WIMAX NETWORK ARCHITECTURE

The mobile WiMAX end-to-end network architecture is based on an All-Internet Protocol (IP) platform, all packet technology, and no circuit switch telephony. The end-to-end architecture makes the greatest possible use of IETF and IEEE standards and protocols along with the adoption of commonly available standard equipment.

The open IP architecture gives network operators great flexibility when selecting solutions that work with legacy networks or that use the most advanced technologies, and in determining what functionality they want their network to support. They can choose from a vertically integrated vendor that provides a turnkey solution or they can pick and choose from a dense ecosystem of best-of-breed players with a more narrow focus. The architecture allows modularity and flexibility to accommodate a broad range of deployment options such as small scale to large scale, urban, suburban, and rural coverage, mesh topologies, flat, hierarchical and their variant, and finally, coexistance of fixed, nomadic portable and mobile usage models.

Mobile WiMAX adds both the mobility and multiple-input multiple-output (MIMO) functionalities to the IEEE 802.16-2005 standard. It is one of two standards adopted by the WiMAX forum with the other one being the IEEE 802.16-2004. Mobile WiMAX network architecture mainly has three components. These include the access services network (ASN), the core services network (CSN), and the application services (AS) network. Figure 12.1 illustrates the interconnection of these networks. The WiMAX network supports the following key functions:

• All-IP access and core service networks
• Support for fixed, nomadic, and mobile access
• Interoperability with existing networks via internetworking functions
• Open interfaces between ASNs and between the ASN and the CSN
• Support for differential quality of service (QoS) depending on the application
• Unbundling of the access, core, and application service networks

 

Figure 1: WiMAX network architecture.

WiMAX—Architecture, Planning, and Business Model

The fixed/portable broadband wireless access is becoming a necessity for many residential and business subscribers worldwide. The demand is exploding as the pricing of broadband services is rapidly decreasing. The worldwide interoperability for microwave access (WiMAX) technology is an integral part of the portfolio by complementing 2G/3G mobile access, Digital Subscriber Line (DSL) broadband fixed access, and Wireless Fidelity (WiFi) hotspot access. An extended overview of WiMAX and its applications in higher generation wireless networks as a cost-effective solution to answering the challenges posed by the digital divide is presented. Technology behind WiMAX and its network architecture, design, and deployment are examined in addition to factors that impact WiMAX planning and performance. A WiMAX radio coverage simulation and analysis at different frequency bands for different demographic is presented. Furthermore, the WiMAX business models and a comparison with two enhanced third-generation (3G) technologies that are potential competitors to WiMAX are explored. 

Telecommunications has grown at a tremendous rate in the last ten to twenty years. Improved semiconductor and electronics manufacturing technology, and the growth of the Internet and mobile telecommunications have been some of the factors which have fueled this growth in telecommunications. The deployment of state-of-the-art telecommunications infrastructure and services has, however, been restricted to the developed world. The least developed countries have been left in the technological dark ages with few or none of the next-generation networks installed. Developed countries now boast high-speed connections with a large percentage of homes having access to the Internet and broadband services at an affordable fee. The underdeveloped countries are yet to enjoy such facilities. This is referred to as the digital divide. During the first World Summit on the Information Society (WSIS) held in Geneva in December 2003, the Digital Divide was defined as the unequal access to information and communication technologies (ICTs), where the least developed countries are separated from the developed countries because of a lack of technology particularly ICT.

The digital divide has persisted due to the relatively high cost of putting up modern telecommunications infrastructure. This is compounded by the fact that there are a number of different services available and each service requires its own technology and network. Therefore, existing technologies such as Wireless Fidelity (WiFi), Digital Subscriber Line (DSL), Global System for Mobile communications (GSM), Integrated Services Digital Network (ISDN), and the relatively new 3G technologies have not been able to provide a total solution to closing the digital divide. Figure 1 illustrates the main network types and the prevalent technologies associated with each, mapped against usage models and access modes.

 

Figure 1: Wireless network type and range. MAN—metropolitan area network (citywide, rural area), LAN—local area network (office, home, campus), and WAN—wide area network (countrywide, international).

WiMAX will boost today’s fragmented broadband wireless access market and mobile WiMAX promises to offer a solution to closing the existing digital divide. WiMAX can address the fixed wireless access and portable Internet market, complementing other broadband wireless technologies. Government initiatives to reduce the digital divide are making gains for broadband wireless countries such as Australia, South Korea, Taiwan, and the United States have programs in place today, and there has been a push by the European Commission for more flexible spectrum policies.

WiMAX access can be easily integrated within both fixed and mobile architectures, enabling operators to integrate it within a single converged core network, thereby providing new capabilities for a user-centric broadband world.

WiMAX addresses the following needs which may answer the question of closing the digital divide:

• Cost effective
• Offers high data rates
• Supports fixed, nomadic, and mobile applications thereby converging the fixed and mobile networks
• Easy to deploy and has flexible network architectures
• Supports interoperability with other networks
• Aimed at being the first truly a global wireless broadband network

WiMAX is a standard that is championed by the WiMAX forum which was formed in June 2001 to promote conformance to IEEE 802.16 standard. The WiMAX forum currently has more than 470 members comprising the majority of operators, component, and equipment companies in the communications ecosystem. The WiMAX forum promotes interoperability by working closely with IEEE and other standards groups such as the European Telecommunications Standards Institute (ETSI) which have their own versions of broadband wireless. Along these lines, the WiMAX forum works closely with service providers and regulators to ensure that WiMAX forum certified systems meet customer and government requirements.

The original WiMAX standard only catered for fixed and nomadic services. It was reviewed to address full mobility applications; hence, the mobile WiMAX standard, defined under the IEEE 802.16e specification was created. Mobile WiMAX supports full mobility, nomadic, and fixed systems to compete against DSL to cover isolated areas such as rural hot spots, private campus networks, and remote neighborhoods. Mobile WiMAX is more promising to be deployed as a cellular network that offers ubiquitous broadband services to mobile users to over large geographical areas. It can be deployed as a central office bypass to avoid using existing wired infrastructure for competitive local exchange carriers and wireless Internet service provider. Figure 2 shows the standard history for 802.16.

 

Figure 2: 802.16 standard evolution.

ROUTING ALGORITHMS | WiMAX Mesh Networks

In this section we will investigate routing and scheduling algorithms for WiMAX mesh networks, with the target application of using WiMAX mesh networks to network cellular BSs and gateways to the Internet. The gateways of general cellular networks can be mobile switching center (MSC) in GSM networks or MSC and serving GPRS support node (SGSN) in WCDMA networks. In this network architecture, the gateways act as 802.16 mesh BSs in the 802.16 backhaul network. They manage the backhaul network and allocate bandwidth to the SSs. The cellular BSs act as 802.16 SSs in the backhaul network. In the remaining part of the chapter, the cellular BSs will be called SSs for simplicity. The SSs forward aggregated traffic from the mobile users to Internet in the uplink direction and deliver traffic to the mobile users from the Internet in the downlink direction via the 802.16 BSs. For simplicity, only uplink traffic with QoS requirement is considered in this chapter. However, the approaches of routing and scheduling can be applied to bidirection traffic. It is assumed that the backhaul topology and traffic demands from the SSs will not change frequently. Therefore, the frequency of updating traffic routes and scheduling will be low. It is feasible and beneficial to use high performance algorithms such as the optimal centralize scheduling and routing algorithms.

CLASSIFICATION

As IEEE 802.16 standards specify the MAC and PHY layer protocols, routing protocols are outside the scope of the standard work. We can identify the following ways of solving routing and scheduling (bandwidth allocation) problems for WiMAX mesh networks:

• Centralized routing and centralized scheduling (CRCS): All the traffic, position information of the stations are sent to the BSs. The BSs jointly solve the routes and schedule problems, and send the routes/schedule information back to the SSs.
• Distributed routing and centralized scheduling (DRCS): SSs distributedly find their routes to the BSs. After the routes are found, the SSs send their information to the BSs. The BSs determine collision-free transmission schedule.
• Distributed routing and distributed scheduling (DRDS): Stations distributively find their routes to the BSs. After SSs find their routes to BSs, they work with their next-hop stations toward the BSs to determine collision-free transmission schedules in a distributed way.
• Hybrid routing and scheduling (HRS): Both routing and scheduling algorithms can be implemented in either a distributed or a centralized way.

In WiMAX mesh networks, distributed scheduling can be used by SSs with their neighbor SSs to reserve time slots. However, the success of reserving time slots will depend on the availability of free time slots not reserved by the centralized scheduling and other neighbor stations. As centralized scheduling has priority over distributed scheduling in WiMAX, it will be easier to provide time slots reservation for end-to-end QoS support. Therefore, we will consider only centralized scheduling in this chapter.

Centralized scheduling algorithm will work together with either centralized or distributed routing algorithms. For the centralized routing algorithm, routing, bandwidth allocation, and scheduling problem can be jointly investigated. We will formulate the joint routing, allocation, and scheduling problem with an optimal mathematical model. The input to the optimal model from the routing point of view will be network connectivity matrix CM, CM_ij = 1 if and only if station i is in the communication range of station j. After the optimal problem is solved, the route from an SS to a BS will be determined.

For the centralized scheduling and distributed routing algorithms, routes from SSs to BSs will be determined first. Then we can also derive network connectivity matrix CM for distributed routing. However, in this case, the station connectivity matrix CM will not only be determined by the communication ranges but also the traffic routes. We have CM_ij = 1 if and only if the link e_ij between station i and station j is in any route of SSs to BSs. Then the connectivity matric CM can be input to the optimization model for centralized scheduling. 

DISTRIBUTED ROUTING

In the WiMAX mesh networks, the BSs will periodically broadcast network configuration messages, which are further forwarded by the neighbor stations. The forwarding process continues toward network edge until the predefined maximum number of hops is reached. For each forwarding neighbor stations, it will add the information of hops from itself to the BSs. Although routing algorithm is not specified in the standards, the distance information together with other local information can be utilized by a new SS to find a route to a BS.

For a new SS to join the WiMAX mesh network, it is required to listen to the network configuration/synchronization from its neighbor stations. After it hears network configuration message at least twice from a station, it can send request to join the network through this station. With the available local information, such as the signal strength, distance between, the new station can select which station to be its sponsor station to a BS. Therefore the distributed routing algorithm can be reduced to find a sponsor station for a route toward a BS. We simply give four methods.

• Random selection (RS): In this method, a new station will choose a neighbor station as its sponsor station randomly among the candidate stations with the same minimum number of hops to a BS.
• Minimum node ID (MID): Among all the neighbor stations with the same minimum number of hops to any BSs, the station with minimum node ID will be selected as the sponsor station.
• Maximum signal strength (MSS): The station with maximum average signal strength will be selected as the sponsor station among the candidate stations, which can achieve higher transmission data rate.
• Minimum aggregate traffic (MAT): To balance traffic routed over the neighbor stations, a new station can also choose the station with minimum aggregated traffic load among the candidate stations.