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There has been growing excitement in the wireless industry about the opening of TV white spaces radio spectrum. This is the spectrum band that was vacated by broadcasters in the shift from analog to digital television in 2007. Like Wi-Fi, it resides on unlicensed (public) airwaves, which means it will allow enterprises to create networks that are customized to address specific communications needs.
Technologies being developed for deployment in these frequencies have wide-ranging potential to create exciting business opportunities. A study commissioned by Microsoft found that the use of white spaces could generate an additional $3.9 billion to $7.3 billion per year in economic activity. Among the uses envisioned for white spaces include helping deliver broadband to rural and underserved areas, building campus-wide corporate networks, enabling the delivery of new applications to subscribers’ homes and allowing service providers to create new (or expand existing) broadband networks.
There are key technical considerations to address before white spaces can fully support the deployment of cost-effective networks and fulfill their lofty promise.
An initial step in fostering white spaces development was taken in 2010 when the Federal Communications Commission named the first companies to be eligible to act as administrators of geolocation databases. These databases provide continually updated information about what communications channels are available for non-TV broadcast use in a particular area. They take into account channels being used for broadcasting at specific locations and those being used for other purposes, such as wireless microphones. One such database has already been certified by the FCC, while another is just ending the public review phase of the process.
The geolocation capability these databases provide is crucial to ensuring that unlicensed white space devices will not cause undue interference to over-the-air TV broadcasts or wireless microphone systems. However, while these databases will protect incumbents, they do little to address the efficient operation of the white space devices themselves. Because of the extended range possible in white spaces frequencies, these devices will be able to cause interference with other white space devices over much larger areas than current Wi-Fi networks. These devices therefore must incorporate additional capabilities beyond database access to enable true carrier-class wide-area networks in white spaces. This will require white spaces systems that are able to perform cognitive (intelligent) sensing and decision-making at the edge of the network – where subscribers are actually using their devices. Simply stated, while the spectrum databases will let devices know which channels can be used, cognitive radio technologies can take that information, enhance it with their own spectrum sensing capabilities and determine which channels they should use to maximize throughput and minimize self-interference.
Cognitive technology also enables other key benefits, like the ability to dynamically switch between different frequency bands if one set of frequencies gets too crowded or is not available. For example, in areas where white spaces are not readily available (as is the case in a few larger cities), cognitive devices would automatically switch to channels in other unlicensed bands (the 900 MHz or 2.4 GHz bands, for instance) when needed. This would ensure uninterrupted connectivity and coverage in areas congested with white spaces wireless traffic.
Another area of application that is currently being overlooked by white space proponents is mobile voice and broadband networking.
The FCC approved the operation of both fixed and portable devices in white spaces spectrum. Fixed devices are limited to operating at power levels of up to 4 watts EIRP (effective isotropically radiated power, or the effective transmitting power of a radio/antenna toward a receiver). Portable wireless devices, on the other hand, are limited to 100 milliwatts EIRP, due to their greater risk of being located close to a TV receiver and potentially causing interference to licensed TV broadcasts. Due to this very low power limit, conventional wisdom in the wireless industry has relegated portable white spaces devices to short-range, in-home applications. However, detailed simulations presented by xG Technology at various industry forums clearly demonstrate that cost-effective, wide-area mobile broadband networks can be deployed using white spaces.
To make this possible and practical, these systems must incorporate features like advanced interference mitigation and innovative antenna technologies like MIMO (multiple input, multiple output). By using these and other cognitive radio technologies, networks are able to boost base station receiver gain (i.e., reliably receive the weak signals coming from the portable devices) and thus make the current white spaces power rules very workable for mobile applications. Note that by enabling these devices to operate at such low power, battery drain and weight can be kept to a minimum.
The bottom line is that, while it was previously assumed that wide-area coverage was only economical between high-powered fixed devices, cognitive radio systems bring exciting capabilities and economics to fully mobile white spaces networks.
As the possibilities for white spaces receive increased attention from spectrum regulators around the world, the potential for international cognitive radio system deployments grows commensurately. For example, U.K. telecom regulator Ofcom is very actively pursuing a white spaces strategy and is currently allowing multiple white spaces trials to take place around the country. Canada, Brazil and other countries are also beginning to look at freeing up TV white spaces in their markets.
There is clearly a bright future for white spaces to enhance mobile broadband communications. Technologically-advanced cognitive radios will help unleash their full potential as an economical wireless broadband solution, one that is capable of ushering in a new era in low-cost mobile communications.
