WiMAX and Its Applications

WiMAX is the abbreviation of World Interoperability for Microwave Access. It is a nongovernmental industry alliance established in April 2001, with telecom equipment manufacturers as its major members. The alliance aims at promoting the development of IEEE 802.16/ETSI HIPERMAN broadband wireless access equipment to ensure the compatibility and interoperability of equipment from different providers.

1 IEEE 802.16 Standards

1.1 IEEE 802.16 Working Group
The IEEE 802 LAN/MAN Standards Committee set up the IEEE 802.16 Working Group on broadband wireless access standards in 1999. The working group is divided into three task groups. IEEE 802.16.1 Task Group led the development of the air interface standard for fixed broadband wireless access systems working in 10-66 GHz, IEEE 802.16.2 Task Group was responsible for the coexistence standard of fixed broadband wireless access systems, and IEEE 802.16.3 Task Group has been chartered to make air interface standards for the licensed band of 2-11 GHz.
New task groups have been established, such as IEEE 802.16e Task Group that aims at free handover and roaming of Subscriber Stations (SS) between IEEE 802.16 Base Stations (BS) and IEEE 802.16f Task Group, the mesh Ad Hoc committee, for the purpose of improving base station coverage.  

1.2 IEEE 802.16 Standards
According to the tasks, the IEEE 802.16 Task Groups have done research in broadband wireless access standards for 10-66 GHz and 2-11 GHz separately, and developed the following standards for different wireless Metropolitan Area Networks (MAN).

• Approved in December 2001, IEEE 802.16 defines the air interface specifications for fixed broadband wireless access systems. The standard designs single-carrier modulation for 10-66 GHz operation. Since the working wavelengths are short, line-of-sight propagation is requested. IEEE 802.16 standards defines a two-layer structure of the air interface: the Physical Layer and Medium Access Control (MAC) Layer.
• In 2001, IEEE 802.16.2 was approved for the coexistence of fixed broadband wireless access systems working in 10-66 GHz.
• IEEE 802.16c was approved in December 2002 to guarantee interoperability between systems working in 10-66 GHz.
• IEEE 802.16a, an amendment to IEEE 802.16, was approved in January 2003. It defines specifications for Wireless MAN operating in licensed and license-exempt spectra between 2-11 GHz.
  WiMAX, we are discussing here, is based on IEEE 802.16a.
  

2 WiMAX and Its Characteristics

2.1 Main WiMAX Technologies
As a latest standard from the IEEE 802.16 Working Group and an amendment to IEEE 802.16, IEEE 802.16a develops additional physical layer specifications and enhances the MAC layer for 2-11GHz. It is the base of WiMAX technologies and represents technical trends towards the mesh architecture, multiple carrier modulation technologies, QoS support and wireless security.

2.1.1 Mesh Architecture
IEEE 802.16a supports the mesh architecture that is fit for licensed and license-exempt spectra in 2-11 GHz, while IEEE 802.16 developed a point-to-multipoint architecture. IEEE 802.16a defines specific MAC service and message specifications for this architecture. There are at least 2 WiMAX nodes adopting multipoint-to-multipoint wireless connection in the mesh architecture, which also follows service and network specifications of IEEE 802.16a MAC and physical layers.

2.1.2 Multiple Carrier Modulation Schemes
IEEE 802.16 standards adopt Single-Carrier (SC) modulation, which is called WirelessMAN-SC. But IEEE 802.16a defines 3 modulation modes:
    (1) Single-carrier Modulation: It, called WirelessMAN-SCa, is retained for
special-purpose networks.

    (2) 256-carrier Orthogonal 
    Frequency-Division Multiplex (OFDM): The modulation is implemented by 256 subcarriers with the same intervals. With capabilities of resistance to multipath fading and delay spread, it is used by most applications. It is called WirelessMAN-OFDM.

    (3) 2048-carrier Orthogonal 
    Frequency-Division Multiple Access (OFDMA): With this modulation, a frequency band is divided into multiple subchannels that are orthogonalized in turn. Spectra can be superposed. This modulation implements multiplex/demultiplex access of multiple subscribers, and can support specific multi-object communication. It is called WirelessMAN-OFDMA. 

    In addition, modulation for high-speed unlicensed MAN is OFDM for license-exempt spectra in 2-11 GHz, and called WirelessHUMAN.

    In order to support multiple carrier modulation schemes, IEEE 802.16a has developed a large number of additional functions on both physical and MAC layers, such as the Adaptive Antenna System (AAS) on the MAC layer, Automatic Repeat Request (ARQ) on the MAC layer, Space-Time Coding (STC)  on the MAC layer and Dynamic Frequency Selection (DFS) on the MAC layer.

    Besides, IEEE 802.16 adopts dynamic adaptive signal modulation, that is to say, a BS can build up connection by 16QAM or Quadrature Phase Shift Keying (QPSK) modulation if it fails by 64QAM. With such a modulation scheme, operators can promptly adjust BS bandwidths according to signal strength, and accordingly ensure users’ normal connection.

2.1.3 QoS Support
IEEE 802.16 is defined with full QoS support. The MAC layer supports connection-oriented transport, and can transport and control physical-layer data with different QoS requirements. Almost all the MAC message heads carry service parameters to represent different QoS requirements. Parameters of the constant bit rate, real-time stream,
non-real-time stream and best effort services are supported.

    The MAC layer adopts self-correcting bandwidth request/grant scheme. According to QoS requirements and service stream parameters, a 6-byte bandwidth request frame header, located at the frame header of common MAC, requests bandwidth individually or in groups or adjusts allocated bandwidth for new applications to ensure minimum requested delay of real-time voice and video applications. The standard also defines ARQ based on individual application streams, which guarantees automatic data resending of the MAC Service Data Unit (MSDU) in the case of with high wireless error rate and loss tolerance and provides QoS for applications with strict limits on loss tolerance.

2.1.4 Wireless Security
In order to protect users’ information security, and especially to meet enterprisers’ security demands for commercial application data, the privacy sublayer in IEEE 802.16 defines specifications on authentication, secure key exchange and encapsulation for the air interface. With encapsulating data from authorized users, BS limits the access of unauthorized users. Besides, it supports the Privacy Key Management (PKM) protocol for secure
two-layer-key distribution and exchange and real-time confirmation of subscribers’ identification, which ensures secure wireless data transport.

2.2 Characteristics and Strengths of WiMAX Technologies
Wi-Fi, the Wireless Local Area Network technology, is the biggest competitor but partner of WiMAX. IEEE 802.11a and 802.11b are currently main Wi-Fi standards. Besides, the latest approved IEEE 802.11g supports
high-speed rate defined by IEEE 802.11a while compatible with IEEE 802.11b equipment. It is also a noteworthy Wi-Fi standard. WiMAX and Wi-Fi are compared in Table 1.

    Compared with Wi-Fi, WiMAX supports further transport distance and higher rate, so some researchers think WiMAX technologies are making a tremendous impact on Wi-Fi and even tending to replace it.

    WiMAX has the following strengths when compared with traditional broadband fixed access technologies such as ADSL and LAN:

• Good flexibility: The wireless broadband access can be quickly and easily setup on any temporal sites, saving a few weeks for laying T1 or DSL lines. Besides, WiMAX can, according to users’ demands and channels’ states, dynamically allocate system resources, which helps improve investment efficiency.
• Broad application: WiMAX can be applied in the areas where fixed broadband access technologies are helpless. For example, the effective coverage of the ADSL access is limited in 5 km, but WiMAX has broken the limitation.
• Low cost: Deployment of traditional fixed access in the suburbs and rural areas needs high cost. But with wireless access, such cost will be effectively reduced, for an individual BS can support thousands of SSs.
  However, WiMAX has its inherent limitations. Its access equipment at the user’s side still costs much in urban areas where the fixed broadband access is suitable. Besides, the rate of 75 Mb/s can be actually realized only in 3-5 km, although the maximum transport distance of its signals can reach 50 km. Compared with fixed access, the security of wireless communication inevitably becomes one of its weaknesses. In addition, before the development of IEEE 802.16e is completed, WiMAX cannot implement SS’s roaming between BSs yet, that is to say, it cannot implement IEEE 802.16b-based seamless access of wireless terminals yet.

3 Architecture and Applications of  WiMAX System

3.1 WiMAX System Architecture
As shown in Figure 1, a WiMAX system and its surrounding systems, consist of the core network, BS, SS, Relay Station (RS), Terminal Equipment (TE) and network management system. One WiMAX system usually includes only one BS and multiple SSs. But it can, according to specific requirements, also be a multipoint-to-multipoint structure by deploying several RSs.

    (1) Core Network
    The core network connected with the WiMAX system is usually a traditional switching network or the Internet. The WiMAX system provides the interface between the core network and the BS, but it does not include the core network.

    (2) Base Station
    It implements connection between the core network and SSs, and usually has the sector/directional or omnidirectional antenna. It supports flexible subchannel deployment and allocation.

    (3) Subscriber Station
    Unique to the WiMAX system, SSs implement relay between the BS and TEs. The IEEE 802.16 SS usually has a fixed antenna that is located on roofs. Dynamic adaptive signal modulation is adopted between the BS and SSs. With such a modulation, the BS can adjust bandwidths to each SS according to signal strength, which ensures every SS a normal access.

    (4) Relay Station
    In the point-to-multipoint structure, RS is used to improve the coverage of the BS, that is to say, it is a relay of information between the BS and multiple SSs or TEs. Its downlink frequency (to SSs) can keep the same with or different from its uplink (to BS).

    (5) Terminal Equipment
    The WiMAX system defines an interface between the TE and SS and supports TE access. However, the TE itself does not belong to the system.

    (6) Network Management System
    It monitors and controls all BSs and SSs of the WiMAX system, offering functionalities such as inquiry, state monitoring, software download and system parameter deployment.

3.2 WiMAX Applications
According to the characteristics and strengths of the WiMAX technology, WiMAX applications are focused on Wireless MAN. On one hand, it can be used as a complement to wired broadband access technologies to improve the coverage and flexibility of broadband access. On the other hand, it is regarded as a Wireless MAN technology on a par with Wi-Fi. The two technologies separately serve MAN and LAN that are complementary to each other. Shown in Figure 2 is a WiMAX application solution.

    In this solution, WiMAX is used as a supplement to wired broadband access modes for wireless access of individual broadband users and subscribers of the enterprise’s network to MAN. The solution can surmount physical difficulties that traditional wired systems have and be fit for downtown buildings with access problems, suburbs far from switch offices and rural areas with a sparse population and poor telecom infrastructure. It helps greatly reduce the construction investment of broadband applications in these areas. For example, exhibition centers in cities are typical application sites of the solution. Only one T1 access is used to meet daily demands, while WiMAX services can be just applied for a large amount of Internet and VPN access demands when there are conferences or exhibitions there. Another application of the solution is to use WiMAX as a Wireless MAN technology to directly implement wireless access for TEs. A typical application is, for example, to provide wireless terminal users with "best wireless access". Wireless terminal users realize access via IEEE 802.11 in hotspots and IEEE 802.16 in other areas of MAN except hotspots. In this way, the users can always keep wireless access in MAN. Besides, a WiMAX system can be used as a wireless loop of IEEE 802.11 access hotspots, that is to say, its SSs follow IEEE 802.11 specifications and the WiMAX technology is used to connect Wireless MAN with operator’s backbone network. Accordingly, the problem of deploying IEEE 802.11 networks in areas with difficulties in building wired loops is solved. 

4 Summary
The WiMAX technology integrates the mobility and flexibility of wireless broadband access technologies and the wide bandwidth and good QoS of traditional wired broadband access technologies. It may become the final wireless solution of "Last Mile Broadband Access". However, its standard is not perfect yet, and equipment for real commercial use is not available either. Besides, it is necessary to carefully consider its operation service platform and frequency planning in its actual applications. Therefore, it is still a long way for WiMAX to implement its large-scale application.

References
[1] IEEE802.16 Air Interface for Fixed Broadband Wireless Access Systems[S].
[2] IEEE802.16a Air Interface for Fixed Broadband Wireless Access Systems—Amendment 2: Medium Access Control Modification and Additional Physical Layer Specifications for 2-11 GHz[S].

Manuscript received: 2004-09-20