Norway is about to reap the rewards of the digital dividend and ATDI is helping bring in the harvest.
The country’s spectrum regulator, NPT, is preparing to auction slots in the 790-862 MHz range to operators of long term evolution (LTE) mobile networks and has asked ATDI to define with precision what resources are available and potential interference problems.
“Meaningful modelling using realistic scenarios is our stock-in-trade,” says ATDI managing director Cyprien de Cosson, “and never is it more vital than in this kind of spectrum auction. There are vast sums involved here, both in the purchase price and the subsequent investment, and everybody needs to know extremely clearly what is being sold and what is being bought and what it will do.”
Specifically, ATDI assessed the potential impact of an LTE mobile service in the 800 MHz band on the existing UHF networks using the Digital Video Broadcast Terrestrial system (DVB-T) that provides a public television service in Norway.
The challenge for the company was that the lowest block of spectrum due to be auctioned is immediately adjacent to the allocation for digital broadcasting.
“While there is a small guard band between the two services, there is the potential for interference between the digital broadcast network and LTE wireless broadband network,” Cyprien notes. “This problem is particularly acute when considering LTE downlink interference in the lowest block into the topmost broadcast channels.”
He points out that there is a lot at stake in such areas: “These issues can sometimes sound esoteric and relevant only to engineers but they may have an impact on a lot of people – the users of the services, of course, plus the investors paying to put those services in place and, on top of that, the people upholding a political ideal. Public television across Europe was established on the platform of some lofty beliefs, and those standards cannot be met if a large proportion of the population are not getting a signal. It is, then, particularly exacting to be charged with ensuring that all works as it was intended to work – and those purchasers of spectrum resources get an allocation they can work with.”
To reduce the interference, several mitigation techniques were investigated in a sample area around Oslo, where there is potentially a large population that could suffer from interference, particularly due to adjacent channel interference due to the use of channels at the top of UHF television allocation.
Amongst the mitigation techniques modelled included: LTE base station filtering to improve emissions, low cost in-line filtering at the domestic receiver through to improvements in the rooftop antenna, and limiting the use of polarisation of emission at the LTE base station.
The most efficient mitigation technique was found to be to limit interfering LTE base station to utilise only vertical polarisation; this is based upon the fact that, with a few minor exceptions, the Norwegian DVB-T network is horizontally polarised.
“We suggested that while network designs are being completed, LTE operators could consider the interference potential to the DVB-T networks and provide mitigation techniques they intend to deploy in order to overcome the effects,” says Cyprien. “While very challenging, we also advocated that a feasibility study is completed to assess the re-assignment of spectrum of affected areas away from channels 58 to 60; the study would also look at the effect of not using these channels in highly populated areas to determine how helpful this would be in combating the interference effects given dense LTE networks.
“The key is, though, that, no matter how the problems are addressed, everybody understands the issues and options so that they can make informed decisions leading up to getting spectrum that works.”