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FCC Approves Unlicensed Use of White Space Spectrum

by Chris Taylor | Oct 01, 2010

In a five to zero vote, the US FCC approved the use of so-called “white space spectrum,” which consists of spaces between TV channels and unused TV channels themselves from 54 MHz to 698 MHz (channels 2 – 51 in the US, each 6 MHz wide). The ruling allows the unlicensed use of this spectrum, in locations where available, for new wireless consumer devices and services. The main application in mind by backers of unlicensed use of TV white space spectrum appears to be "White-Fi," which resembles Wi-Fi but uses spectrum below 700 MHz for better penetration of walls compared to 2.4 GHz and 5.8 GHz. Several companies want to offer rural broadband service using white space spectrum by setting up White-Fi with pole-mounted access points connected to the internet using wired backhaul, or connected to each other in a mesh topology to one large internet pipe. Each AP would typically cover up to a few hundred meters of range with good indoor penetration, better than 2.4 GHz and 5.8 GHz in any case. If enough spectrum is available locally, White-Fi can potential handle single-user video streaming at 50 MBps to 100 MBps (megabytes per second). These are higher data rates than FIOS (fiber-to-the home) delivers at the moment in the US. The FCC ruling specifies a maximum EIRP of 4 watts for fixed, outdoor devices, and a maximum antenna height of 30 meters compared to average terrain. Microsoft claims to have attained unobstructed ranges up to 1 mile between two fixed white space devices in recent tests. Indoors, it appears that the 30 m antenna limit does not apply. The FCC assumes that attenuation by walls will limit the outdoor range and reduce long-range interference. Personal / portable devices are limited to 100 mW RF out, or 40 mW if adjacent TV channels are in use. The FCC also specifies spectral density limits of 16.7 mW / 100 kHz or less depending on radio traffic conditions. An earlier draft of the ruling required white-space devices to check for interference from TV stations for 30 seconds on initial power-up, then check once every 60 seconds thereafter, changing to open TV channels to avoid disrupting over-the-air TV reception. Device OEMs objected to this “sensing requirement” (in the words of the FCC) as too difficult and expensive to implement, so the FCC dropped sensing (aka cognitive radio) from its recent ruling. Instead of sensing, a fixed device has to have geolocation capabilities (GPS), connect to the internet to consult a list of ‘registered sites,’ and then determine available spectrum not used by local TV stations or other registered sites. The fixed device would then configure itself and any portable devices on the network to use only those frequencies available locally. Registered sites would include TV stations, sports and entertainment venues using wireless microphones. Channel 37 (radio astronomy) is off limits everywhere. With the database look-up requirement, fixed white space devices will cost more than unlicensed devices would, but this should work well enough for many locations. One factor is that many urban areas will not have many white space channels available, in contrast to rural areas. White space access points from Dell, Microsoft, Motorola, Google, Alvarion and Ubiquiti Networks could reach the market within a few months, but issues such as the FCC’s strict out-of-channel emission and complex RF power level management requirements make this uncertain. This seems an example of a small group of large corporations influencing regulations at the expense of smaller corporations and interests, a nail in the coffin for free over-the-air TV (local broadcasters) and wireless microphones (Shure, Sennheiser) just to name two interests affected. However, the FCC has attempted to prevent and resolve interference issues without onerous requirements on white space device makers. This will make an interesting test-case for Europe and other parts of the world looking into ways to use white space and digital dividend spectrum freed up by the switch from analog to digital TV.

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