The Roadmap to Mobile Wimax by Rupert Baines

Trying to replicate what Wi-Fi did for wireless LANs, WiMAX has arrived with the promise of standardisation in the wireless broadband access, which effectively means bringing costs down through the introduction of volume, off-the-shelf components and subsystems. While the ultimate promise is the extension of such concept to the mobile environment, several key elements must be addressed before we can get there.

Although there has been an undeniable element of hype, the importance of WiMAX is hard to deny. Most analysts agree that standardisation under the WiMAX banner will help drive the fixed wireless market forward after many false starts. It provides equipment vendors with off-the-shelf silicon and reduces costs to levels where volume deployment is possible. Just as many of the two billion cellular users are in areas with no fixed network, WiMAX has the huge potential of bringing broadband access to the hundreds of millions of people around the world who do not have access to cable or DSL.

Even though it has been designed to maximise 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 different options, each with slightly different technology or requirements. Of these, the most significant is arguably mobile WiMAX, or IEEE 802.16e. Many see this as the most exciting area in wireless today, with the potential to leapfrog 3G.

One of the most fascinating things of this technology is how it encompasses a wide range of applications – more than address a market it serves a complex portfolio of different markets, each with somewhat distinct characteristics. Another unique aspect is the wide variety of radio options supported within the one standard.

DEPLOYMENT CHOICES
There are several ways in which WiMAX can be deployed. One is high-bandwidth, point-to-point backhaul (for example from 2G/3G sites or Wi-Fi hotspots). A second market is “metro Ethernet”, where bandwidths of 10Mbit/s and above are provided on a point-to-multipoint basis, competing with fibre.

Another possibility is commercial broadband access, where competitive operators can use WiMAX in the 1Mbit/s to 10Mbit/s range as an alternative to DSL or cable modem, 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 – a market of potentially billions of users worldwide.

And then there is of course mobility, ranging from nomadic use (“super hotspots”) through portable to true high-speed mobile data services. Uniquely for wireless standards, WiMAX does not specify the radio types, and allows for both FDD & TDD with channels of 1.75MHz-28MHz and almost any carrier (there are, for example, operators using WiMAX at 450MHz, 700MHz, 1.9GHz, 2.3GHz, 3.5GHz, 4.9GHz and 5.8GHz – and probably others).

This variety of frequencies and modes is a big difference to other wireless standards – especially Wi-Fi, which uses spectrum in bands that are unlicensed in most of the world’s regions, 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, which spans both licensed and unlicensed bands that also differ from country to country. As such, WiMAX deployments will range from 450MHz to 6GHz. (Just compare this easygoing attitude to that of the cellular philosophy, which invents a new description for the occasion of shifting a standard to a new frequency: CDMA450 or PCS as distinct from GSM)

Licensed bands (e.g. 3.5GHz in many countries) allow operators to manage frequency planning; for unlicensed bands (e.g. 5.8GHz), different techniques are needed. While carrier-sense multiple access (CSMA) 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 MAC layers.

Another distinction to mention is simply the environment. Wi-Fi is intended for local coverage with relatively short distances and simple radio environments. In contrast, WiMAX can be used in a huge variety of ways, many with extremely long range and corresponding variety in channel conditions. Mobility, with its fast fades, further complicates things. This is why the PHY signal processing is so much more sophisticated in .16 than in its “little brother” .11, and the DSP algorithms are dramatically harder.

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

Although WiMAX was created to promote 802.16, it has deliberately defined a small subset of options and pre-defined profiles in order to simplify implementation. For example, WiMAX only supports one of three possible options in 16d: 256OFDM (i.e. orthogonal frequency division multiplexing with 256 tones), which is highly suited to non-line-of-sight environments and excludes the single carrier and OFDMA2048 modes.

With 802.16-2004 published, attention has shifted to developing the 802.16e mobile standard, which opens up competition with 3G cellular networks (see box ‘Battling it out with HSDPA’). 16e extends this with a scalable OFDMA system, delivering further improvements at the expense of complexity, with a scalable FFT size (proportional to channel). This standard will add further complex PHY-layer processing together with handoff signals, to allow users in vehicles to seamlessly switch from one base station to another. Forward error correction (FEC) in 16d used convolutional coding; in 16e that is optionally extended with a very powerful – but very complex – convolutional turbo code (CTC). In WiBRO (the Korean version of WiMAX), that is a mandatory option.

Uplink subchannelisation is an optional feature in 16d, and is generating a lot of interest from operators. This allows a subscriber station to concentrate its transmit power on a subset of the total OFDM subcarriers, leading to link budget improvements in the uplink (which translates into coverage and capacity benefits). Multiple subscriber stations can be scheduled to transmit simultaneously on different subchannels.

WiMAX was designed from the start to support smart-antenna systems, including RX antenna diversity, TX antenna diversity, beam forming, space time coding (STC) and multiple-input, multiple-output (MIMO). These systems 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. A WiMAX Forum White Paper1 suggests that for the same circumstances (3.5MHz FDD, 3.5GHz band) adding subchannelisation, diversity and STC to a base station could increase coverage from 2km to 9km range. That is a twenty-fold increase in coverage, and potentially in subscribers. Consequently, most practical systems will choose to support these options, despite the complication they bring.

Mobile WiMAX does compete with several other mobile broadband techlogies, including WCDMA, CDMA2000 EVDO, TD-SCDMA and Flash-OFDM. Among the advantages that mobile WiMAX claims over these are:

• Superior airlink technology. Scalable OFDMA is a very modern, sophisticated modulation method that can reliably deliver high performance (bit/s/Hz) even in challenging multi-path environments
• Network efficiency. WiMAX is an inherently IP-based system and intends to create an open architecture for mobile data networks, significantly reducing complexity and cost
• Full QoS. WiMAX includes a sophisticated and versatile MAC layer, with extremely good support for management of QoS. This is especially important for multimedia and voice (VoIP) services
• Applications. Superior transparency to applications in WiMAX will encourage faster adoption of the service by enabling performance equivalent to and – in some cases – better than wireline access technologies. Mobility data with this level of performance promises to open up new applications as well

While there is some truth in these, there is an element of caution. In reality, the laws of physics do impose some hard limits; better air interfaces are an improvement, but Shannon’s Law still holds.

However, an area where the laws of physics don’t have so much sway is the financial space, and here there are very good reasons why WiMAX has advantages. Fewer royalties, less expensive system architecture, increased competition from open markets all deliver lower costs. The possibility of higher performance yields higher average revenues for significantly better economics.

MOBILITY
The present WiMAX standard, 802.16-2004 (also known as 802.16d), is for fixed applications only: 16e adds support for mobility (although it is also expected to be adopted for some fixed installations, since it offers a better link budget than 16d).

Despite the name, there is no backward compatibility between 16e and 16d. Another variant is Korea’s WiBRO, which is essentially the first version of 16e, in much the same way as FOMA was for WCDMA. Licences for WiBRO have already been issued and the service is due to launch next Spring.

A major consideration is seamless handoff. Cellular-based standards have the advantage of many years experience in handoff for voice calls, while for broadband mobility in itself is no mean feat, and handoff is still a challenge. Mobile IP, with “slow” handoff, will be fine for web-browsing but not enough for decent voice. Many services require the appearance of seamless connections (VoIP, VPNs, etc). Much of the complexity (and latency) in the cellular network is from maintaining these connections across cell boundaries.

WiBRO will launch with mobility (i.e. support for fast-moving users in a train) but with “simple” handover, where the session is passed but not totally transparently. True mobility will follow, perhaps in the 2007/08 timeframe.

Because there are so many options under the WiMAX umbrella, it will clearly be a much more complex standard to support than Wi-Fi. Flexibility will be the key to successful adoption, as deployment challenges in the field will lead to changes being demanded. This will be a prime consideration when it comes to selecting an architecture that supports the evolution of WiMAX from its current position, where standardisation is not yet complete.