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Keeping digital TV and LTE from colliding

03 Dec 2013  | Glenn Chang

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Homes or apartments with outdoor or indoor antennas can experience LTE interference on their DTV from nearby base stations. In either case, whether the interference is from the mobile device or base station, the interference can cause DTV picture quality to be severely degraded or even lose the video altogether.

What can be done?
The solution requires defining new standards in each country so that STB manufacturers can select tuners that will prevent the problem. This means standards bodies developing practical test cases for both base station and user equipment interference. The test cases must be specific to the exact LTE frequency plan in each country. With specifications in place, STB manufacturers can select any of the DTV tuners with the performance needed to coexist with LTE signals.

Brazil is just now going through this process to support its analogue shut off, which is scheduled to begin in 2015. The country's national telecommunications agency (ANATEL) has decided to release the 700MHz RF spectrum for mobile broadband services. Terrestrial TV services are very popular in the country, so millions of digital converter boxes will be needed to allow people to continue using their existing analogue televisions.

Brazil has a legacy installed base of more than 20 million digital televisions and many of these will experience DTV interference when LTE broadcasting becomes widespread. This is because most of thefirst- and second-generation DTVs in Brazil used discrete (non-silicon) "can" tuners that have poor performance to reject the unwanted interference signals. Consumers with older DTVs may need to upgrade to newer models with higher performance silicon tuners inside, or purchase an external filter to resolve the problem.

Fortunately, for new DTVs and STBs, the solution is much simpler. Careful design with high-performance silicon tuners that feature built-in filtering that delivers a good protection ratio against LTE interference (also called the desired-to-undesired, D/U, ratio) to meet the interference test cases is the best way to avoid interference issues in the future.

An example of such test cases is documented in a whitepaper that I wrote with Dr. Gunnar Bedicks, Chief Researcher in the DTV Laboratory at Mackenzie University in Brazil. In the document, we look at the interference issue in Brazil and specify five different digital TV / LTE use cases that define the amount of LTE power – either from a base station or a mobile device – that one can expect at the DTV receiver input.

The outcome of this analysis was a protection ratio specification for each use case that a TV tuner from any manufacturer must accommodate in order to coexist with the expected LTE interference. If the tuner doesn't support these, then expensive frequency filters must be installed to eliminate the interference sources.

The correct tuner choice can alleviate the problem of LTE interference, but testing is needed to ensure that the standards will cover the most likely use cases expected in each country.

About the author
Glenn Chang is wih MaxLinear.

To download the PDF version of this article, click here.

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