<font color=Purple>802.16a preps wireless for prime time
By John Liebetreu -- EE Times -- May 16, 2003 (11:36 a.m. ET)
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The 802.16 standard-amended in 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 services for last-mile broadband access and provide backhaul for 801.11 hotspots.
The new 802.16a standard specifies a protocol that provides broadband connectivity without requiring a direct line of sight to close the link between subscriber terminals and the basestation. The standard will 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.
In addition, a new, nonprofit organization, the WiMax Forum, has been chartered to remove an important barrier to adoption of the standard by assuring demonstrable interoperability among system components developed by OEMs. WiMax will develop conformance test plans, select certification labs and host interoperability events for IEEE 802.16 equipment vendors.
By defining and conducting interoperability testing, and by awarding vendor systems a "WiMax-certified" label, WiMax will model the approach pioneered by the Wi-Fi Alliance to ignite the wireless-LAN industry, bringing the same benefits to the broadband wireless-access market segment.
The 802.16a standard was designed from the ground up to deliver scalable, long-range, high-capacity "last-mile" wireless communication for Internet and voice service providers. <font color=Black>Although it already addresses fixed-service requirements, development is now under way to specify enhancements to the technology that will support mobile broadband wireless-access applications, including nomadic coverage.
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The 802.16 standard addresses line-of-sight environments at high-frequency bands in the 10- to 66-GHz range, whereas the 802.16a standard is designed for systems operating in bands between 2 and 11 GHz, where non-line-of-sight (NLOS) environments predominate. To address NLOS requirements, the 802.16a amendment to the standard required major changes to the physical (PHY)-layer specification-achieved through the introduction of three new PHY-layer specifications: single carrier, 256-carrier orthogonal frequency-division multiplex (OFDM) and orthogonal frequency-division multiple access (OFDMA)-as well as significant media-access control (MAC)-layer enhancements. Although multiple PHYs are specified, the overwhelming majority of vendors are expected to implement the 256-carrier OFDM PHY.
Simpler design
The OFDM signaling format was selected along with single-carrier (SC) instead of competing technologies such as CDMA. <font color=Black>This is due to OFDM's superior NLOS performance, a characteristic that permits significant design simplification for equalizers needed to support operation in multipath environments. For SC, severe multipath environments require long equalizers, and degraded performance can result if the equalizer is not appropriately matched to the character of the multipath interference. In the case of CDMA (prevalent in 2G and 3G standards), the RF bandwidth must be much larger than the data throughput, in order to maintain processing gain adequate to overcome interference. This is clearly impractical for broadband wireless below 11 GHz since, for example, data rates up to 70 Mbits/second require RF bandwidths exceeding 200 MHz to deliver comparable processing gains.
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Features of 802.16a that are instrumental in giving this technology the power to deliver robust performance in a broad range of channel environments are adaptive burst profiles, forward error correction with concatenated Reed-Solomon and convolutional encoding (block turbo codes and convolutional turbo codes are optional), advanced antenna systems to improve range and capacity, dynamic frequency selection-which helps in minimizing interference-and space-time coding to enhance performance in fading environments through spatial diversity.
Every wireless network uses a shared medium-in the case of broadband wireless, the atmospheric medium supporting radio wave propagation-that requires a mechanism for controlling access. The 802.16a standard uses a slotted protocol scheduled by the basestation to allocate capacity to subscribers in a point-to-multipoint network topology.
Since the 802.16a MAC relies on a grant/request protocol for capacity assignment, it supports differentiated service levels (e.g., Tx/OCx for business and best effort for residential). The protocol employs time-division multiplex (TDM) data streams on the downlink and TDMA (time-division multiple access) on the uplink, with a centralized scheduler to guarantee efficient and prioritized allocations. That makes it a natural fit for 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, compared with contention-based access techniques like carrier-sense multiple access, collision avoidance (CSMA-CA).
Smart power savings
The .16a MAC also ensures bounded delay on the data (CSMA-CA, by contrast, offers no guarantees on delay). The TDM/TDMA access technique additionally ensures easier support for multicast and broadcast services. And since subscriber allocations are predetermined, smart power-saving algorithms can be developed to support low-power terminals.
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 transmit power control and channel quality measurements as additional tools to support cell planning and efficient spectrum use. The standard has been designed to scale up to hundreds or even thousands of users within one RF channel. Operators can reallocate spectrum through sectorization and cell splitting as the number of subscribers grows. Also, support for multiple channel bandwidths enables equipment makers to provide a way to address the unique spectrum use and allocation regulations faced by operators in diverse international markets.
Fiberlike performance
Meanwhile, the standard is designed for optimal performance in all types of propagation environments, including line-of-sight, near-LOS and non-LOS, and delivers fiberlike performance even in cases where extreme link pathologies have been introduced by signal reflections. The robust OFDM modulation scheme yields high spectral efficiency (bits/second/Hz) over ranges from 2 to 40 kilometers with up to 70 Mbits/second in a single RF channel. Advanced topologies (mesh networks) and antenna techniques (beam forming, space-time coding, 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.
John Liebetreu is the chief technologist in broadband wireless access at Intel Corp. (Phoenix).
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