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

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 in Table 4.

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 Turbo

Code (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 2 summarizes the coding and modulation schemes supported in the Mobile

WiMAX profile the optional UL codes and modulation are shown in italics.
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.

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. 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. MAC Layer Description
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 [9]
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.

 

TDD Frame Structure

The 802.16e PHY [3] 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 7 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 length and coding scheme and

usable 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.




 

OFDMA Symbol Structure and Sub-Channelization

The OFDMA symbol structure consists of three types of sub-carriers as shown in Figure

4:

 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

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

The WiMAX OFDMA PHY [3] 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 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 as shown in Figure 5.

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
whose format is shown in Figure 6.

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 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.

 

WiMAX-3

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 downlink

sub-channels and uplink multiple access by means of uplink sub-channels.

 

WiMAX-2

Introduction

The WiMAX technology, based on the IEEE 802.16-2004 Air Interface Standard is

rapidly proving itself as a technology that will play a key role in fixed broadband wireless

metropolitan area networks. The first certification lab, established at Cetecom Labs in

Malaga, Spain is fully operational and more than 150 WiMAX trials are underway in

Europe, Asia, Africa and North and South America. Unquestionably, Fixed WiMAX,

based on the IEEE 802.16-2004 [1] Air Interface Standard, has proven to be a costeffective

fixed wireless alternative to cable and DSL services. In December, 2005 the

IEEE ratified the 802.16e amendment [2] to the 802.16 standard. This amendment adds

the features and attributes to the standard that are necessary to support mobility. The

WiMAX Forum is now defining system performance and certification profiles based on

the IEEE 802.16e Mobile Amendment and, going beyond the air interface, the WiMAX

Forum is defining the network architecture necessary for implementing an end-to-end

Mobile WiMAX1 network. Release-1 system profiles will be completed in early 2006.

Mobile WiMAX is a broadband wireless solution that enables convergence of mobile and

fixed broadband networks through a common wide area broadband radio access

technology and flexible network architecture. The Mobile WiMAX Air Interface adopts

Orthogonal Frequency Division Multiple Access (OFDMA) for improved multi-path

performance in non-line-of-sight environments. Scalable OFDMA (SOFDMA) [3] is

introduced in the IEEE 802.16e Amendment to support scalable channel bandwidths from

1.25 to 20 MHz. The Mobile Technical Group (MTG) in the WiMAX Forum is

developing the Mobile WiMAX system profiles that will define the mandatory and

optional features of the IEEE standard that are necessary to build a Mobile WiMAXcompliant

air interface that can be certified by the WiMAX Forum. The Mobile WiMAX

System Profile enables mobile systems to be configured based on a common base feature

set thus ensuring baseline functionality for terminals and base stations that are fully

interoperable. Some elements of the base station profiles are specified as optional to

provide additional flexibility for deployment based on specific deployment scenarios that

may require different configurations that are either capacity-optimized or coverageoptimized.

Release-1 Mobile WiMAX profiles will cover 5, 7, 8.75, and 10 MHz channel

bandwidths for licensed worldwide spectrum allocations in the 2.3 GHz, 2.5 GHz, and

3.5 GHz frequency bands.

1 The term WiMAX has been used generically to describe wireless systems based on the WiMAX

certification profiles based on the IEEE 802.16-2004 Air Interface Standard. With additional profiles

pending based on the IEEE 802.16e Mobile Amendment, it is necessary to differentiate between the two

WiMAX systems. “Fixed” WiMAX is used to describe 802.16-2004 based systems and “Mobile” WiMAX

is used to describe 802.16e-based systems.

The WiMAX Forum Network Working Group (NWG) is developing the higher-level

networking specifications [4] for Mobile WiMAX systems beyond what is defined in the

IEEE 802.16 standard that simply addresses the air interface specifications. The

combined effort of IEEE 802.16 and the WiMAX Forum help define the end-to-end

system solution for a Mobile WiMAX network.

Mobile WiMAX systems offer scalability in both radio access technology and network

architecture, thus providing a great deal of flexibility in network deployment options and

service offerings. Some of the salient features supported by Mobile WiMAX are:

High Data Rates: The inclusion of MIMO antenna techniques along with flexible

sub-channelization schemes, Advanced Coding and Modulation all enable the

Mobile WiMAX technology to support peak DL data rates up to 63 Mbps per sector

and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.

Quality of Service (QoS): The fundamental premise of the IEEE 802.16 MAC

architecture is QoS. It defines Service Flows which can map to DiffServ code points

or MPLS flow labels that enable end-to-end IP based QoS. Additionally, subchannelization

and MAP-based signaling schemes provide a flexible mechanism for

optimal scheduling of space, frequency and time resources over the air interface on a

frame-by-frame basis.

Scalability: Despite an increasingly globalized economy, spectrum resources for

wireless broadband worldwide are still quite disparate in its allocations. Mobile

WiMAX technology therefore, is designed to be able to scale to work in different

channelizations from 1.25 to 20 MHz to comply with varied worldwide requirements

as efforts proceed to achieve spectrum harmonization in the longer term. This also

allows diverse economies to realize the multi-faceted benefits of the Mobile WiMAX

technology for their specific geographic needs such as providing affordable internet

access in rural settings versus enhancing the capacity of mobile broadband access in

metro and suburban areas.

Security: The features provided for Mobile WiMAX security aspects are best in

class with EAP-based authentication, AES-CCM-based authenticated encryption,

and CMAC and HMAC based control message protection schemes. Support for a

diverse set of user credentials exists including; SIM/USIM cards, Smart Cards,

Digital Certificates, and Username/Password schemes based on the relevant EAP

methods for the credential type.

Mobility: Mobile WiMAX supports optimized handover schemes with latencies less

than 50 milliseconds to ensure real-time applications such as VoIP perform without

service degradation. Flexible key management schemes assure that security is

maintained during handover.

While the Mobile WiMAX standards activity has been progressing, equipment suppliers

have been aggressively developing equipment that will be WiMAX/802.16e compliant.

With commercial availability of Mobile WiMAX-compliant equipment anticipated in the

very near future and the launch of WiBro services (also based on 802.16e) this year in

Korea, it begs the question as to how the Mobile WiMAX technology relates to and

impacts concurrent advances in 3G cellular technology. To address this question it is

necessary to gain an understanding of the underlying technology for Mobile WiMAX as

well as the planned 3G enhancements. The white paper is comprised of two parts. Part I

is focused on Mobile WiMAX. It provides a detailed discussion of the Mobile WiMAX

technology based on the planned WiMAX Forum Certification profiles and includes a

detailed analysis of Mobile WiMAX performance projections in a mobile environment.

An extensive list of references is also provided for the reader seeking further information

on any of WiMAX features and attributes discussed in the paper. Part II [5] of the white

paper provides an overview of enhancements to CDMA-based 3G systems and offers

both a qualitative and quantitative comparative analysis of Mobile WiMAX relative to

the 3G cellular technologies.

2. Physical Layer Description

2.1 OFDMA Basics

Orthogonal Frequency Division Multiplexing (OFDM) [6,7] is a multiplexing technique

that subdivides the bandwidth into multiple frequency sub-carriers as shown in Figure 2.

In an OFDM system, the input data stream is divided into several parallel sub-streams of

reduced data rate (thus increased symbol duration) and each sub-stream is modulated and

transmitted on a separate orthogonal sub-carrier. The increased symbol duration improves

the robustness of OFDM to delay spread. Furthermore, the introduction of the cyclic

prefix (CP) can completely eliminate Inter-Symbol Interference (ISI) as long as the CP

duration is longer than the channel delay spread. The CP is typically a repetition of thelast samples of data portion of the block that is appended to the beginning of the data
payload as shown in Figure 3. The CP prevents inter-block interference and makes the
channel appear circular and permits low-complexity frequency domain equalization. A
perceived drawback of CP is that it introduces overhead, which effectively reduces
bandwidth efficiency. While the CP does reduce bandwidth efficiency somewhat, the
impact of the CP is similar to the “roll-off factor” in raised-cosine filtered single-carrier
systems. Since OFDM has a very sharp, almost “brick-wall” spectrum, a large fraction of
the allocated channel bandwidth can be utilized for data transmission, which helps to
moderate the loss in efficiency due to the cyclic prefix.

 

 

coming to actual Wi MAX

Mobile WiMAX – Part 1: A Technical Overview and

Performance Evaluation

This paper has been prepared on behalf of the WiMAX Forum and the material presented

represents the combined efforts of many people from several WiMAX Forum

organizations with long-standing experience in wireless technologies. Additionally, a

broader range of WiMAX Forum members have had the opportunity to review and

critique the material and every attempt has been made to assure accuracy of the material.

WiMAX Forum member organizations that have made contributions to the material

presented in this paper are:

Alvarion

Arraycomm

ATT

Beceem

Intel

Motorola

Nortel

Samsung

The WiMAX Forum is especially grateful to participants from Intel, Motorola, and Nortel

for their dedicated efforts and substantial contributions towards completing this paper in a

timely manner. The overall preparation and editing was done by Doug Gray, a

Telecommunications Consultant under contract to the WiMAX Forum.

Acronyms

3GPP 3G Partnership Project

3GPP2 3G Partnership Project 2

AAS Adaptive Antenna System also Advanced Antenna System

ACK Acknowledge

AES Advanced Encryption Standard

AG Absolute Grant

AMC Adaptive Modulation and Coding

A-MIMO Adaptive Multiple Input Multiple Output (Antenna)

ASM Adaptive MIMO Switching

ARQ Automatic Repeat reQuest

ASN Access Service Network

ASP Application Service Provider

BE Best Effort

BRAN Broadband Radio Access Network

CC Chase Combining (also Convolutional Code)

CCI Co-Channel Interference

CCM Counter with Cipher-block chaining Message authentication code

CDF Cumulative Distribution Function

CDMA Code Division Multiple Access

CINR Carrier to Interference + Noise Ratio

CMAC block Cipher-based Message Authentication Code

CP Cyclic Prefix

CQI Channel Quality Indicator

CSN Connectivity Service Network

CSTD Cyclic Shift Transmit Diversity

CTC Convolutional Turbo Code

DL Downlink

DOCSIS Data Over Cable Service Interface Specification

DSL Digital Subscriber Line

DVB Digital Video Broadcast

EAP Extensible Authentication Protocol

EESM Exponential Effective SIR Mapping

EIRP Effective Isotropic Radiated Power

ErtVR Extended Real-Time Variable Rate

ETSI European Telecommunications Standards Institute

FBSS Fast Base Station Switch

FCH Frame Control Header

FDD Frequency Division Duplex

FFT Fast Fourier Transform

FTP File Transfer Protocol

FUSC Fully Used Sub-Channel

HARQ Hybrid Automatic Repeat reQuest

HHO Hard Hand-Off

HiperMAN High Performance Metropolitan Area Network

HMAC keyed Hash Message Authentication Code

HO Hand-Off

HTTP Hyper Text Transfer Protocol

IE Information Element

IEFT Internet Engineering Task Force

IFFT Inverse Fast Fourier Transform

IR Incremental Redundancy

ISI Inter-Symbol Interference

LDPC Low-Density-Parity-Check

LOS Line of Sight

MAC Media Access Control

MAI Multiple Access Interference

MAN Metropolitan Area Network

MAP Media Access Protocol

MBS Multicast and Broadcast Service

MDHO Macro Diversity Hand Over

MIMO Multiple Input Multiple Output (Antenna)

MMS Multimedia Message Service

MPLS Multi-Protocol Label Switching

MS Mobile Station

MSO Multi-Services Operator

NACK Not Acknowledge

NAP Network Access Provider

NLOS Non Line-of-Sight

NRM Network Reference Model

nrtPS Non-Real-Time Packet Service

NSP Network Service Provider

OFDM Orthogonal Frequency Division Multiplex

OFDMA Orthogonal Frequency Division Multiple Access

PER Packet Error Rate

PF Proportional Fair (Scheduler)

PKM Public Key Management

PUSC Partially Used Sub-Channel

QAM Quadrature Amplitude Modulation

QPSK Quadrature Phase Shift Keying

RG Relative Grant

RR Round Robin (Scheduler)

RRI Reverse Rate Indicator

RTG Receive/transmit Transition Gap

rtPS Real-Time Packet Service

RUIM Removable User Identify Module

SDMA Space (or Spatial) Division (or Diversity) Multiple Access

SF Spreading Factor

SFN Single Frequency Network

SGSN Serving GPRS Support Node

SHO Soft Hand-Off

SIM Subscriber Identify Module

SINR Signal to Interference + Noise Ratio

SISO Single Input Single Output (Antenna)

SLA Service Level Agreement

SM Spatial Multiplexing

SMS Short Message Service

SNIR Signal to Noise + Interference Ratio

SNR Signal to Noise Ratio

S-OFDMA Scalable Orthogonal Frequency Division Multiple Access

SS Subscriber Station

STC Space Time Coding

TDD Time Division Duplex

TEK Traffic Encryption Key

TTG Transmit/receive Transition Gap

TTI Transmission Time Interval

TU Typical Urban (as in channel model)

UE User Equipment

UGS Unsolicited Grant Service

UL Uplink

UMTS Universal Mobile Telephone System

USIM Universal Subscriber Identify Module

VoIP Voice over Internet Protocol

VPN Virtual Private Network

VSF Variable Spreading Factor

WiFi Wireless Fidelity

WAP Wireless Application Protocol

WiBro Wireless Broadband (Service)

WiMAX Worldwide Interoperability for Microwave Access

EXECUTIVE SUMMARY

In this document we provide an overview of Mobile WiMAX and provide the

performance for the basic minimal configuration based on the WiMAX Forum Release-1

system profiles. We show that mobile WiMAX can provide tens of megabits per second

of capacity per channel from each base station with a baseline configuration. Some of the

advanced features such as adaptive antenna systems (AAS) which can significantly

improve the performance are discussed but not included in the performance analysis.

The high data throughput enables efficient data multiplexing and low data latency.

Attributes essential to enable broadband data services including data, streaming video and

VoIP with high quality of service (QoS). The performance will enable transparency of

quality of service between Mobile WiMAX and broadband wired services such as Cable

and DSL, an important requirement for the success of the targeted Mobile Internet

application for Mobile WiMAX.

The scalable architecture, high data throughput and low cost deployment make Mobile

WiMAX a leading solution for wireless broadband services. Other advantages of

WiMAX include an open standards approach, friendly IPR structure and healthy

ecosystem. Hundreds of companies have contributed to the development of the

technology and many companies have announced product plans for this technology. This

addresses another important requirement for the success of the technology, which is low

cost of subscription services for mobile internet. The broad industry participation will

ensure economies of scale that will help drive down the costs of subscription and enable

the deployment of mobile internet services globally, including emerging countries.

A companion paper, Mobile WiMAX - Part II: A Competitive Analysis, provides a

comparison with contemporary cellular alternatives. The comparison is carried out in

qualitative (feature comparison) and quantitative terms to demonstrate the advantages of

Mobile WiMAX compared to the available mobile wireless alternatives.

 

Wi MAX

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Hello Everyone

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