Will Base Station Signals Interfere with TV Reception?

My colleague Linley Gumm and I recently conducted a series of measurements of the threshold protection (desired/undesired (D/U)) ratios required for modern ATSC receivers to reject interference from LTE signals radiated by base stations in the 600 MHz Band. We tested 21 receivers and forwarded our results to the International Telecommunications Union through Dr. Charles Einolf, a U.S. member of the ITU working group dealing with such interference issues. Our paper described how we conducted these tests and our results. This report is available from the ITU website www.itu.int/pub/R-REP-BR.2215-6-2016, entitled Measurement of Protection Ratios and Overload Threshold for Broadcast TV Receivers. Our contribution is Annex 3F.

THE PROBLEM

Our problem was to find a source of real, live LTE signals for testing purposes. We found two such LTE signals on-the-air in the 700 MHz band with strong signals, each 10 MHz wide centered at 739 MHz and 751 MHz. These we heterodyned down into the 600 MHz band. Such signals in the 600 MHz band would consist of two 5 MHz blocks of contiguous frequencies, so we were emulating what we believe will be common practice in the 600 MHz band after the 600 MHz band is repacked following the spectrum auctions.

By now, it is generally accepted that ATSC reception may be jammed by pairs of undesired signals on Channels N K and N 2K with the desired signal on Channel N or N 3 if third-order intermodulation (IM3) products are being generated in the frontend of the affected receivers.

Gumm proposed to heterodyne our LTE signals into Channels 18 MHz above and below the desired ATSC signal on Channel N. None of the third-order distortion products fall within the desired Channel N. The desired ATSC signal was being jammed. That is what we reported in Table 3 on page 8 of what today is Annex 3F to the ITU Report BT-2215. This data is given in the column marked No IM3. Without any IM3 in desired Channel N, we found that the threshold D/U ratio was, for 90 percent of the receivers, to perform -32 dB. Two LTE signals 18 MHz above and below the desired ATSC signal power cannot be more than 32 dB greater than the ATSC signal power. That is a long way from the -60 dB D/U threshold, which the FCC assumes for planning purposes. When our two LTE signals were centered on Channels N K and N 2K, the undesired threshold was -28 dB, only 4 dB worse. This is shown for FCC Scenarios 7, 10 and 12 in the abovementioned table in Annex 3F. Even with a perfectly linear receiver front-end (which would not generate IM3), interference was only slightly reduced.

What could explain why we saw interference, which could not have been generated in the receiver? Well, we saw desensitization. All modern DTV receivers have wideband RF automatic gain control (AGC). Strong undesired signals on Channels N /-3 forced the wideband RF AGC system to reduce the gain of the RF amplifier to protect the mixer from being overloaded. Table 1 summarizes our test results for the 21 modern ATSC receivers detailed in Annex 3F of the ITU Report.

Table 1: Threshold D/U (dB) for 21 modern ATSC Receivers from LTE signals

LINK:	http://www.tvtechnology.com/resources/0006/will-base-station-signals-i...
	See more stories from tvtechnology

Most recent headlines