol duration such that it covers integer number of cycles. Moreover, the

consecutive subcarriers differ by 1 full cycle only (Figure 3.1). If the system is

to operate in a multipath environment, then each subcarrier should experience a

flat fading, hence the subcarrier spacing should be less than the coherence

bandwidth and each symbol should experience a time-invariant channel, hence

the symbol time should be less than the coherence time else the complexity of

receiver increases when overcoming the fading effect.

2.3 How to Overcome Inter Symbol Interference (ISI)

A guide time is added.

Reduction of inter symbol interference, which would require bulky

equalizer to be constructed at the receiver in a single carrier system, is

overcome by the use of guard time in an OFDM system. A guard time is added

in time domain between two OFDM symbols and the FFT is considered over

duration such that there is no component from the previous or next symbol,

(Figure 3.3) which nulls the ISI and thus avoiding the bulky equalizer. ISI is

completely eliminated when the multipath signal delay is within the guard


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time. When designing an OFDM system proper values are selected depending

on the environment so as to satisfy the above condition.

Figure 3.4 subcarriers and multipath component shown separately, in

practice the signal is a sum of all subcarriers

2.4 How to Overcome Inter Carrier Interference (ICI)

Cyclic prefix is fiiled.

Multi carrier systems have the problem of inter carrier interference (ICI),

which results from loss of orthogonality between the subcarriers. This happenswhen the FFT is considered over duration where the subcarrier is not present

(non-integer number of cycles), which would be the case when multipath is

present and the guard time has amplitude zero. This is reduced by use of cyclic

prefix, where we transmit a copy the last part of the symbol followed by the

symbol itself. This ensures orthogonality over the FFT period in case of delayed

multipath (Figure 3.3 and Figure3.4).

Figure 3.5 Cyclicprefix


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Section3 OFDMA

Knowledge

OFDMA Basics -----------------------------------------------------------------Level 1 2

OFDMA Symbol Stucture-----------------------------------------------------Level 1 2

Scalable OFDMA---------------------------------------------------------------Level 1 2

TDD Frame Structure----------------------------------------------------------Level 1 2

Advanced PHY Features------------------------------------------------------Level 1 2

Difference between OFDMA and OFDM-----------------------------------Level 1 2

3.1 OFDMA Basics

OFDMA = Orthogonal Frequency Division Multiple Access

In Scalable OFDMA, subcarrier spacing is independent of bandwidth

FFT size is scaled with bandwidth

Subchannel size is fixed and independent of bandwidth and other modes of

operation

The number of subchannels scales with FFT size rather than with the

capacity of subchannels

OFDM exploits the frequency diversity of the multipath channel by coding

and interleaving the information across the sub-carriers prior to transmissions.

OFDM modulation can be realized with efficient Inverse Fast Fourier

Transform (IFFT), which enables a large number of sub-carriers (up to 2048)

with low complexity. In an OFDM system, resources are available in the time

domain by means of OFDM symbols and in the frequency domain by means of

sub-carriers. The time and frequency resources can be organized into

sub-channels for allocation to individual users. Orthogonal Frequency Division

Multiple Access (OFDMA) is a multiple-access/multiplexing scheme that

provides multiplexing operation of data streams from multiple users onto the

downlinksub-channels and uplink multiple access by means of uplink

sub-channels (Figure 4.1).


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Figure 4.1 Basic Architeture of OFDMA system

3.2 OFDMA SymbolStructure and Sub-ChannelizationThe OFDMA symbol structure consists of three types of sub-carriers (Figure

4.2).

Data sub-carriers for data transmission

Pilot sub-carriers for estimation and synchronization purposes

Null sub-carriers for no transmission; used for guard bands and DC carriers

Figure 4.2 OFDMA Sub-carrier Structure

Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers

called subchannels.The WiMAX OFDMA PHY supports sub-channelization in both

DL and UL. The minimum frequency-time resource unit of sub-channelization is

one slot, which is equal to 48 data tones (sub-carriers).

There are two types of sub-carrier permutations for sub-channelization;

diversity and contiguous. The diversity permutation draws sub-carriers

pseudo-randomly to form a sub-channel. It provides frequency diversity and


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inter-cell interference averaging. The diversity permutations include DL FUSC

(Fully Used Sub-Carrier), DL PUSC (Partially Used Sub-Carrier) and UL PUSC

and additional optional permutations. With DL PUSC, for each pair of OFDM

symbols, the available or usable sub-carriers are grouped into clusters containing 14

contiguous sub-carriers per symbol, with pilot and data allocations in each cluster in

the even and odd symbols (Figure 4.3).

Figure 4.3 DL Frequency Diverse Sub-Channel

A re-arranging scheme is used to form groups of clusters such that each group is made

up of clusters that are distributed throughout the sub-carrier space. A sub-channel in a group

contains two (2) clusters and is comprised of 48 data sub-carriers and eight (8) pilot

subcarriers. Analogous to the cluster structure for DL, a tile structure is defined for the UL

PUSC (Figure 4.4).

Figure 4.4 Tile Structure for UL PUSC

The available sub-carrier space is split into tiles and six (6) tiles, chosen from across

the entire spectrum by means of a re-arranging/permutation scheme, are grouped together to

form a slot. The slot is comprised of 48 data sub-carriers and 24 pilot sub-carriers in 3

OFDM symbols.

The contiguous permutation groups a block of contiguous sub-carriers to form a

subchannel.The contiguous permutations include DL AMC and UL AMC, and have the

same structure. A bin consists of 9 contiguous sub-carriers in a symbol, with 8 assigned for


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data and one assigned for a pilot. A slot in AMC is defined as a collection of bins of the type

(N x M = 6), where N is the number of contiguous bins and M is the number of contiguous

symbols. Thus the allowed combinations are [(6 bins, 1 symbol), (3 bins, 2 symbols), (2 bins,

3 symbols), (1 bin, 6 symbols)]. AMC permutation enables multi-user diversity by choosing

the sub-channel with the best frequency response.

In general, diversity sub-carrier permutations perform well in mobile applications

while contiguous sub-carrier permutations are well suited for fixed, portable, or low

mobility environments. These options enable the system designer to trade-off mobility for

throughput.

3.3 ScalableOFDMAThe IEEE 802.16e Wireless MAN OFDMA mode is based on the concept of scalable

OFDMA (S-OFDMA). S-OFDMA supports a wide range of bandwidths to flexibly address

the need for various spectrum allocation and usage model requirements. The scalability is

supported by adjusting the FFT size while fixing the sub-carrier frequency spacing at 10.94

kHz. Since the resource unit sub-carrier bandwidth and symbol duration is fixed, the impact

to higher layers is minimal when scaling the bandwidth. The SOFDMA parameters are

listed in Table 1. The system bandwidths for the initial planned profiles being developed by

the WiMAX Forum Technical Working Group for Release-1 are 5 and 10 MHz (Table 4.1).

Table 4.1 OFDMA Scalablity Parameters


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3.4 TDD Frame Structure

The 802.16e PHY supports TDD, FDD, and Half-Duplex FDD operation;

however the initial release of Mobile WiMAX certification profiles will only include

TDD. With ongoing releases, FDD profiles will be considered by the WiMAX Forum

to address specific market opportunities where local spectrum regulatory requirements

either prohibit TDD or are more suitable for FDD deployments.

To counter interference issues, TDD does require system-wide synchronization;

nevertheless, 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, while with FDD, downlink and

uplink always have fixed and generally, equal DL and UL bandwidths.

TDD assures channel reciprocity for better support of link adaptation,

MIMO and other closed loop advanced antenna technologies.

Unlike FDD, which requires a pair of channels, TDD only requires a single

channel for both downlink and uplink providing greater flexibility for

adaptation to varied global spectrum allocations.

Transceiver designs for TDD implementations are less complex and

therefore less expensive.

Figure 4.5 illustrates the OFDM frame structure for a Time Division Duplex

(TDD) implementation. Each frame is divided into DL and UL sub-frames separated

by Transmit/Receive and Receive/Transmit Transition Gaps (TTG and RTG,

respectively) to prevent DL and UL transmission collisions. In a frame, the following

control information is used to ensure optimal system operation:

Preamble: The preamble, used for synchronization, is the first OFDM

symbol of the frame.

Frame Control Head (FCH): The FCH follows the preamble. It

provides the frame configuration information such as MAP message


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length and coding scheme andusable sub-channels.

DL-MAP and UL-MAP: The DL-MAP and UL-MAP provide

sub-channel allocation and other control information for the DL and

UL sub-frames respectively.

UL Ranging: The UL ranging sub-channel is allocated for mobile

stations (MS) to perform closed-loop time, frequency, and power

adjustment as well as bandwidth requests.

UL CQICH: The UL CQICH channel is allocated for the MS to

feedback channelstate information.

UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ

acknowledgement.

Figure 4.5 OFDMA Frame Structure

3.5 Other Advanced PHY Layer Features

Adaptive modulation and coding (AMC), Hybrid Automatic Repeat Request

(HARQ) and Fast Channel Feedback (CQICH) were introduced with Mobile WiMAX

to enhance coverage and capacity for WiMAX in mobile applications.

Support for QPSK, 16QAM and 64QAM are mandatory in the DL with Mobile

WiMAX. In the UL, 64QAM is optional. Both Convolutional Code (CC) and

Convolutional TurboCode (CTC) with variable code rate and repetition coding are

supported. Block Turbo Code and Low Density Parity Check Code (LDPC) are

supported as optional features Table 4.2 summarizes the coding and modulation


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schemes supported in the Mobile WiMAX profile the optional UL codes and

modulation are shown in italics.

Table 4.2 Supported Code and Modulations

The combinations of various modulations and code rates provide a fine resolution

of data rates as shown in Table 3 which shows the data rates for 5 and 10 MHz

channels with PUSC sub-channels. The frame duration is 5 milliseconds. Each frame

has 48 OFDM symbols, with 44 OFDM symbols available for data transmission. The

highlighted values indicate data rates for optional 64QAM in the UL.

Table 4.3Mobile WiMAX PHY Data Rates with PUSC Sub-Channel

The base station scheduler determines the appropriate data rate (or burst profile)

for each burst allocation based on the buffer size, channel propagation conditions at the

receiver, etc. A Channel Quality Indicator (CQI) channel is utilized to provide

channel-state information from the user terminals to the base station scheduler.


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Relevant channel-state information can be fed back by the CQICH including: Physical

CINR, effective CINR, MIMO mode selection and frequency selective sub-channel

selection. With TDD implementations, link adaptation can also take advantage of

channel reciprocity to provide a more accurate measure of the channel condition (such

as sounding). Hybrid Auto Repeat Request (HARQ) is supported by Mobile WiMAX.

HARQ is enabled using N channel Stop and Wait protocol which provides fast

response to packet errors and improves cell edge coverage. Chase Combining and

optionally, Incremental Redundancy are supported to further improve the reliability of

the retransmission. A dedicated ACK channel is also provided in the uplink for HARQ

ACK/NACK signaling. Multi-channel HARQ operation is supported. Multi-channel

stop-and-wait ARQ with a small number of channels is an efficient, simple protocol

that minimizes the memory required for HARQ and stalling [8]. WiMAX provides

signaling to allow fully asynchronous operation. The asynchronous operation allows

variable delay between retransmissions which gives more flexibility to the scheduler at

the cost of additional overhead for each retransmission allocation. HARQ combined

together with CQICH and AMC provides robust link adaptation in mobile

environments at vehicular speeds in excess of 120 km/hr.

3.6 Difference betweenOFDMA and OFDMIEEE 802.16d (fixed service) uses Orthogonal Frequency Division Multiplexing

(OFDM). IEEE 802.16e (mobile) uses Orthogonal Frequency Division Multiple Access

(OFDMA). So, whats the difference between the two, and why is there a difference?

(Figure 4.6)

Figure 4.6 Compirement of OFDM and OFDMA


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OFDM allows only one user on the channel at any given time. To

accommodate multiple users, a strictly OFDM system must employ Time Division

Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple

Access (FDMA) (separate channels). Neither of these techniques is time or

frequency efficient: TDMA is a time hog and FDMA is a bandwidth hog.

OFDMA is a multi-user OFDM that allows multiple access on the same

channel (a channel being a group of evenly spaced subcarriers, as discussed

above). WiMAX uses OFDMA, extended OFDM, to accommodate many users inthe same channel at the same time.


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Section4 WiMAX MAC

Knowledge

Kinds of Qos ----------------------------------------------------------------Level 1 2

MAC Scheduling Service--------------------------------------------------Level 1 2

Mobility Management------------------------------------------------------Level 1 2

Security-----------------------------------------------------------------------Level 1 2

The 802.16 standard was developed from the outset for the delivery of broadband

services including voice, data, and video. The MAC layer is based on the time-proven

DOCSIS standard and can support bursty data traffic with high peak rate demand while

simultaneously supporting streaming video and latency-sensitive voice traffic over the same

channel. The resource allocated to one terminal by the MAC scheduler can vary from a

single time slot to the entire frame, thus providing a very large dynamic range of throughput

to a specific user terminal at any given time. Furthermore, since the resource allocation

information is conveyed in the MAP messages at the beginning of each frame, the scheduler

can effectively change the resource allocation on a frame-by-frame basis to adapt to the

bursty nature of the traffic.

4.1 Common MAC Concepts

The MAC layer consists of Convergence Sublayer, MAC CPS, and Security Sublayer.

4.1.1 CS Sublayer

CS is a transition sublayer, on which the SAP is used to receive data from external

networks, and then transfer or map the data. This operation involves classifying of

external network SDUs, and assignment of an appropriate MAC-layer SFID and CID

to each classification. It also includes the PSH function.

The CS is used to process the objects of upper-layer data packets (core network PDU)

and upper and lower-layer QoS features.

The CS is used to implement the classifier and PHS functions.


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4.1.2 MAC CPS Sublayer

The CPS sublayer receives data from different CSs through the MAC SAP, and

classifies the data to specific MAC connections. Through QoS scheduling, bandwidth

is allocated, and SDUs are formed into PDUs. In each PUD MAC header, the CID field

is used to identify connection.

The formed PDUs are transferred to the PHY layer through the PHY SAP.

4.1.2.1 MAC-Layer PDU Format

A MAC message consists of a MAC header, MAC data, and CRC.

For OFDMA, CRC is a required part.

Figure 5.1 MAC PDU format

The MAC header takes the format as follows:


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Figure 5.2 MAC header format

4.1.2.2 MAC-Layer Management Message

A MAC-layer management message takes the format as follows:

Figure 5.3 MAC management message format


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MAC-layer management messages are listed in the table below. The applications of

MAC messages are detailed in each optimization topic.

Table 5-1 MAC management messages


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4.1.2.3 Composition of Burst and MAC PDU

The MAC-layer messages will be ultimately mapped to the Burst and transferred on the

physical layer, as shown in the figure below.

Burst

MAC Msg 1

MAC PDU 1

MAC Msg n

MAC PDU nPad

MAC HeaderMAC msg payload

(optional)

CRC

(optional)

Figure 5.6 Composition of Burst and MAC PDU

4.2 Quality of Service (QoS) Support

With fast air link, symmetric downlink/uplink capacity, fine resource granularity and a

flexible resource allocation mechanism, Mobile WiMAX can meet QoS requirements for a

wide range of data services and applications.

In the Mobile WiMAX MAC layer, QoS is provided via service flows as illustrated in

Figure 5.7. This is a unidirectional flow of packets that is provided with a particular set of


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QoS parameters. Before providing a certain type of data service, the base station and

user-terminal first establish a unidirectional logical link between the peer MACs called a

connection. The outbound MAC then associates packets traversing the MAC interface into a

service flow to be delivered over the connection. The QoS parameters associated with the

service flow define the transmission ordering and scheduling on the air interface.

The connection-oriented QoS therefore, can provide accurate control over the air

interface. Since the air interface is usually the bottleneck, the connection-oriented QoS can

effectively enable the end-to-end QoS control. The service flow parameters can be

dynamically managed through MAC messages to accommodate the dynamic service

demand. The service flow based QoS mechanism applies to both DL and UL to provide

improved QoS in both directions. Mobile WiMAX supports a wide range of data services

and applications with varied QoS requirements. These are summarized (Table 5.2).

Figure 5.7 Mobile WiMAX QoS Support


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Table 5.2 Mobile WiMAX Application and Quality of service

4.3 MAC Scheduling Service

The Mobile WiMAX MAC scheduling service is designed to efficiently deliver

broadband data services including voice, data, and video over time varying broadband

wireless channel. The MAC scheduling service has the following properties that enable

the broadband data service:

Fast Data Scheduler: The MAC scheduler must efficiently allocate

available resources in response to bursty data traffic and time-varying

channel conditions. Thescheduler is located at each base station to enable

rapid response to traffic requirements and channel conditions. The datapackets are associated to service flows with well defined QoS parameters in

the MAC layer so that the scheduler can correctly determine the packet

transmission ordering over the air interface. The CQICH channel provides

fast channel information feedback to enable the scheduler to choose the

appropriate coding and modulation for each allocation. The adaptive

modulation/coding combined with HARQ provide robust transmission over

the timevarying channel.


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Scheduling for both DL and UL: The scheduling service is provided for

both DL and UL traffic. In order for the MAC scheduler to make an efficient

resource allocation and provide the desired QoS in the UL, the UL must

feedback accurate and timely information as to the traffic conditions and QoS

requirements. Multiple uplink bandwidth request mechanisms, such as

bandwidth request through ranging channel, piggyback request and polling

are designed to support UL bandwidth requests. The UL service flow defines

the feedback mechanism for each uplink connection to ensure predictable UL

scheduler behavior. Furthermore, with orthogonal UL sub-channels, there is

no intra-cell interference. UL scheduling can allocate resource more

efficiently and better enforce QoS.

Dynamic Resource Allocation: The MAC supports frequency-time resource

allocation in both DL and UL on a per-frame basis. The resource allocation is

delivered in MAP messages at the beginning of each frame. Therefore, the

resource allocation can be changed on frame-by-frame in response to traffic

and channel conditions. Additionally, the amount of resource in each

allocation can range from one slot to the entire frame. The fast and fine

granular resource allocation allows superior QoS for data traffic.

QoS Oriented: The MAC scheduler handles data transport on aconnection-byconnection basis. Each connection is associated with a single

data service with a set of QoS parameters that quantify the aspects of its

behavior. With the ability to dynamically allocate resources in both DL and

UL, the scheduler can provide superior QoS for both DL and UL traffic.

Particularly with uplink scheduling the uplink resource is more efficiently

allocated, performance is more predictable, and QoS is better enforced.

Frequency Selective Scheduling: The scheduler can operate on different

types of sub-channels. For frequency-diverse sub-channels such as PUSC

permutation, where sub-carriers in the sub-channels are pseudo-randomly

distributed across the bandwidth, sub-channels are of similar quality.

Frequency-diversity scheduling can support a QoS with fine granularity and

flexible time-frequency resource scheduling. With contiguous permutation

such as AMC permutation, the sub-channels may experience different

attenuation. The frequency-selective scheduling can allocate mobile users to

their corresponding strongest sub-channels. The frequency-selective


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scheduling can enhance system capacity with a moderate increase in CQI

overhead in the UL.

4.4 Mobility Management

Battery life and handoff are two critical issues for mobile applications. Mobile

WiMAX supports Sleep Mode and Idle Mode to enable power-efficient MS operation.

Mobile WiMAX also supports seamless handoff to enable the MS to switch from one

base station to another at vehicular speeds without interrupting the connection.

4.4.1 Power Management

Mobile WiMAX supports two modes for power efficient operation Sleep Mode

and Idle Mode. Sleep Mode is a state in which the MS conducts pre-negotiated periods

of absence from the Serving Base Station air interface. These periods are characterized

by the unavailability of the MS, as observed from the Serving Base Station, to DL or

UL traffic. Sleep Mode is intended to minimize MS power usage and minimize the

usage of the Serving Base Station air interface resources. The Sleep Mode also

provides flexibility for the MS to scan other base stations to collect information to

assist handoff during the Sleep Mode. Idle Mode provides a mechanism for the MS to

become periodically available for DL broadcast traffic messaging without registration

at a specific base station as the MS traverses an air link environment populated by

multiple base stations. Idle Mode benefits the MS by removing the requirement for

handoff and other normal operations and benefits the network and base station by

eliminating air interface and network handoff traffic from essentially inactive MSs

while still providing a simple and timely method (paging) for alerting the MS about

pending DL traffic.

4.4.2 Handoff

There are three handoff methods supported within the 802.16e standard Hard

Handoff (HHO), Fast Base Station Switching (FBSS) and Macro Diversity Handover

(MDHO). Of these, the HHO is mandatory while FBSS and MDHO are two optional

modes. The WiMAX Forum has developed several techniques for optimizing hard


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handoff within the framework of the 802.16e standard. These improvements have been

developed with the goal of keeping Layer 2 handoff delays to less than 50 milliseconds.

When FBSS is supported, the MS and BS maintain a list of BSs that are involved

in FBSS with the MS. This set is called an Active Set. In FBSS, the MS continuously

monitors the base stations in the Active Set. Among the BSs in the Active Set, an

Anchor BS is defined. When operating in FBSS, the MS only communicates with the

Anchor BS for uplink and downlink messages including management and traffic

connections.

Transition from one Anchor BS to another (i.e. BS switching) is performedwithout invocation of explicit HO signaling messages. Anchor update procedures are

enabled by communicating signal strength of the serving BS via the CQI channel. A

FBSS handover begins with a decision by an MS to receive or transmit data from the

Anchor BS that may change within the active set. The MS scans the neighbor BSs and

selects those that are suitable to be included in the active set. The MS reports the

selected BSs and the active set update procedure is performed by the BS and MS. The

MS continuously monitors the signal strength of the BSs that are in the active set and

selects one BS from the set to be the Anchor BS. The MS reports the selected Anchor

BS on CQICH or MS initiated HO request message. An important requirement of

FBSS is that the data is simultaneously transmitted to all members of an active set of

BSs that are able to serve the MS.

For MSs and BSs that support MDHO, the MS and BS maintain an active set of

BSs that are involved in MDHO with the MS. Among the BSs in the active set, an

Anchor BS is defined. The regular mode of operation refers to a particular case of

MDHO with the active set consisting of a single BS. When operating in MDHO, the

MS communicates with all BSs in the active set of uplink and downlink unicast

messages and traffic. A MDHO begins when a MS decides to transmit or receive

unicast messages and traffic from multiple BSs in the same time interval. For downlink

MDHO, two or more BSs provide synchronized transmission of MS downlink data

such that diversity combining is performed at the MS. For uplink MDHO, the

transmission from a MS is received by multiple BSs where selection diversity of the

information received is performed.


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4.5 Security

Mobile WiMAX supports best in class security features by adopting the best

technologies available today. Support exists for mutual device/user authentication,

flexible key management protocol, strong traffic encryption, control and management

plane message protection and security protocol optimizations for fast handovers.The

usage aspects of the security features are:

Key Management Protocol: Privacy and Key Management Protocol Version

2 (PKMv2) is the basis of Mobile WiMAX security as defined in 802.16e.

This protocol manages the MAC security using PKM-REQ/RSP messages.

PKM EAP authentication, Traffic Encryption Control, Handover Key

Exchange and Multicast/Broadcast security messages all are based on this

protocol.

Device/User Authentication: Mobile WiMAX supports Device and User

Authentication using IETF EAP protocol by providing support for credentials

that are SIM-based, USIM-based or Digital Certificate or

UserName/Password-based. Corresponding EAP-SIM, EAP-AKA, EAP-TLS

or EAP-MSCHAPv2 authentication methods are supported through the EAP

protocol. Key deriving methods are the only EAP methods supported.

Traffic Encryption: AES-CCM is the cipher used for protecting all the user

data over the Mobile WiMAX MAC interface. The keys used for driving the

cipher are generated from the EAP authentication. A Traffic Encryption State

machine that has a periodic key (TEK) refresh mechanism enables sustained

transition of keys to further improve protection.

Control Message Protection: Control data is protected using AES based

CMAC, or MD5-based HMAC schemes.

Fast Handover Support: A 3-way Handshake scheme is supported by

Mobile WiMAX to optimize the re-authentication mechanisms for supporting

fast handovers. This mechanism is also useful to prevent any

man-in-the-middle-attacks.


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Section5 WiMAX Advanced Features

Knowledge

AntennaTechnolegies Basics-------------------------------------------------Level 1 2

FFR-------------------------------------------------------------------------------Level 1 2

MBS------------------------------------------------------------------------------Level 1 2

5.1 Smart Antenna Technologies

Smart antenna technologies typically involve complex vector or matrix operations

on signals due to multiple antennas. OFDMA allows smart antenna operations to be

performed on vector-flat sub-carriers. Complex equalizers are not required to

compensate for frequency selective fading. OFDMA therefore, is very well-suited to

support smart antenna technologies. In fact, MIMO-OFDM/OFDMA is envisioned as

the corner-stone for next generation broadband communication systems. Mobile

WiMAX supports a full range of smart antenna technologies to enhance system

performance. The smart antenna technologies supported include:

Beamforming: With beamforming, the system uses multiple-antennas to

transmit weighted signals to improve coverage and capacity of the system

and reduce outage probability.

Space-Time Code (STC): Transmit diversity such as Alamouti code is

supported to provide spatial diversity and reduce fade margin.

Spatial Multiplexing (SM): Spatial multiplexing is supported to take

advantage of higher peak rates and increased throughput. With spatial

multiplexing, multiple streams are transmitted over multiple antennas. If the

receiver also has multiple antennas, it can separate the different streams to

achieve higher throughput compared to single antenna systems. With 2x2

MIMO, SM increases the peak data rate two-fold by transmitting two data

streams. In UL, each user has only one transmit antenna, two users can

transmit collaboratively in the same slot as if two streams are spatially

multiplexed from two antennas of the same user. This is called UL

collaborative SM.


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The supported features in the Mobile WiMAX performance profile are listed in

the following table 6.1.

Table 6.1 Advanced Antenna Options

Mobile WiMAX supports adiptive switching between these options to

maximize the benefit of smart antenna technologies under different channelconditions. For instance, SM improves peak throughput. However, when channel

conditions are poor, the Packet Error Rate (PER) can be high and thus the

coverage area where target PER is met may be limited. STC on the other hand

provides large coverage regardless of the channel condition but does not improve

the peak data rate. Mobile WiMAX supports adaptive switching between multiple

MIMO modes to maximize spectral efficiency with no reduction in coverage area.

Figure 6.1 shows the architecture for supporting the smart antenna features.

The following table provides a summary of the theoretical peak data rates for

various DL/UL ratios assuming a 10 MHz channel bandwidth, 5 ms frame

duration with 44 OFDM data symbols (out of 48 total OFDM symbols) and PUSC

subchannelization. With 2x2 MIMO, the DL user and sector peak data rate are

doubled. The maximum DL peak data rate is 63.36 Mbps when all the data

symbols are dedicated to DL.

With UL collaborative SM, the UL sector peak data rate is doubled while the

user peak data rate is unchanged. The UL user peak data rate and sector peak datarate are 14.11 Mbps and 28.22 Mbps respectively when all the data symbols are

dedicated to UL. By applying different DL/UL ratio, the bandwidth can by

adjusted between DL and UL to accommodate different traffic pattern. It should

be noted that the extreme cases such as all DL and all UL partition are rarely used.

WiMAX profile supports DL/UL ratio ranging from 3:1 to 1:1 to accommodate

different traffic profiles. The resulting peak data rates that will typically be


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encountered are in between the two extreme cases.

Table 6.2 Data Rates for SIMO/MIMO Configurations

(For 10 MHz channel, 5 ms frame, PUSC sub-channel, 44 data OFDMsymbols)

Figure 6.1 Adaptive Switching for Smart Antennas

5.2 Fractional Frequency Reuse

Mobile WiMAX supports frequency reuse of one, i.e. all cells/sectors operate

on the same frequency channel to maximize spectral efficiency. However, due to

heavy cochannel interference (CCI) in frequency reuse one deployment, users at

the cell edge may suffer degradation in connection quality. With Mobile WiMAX,

users operate on subchannels, which only occupy a small fraction of the whole

channel bandwidth; the cell edge interference problem can be easily addressed by

appropriately configuring subchannel usage without resorting to traditional

frequency planning.


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In Mobile WiMAX, the flexible sub-channel reuse is facilitated by

sub-channel segmentation and permutation zone. A segment is a subdivision of the

available OFDMA sub-channels (one segment may include all sub-channels). One

segment is used for deploying a single instance of MAC.

Permutation Zone is a number of contiguous OFDMA symbols in DL or UL

that use the same permutation. The DL or UL sub-frame may contain more than

one permutation zone as shown in the followingfigure 6.2.

Figure 6.2 Multi-Zone Frame Structure

The sub-channel reuse pattern can be configured so that users close to the

base station operate on the zone with all sub-channels available. While for the

edge users, each cell or sector operates on the zone with a fraction of all

sub-channels available. In Figure 6.3, F1, F2, and F3 represent different sets of

sub-channels in the same frequency channel. With this configuration, the full load

frequency reuse one is maintained for center users to maximize spectral efficiency

and fractional frequency reuse is implemented for edge users to assure edge-user

connection quality and throughput. The sub-channel reuse planning can be

dynamically optimized across sectors or cells based on network load and

interference conditions on a frame by frame basis. All the cells and sectors

therefore, can operate on the same frequency channel without the need for

frequency planning.


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Figure 6.3 Fractional Frequency Reuse

5.3 Multicast and Broadcast Service (MBS)Multicast and Broadcast Service (MBS) supported by Mobile WiMAX combines the

best features of DVB-H, MediaFLO and 3GPP E-UTRA and satisfies the following

requirements:

High data rate and coverage using a Single Frequency Network (SFN)

Flexible allocation of radio resources

Low MS power consumption

Support of data-casting in addition to audio and video streams

Low channel switching time

The Mobile WiMAX Release-1 profile defines a toolbox for initial MBS service

delivery. The MBS service can be supported by either constructing a separate MBS zone in

the DL frame along with unicast service (embedded MBS) or the whole frame can be

dedicated to MBS (DL only) for standalone broadcast service. Figure 6.4 shows the DL/UL

zone construction when a mix of unicast and broadcast service are supported. The MBS

zone supports multi-BS MBS mode using Single Frequency Network (SFN) operation and

flexible duration of MBS zones permits scalable assignment of radio resources to MBS

traffic. It may be noted that multiple MBS zones are also feasible. There is one MBS zone

MAP IE descriptor per MBS zone. The MS accesses the DL MAP to initially identify MBS

zones and locations of the associated MBS MAPs in each zone. The MS can then


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subsequently read the MBS MAPs without reference to DL MAP unless synchronization to

MBS MAP is lost. The MBS MAP IE specifies MBS zone PHY configuration and defines

the location of each MBS zone via the OFDMA Symbol Offset parameter. The MBS MAP is

located at the 1st sub-channel of the 1st OFDM symbol of the associated MBS zone. The

multi-BS MBS does not require the MS be registered to any base station. MBS can be

accessed when MS in Idle mode to allow low MS power consumption. The flexibility of

Mobile WiMAX to support integrated MBS and uni-cast services enables a broader range of

applications.

Figure 6.4 Embedded MBS Support with Mobile WiMAX MBS Zones


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Section6 WiMAX Network Architecture

Knowledge

Network Architecture -------------------------------------------------------------Level 1 2

6.1 WiMAX Network Architecture

The IEEE 802.16e-2005 standard provides the air interface for WiMAX but does

not define the full end-to-end WiMAX network. The WiMAX Forum's NetworkWorking Group (NWG), is responsible for developing the end-to-end network

requirements, architecture, and protocols for WiMAX, using IEEE 802.16e-2005 as the

air interface.

The WiMAX NWG has developed a network reference model to serve as an

architecture framework for WiMAX deployments and to ensure interoperability among

various WiMAX equipment and operators.

The network reference model envisions a unified network architecture for

supporting fixed, nomadic, and mobile deployments and is based on an IP service model.

Below is simplified illustration of an IP-based WiMAX network architecture. The

overall network may be logically divided into three parts:

1. Mobile Stations (MS) used by the end user to access the network.

2. The access service network (ASN), which comprises one or more base stations

and one or more ASN gateways that form the radio access network at the edge.

3. Connectivity service network (CSN), which provides IP connectivity and all

the IP core network functions.

The network reference model developed by the WiMAX Forum NWG defines a

number of functional entities and interfaces between those entities. Fig below shows

some of the more important functional entities.


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Base station (BS): The BS is responsible for providing the air interface to the MS.

Additional functions that may be part of the BS are micromobility management

functions, such as handoff triggering and tunnel establishment, radio resource

management, QoS policy enforcement, traffic classification, DHCP (Dynamic

Host Control Protocol) proxy, key management, session management, and

multicast group management.

Access service network gateway (ASN-GW): The ASN gateway typically acts

as a layer 2 traffic aggregation point within an ASN. Additional functions that may

be part of the ASN gateway include intra-ASN location management and paging,

radio resource management and admission control, caching of subscriber profiles

and encryption keys, AAA client functionality, establishment and management of

mobility tunnel with base stations, QoS and policy enforcement, foreign agent

functionality for mobile IP, and routing to the selected CSN.

Connectivity service network (CSN): The CSN provides connectivity to the

Internet, ASP, other public networks, and corporate networks. The CSN is owned

by the NSP and includes AAA servers that support authentication for the devices,

users, and specific services. The CSN also provides per user policy management

of QoS and security. The CSN is also responsible for IP address management,

support for roaming between different NSPs, location management between ASNs,

and mobility and roaming between ASNs.

The WiMAX architecture framework allows for the flexible decomposition and/or

combination of functional entities when building the physical entities. For example, the

ASN may be decomposed into base station transceivers (BST), base station controllers


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(BSC), and an ASNGW analogous to the GSM model of BTS, BSC, and Serving

GPRS Support Node (SGSN).


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Section7 WiMAX Channel Estimation

Knowledge

Introduction -----------------------------------------------------------------------Level 1 2

Uplink Transmittion--------------------------------------------------------------Level 1 2

Channel Estimation_Transmitter------------------------------------------------Level 1 2

Channel Estimation_Channel----------------------------------------------------Level 1 2

Channel Estimation_Reciever----------------------------------------------------Level 1 2

7.1 Introduction

A general communication system consists of two blocks, a transmitter and

receiver, connected by a channel. The information transmitted by the transmitter passes

through the channel and then reaches the receiver. If the channel does not distort the

transmitted signal, then the receiver can retrieve the transmitted information

successfully, but in practice the channel alters the transmitted information making thetask difficult for the receiver. The main aim of the designer is to reduce the number of

errors made at the receiver. To achieve this, information is required at the receiver, as

to how the channel alters the information, so that the channel impairments can be

mitigated.

When the user is mobile, the channel characteristics do not remain constant for a

very long time. Hence the channel parameters need to be tracked, so that the effect can

be mitigated and reconstruct the transmitted data. This part deals with the requirements

of Channel estimation at the Base station (BS) for an 802.16e uplink. Symbol time has

an effect on system performance depending on the channel conditions. Different

symbol times are proposed in and each one has been simulated and compared for

various channel condition. In addition a solution proposed by Intel coop. has also been

analyzed. It is concluded that the performance of the system, for few proposed symbol

times, is relatively good in all conditions.


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7.2 Channel Estimation

The Block diagram (Figure 7.5) represents the whole system model or the signal

chain at base band. The block system is divided into 3 main sections namely the

transmitter, receiver and the channel. The model has been tested with and without the

channel coding (part in doted box representing the channel coding and decoding). The

bit error rate (BER) plots have been obtained for at least 2000 errors to get a good

confidence limit.

7.2.1 Transmitter

Data Generation: The data is generated from a random source, consists of a

series of ones and zeros. Since the transmission is done block wise, when forward error

correction (FEC) is used, the size of the data generated depends on the block size used,

modulation scheme used to map the bits to symbols (QPSK, 16QAM), and whether

FEC is used or not [1]. The generated data is passed on to the next stage, either to the

FEC block or directly to the symbol mapping if FEC is not used.

Forward error correction: In case error correcting codes are used, the data

generated is randomized so as to avoid long run of zeros or ones, the result is ease in

carrier recovery at the receiver. The randomized data is encoded using tail biting

convolutional codes (CC) with a coding rate of (puncturing of codes is provided in

the standard, but not simulated here). Finally interleaving is done by two stage

permutation, first to avoid mapping of adjacent coded bits on adjacent subcarriers and

the second permutation insures that adjacent coded bits are mapped alternately onto

less or more significant bits of the constellation, thus avoiding long runs of lowly

reliable bits.

Symbol mapping: The coded bits (uncoded, if FEC not used) are then mapped to

form symbols. Modulation scheme used is QPSK or 16QAM (QPSK unless otherwise

specified) with gray coding in the constellation map. In any case the symbol is

normalized so that the average power is unity, irrespective of the modulation scheme

used.

Subcarrier allocation: The subcarrier allocation is mentioned in the section 1.2

(Uplink transmission). This separates data into set of 4 subcarriers for 3 time symbols,


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named as the tile structure. Symbols are allocated indices representing the subcarriers

and OFDM time symbol, and then passed onto the next stage, the IFFT, to convert into

time domain.

IFFT and cyclic prefix: An N point inverse discrete fourier transform (IDFT)

of X(k)is defined as

X(n) =N

1

1

0

2

)(N

n

N

nj

ekX

for n = 1, 2, N-1. ----------------------eq. 7.1

From the equation we can infer that this is equivalent to generation of OFDMsymbol. An efficient way of implementing IDFT is by inverse fast fourier transform

(IFFT). Hence IFFT is used in generation of OFDM symbol. The addition of cyclic

prefix is done on the time domain symbol obtained after IFFT. The IFFT size (N

value) is considered as 2048 in simulations. This data is fed to the channel which

represents Rayleigh fading channel model and also implements multipath as shown in

block diagram.

7.2.2 Channel

In NLOS wireless communication, the received signal is a combination of many

multipath signals, which are result of reflections from surrounding objects. These

multipaths have different amplitude and phase and may add either constructively or

destructively leading to a complex envelope, i.e. fading. Fading characteristics depend

on the channel parameters (rms delay spread and Doppler spread) and signal

parameters (symbol period and bandwidth). Multipath delay spread leads to time

dispersion and frequency selective fading and Doppler spread leads to frequency

dispersion and time selective fading. Any mobile channel is one of the four mentioned

below.

Based on multipath time delay spread

Flat fading Freq selective fading

BW of Signal < BW of channel [BsBc]


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Delay spread < symbol period [Ts>> ] [Ts


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to shape it in the frequency domain. By using an IFFT (r (t) = IFFT (S (f).*G)), we

get an accurate time domain waveform of Doppler fading.

Figure 7.1: Simulated Doppler spectrum

Using Smiths method, the system generates time samples of the fading channel.

The data is multiplied in time domain with the fading channel output.

Figure 7.2: A typical Rayleigh fading channel


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Figure 7.2 shows simulated Rayleigh fading channel for the speed of 125

KMPH, and frequency of 5.9GHz.

Output = fading * input

r(t) = a(t)e)(j ts(t)------------------------------------ (eq. 7.3)

s(t) is the transmitted signal

a(t) is the amplitude of the fading channel (Rayleigh distributed)

(t) is the phase of the fading channel (uniformly distributed)

According to the standard the maximum supported speed of mobile is 125 KMPH

and the operating frequency range is between 2 6 GHz. The system has been

simulated for speeds 30, 80, 125 KMPH and frequency band of 3 GHz and 5.9GHz.

Three multipaths were simulated with uniformly distributed phase. For multipath the

amplitude and delay has been chosen as a random parameter, the first path does not

have any excess delay and the amplitude is scaled by a uniformly distributed number in

the range of 0 to 1. The other 2 paths have their amplitude scaled by uniformly

distributed number between 0 to 0.9 and 0 to 0.7. The excess delay is selected as a

uniformly distributed random parameter. Finally additive white Gaussian noise

(AWGN) is added as a last component in the channel.

Dopplerd_frequency(fd) =)/(__

)(*)/(velocy

smlightofspeed

Hzfrequencysm--- (eq. 7.4)

Coherence_Time(Tc) = 0.423/fd----------------------------------------- (eq. 7.5)

7.2.3 Reciever

The first thing done at receiver (in simulation) is removal of cyclic prefix, thus

eliminating the inter symbol interference (ISI). Data is then passed through the serial to

parallel converter of size 2048 and then fed to the FFT for frequency domain

transformation. The signal was distorted by the channel, to reconstruct the original

signal we need information as to how the channel acted on the transmitted signal so

that we can mitigate its effect. This is called equalization

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