An Ofcom commissioned report into the possibility of interference from LTE handsets to DVB-T receivers has concluded the installation of high quality filters and aerial flyleads can resolve the majority of issues.
Cobham Technical Services’ Era Technology division investigated the possibility of interference from LTE handsets situated 2.5 metres or more from digital TV receivers. There has been industry concern at the possibility of major interference from the devices if LTE services are licensed to use capacity within the television spectrum freed up through the Digital Dividend.
Cobham tested five receivers comprising a super-heterodyne-based integrated digital television (IDTV), two IDTVs with silicon-based digital tuners, a personal video recorder and a set-top box. The tests were made with and without the use of filters and better flyleads. Ofcom considers the use of a filter as a possible method of reducing interference at higher frequencies from base stations and mobile handsets.
When used with an outdoor aerial no interference was found when the five DVB-T receivers were subjected to a source signal of 23 dBm. However, when the source power was increased to 28 dBm interference was found at 849MHz, just 1 MHz below the centre frequency of the receivers.
The interference was tracked down to the aerial flylead with Cobham suggesting that a higher quality replacement “may either substantially reduce or cure the problem”.
Predictably performance varied between the tested devices, but Cobham concludes most of the resultant interference could be removed with the use of a low pass UHF filters. Performance was at its worst on receivers based on a super-heterodyne architect cure, some of which have protection ratios as low as -40 to -30dB. The problem was particularly acute when an indoor aerial was used.
Separate tests of a Sky Digibox, where the signal is distributed to a second set, were conducted using standard co-axial cable and the better quality CT100. Co-channel results were designated as Grade 3 or ‘slightly annoying’ with the impairment on the co-axial cable being measured at 37 to 41 dB worse than on the CT100, which required an EIRP of 15 -16 dBm for the effects to be noticeable. First and second channel interference was virtually unperceivable.
The findings will come as a relief to telecoms operators that may be eying up the spectrum, less so to the long suffering consumer who may be faced with unwanted inconvenience and expenditure.
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