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Frequency
WiMAX is a flexible and scalable standard that may be adapted to different frequency bands. The standard is torn between two opposite goals. On the one hand, limiting frequency bands and channel bandwidths narrow down the standard and make interoperability easier while on the other, profiles in different bands and using different channel widths make the standard more flexible.
Frequency bands and frequency channel widths are standardized in different WiMAX profiles. There are many reasons behind the choices made
for these bands, including spectrum availability and regulations in different countries. The bands of highest interest for WiMAX are presented below:
2.3 GHz: In the United States, a 1997 auction for wireless communi- cations service (WCS) addressed 30 MHz of spectrum, which was then left unused for a long time. WiBro-related products may soon change that.
2.5 GHz: Educational broadband services (EBS) and broadband radio services (BRS) occupy a large band of spectrum above 2.5 GHz.∗ Renewed interest comes from the high priority given to these bands for 802.16e mobile WiMAX products.
Broadband access at 3.4–3.7 GHz: In many countries, the spectrum between 3.4 and 3.6 GHz was allocated (in most cases auctioned) for fixed broadband wireless access. This band was the first to see WiMAX certified products. In the United States, 3.65–3.7 GHz was allocated in March 2005 for fixed and mobile service, which unfortu- nately provides much less spectrum. Operations in the band should be licensed on a nationwide nonexclusive basis with all licensees registering their fixed stations in a common database.† Protection zones of 150 km were established around the grandfathered fixed satellite stations.
Unlicensed spectrum at 5.4–5.8 GHz: In the WiMAX community, some equipment manufacturers and service providers are interested in unlicensed (or license exempt) bands of spectrum. In the United States, these bands are governed by Part 15 of the FCC Rules & Reg- ulations: they may not cause harmful interference to authorized services and have to follow listen-before-talk rules.
Several unlicensed bands exist and have great potential for fixed access, but only the highest is the focus of WiMAX. There are several reasons for this: the 900 MHz band benefits from great propagation characteristics but is limited in power and bandwidth; the 2.4 GHz band is wider but has recently seen heavy deployment of Wi-Fi LANs.
The 5 GHz band is referred to as the unlicensed national infor- mation and infrastructure (UNII) band. Its upper portion (UNII-3, 5.725–5.825 GHz) is intended for community networking com- munications devices operating over a range of several kilometers.
∗ Formerly MMDS and ITFS, these spectrum bands are now referred to as EBS and BRS spectrum bands. A new band plan was proposed by FCC to transition the old 6 MHz analog TV channels to 5.5 MHz channels.
† The WiMAX Forum and several member companies have asked the FCC to adopt an exclusive licensing regime for the 3.65–3.7 GHz band in the top 50 metropolitan statistical areas (MSAs), while retaining its nonexclusive licensed approach in smaller markets.
Combined with a new 5.475–5.725 GHz∗ band recently opened by the FCC, over 400 MHz of spectrum is now available for unlicensed operations.
Other bands of spectrum are of interest to the WiMAX Forum, although no specific profiles have been defined for them yet.
UHF channels at 700 MHz: TV broadcasting spectrum is very attrac- tive for broadband wireless applications because of its excellent propagation and in-building penetration characteristics. In the United States, TV broadcasters must transition to digital television and return their 700 MHz analog frequencies by February 18, 2009. This opens large bands of spectrum for potential use in wireless communications. Suppliers are already developing equipment in these bands based on 802.16 and WiMAX.
AWS at 1.7–2.1 GHz: In August 2006, the FCC auctioned 90 MHz of spectrum for advanced wireless services (AWS). This band was somewhat puzzling to equipment manufacturers because of its pairing with a rather large interval between forward and reverse links (400 MHz); still, WiMAX and 3G services can be expected in this band.
Public safety at 4.9 GHz: In 2002, the FCC designated 50 MHz of spec- trum in the 4.9 GHz band for exclusive public safety use. WiMAX services are appropriate for public safety applications. Products exist in that band and plugfest initiatives started in 2006 for operations between suppliers.
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