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.