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Chapter 2
Fundamental of WiMAX
What is WiMAX?
WiMAX (Worldwide Interoperability for Microwave Access) is a wireless communications standard designed to provide 30 to 40 megabit-per-second data rates, with the 2011 update providing up to 1 Gbit/s for fixed stations.
With the further development of the communication network, WiMAX has major realistic significance and strategic value as a standard facing to “the last kilometer” access, especially when no globally uniform standard is established for broadband wireless access. There are two main types of such standard: the IEEE 802.16d supporting air interface of fixed broadband wireless access system, and the IEEE
802.16e in the works supporting the air interface of both fixed and mobile broadband wireless access systems.WiMAX is a Broadband Wireless Access Metropolitan Area
Network (BWA-MAN) technology based on the IEEE 802.16 standard, which is also called the IEEE Wireless MAN. It is a new air interface standard in connection with the frequency ranges of microwave and millimeter wave. Its main purpose is to provide a broadband wireless access approach which can be interoperated effectively in the environment of multiple manufacturers with "one-point to multi-point" in the metropolitan area network. [1]

2.1 Types of WiMAX
The WiMAX family of standards addresses two types of usage models: a fixed-usage model (IEEE 802.16-2004) and a portable usage model (802.16 REV E, scheduled for ratification in current year). The basic feature that differentiates these systems is the ground speed at which the systems are designed to operate. Based on mobility, wireless access can be divided into four classes: stationary (0 km/hr), pedestrian (up to 10 km/hr), and vehicular (sub classified as “typical” up to 100 km/hr and “high speed” up to 500 km/hr). [2]
A mobile wireless access system is one that can address the vehicular class, whereas the fixed serves the stationary and pedestrian classes. The nomadic wireless access system, which is referred to as a system that worksas a fixed wireless access system but can change its location. An example is a WiMAX subscriber operating from one

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location, i.e., the office during daytime, and moving to another location, i.e., the residence in the evening. If the wireless access system works at both the locations, it can be referred to as nomadic.

2.1.1 Fixed
Service and consumer usage of 802.16 for fixed access is expected to mirror that of fixed wire line service, with many of the standards-based requirements being confined to the air interface. Because communication takes place via wireless links from CPE to a remote NLOS base station, requirements for link security are greater than those needed for wire line service. The security mechanisms within the IEEE 802.16 standards are adequate for fixed access service. An additional challenge for the fixed-access air interface is the need to establish high-performance radio links capable of data rates comparable to wired broadband service, using equipment that can be self installed indoors by users, as is the case for DSL and cable modems. IEEE 802.16 standards provide advanced physical (PHY) layer techniques to achieve link margins capable of supporting high throughput in NLOS environments.

Figure 2.1: Types of WiMAX

2.1.2 Portable or Mobile
The 802.16a extension, ratified in January 2003, uses a lower frequency of 2 to 11
GHz, enabling NLOS connections. The latest 802.16e task group is capitalizing on the new capabilities this provides by working on developing a specification to enable

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mobile 802.16 clients. These clients will be able to hand off between 802.16 base stations, enabling users to roam between service areas. There can be two cases of portability: full mobility or limited mobility. The simplest case of portable service
(referred to as Nomad city) involves a user transporting an 802.16 modem to a different location. Provided this visited location is served by wireless broadband service, in this scenario the user re authenticates and manually reestablishes new IP connections and is afforded broadband service at the visited location. In the fully mobile scenario, user expectations for connectivity are comparable to facilities available in third-generation (3G) voice/data systems. Users may move around while engaged in a broadband data access or multimedia streaming session. Mobile wireless access systems need to be robust against rapid channel variation to support vehicular speeds. There are significant implications of mobility on the IP layer owing to the need to maintain rout ability of the host IP address to preserve in-flight packets during IP handoff. This may require authentication and handoffs for uplink and downlink IP packets and MAC frames. The need to support low latency and low-packet-loss handovers of data streams as users’ transition from one base station to another is clearly a challenging task. For mobile data services, users will not easily adapt their service expectations because of environmental limitations that are technically challenging but not directly relevant to the mode of user (such as being stationary or moving). For these reasons, the network and air interface must be designed to anticipate these user expectations and deliver accordingly. [3]
IEEE 802.16e will add mobility and portability to applications such as notebooks and
PDAs. Both licensed and unlicensed spectrums will be utilized in these deployments.
802.16e is tentatively scheduled to be approved in the second half of this year.

2.2 How does WiMAX Work?
In practical terms, WiMAX would operate similar to Wi-Fi but at higher speeds, over greater distances and for a greater number of users. WiMAX could potentially erase the suburban and rural blackout areas that currently have no broadband Internet access because phone and cable companies have not yet run the necessary wires to those remote locations
A WiMAX system consists of two parts:
A WiMAX tower, similar in concept to a cell-phone tower - A single WiMAX tower can provide coverage to a very large area -- as big as 3,000 square miles (~8,000 square km). A WiMAX receiver, the receiver and antenna could be a small box or PCMCIA card, or

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they could be built into a laptop the way Wi-Fi access is today.
A WiMAX tower station can connect directly to the Internet using a high-bandwidth, wired connection (for example, a T3 line). It can also connect to another WiMAX tower using a line-of-sight, microwave link. This connection to a second tower (often referred to as a backhaul), along with the ability of a single tower to cover up to 3,000 square miles, is what allows WiMAX to provide coverage to remote rural area.

Figure 2.2: Simple diagram of WiMAX’s working process.

What this points out is that WiMAX actually can provide two forms of wireless service:


There is the non-line-of-sight, Wi-Fi sort of service, where a small antenna on your computer connects to the tower. In this mode, WiMAX uses a lowerfrequency range - 2
GHz to 11 GHz (similar to Wi-Fi). Lower-wavelength transmissions are not as easily disrupted by physical obstructions -- they are better able to diffract, or bend, around obstacles. 

There is line-of-sight service, where a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. The line-of-sight connection is stronger and more stable, so it's able to send a lot of data with fewer errors. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz. At higher frequencies, there is less interference and lots more bandwidth.
Wi-Fi style access will be limited to a 4-to-6 mile radius (perhaps 25 square mile¬s or
65 square km of coverage, which is similar in range to a cell-phone zone). Through the

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stronger line-of-sight antennas, the WiMAX transmitting station would send data to
WiMAX-enabled computers or routers set up within the transmitter's 30-mile radius
(2,800 square miles or 9,300 square km of coverage). This is what allows WiMAX to achieve its maximum range.
The final step in the area network scale is the global area network (GAN). The proposal for GAN is IEEE 802.20. A true GAN would work a lot like today's cell phone networks, with users able to travel across the country and still have access to the network the whole time. This network would have enough bandwidth to offer Internet access comparable to cable modem service, but it would be accessible to mobile, always-connected devices like laptops or next-generation cell phones. [4]

2.3 Network Architecture of WiMAX
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 micro mobility management functions, such as handoff triggering and tunnel establishment, radio resource management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host
Control Protocol) proxy, key management, session management, and multicast group management .
Access service network gateway (ASN-GW): The ASN gateway typically acts as a layer
2 traffic aggregation point within an ASN. Additional functions that may be part of the
ASN gateway include intra-ASN location management and paging, radio resource management, and admission control, caching of subscriber profiles, and encryption keys, AAA client functionality, establishment, and management of mobility tunnel with base stations, QoS and policy enforcement, foreign agent functionality for mobile IP, and routing to the selected CSN.

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Figure 2.3: IP-Based WiMAX network architecture
Connectivity service network (CSN): The CSN provides connectivity to the Internet,
ASP, other public networks, and corporate networks. The CSN is owned by the NSP and includes AAA servers that support authentication for the devices, users, and specific services. The CSN also provides per user policy management of QoS and security. The CSN is also responsible for IP address management, support for roaming between different NSPs, location management between ASNs, and mobility and roaming between ASNs.
The WiMAX architecture framework allows for the flexible decomposition and/or combination of functional entities when building the physical entities. For example, the
ASN may be decomposed into base station transceivers (BST), base station controllers (BSC), and an ASNGW analogous to the GSM model of BTS, BSC, and
Serving GPRS Support Node (SGSN). [5]

2.4 Protocol Architecture of WiMAX
This article describes WiMAXprotocol stack with functions of each in the system, which include WiMAX physical layer WiMAX MAC layer and upper layers. The same protocol stack is depicted below in figure with sub layers.

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Figure 2.4: WiMAX protocol stack
Physical Layer
WiMAX physical layer are of five types viz. SC, SCa, HUMAN, OFDM and OFDMA as described in IEEE 802.16-2004/802.16e standards. Any one out of these will be used in the system. For example fixed WiMAX uses OFDM type of physical layer and mobile
WiMAX uses OFDMA type. Physical layer takes MAC PDU consisting of MAC GMH,
MAC payload and CRC and perform following functions.
1.
2.
3.
4.
5.
6.

Scrambling
Forward error correction
Interleaving
Modulation
IFFT
Cyclic prefix insertion

7. Pass the IQ data to RF module for radio frequency modulation and transmission into the air, hence often this layer is referred as transmission layer. There are
2.5 GHz/3.5 GHz and 5.8 GHz frequency bands supported in WiMAX. [6]
MAC Layer

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MAC layer consists of three sub layers viz. MAC privacy sub layer, MAC common part sub layer and MAC convergence sub layer as mentioned in the figure.
MAC privacy sub layer
It does Authentication, encryption and key management functions.
MAC common part sub layer
It does ranging, scheduling, connection setup, bandwidth allocation, hybrid ARQ and
QoS functions. Various QoS schemes and applications of each supported in WiMAX are as follows. It is Connection oriented protocol, which assigns connection ID (A
16-bit value that identifies a connection to equivalent peers in the MAC) to each service flow on both uplink and downlink pair between BS and SS. Each service flow
(uniquely identified by a SFID, 32-bit value) has its own QoS parameter setting (latency, jitter & throughput).

MAC convergence sub layer
Functions performed by MAC convergence specific sub layer are mentioned below.
-Make upper layer frames compatible to be used by WiMAX MAC/PHY layers.
-Map upper layer addresses into WiMAX protocol addresses.
-Translate upper layer QoS fields into WiMAX MAC format and more
-Classify external network data and associate them to proper MAC service flow identifier (SFID) and connection id (CID)
- Handle TCP/IP based traffic.

2.5 Features of WiMAX
WiMAX is an adopted global standard for low-cost, high performance wireless broadband networks. One of the main goals of WiMAX is to make high quality, long-range data and voice communications affordable. This Next Generation technology platform offers several improvements in the operation of wireless broadband access networks.

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Coverage and Range: The maximum cell radius for a WiFi network is an eighth to a quarter of a mile. The range of WiMAX is 3 to 5 miles from the antenna, depending on the frequency and topography. This wider coverage for WiMAX means that more customers can be connected to the network with at a lower cost. With a lower investment in infrastructure, it is now possible to profitably serve rural areas with lower population densities, as well as economically depressed areas in cities with lower broadband adoption rates. WiMAX is designed to serve entire communities, a
Metropolitan Area Network (MAN), as municipal Wi-Fi, which uses unlicensed spectrum, failed to do. For rural areas, the current range of WiMAX networks is comparable to mobile phones.
Non-Line-of-Sight (NLOS) Service: Many 3G mobile phones and radio devices require a line-of-sight (LOS) connection between the access point and user’s modem. Mobile
WiMAX standard features a 256 carrier OFDM technology which significantly enhances the NLOS capabilities of the radio. This allows the operator to support more customers per cell site due to better signal penetration.
Higher Data Rates: WiMAX is able to achieve download speeds ranging up to 6
Megabits per second for fixed residential service and up to 4 Mbps for mobile users.
Residential WiMAX is able to achieve a higher data rate because it has a longer antenna. In the future, WiMAX data rates will be able to be increased by decreasing cell size in more populous areas, thereby giving users multiple access points.
Lower Consumer Premises Equipment (CPE) Cost: The cost of WiMAX modems and compliant devices will fall as multiple equipment vendors compete with one another in introducing their products. A proprietary system, such as LTE, does not take advantage of economies of scale, as much as an open standards technology platforms. Improved Quality of Service (QoS): The improved Quality of Service built into the
WiMAX technology platform will enable carriers to offer features designed for use by commercial enterprises. WiMAX will also be a pipeline for media-rich applications for personal use, such as gaming or multicast video (once IPv6 is introduced). [7]

2.6 Multiple Access Technology of WiMAX
The aim of this part is to describe the multiple access of WiMAX/802.16. It will be seen that the mechanisms of multiple access and radio resource sharing are rather complex. It can be said that they are more complex than in other known wireless

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systems such as GSM, Wi-Fi/IEEE 802.11 or even UMTS. Yet, globally, WiMAX multiple access is an extremely flexible F/TDMA (Frequency and Time Division Multiple
Access).
The concept of a service flow on a connection is central to the operation of the MAC protocol. Service flows in the 802.16 standard provide a mechanism for QoS management in both the uplink and downlink. Service flows are integral to the bandwidth allocation process. In this process, an SS requests an uplink bandwidth on a per-connection basis (implicitly identifying the service flow). Bandwidth is granted by the BS to an SS in response to per-connection requests from the SS. WiMAX has been called a Demand Assigned Multiple Access (DAMA) system.

2.6.1 Duplexing: Both FDD and TDD are Possible
The WiMAX/802.16 standard includes the two main duplexing techniques: Time
Division Duplexing (TDD) and Frequency Division Duplexing (FDD). The choice of one duplexing technique or the other may affect certain PHY parameters as well as impact on the features that can be supported. Next, each of these duplexing techniques will be discussed. Figure 2.5:Illustration of different FDD mode operations: broadcast, full duplex and half duplex. Half duplex SSs as SS 1 and 2.

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2.6.2 FDD Mode
In an FDD system, the uplink and downlink channels are located on separate frequencies. A fixed duration frame is used for both uplink and downlink transmissions. This facilitates the use of different modulation types. It also allows simultaneous use of both full-duplex SSs, which can transmit and receive simultaneously and, optionally, half-duplex SSs (H-FDD for Half-duplex Frequency
Division Duplex), which cannot. A full-duplex SS is capable of continuously listening to the downlink channel, while a half-duplex SS can listen to the downlink channel only when it is not transmitting on the uplink channel. Figure 6.1 illustrates different cases of the FDD mode of operation.
When half-duplex SSs are used, the bandwidth controller does not allocate an uplink bandwidth for a half-duplex SS at the same time as the latter is expected to receive data on the downlink channel, including allowance for the propagation delay uplink/downlink transmission shift delays.
2.6.3 TDD Mode
In the case of TDD, the uplink and downlink transmissions share the same frequency but they take place at different times. A TDD frame (see Figures 6.2 and 6.3) has a fixed duration and contains one downlink and one uplink sub frame. The frame is divided into an integer number of Physical Slots (PSs), which help to partition the bandwidth easily. For OFDM and OFDMA physical layers, a PS is defined as the duration of four modulation symbols. The frame is not necessarily divided into two equal parts. The TDD framing is adaptive in that the bandwidth allocated to the downlink versus the uplink can change. The split between the uplink and downlink is a system parameter and the 802.16 standard states that it is controlled at higher layers within the system.Mesh topology supports only TDD duplexing.

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Figure 2.6: TDD frame: uplink and downlink transmissions share the same frequency but have different transmission times. (From IEEE Std 802.16-2004. Copyright IEEE
2004, IEEE. All rights reserved.)

Figure 2.7:General format of a TDD frame (OFDM PHY). In the FDD mode, the downlink sub frame and uplink sub frames are transmitted on two separate frequencies as the uplink frame and downlink frame. The contents are the same for FDD and TDD.
Comparing the two modes, a fixed duration frame is used for both uplink and downlink transmissions in FDD while the TDD distribution is adaptive. Therefore TDD duplexing is more suitable when data rates are asymmetrical (between the uplink and downlink),
e.g. for an Internet transmission.
After settling the question of duplexing, many users have to share the bandwidth resource in each kind of transmission.

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2.7 Frequency of WiMAX
The WiMAX Forum currently supports continued rapid WiMAX user adoption in the 2.3
GHz, 2.5 GHz, and 3.5 GHz frequency bands, with additional spectrum bands to come.
At the heart of WiMAX technology is the base transceiver station, a central antenna which communicates with subscribers' antennas. The term point-multipoint link is used for WiMAX's method of communication.

2.7.1 Fixed WiMAX and WiMAX portable
The revisions of the IEEE 802.16 standard fall into two categories:
Fixed WiMAX, also called IEEE 802.16-2004, provides for a fixed-line connection with an antenna mounted on a rooftop, like a TV antenna. Fixed WiMAX operates in the 2.5
GHz and 3.5 GHz frequency bands, which require a license, as well as the license-free
5.8 GHz band.
Mobile WiMAX, also called IEEE 802.16e, allows mobile client machines to be connected to the Internet. Mobile WiMAX opens the doors to mobile phone use over IP, and even high-speed mobile services

Table 2.1:Frequency of fixed & mobile WiMAX
2.7.2 WiMAX and Quality of Service
The WiMAX standard natively supports Quality of Service(often called QoS for short), the ability to ensure that a service works when used. In practice, WiMAX lets bandwidth be reserved for a given purpose. Some applications cannot work properly when bottlenecks occur. This is the case for Voice over IP (VOIP), as spoken communication is ineffective when gaps a second long are introduced. [8]

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Table 2.2: WiMAX Standard Frequency

References
[1] WIMAX New Developments by Upena D Dalal and Y P Kosta.
[2] WiMAX Technologies Architectures, Protocols Resource Management and
Applications by Eugen Borcoci, University POLITEHNICA Bucharest, Electronics,
Telecommunication and Information Technology Faculty.

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[3] WiMAX A Study of Mobility and a MAC layer Implementation in GloMoSim by
Michael Carlberg Lax and Annelie Dammander.
[4] WiMAX Overview by Andrew Burnette Principal Analyst.
[5] http://www.tutorialspoint.com/wimax/wimax_quick_guide.htm