WiMAX Networking Technologies and Solutions

The demands of broadband wireless services have been growing, and the relevant technologies are developing rapidly. Under such circumstances, IEEE released IEEE 802.16, the Wireless Metropolitan Area Network (MAN) Protocol, with the cooperation and efforts of thousands of technical experts from the governments, operators, research institutes and equipment vendors[1]. The protocol has attracted wide attention from the industry. Moreover, several telecommunication corporations including equipment vendors and operators set up the WiMAX Forum. The purpose of the WiMAX Forum is to promote the compatibility test of broadband wireless equipment based on IEEE 802.16[2], and to implement the interoperability of the equipment from different vendors.

    The WiMAX technology is able to implement the coverage of tens of  kilometers and a peak rate of the single carrier of about 70 Mb/s. Besides, it has potentials to support roaming and mobility application. Therefore, it will have a wide application in the future. Currently, WiMAX can be applied in the market of backhaul, data access and roaming and mobility.

    The main service characteristic of the backhaul market is the fixed point-to-point distribution. Besides, its corresponding wired-access market is the service market of the point-to-point optical transmission equipment with E1/T1 interfaces.

    The main service characteristics of the data access market are the multiple broadband data access services based on the IP platform, especially the Internet access of IP data. Its corresponding wired-access market is the service market of Digital Subscriber Line (DSL) and the cable modem.

    The main service characteristic of the roaming and mobile data access market is the support of the data access of portable terminals. With broadband wireless equipment, operators offer data services to portable electronic equipment such as the notebook computer, Personal Digital Assistant (PDA) and handset. The system based on the WiMAX/IEEE 802.16 is expected to become widely applied and leading broadband wireless equipment.

    The current IEEE 802.16 standard and the relevant WiMAX test specifications just aim at the radio air interface technologies. In addition, all the standards issued only involve the physical layer and the Media Access Control (MAC) layer of the Open System Interconnection (OSI). There are no clear networking technologies and solutions for the WiMAX networks yet. Therefore, diversified opinions about the development of WiMAX still exist. For example, some people think it will compete with DSL products, while others regard it as a competitor to 3G networks. In fact, its development is closely related to its networking technologies and service applications. It particularly requires a deep research if the relationship between WiMAX and existing equipment is the revolutionary replacement or the convergence and evolution.

1 Key Technologies for WiMAX Networking
Referring to the general architecture of the wireless communication systems, the architecture of a WiMAX network can be divided into three parts: the terminals, the access network and the core network. As shown in Figure 1, the WiMAX terminals include fixed, roaming and mobile terminals. The WiMAX core network supports such functions as the customer authentication and roaming, while provides interfaces between the WiMAX network and other networks. The key technologies for the WiMAX networking include the frequency plan, Authentication Authorization Accounting (AAA), and the mobility management.

1.1 Frequency Plan
Given the rarity of the frequency resources every government pays much attention to its management. Since the frequency resource of the WiMAX network is very limited, it is necessary the operators do an overall planning for it, while considering the network capacity and the frequency interference. The frequency planning of the WiMAX network follows the general planning model for wireless networks. However, there are actually diversified planning solutions when considering different applications. Take a WiMAX application with fixed access as an example. The fixed access may make full use of the polarity isolation and the directional antenna; therefore, the followings are the principles of the frequency planning:

(1) Two adjacent sites are forbidden to use co-channel multiplexing with the same direction, except with 5-7 times of the distance or with effective obstacles between them. If the multiplexing of the two sites is necessary, different polarity directions may be used to gain the extra isolation of 20 dB.

(2) It is best not to use adjacent frequencies in one site or in one cell. The staggered angles help ensure the best registered frequencies in a service cell.

(3) It is forbidden to use the same frequency repeatedly in one site or in one cell.

(4) It is forbidden to use the same frequency repeatedly in adjacent cells of a site. If a high-performance antenna is used, and if a Subscriber Unit (SU) is not in the overlapped area of the two adjacent cells, the cross polarization of the frequency can be used.

(5) The back-to-back cells of a site may use the same frequency with the same polarity only if the isolation between antennas reaches 30 dB.

    IEEE 802.16 particularly specifies the dynamic frequency selection for the license-exempt spectra. Moreover, the WiMAX system supports Multi-Input Multi-Output (MIMO), intelligent antenna and Orthogonal Frequency Division Multiple Access (OFDMA) to alleviate the frequency interference and improve the network capacity.

1.2 AAA
The AAA system is an important part of the WiMAX network. It means that the authentication, authorization and accounting (AAA) is an important part of the WiMAX network security strategy. The typical implementation of the AAA access control of the broadband customer is: Customer-end Equipment→AAA Customer Host→AAA Server→Accounting System. The authentication of customers of the WiMAX network is a similar one. The RADIUS protocol is usually adopted between the AAA customer host and the AAA server. The AAA server and the accounting system are generally centralized to support the roaming of the customers, and to lower the cost of network construction. The Point-to-Point Protocol over Ethernet (PPPoE), 802.1X and Web authentications are usually used between the customer-end equipment and the AAA customer host (such as the broadband access server).

    Available networking solutions of the WiMAX system include building an independent network, overlapping with the data MAN and overlapping with the mobile cellular network. Therefore, it is possible for a WiMAX network to borrow the implementation technologies or corresponding equipment of the existing networks to act as its AAA customer host, AAA server and accounting system. In this way, the resources are shared, and the investment in network equipment is accordingly saved.

1.3 Mobility Management
The WiMAX network supports roaming and mobile services. Its mobility management mode has a great influence on the network structure. The mobility management aims at effectively supporting the roaming and mobility of terminal stations in the whole service network. It is composed of location management and handover management. The location management confirms the exact location of a legal terminal in the network, ensuring the quick building of the service route once a service is initiated. The handover management guarantees the QoS of the enabled services when the terminal is in a moving status.

    A moving WiMAX terminal may use the Simple IP or Mobile IP to implement the mobility management. When following the Simple IP, dynamic and changeable IP addresses are assigned to the terminal. However, any enabled services for it will be interrupted when a handover from one cell to another occurs, and the reconnection will be implemented only by special protocols of the application layer. Therefore, the Simple IP is only fit for services that are not sensitive to time, such as the WWW and e-mail, with a failure to well support the real-time services with mobility in a large scope like VoIP. With the Mobile IP, the terminal has a permanent IP address, and may supports all types of IP services. However, this protocol faces the shortage of the IPv4 addresses, and is not able to guarantee good time delay and packet loss tolerance. Solutions such as the address transfer, IPv6 and multiplayer handover are used to solve these problems.

2 WiMAX Networking Solutions

2.1 Networking with Fixed Applications
With fixed applications, a WiMAX network may be overlapped to a broadband MAN. The functions of its core network can be directly mapped to the relevant equipment of the existing IP network, such as the broadband access server. This networking solution involves the frequency planning and the AAA of the terminal.

    Having limited frequency resources, operators may divide their frequencies into multiple independent carrier waves according to the width of wave channels. This frequency planning solution will directly influence the peak rate at the user end and the network capacity of the planned area. Therefore, a careful planning is inevitable. Take the 3.5 GHz as an example. It is typically divided into 1.75 MHz, 3.5 MHz and 7 MHz. When considering various factors,
3.5 MHz is suggested as a major carrier wave band during the frequency planning. Figure 2 demonstrates the typical networking with three frequencies and multiple base stations. The H and V represent Horizontal Polarization and Vertical Polarization respectively.

    There are three major solutions for authentication and accounting: the PPPoE authentication, the Web authentication and the 802.1x authentication.

(1) PPPoE, i.e. the Standard RFC2516, is a specification for connecting the users to the Internet by Ethernet. It can be applied in the environments of xDSL and Local Area Network (LAN). It features easy management, support of multiple services and easy traffic control. However, it is inefficient, and the cost of isolation in Ethernet is very high.

(2) The Web authentication must cooperate with the Dynamic Host Control protocol (DHCP) server and the portal server. It doesn’t require any software, and is able to support the value-added services of the portal server. With it, the network cost is low. However, it is not safe.

(3) IEEE 802.1x is a port-based network access control standard. The port access will be stopped once the authentication fails. IEEE 802.1x is safe, simple and easy to operate. Therefore, IEEE 802.1x is a priority solution for the AAA of the WiMAX network.

2.2 Networking with Mobile Applications
With mobile applications, the WiMAX network may be overlapped to the broadband MAN or to the mobile cellular network. The frequency planning, AAA and mobility management are the three key parts of this networking solution. A typical networking solution with three frequencies and multiple base stations are demonstrated in Figure 3, where f1, f2 and f3 represent three frequencies. A single cell is required to support the MIMO and OFDMA, in order to implement the cellular networking.

    The overlap of the WiMAX network to the broadband MAN can be treated as a special case of its overlap to the mobile cellular network. Therefore, only the networking architecture of the WiMAX network and the mobile cellular network will be discussed in detail here.

    According to the tightness between the WiMAX network and the mobile cellular network, the solution can be divided into two models: the loose coupling and the tight coupling. There are six working modes between the two networks. From the loosest coupling to the tightest coupling, the modes are respectively:

(1) the integrated management for accounting and the customers.

(2) the authentication and accounting of the WiMAX network that is based on the mobile cellular network.

(3) the standard service mode of the packet domain when the WiMAX network accesses the mobile cellular network.

(4) the service consistency and continuity.

(5) the seamless handover of the services of the packet domain.

(6) and the standard circuit domain mode when the WiMAX network accesses the mobile cellular network.

    The first two modes are belonging to the loose coupling, while the last four are tight coupling. (The convergence networking of the WiMAX network and the 3G network is demonstrated in Figure 4.) In the first mode, an additional network is built between the two systems, and AAA is implemented in the additional network. In the second mode, the WiMAX network is a complement of the mobile cellular network. Its authentication and accounting needs to use the Home Location Register (HLR) and the AAA of the mobile cellular network, and its traffic outlet is directly connected to the MAN. In the third mode, the WiMAX authentication and accounting has a similar implementation with that of the second mode, but the WiMAX traffic outlet is the packet domain gateway of the mobile cellular network. In the fourth mode, the WiMAX may directly visit all the services of the mobile cellular network. In the fifth mode, handover in the WiMAX network is controlled by the mobile cellular network, and the VoIP services of the WiMAX network are able to be handovered to the mobile cellular network. In the sixth mode, the wireless resources of the WiMAX network and those of the mobile cellular network are integrated.

    Therefore, for the WiMAX networking, the first mode can be used at the early stage of the networking and it evolves up to the fourth and the fifth mode through upgrade of the mobile cellular network. The sixth mode is the ultimate goal. The AAA of the WiMAX terminal is implemented in the corresponding equipment of the mobile cellular system. Special equipment of the WiMAX network is used to implement the mobility management in loose coupling, while in tight coupling the equipment of the mobile cellular network takes part in the mobility management of the WiMAX terminal.

3 Evolution of WiMAX Networking
The WiMAX technology is developing. The networking based on the process as it will go through four phases: the outdoor access phase, the indoor access phase, the seamed roaming phase and the seamless mobility phase.

(1) The outdoor access phase
    As the WiMAX solutions that are based on the unified air standards have just been issued, it becomes a good operation mode to make use of outdoor access equipment to implement the coverage of commercial customers in a large area. In this phase, the outdoor access system adopts the 15-23 dB directional antenna, and the base station covers 3-10 km. The terminal interface at the first stage is 10BaseT/RJ11 (VoIP), while those at the second stage are V.35, E1/T1 and RS530.

(2) The indoor access phase
    After the standard WiMAX equipment is launched, the indoor access equipment will gradually enter the market of fixed data access for residents. In this phase, the indoor access system adopts 5-15 dB omni and directional antennas, and the base station covers 1-3 km. The terminal interfaces adopt such interfaces as 10/100BaseT and USB.

(3) The seamed roaming phase
    In this phase, the base station based on standard protocols will adopt newer radio technologies to improve the radio coverage capability. The seamed mobile broadband wireless access system features a terminal with the 3-7 dB omni antenna and a centralized authentication and authorization center of the network. It primarily serves portable computers, and supports the Internet data access services and a few video services.

(4) The seamless mobility phase
    The typical base station of the seamless mobile broadband wireless access system adopts the model of "the base station + the base station controller" to implement the support of centralized management of wireless resources.

    Another networking model adopts the Dynamic Frequency Selection (DFS) mechanism under the micro cellular status to make full use of the frequency resources.

4 Conclusions
The technology of WiMAX is a wireless access technology of MAN. It has been developed with the growing trend of the telecommunication networks towards the IP networks. The WiMAX network has the capability to seamlessly converge with all-IP networks. From the point of view of networking, the WiMAX network will not only borrow the networking technologies from the IP networks, NGN and mobile cellular networks, but also fully converge with them. The Network Working Group (NWG) of the WiMAX Forum is studying the relevant specifications. With the development and improvement of its networking technologies, the WiMAX network with broad bandwidth and low cost is expected to play an important role in the future wireless Internet.

References
[1] IEEE 802.16-2004 Part16: Air Interface for Fixed Broadband Wireless Access System [S].
[2] WiMAX Forum. PICS for WirelessMAN-OFDM and WirelessHUMAN(-OFDM) [Z].

Manuscript received：2005-01-27