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With the compress of the 3RD generation cellular networks, 3G, the seek is already set on the way to the next generation. Future generation networks will be exemplified by unpredictable and high data rates, quality of services and flawless mobility both within a network and among networks of diverse technologies and service providers. An significant stage of mechanism in a universal next generation network is standardization to allow vendor independence and interoperability.
Technology development to achieve these properties, standardized by IEEE, is 802.16. It is generally called to as Worldwide Interoperability for Microwave Access (WiMAX).
IEEE 802.16 and WiMAX
The IEEE 802.16 group is produced in 1998 to expand an air-interface standard for wireless broadband. The group's original focal point was the expansion of a LOS-based point-to-multipoint wireless broadband system for stroke in the 10GHz-66GHz millimeter wave band. The follow-on standard the original 802.16 standard, finished in December 2001 was supported on a single-carrier physical (PHY) layer with a burst time division multiplexed (TDM) MAC layer. The group consequently formed 802.16a, an modification to the standard, to contain NLOS applications in the 2GHz 11GHz band, using an orthogonal frequency division multiplexing (OFDM)-based physical layer. More over revisions resulted in a new standard in 2004, called IEEE 802.16 2004, which substituted all previous versions and created the basis for the first WiMAX solution.
Table 2.1: Shows basic characteristics of WiMAX
The 802.16 standard delineates a figure of air interfaces that make use of a frequency band that is categorized into one of three categories: 10-66 GHz licensed bands, licensed bands below 11 GHz, and unlicensed bands below 11 GHz. Table 3.1 reviews the air interface designations and their applicable category of frequency bands.
Table 2.2: Shows different frequency band
The 10-66 GHz Licensed Bands
One of the first category of frequency bands is the 10-66 GHz licensed bands, these frequency bands with a petite wavelength that requires line-of-sight (LOS); however, the multipath interference is negligible. The bands permit for data rates in excess of 120 Mb/s and together with surroundings that is well suitable to the accomplishment of point-to-multipoint (PMP) access and the frequency bands fine suitable to small office/home office (SOHO) towards up to large office applications.
Below 11 GHz Licensed Bands
whereas still an element of the 10-66 GHz licensed bands category, the second category of licensed bands below 11 GHz needs a view due to the distinctiveness of the frequency bands. LOS is not necessary and the multipath interference may be significant Just because of longer bandwidth of the frequency range,. Since, the aptitude to encourage near-LOS and non-LOS (NLOS) applications requires additional PHY requirements to be fulfilled such as advanced power management techniques, interference mitigation/coexistence, and multiple antennas.
Below 11 GHz License-exempt Bands
The closing category of frequency bands are the license except frequencies below 11 GHz, classically using the 5-6 GHz bands. The operating environment of these bands is like to those of the 11 GHz licensed bands; though, the license-exempt position of the bands bring in additional interference from the coexistence of other devices within the range, as well as rigid constraints that bound the radiated power. To trounce these harms mechanisms such as dynamic frequency selection is introduced to the PHY and MAC of the air interface.
Different version of IEEE 802.16 standards
Different versions of WiMAX are also shown in Figure 2.1.
IEEE 802.16 - 2001
IEEE 802.16 -2001 released in December 2001. It runs in the 10-66 GHz range, and it have only with line-of sight operations. The data rate is operated up to 134 Mbps. The original IEEE 802.16-2001 specification defined a set of MAC and PHY layer standards proposed to give fixed, broadband wireless access in a point-to-point or point-to-multipoint (PMP) setting.. Where IEEE 802.11 relies on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to determine when nodes in the network are allowed to transmit, the IEEE 802.16-2001 MAC uses an entirely different paradigm to control transmissions.  Importantly, IEEE 802.16-2001 incorporates features that provide differential Quality of Service (QoS) down to the PHY layer. QoS support is built around the concept of service flows that are identified, appropriately enough, by a Service Flow ID. Service flows are characterized by their QoS Parameters, which can be used to specify parameters such as maximum latency and tolerated jitter. Service flows are unidirectional, and may originate at either BS or SS. Higher layer mechanisms, such as Diff-Serv (differentiated service architecture), must be employed in conjunction with IEEE 802.16's service flows to ensure end-to-end QoS. 
IEEE 802.16a - 2003
IEEE 802.16a -2003 released in January 2003. It operates in 2-11 GHz range, and support NLOS. The data rate can be operated at 70Mbps. IEEE 802.16a was a major revision to the basic standard, ratified by the IEEE Standards Board in January 2003 . Most importantly, the IEEE 802.16a extension added support in the 2 - 11 GHz Licensed bands, which opens up many potential markets for the technology. Non Line of Sight (NLOS) operation becomes possible when operating in the 2 - 11 GHz range, extending the geographic reach of the network. Multipath propagation can also become an issue. IEEE 802.16a includes both PHY specification and enhancements to the MAC layer to deal with multipath propagation and interference mitigation. Features were added to allow advanced power management techniques and adaptive antenna arrays.
Security enhanced, with many of the privacy layer features now required elements.IEEE 802.16a also affix voluntary support for Mesh networks, where traffic routed from subscriber station to subscriber. This is a transform from the PMP mode, where traffic is only permissible between BS and SS. suitable accompaniments to the MAC layer specification made to let permit for scheduling the transmissions of SS are part of the Mesh, but not visible to the BS. 
IEEE 802.16- 2004
IEEE 802.16-2001, 802.16a and 802.16c were incorporated into IEEE 802.16-2004 approved on 24 June 2004 and was available in September 2004. The amendment initially developd as a set of system provisions titled as IEEE 802.16-REVd, but was broad enough to classify as a complete reissue of the basic IEEE 802.16 standard.
802.16 2001 10-66GHz
802.16 2004 2-66GHz
802.16 C 10-66GHz
802.16 D 2-11GHz
802.16 E 2-6GHz
Original Fixed Wireless Broadband
Fixed Wireless broadband (Lower Frequency)
Updated Fixed Wireless Broadband
Mobile Wireless Broadband
Figure 2.1: Shows emerging of WiMAX standards
WiMAX is broadband wireless that offer loaded features with a lot of elasticity in term of potential service and consumption. Some of the features of WiMAX are discussed below; 
WiMAX ropes tough encryption, using Advanced Encyption Standard (AES), and has a robust privcy and key-management protocol.
The mobile WiMAX departure has mechanism to hold up protected seamless handovers for delay-tolerant full-moblity applications, example VoIP and video sensitive traffic.
High Quality of Service
The WiMAX MAC layer connection-oriented architecture to support a diversity applications, together with voice and multmedia services.
Magnificent Peak Data Rate
WiMAX is proficient of sustaining very high peek data rates. Even, the peak PHY data rate can be as elevated as 74Mbps when in use using a 20MHz wide spectrum.. These peak PHY data rates are reached using 64 QAM modulations with rate 5/6 error-corection coding. below very good signal conditions, yet higher peak rates may be accomplished by using multiple antennas and spatial multiplexing.
OFDM Base Physical Layer
The WiMAX physical layer (PHY) is stands on orthogonal frequency division multiplexing, a system presents good resistance to multipath, permits WiMAX to work in NLOS circumstances. OFDM is now extensively known as the method for extenuating multipath for broadband wireless.
Support for TDD and FDD
WiMAX time division duplexing and frequency division duplexing, and half-duplex FDD, which allocates for a low-cost implementation. TDD is preferential by a bulk of implementations because of its recompenses: (1) flexibility in choosing uplink-to-downlink data rate ratios, (2) capability to make use of channel reciprocity. All the initial WiMAX profile are based on TDD, except for two fixed WiMAX profiles in 3.5GHz.
Link Layer Retransmission
connections which require improved reliability, WiMAX goes for automatic retransmission requests (ARQ) over the link layer. ARQ-enabled connections have need of each transmitted packet to be accepted by the receiver; unacknowledged packets are considered to be lost and are retransmitted.
Support for Advance Antenna Technique
The WiMAX has a enough number of clips assembled into the physical-layer plan, which allocates for the use of multiple-antenna techniques, as beam forming, space-time coding, and spatial multiplexing. To recover the overall system capacity these schemes used and spectral efficiency by installing multiple antennas at the transmitter and/or the receiver.
Adaptive Modulation and Coding (AMC)
WiMAX ropes modulations and forward error correction (FEC) coding schemes and allows the scheme to be altered on per user and per frame basis, based on channel situations. AMC is an efficient mechanism to make the most of throughput in a time-varying channel. Where as adaptation algorithm naturally labels for the use of the maximum modulation and coding scheme such that each user is connected with the highest possible data rate can be sustained in their respective links.
The IEEE 802.16 operational group does not intend to restore the IEEE Std 802.11 with the IEEE Std 802.16, rather the IEEE Std 802.16 serve as a balance of the IEEE Std 802.11. The IEEE Std 802.16 is planned for WMANs, whereas the IEEE Std 802.11 is planned for WLANs. Therefore, the IEEE Std 802.16 enable users to access into wireless network on a superior scale. Because of its better coverage, faster speed, and larger figure of users per base station, the IEEE Std 802.16 tends to be used widely as backhaul and last-mile solutions for Wi-Fi networks. Wi-Fi access points, wireless network to supply the core network baekholeuro signal is able to use the IEEE 802.16 standard solution. As a last-mile key, an internet service provider (ISP) could mount the IEEE 802.16 technology, as an alternative of cables, DSL, and T1, to deliver services to subscribers. It gives advantages over the traditional wired technology in case that cabling is not feasible or too expensive, for example, implementing in rural areas or developing countries where lack wired network infrastructures.
Figure. 2.2: An example of the IEEE 802.16 network
Figure 2.2 shows two usage models of the IEEE Std 802.16. The IEEE Std 802.16 base station provides wireless access services to fixed and portable subscribers. The fixed subscribers could be residential subscribers or businesses in buildings. For residential subscribers, the standard provides broadband wireless access as a last-mile solution, in place of using cable modem. The IEEE 802.16 technology can be a substitute to DSL or leased line services for small and medium size businesses, as well as a substitute to T1/E1 level services for enterprises. Users in the buildings can join to the network via Ethernet/IEEE 802.3 or WLAN/IEEE 802.11 standards. For mobile users, the IEEE Std 802.16e which is an amendment to IEEE Std 802.16-2004 adds the capability to provide services. The users can openly connect to the network using devices, as personal digital assistants (PDAs), smart phones, and mobile laptops.