Electronic Design

Big Changes In The Frequency Spectrum Cultivate New Wireless Possibilities

Mark Twain once said, “Buy land. They’re not making it anymore.” Such is the case with electromagnetic frequency spectrum. They’re scarce, coveted, and very expensive. Furthermore, they’re subject to regulation both nationally and internationally.

The general consensus is that most of the good spectrum are already allocated. But the opposing view is that lots of existing unused or underutilized spectrum could be used (see “A Technology Solution To Spectrum Limitations”). Both views are correct up to a point. The recent transition from analog to digital TV has freed up some great spectrum in the U.S., and the Federal Communications Commission (FCC) is attempting is to make more efficient use of it.

The frequency spectrum is a continuously changing landscape. The best way to keep up with changes is to monitor the two U.S. agencies that control it: the FCC and the National Telecommunications and Information Administration (NTIA).

The FCC deals with personal and commercial spectrum, while the NTIA handles government and military spectrum assignments. Their Web sites offer a mass of information for those who need to follow regulatory actions. And while the U.S. spectrum is relatively settled, two recent events have opened an unusually large segment previously tied up by TV broadcasters.

First, the broadcasters abandoned their assignments from channels 52 to 69 (698 to 806 MHz) as part of the FCC-mandated digital TV switchover. This is the so-called 700-MHz spectrum. It was auctioned off last year in Auction 73. A total of 101 bidders won 1090 licenses for a net income to the U.S. Treasury of $19,592,420,000.

AT&T and Verizon won most of this spectrum for cellular expansion, but other licenses will support mobile TV, wireless broadband, and a mix of other services. Little or none of the spectrum is in use so far, but it will be soon because of the final digital TV switchover on June 12.

Second, in another action related to the DTV transition, most TV stations were assigned new operating frequencies in the remaining spectrum from channels 2 through 51 (54 to 698 MHz). These new assignments don’t use all of the available spectrum. Many gaps exist, and they vary in frequency depending on the local TV station’s frequency assignments. These gaps of unused spectrum are called “white spaces.”

The FCC recently declared that these unused frequencies may be applied to new wireless services. These new services will be unlicensed, and they will come under the FCC’s CFR 47 Part 15 as recently amended. This spectrum still retains its 6-MHz channel assignments, though, and the available bands vary from locale to locale.

The spectrum assignments in the U.S. are a nightmarish tangle of regulations and recordkeeping. Changes occur daily, so keeping track of it all is difficult. While who owns and uses which spectrum is generally known, some forgotten assignments and recent changes slip through the cracks. This lack of exact knowledge of the spectrum assignments recently led to the Radio Spectrum Inventory Act, a bill sponsored by U.S. Senators John Kerry (D) of Massachusetts and Olympia Snowe (R) of Maine.

The bill directs the FCC and the NTIA to perform a U.S. spectrum audit and report on the use of all spectrum from 300 MHz to 3.5 GHz. The outcome of this bill should more clearly define spectrum usage. It will determine if the spectrum is overcrowded as well as if unused segments could be put to good use (see “What’s So Special About The White Space And 700-MHz Bands?”).

Also, it should help ensure that our available spectrum is fully used for the benefit of U.S citizens, increasing services for consumers and profit for businesses, especially in the broadband service sector. Never before has so much good spectrum opened up at the same time. It no doubt presents many new products and services possibilities.

That’s the total amount of available spectrum in the 700-MHz block. Spectrum blocks don’t always follow the original 6-MHz channel assignments. The gray C and D blocks in Figure 1 in the lower 700-MHz band and the gray A and B blocks in the upper 700-MHz band were auctioned off before Auction 73 last year. The remaining blocks were sold off in Auction 73, except for the D block in the upper 700-MHz segment.

So, how will these bands be used? Most of the spectrum will go to cellular service expansion. Cellular service continues to grow as more new subscribers come aboard, including those dropping their wired telephone service in favor of wireless only. Consequently, there’s always a demand for more spectrum among carriers and operators.

Another factor is the increased use of data services—specifically messaging, e-mail, and Internet access. And much of that Internet access involves video. Data services require more bandwidth to push data speeds to a level that’s satisfactory for subscribers expecting data rates comparable to their home broadband services. Most carriers are seeing steady growth in 3G data services.

Also, new 4G cellular technologies like Long-Term Evolution (LTE) will be implemented in 2010 and beyond. LTE will bring even faster data services (Fig. 2). Yet this orthogonal frequencydivision multiple-access (OFDMA) technology, which uses multiple- input multiple-output (MIMO), needs more bandwidth.

Some of the spectrum has already been allocated to mobile TV. MediaFLO is transmitting on old TV channel 55 to cell phones with built-in receivers. Additional spectrum was assigned to mobile TV. AT&T recently announced the beginning of its Mobile TV effort in 20 new markets nationwide. It includes CBS, Comedy Central, and MTV. Both LG and Samsung offer phones with mobile TV capability for this new service (Fig. 3).

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Some of the 700-MHz spectrum is expected to be assigned to nationwide broadband expansion. The federal stimulus bill set aside $7.2 billion for increased penetration of broadband services to all citizens.

However, many underserved or totally unserved segments still remain throughout the U.S., particularly in rural areas. Wireless is expected to fill those gaps in broadband coverage, led by 4G WiMAX. While WiMAX is generally assigned frequencies in the 2.5- or 3.5-GHz bands, the WiMAX Forum is just completing a design profile for mobile WiMAX in the 700-MHz band.

Finally, a relatively large chunk of the 700-MHz band has been set aside for public safety. The basic goal of this spectrum is for some entity to build a nationwide public safety network for first responders such as fire, police, and others who must communicate effectively during an emergency.

Today, a massive communications disconnect continues because of the widely varying spectrum and technologies used by local public safety organizations. By building a compatible nationwide system, all fire, police, and other first responder agencies will be able to easily communicate with one another.

The 10-MHz D block assigned for this purpose wasn’t auctioned off last year with the other 700-MHz blocks. No one met the FCC’s minimum bid requirement, so this D block remains unassigned. The FCC hoped a commercial company would acquire the spectrum and establish a private-public partnership with the Public Safety Spectrum Trust (PSST), which currently owns another 10-MHz chunk of spectrum in the 700-MHz range.

The company would build out a nationwide network for use among public safety groups. Also, the company could employ the spectrum for its own commercial profit, though public safety would always be prioritized. The plan sounds like it was conceived by some government policy wonk with no real business experience. The whole deal is a turnoff—no wonder the spectrum didn’t get auctioned off.

Nonetheless, public safety groups still need a solution. A number of these organizations recently met and asked Congress to allocate the available 10-MHz chunk of spectrum strictly for public safety use. The spectrum would go to the PSST, giving it a full 20 MHz of space to implement a nationwide communications system. While no standards have been set, most of the organizations agreed that the LTE cell-phone technology would be the preferred radio system.

Other proposals continue to flood the FCC. With the agency just emerging from its limbo position due to the presidential transition, it hasn’t been in a position to form a policy and put forth a decision. Now that Congress has confirmed new FCC chairman Julius Genachowski, an old school buddy of President Obama, hopefully a suitable solution will found.

White space refers to the 6-MHz channels left unused by TV broadcasters in the 54- to 698-MHz range—channels 2 through 51 (see the table). In any given local region, TV stations are assigned frequencies several channels apart from one another to prevent interference. Thus, unused gaps pop up.

These unused channels or guard bands can be assigned to other activities. TV broadcasters are horrified—the National Association of Broadcasters and wireless microphone makers are solidly against white-space usage, claiming the potential for all sorts of terrible interference problems.

On the other side of the white-space debate is a group of major technology companies such as Dell, Google, HP, Intel, Microsoft, Motorola, and Philips, which would like to see open spectrum for unlicensed white-space devices (WSDs), also called TV band devices (TVBDs). That could potentially open the market for new wireless services, including broadband access as well as some interesting new wireless devices.

These companies belong to the Wireless Innovation Alliance (WIA), the organization that promotes white space. With the use of modern wireless technology, these companies feel that interference can be controlled or eliminated. The FCC agreed with that, and after some testing of its own, it issued approval for unlicensed white-space devices in November 2008.

To get a sense of the available white space in your area, go to www.showmywhitespace.com. This Web site is owned by Spectrum Bridge Inc., a company that brokers spectrum space. The company’s other Web site, www.SpecEx.com, is a place to buy and sell licensed spectrum.

Spectrum Bridge chief marketing officer and founder Rick Rotondo indicated that at the “Show My White Space” site, you can find your area on the map and locate the open channels for WSDs. Tens to hundreds of channels are available, depending on whether you’re in the big city or out in the boonies.

All sorts of guidelines and safeguards prevent interference. For example, using a white-space channel as a basestation or a fixed master requires an unused 6-MHz channel on either side of it. Each basestation or fixed-master device must also have a location capability (e.g., GPS) to designate its position precisely. Also, the WSDs must be able to access a database of TV broadcast locations as well as licensed wireless microphone users in local areas. The unit will automatically check for available local channels and attempt to use them.

Other than the NAB, no one group put up more of a fight against white-space usage than the users of wireless mikes. You wouldn’t think that there were enough of them to be an issue, but not so. Wireless mikes are everywhere, widely used in theaters, churches, radio and TV studios, and wireless video recording.

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All of these devices operate in the 500- to 650-MHz range, which is a key part of the white-space territory. Interference is expected. As a result, wireless mike users, who are licensed users of that spectrum, can be protected. They will be entered into the national database so they will be protected by a 1-km radius from their location.

In addition, there’s a listen-before-you-transmit rule. All WSDs must have a super-sensitive receiver (–94- to –116-dBm range) that listens to the desired channel to see if other signals are present. If so, the unit can’t transmit. Instead, it will jump to another potential channel and repeat the listening process. The frequencyagile WSD will eventually find a channel and transmit. WSDs will, for the most part, be cognitive radios.

Furthermore, according to the official guidelines, fixed stations on channels 2, 3, and 37 won’t be operable. Portable or mobile WSDs are restricted to channels 21 through 51 except channel 37. (Channel 37 is a protected channel for radio astronomy and for medical telemetry.) All potential devices will be tested by the FCC and approved as either a fixed or master device or as a portable client device.

In addition, basestations or fixed-master wireless nodes can’t use more than 1 W of power or 4 W of effective isotropic radiated power (EIRP). In other words, the transmitter power is no more than 1 W, but the antenna gain makes up for it. A mobile WSD is limited to 100 mW or 40 mW if it’s operating on a channel adjacent to a nearby TV station. Furthermore, all units must have adaptive power control that can automatically adjust for minimum power to maintain a link.

As indicated earlier, WSDs will operate under the FCC’s Part 15 unlicensed rules and regulations. No formal wireless radio standards exist for WSDs, but several technologies are under test. One is Wi-Fi. Why not use the 802.11 standard in the white spaces? The same goes for 802.16 WiMAX.

The IEEE has joined the mix, too, with an effort designated the 802.22 Wireless Regional Area Network working group. It’s developing a physical layer (PHY) and media-access-controller (MAC) layer standard for cognitive radios that can operate within the white-space guidelines. Another IEEE activity involves the P1900 standards group. A recent project dubbed P1900.4a will look at new architectures and interfaces that can provide dynamic spectrum access in the white-space bands.

With approval in tow and great potential on tap, what new services and products can we expect? First, there’s wireless localarea networks (WLANs). Just like Wi-Fi, WLAN could be used in the home, office, or public spaces. It also offers longer range and greater reliability.

Wireless broadband could provide an alternative to 3G and 4G cellular data services and WiMAX. A wireless Internet service provider (WISP) may be able to offer services in rural areas or sections of the country where there’s no broadband. Stimulus money could help encourage its growth.

Then, there’s consumer entertainment or home monitoring and control wireless. Industry, municipalities, and utilities could use it for telemetry and remote control. Also, consider cordless telephones, video surveillance, and wireless backhaul.

Many WSDs are under development, and they could start to arrive in the coming years. Some manufacturers may be waiting on a standard, but some are not. Is a single standard really necessary with its implied interoperability?

No one knows for sure how this will unfold. Is it just a big boondoggle where interference is rampant and spectrum is wasted? Or will we really enjoy and benefit from some new and better wireless products and services? We’ll find out soon enough.

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