WiMax opens wide range of design options

Even though it has been designed to maximize interoperability by taking only a subset of the much wider IEEE 802.16 standards on which it is based, WiMax encompasses a wide range of options, each with a slightly different technology or set of requirements: There is not one WiMax market, but a portfolio of distinct niches.

Some of these niches arise through regulation. Wi-Fi uses spectrum in bands that are unlicensed in most regions of the world, allowing the development of a large, homogeneous market. The situation for WiMax is much more complex due to the higher transmit-power levels and the fragmented radio spectrum in both licensed and unlicensed bands, which differ from country to country. As such, WiMax deployments are envisaged ranging from 450 MHz to more than 6 GHz. While carrier-sense multiple access is sufficient for Wi-Fi, a much more rigorous radio access control mechanism is required for WiMax, leading to increased complexity in the physical and media-access control (MAC) layers. Coexistence issues for WiMax are a further wrinkle.

Further, the standardization of WiMax has yet to be completed, so equipment builders are still chasing a moving target.

There are several ways WiMax can be deployed. One is high-bandwidth, point-to-point back haul, for example from 2G/3G sites or Wi-Fi hotspots. A second market is "metro Ethernet," where bandwidths from 10 Mbits/second and upward are provided on a point-to-multipoint basis, competing with fiber. New and rural operators can use WiMax in the 1-Mbit/s to 10-Mbit/s range as an alternative to DSL or cable modems, potentially with longer range and, hence, better economics.

While this can be a competitive offering, the opportunities are most powerful in territories without much installed copper plant, using WiMax to obtain access to the Internet-a potential market of billions of users worldwide. Finally, of course, there is mobility: It ranges from nomadic use ("super hotspots") through portable to high-speed mobile data services, adding a further range of options.

The first version of the IEEE 802.16 standard released addressed line-of-sight environments using comparatively high-frequency bands in the 10- to 66-GHz range. The most recently published standard, 802.16-2004, describes 2 to 11 GHz, allowing support for non-line-of-sight (NLOS) environments. Three completely new physical (PHY) layers were added, together with a number of modifications to the MAC, with knock-on effects in terms of the digital processing needed. Further changes have been proposed to allow more efficient use of the radio spectrum at lower frequencies-for example, 450 MHz.

With 802.16-2004 published, attention has shifted to developing the 802.16e standard, adding mobility and opening up competition with 3G cellular networks. This standard will add further complex PHY-layer processing together with handoff signals to allow users in vehicles to switch from basestation to basestation seamlessly.

WiMax was created to promote 802.16, but the two are not identical: WiMax has very deliberately defined a small subset of options and predefined profiles. This simplifies implementation, although there are still many configurations and optional features that designers have to cater to. WiMax has specified the OFDM and 256-point fast Fourier transform (FFT) mode of the 802.16-2004 standard, ignoring the single-carrier and orthogonal frequency-division multiple-access (OFDMA) 2,048 modes. This specified mode is highly suited to NLOS environments, as the symbol period for each subcarrier is much greater than the maximum delay spread, simplifying equalization. Forward error correction (FEC) makes it possible to tolerate deep fades, caused by multipathing, on multiple subcarriers. The 16e specification extends this with a scalable OFDMA system, delivering further improvements at the expense of complexity.

Subchannelization is an optional feature in OFDM 256 that is generating a lot of interest from operators. It allows a subscriber station to concentrate its transmit power on a subset (subchannel) of the total OFDM subcarriers, leading to link budget improvements in the uplink. These translate into coverage and capacity benefits. Multiple subscriber stations can be scheduled to transmit simultaneously on different subchannels.

A sophisticated intelligent scheduler delivers the appropriate quality of service to each subscriber station using the optimum coding and modulation, calculated from channel conditions measured at the basestation and also reported by the subscriber station. Coding and modulation interact to provide the most efficient form of transmission for each channel condition, for each user.

WiMax was designed from the start to support smart-antenna systems. These are becoming more affordable and their ability to suppress interference and increase system gain will see them introduced to WiMax implementations in the near term. There are several main types of smart antennas. Two varieties are commonly used today, often with subchannelization.

The WiMax Forum has published a white paper discussing the impact of different features and the importance of non-line-of-sight coverage (www.wimaxforum.org/news/downloads/WiMAXNLOSgeneral-versionaug04.pdf). It suggests that for the same circumstances (3.5-MHz frequency-division duplex, 3.5-GHz band), adding subchannelization, diversity and space-time coding to a basestation could increase the coverage range from 2 to 9 km-a twentyfold increase in coverage and, potentially, in subscribers. The other varieties will become increasingly important, especially for higher frequencies where propagation becomes harder.

Along with mobility support, 16e adds several features to the PHY, including OFDMA, a scalable FFT size (proportional to channel) and better FEC schemes.

But all of these options come under the WiMax banner, making it a much more complex standard to support than Wi-Fi ever will be. Flexibility will be the key to successful adoption as deployment challenges in the field will lead to a demand for changes. This will be a prime consideration when it comes to selecting an architecture that supports WiMax not only as it stands today but in how it will evolve in the future.

Patrick Fuller (patrickf@picochip.com), senior applications engineer and lead applications engineer for IEEE 802.16 at picoChip Designs Ltd. (Bath, England).