OVERVIEW OF IEEE 802.16J

In IEEE 802.16j low cost RSs are introduced to provide enhanced coverage and capacity. Using such stations, an operator could deploy a network with wide coverage at a lower cost than using only (more) expensive BSs to provide good coverage, and increasing significantly the system throughput. As network utilization increases, these RSs could be replaced by BSs as required. The mesh architecture defined in WiMAX is already used to increase the coverage and the throughput of the system. However, this mode is not compatible with the point-to-multipoint (PMP) mode with no support of the OFDMA PHY, fast route change for mobile station (MS), etc. Hence, the standards organization has recognized this as an important area of development, and today a task group is charged with drafting a new standard: the IEEE 802.16j mobile multihop relay design to address these issues. The first draft of the IEEE 802.16j standard has just finished in August 2007.

IEEE 802.16J SCOPE

The IEEE 802.16j is aiming to develop a relay mode based on IEEE 802.16e by introducing RSs depending on the usage model:

• Coverage extension
• Capacity enhancement

In other words, the relay technology is first expected to improve the coverage reliability in geographic areas that are severely shadowed from the BS or to extend the range of a BS. In both cases, the RS enhances coverage by transmitting from an advantageous location closer to a disadvantaged SS than the BS. Second, it is expected to improve the throughput for users at the edges of an 802.16 cell. It has been recognized in previous 802.16 contributions that subscribers at the edges of a cell may be required to communicate at reduced rates. This is because received signal strength is lower at the cell edge. Finally, it is expected to increase system capacity by deploying RSs in a manner that enables more aggressive frequency reuse. Figure 1 illustrates the different scenarios in which relay mode could be used. However, introducing such RSs considerably alters the architecture of the network and raises many issues and questions. It is still unclear what system design is appropriate and can be realized at a low cost while still providing good coverage with an enhancement of the throughput.

 
Figure 1: IEEE 802.16j example use cases.

The 802.16j task group’s scope is to specify OFDMA PHY and MAC enhancement to the IEEE 802.16 standards for licensed bands. These specifications aim to enable the operation of fixed, nomadic, and mobile RSs by keeping the backward compatibility with SS/MS. In other words, the standard will define a new RS entity and modify the BS to support Mobile Multihop Relay (MMR) links and aggregation of traffic from multiple sources. An MMR link represents a radio link between an MMR-BS and an RS or between a pair of RSs. Such link can support fixed, portable, and mobile RSs and multihop communications between a BS and RSs on the path. An access link is a radio link that originates or terminates at an SS/MS. Table 1 illustrates the main scope of the project.

Table 1: IEEE 802.16j Project Scope 

		Define New
No Change	Changes to BS	RS Entity	“802.16j Relay” Link Air Interface
To SS/MS To 802.16e OFDMA PMP link	Add support for MMR links Add support for aggregation of traffic from multiple RSs	Supports PMP links Supports MMR links Supports aggregation of traffic from multiple RSs	Support fixed, portable, and mobile RSs Based on OFDMA PHY MAC to support multi-hop communication Security and management

MAC LAYER OVERVIEW | MOBILE WiMAX

The WiMAX forum has designed the physical and MAC layers of the mobile WiMAX based on the amendments of the IEEE 802.16e to offer broadband services including voice, data, and video at the metropolitan scale for mobile users. The mobile WiMAX network comprises BSs and subscriber stations (SSs). Each SS is assigned a 48-bit MAC universal address that it used to uniquely identify it toward a BS. Mobile WiMAX uses UL and DL maps to prevent collisions. More specifically, SSs implement the time division multiple access (TDMA) to share the UL while BSs use the time division multiplexing. 

 

UL and DL schedules are exchanged between the BS and the managed SSs in every frame using the UL-MAP and the DL-MAP messages. The MAC layer is connection-oriented; besides, all data communication is associated with a connection. Each connection with its QoS parameters forms a service flow and is identified by a 16-bit connection identifier (CID). MAC layer connections can be compared to TCP connections. 

 

In fact, thanks to TCP, a computer may have simultaneously different active connections for different applications using different ports. With MAC connections, an SS may have many connections to a BS for different services such as network management or user data transport; every connection is characterized by its own bandwidth, security, and priority parameters. When a new SS joins the network, the managing BS assigns to it three CIDs with different QoS requirements used by different management levels which are the basic, the primary, and the secondary management connections. The basic connection enables the transfer of short, time-critical MAC and radio link control messages; the primary management connection is used to transfer larger but more delay-tolerant messages while the secondary management connection is used to transfer standards-based management messages such as DHCP ones. Note that a CID may carry traffic for many different higher-layer sessions. The mobile WiMAX version adapts dynamic modulation and forward error codes to correctly serve the SSs located far from the BS in the rural areas and resists the weather conditions. 

 

Both the BS and the SS can adapt the transmission of the burst profiles by lowering the bandwidth for higher robustness. For instance, the BS always begins by adopting the most robust modulation and forwards error code scheme so that all SSs in the coverage area can correctly receive the DL-MAP and the UL-MAP messages. Meanwhile, an SS may ask its BS to have a longer UL window when needed. The BS and the managed SSs exchange MAC protocol data units (PDUs) carrying MAC management messages or convergence sublayer MAC service data units (MAC SDUs). The MAC PDU has a fixed MAC header, a variable length payload and a cyclic redundancy check (CRC) field. The MAC header may be a generic MAC header (GMH) or a bandwidth request header. The GMH is used to transfer the standard MAC management messages while the bandwidth request header is a header sent without payload to request additional bandwidth. 

 

Both the payload and the CRC fields are optional. It is worth noticing that the MAC header and the MAC management messages are never encrypted to facilitate registration, ranging, and normal operation of the MAC sublayer; however, this decision has opened the door to eavesdropping and other serious attacks. An SS which enters the network may be programmed to register with a certain BS; but generally speaking, it begins by scanning its frequency to detect an operating channel. Scanning consists in listening to each possible frequency until the frame preamble is heard. After detecting that channel, the SS tries to synchronize to the DL transmission by waiting for the DL-MAP stating the map of the timeslot locations in use for the frame. After that, the MS waits for the downlink channel descriptor (DCD) and the uplink channel descriptor (UCD) messages that are periodically broadcasted to specify the modulation and the FEC schemes used on the carrier. After gathering the required information describing the parameters needed for initial ranging transmission, the SS scans the UL-MAP to find an opportunity to perform the ranging. Initial ranging is used to determine the transmit power requirements of the MS to reach the BS. It is worth noticing that each SS should be informed about when to send the ranging request as many SSs may try to join simultaneously the network and highly affect the networks efficiency. Therefore, each new SS will send a ranging request (RNG-REQ) message and wait for the corresponding ranging response (RNG-RSP) message, indicating the timing advance, the power adjustment, and the basic and the primary management CIDs. After correctly determining the timing advance of SS transmissions, the SS and the BS will continue exchanging RNG-REQ and RNG-RSP messages until an acceptable radio link is established. Then, the SS should perform the authentication and the registration processes to enter the network. First, the BS asks the SS for strong authentication. Upon successful authentication, the SS will be able to register to the network. The SS will then establish a secondary management CID to receive secondary messages for different services. For instance, the SS will get an IP address through DHCP and a Trivial File Transfer Protocol (TFTP) address to request configuration files when needed.

QoS is guaranteed thanks to five QoS classes implementing different scheduling mechanisms. Those classes are the unsolicited grant service (UGS), the real-time polling service (rtPS), the extended rtPS (ErtPS), the nonreal-time polling service (nrtPS), and the best effort (BE). UGS fulfils the requirements of real-time communications occurring at periodic intervals such as voice over IP (VoIP); it is given a grant by BS that accommodates the maximum sustained traffic rate of such applications. rtPS uses unicast polling to support periodic real-time variable-size transmissions such as MPEG video streams, but generates more overhead than UGS. ErtPS is a combination of UGS and rtPS; in fact, the bandwidth is allocated without solicitation, but the allocation is done in a dynamic fashion. ErtPS fits well in the case of voice with activity detection like VoIP with silence suppression applications. nrtPS uses unicast polling regularly and allows contention requests to guarantee a minimum data rate for delay-tolerant applications, which generate variable-sized traffic such as FTP. Finally, the BE class guarantees a minimum QoS level for the associated traffic. Therefore, the SSs are never polled individually but are rather permitted to use contention requests and unicast requests.

PHYSICAL LAYER OVERVIEW | MOBILE WiMAX

802.16e WiMAX system profiles will cover 5, 7, 8.75, and 10MHz channel bandwidths for licensed spectrum allocations in the 2.3, 2.5, 3.3, and 3.5 GHz frequency bands. Targeting a worldwide coverage, the WiMAX forum intends to promote the allocation of frequency bands inferior to 6 GHz for civilian applications while considering the available spectrum all over the world. For instance, the 3.5 GHz frequency band is already assigned to fixed services in many countries, the 2.3 GHz band was reserved for the deployment of the WiBro solution in South Korea, while the 2.5 and 2.7 GHz bands have been assigned by the United States for fixed and mobile WiMAX deployment. 

 

Considering the modulation technique, mobile WiMAX is based on the orthogonal frequency division multiple access (OFDMA) technique and particularly on the SOFDMA variant. The frequency division multiplexing (FDM) principle consists in using multiple frequencies to transmit different signals in parallel. This is achieved by assigning a frequency range or subcarrier to each signal then modulating it by data. The multiple subcarriers used for transmitting different signals should be separated by guard bands to prevent interferences. 

 

Orthogonal FDM (OFDM) eliminates the guard bands by using overlapping subcarriers that are spaced apart at precise frequencies; thus achieving orthogonality. With OFDM, the center of the modulated carrier coincides with the edge of the adjacent carrier so that the independent demodulators performing a discrete Fourier transform see only their own frequencies. Redundancy may be guaranteed by scattering some bits over some sets of distant subcarriers. The OFDMA is a multiple access and multiplexing scheme that multiplexes data streams generated by multiple users onto the downlink (DL) subchannels and fulfils uplink (UL) multiple access by means of UL subchannels. The OFDMA symbol structure is made up of data subcarriers for data transmission, pilot subcarriers for estimation and synchronisation purposes, and null subcarriers used for guard bands and DC carriers. Data and pilot subcarriers are organized in a subset of subcarriers called subchannels. The minimum frequency–time resource unit of subchannelization is one slot equal to 48 data tones. 

 

It is worth noticing that there exist two types of subcarrier permutations for subchannelization which are the diversity permutation and the contiguous permutation. The diversity permutation organizes the subcarriers in a pseudorandom fashion to form a subchannel while the contiguous permutation organizes a block of contiguous subcarriers forming a bin. The bin is formed by nine contiguous subcarriers in a symbol with eight assigned for data, and one assigned for a pilot. Generally speaking, diversity subcarriers permutation achieves a high performance with the mobile applications while the contiguous subcarrier permutation fits well to fixed, portable, or low mobility environments. 

 

The SOFDMA technology optimizes the mobile access by assigning a set of subcarriers to particular users. For instance, subcarriers 1, 3, and 7 may be assigned to user 1 while subchannels 2, 5, and 9 to user 2 and so on. Users close to the base station (BS) will benefit from a larger throughput by getting an important number of subchannels with a high modulation scheme. SOFDMA offers multiple bandwidths to allow multiple spectrum allocation and fulfil different usage-model requirements. In fact, the scalability is guaranteed by adjusting the fast Fourier transform (FFT) size with respect to the available bandwidth while fixing the subcarrier spacing at 10.94 kHz. As the resource unit-subcarrier bandwidth and the symbol duration are fixed, scaling bandwidth will not affect higher layers. 

 

The first release of mobile WiMAX will adopt a system channel bandwidth of, respectively, 5 and 10 MHz, with a sampling frequency of, respectively, 5.6 and 11.2 MHz. The FFT size will be either 512 or 1024 while the number of subchannels will be either 8 or 16 and the useful symbol time will be fixed to 91.4 ms. The OFDMA symbol duration is 102.9 ms while the number of OFDMA symbol within a 5 ms frame is 48. The modulation scheme will gradually vary from 16 quadrature amplitude modulation (QAM) to quaternary phase shift keying (QPSK) (four channels) and even binary phase shift keying (BPSK) (two channels) at longer ranges while the power allotted to each channel will be increased. The adaptation of multiple-input multiple-output (MIMO) antenna technique along with advanced coding and modulation achieve peak DL data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.

 

IEEE 802.16e supports both time division duplex (TDD) and frequency division duplex (FDD) duplexing modes. TDD is adopted when the license-exempt spectrum is used because a unique channel is shared between the UL and the DL traffics which occupy different time slots. Contrarily to TDD, FDD operates with two channels, one is dedicated to the UL traffic and the second is reserved to the DL traffic. Mobile WiMAX in its first release supports only the TDD duplexing mode although the WiMAX forum intends to address particular market opportunities by supporting FDD in future releases. With TDD, it becomes feasible to adjust the DL/UL ratio with respect to the nature of the ongoing traffic so that the DL/UL asymmetric traffic is efficiently supported. Besides, TDD guarantees channel reciprocity for better support of link adaptation, MIMO, and other advanced antenna technologies. Meanwhile, TDD offers a greater flexibility for adaptation to varied spectrum allocations as it uses the same channel for both UL and DL traffics. Last but not least, transceivers designed for TDD implementations are less complex and less expensive. An OFDM frame structure for a TDD implementation is divided into DL and UL subframes separated by transmit/receive and receive/transmit transition gaps (TTG and RTG) to eliminate DL and UL transmission collisions. The frame control information is carried by the preamble, the frame control header (FCH), the DL-MAP, and UL-MAP, the UL ranging, and the UL fast channel feedback (UL CQICH) fields. More specifically, the preamble field appears as the first OFDM symbol in the frame and it is used for synchronization. The FCH field identifies the frame configuration information including the MAP message length and coding scheme and usable subchannels. DL-MAP and UL-MAP provide subchannel allocation for the DL and the UL subframes while the UL ranging subchannel, which is allocated for the mobile stations (MSs), is used for closed-loop time, frequency, and power adjustments as well as bandwidth requests. Finally, the UL CQICH enables the MSs feedbacking the channel-state information to the managing entities.

The WiMAX forum included advanced physical layer features to enhance the mobile WiMAX network coverage and capacity. For instance, mobile WiMAX supports the mandatory QPSK, 16 quadrature amplitude modulation (QAM), 64 QAM schemes in the DL, and the optional 64 QAM in the DL. Meanwhile, both convolutional code (CC) and convolutional turbo code (CTC) coding schemes are supported along with two other optional coding schemes, which are the block turbo code (BTC) and low density parity check code (LDPC). The combination of various modulation and code rates results in a fine resolution of data rates so that the BS scheduler may determine the best suited data rate for each burst allocation, based on the buffer size and the channel propagation conditions at the receiver. Moreover, a channel quality indicator (CQI) channel is used for providing channel-state information from the MSs to the base station scheduler while other channel-state information can be provided to the BS by the CQICH, which includes the physical carrier to interference plus noise ratio (CINR), the effective CINR, the MIMO mode selection, and the frequency selective subchannel selection. Hybrid auto repeat request (HARQ) is supported to provide fast response to packet errors and improve the cell edge coverage through using N channel Stop and Wait protocol. The previously stated adaptive modulation and coding, CQICH and HARQ, guarantee robust link adaptation in mobile environments at vehicular speeds exceeding 120 km/h.

Overview of the Mobile WiMAX Specification

IEEE 802.16 Standard

The IEEE 802.16 standard, which includes Medium Access Control (MAC) and physical (PHY) layer specifications, aims at supporting Internet services over wireless metropolitan area networks (WMAN). It is also an alternative to traditional wired networks, such as Digital Subscriber Line (DSL) and cable-modem. There are two modes defined in WiMAX networks: point-to-multiple-points (PMP) mode and mesh mode.

In the PHY layer, the IEEE 802.16 standard adopts the orthogonal frequency division multiplexing (OFDM), which is a multicarrier modulation scheme. The IEEE 802.16 standard has two OFDM-based modes: OFDM and orthogonal frequency division multiplexing access (OFDMA). Both of these technologies allow subcarriers to be adaptively modulated (e.g., QPSK, 16-QAM, and 64-QAM), depending on transmission distance and noise. Moreover, OFDMA has scalability to provide efficient use of bandwidth.

The MAC layer of IEEE 802.16 standard was originally designed for the PMP mode. On the later amendments of the IEEE 802.16a and the IEEE 802.16d, the mesh mode was included. The IEEE 802.16a adopts OFDM to provide greater spectral efficiency and to mitigate interference. IEEE 802.16b covers most of the quality of service (QoS) aspects. The IEEE 802.16e introduces scalable OFDMA into the standard, and supports mobile communications. With handover mechanisms, WiMAX is thus able to support mobile communications at vehicular speeds. We summarize the history of the evolution of the IEEE 802.16 standard in Figure 1.

Figure 1: Evolution of the IEEE 802.16 standard.

The IEEE 802.16 working groups on broadband access standards developed the IEEE 802.16 WirelessMAN standard for WMANs. On the other hand, the WiMAX forum was formed in June of 2001 to ensure interoperability among 802.16 products from different vendors. These groups and their activities may help popularizing WiMAX networks and systems by bringing vendors together and improving the specifications.

According to the IEEE 802.16 specification, the non-line-of-sight (NLOS) transmission range is 4 miles. With the combination of soft-switch technologies, a WiMAX network can work as a wireless "last mile" and make a viable alternative to the Public switched telephone network (PSTN) for VoIP services. In addition, a WiMAX network can work as a point-to-point backhaul trunk with a transmission capability of 72 Mbps at a transmission distance over 30 miles. With its technological advantages of throughput, power, transmission range, and versatility, WiMAX might be a strong competitor of other technologies, such as WiFi and 3G. Therefore, from both economical and technical points of view, WiMAX could be an appealing choice for broadband wireless services.

WiMAX Network Architecture

WiMAX has an IP-based wireless access architecture, which contains three parts: user terminal devices, access service network (ASN), and core service network (CSN). A user terminal device can be a fixed or portable/mobile terminal device, which supports the fixed/nomadic/mobile usage scenarios. Each device can establish a connection link to a WiMAX Base Station (BS), and perform authentication and registration through an access gateway in the CSN. The system architecture is illustrated in Figure 2.

Figure 2: Internet Protocol (IP)-based wireless access architecture of WiMAX.

A mobile WiMAX network has a similar architecture as a cellular network, where PMP links are between each BS and multiple Subscriber Stations (SSs). Each BS provides frequency and timing reference to SSs for synchronization purpose. The detailed MAC layer protocols and message sequences will be described later.

Scheduling Services

Overview

Scheduling services are the medium access control functions (data flow control) that define how and when devices will receive and transmit on a communication system. The types of services that WiMAX can provide range from guaranteed bandwidth with low delay unsolicited grant service (UGS) to random access best effort (BE) service. WiMAX systems use a grant management system to coordinate the request for new services and changes to existing services (such as requesting more bandwidth). The WiMAX system uses a combination of time division multiple access, polling and contention based flow control to provide specific types of services to users.

Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a subscriber communicates on a WiMAX system using TDMA, he/she is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems can guarantee a constant data rate with a minimal amount of flow control overhead.

Polling is the process of sending a request message (usually periodically) for the purpose of collecting events or information from a network device. The receipt of a polling message by a device starts an information transfer operation for a specific time period. Polling may be performed with specific units (unicast), to groups of units (multicast) or to all units (broadcast).

Unicast polls are requests for data transmission or responses to commands that are only sent between a sender (polling device) and receiver (polled device). When a subscriber station is responding to a unicast polling message, no other devices are allowed to transmit.

Multicast polls are requests for data transmission or responses to commands that are sent from a polling device to several receiving devices which are part of a multicast group. When a device receives a multicast polling message for its group, it will respond if it has data to send. When a subscriber station is responding to a multicast polling message, others may also have information to transmit. For multicast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

Broadcast polls are requests for data transmission or responses to commands that are sent from a polling device to all devices that are able to receive its broadcasted polling message. When a device receives a broadcast polling message, it will respond if it has data to send. For broadcast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

The amount of time between polling messages is called the polling cycle. The time between polling cycles is a balance between delay (more polling messages is less delay) and overhead (more polling messages increases the percentage of data that is used for control messages).

Figure 1 illustrates the different types of polling that are used in the WiMAX system. A device that is part of a multicast group, has received a multicast polling message, must compete for access to send its data. Finally, for a broadcast polling message, any device that has data will compete for access to send its data.

Figure 1: WiMax Polling Types

Contention based access control is the independent operation (distributed access control) of communication devices (stations). In a contention-based system, communication devices randomly request service from channels within a communication system. Because communication requests occur randomly, two or more communication devices may request service simultaneously. The access control portion of a contention based session usually involves requiring the communication device to sense for activity before transmitting and listening for message collisions after sending its service request. If the requesting device does not hear a response to its request, it will wait a random amount of time before repeating the access attempt. The amount of time waited between retransmission requests increases each time a collision occurs.

The WiMAX system defines time periods that subscriber stations can use for contention based access. When subscriber units desire to initiate requests to the system that are not scheduled from a polling message, they must access the process during the contention time slots period. The contention time slot period is periodically broadcast on the downlink channel along with other channel access control information.

Figure 2 shows how contention based access control can be performed on a WiMAX system. Channel descriptors are periodically broadcasted on the downlink radio channel that provides the time intervals for the contention slots. Subscriber devices that use contention based access must compete during these time periods. The WiMAX subscriber station will initially attempt to access the system at a relatively low power level. If the subscriber station does not hear a response to its request, it will wait a random amount of time, increase its transmitted power level and attempt access again. The subscriber station will continue to wait increasing amounts of time each time and increases its transmitted power level each time an access attempt fails until it receives a response from the system.

Figure 2: WiMax Contention Based Access Control

The WiMAX system uses a grant management process for the requesting and allocation (granting) of resources (such as transmission time or bandwidth). Subscriber stations can request changes to the type of services they require (e.g. increases or decreases in bandwidth) by transmitting a bandwidth request header and the system can decide to grant, adjust or not authorize the grant request.

The WiMAX system can grant resources based on a connection or based on a specific subscriber station. A grant per subscriber station is the allocation of transmission bandwidth that affects the transmission for all the connections associated with a subscriber station. A grant per connection is the assignment of bandwidth which only affects the transmission for a specific connection on a subscriber device.

Bandwidth requests can be in aggregate or incremental form. An aggregate request is a message that defines the amount of a resource (such as transmission bandwidth) that is requested to provide for a combined group of applications or services. An incremental request is a message that defines the additional amount of a resource (such as transmission bandwidth) that is requested to provide for an application or service. Bandwidth request messages may be sent as stand alone messages or they may be piggybacked in the payload of another packet of data.

WiMAX Radio Overview

A WiMAX radio channel is a communications channel that uses radio waves to transfer information from a source to a destination. It may transport one or many communication channels and communication circuits on a single RF channel.

WiMAX radio channels may operate within different frequency bands, have different radio channel bandwidths, dynamically change modulation types, use a variety of access technologies and other characteristics that allow WiMAX to reliably provide a variety of types of communication services.

The WiMAX radio systems can use a single carrier (SC) or multi-carrier (MC) transmission. Single carrier transmission is the use of a single carrier wave that is modified to carry (transport) all of the information. Multi-carrier is a communication system that combines or binds together two or more communication carrier signals (carrier channels) to produce a single communication channel. This single communication channel has capabilities (capacity) beyond any of the individual carriers that have been combined. When each of the carriers in a multi-carrier system is mutually independent (orthogonal) to each other, it is called orthogonal frequency division multiplexing (OFDM).

Figure 1 shows the key components of a basic WiMAX radio system. The major component of a WiMAX system include subscriber station (SS), a base station (BS) and interconnection gateways to datacom (e.g. Internet) and telecom (e.g. PSTN). An antenna and receiver (subscriber station) in the home or business converts the microwave radio signals into broadband data signals for distribution. In the example, a WiMAX system is being used to provide telephone and broadband data communication services. When used for telephone services, the WiMAX system converts broadcast signals to an audio format (such as VoIP) for distribution to IP telephones or analog telephone adapter (ATA) boxes. When WiMAX is used for broadband data, the WiMAX system also connects the Internet through a gateway to the Internet. The WiMAX system can reach distances of up to 50 km when operating at lower frequencies (2-11 GHz).

Figure 1: WiMax System

What is WiMax

Worldwide Interoperability for Microwave Access (WiMAX) is a wireless communication system that allows computers and workstations to connect to high-speed data networks (such as the Internet) using radio waves as the transmission medium with data transmission rates that can exceed 120 Mbps for each radio channel. The WiMAX system is defined in a group of IEEE 802.16 industry standards and its various revisions are used for particular forms of fixed and mobile broadband wireless access.

WiMAX is primarily used as a wireless metropolitan area network (WMAN). Derived from wireless metropolitan area networks (WMAN), WiMAX can provide broadband data communication access services ranging from urban to rural settings.

Used throughout the world, WiMAX broadband competes with DSO, cable modem and optical broadband connections by offering applications which include consumer broadband wireless Internet services, interconnecting lines (leased lines), and transport of digital television (IPTV) services.

Figure 1 depicts a number of the applications compatible/suitable for the wireless WiMAX systems including broadband Internet access, telephone access services, television service access and mobile telephone services.

Figure 1: WiMax Applications

The 802.16 system was initially designed for fixed location nomadic service in order to provide communication services to more than one location. While nomadic service may be provided to many locations, it typically requires the transportable communication device to be in a fixed location during the usage of communication services.

Developed for mobile service, the 802.16e specification adds mobility management, extensible authentication protocol (EAP), handoff (call transfer), and power saving sleep modes.

WiMAX has several different physical radio transmission options which allows it to be deployed in areas with different regulatory and frequency availability requirements. Moreover, the system was designed with the ability to be used in licensed or unlicensed frequency bands using narrow or wide frequency channels.

Figure 2 illustrates a variety of uses that WiMAX networks can provide including point-to-point links, residential broadband and high-speed business connections. As shown, the point to point (PTP) connection may be independent from all other systems or networks. The point to multipoint (PMP) system allows a radio system to provide services to multiple users. WiMAX systems can also be established as mesh networks allowing the WiMAX system to forward packets between base stations and subscribers without having to install communication lines between base stations.

Figure 2: Types of WiMAX Systems

WiMAX systems are composed of subscriber stations, base stations, interconnecting switches, and databases. Subscriber stations receive and convert radio signals into user information, while base stations are the radio part of a radio transmission site (cell site). Base stations convert signals from subscriber stations into a form that can be transferred into the wireless network. Interconnecting switches and transmission lines transfer signals between base stations and other systems (such as the public telephone network or the Internet). Databases are collections of data that is interrelated and stored in memory (disk, computer, or other data storage medium). WiMAX systems typically contain several databases that hold subscriber information, equipment configuration, feature lists and security codes.

Figure 3 illustrates the key components of a WiMAX radio system. The major components of a WiMAX system include; a subscriber station (SS), a base station (BS) and interconnection gateways to datacom (e.g. Internet), telecom (e.g. PSTN) and television (e.g. IPTV). An antenna and receiver ( subscriber station) in the home or business converts the high frequency (microwave) radio signals into broadband data signals for distribution. In Figure 2 WiMAX system is being used to provide television and broadband data communication services. When used for television services, the WiMAX system converts broadcast signals to a data format (such as IPTV) for distribution to IP set top boxes. When WiMAX is used for broadband data, the WiMAX system also connects the Internet through a gateway to the Internet. This example also shows that the WiMAX system can reach distances of up to 50 km for fixed point to point operation.

Figure 3: WiMax Radio System

To develop a cost effective, high-speed data transmission WMAN system, the IEEE created the 802.16 industry specification. The original 802.16 systems were a line of sight system that operates in the 10 GHz to 66 GHz of radio spectrum. To allow the 802.16 systems to operate in the 2 GHz to 11 GHz bands, the 802.16A specification was created.

The radio channel bandwidth of a WiMAX system can be very wide (e.g. greater than 20 MHz) and the radio access technology uses dynamically assigned burst transmission. This allows WiMAX systems to provide data transmission rates that can exceed 120 Mbps .

To help ensure WiMAX products perform correctly and are interoperable with each other, the WiMAX Forum was created. The WiMAX Forum is a non-profit organization that certifies products conform to the industry specification and interoperate with each other. WiMAX™ is a registered trademark of the WiMAX Alliance and the indication that the product is WiMAX Certified™ indicates products have been tested and should be interoperable with other products regardless of who manufactured the product.

Because the fundamental technology used in the 802.16 system is similar to 802.11 (wireless LAN), which is similar to 802.3 (Ethernet), wired or wireless LANs systems can be connected to a WiMAX system as an extension. In some cases, the WiMAX system can be operated independently to provide direct data connections between all the computers that can connect to the WiMAX system.