World Interoperability for Microwave Access (WiMAX)
WiMAX refers to broadband wireless networks that are based on the IEEE 802.16 standard, which ensures compatibility and interoperability between broadband wireless access equipment. The WiMAX Forum advocacy group was started in 2003 by wireless equipment suppliers to further the standards effort and establish product certification rules. WiMAX promises to extend Wi-Fi networks across greater distances, such as a campus, and provide last-mile connectivity to ISPs and other carriers that might be miles away.

Technical overview

WiMAX is a term coined to describe standard, interoperable implementations of IEEE 802.16 wireless networks, in a rather similar way to Wi-Fi being interoperable implementations of the IEEE 802.11 Wireless LAN standard. However, WiMAX is very different from Wi-Fi in the way it works.

In Wi-Fi, the media access controller (MAC) uses contention access — all subscriber stations that wish to pass data through a wireless access point (AP) are competing for the AP's attention on a random interrupt basis. This can cause subscriber stations distant from the AP to be repeatedly interrupted by closer stations, greatly reducing their throughput. This makes services such as Voice over IP (VoIP) or IPTV, which depend on a predetermined type of Quality of Service (QoS), difficult to maintain for large numbers of users.

In contrast, the 802.16 MAC uses a scheduling algorithm, where the subscriber station only has to compete once (for initial entry into the network). After that it is allocated a time slot by the base station. The time slot can enlarge and contract, but it remains assigned to the subscriber station, meaning that other subscribers cannot use it. This scheduling algorithm is stable under overload and over-subscription (unlike 802.11). It can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

The original WiMAX standard (IEEE 802.16) specified WiMAX in the 10 to 66 GHz range. 802.16a, updated in 2004 to 802.16-2004 (also known as 802.16d), added support for the 2 to 11 GHz range. 802.16d was updated to 802.16e in 2005. Revision 802.16e uses scalable orthogonal frequency-division multiplexing (OFDM) as opposed to the non-scalable version used in revision .16d. This brings potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency. Revision .16e also adds a capability for full mobility support.

Most interest will probably be in the 802.16d and .16e standards, since the lower frequencies suffer less from signal attenuation and therefore give improved range and in-building penetration.

The WiMAX specification improves upon many of the limitations of the Wi-Fi standard by providing increased bandwidth and range and stronger encryption. It provides connectivity between network endpoints without need for direct line of sight in favourable circumstances. The non-line-of-sight propagation (NLOS) performance requires the .16d or .16e revisions, since the lower frequencies are needed. It relies upon clever use of multi-path signals.

Uses for WiMAX

A commonly held misconception is that WiMAX will deliver 70 Mbit/s, over 70 miles. Each of these may be true individually, given ideal circumstances, but they are not simultaneously true. WiMAX has some similarities to DSL in this respect, where one can either have high bandwidth or long reach, but not both simultaneously. The other feature to consider with WiMAX is that the bandwidth is shared between users in a given radio sector, so if there are many active users in a single sector, each will get reduced bandwidth.

The bandwidth and reach of WiMAX make it suitable for the following potential applications:

• Connecting Wi-Fi hotspots with each other and to other parts of the Internet
• Providing a wireless alternative to cable and DSL for last mile (last km) broadband access.
• Providing high-speed mobile data and telecommunications services (4G)

Broadband Access

Many cable and traditional telephone companies are closely examining or actively trialling the potential of WiMAX for "last mile" connectivity. This could result in better price-points for both home and business customers as based on the benefits of competition. In areas without pre-existing physical cable or telephone networks, WiMAX could allow broadband access that has hitherto been unavailable. Home units the size of a paperback book that provide both phone and network connection points are already available and easy to install.

Mobile applications

There is potential for using WiMAX with legacy cellular networks. WiMAX antennas can "share" a cell tower without compromising the function of cellular arrays already in place. Some cellular companies are evaluating WiMAX as a means of increasing bandwidth for a variety of data-intensive applications; indeed, Sprint Nextel has announced in mid-2006 that it will be investing about US$ 3 billion in a WiMAX technology buildout over the next few years. In line with these possible applications is the technology's ability to serve as a high bandwidth "backhaul" for Internet or cellular phone traffic from remote areas back to an internet backbone. Although the cost-effectiveness of WiMAX in a remote application will be higher, it is definitely not limited to such applications, and may in fact be an answer to reducing the cost of T1/E1 backhaul as well. Given the limited wired infrastructure in some developing countries, the costs to install a WiMAX station in conjunction with an existing cellular tower or even as a solitary hub are likely to be small in comparison to developing a wired solution. Areas of low population density and flat terrain are particularily suited to WiMAX and its range. For countries that have skipped wired infrastructure as a result of inhibitive costs and unsympathetic geography, WiMAX can enhance wireless infrastructure in an inexpensive, decentralized, deployment-friendly and effective manner.

Spectrum Allocations for WiMAX

The 802.16 specification applies across a wide swath of RF spectrum. However, specification is not the same as permission to use. There is no uniform global licensed spectrum for WiMAX. In the US, the biggest segment available is around 2.5 GHz, and is already assigned, primarily to Sprint Nextel, along with Clearwire in more rural areas. Elsewhere in the world, the most likely bands used will be around 3.5 GHz, 2.3/2.5 GHz, or 5 GHz, with 2.3/2.5 GHz probably being most important in Asia.

There is some prospect in the U. S. that some of a 700 MHz band might be made available for WiMAX use, but it is currently assigned to analog TV and awaits the complete rollout of digital TV before it can become available, likely by 2009. In any case, there will be other uses suggested for that spectrum if and when it actually becomes open.

It seems likely that there will be several variants of 802.16, depending on local regulatory conditions and thus on which spectrum is used, even if everything but the underlying radio frequencies is the same. WiMAX equipment will not, therefore, be as portable as it might have been - perhaps even less so than WiFi, whose assigned channels in unlicensed spectrum varies little from jurisdiction to jurisdiction.

The actual radio bandwidth of spectrum allocations is also likely to vary. Typical allocations are likely to provide channels of 5 MHz or 7 MHz. In principle the larger the bandwidth allocation of the spectrum, the higher the bandwidth that WiMAX can support for user traffic.

Standards

The current 802.16 standard is IEEE Std 802.16e-2005[1], approved in December 2005. It followed on from IEEE Std 802.16-2004[2], which replaced IEEE Standards 802.16-2001, 802.16c-2002, and 802.16a-2003.

IEEE Std 802.16-2004 (802.16d) addresses only fixed systems. 802.16e adds mobility components to the standard.

IEEE 802.16e

IEEE 802.16-2005 (formerly named, but still best known as, 802.16e or Mobile WiMAX) provides an improvement on the modulation schemes stipulated in the original (fixed) WiMAX standard. It allows for fixed wireless and mobile Non Line of Sight (NLOS) applications primarily by enhancing the OFDMA (Orthogonal Frequency Division Multiple Access).

SOFDMA improves upon OFDM256 for NLOS applications by:

• Improving NLOS coverage by utilizing advanced antenna diversity schemes, and hybrid-Automatic Retransmission Request (hARQ)
• Increasing system gain by use of denser sub-channelization, thereby improving indoor penetration
• Introducing high-performance coding techniques such as Turbo Coding and Low-Density Parity Check (LDPC), enhancing security and NLOS performance
• Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa
• Improving coverage by introducing Adaptive Antenna Systems (AAS) and Multiple Input Multiple Output (MIMO) technology
• Eliminating channel bandwidth dependencies on sub-carrier spacing, allowing for equal performance under any RF channel spacing (1.25-14 MHz)
• Enhanced Fast Fourier transform (FFT) algorithm can tolerate larger delay spreads, increasing resistance to multipath interference

SOFDMA and OFDMA256 are not compatible so most equipment will have to be replaced. However, some manufacturers are planning to provide a migration path for older equipment to SOFDMA compatibility which would ease the transition for those networks which have already made the OFDMA256 investment.

The equivalent of 802.16 in Europe is HIPERMAN. The WiMAX Forum is working to ensure that 802.16 and HIPERMAN inter-operate seamlessly.

WiBro

Korea's telecoms industry has developed its own standard, WiBro. In late 2004, Intel and LG Electronics have agreed on interoperability between WiBro and WiMAX.

WiBro has South Korean government support with the requirement for each carrier to spend over US$1 billion for deployments. The Koreans sought to develop WiBro as a regional and potentially international alternative to 3.5-4G systems. But given the lack of self-developed momentum as a standard, WiBro has joined WiMAX and agreed to harmonize with the similar OFDMA 802.16e version of the standard. What makes WiBro roll-outs, which will start in April 2006, a good 'test case' for the overall WiMAX effort is that it is mobile, well thought out for delivery of wireless broadband services, and the fact that the deployment is taking place in a highly sophisticated, broadband-saturated market. WiBro will go up against 3G and very high bandwidth wire-line services rather than as gap-filler or rural under-served market deployments as is often exampled as the 'best fit' markets for WiMAX.

Associations

WiMAX Forum

The WiMAX Forum is "the exclusive organization dedicated to certifying the interoperability of BWA products, the WiMAX Forum defines and conducts conformance and interoperability testing to ensure that different vendor systems work seamlessly with one another." Those that pass conformance and interoperability testing achieve the "WiMAX Forum Certified" designation and display this mark on their products and marketing materials. Vendors claiming their equipment is "WiMAX-ready", "WiMAX-compliant", or "pre-WiMAX" are not WiMAX Forum Certified, according to the Forum. ^[2]

WiMAX Spectrum Owners Alliance - WiSOA

WiSOA is the first global organisation composed exclusively of owners of WiMAX spectrum. WiSOA is focussed on the regulation, commercialisation, and deployment of WiMAX spectrum in the 2.3–2.5 GHz and the 3.4–3.5 GHz ranges. WiSOA is dedicated to educating and informing its members, industry representatives and government regulators of the importance of WiMAX spectrum, its use, and the potential for WiMAX to revolutionise broadband.^[3]

Competing technologies

WiMAX is a framework for wireless development based on a forward-looking core set of technologies. More recently 3GPP cellular's 4G, 802.22 Cognitive Radio RAN (Rural Area Network), and 802.20, the High Speed Mobile Broadband Wireless Access (MBWA) Working Group, have shifted toward use of similar constructs of multi-channel scalable OFDM, HARQ, FEC, MIMO-AAS and other complementary technologies as are part of WiMAX.

UMTS

For some applications, UMTS could be a direct competitor to WiMAX. UMTS has been deployed in Europe and elsewhere mostly by Mobile Telephone operators. The HSDPA technology enables down-link with data transmission up to 8-10 Mbit/s (see above for comparison). UMTS also provides a circuit channel optimized for voice and video traffic. In July 2005 EU frequency allocation for WiMAX was blocked by France and Finland, where manufacturers have invested heavily in UMTS technology.

The most recent 3GPP standardization activities are development of advanced systems based on OFDM rather than CDMA. The 3G Long Term Evolution (LTE) platform will be based on MIMO-OFDM similar to WiMAX/802.16e-2005.

EV-DO

Evolution-Data Optimized is a wireless radio broadband data standard adopted by many CDMA mobile phone service providers around the globe. It is standardized by 3GPP2, as part of the CDMA family of standards.

Wi-Fi

Wi-Fi is a Wireless Local Area Network (LAN) technology that works in unlicensed spectrum, using the 2.4GHz and 5 GHz bands. Wi-Fi is a cheap and easy way of providing local connectivity at high speed. WiMAX uses licenced spectrum and has strong authentication mechanisms built in. It has considerably greater range than Wi-Fi. Taken together, this means that WiMAX and Wi-Fi are generally complementary rather than competing.

Mobile Broadband Wireless Access

MBWA is a technology under development by IEEE 802.20. It is a future technology for true wireless broadband or 4G.

Deployment

The following are current and planned deployments, the bands in which they operate and the standards they use.

Australia:

• Unwired holds licences in the 3.5GHz and 2.3GHz bands and is gradually dropping its proprietary system for a 802.16e based system.
• Austar holds licences in the 3.5GHz and 2.3GHz bands.

Austria:

• WiMAX Telecom holds 3.5GHz licences for the entire country.

Brazil:

• Brazilian Telecomunications Agency to sell licences for WiMAX. See news (in portuguese)

Croatia:

• VIPnet and Odašiljaèi hold 3.5GHz licences for the capital, Zagreb.
• Portus, Optima Telekom, WiMAX Telecom and OiV hold 3.5GHz licences for some regions.

Colombia:

• Cali was the first city in Colombia and SouthAmerica with an official WiMAX network. Orbitel, ETB and EPM hold 3.5GHz licences for the entire country.

Ireland:

• Irish Broadband Irish Broadband holds 3.5 GHz licenses in Ireland.
• Lastmile Broadband also hold regional 3.5GHz licences and plan to deploy a WiMAX network in the fourth quarter of 2006. [3]

Slovakia:

• WiMAX Telecom holds 3.5GHz licences.

UK:

• UK Broadband, a subsidiary of PCCW, owns 3.5 GHz licenses.

USA:

• Clearwire holds 2.5GHz licences in several regions.
• Sprint Nextel holds licences in the 2.5GHz band.

Source: Wikipedia.