White space refers to unoccupied parts of the wireless spectrum. With many TV channels typically left vacant, white-space communications technology aims to utilize the available bandwidth. At the same time, it allows underused frequencies within other ultra-high-frequency (UHF) licensed and unlicensed bands to be used for wireless communication. Known misleadingly as Super WiFi, white-space communications is rapidly gaining ground in Europe (see sidebar). Efforts are building from sources ranging from industry-standards groups to firms and research centers.
In the UK, for example, the Weightless open-standards group is aiming to kick off the “Internet of Things” via the use of white-space communications. The group plans to deliver its first complete specification this quarter. Meanwhile, mobile wireless-data service provider Neul has issued version 0.9 of the Weightless Specification. The company also has deployed a city-wide, fully functional wireless network in white space in its home city of Cambridge, England. In nearby France, communications research center CEA-Leti has been granted a government license to experiment with television white-space equipment in the field.
Neul’s Cambridge network builds upon the completion of the first phase of the Cambridge White Space Consortium’s wireless network. This network uses Neul’s equipment and cloud interface together with the Weightless communications standard. In doing so, it proves that its white-space network co-exists with televisions and wireless microphones without causing interference or disruption.
Of course, city space does create a challenging propagation environment. There is more than 120-dB link loss through buildings, foliage, walls, furniture, and human beings. Neul believes that its network opens up several possibilities for the Smart City of the future. It could enable intelligent transport and traffic management, city lighting, and other municipal services.
Cognitive Radios
CEA-Leti has developed opportunistic radio technologies (or cognitive radio) since 2005. In fact, the French government decided to issue a license to the research center because of the wireless cognitive-radio systems that were developed by Leti’s Communication and Security Department (STCS). Cognitive radios are able to both allocate and exploit spectral resources. In doing so, they optimize the capacity and capability of wireless-communications networks. Dynamic spectrum management is not currently permitted in France, however. Because it is perceived as an efficient means to de-saturate the crowded radio spectrum, many industry experts expect permission for such management to be given.
It’s a different matter in the US, where unlicensed opportunistic spectrum usage has been authorized in the free channels of the television spectrum or television white space. Studies have been evaluating whether a similar paradigm could be applied in Europe—for instance, in the framework of the European Conference of Postal and Telecommunications Administrations. The technology developed by Leti can be applied in both Europe and the US. The STCS’s cognitive-radio research focuses on two fundamental issues related to dynamic spectrum access: free-channel detection and flexible, high-spectrum-efficiency communication systems with low power leakage in the adjacent channels (known as adjacent-channel leakage ratio).
The licensing of spectrum space is always a key issue in wireless communication. Here in the UK, however, national communications regulator Ofcom has said that it would not seek to license access to the spectrum relative to white-space communications. Ofcom expects the first examples of white-space technology to come online this year.
What’s Not In A Name?
The name “Super WiFi” is misleading because white-space communications is not based on WiFi technology. Instead of using the 2.4-GHz frequency, white-space communications employ the lower-frequency white spaces between television channel frequencies. These allow signals to travel further and penetrate walls better than technologies based on the higher frequencies.
The term “white spaces” refers to the frequencies allocated to a broadcasting service. This allocation process creates a bandplan that assigns white space.
However, these white spaces usually exist naturally between used channels. In addition to these, there is also unused radio spectrum that has either never been used or has become free. The switch from analog to digital TV is a case in point, as it made available large areas between about 50 and 700 MHz.