It has been a long time since the debate about TV white spaces (TVWS) started. TVWS are the unassigned frequencies between broadcast TV channels and are to be made available on an unlicensed basis. Intended for use by certified devices for offering high-speed wireless Internet access, TVWS started a long debate about spectrum sharing that may never end.
Towards the acceptance of the TVWS concept
Back in 2004, the TVWS concept was considered a radical change. The chairman of the US Federal Communications Commission (FCC), Michael Powell, went so far as to say that it "promises to dramatically increase the availability and quality of wireless Internet connections – the equivalent of doubling the number of lanes on a congested highway.” The belief was that TVWS had the potential to take the success of Wi-Fi to a whole new level, with radio channels that could travel farther and better penetrate walls, buildings, and other obstructions.
Through the use of spectrum sensing and cognitive radios, the concept of detecting and using available channels seemed promising. Despite appearing relatively simple, in reality it posed a lot of technical challenges:
- There is the problem of having a white space device (WSD) assess channel availability when there is a very weak signal from a TV station very far away, which creates the risk of the WSD interfering with it.
- There is the problem of a high-power TV broadcast station that can interfere or even saturate a WSD receiver operating rightfully on an adjacent vacant channel.
- There is the problem of a WSD rightfully operating on a vacant channel that can interfere with a nearby TV receiver tuned to a TV station on an adjacent channel.
To assess TVWS and its potential issues, a coalition of companies, including Microsoft, Intel and Google, proposed in 2007 to build a prototype device for conducting field trials with the FCC. Unfortunately, the trials did not live up to the expectations. On July 31, 2007, the FCC reported that the device did not reliably detect unoccupied spectrum and could interfere with other TV programming and wireless microphone signals.
Without getting into the details of the subsequent two years of debate1, let’s simply note that on November 4th, 2009, by a majority vote of 5-0, the FCC formally agreed to open up the white space spectrum based on the concept of white space databases (WSDBs), thus alleviating the need for WSDs to detect TV and wireless microphone signals.
Towards the commercial rollout of WSDs
Based on geolocation, the WSDBs are responsible for identifying which channels can safely be used at a given location without interfering with the incumbents (TV stations and low-power wireless microphones). For certification, a WSD must not only comply with the radio emission certification but also comply with the WSDB interfacing requirements.
Since the FCC regulation (“Part 15 TV Bands Devices”, May 19, 2011) was enacted, many commercial products have emerged, starting with Koos Technical Services (KTS) in December 2011. According to presentations at the Super Wi-Fi Summit held last August in Las Vegas, several trials have been conducted this year in US, Europe, Asia, and Africa. The focus is now shifting towards commercial rollouts. There is one major caveat, however: the lack of an international standard.
Towards TV spectrum standardization
As it stands today, WSDs are based on proprietary radio access technologies. Although some standards exist, such as IEEE 802.22 for the rural market and 802.11af for the Super-Wi-Fi market, they have not been adopted by the industry and no dedicated low-cost ASICs are currently available. TVWS vendors have relied on general-purpose processors, which offer the flexibility required for this emerging market.
For example, Nutaq has been selling software-defined radio (SDR) systems based on the latest FPGAs and SoCs for several years to developers who required a flexible platform to implement various radio air interfaces such as those from the IEEE or 3GPP standards.
For TVWS to attract semiconductor vendors and become low-cost devices, regulations will need to be broadly adopted and harmonized. As it stands now, Canada, Singapore, and Europe are preparing their regulations. It is not clear however when developing countries in Africa and South America will follow. So far the trials in those countries have been using experimental licenses.
Will the fight for TV Spectrum ever end?
While TVWS regulations are being adopted, other technologies are looking at the TV spectrum as well. For example, 450 MHz was added to 3GPP as LTE band 31 for Anatel in Brazil and SRFC in Russia. Although the bandwidth is limited and not necessarily compromising much of the TVWS spectrum, it is still competing for the rural markets. Similarly, 600 MHz is being considered by FCC (almost 100 MHz of TVWS spectrum) and it's not clear if other countries will follow.
TVWS has come a long way but it is clear that coexistence will continue to be a prime concern for many more years to come.
1One of the WSDs used during the trial was found to be defective.