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Wednesday, June 29, 2011

TV White Spaces: Spectrum Sharing in Licensed Bands

The main interest in the TV bands is driven by the worldwide switchover from analogue to digital terrestrial TV which will release a large portion of spectrum known as the “digital dividend”, and might also make available the guard bands between existing or new broadcasting channels (the so-called TV White Spaces or TVWS) available for opportunistic use. These frequency bands are very valuable as they provide propagation characteristics that enable signals to penetrate thick walls and travel long distances. The general consensus worldwide is that at least part of this digital dividend and TVWS spectrum should be allocated for mobile broadband services.

The debate on these so-called ‘white spaces’, is regularly grabbing headlines in the media with a noticeable interest from and role for new players in the wireless field. The debates centers on the free use of White Space Devices (WSD), i.e. devices that could opportunistically make use of free spectrum for various goals. Parties in favor of these devices see major opportunities for the freed up spectrum, including more mobile broadband services, offering better quality of service, to more users. Traditional users of the freed-up spectrum bands and adjacent spectrum bands such as broadcasters are concerned about the consequences of this type of unlicensed use that could interfere with licensed use. They demand a highly regulated approach.

In the US, the FCC adopted rules for unlicensed use of television white spaces already in 2008. This announcement was a major endorsement for Cognitive Radio and will softly introduce spectrum sensing technologies in the market in the coming years. The FCC has proposed to use geo-location technology as a primary measure to combat interference: i.e. to check in a database whether a certain frequency band is available or not at a certain location before authorizing the unlicensed use. However, the FCC also stipulates that the geo-location based devices should be complemented with sensing technology: The Commission also has required that devices include the ability to listen to the airwaves to sense wireless microphones as an additional measure of protection for these devices. To implement reuse of the TV white spaces, from a platform point of view, the main challenges are related to the sensing requirements. Indeed, as illustrated below, it is impossible to sense the exact impact of the secondary transmitter on the primary receiver that should be protected.
The IEEE 802.22 Working Group (WG) was formed in November 2004, after the FCC released its Notice of Proposed Rule Making (NPRM) for the TV bands in May 2004. This WG was specifying an air interface (including PHY and MAC specifications) for Wireless Regional Area Networks (WRAN) to coexist with legacy TV transmission relying on cognitive capability. It was the first standard designed for opportunistic spectrum access. It is only possible to sense the channel
from the primary transmitter to the secondary transmitter, and huge safety margins are added in order to accommodate unknown blocking in the sensed channel. Indeed, if the sensed channel would be blocked, the primary transmitter would seem to be further away than it actually is, and the secondary transmitter could falsely assume that the channel is free. For the TV white spaces, sensing up to −116 dBm is hence targeted, which is well below the noise floor in those bands. The FCC’s conclusion however stated that devices do not consistently sense TV or wireless microphone signals and the transmitter is capable of causing interference to these signals.

IEEE 802.22 only focuses on fixed devices, while the IEEE 802.22a is to add mobile and portable device functionality to IEEE 802.22. The use of the lower frequency bands are particularly useful for rural access because of the favorable propagation conditions encountered for those lower frequencies. Although the population density is often very small in rural areas, large coverage areas might render the deployment of 802.22 Base Stations (BSs) a profitable business. These lower frequency bands are licensed for TV broadcasting and Wireless Microphones. However, many TV channels are largely unoccupied in many parts of The world and often TV is delivered through cable access or satellite. As a result, opening up those bands for WRAN systems could make a good case, both from business and technical points of view. Next to the main WRAN application domain, 802.22 networks can also be used for smaller markets such as small businesses or home offices. But the main goal is delivering broadband to households in rural areas. An example of a deployed 802.22 network is given in the illustration below.
The 802.22 networks operate in a fixed point-to-multi-point topology where a BS controls a cell consisting of a number of Consumer Premise Equipments (CPEs). The BS is an entity installed by an operator, and controls the cell strictly. Next to more traditional medium access control, that addresses when to transmit, it decides on how CPEs should access the spectrum. Moreover, the BS maintains control of a distributed sensing strategy to keep track of potential primary users (TV or wireless microphone signals). Clearly, it possible to have multiple 802.22 cells that interfere. This is aggravated because of the very large transmission area of those systems. Coexistence issues of 802.22 cells are hence also addressed in the 802.22 standard.

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