Cognitive Radio

21 October 2005

How does it help?

Investigation of spectrum utilisation suggests that not all the spectrum is in use for all of the time. Cognitive radio (CR) is one technology which offers the potential to make efficient use of this unused spectrum, potentially allowing large amounts of spectrum to become available for future high bandwidth applications. The long term vision of cognitive radio technology is one in which handsets would automatically make use of underutilised spectrum across a broad frequency range, allowing the high bandwidth requirements of the future set out in our vision to be realised.

The hidden terminal problem

A cognitive radio user might make a measurement and not spot any activity on a piece of spectrum. However, there might be a legitimate user of that spectrum behind the next building, transmitting to a tower on the hill. Because the building is between the users, the cognitive radio user does not receive the legitimate signal and so concludes the spectrum is unoccupied. But because both users are visible to the tower on the hill, when the cognitive radio user transmits its signal it is received as interference at the tower.

Figure 1 - click for text version

Figure 1 The hidden terminal problem

This problem is solved by the tower on the hill transmitting a signal indicating whether the spectrum is free. A terminal then requests usage of the spectrum, and if granted, the tower indicates that the spectrum is busy. Such an approach works well but it requires central management by the owner of the band. Hence, it becomes a choice of the owner of the spectrum as to whether they wish to allow this kind of access and if so under what conditions.

Although the technology holds much promise, it is a radical departure from the existing methods of spectrum regulation. Complications exist from both a technical and a regulatory standpoint with the introduction of this new technology. On the technical front, the “hidden terminal problem” must be overcome to ensure that primary users of a band are protected from interference. On the regulatory side consideration must be given as to the most appropriate mechanisms to allow CR.

Over the coming year we are undertaking a study to investigate the issues surrounding introduction of CR technology and to develop a simple software demonstrator system to investigate issues. The study will address areas such as:

  • Identifying applications which could potentially share or partially share spectrum (in space and/or time).
  • Quantifying the gains in spectrum efficiency and utilisation that would result from the CR approach, and estimating the increased level of activity possible and the consequential economic benefit.
  • Considering the regulatory impact of CR and what actions could be taken to facilitate deployment and maximise the benefits of the technology.
  • Identifying simple, low cost, and potentially nation-wide control mechanisms which can be easily adapted to today’s deployed technologies.

Spectrum Utilisation

There is potential to make considerably better usage of the radio-spectrum. Studies undertaken by Ofcom have investigated the usage of the radio spectrum. These measurements appear to indicate that there are many areas of the radio spectrum which are not fully utilised in different geographical areas of the country.

Figure 2 - click for text version

Figure 2 Spectrum occupancy measurements in a rural area (top), near Heathrow airport (middle) and in central London (bottom)

There are some caveats which must accompany the figures, for example some low level signals may not have been picked up and thus activity levels will be indicated as lower than is truly the case. Nevertheless, such measurements indicate the potential gains that might be accrued from a system which was able to sense its radio environment and utilise it effectively.

Our research in this area will establish the likely timescales for deployment of practical applications employing CR technology, bringing together the existing work in the different underpinning technology areas such as wideband antenna development and SDR.

Project status

This project has concluded and the final report can be downloaded below.

Cognitive Radio Final Report - Summary (PDF, 813.8 KB)

Cognitive Radio Final Report - Main Report (PDF, 1.7 MB)

Cognitive Radio Final Report - Appendices A-J (PDF, 1.1 MB)

Cognitive Radio Final Report - Appendices K (PDF, 2.0 MB)

Cognitive Radio Final Report - Appendices L-M (PDF, 820.6 KB)