The wireless business totally depends on having spectrum in which to operate. There’s no such thing as too much. White spaces—unlicensed chunks of essentially free spectrum—are available for a wide range of applications, but so far few, if any, have ventured into this space. Why aren’t we tapping this excellent spectrum offering, and how can we begin to do so?
White space refers to the unused spectrum assigned to TV broadcast stations. Also called TV white spaces (TVWS), these are open 6-MHz VHF and UHF channels 2 to 51 in the 54- to 698-MHz range. Available channels vary by location depending upon use by local stations.
Several years ago, the Federal Communications Commission (FCC) gave permission for open access to this unlicensed spectrum if certain conditions were met, and many new channels became available with the 2009 transition from analog to digital TV. This recycling is a real benefit where spectrum is limited or expensive—common circumstances in many parts of the world.
Good And Bad News
First, the bad news:
- White space is variable and unlicensed. An assigned channel in one area may not be available in another. This could be a problem for some applications.
- Radios designed to use white spaces must be widely frequency agile. Wide band frequency synthesizers are available to make frequency assignment less of a problem, but power amplifiers are more difficult to make for such a wide band.
- When using unlicensed spectrum, you may at times be sharing space or adjacent channels. That means potential interference, such as from or to wireless microphones. For applications requiring good channel reliability, that may be a knockout factor.
- To be effective, any wireless device needs a good antenna. On the higher channels (500 MHz+), antennas are small. For example, a half wave antenna at 700 MHz is only about 8 inches. At 300 MHz, it would be 18 inches. At 54 MHz, that translates to almost 9 feet for a half wave. Long antennas are not a big problem for fixed applications but become a major obstacle in portable devices.
- The FCC requires radios to access certified databases in real time to find an open channel, ensuring minimum interference. These databases list TV stations and other licensed and unlicensed wireless devices such as wireless microphones that can potentially interfere with one another in a given area. Device data such as location, type (fixed or mobile), power level, and other factors are included. It has taken time for these databases to come on line, and the FCC is just now in the process of certifying them for use.
- There are no radio/network standards in place. Numerous proprietary standards have been tested but none has emerged as a clear favorite. Two are now available: the IEEE’s recently approved 802.22 standard and the forthcoming 802.11af Wi-Fi variation. (We’ll take a look at these later.)
- Is there a workable business model? The technology is here and now. All we need are the killer apps. That of course depends on the markets.
Despite these impediments, there is some good news. There’s no cost and no license is needed, which are the real benefits of white spaces. And the propagation effects are truly positive. VHF and UHF provide longer-range links than the usual 900-MHz and 2.4-GHz bands. Ranges to 60 miles or more are not out of the question in some conditions.
Penetration of buildings and other obstacles is also far greater than at the microwave frequencies. Multipath is less of a problem, too. This means products can be more non-line of sight (NLOS) rather than the almost essential LOS conditions needed for the higher-frequency bands. I suspect that the chip companies already have plans for ICs that are based on existing designs.
A Look At The Standards
The IEEE 802.22 standard, in the works since 2004, was finally approved in July 2011. It was the first of the wireless standards to look at incorporating cognitive radio (CR) technology. CR is an intelligent form of software-defined radio (SDR) that relies on frequency agility and the ability to sense both interference and the availability of open channels—a key technology for white space adoption.
The 802.22 standard is referred to as wireless regional area network (WRAN) technology. Ideal for white space usage, it employs a point-to-multipoint arrangement where a basestation (BS) serves multiple customer premise unit (CPE) modems. The BS has geo-location capability with GPS built in. The BS also has access to a remote database that catalogs available channels and adjacent assignments that may result in interference.
The CPE units sense conditions locally, determine their location by GPS, and then send that information to the BS, which determines what channel to use for the communications. Not all radios will be required to sense adjacent channels, but their geo-location is the key factor in determining what channel to use.
The physical layer of 802.22 is orthogonal frequency-division multiple access (OFDMA) with adaptable modulations of binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and 16-state quadrature amplitude modulation (16QAM). The OFDMA uses a basic 6-MHz channel to achieve a maximum data rate of about 19 Mbits/s with a range of about 18 miles. A scheme for channel bonding, using two or more adjacent or contiguous channels, allows higher data rates.
A competing technology, the popular 802.11 standard, is widely used for Wi-Fi. The goal of the 802.11af task group is to apply 802.11b/g/n technology to white spaces with appropriate modifications to meet FCC regulations for TVWS. Sometimes referred to as Super Wi-Fi, this technology uses orthogonal frequency-division multiplexing (OFDM) and involves two or more contiguous and/or non-contiguous TV channels to get the higher data rates. The media access control (MAC) layer is modified to meet the requirements for frequency agility and channel sensing. The 802.11af standard is still a work in progress but may be complete by the end of the year.
It is not clear if the two standards will compete directly. The 802.22 standard seems more appropriate for commercial wireless service vendors, while the 802.11af standard appears better suited to shorter-range personal applications. Proprietary standards, mostly based on narrowband modulation schemes using frequency shift keying (FSK), will likely continue as well.
Regardless of the standard used, white space radio is being called Super Wi-Fi because of its longer-range potential. I have no doubt that the Wi-Fi Alliance will eventually certify white space radios.
Applications For White Space
The most likely application is wireless broadband in rural areas. There are still many places where good, high-speed Internet connections are not available. WRAN (802.22) seems well suited to this use. The lower-frequency and NLOS characteristics make long-range connections of several miles not only possible but also reliable. In one potential business model, wireless Internet service providers (WISPs) would help fulfill the federal government’s National Broadband Initiative. The costs should be relatively low for such systems.
Remote monitoring and control is another great possibility. White spaces should find a niche in machine-to-machine (M2M) applications, Smart Grid connections, video surveillance cameras, medical patient monitoring, and sensor networks. White space spectrum outpaces technologies like Wi-Fi and ZigBee because of its capacity for longer ranges. TVWS could help bring about the predicted “Internet of Things” where we connect 50 billion items to the Internet.
Then there are the backhaul possibilities. TVWS does not match up well with cellular carriers’ need for extreme reliability and quality of service (QoS) in backhaul, but it may offer opportunities for data offload of their 3G and 4G systems. TVWS could work well in WiMAX and Wi-Fi hot spot backhaul.
There is obviously lots of interest in TVWS, but it is not yet clear who is doing what. Big names such as Google and Microsoft have been associated with tests and trials. The Cambridge Consultants of the U.K. recently conducted a workshop of interested parties including companies like Broadcom, CSR, Nokia, NXP, Samsung, Nokia Siemens Networks, and Neul. Neul recently announced some commercial TVWS products for M2M (see http://electronicdesign.com/article/communications/First-Commercial-White-Space-Radios-Target-M2M-And-Broadband-Applications.aspx and http://electronicdesign.com/article/communications/Consortium-Explores-Wireless-Viability-Of-TV-White-Spaces-.aspx).
Cambridge Consultants also released a new white paper on white spaces called “White Space Radio: highstreet hit or left in the lab?”You can get a copy at www.cambridgeconsultants.com. This paper is an excellent overview of the white space opportunity and a summary of the progress to date based upon the workshop.
The U.K.’s Office of Communications (Ofcom) just announced its approval of white spaces for wireless broadband in the U.K. The white spaces will be license-free, and third-party providers can develop the required databases. Applications include rural broadband and M2M.
For other good information on white spaces try these resources:
- Wireless Innovation Alliance (www.wirelessinnovationalliance.org)
- Spectrum Bridge (www.spectrumbridge.com): This organization maintains a white space database online that the FCC has recently announced is available for public testing. If you want to see how this works, go to http://whitespaces.spectrumbridge.com/Trial.aspx.
- Telcordia Technologies (www.telcordia.com): This organization is also building a white space database. At ITExpo, Telcordia Technologies and radio manufacturer Carlson Wireless demonstrated the ability of the Carlson White Space Device radio to query the Telcordia geolocation database and obtain a list of available vacant (white space) VHF and UHF frequencies between channels 2 and 51. Telcordia also demonstrated its public visualization tool, which displays the protected entities and their associated contours in and around Texas. John Malyar, chief architect, stated that Telcordia is pleased to be part of the ecosystem as one the conditionally designated database providers to support unlicensed access to TVWS. “These frequencies can provide social and economic value by serving the underserved rural communities providing low cost high speed broadband. In the future these frequencies may effectively support other services given their excellent propagation characteristics such as M2M,” he said.
- The FCC (www.fcc.gov/oet/whitespace/): The related regulations are Dockets 08-260, 10-174, 04-186 and 02-380. White spaces are regulated under Section 47 of the Code of Federal Regulations (CFR) Part 15. The FCC also maintains the TV channel database that includes other services in the channels, such as wireless microphones.
In summary, the white spaces are an excellent example of the efficient repurposing of existing spectrum. They also provide a great learning experience for developing cognitive radio concepts and practices. With formal regulations in place in the U.S. (and with the U.K. not far behind), it is safe to begin making products and building service businesses. The general prediction is that there may be some white space products available late this year and surely next year. Most expect the white space business to develop over the next five years.
However, some people think that white space is too good to be true. With tens of megahertz available, white spaces could easily set off a wireless revolution similar to the great expansion of Wi-Fi, Bluetooth, and ZigBee, which occurred when 83.5 MHz of license-free spectrum opened up in the 2.4-GHz industrial, scientific, and medical (ISM) band.
With white space efforts just now coming into their own, careless legislation could wreak havoc. As part of Congress’ efforts to get control of the budget and work on the president’s jobs bill, new legislation could cause the FCC to relocate and concentrate the TV bands, freeing up more spectrum that would be auctioned off for federal revenue. That would mean less white space available for wireless applications. Hopefully, Congress will educate itself before passing bills that kill innovation and jobs in the wireless industry.