WiMAX

The 802.16 standard, amended this January by the IEEE to cover frequency bands in the range between 2 GHz and 11 GHz, specifies a metropolitan area networking protocol that will enable a wireless alternative for cable, DSL and T1 level services for last mile broadband access, as well as providing backhaul for 801.11 hotspots. The new 802.16a standard specifies a protocol that among other things supports low latency applications such as voice and video, provides broadband connectivity without requiring a direct line of sight between subscriber terminals and the base station (BTS) and will support hundreds if not thousands of subscribers from a single BTS.

The standard will help accelerate the introduction of wireless broadband equipment into the marketplace, speeding up last-mile broadband deployment worldwide by enabling service providers to increase system performance and reliability while reducing their equipment costs and investment risks.

Overview of the IEEE 802.16a Standard

Satisfying the growing demand for Broadband Wireless Access (BWA) in underserved markets has been a continuing challenge for service providers, due to the absence of a truly global standard. A standard that would enable companies to build systems that will effectively reach almost every possible area of the city without much infrastructure hassles like those which come up in case of DSL, Cable, etc. For years, the wildly successful 802.11x or WiFi wireless LAN technology has been used in BWA applications along with a host of proprietary based solutions. The WLAN technology was not well suited for outdoor BWA applications. It could be done, but with limited capacity in terms of bandwidth and subscribers, range and a host of other issues made it clear this approach while a great fit for indoor WLAN was a poor fit for outdoor BWA.

This analysis and review was conducted by the IEEE and it was decided that a new, more complex and fully developed standard would be required to address both the physical layer environment (outdoor versus indoor RF transmissions) and the Quality of Service (QoS) needs demanded by the BWA and last mile access market. The IEEE conducted a multi-year effort to develop this new standard, culminating in final approval of the 802.16a Air-Interface Specification in January 2003. This standard has since received broad industry support from leading equipment makers.

The 802.16a standard delivers carrier class performance in terms of robustness and QoS and has been designed from the ground up to deliver a suite of services over a scalable, long range, high capacity “last mile” wireless communications for carriers and service providers around
the world.

In BWA, applications include residential broadband access– DSL-level service for SOHO and small businesses, T1/E1 level service for enterprise, all supporting not just data but voice and video as well, wireless backhaul for hotspots and cellular tower backhaul service to name a few.

In reviewing the standard, the technical details and features that differentiate WiMAX certified equipment from WiFi or other technologies can best be illustrated by focusing on the two layers addressed in the standard, the physical (PHY) or RF transmissions and the media access control (MAC) layer design.

802.16a PHY Features

802.16a MAC Features

Differentiating the IEEE 802.16a and 802.11 Standards - WiFi versus WiMAX Scalability

At the PHY layer the standard supports flexible RF channel bandwidths and reuse of these channels (frequency reuse) as a way to increase cell capacity as the network grows. The standard also specifies support for automatic transmit power control and channel quality measurements as additional PHY layer tools to support cell planning/deployment and efficient
spectrum use. Operators can re-allocate spectrum through sectorization and cell splitting as the number of subscribers grows. Also, support for multiple channel bandwidths enables equipment makers to provide a means to address the unique government spectrum use and allocation regulations faced by operators in diverse international markets. The IEEE 802.16a standard specifies channel sizes ranging form 1.75MHz up to 20MHz with many options in between. WiFi based products on the other hand require at least 20MHz for each channel (22MHz in the 2.4GHz band for 802.11b), and have specified only the license exempt bands 2.4GHz ISM, 5GHz ISM and 5GHz UNII for operation. In the MAC layer, the CSMA/CA foundation of 802.11, basically a wireless Ethernet protocol, scales about as well as does Ethernet. That is to say - poorly. Just as in an Ethernet LAN, more users results in a geometric reduction of throughput, so does the CSMA/CA MAC for WLANs. In contrast the MAC layer in the 802.16 standard has been designed to scale from one up to 100’s of users within one RF channel, a feat the 802.11 MAC was never designed for and is incapable of supporting.

Coverage

The BWA standard is designed for optimal performance in all types of propagation environments, including LOS, near LOS and NLOS environments, and delivers reliable robust performance even in cases where extreme link pathologies have been introduced. The robust OFDM waveform supports high spectral efficiency (bits per second per Hertz) over ranges from 2 to 40 kilometers with up to 70 Mbps in a single RF channel. Advanced topologies (mesh networks) and antenna techniques (beam-forming, STC, antenna diversity) can be employed to improve coverage even further. These advanced techniques can also be used to increase spectral efficiency, capacity, reuse, and average and peak throughput per RF channel. In addition, not all OFDM is the same. The OFDM designed for BWA has in it the ability to support longer range transmissions and the multi-path or reflections encountered.

In contrast, WLANs and 802.11 systems have at their core either a basic CDMA approach or use OFDM with a much different design, and have as a requirement low power consumption limiting the range. OFDM in the WLAN was created with the vision of the systems covering tens and maybe a few hundreds of meters versus 802.16 which is designed for higher power and an OFDM approach that supports deployments in the tens of kilometers.

QoS

The 802.16a MAC relies on a Grant/Request protocol for access to the medium and it supports differentiated service levels (e.g., dedicated T1/E1 for business and best effort for residential). The protocol employs TDM data streams on the DL (downlink) and TDMA on the UL (uplink), with the hooks for a centralized scheduler to support delay-sensitive services like
voice and video. By assuring collision-free data access to the channel, the 16a MAC improves total system throughput and bandwidth efficiency, in comparison with contention-based access techniques like the CSMA-CA protocol used in WLANs. The 16a MAC also assures bounded delay on the data (CSMA-CA by contrast, offers no guarantees on delay).
The TDM/TDMA access technique also ensures easier support for multicast and broadcast services.

With a CSMA/CA approach at its core, WLANs in their current implementation will never be able to deliver the QoS of a BWA, 802.16 system.