The UHF broadcasting spectrum is a popular resource. Broadcasters have started to free the higher part of the UHF band (790-862 MHz, the so-called 'Digital Dividend') for use by telecom operators. The remaining lower part (470-790 MHz) stays in use for broadcasting. This means guarantees are needed to make sure mobile base stations or handhelds will not cause interference into the lower band. EBU frequency expert Dr Walid Sami (EBU), has written an article to help regulators understand the interference risks, and adequate means to address them, when they appear. This information is important, as it can influence the licences of the Digital Dividend users.
The EBU Technical Review article presents briefly the results of field-trials performed jointly by FreeTV Australia and the EBU in 2008-2009. It mostly describes in detail the theoretical studies made by the CEPT on adjacent-band interference in 2009, which have lead to the specification of harmonized technical conditions for the use of the band 790-862 MHz as published in the CEPT Report 30. The tests and studies showed that mobile base stations do indeed present a significant risk for creating interference to adjacent band DTT (Digital Terrestrial Television) services. Two requirements result from this work:
1. Limitations on the transmission characteristics of the mobile network are needed in order to reduce the interference probability on broadcasting. The main limitation is in terms of out-of-band emission limits for the base station;
2. Additional mitigation techniques are needed as well, such as co-siting the DTT transmitters and mobile base stations, use of cross-polarization, further reduction of the base station out-of-band emission level, filtering the RF input signal in the DTT receiver, and others.
Graphic illustrating the channelling arrangements of FDD mobile down- and uplinks in Europe.
It shows the separation from the lower UHF broadcasting band by a 1 MHz band (left in the image).
The mobile terminals were also found to pose an interference risk, especially for portable indoor DTT reception. Again restricting the out-of-band emissions and using a (large) guard interval are needed to help prevent this problem. Additionally, the main mitigation technique needed is to filter the mobile uplink signals, at the receiving input of the DTT receivers. For more information on this topic, see the joint recommendations issued by DigiTAG, the EBU, BNE and ACT in November 2010 or send an e-mail to Dr Walid Sami (EBU).