Introduction to Mobile WiMAX

8/3/2019 Introduction to Mobile WiMAX

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2007 NTUEE Mobile Communications

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Introduction to Mobile WiMAX

Kwang-Cheng Chen, National Taiwan University

Abstract

We briefly introduce the origin and development of Mobile WiMAX in this chapter. Then, we

orient the technology used in mobile WiMAX in physical transmission and medium access

control, along with recent progress related to the IEEE 802.16 and mobile WiMAX.

1 IEEE 802.16

Before talking about WiMAX, we must start from the IEEE 802.16. IEEE 802 defines

international standards (more precisely, to be recognized by the ISO later) for local area

networks (LAN) and metropolitan area networks (MAN), such as IEEE 802.3 well known as

Ethernet. IEEE 802 projects generally consider physical layer transmission (PHY) and

medium access control (MAC), while left network layer and above to other international

standards such ISO. Since 1990, there are a few wireless standards in IEEE Project 802:

IEEE 802.11 wireless LANs (WLAN)

IEEE 802.15 wireless personal area networks (WPAN)

IEEE 802.16 wireless metropolitan area networks (WMAN)

IEEE 802.20 and more others

With popular WiFi applications (i.e. wireless LANs) especially after hot-spot deployment,more reliable wireless broadband technology for Internet access attracts great interests. The

concept for wireless metropolitan area networks (WMAN) has therefore been introduced in

recent years. Among many efforts, IEEE 802.16 standard defining fixed broadband wireless

(FBW) is widely considered as a new generation technology to replace past wireless local

loop (WLL) in telecommunications, and to deliver performance comparable to traditional

cable, T1, xDSL, etc. The advantages of IEEE 802.16 include

Quick deployment, even in those areas hard for wired infrastructure to reach

Ability to overcome physical limitation of traditional wired infrastructure

Reasonable installation cost to support high rate access

In other words, standardized FBW can support flexible, cost-effective, broadband accessservices in a wide range of devices. WiMAX (Worldwide Interoperability for Microwave




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Access) Forum is a non-profit corporation formed by equipment and component suppliers to

promote the adoption of IEEE 802.16 compliant equipment by operators of broadband

wireless access systems, which is comparable to WiFi Alliance in promoting IEEE 802.11

wireless LANs. WiMAX is establishing “System Profiles” for all compliant equipment,

which can also address regulatory spectrum constraints faces by operators in different

geographical regions. WiMAX forum also develops higher-layer specifications to match

IEEE 802.16. In the mean time, WiMAX defining conformance tests in conjunction withinteroperability enables service providers to choose multiple vendors. WiMAX works with

ETSI (European Telecommunications Standards Institute) to develop for the HIPERMAN

standard.

In April 2002, IEEE 802.16 was published for 10-66G Hz operations, while line-of-sight

transmission is considered as primary application. To promote immediate wider applications,

IEEE 802.16a was published in January 2003, which aims at 2-11G Hz operations for non-

line-of-sight performance.

Fixed broadband wireless (FBW) access applications based on point-to-multipoint network

topology primarily include

Cellular (or Fixed-Network) backhaul Broadband on-demand Residential broadband

Underserved areas services

Nomadic wireless services

As a consequence, FBW (later refined as Broadband Wireless Access, BWA, for the IEEE

802.16) systems and networks shall support

High throughput

High degree of scalability

Quality-of-service (QoS) capability

High degree of security

Excellent radio coverage

IEEE 802.16 Wireless MAN has a connection-oriented MAC and PHY is based on non-line-

of-sight radio operation in 2-11 GHz. For licensed bands, channel bandwidth shall be

limited to the regulatory provisioned bandwidth divided by any power of 2, no less than

1.25M Hz. Three technologies have been defined:

Single carrier (SC)

Orthogonal frequency division multiplexing (OFDM)

Orthogonal frequency division multiple access (OFDMA)

The communication of frame-based IEEE 802.16 is based on the fundamental concept by

defining burst profiles in each BS-SS communication link.

IEEE 802.16 has a revision published in October 2004, which is known as IEEE 802.16-

2004. The mobile version of IEEE 802.16 has been developed in the IEEE 802.16e (official




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name as “Physical and Medium Access Control Layers for Combined Fixed and Mobile

Operation in Licensed Bands”), which is commonly known as Mobile WiMAX , especially

considering its OFDMA (orthogonal frequency division multiple access) PHY. Such a

mobile enhancement of IEEE 802.16e are primarily specified for licensed bands Korean

WiBro provides mobile services based on IEEE 802.16-2004 and IEEE 802.16e. In ITU-R

May 2007 meeting in Japan, mobile WiMAX has been recommended as OFDMA TDD

WMAN (though still subject to further formal approval), and thus leaving 50M Hzbandwidth internationally available at 2.57-2.62 GHz from 3G TDD spectrum, on per nation

basis.

Since December 2006, IEEE 802.16m has started as a new amendment project to study the

IEEE 802.16 WirelessMAN-OFDMA specification to provide an advanced air interface for

operation in licensed bands, and to meet the cellular layer requirements for IMT-Advanced

toward next generation mobile networks, of course, with continuing support for legacy

WirelessMAN-OFDMA equipment and devices. The target speed for IEEE 802.16m is100M bps, with supporting high mobility, so that it may serve as a candidate of IMT-

Advanced. Consequently, 3G LTE (long-term evolution) from 3GPP, UMB (ultra mobile

broadband) from 3GPP2, and IEEE 802.16e and 802.16m, are all adopting OFDMA based

technology.

1.2 IEEE 802.16 MAC

IEEE 802.16 Medium Access Control (MAC), while IEEE 802.16e MAC generally follows,

has a network topology of point to multi-point, with support to mesh network topology. Its

backhaul can be either ATM (asynchronous transfer mode) or packet-based (such as IP

networks). From the reference model as illustrated in the Figure 1.1, there are three sub-

layers in the MAC:

Service Specific Convergence Sub-layer (CS): providing any transformation or

mapping of external network data through CS SAP (CS service access point).

MAC Common Part Sub-layer (MAC CPS): classifying external network

service data units (SDUs) and associating these SDUs to proper MAC service

flow and Connection Identifier (CID). Multiple CS specifications are providedfor interfacing with various protocols.

Privacy (or Security) Sub-layer: supporting authentication, secure key exchange,

and encryption.

Different from typical MACs using random access techniques in the IEEE 802, IEEE 802.16

MAC is connection oriented, and similar to time division multiple access (TDMA). Once a

subscriber station (SS) enters the network, it creates one or more connections to

communicate with base station (BS). It also performs link adaptation and automatic repeat

request (ARQ) functions to maintain target bit error rate. To further support multimedia

traffic, IEEE 802.16 MAC may have to execute usage of radio resources, and provide

quality-of-service (QoS) differentiation in services, which are not considered as typical

MAC functions. To support OFDMA PHY, the MAC layer is responsible for assigning

frames into proper zones and exchanging this structure information to the SSs in the DL andUL maps. Transmit diversity and adaptive antenna system (AAS), as well as MIMO zone,

are included.




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Scope of standard

CS SAP

PHY SAP

MAC SAP

Physical Layer(PHY)

Privacy Sublayer

MAC Common PartSublayer (MAC CPS)

Service Specific

Convergence Sublayer(CS)

M A C

P H Y

Date/Control Plane

Management Entity

Service Specific

Convergence SublayerManagement Entity

MAC CommonPart Sublayer

Management Entity

PHY Layer

Management Plane

Privacy Sublayer

N e t w o r k M a n a g e m e n t S y s t e m

Figure 1.1 Reference Model of IEEE 802.16

The IEEE 802.16e MAC provides QoS differentiation for different types of applications,

and defines 4 types of services:

Unsolicited Grant Services (UGS): UGS is designated for constant bit rate

(CBR) services, such as T1/E1 emulation and VoIP without silence

suppression. Real-Time Polling Services (rtPS): rtPS is designated for real-time services

that generate variable size of data packets on a periodic basis, such as MPEG

video and VoIP with silence suppression.

Non-Real-Time Polling Services (nrtPS): nrtPS is designated for non-real-

time services that require variable size data grant burst types on a regular

basis.

Best Effort Services (BE): It counts typical data traffic such as Internet webbrowsing and ftp file transfer.

Closer to the concept of cellular layer-2/3, IEEE 802.16 MAC has the radio link control

(RLC) to control PHY transition from one burst profile to another, in addition to traditional

power control and ranging.

Another important sub-layer in the IEEE 802.16 MAC is security sub-layer, and an

improved version has been developed for the IEEE 802.16e. Privacy and Key Management

Protocol version 2 (PMKv2) is the basis of mobile WiMAX security. Device and user

authentication adopts IETF EAP protocol. The traffic encryption follows the IEEE 802.11i

using AES-CCM to protect traffic data. The keys used for deriving the ciphertext are

generated from the EAP authentication. To avoid further attacks and hostile analysis, a

periodic key (TEK) refreshing mechanism enables improved protection. A 3-way handshakescheme in mobile WiMAX optimizes the re-authentication mechanism for fast handover by

preventing man-in-the-middle-attacks.




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1.3 IEEE 802.16e Mobile WiMAX

Mobile WiMAX is generally considered as the IEEE 802.16e-2005 adopting OFDMA PHY.

In this book, we shall describe recent advances in mobile WiMAX from technology to

services and applications. Prior to these chapters, we shall briefly introduce Mobile WiMAX

in this section.

The IEEE 802.16e-2005 supports both time division duplexing (TDD) and frequency

division duplexing (FDD) modes. However, the initial release of mobile WiMAX profiles

only considers the TDD mode of operation by the following reasons:

It enables dynamic allocation of downlink (DL) and uplink (UL) radio

resources to effectively support asymmetric DL/UL traffic that is common in

Internet applications. The allocation of radio resources in DL and UL isdetermined by the DL/UL switching point(s).

Both DL and UL are in the same frequency channel to yield better channel

reciprocity and to better support link adaptation, multi-input-multi-output

(MIMO) techniques, and closed-loop advanced antenna technique such as

beam-forming. Single frequency channel in DL and UL can provide more flexibility for

spectrum allocation.

To further alleviate spectrum allocation efforts, mobile WiMAX adopts simple frequency

reuse schemes by reusing 1 and 3 with PUSC.

F1

F1F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1

F1,S1

F1,S2F1,S3

F1,S1

F1,S2

F1,S3

F1,S1

F1,S2

F1,S3

F1,S1

F1,S2

F1,S3

F1,S1

F1,S2

F1,S3

F1,S1

F1,S2

F1,S3

F1,S1

F1,S2

F1,S3

F1= B MHz

S1=SubCH {0, N-1}

S2=SubCH {N, 2N-1}S3=SubCH {2N, 3N-1}

Reuse 1x3x1 Reuse 1x3x3

Figure 1.2 Frequency Reuse Schemes (a) 1×3×1 (b) 1×3×3

As pointed in [3], time division DL/UL in multiple-cell wireless networks can create up-down collisions (or interference) to result in performance loss. Fractional frequency reuse




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(FFR) as Figure 1.3 can be applied by utilizing frequency reuse 1×3×1 near the center and

frequency reuse 1×3×3 near the cell edges. There is no need for frequency planning and is

very flexible to configure the networks. Frequency reuse factor 1 at the center of the cell to

maximize the network spectral efficiency, while higher reuse factor at cell edges to alleviate

(co-channel) interference.

F

F

FF1

F2

F3

F

F

FF1

F2

F3

F

F

FF1

F2

F3

1X3X1 Reuse

1X3X3 Reuse

F=F1+F2+F3 F1: F,S1 F2: F,S2 F3: F,S3

Figure 1.3 Fractional Frequency Reuse

When we design a mobile WiMAX system, we usually consider the wide-sense stationary

uncorrelated scattering (WSSUS) to stochastically model the time-varying fading wireless

channels in time and frequency domains. Two main factors from this model are used in

developing the system parameters: Doppler spread and thus coherence time of the channel,

and multipath delay spread and thus coherence bandwidth. Stanford University Interim (SUI)

channel models are widely accepted in study of WiMAX systems.

A very special feature in (mobile) WiMAX is ranging while SS at initial entry and alsoperiodically in normal operation, in which the mobile subscriber station (MS) acquires

frequency, time, and power adjustments, so that all MS transmissions can align with the UL

sub-frame received by the base station (BS). Ranging process proceeds via MS transmitting

a signal and BS responding with required adjustments, which is a closed loop control

process critical to OFDMA communications in (mobile) WiMAX. Ranging happens in 3

types: initial/handoff ranging, periodic ranging, and BW request ranging.

Mobile WiMAX OFDMA PHY adopts scalable OFDMA with 1.25× 2n MHz bandwidth,

n=0, 1, 2, 3, 4, at fixed sub-carrier spacing. There are 3 types of OFDMA sub-carriers:

Data sub-carriers for data transmission

Pilot sub-carriers for estimation and synchronization purposes Null sub-carriers for guard band and DC carriers, without transmission at all




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The pilot sub-carrier allocation can be performed in different modes. For DL Fully Used

Subchannelization (FUSC), the pilot tones (or sub-carriers) are allocated first and then the

remaining sub-carriers are arranged for data sub-channels. For DL Partially Used

Subchannelization (PUSC) and all UL modes, the set of all used sub-carriers (pilot and data)

is partitioned into sub-channels, and then pilot sub-carrier(s) are allocated within each sub-

channel.

Adaptive modulation and coding (AMC) is adopted by using QPSK, 16QAM, 64QAM

(optional in UL) as modulation, and convolutional codes (mandatory), turbo codes, low-

density parity check codes for forward error correcting codes (FEC). Of most interests,

space-time codes (STC) and spatial multiplexing (SM) are used to enhance PHY

transmission speed and reception quality of signals from mobile stations.

STC is originated from pioneer work of transmit diversity coding by S. Alamouti [9], and

Figure 1.4 depicts the realization of closed-loop STC for mobile WiMAX, and Alamouticode as shown. There are 3 kinds of STC in the IEEE 802.16e OFDMA by using 2 or 3

antennas, while Alamouti code is one of them.

STCCoder

⎥⎥⎦

⎤

⎢⎢⎣

⎡ −*

01

*

10

ss

ss

[ ]10 ss

MatrixBook

STCCombining

ChannelEstimator

+

+

⎥⎥⎦

⎤

⎢⎢⎣

⎡

32

10

hh

hh

Feedback Channel

0h

⎥⎥⎦

⎤

⎢⎢⎣

⎡

32

10

ˆ ˆ

ˆ ˆ

hh

hh

1h

3h

2h

0n

1n[ ]10

ˆ ˆ ss

Figure 1.4 Closed-Loop Space Time Coding (Alamouti codes as an example)

In addition to STC, spatial multiplexing can be further used, which is depicted in Figure 1.5.

Spatial multiplexing transmits data streams via different spatial domains (typically multiple

antennas). STC and spatial multiplexing can form the foundation of IEEE 802.16e MIMO

processing. BS can send control messages to indicate subsequent allocation shall use certain

permutation with a specific transmit diversity mode, and to describe DL allocations assigned

to MIMO enabled SSs by defining one of the 3 STC matrices.




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D

EC

O

D

E

R

Channel

Estimator

+

+

0n

1n

+

2n

Data Stream 1

Data Stream 2

Data Stream 3

One Sub-carrier

Data Stream 1

Data Stream 2

Data Stream 3

Figure 1.5 Spatial Multiplexing

To support STC and spatial multiplexing, pilots for multiple transmission antennas should

be disjunct to avoid inter-stream interference. It is worth noting that OFDM particularly fits

MIMO and adaptive antenna techniques, comparing with CDMA and single carrier

transmission.

1.4 Mobile WiMAX End-to-End Network Architecture

IEEE 802.16e only defines PHY and MAC. However, in light of the needs of interfaces at

higher layers to allow multi-vendor supply as typical wireless communication standards,

WiMAX Forum has working groups beyond the IEEE 802.16. The mobile WiMAX End-to-End Network Architecture is developed on an all-IP platform with all packet technology and

without any legacy circuit telephony.

Figure 1.6 depicts an IP-based WiMAX network architecture, which consists of 3 major

parts: user terminals (i.e. subscriber/mobile stations), access service network (ASN), and

connectivity service network (CSN). ASN defines a logical boundary to describeaggregation of functional entities and corresponding message flows associated with the

access services. Connectivity service network (CSN) represents a set of network functions

providing IP connectivity services to WiMAX subscribers. A CSN may compromise

network elements such as AAA proxy and servers, routers, user database, and

internetworking gateway.

The end-to-end WiMAX network architecture extensively supports mobility and handover,

which includes

Vertical or inter-technology handovers under multi-mode operation IPv4 and IPv6 based mobility management

Roaming between network service providers (NSPs)




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Seamless handover up to vehicular speed satisfying bounds of service

disruptions

MobileWiMAX

Terminal

PortableWiMAX

Terminal

FixedWiMAX

Terminal

UserTerminals

MobileWiMAXBase

Station

AccessServiceNetworkGateway

(ASN-GW)

Access Service Network Connectivity Service Network

AAAServer

MIP HA

BillingSupportSystem

ContentServices

IMS

OperationSupportSystems

ServiceProvider IP Based

Core Networks

Air Interface Roaming Interface

NetworkInteroperability

Interfaces

Figure 1.6 WiMAX Network IP-Based Architecture

WiMAX network architecture surely has provisions to support QoS via differentiated levels

of QoS, admission control, bandwidth management, and other appropriate policies.

References:

[1] C. Eklund, R.B. Marks, K.L. Stanwood, S. Wang, “IEEE Standard 802.16: A Technical Overview of the

Wireless MAN Air Interface for Broadband Wireless Access”, IEEE Communications Magazine , June 2002.

[2] INTEL Technology Journal , special issue on WiMAX, Vol. 8, No. 3, 2004.

[3] K.C. Chen, “Medium Access Control of Wireless Local Area Networks for Mobile Computing”, IEEE

Networks, 1994.

[4] X. Fu, Y. Li, H. Minn, “A New Ranging Method for OFDMA Systems”, IEEE Tr. On Wireless

Communications, vol. 6, no. 2, Feb. 2007.

[5] C. Cicconetti, et al., “Quality of Service Support in IEEE 802.16 Networks”, IEEE Network , March/April,

2006.

[6] J. Wang, M. Venkatachalam, Y. Fang, “System Architecture and Cross-Layer Optimization of Video

Broadcast over WiMAX”, IEEE Journal on Selected Areas in Communications, Vol. 25, No. 4, May 2007.

[7] Q. Ni, et al., “Investigation of Bandwidth Request Mechanisms under Point-to-Multipoint Mode of

WiMAX Networks”, IEEE Communications Magazine , May 2007.

[8] K. Lu, Y. Qian, H-H. Chen, “A Secure and Service-Oriented Network Control Framework for WiMAX

Networks”, IEEE Communications Magazine , May 2007.

[9] S. Alamouti, “Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on

Selected Areas in Communications, Vol. 16, No. 8, October 1998.

Documents

Introduction to Mobile WiMAX