January 15, 2008

start:The designer's basic guide to WiMAX technology, standards and implementation

This reference will be useful to novices and experienced wireless designers alike because it sets forth many WiMAX's fundamentals in easy-to-understand language.

By Dr. S. Jagannathan, Tata Elxsi Ltd.

As communications distances grow, it becomes infeasible economically to connect information sources and information users using coaxial cables and fiber optic links. But wireless technology can provide cost-effective access to remote, difficult-to-reach areas.

WiMAX (Worldwide Interoperability for Microwave Access)—or IEE 802.16 or Wireless MAN—provides such broadband wireless access over larger areas than 802.11 (Wireless LAN), and it does so at broadband speeds.

WiMAX highlights
There are four key defining elements that differentiate WiMAX and other wireless technologies.

• WiMAX is a broadband wireless access (BWA) technique, offering fast broadband connections over long distances. It is an IEEE 802.16/ETSI HiperMAN based certificate for equipment fulfilling the interoperability requirements set by the WiMAX Forum (comprising of industry leaders committed to the open interoperability of all products used for broadband wireless access).
• The technology underlying the standard is often referred to as "Wireless MAN wireless local loop" or "WiMAX".
• A key aspect of WiMAX is the interoperability between products from different vendors.
• WiMAX uses different modulation schemes between the consumer and the base station along different distances.

WiMAX architecture
The IEEE Working Group 16 defined two access options for a WiMAX network: fixed (IEEE 802.16TM- 2004) and portable (IEEE 802.16eTM). In the fixed option, access is provided through a fixed antenna as in a satellite television subscriber station. In portable option, the subscriber stations are very similar to IEEE 802.11 Wi-Fi stations.

WiMAX point-to-point backhaul can be used to interconnect Wi-Fi mesh networks through dual mode Wi-Fi and WiMAX cells. It can also be used to offer access to fixed WiMAX point- to-point backhaul, interconnecting Wi-Fi mesh networks through a dual mode Wi-Fi and WiMAX cells.

Furthermore, it can be used to offer access to fixed Subscriber Stations (SS) through a Point to Multi Point (PMP) topology. Mesh topology can be used to reach users, who cannot otherwise be reached without a new Base Station (BS). Portable SS's will be supported in the near future.

The WiMAX architecture depends on: topology and Wi-Fi interconnection. With the emergence of WiMAX in near future, deployments that combine the two technologies can be constructed to take advantage of the strengths of both Wi-Fi and WiMAX. The WiMAX cells will interoperate seamlessly with existing Wi-Fi cells always selecting the best path for delivering maximum user throughput end-to-end.

The scenario will become increasingly complicated with the introduction of five other factors:

• PHY air interface
• Duplexing technique (FDD—Frequency Division Duplexing/TDD—Time Division Duplexing)
• Transmission mode (half-duplex/full-duplex)
• Operation (licensed/unlicensed)
• Adaptive burst profiles

Click here for Figure 1
Figure 1: Protocol stacks as per IEEE 802.16 MAC.

How WiMAX works
WiMAX requires two main components to form an operational network:

• Base Station (BS), which can serve as a repeater or can be connected to the Internet backbone
• Subscriber/end user, enjoying broadband wireless access through the base station

WiMAX provides two types of wireless services:

• Non-line-of-sight (NLOS):Protocol stacks as per IEEE 802.16 MAC This is a Wi-Fi kind of service in which a small antenna on the subscriber's computer connects to the tower. In NLOS, WiMAX uses a lower frequency range similar to Wi-Fi: 2 GHz to 11 GHz. NLOS services can, by virtue of their lower-wavelengths, easily diffract or bend around obstacles.
• Line-of-sight service (LOS): In this service type, a dish antenna fixed on a rooftop or pole points to a WiMAX tower. LOS connections are stronger and more stable, and can send large amount of data with fewer errors.
LOS transmissions use higher frequencies of, possibly, up to 66 GHz. At higher frequencies, there is less interference and lot more of bandwidth. Using strong LOS antennas, the WiMAX transmitting station can send data to WiMAX-enabled computers/routers set up within a 30-mile radius, and this allows WiMAX to achieve its maximum range.

Click here for Figure 2
Figure 2: Protocol stacks as per IEEE 802.16 MAC.

The LAN, usually established through Wi-Fi, operates in tandem with WiMAX entities. The NLOS is usually used between subscriber LANs and base stations, while LOS is used between base stations.

IEEE Standard 802.16-2001, completed in October 2001 and published on April 8, 2002, defines the Wireless MAN air interface specification for wireless metropolitan area networks (MANs). The completion of this standard heralded the entry of broadband wireless access as a major new tool linking homes and businesses to core telecommunication networks worldwide.

As currently defined through IEEE Standard 802.16, a wireless MAN can provide network access to buildings through exterior antennas communicating with central radio base stations (BS).

The wireless MAN represents an alternative to cabled access networks such as fiber optic links, coaxial systems using cable modems, and digital subscriber line (DSL) links. Because wireless systems have the capacity to address broad geographic areas without costly infrastructure (for deploying cable links to individual sites), the technology may lead to more ubiquitous broadband access.

Although these systems have been in use for years, the development of the new standard marks the industry's maturation and success in second-generation equipment. With Wireless MAN technology bringing the network to buildings, users within buildings can connect to it via conventional in-building networks such as Ethernet IEEE Standard 802.3 for data and wireless LANs IEEE Standard 802.11. Despite of this, the fundamental design of the standard may eventually allow for efficient extension of the Wireless MAN networking protocols direct to individual users.

Consider, for instance, that a central BS might someday exchange medium access control (MAC) protocol data with an individual laptop computer in a home. The links from the BS to the home receiver and from the home receiver to the laptop are likely to use physical layers that are quite different.

Nevertheless, it might be possible for the Wireless MAN MAC design to accommodate such a connection together with full quality of service (QoS). With technology pushing toward this horizon, it is conceivable that the standard could evolve to support increasingly nomadic and mobile usages such as in stationary or slow moving vehicles, for instance.

This being the case, IEEE Standard 802.16 was designed to evolve as a set of air interfaces based on a common MAC protocol, albeit with physical layer specifications dependent on spectrum usage and related regulations.

Part 2 is available at The designer's basic guide to WiMAX technology: Part 2.

About the author
Dr. S. Jagannathan is the head of patents and publications for Tata Elxsi, Bangalore, India. He has over 21 years experience in systems design with technologies such as software / image coding, network wired and wireless protocols, IPv6, XDSL and benchmarking. He can be reached at Jagannathans@tataelxsi.co.in.