Keeping white space transmissions safe from interference

While local broadcasters, video production companies and others that use wireless devices are concerned about interference from unlicensed devices operating on the same frequency bands — commonly known as TV white spaces because the frequencies are between occupied channels — no one is more cautious about stepping on occupied channels than Narayan Menon, vice president of R&D at a company called InterDigital.

The patent licensing company is a veteran in developing wireless technology transmission and reception technology, dating back to the 1980s. In the 1990s, it did a lot of pioneering work in the area of broadband CDMA, which morphed into the 3G standard. Later, its 3G protocol stacks — used to improve transmission and reception on cell phones — were used in the Apple iPhone 4. The InterDigital-developed algorithms detect when data is lost and retransmits it within seconds. In addition, a lot of the handshaking that goes on between the wireless device and the network are improved with InterDigital’s patented technology.

Today, with the proliferation of a wide variety of cell phones and the subsequent massive amount of wireless data traffic, Menon said, the need for white space bandwidth is unavoidable. He and his fellow engineers are proposing a number of spectrum sharing technologies to make the highly desirable 500MHz-700MHz band practical and safe for nonbroadcast use.

In order to harvest that bandwidth, InterDigital has developed a number of dynamic spectrum management (DSM) systems that exploit and aggregate the capacity of underutilized bands and provide more capacity to commercial LTE and Wi-Fi systems.

DSM combines radio frequency (RF) sensing, geo-location database-coordinated access (in cooperation with a company called Spectrum Bridge), radio resource management and wireless access extensions to dramatically increase capacity, significantly improve coverage and provide substantial mitigation of interference.

As a result, new applications and services, such as machine-to-machine type systems (medial sensors, parking meter and utility meter monitoring, corporate asset tracking, etc.) as well as video backhaul, small cell deployments and last-mile access could reliably be supported over a commercial-grade Wi-Fi infrastructure.

“In our implementation, we have brought many of these technology components together to help identify the best available channels at any given time of the day or night, at any location,” said Menon. “Spectrum sensing also tells us which channels are most appropriate for supplemental use. These channels might be available for one part of the day and not another, so it has to work in a very sophisticated way.”

The other challenge Menon and his team are tackling is that within the TV white spaces band, inevitably you won’t be able to find available channels next to each other, though you might find available channels on either end of the band. So InterDigital has developed a radio system that can aggregate up to four channels, which don’t have to be next to each other, and combine the bandwidth of those four channels together. Menon said they could get in excess 40Mb/s by combining those four channels. And if one of the channels somehow disappears (perhaps due to someone else occupying the channel), the DSM system is able to scan across the spectrum and find another empty one quickly.

One of the other advantages of TV white space is that it allows signals to travel farther, which makes it attractive for video backhaul.

“As you see small cells starting to be deployed in various places around the country, backhaul is going to become a major issue, because how do you get the data from all of these small cells back to a central hub,” Menon said. “TV white spaces can be used for all of those different types of applications, primarily because of the extended range it provides.

“We’ve taken a Wi-Fi solution, which normally operates in the couple of hundred meters type ranges and we’ve shown that TV white space can be used to transmit signals over at least a couple of kilometers,” he said. “Extended range is a key value add when using white space spectrum. We refer to this solution as addressing managed spectrum at the macro cell level.”

In addition, InterDigital’s Wi-Fi (DSM-WiFi) and LTE (DSM-LTE) systems are being designed for standards-based interoperability to make them highly scalable and cost-effective.

Menon described how new bands of spectrum could be freed up by the government in the near future, helping to alleviate the data traffic crunch. The FCC released an NPR in 2012 that proposes to open up that 3.5GHz band (now used for federal applications such as naval radar) in the U.S. It equals up to 150MHz of capacity and could provide a solution to the wireless bandwidth congestion problem. Eventually, the release of other federal spectrum bands by the government could free up as much as 1GHz of bandwidth for spectrum sharing.

“The government has expressed interest in freeing up some of that capacity for spectrum sharing,” he said. “So, if the incumbent is not using it, a secondary user can come in and temporarily occupy that channel or channels.”

The idea is that a number of new applications could take advantage of Wi-Fi in ways not possible today.

“We feel that there is enough spectrum for everyone, but it is currently not managed and used to its greatest potential,” Menon said. “We just have to use the wireless spectrum a lot more efficiently and the technologies for doing that are now coming into focus. We are being careful to avoid interfering with local TV stations because that would not be good for anyone involved.

“So, FCC rules dictate what we can do, so we operate within those guidelines while still providing the highest quality, completely reliable signal transmissions for the user,” he said. “And there are ample safeguards in place to ensure we do not interfere with TV channels.”

InterDigital is headquartered in Wilmington, DE, and maintains development sites in New York, Pennsylvania, California and Montreal, Quebec, Canada.