Interference analysis of co-sited DTV and NTSC translators

Digital Television (DTV) service has begun in all the major urban areas within the United States but has yet to significantly reach the rural areas. Often, translators are the only means of providing free, over-the-air television service to people living in these areas. One challenge is that while specific rules for operation and spectrum allocation have existed for quite some time for full-service stations in urban areas, rules for DTV translator stations in rural areas have just been created (September 2004). Another challenge is overcoming the scarcity of spectrum that exists under the old analog taboo interference rules. A key component in overcoming these challenges is to determine interference parameters in the form of desired-to-undesired (D/U) field strength ratios that will allow as many DTV and NTSC signals to co-exist as possible. Also, co-sited analog and digital translator sites utilizing low radiated power can be used for adjacent/taboo channel operation (NTSC next to DTV or DTV next to DTV) if careful engineering and planning are employed. Even first adjacent channel operation can be accomplished if proper effective radiated power (ERP) ratios are selected. This paper shows that a relative 10-dB NTSC-to-DTV power ratio and a relative 0-dB DTV-to-DTV power ratio is acceptable at a co-sited repeater site in order to avoid receiver interference among the co-sited signals. The choice of absolute radiated power levels depends on the desired coverage area, the surrounding terrain, and the required interference avoidance of potential weak signals from distant stations (if present). Co-siting repeaters and sharing as many of the common transmission elements as possible such as combiners, feedlines, broadband antennas, test equipment, etc. allow many low-power DTV and NTSC signals to be transmitted together under well-controlled conditions, thereby providing the most efficient use of scarce spectrum. Further independent laboratory testing with current consumer DTV receivers is also vital for improved channel allocation planning.