White Space & Broadcasting Networks
Mark Waddell is a lead research engineer here in BBC R&D working on our research into "white space"- the potentially useful bits and pieces of spare radio frequency spectrum that the move from analogue to digital broadcasting might make available. The BBC's position on white space is probably best articulated in our recent reply to an Ofcom consultation. This post outlines where we are to date with our research in this area, what we plan to do and some of the interesting issues as Mark see's them:
In 2009 my IBC conference paper "Compatibility Challenges for Whitespace Devices and Broadcast Networks" discussed the opportunities and issues with emerging devices that offered the possibility of delivering broadband applications in potentially under utilised TV spectrum known as the white space. The switchover from analogue to digital broadcasting offered the potential of using this spectrum as the digital TV signals are significantly more resilient to interference than their analogue predecessors. The main concern related to the proposed access schemes required to use the fragmented spectrum and the proposals to consider spectrum sensing alone, aka cognitive radio, to detect and avoid the incumbent TV and microphone signals. Issues relating to adjacent channel interference were also expressed requiring location-dependent power limits for the white space devices to prevent interference to TV reception.
Image of Spectrum Analysis for Southwark in 2004 from OfCom's initial Spectrum Framework Review Consultation Document
Two years on, the issues with spectrum sensing are being addressed by new regulator proposals advocating the use of geolocation databases to control access in preference to sensing. The new devices will use a location service (e.g. GPS) and an internet-hosted database to identify the available channels and the permitted power limits. Details of how to compute the database from the broadcast coverage have been the subject of studies by CEPT in study group SE43 which reported its preliminary findings earlier this year. Ofcom consulted stakeholders on the proposed techniques in December 2010, but no firm decisions have been made.
Candidate technologies are now emerging from IEEE (802.11 and 802.16), ECMA and a new start up company in the UK (Nuel).The Nuel technology has been selected for the trial in Cambridge as they are based locally and are partners in the trial.
For the BBC, the TV White Space might offer the possibility of improved home and business campus networking, rural broadband connectivity and enhanced access to broadband services like iPlayer and bbc.co.uk. The challenge is to balance this against the risk of interference and consequent disruption to Freeview and Freeview-HD services. The BBC has invested heavily in these services, which have proved very popular and cost effective for consumers. Ofcom’s statistics show that 45% of households use the terrestrial platform as the main TV set and 80-90% of homes use terrestrial for 2nd, 3rd or 4th sets. Many homes with cable or satellite subscription services for the main TV also use DTT for video recording with simple PVR devices
Demand for wireless broadband is increasing and TV White Space technology could play a part in servicing this. Many questions remain and the technology is still very immature. Will the location services prove reliable enough and are the coverage predictions sufficiently accurate to allow the White Space Device transmitter power levels to be positioned sufficiently below the digital cliff of picture failure to maintain reliable Freeview reception? What happens if the devices or databases are hacked and do the benefits of the technology out weigh the possible disruption to the free to air DTT service enjoyed by over 80% of the population? Is there a business model for operating the databases and can frequency agile radios with the required spectral purity be produced to tune over 320MHz of bandwidth? Do Ofcom's regulatory proposals, which permit interference to 1% of TV services, allow sufficient device power to build a useful service? Is it ok for 270 thousand households to lose their DTT Freeview reception to enable the TV White Space or should power limits be set at a lower level to limit the damage? Is it appropriate to protect only fixed reception, as Ofcom propose, or should portable and set top aerials, commonly used for second and third TVs also be protected from interference?
A number of field trials are now underway with test licenses issued in Scotland and in Cambridge. These will explore some of the questions and the technology may prove to be a valuable piece in the wireless broadband jigsaw. There is considerable hype and some advocates claim a TVWS broadband network radiating 100mW- 4W will outperform proposed 4G LTE networks radiating 3kW. The BBC is participating in these trials to understand what the technology can offer in broadband connectivity and what this will be the cost in terms of interference to DTT.