Dynamic Spectrum Access technologies have evolved significantly in and out of the lab in the past decade, but more work remains to bring the approach to mainstream use.
The last decade witnessed tremendous innovations in dynamic spectrum access (DSA), an approach to letting secondary users access the spectrum holes called "white spaces" in licensed spectrum bands. More work still needs to be done.
People in the field have developed techniques to determine if a frequency band is occupied by a primary user and built radios, protocols, and cloud infrastructure that can efficiently use shared spectrum. The work has involved research in areas such as information theory, physical layer technologies, antenna design, speech, audio, networking, and security.
There are two main ways to determine unused spectrum: sensing or using a geo-location database. The core idea behind sensing is for each device to quickly sample the spectrum, and look for signatures of the primary users to determine if the spectrum is free or occupied. Since the spectrum is noisy, and the primary signal might be weak, machine learning is commonly used to detect primary users with high probability.
Among its drawbacks, spectrum sensing requires another chipset (or at least an RF front end) to constantly check that its spectrum is available, increasing costs and power consumption. In addition, sensing is still in a research stage. Accurate sensing is very difficult, especially with off-the-shelf electronic components that are most likely to be used in commercial radios.
To counter these limitations, researchers proposed using a geo-location database. A device reports its location to a cloud database service, which returns data such as usable spectrum, power levels, and time of use. The cloud service periodically downloads information about known transmitters from sources such as databases maintained by government regulators. It then applies propagation modeling algorithms, such as Longley-Rice or ITU 1812 to infer what spectrum is available at a specific location.
This technique has been adopted by the US FCC, UK OFCOM, and other regulators for the TV white spaces and is being investigated for other portions of spectrum as well. Spectrum Bridge, Google, and iConnectiv are among several authorized geo-location database providers for TV white spaces.
At the device end, researchers started work on a system called White-Fi. We showed the minimal changes needed to WiFi to allow it to work in narrower channels, while not interfering with TV transmitters, using different, non-contiguous channel bandwidths than the constant 20 MHz used for WiFi at that time. Several of these ideas, along with many new ones, were incorporated in the IEEE 802.11af standard, which was approved earlier this year.
Separately, the IEEE 802.22 standard showed how sensing-based systems could efficiently use the fragmented white space spectrum. This standard for wireless regional area networks (RANs) was also approved last year.
Several hardware vendors are building radios to use this spectrum. Adaptrum, 6Harmonics, and Carlson Wireless -- all members of our Alliance -- have radios certified by the FCC to operate in the TV white spaces. They periodically communicate with the white space databases, and only operate in the empty spectrum in such a way that their transmissions do not interfere with those in adjacent bands.
These networking technologies have been demonstrated in several pilot deployments worldwide. From the first deployments on the Microsoft campus in Redmond, Wash., and the Spectrum Bridge Wilmington, N.C., deployment in 2009, this technology has since been showcased in larger deployments in the UK, Singapore, Africa, and the Philippines. Several people who were previously disconnected from the Internet, are now connected because of this technology.
In June 2014 the University of Strathclyde, UK, piloted the 802.11af standard, also called SuperWiFi. This was the first tri-band WiFi pilot (2.4, 5, and sub-1 GHz). It used radios from MediaTek in partnership with Aviacomm.
The second key TV white space standard, IEEE 802.22, also called Wi-Far or Last Mile, evolved out of the WiMax stable for rural broadband. Another important standard is the IETF's Protocol to Access White Space database. It provides the mechanisms white space devices use to report their locations to the database and receive frequencies they can use.
Open research questions remain for DSA to achieve its full potential.
For example, spectrum sensing is still more efficient than modeling-based techniques because it gets an instantaneous, real-time view of spectrum use that is very difficult to obtain from a cloud service. In addition, the databases right now are very conservative. Technologies to populate them with real spectrum measurements, or to make the propagation modeling more accurate, will increase the amount of reusable spectrum.
Finally, most of the current research has focused on coexistence mechanisms between primary and secondary users. A largely overlooked problem is of coexistence among secondary users themselves, which we believe will become increasingly important as the DSA technology becomes mainstream.
— Ranveer Chandra is a senior researcher with Microsoft Research. H. Nwana is executive director of the Dynamic Spectrum Alliance.