[99-25 NPRM] Appendix D: Spectrum Availability Analysis

To investigate the feasibility of a low power radio service, we conducted spectrum availability analyses for sixty communities of various sizes throughout the United States. Twenty cities were chosen within each of three population "tiers." The first tier consisted of cities with populations of more than 500,000 persons; the second tier, cities with populations between 200,000 and 500,000 persons; and the third tier, cities with populations between 50,000 and 200,000 persons.131

Grids. We established a uniform distribution of study locations centered on each city by overlaying a coordinate grid consisting of grid cells of a size one minute latitude by one minute longitude. Throughout much of the country, a one minute variation in longitude is slightly less than one mile and a one minute variation in latitude is slightly more than one mile. The study locations correspond to where the grid lines intersect. For Tier 1 and Tier 2 cities, the grid extended 30 minutes a side. This yields a total of 961 intersections (study locations). For Tier 3 cities the grid extended 20 minutes on each side, yielding a total of 441 study locations.

Interference with respect to other services. At each study location, we determined whether or not a proposed low power FM station could operate on each of the 100 FM channels without causing or receiving objectionable interference. 132 We based these determinations entirely on minimum distance separation tables. In all cases, we used the larger of the two spacing requirements set forth in Appendix B. We applied these separation requirements to all full service FM licensed facilities, construction permits, pending applications, and vacant allotments. 133 Additionally, low power FM stations operating in the reserved band (channels 201-220) or on channel 253 were required to provide protection to nearby TV channel 6 stations.134 Additionally, in one of the studies, LP100 stations were restricted from causing or receiving interference with respect to FM translators.135

Interference between low power stations. Our model provided interference protection between co-channel or first-adjacent channel low power stations.136 Because of this, some stations are precluded from assignment solely because of previously assigned low power stations. Thus, the grid location assigned to a station becomes an important factor in its preclusive effect on the assignment of other co-channel and first-adjacent channel low power stations within the grid. For example, a channel 202 assignment near the center of the grid may preclude any other channel 202 station from being assigned, whereas two or perhaps four channel 202 stations could be assigned if they were located at the corners of the grid.

Assignment methodology. For each of the 100 FM frequencies, the analysis program determines which grid points are precluded because of interference considerations with respect to other services as described above. For each grid location available for a frequency assignment (e.g., channel 202), the program determines how many assignments on other available co-channel (channel 202) and first-adjacent channel (channels 201 and 203) grid locations would be precluded by this assignment. The program repeats this process for each available grid location, recording the preclusive effect until all available locations have been considered. Then the assignment process begins. The program makes assignments at the most preclusive grid locations. Between equally preclusive locations, the location nearest the center of the grid is selected. We selected the most preclusive locations, rather than the least preclusive locations, in our analysis for several reasons. First, we wanted a realistic, rather than an overly optimistic assessment of the spectrum available for this proposed service. Also, transmitter sites will most likely be selected based on coverage considerations, not preclusion considerations. Finally, a great many of the grid locations theoretically available for a low power station will, in fact, not be available due to a variety of environmental considerations (e.g., zoning restrictions, proximity to airports, swamps, rivers or water, etc.).137


131 Population figures were based upon the 1996 U.S. Census estimates.

132 This protection criterion differs somewhat from the criteria proposed in this Notice. Specifically, the Notice proposes a secondary status for LP100 stations, which means that they would not be protected against interference received. Thus, our analysis, which assumes full protection against interference received by the low power station, may significantly underestimate the number of low power stations that could be assigned if they were permitted to receive interference.

133 These studies were based upon the Mass Media Bureau's FM Engineering Database as of December 9, 1998. Subsequent staff actions or application filings could alter the results of this analysis.

134 We used the TV channel 6 spacing requirements listed in the FM translator rules, 47 C.F.R. ' 74.1205(a), for stations in the reserved band. We required low power stations operating on channel 253 in Zone I to be spaced at least 16 kilometers from TV channel 6 stations and those in Zone II to be spaced at least 20 kilometers. 135 Because FM translator stations are not specified by class, we provided protection to and from translators in accordance with the following table based on the translator's ERP and HAAT in the azimuth towards the LP100 station.

FM Translator 1 mV/m contour distance (km) LP100 co-channel (km) LP100 1st-adjacent channel (km) LP100 2nd/3rd adjacent channel (km) LP100 IF Channel (km)
13.3 or greater 67 35 21 5
Greater than 7.3 but less than 13.3 51 26 14 4
7.3 or less 30 16 8 5

136 We used the minimum distance separations listed in Appendix B. The model did not provide any 2nd- or 3rd-adjacent channel protection between low power stations. No studies were made mixing LP1000 stations with LP100 stations. Similarly, no studies were made involving microradio stations.

137 In several cities located in coastal areas or bordering on large bodies of water, the program excludes from consideration grid points likely to be over water.