This paper uses data from the FCC and the 2000 USA Census to estimate the actual white space (in terms of area/population recovered) available for cognitive radios in TV bands in the continental USA. The available white space is the intersection of white spaces resulting from two different viewpoints. The "pollution" viewpoint reflects the cognitive radios' perspective: where is interference from the primary tolerable? The primary users' perspective is called the "protection" viewpoint: where can a secondary operate without generating excessive 'harmful interference' to the primary? For most practical systems that may operate in the TV bands, it turns out that the operational limit will largely come from the pollution-oriented view.

The amount of white space resulting from the protection oriented view is greatly effected by the amount of 'harmful interference' allowed. Harmful interference is crisply quantified by the fading margin eroded by the secondary's potential operation. Based on the FCC's November 14th ruling in 2008, the median (across TV towers) erosion of the fading margin is ~1dB. Furthermore, the available white space depends on the scale of secondary users --- while the current DTV channel allocation cannot accommodate new TV stations, about five channels per person are available for 4W fixed transmitters.

We propose a principled way for regulators to choose the protection margin for primaries that can be eroded by secondary operation. This approach quantifies the political tradeoff between person-channels gained for potential whitespace usage versus person-channels lost for broadcasters as we vary the protection margin. For the choice of protection margin(s) used by the FCC, the overall tradeoff is at least 30:1 while being approximately 3:1 at the margin --- that is three additional people gain a channel for potential white-space use for every additional person that potentially loses reception of a channel of broadcast television.

Finally, the data validates the conservatism of fixed threshold rules for white-space detection --- the -114dBm rule for ATSC signals fails to recover most available white space except in areas of low population density.




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