While the FCC last month continued its second round of white space testing, online search and ads giant Google submitted a new proposal that attempts to address interference concerns raised by broadcasters and wireless RF users who oppose the use of unlicensed devices within the whites spaces of the DTV spectrum.
At issue is whether to allow unlicensed devices to operate in the 54 MHz to 698 MHz spectrum portion of broadcasters’ signals. In February the commission’s Office of Engineering and Technology began its second round of tests to determine the DTV sensitivity of four devices whose makers hope to operate in the these so-called “white spaces.” Results from the first round of tests, conducted last summer by the FCC, showed that prototypes consistently failed to detect the presence of operational television transmitters and also failed to acknowledge, or were confused about wireless microphone signals.
Google’s new proposal concedes some of the problems with the spectrum detection technologies so far. The Google “enhanced protection proposal” included a combination of methods suggested by Motorola—geolocation technology (to protect broadcast TV) and beacons (to protect wireless microphones)—along with a “permission to transmit” signal from existing devices before the white space device could transmit.
Geolocation, unlike spectrum sensing, does not rely upon detecting signals off the air, but instead ties into a database of known TV stations in the area, so the devices don’t operate on those channels.
The Broadway League, which opposes devices that could interfere with the use of RF in theaters, called the proposal “inadequate.”
(click thumbnail)Motorola was among four companies that submitted a prototype device for the FCC’s “white space” tests.“Google’s plan, correctly, does not intend interference protection based on spectrum sensing technology,” the group said in a statement. “However, Google’s vision of a beacon system, which would be purchased by current wireless microphone users, actually relies on the aforementioned unproven spectrum sensing technology.”
NAB and the Association for Maximum Service Television also dismissed Google’s latest effort.
“The letter provides no new technical information demonstrating that its proposals will work,” said MSTV President David Donovan. “Given Google’s approach not to pay a market price for spectrum in the 700 MHz band, it is not surprising that it is turning its attention to the TV band.”
CTIA-The Wireless Association also offered a white space plan it called balanced. The group called for an areawide licensing approach for the majority of the band, with a portion reserved for further study. MSTV, which has advocated a licensed approach, said the CTIA plan “may provide opportunities that warrant further explanation.”
REAL WORLD TESTS
Meanwhile, as competing industries consider the recent spate of white space proposals, the FCC continues its second round of white space tests at its Columbia, Md.-based lab.
In the current round of tests, which are open to the public, Microsoft, Motorola, Philips and Adaptrum, a Mountain View, Calif.-based developer of radio sensing technology, each submitted one device.
The FCC’s current plans call for DTV sensing tests for these devices in the presence of DTV signals in adjacent channels; earlier it focused on sensing static single microphone signals. The commission has not issued a timeline for when the tests will be completed or results made public.
Manufacturers, NAB and MSTV, the device providers, and others have witnessed the tests. One company that is closely monitoring the tests is Shure, which has also submitted wireless microphone products for the FCC to use in their tests, according to the company’s director of Platform Engineering, Ahren Hartman.
The FCC declined to comment on the specific signal levels it has used in its tests. According to a filing by Shure last month, DTV signals from Channels 43 and 45 and signals from analog and digital mics from Channel 44, were injected directly into the unlicensed device under three scenarios. The first scenario involved a moderately strong DTV signal (–28 dBm) paired with a mic signal of –80 dBm. Scenario two was an average DTV signal (–68 dBm) and a mic signal of –100 dBm. The final scenario paired a low DTV signal (–84 dBm) with a mic signal of –100 dBm.
Shure’s filing concluded that “the tests do not yet take into account real-world factors such as signal fading and the sub-optimal antenna performance of the unlicensed de-vice.” And it pointed out that the device antenna was not being factored into the sensing performance.
“The microphone and DTV signals are currently being injected directly into the unlicensed device prototype antenna input via coaxial cables,” Shure said.
Moreover, Shure indicated the “need to test the cognitive ability against the proposed DFS [dynamic frequency selection] model to stop unlicensed transmissions after microphones are detected.” Suggestions for several modifications to the tests were made, including the recommended use of a DTV signal strength of –20 dBm, a level that Shure insisted “exists in urban environments where wireless microphones are constantly used in broadcasting.”
Hartman noted that the White Spaces Coalition, an alliance of high-tech firms including Microsoft and Philips that supports the use of unlicensed devices in white spaces, has “repeatedly touted cognitive or ‘smart’ radio as their answer to protecting incumbent users from interference. The cognitive nature of the unlicensed devices must be characterized and the technology proven to protect incumbents prior to the FCC authorizing any new unlicensed devices to operate in the TV bands.”
Shure’s suggestions for field test venues included arenas hosting the NBA finals and NCAA basketball tournament and stadiums for Major League Baseball games. For the second round of testing, last November, Shure urged the FCC to examine sensing performance when mic signals are located in open TV channels adjacent to DTV signals.
The FCC confirmed that Shure was granted a Part 5 experimental license which, the agency explained, lets companies conduct experiments by transmitting on bandwidth. The FCC also confirmed that it granted Shure Special Temporary Authority to operate a device for a couple of weeks for demonstrations or testing.
However, the company does not have physical access to the unlicensed device prototypes to conduct their own interference tests, Hartman said.
“We only have access to monitor and record the FCC test results sitting side-by-side with the FCC engineers,” he said. “If the unlicensed device prototypes are made available to incumbent users, Shure will conduct our own interference tests. This was the reason we applied for the experimental license over two years ago, since no devices actually existed for Shure to test the interference potential into wireless microphones.”
Hartman said that, using its experimental license, Shure is simulating the unlicensed device signal transmissions and data traffic models with test equipment and modified 802.11 radios to characterize and predict the interference levels into wireless microphones.
Results of the tests, like the above comments, were filed with the FCC. According to an agency spokesman, all filings are considered in its guidance for the rulemaking, not an obligation for further testing.
The former chief of the FCC’s Office of Engineering and Technology, Ed Thomas, is now a partner at the law firm Harris, Wiltshire & Grannis LLP and technical consultant to the White Spaces Coalition. He views Shure’s –20 dBM suggestion as just another condition to “increase the probability of failure.”
“The higher the power in the adjacent channels, the higher the probability you overload a device and you create overflow into the white spaces,” said Thomas. “The odds of getting signals the strength that are being suggested by the outsiders is very, very remote: most TV signals are below –70 dBM, so they’re talking about 50 dBm hotter, which is about 100,000 times more powerful.”
Thomas was quick to point out that the devices submitted for testing are data gathering vehicles, not prototypes.
“They’re basically test sets that have been provided to the FCC to collect data,” he said. “A prototype implies a first step in the manufacturing process. These devices have no resemblance whatsoever to any commercial product that may be built in the future. These devices are huge, they’re about the size of two cinderblocks. Commercial devices will be the size of a WiFi card.”
Although he conceded that his group may make some suggestions for the field tests, Thomas thinks the FCC approach is reasonable, disagreeing with Motorola and Google that testing should include geolocation database information. “We think that sensing is more than sufficient,” he said.
Stu Overby, Motorola’s senior director of Global Spectrum Strategy, disagrees. “Our primary means of protection is not sensing, it’s geolocation,” he said.
In October 2007, Motorola also recommended that the FCC establish two classes of unlicensed devices—relatively high-powered (up to 36 dBm), and lower-powered consumer devices (up to 10 dBm)—based on their potential to cause interference.
High-powered devices would include a geolocation database, means for sensing (as a secondary protection) and the ability to detect the type of beacon defined by IEEE-802.22.1, a proposed standard addressing the use of low-power devices in the TV band. Lower-powered devices would not require a geolocation database.