Basically difference between WiMAX and WiFi are distance, speed, cost, and so on.
WiMAX coverage area about 30 miles (50 km) and WiFi coverage area about 10 km.

WiMax (802.16) is is a newer standard of wireless networking designed to provide the last mile of high speed internet access to the end user. Some people would call Wimax WiFi on steroids but this would be to broad of an assessment. Wifi was and still will be used in LAN environments for the foreseeable future. WiMax was designed to provide (MAN) Metropolitan Area Access, to homes and businesses.

WiMax base stations will have the ability to provide approximately 60 businesses with T1 access and hundreds of homes with DSL/Cable speed access…in theory. Engineers are stating that WiMax has the capability of reaching 30 Miles but real world testing has shown 4-8 mile working radius.

WiMax (MAN) deployments are similar to a WiFi network. First the ISP would have their T3 or higher access. The ISP would then use line of sight antennas (Bridges) to connect to towers that would distribute the non line of sight signal to (MAN) residential/business clients.

WiMax line of sight antennas operate at a higher Frequency up to 60mhz. Distribution antennas do not have to be in the line of sight with their clients. Non – line of sight towers operate on a range similar to WiFi . WiMax can operate right next to cell phone towers with no interference.

WiMax networks are similar to Wifi in deployment. The Wimax Base station/Tower will beam a signal to a WiMax Receiver. Similar to a WiFi access point sending a signal to a laptop. As far as I can tell laptops will be shipping with Wimax receivers in 2006.

QOS (Quality of Service) is an major issue with WiMax because of the number of people accessing a tower at once. Some would think that a tower could be easily overloaded with a lot of people accessing it at once. Built into the WiMax standard is an algorithm that when the tower/base station is nearing capacity then it automatically will transfer the user to another WiMax tower or cell. Unlike a Wifi clients who have to kind of fight to stay associated with a given access point; WiMax will only have to perform this hand shake at the MAC level the first time they access the network.

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1 Cell

A Cell is the basic geographical unit of a cellular system. Commonly represented as a hexagon.

The term cellular comes from the hexagonal or honeycomb shape of the coverage area. Each cell has a BS transmitting over a cell. Because of constraints imposed by natural terrain and man made structure . The true shapes of cell are not hexagonal. The coverage area of cell is called the footprint. The BS simultaneously communicates with many mobile using one channel (pair of frequencies) per mobile. One frequency is for the forward link (Base station to the mobile) and other frequency is for the reverse link (Mobile to the Base station). Each cell size varies depending on landscape, subscriber density and demand within particular region. Cells can be added to accommodate growth e.g.; creating new cells by overlaying, splitting, or sectoring existing cells. This technique increases the capacity of the system. Sectoring existing cells and then using directional antenna can also increase capacity.

2 Cluster

A cluster is a group of cells. No frequencies are reused within a cluster. Frequencies used in one cell cluster can be reused in another cluster of cells. A large number of cells per cluster arrangement reduces interference to the system.

3 Frequency Reuse

Frequency reuse is a technique of allocating channels to the cellular system. Because of the unavailability of the spectrum at the cellular band, channels frequencies must be reused. Cells are assigned group of channels that are completely different from those of neighboring cells. Cells with the same number have the same set of frequencies, if the number of available frequency is 7, the frequency reuse factor is 1/7, which implies that each cell is using 1/7 of available frequencies . Frequency reuse introduces interference into the system.

In Cellular Communication there are two types Interference when we want to do Frequency Re-use. They are Known as: Adjacent Channel Interference (ACI), Co-Channel Interference (CCI).

1.Adjacent Channel Interference(ACI):

ACI is the interference between signal's having frequencies close together.

For example suppose channel 1 has frequencies 825.030 MHz (Mobile) and 870.030 MHz (BS) and channel 2 has frequencies 825.060 and 870.060 MHz. Channels 1 and 2 have frequencies close to one another, which will results in ACI.

2.Co-Channel Interference (CCI):
CCI is the interference between signal's having the same frequency(i.e; re-use frequency).
For example suppose channel 1 has frequencies 825.030 MHz (Mobile) and 870.030 MHz (BS).

Any othe signals having the frequencies of channel 1 825.030 (Mobile) and 870.030MHz (BS) are co-channel signals and will suffer from co-channel interference (CCI).

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Smart antenna systems refer to a class of antenna technologies designed to improve the received signal strength in a wireless access network. The intent is to improve the carrier to-interference plus noise ratio (CINR). The use of ‘smart’ antenna technologies can both increase received signal strength and decrease interference levels to greatly enhance performance in a mobile communication network

Smart antenna technologies involve complex vector or matrix operations on signals due to the use of multiple antennas. OFDMA allows smart antenna operations to be performed on vector-flat sub-carriers. Mobile WiMAX supports a full range of smart antenna technologies to enhance system performance. The smart antenna technologies supported are:

• Space-Time Code (STC)
• Beam forming
• Spatial Multiplexing (SM)

1 Space-Time Coding

Space-time coding (STC) is a family of techniques for implementing transmission diversity. Mobile WiMAX uses transmit diversity in the downlink direction to provide spatial diversity that enhance the signal quality to a specific subscriber located anywhere within the range of the antenna beam. Although providing less signal gain than beam-forming, transmit diversity is more robust for mobile users since it does not require prior knowledge of the path characteristics of a subscriber’s particular frequency channel. One such STC technique, known as the Alamouti Code, was published in 1998 and has been incorporated in the WiMAX standard

2 Beam-Forming

The transmission of signals from several antennas at specific relative phases can be used to create a much narrower antenna beam giving rise to the name ‘beam-forming’. Beam-forming provides substantial improvement in the link budget in both the downlink and uplink directions by increasing the effective antenna gain in addition to reducing fade margin requirements due to interference. Beam-forming does require knowledge of a subscriber’s location making it more challenging to implement for subscribers moving at high speeds. According to cellular network statistics however, the majority of subscribers are either stationary or only moving at pedestrian speeds thus enabling beam-forming to provide significant benefits for most usage models.

3 Spatial Multiplexing (SM)

Spatial multiplexing [30, 31] is supported to take advantage of higher peak rates and increased throughput. With spatial multiplexing, 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 Multiple Input Multiple Output (MIMO), SM doubles the peak date rate by transmitting two data streams. In up-load direction, each user has only one transmit antenna, two users can transmit in the same slot as if two streams are spatially multiplexed from two antennas of the same user.

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Mobility support in Mobile WiMAX is realized by three schemes of handoff: Hard Handoff (HHO), Fast Base Station Switching (FBSS) and Macro Diversity Handover (MDHO), where only the HHO is mandatory and the other two schemes are optional

1 FBSS

When the FBSS is supported, the Mobile Station (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 BSs in the Active Set.

Among the BSs in the Active Set, the anchor 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 an 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. An FBSS begins with a decision by an MS to receive or transmit data from the anchor BS that may change within the Active Set.

2 MDHO

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, the anchor BS is defined. When operating in MDHO, the MS communicates with all BSs in the Active Set for uplink and downlink unicast messages and traffic. MDHO begins when an MS decides to transmit or receive unicast messages and traffic from multiple BSs in the same time interval.

3 HHO

When the Mobile WiMAX unit switches from one Active Set area to another, it performs an HHO. Where the Break-Before-Make HO scheme is applied, service with the target BS starts after disconnection of service with the previous serving BS. Meanwhile, service starts before disconnection if the Make-Before-Break HO scheme is used

4. HARQ

HARQ is the function that integrates the existing Automatic Repeat Request (ARQ) function and an error correction technique (i.e., Chase combining (CC) and optionally Incremental Redundancy (IR)) that combines an error-detected packet with retransmission packets.

Error detection in the HARQ system is implemented by allocating a dedicated ACK channel in the uplink to provide feedback (i.e., ACK or NACK signaling) for fast retransmission in case the packet is in error. The receiver will keep the error packet and implement CC or IR to jointly process the packets in error and new transmission to improve the packet reception.

Mobile WiMAX supports multi-channel HARQ operation. 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. Mobile WiMAX provides signaling to allow fully asynchronous operation where the packet retransmission after receiving an NACK is determined by the Base Station scheduler. The asynchronous operation allows variable delay between retransmissions which gives more flexibility to the scheduler at the cost of additional overhead for each retransmission allocation.

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Mobile WiMAX

Mobile WiMAX is a rapidly growing broadband wireless access technology based on IEEE 802.16-2004 and IEEE 802.16e-2005 air-interface standards. Its a fast growing broadband access technology that enables low-cost mobile Internet applications, and realizes the convergence of mobile and fixed broadband access in a single air interface and network architecture. Mobile WiMAX combines OFDMA and advanced MIMO schemes along with flexible bandwidth and fast link adaptation, creating a highly efficient air interface that exceeds the capacity of existing and evolving 3G radio access networks. WiMAX networks, built on all-IP network architecture for plug and play network deployments, can support a mix of different usage and service models. While some consider mobile WiMAX as a candidate for the fourth generation of mobile networks, others view it as the first generation of mobile Internet technologies emerging from a wider ecosystem targeting to extend the success of WiFi over wide area networks supporting mobility.

Mobile WiMAX operates in 2 to 6 GHz range which is mainly licensed bands. Mobile WiMAX will be the most famous technology in the coming years .the figure below shows the mobile subscribers in the coming years, it has been predicted that up till 2010 WiMAX industry.

Main Features of Mobile WiMAX:

1. Mobile WiMAX Support Orthogonal Frequency Division Multiple Access as the radio access method for improved Multipath performance in Non-Line of Sight(NLOS) environments.

2. High Data Rates: The use of smart antenna technologies(ex: AAS, MIMO) sub-channelization schemes(OFDM) and adaptive modulation and coding schemes enables mobile WiMAX to support peakUL/DL data rate of 18.5Mbps per sector in a 10MHz channel.

3. Quality of Services: Mobile WiMAX supports connection-oriented QoS, providing accurate control for each service flow over the air interface. Moreover, since the orthogonal sub-channels develop no intra-cell interference in both downlink and uplink, the link quality and QoS can be easily controlled.

4. Scalability: The Mobile WiMAX technology utilizes Scalable-OFDMA(S-OFDMA) and has the capability to operate in scable bandwidths from 1.25 to 20MHz to comply with various spectrum allocations worldwide.

5. Security: The Mobile WiMAX incorporates the most advanced security features that are currently used in wireless acces system.These include Extensible Authentication Protocol(AEP) based authentication, Advanced Encryption Stssndard(AES) based authenticaton encryption and Cipher-based message authentication code(CMAC) and Hashed Message Authentication Code(HMAC) based control message protection schemes.

6. Mobility: Mobile WiMAX supports optimized handoff schemes, ensuring seanless services continuty with latency less than 50 milliseconds. The user mobility support is avantegeous for handling voice and other latency -sensitive applications.

7. Easy Implementation: Mobile WiMAX uses a simple protocol structure that contains fewer protocols as compared with 3G cellular system, thus enabling Easy Implementation.

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