The re-use of 800 MHz spectrum for mobile broadband has led governments and regulators to instigate major programmes of interference mitigation. The resulting work programmes have led to a greater understanding of coexistence of TV and mobile services in the same band.
This paper summarises the new and important advances in technical understanding of coexistence from the mitigation programme in the UK.
The new licences for the 800 MHz spectrum in the UK required licensees to set up a company to mitigate interference. It is required to operate an interference prediction model, to communicate with the public, to provide practical assistance to mitigate interference and to report on its performance to an independent Oversight Board.
In practice, television interference has been reported by only a small number of those households predicted to be at risk. A combination of operational experience, practical observations and research have led to improved modelling, more targeted communications and a revised approach to viewer support.
Study results have fed back to improve models and specifications
An interference mitigation programme is underway in the UK and this has led to an improved understanding of real-world TV reception and interference prediction and mitigation.
This paper reports the key results to date, including a number of improvements to predictive modelling of which homes are more likely to be affected by TV interference from LTE base stations.
The results are important because they provide background information for future spectrum reuse programmes and provide insights into how to improve products and system designs for better coexistence.
Amongst the results is the finding that predictive modelling can be improved significantly by adopting household antenna gains from measurement campaigns, rather than assuming a standard antenna response.
Adoption of this and other findings summarised in this paper can lead to a worthwhile improvement in the accuracy of predictive modelling.
Background to the LTE/TV Interference mitigation programme
The current interference mitigation programme is associated with the reuse, for mobile broadband (LTE), of UHF 800 MHz spectrum. This was formerly used mainly for terrestrial television.
It was recognised early in planning for Digital Terrestrial TV (DTT) in the 1990s that the increase in spectrum efficiency DTT brings would mean that a switch to all digital operation would free up some spectrum for other uses.
Agreements between Western European countries later led to the repurposing of the 800 MHz band for mobile broadband use. The technical issues relating to this mobile allocation were first studied by a European working group, CEPT WG-SE42 .
During the lead up to the auction of this spectrum in the UK in March 2013, Ofcom (The UK’s independent regulator for communications industries and spectrum) led a comprehensive programme of work with stakeholders in 2010 which included measurements, computer simulations and a field trial.
The reports from the Ofcom studies [2,3,4] illustrated an interference risk that could largely be mitigated by the use of DTT receiver filters. This and subsequent Government policy decisions about the extent and nature of mitigation  led to a mitigation programme.
The LTE licensees were required to set up and jointly fund a company to implement the mitigation policy. The resulting company is Digital Mobile Spectrum Limited (DMSL, referred to in some policy documents as “mitco”), which operates under the brand at800.
The mitigation process includes using a computer model to predict which homes might be affected by interference in the near future, based on DTT coverage data from the UK Planning Model , and data on prospective LTE base station deployments provided by the 800 MHz licensees.
The data is used for communication with people living in areas at risk of interference. This consists of the targeted mailings of postcards to properties identified as at risk, as well as broader public relations, advertising, stakeholder relations and social media marketing.
The work of DMSL is monitored from a regulatory point of view by the 4G/TV Coexistence Oversight Board, made up of representatives of the 800 MHz licensees, terrestrial TV broadcasters and three independent members including a chairman.The board also includes observers from Ofcom, Government and DMSL.
This Oversight Board asked technical representatives from DMSL, Ofcom, and the broadcasters to undertake support work to monitor technical aspects of the mitigation programme with a view to improving interference prediction and mitigation; and it is that work that forms the basis of this paper.
Two important reasons for creating a joint Oversight Board are to build a collaborative environment where mobile network operators, DTT broadcasters, the regulator and the Government can work together to ensure an effective and efficient mitigation process; and to allow flexibility to agree changes to the mitigation programme as it progresses to allow it to adapt in line with experience, maintaining and improving its effectiveness.
From the outset of the planning for reuse of the 800 MHz spectrum, it was envisaged that computer predictions of interference would be used as the basis for the interference mitigation programme.
The UK has a well-established model for planning and predicting DTT coverage (the UK Planning Model ) and outputs from this model are used for calculating interference.
Two aspects of interference prediction are of interest: prediction of the specific areas that might be affected; and the overall numbers of homes that could be affected, for example to plan the mitigation programme.
The following paragraphs give a brief outline of the UK Planning Model and the interference prediction modelling.