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7/27/2019 WiMAX Basic Theory Level III
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WiMAX Basic Theory
WiMAX Basic Theory
Level III
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WiMAX Basic Theory
Version Date Author Approved By Remarks
V1.0 2010/12/23 Not open to the Third Party
2010 ZTE Corporation. All rights reserved.ZTE CONFIDENTIAL: This document contains proprietary information of ZTE and is not to bedisclosed or used without the prior written permission of ZTE.Due to update and improvement of ZTE products and technologies, information in this documentis subjected to change without notice.
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CONTENT
1 Chapter1 Introduction to WiMAX.....................................................................71.1 Wireless Introduction...........................................................................................71.1.1 Wireless Network Topologies..............................................................................21.1.2 Wireless Technologies ........................................................................................21.1.3 Kinds of Wireless Networks ................................................................................21.1.4 Wireless Broadband Access (WBA)....................................................................31.2 Related Organization ..........................................................................................31.2.1 IEEE....................................................................................................................31.2.2 WiMAX Forum.....................................................................................................41.3 What is WiMAX ...................................................................................................41.3.1 What is WiMAX ...................................................................................................5
1.3.2 What is 802.16d ..................................................................................................51.3.3 What is 802.16e ..................................................................................................51.3.4 WiMAX Speed and Range ..................................................................................61.3.5 Why WiMAX........................................................................................................61.3.6 WiMAX Goals......................................................................................................71.4 Salient Features of WiMAX.................................................................................71.4.1 OFDM-based physical layer................................................................................71.4.2 Very High Peak Date Rate ..................................................................................81.4.3 Scalable bandwidth and rate support..................................................................81.4.4 Adaptive modulation and coding (AMC)..............................................................81.4.5 Link-layer retransmissions ..................................................................................81.4.6 Support for TDD and FDD...................................................................................8
1.4.7 Orthogonal frequency division multiple access (OFDMA)...................................91.4.8 Flexible and dynamic per user resource allocation .............................................91.4.9 Support for advanced antenna techniques .........................................................91.4.10 Qulity of service support......................................................................................91.4.11 Robust security .................................................................................................101.4.12 Support for mobility ...........................................................................................101.4.13 IP-based architecture........................................................................................10
2 Chapter2 OFDM & OFDMA .............................................................................112.1 OFDM System Description................................................................................112.2 Difference between OFDMA and OFDM...........................................................132.3 OFDM Orthogonality .........................................................................................142.4 How to Overcome Inter Symbol Interference (ISI) ............................................142.5 How to Overcome Inter Carrier Interference (ICI) .............................................15
3 Chapter3 Physical Layer Description............................................................163.1 OFDMA Basics..................................................................................................163.2 OFDMA SymbolStructure and Sub-Channelization .........................................173.3 Scalable OFDMA ..............................................................................................193.4 TDD Frame Structure........................................................................................203.5 Other Advanced PHY Layer Features...............................................................21
4 Chapter4 WiMAX MAC Layer .........................................................................23
4.1 Common MAC Concepts ..................................................................................244.1.1 CS Sublayer......................................................................................................24
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4.1.2 MAC CPS Sublayer...........................................................................................244.2 Quality of Service (QoS) Support......................................................................294.3 MAC Scheduling Service ..................................................................................31
4.4 Mobility Management ........................................................................................324.4.1 Power Management ..........................................................................................324.4.2 Handoff..............................................................................................................33 4.5 Security .............................................................................................................34
5 Chapter5 WiMAX Advanced Features ...........................................................355.1 Smart Antenna Technologies............................................................................355.2 Fractional Frequency Reuse.............................................................................37
5.3 Multicast and Broadcast Service (MBS)............................................................39
6 Chapter6 WiMAX Network Architecture........................................................40
6.1 WiMAX Network Architecture............................................................................40
7 Chapter7 WiMAX Channel Estimation ...........................................................447.1 Introduction .......................................................................................................447.2 Channel Estimation...........................................................................................447.2.1 Transmitter........................................................................................................457.2.2 Channel.............................................................................................................46 7.2.3 Receiver............................................................................................................49
8 Chapter8 WiMAX Major Benefi ts ...................................................................518.1 WiMAX Major Benefits ......................................................................................518.1.1 Benefits to Component Makers.........................................................................51
8.1.2 Benefits to Equipment Makers ..........................................................................518.1.3 Benefits to Operators ........................................................................................518.1.4 Benefits to Consumers......................................................................................51
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FIGURESFigure 2-1 OFDM description in both time and frequency division ......................................11
Figure 2-2 Orthogonal subcarriers.......................................................................................12
Figure 2-3 Comparing of FDMA and OFDM ........................................................................12
Figure 2-4 Compirement of OFDM and OFDMA .................................................................13
Figure 2-5 Subcarriers and multipath component shown ....................................................15
Figure 2-6 Cyclic prefix ........................................................................................................15
Figure 3-1 Basic Architeture of OFDMA system..................................................................17
Figure 3-2 OFDMA Sub-carrier Structure ............................................................................18
Figure 3-3 DL Frequency Diverse Sub-Channel..................................................................18
Figure 3-4 Tile Structure for UL PUSC ................................................................................19
Figure 3-5 OFDMA Scalablity Parameters...........................................................................20
Figure 3-6 OFDMA Frame Structure....................................................................................21
Figure 4-1 MAC PDU format................................................................................................24
Figure 4-2 MAC header format ............................................................................................25
Figure 4-3 MAC management message format...................................................................25
Figure 4-4 Composition of Burst and MAC PDU..................................................................29
Figure 4-5 Mobile WiMAX QoS Support ..............................................................................30
Figure 5-1 Adaptive Switching for Smart Antennas .............................................................37
Figure 5-2 Multi-Zone Frame Structure................................................................................38
Figure 5-3 Fractional Frequency Reuse ..............................................................................38
Figure 5-4 MBS Zones.........................................................................................................40
Figure 6-1 IP-Based WiMAX Network Architecture .............................................................41
Figure 7-1 Simulated Doppler spectrum ..............................................................................47
Figure 7-2 Rayleigh fading channel .....................................................................................48
Figure 7-3 The Block Diagram.............................................................................................50
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TABLESTable 3-1 Supported Code and Modulations .......................................................................22
Table 3-2 PHY Data Rates with PUSC Sub-Channel ..........................................................22
Table 4-1 MAC management messages .............................................................................26
Table 4-2 Mobile WiMAX Application and Quality of service ...............................................30
Table 5-1 Advanced Antenna Options.................................................................................36
Table 5-2 Data Rates for SIMO/MIMO Configurations ........................................................36
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1 Chapter1 Introduction to WiMAX
Knowledge
zWhat is WiMAX ------------------------------------------------------------------Level 1 2
zWiMAX Benefits -----------------------------------------------------------------Level 1 2
z 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
beyond their 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 incomparision 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.
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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
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
Internet in 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.
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These networks provide a very fast data speed compared with the data rates of
mobile telecommunications technology, and their range is very limited. Wi-Fi is the
most widespread and popular example of WLAN technology.
Wireless Metropolitan Area Network (WMAN):
This network enables you to access the Internet and multimedia streaming
services via a wireless region area network (WRAN).
These networks provide a very fast data speed compared with the data rates of
mobile 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 andsubscriber 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.
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1.2.2 WiMAX Forum
WiMAX Forum (WMF) is a nonprofit production group founded on April 9, 2001 byequipment 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.
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.
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1.3.1 What is WiMAX
z Acronym for Worldwide Interoperability for Microwave Access
z Based on Wireless MAN technology.
z A wireless technology optimized for the delivery of IP centric services over a
wide area.
z A scaleable wireless platform for constructing alternative and complementary
broadband networks.
z A certification that denotes interoperability of equipment built to the IEEE
802.16 or compatible standard. The IEEE 802.16 Working Group developsstandards 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 (portabledevices) 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.. 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
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complexity. Similar to 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, but more 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
technical configuration 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 3 km 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
1.3.5 Why WiMAX
z 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.
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z 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.
z 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.
z 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.
z 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.
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.
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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 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 acknowledged by the receiver; unacknowledged
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
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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 multip le 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 theexploitation 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 spatialdomain 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.
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.
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1.4.11 Robust securi ty
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 mobi lity
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 asmore frequent channel estimation, uplink subchannelization, and power control, are
also specified in support of moble applications.
1.4.13 IP-based archi tecture
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 with other networks, and
exploit the rich ecosystem for application development that exsits for IP.
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2 Chapter2 OFDM & OFDMA Knowledgez OFDM System Basics-------------------------------------------------------------Level 1 2
z OFDM Orthogonality -------------------------------------------------------------Level 1 2
z Overcome ISI-----------------------------------------------------------------------Level 1 2
z Overcome ICI----------------------------------------------------------------------Level 1 2
2.1 OFDM System Descript ion
z OFDM = Orthogonal Frequency Division Multiplexing .
z OFDM converts a high rate broadband signal into many parallel low rate
narrowband signals.
z Low rate signals have large symbol periods, which make OFDM signal
resistant to multipath delay spread.
z OFDM uses a Fast Fourier Transform (FFT) to allow overlap in frequency of
individual narrowband signals.
z More efficient than conventional multi-carrier.
Figure 2-1 OFDM description in both time and frequency division
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OFDM is a multi carrier transmission scheme where the information is transmitted
on multiple subcarriers, with a lower data rate, instead of one high data rate carrier
Figure 2-2 Orthogonal subcarriers
Figure 2.1-2
Figure 2-3 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
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systems can be implemented very efficiently by using the Inverse Fast Fourier
transform (IFFT) at the transmitter and Fast Fourier transform (FFT) at 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 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 adifference?
Figure 2-4 Compirement of OFDM and OFDMA
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 in the same channel
at the same time.
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2.3 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 be fulfilled by the transmitter and
the receiver.
0dt(2f)tsin2ft2sin
T
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.
2.4 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 time. When designing an OFDM system proper
values are selected depending on the environment so as to satisfy the above
condition.
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Figure 2-5 Subcarriers and multipath component shown
2.5 How to Overcome Inter Carr ier Interference (ICI)
Cyclic prefix is fiiled.
Multi carrier systems have the problem of inter carrier interference (ICI), whichresults from loss of orthogonality between the subcarriers. This happens when 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 2-6 Cyclic prefix
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3 Chapter3 Phys ical Layer Description Knowledge
z OFDMA Basics -----------------------------------------------------------------Level 1 2
z OFDMA Symbol Stucture-----------------------------------------------------Level 1 2
z Scalable OFDMA---------------------------------------------------------------Level 1 2
z TDD Frame Structure----------------------------------------------------------Level 1 2
z Advanced PHY Features------------------------------------------------------Level 1 2
z Difference between OFDMA and OFDM-----------------------------------Level 1 2
3.1 OFDMA Basics
z OFDMA = Orthogonal Frequency Division Multiple Access
z In Scalable OFDMA, subcarrier spacing is independent of bandwidth
z FFT size is scaled with bandwidth
z Subchannel size is fixed and independent of bandwidth and other modes
of operation
z The number of subchannels scales with FFT size rather than with the
capacity of subchannels
Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexing technique
that subdivides the bandwidth into multiple frequency sub-carriers.
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 the last
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
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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.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
Figure 3-1 Basic Architeture of OFDMA system
3.2 OFDMA Symbol Structu re and Sub-Channelization
The OFDMA symbol structure consists of three types of sub-carriers.
z Data sub-carriers for data transmission
z Pilot sub-carriers for estimation and synchronization purposes
z Null sub-carriers for no transmission; used for guard bands and DC carriers
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Figure 3-2 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 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 3-3 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 .
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Figure 3-4 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, aregrouped 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)]. AMCpermutation 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 ScalableOFDMA
The 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 .
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Figure 3-5 OFDMA Scalablity Parameters
3.4 TDD Frame Struc ture
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:
z 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.
z TDD assures channel reciprocity for better support of link adaptation,
MIMO and other closed loop advanced antenna technologies.
z Unlike FDD, which requires a pair of channels, TDD only requires a single
channel for both downlink and uplink providing greater flexibility foradaptation to varied global spectrum allocations.
z Transceiver designs for TDD implementations are less complex and
therefore less expensive.
Figure 3-6 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:
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z Preamble: The preamble, used for synchronization, is the first OFDM symbol
of the frame.
z Frame Control Head (FCH): The FCH follows the preamble. It provides the
frame configuration information such as MAP message length and coding
scheme andusable sub-channels.
z 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.
z 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.
z UL CQICH: The UL CQICH channel is allocated for the MS to feedback
channelstate information.
z UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ
acknowledgement.
Figure 3-6 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
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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
schemes supported in the Mobile WiMAX profile the optional UL codes and
modulation are shown in italics.
Table 3-1 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 3-2 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 takeadvantage 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 delaybetween 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.
Knowledge
4 Chapter4 WiMAX MAC Layer
z Kinds of Qos ----------------------------------------------------------------Level 1 2
z MAC Scheduling Service--------------------------------------------------Level 1 2
z Mobility Management------------------------------------------------------Level 1 2
z 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.
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4.1 Common MAC Concepts
The MAC layer consists of Convergence Sublayer, MAC CPS, and SecuritySublayer.
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.
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 4-1 MAC PDU format
The MAC header takes the format as follows:
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Figure 4-2 MAC header format
4.1.2.2 MAC-Layer Management Message
A MAC-layer management message takes the format as follows:
Figure 4-3 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 4-1 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.
Figure 4-4 Composition of Burst and MAC PDU
Burst
MAC Msg 1
MAC PDU 1
MAC Msg n
MAC PDU nPad
MAC HeaderMAC msg payload
(optional)
CRC
(optional)
4.2 Quality of Servi ce (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 4-5. 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.
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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 4-2).
Figure 4-5 Mobile WiMAX QoS Support
Table 4-2 Mobile WiMAX Application and Quality of service
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4.3 MAC Scheduling Servi ce
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:
z Fast Data Scheduler: The MAC scheduler must efficiently allocate availableresources 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 data packets 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.
z 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.
z Dynamic Resource All ocation: 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.
z QoS Oriented: The MAC scheduler handles data transport on a
connection-byconnection basis. Each connection is associated with a single
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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.
z 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
scheduling can enhance system capacity with a moderate increase in CQI
overhead in the UL.
4.4 Mobil ity Management
Battery life and handoff are two critical issues for mobile applications. Mobile
WiMAX supports Sleep Mode and Idle Mode to enable power-efficient MSoperation. 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 BaseStation, 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.
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4.4.2 Handoff
There are three handoff methods supported within the 802.16e standard HardHandoff (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 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, anAnchor 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 performed without
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:
z Key Management Protocol: Privacy and Key Management Protocol Version
2 (PKMv2) is the basis of Mobile WiMAX security as defined in 802.16e. Thisprotocol 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.
z 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 EAPprotocol. Key deriving methods are the only EAP methods supported.
z 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.
z Control Message Protection: Control data is protected using AES based
CMAC, or MD5-based HMAC schemes.
z 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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5 Chapter5 WiMAX Advanced Features Knowledge
z AntennaTechnolegies Basics-------------------------------------------------Level 1 2
z FFR-------------------------------------------------------------------------------Level 1 2
z MBS------------------------------------------------------------------------------Level 1 2
5.1 Smart Antenna Techno log ies
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:
z Beamforming: With beamforming, the system uses multiple-antennas to
transmit weighted signals to improve coverage and capacity of the system and
reduce outage probability.
z Space-Time Code (STC): Transmit diversity such as Alamouti code is
supported to provide spatial diversity and reduce fade margin.
z Spatial Multiplexing (SM): Spatial multiplexing is supported to take
advantage of higher peak rates and increased throughput. With spatialmultiplexing, 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.
The supported features in the Mobile WiMAX performance profile are listed in the
following table 5-1.
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Table 5-1 Advanced Antenna Options
Mobile WiMAX supports adiptive switching between these options to maximize the
benefit of smart antenna technologies under different channel conditions. 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 tomaximize spectral efficiency with no reduction in coverage area.
Figure 5-1 shows the architecture for supporting the smart antenna feature. 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 encountered are in
between the two extreme cases.
Table 5-2 Data Rates for SIMO/MIMO Configurations
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(For 10 MHz channel, 5 ms frame, PUSC sub-channel, 44 data OFDM symbols)
Figure 5-1 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.
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 following figure 5-2
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Figure 5-2 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 5-3, 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 interferenceconditions 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.
Figure 5-3 Fractional Frequency Reuse
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5.3 Multi cast 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:
z High data rate and coverage using a Single Frequency Network (SFN)
z
Flexible allocation of radio resources
z Low MS power consumption
z Support of data-casting in addition to audio and video streams
z 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 5-4 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
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.
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Figure 5-4 MBS Zones
6 Chapter6 WiMAX Network Architecture
Knowledge
z 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 Network
Working 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 an 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.
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(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.
Figure 6-1 IP-Based WiMAX Network Architecture
z 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.
z 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.
z 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
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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 (BSC), and an ASNGW analogous to the GSM model of BTS, BSC, and
Serving GPRS Support Node (SGSN).
z R1: Between MS and ASN
Protocols and Procedures as per the air interface
MAY include additional protocols related to the management plane
z R2: Between the MS and CSN
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Authentication, Services Authorization and IP Host Configuration
management.
z R3: Between the ASN and CSN
Control plane Protocols support AAA, policy and mobility management
capabilities.
z R4: between ASNs and ASN-GWs.
Control and Bearer plane Protocols
Originating / terminating various functional entities of an ASN that
coordinate MS mobility
z R6: between the BS and the ASN-GW.
Control and Bearer plane protocols for communication
z R8: between the base stations
Control plane message flows
Optionally Bearer plane data flows
Ensure fast and seamless handover.
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7 Chapter7 WiMAX Channel Estimation Knowledgez Introduction -----------------------------------------------------------------------Level 1 2
z Uplink Transmission--------------------------------------------------------------Level 1 2zChannel Estimation_Transmitter------------------------------------------------Level 1 2z Channel Estimation_Channel----------------------------------------------------Level 1 2z Channel Estimation_Receiver----------------------------------------------------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
the task difficult for the receiver. The main aim of the designer is to reduce thenumber 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.
7.2 Channel Estimation
The Block diagram (Figure 7-3) represents the whole system model or the signal
chain at base band. The block system is divided into 3 main Chapters 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
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decoding). The bit error rate (BER) plots have been obtained for at least 2000
errors to get