MSTV FCC Filing Raises Concerns About DTS Interference
February 15, 2006
In comments filed in MB Docket 05-312 regarding DTV distributed transmission technologies, MSTV said that while distributed transmission system technologies have the potential to improve the reliability of DTV service, it urged the FCC to proceed cautiously to "ensure that DTS deployment does not produce new interference to stations or undermine principles of localism through arbitrary service expansion, especially into adjacent markets."
The MSTV comments requested FCC modification of the OET-69 interference standards used to evaluate interference from DTS transmitters and to prohibit DTS users from creating new interference exceeding the limits applicable to single-transmitter systems. The filing also stated that DTS rules should incorporate existing Part 73 parameters concerning power, antenna height and emission masks into DTS rules.
MSTV insists that DTS technical standards should remain "flexible" to encourage development of standards. The filing also requests that "the Commission should ensure that essential patents will be licensed on a fair and nondiscriminatory basis" before any standards are adopted.
MSTV stated that while the FCC should allow LPTV and translator stations to use DTS, it should not permit Class A stations to create single frequency networks using DTS.
It is widely known that the ability of DTV receivers to reject interference, particularly adjacent channel interference, varies with the strength of the signals. Under the MSTV proposal, the OET-69 procedures for evaluating interference would be changed to incorporate variable protection ratios when analyzing DTS interference into single-transmitter systems. The filing explains that the current adjacent channel interference standards were based on adjacent channel transmitters either being collocated or widely separated. When DTS transmitters are located in the same service area as adjacent channel single-transmitter stations, the current interference standard no longer applies.
MSTV proposed the FCC use the values specified in the ATSC A/74 Receiver Performance Guidelines for determining DTV signal levels (strong signal: -28 dBm; moderate signal: -53 dBm; weak signal: -68 dBm. The filing states that OET-69 methodology should be modified to apply the appropriate D/U ratios for each signal level when computing interference caused by a DTS transmitter. The proposal calls for using linear interpolation to determine appropriate D/U ratios for signals between these three levels. For strong signal levels and higher, MSTV recommends modifying OET-69 methodology to ignore adjustment of the receive antenna in the interference calculation, as under strong signal levels indoor reception is likely and use of an outdoor antenna to compute interference most likely would underestimate the interference caused by a DTS transmitter. MSTV said it would refine the approach and work with the FCC in development of a revised methodology for incorporation of variable D/U ratios.
The MSTV comments also outline a proposed procedure for aggregating signal levels from multiple transmitters. This procedure would calculate the interference contribution from each transmitter, then convert the field strength results from decibels above one microvolt per meter to microvolts per meter, to allow addition of interference contributions, and then back to decibels above one microvolt per meter. The aggregate dBµV/m figure would then be used to compute interference based on the variable D/U ratios. As with the variable D/U ratios, MSTV plans to refine the procedure and work with the FCC to develop a revised methodology for aggregation of signals on the same frequency.
For details on these and other MSTV DTS recommendations, including restrictions on coverage, refer to the MSTV comments filed in MB Docket 05-312 regarding DTV distributed transmission technologies. The procedures MSTV is recommending should work well not only for broadcast DTS systems, but for systems such as those being built by Qualcomm and Aloha Partners on lower 700 MHz TV channels adjacent to full service TV stations.