Worldwide Interoperability for Microwave Access - WiMax

WiMax stands for Worldwide Interoperability for Microwave Access. It is a telecommunication technology that provides wireless transmission of data using variety of transmission modes from point to multi-point links to portable and fully mobile internet access.This technology provides up to 72 Mbit/s symmetric broadband speed without the need for cables. This technology is based on IEEE802.16 standard.WiMAX can provide broadband wireless access (BWA) up to 30 miles (50 km) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations. In contrast, the WiFi/802.11 wireless local area network standard is limited in most cases to only 100 - 300 feet (30 - 100m). 


                Working of WiMax
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 WiFi-style access will be limited to a 4-to-6 mile radius (perhaps 25 square miles or 65 square km of coverage, which is similar in range to a cell-phone zone). Through the 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.

                  WiMax Infrastructure
Typically, a WiMax system consists of two parts:

·       A WiMax Base Station- Base station consists of indoor electronics and a WiMax tower. Typically, a base station can cover up to 10 km radius (Theoretically, a base station can cover-up to 50 kilo meter radius or 30 miles, however practical considerations limit it to about 10km or 6 miles). Any wireless node within the coverage area would be able to access the Internet.
·       A WiMax receiver - The receiver and antenna could be a stand-alone box or a PC card that sits in your laptop or computer. Access to WiMax base station is similar to accessing a Wireless Access Point in a WiFi network, but the coverage is more.

                Topologies in urban and rural areas

Several base stations can be connected with one another by use of high-speed backhaul microwave links. This would allow for roaming by a WiMax subscriber from one base station to another base station area, similar to roaming enabled by Cellular phone companies. Several topology and backhauling options are to be supported on the WiMax base stations wire line back hauling (typically over Ethernet), microwave Point-to-Point connection, as well as WiMax backhaul. With the latter option, the base station has the capability to backhaul itself. This can be achieved by reserving part of the bandwidth normally used for the end-user traffic and using it for back hauling purposes.

The WiMax standard has been developed with many objectives in mind. These are summarized below:

· Flexible Architecture: WiMax supports several system architectures, including Point-to-Point, Point-to-Multipoint, and ubiquitous coverage. The WiMax MAC (Media Access Control) supports Point-to-Multipoint and ubiquitous service by scheduling a time slot for each Subscriber Station (SS). If there is only one SS in
the network, the WiMax Base Station (BS) will communicate with the SS on a Point-to-Point basis. A BS in a Point-to-Point configuration may use a narrower
beam antenna to cover longer distances.

· High Security: WiMax supports AES (Advanced Encryption Standard) and 3DES (Triple DES, where DES is the Data Encryption Standard). By encrypting the links between the BS and the SS, WiMax provides subscribers with privacy (against eavesdropping) and security across the broadband wireless interface. Security also provides operators with strong protection against theft of service. WiMax also has built-in VLAN support, which provides protection for data that is being transmitted by different users on the same BS.
· Quick Deployment: Compared with the deployment of wired solutions, WiMax requires little or no external plant construction. For example, excavation to support
the trenching of cables is not required. Operators that have obtained licenses to use one of the licensed bands, or that plan to use one of the unlicensed bands, do not need
to submit further applications to the Government. Once the antenna and equipment are installed and powered, WiMax is ready for service. In most cases, deployment of
WiMax can be completed in a matter of hours, compared with months for other solutions.

· Multi-Level Service: The manner in which QoS is delivered is generally based on the Service Level Agreement (SLA) between the service provider and the end-user. Further, one service provider can offer different SLA s to different subscribers, or even to different users on the same SS.

· Interoperability: WiMax is based on international, vendor-neutral standards, which make it easier for end-users to transport and use their SS at different locations, or with different service providers. Interoperability protects the early investment of an operator since it can select equipment from different equipment vendors, and it will continue to drive the costs of equipment down as a result of mass adoption.

· Portability: As with current cellular systems, once the WiMax SS is powered up, it identifies itself, determines the characteristics of the link with the BS, as long as
the SS is registered in the system database, and then negotiates its transmission characteristics accordingly.

· Mobility: The IEEE 802.16e amendment has added key features in support of mobility. Improvements have been made to the OFDM and OFDMA physical layers to support devices and services in a mobile environment. These improvements, which include Scaleable OFDMA,MIMO, and support for idle/sleep mode and
hand-off, will allow full mobility at speeds up to 160 km/hr.

· Cost-effective: WiMax is based on an open, international standard. Mass adoption of the standard, and the use of low-cost, mass-produced chipsets, will drive costs down dramatically, and the resultant competitive pricing will provide considerable cost savings for service providers and end-users.

· Wider Coverage: WiMax dynamically supports multiple modulation levels, including BPSK, QPSK, 16-QAM, and 64-QAM. When equipped with a highpower amplifier and operating with a low-level modulation (BPSK or QPSK, for example),WiMax systems are able to cover a large geographic area when the path between the BS and the SS is unobstructed.

· Non Line-of-Sight Operation: NLOS usually refers to a radio path with its first Fresnel zone completely blocked. WiMax is based on OFDM technology, whichhas the inherent capability of handling NLOS environments. This capability helps WiMax products deliver broad bandwidth in a NLOS environment, which otherwireless product cannot do.

· High Capacity: Using higher modulation (64-QAM) and channel bandwidth(currently 7 MHz, with planned evolution towards the full bandwidth specified in the standards), WiMax systems can provide significant

· Wireless Services: WiMax actually can provide two forms of wireless services. There is the non-line-of-sight, WiFi sort of service, where a small antenna on subscriber computer connects to the tower. In this mode, WiMAX uses a lower frequency range 2GHz to 11 GHz (similar to WiFi). Lower-wavelength transmissions are not as easily disrupted by physical obstructions -- they are better able to diffract, or bend, around obstacles.
    Advanced Features of WiMax

An important and very challenging function of the WiMax system is the support of various advanced antenna techniques, which are essential to provide high spectral efficiency, capacity, system performance, and reliability:
Beam forming using smart antennas provides additional gain to bridge long distances or to increase indoor coverage; it reduces inter-cell interference and
improves frequency reuse, Transmit diversity and MIMO techniques using multiple antennas take advantage
of multipath reflections to improve reliability and capacity.

Smart Antenna Technologies
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:

· Beam forming. With beam forming, the system uses multiple-antennas to transmit weighted signals to improve coverage and capacity of the system and reduce outage probability.
· Space-Time Code (STC). Transmit diversity such as Alamouti code is supported to provide spatial diversity and reduce fade margin.

· Spatial Multiplexing (SM). Spatial multiplexing 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 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.

Fractional Frequency Reuse
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 co channel interference (CCI) in frequency reuse one deployment, users at the cell edge may suffer degradation in connection quality. Users can operate on sub channels, which only occupy a small fraction of the whole channel bandwidth; the cell edge interference problem can be
easily addressed by appropriately configuring sub channel usage without resorting to traditional frequency planning. 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.
Multicast and Broadcast Service (MBS)
Multicast and Broadcast Service (MBS) supported by WiMax satisfy the following

o  High data rate and coverage using a Single Frequency Network (SFN)
o  Flexible allocation of radio resources
o  Low MS power consumption
o  Support of data-casting in addition to audio and video streams
o  Low channel switching time

The 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.


Some of the salient features supported by Mobile WiMax are:

· High Data Rates. The inclusion of MIMO (Multiple Input Multiple Output antenna techniques along with flexible sub-channelization schemes, Advanced Coding and Modulation all enable the Mobile WiMax technology to support peak DL data rates up to 63Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.

· Quality of Service (QoS) : The fundamental premise of the IEEE 802.16 MAC architecture is QoS. It defines Service Flows which can map to Diff Serv code points that enable end-to end IP based QoS. Additionally, sub channelization schemes provide a flexible mechanism for optimal scheduling of space, frequency
and time resources over the air interface on a frame by-frame basis.

· Scalability :  Despite an increasingly globalize economy, spectrum resources for wireless broadband worldwide are still quite disparate in its allocations. Mobile WiMax technology therefore, is designed to be able to scale to work in different
canalizations from 1.25 to 20 MHz to comply with varied worldwide requirements as efforts proceed to achieve spectrum harmonization in the longer term. This also allows diverse economies to realize the multifaceted benefits of the Mobile WiMax technology for their specific geographic .

· Security : Support for a diverse set of user credentials exists including; SIM/USIM cards, Smart Cards, Digital Certificates, and Username/Password schemes.

· Mobility : Mobile WiMax supports optimized handover schemes with latencies less than 50milliseconds to ensure real-time applications such as VoIP perform
without service degradation. Flexible key management schemes assure that security is maintained during handover.

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