[Note: This item comes from friend Chuck Jackson. DLH]
Progress in the Debate over TV White Space
By Chuck Jackson
Jul 13 2017
Tuesday (July 11, 2017), Microsoft unveiled their current vision for unlicensed radio services in the TV White Space (see “Microsoft calls for U.S. strategy to eliminate rural broadband gap within 5 years”.)
Microsoft’s proposal demonstrates progress in the debate over use of the TV White Space. A decade ago, proponents of unlicensed operation in the TV White Space stated that it “will transform every aspect of civil society.” (No cite given because I don’t want to be cruel. But, search engines are your friend.) Yesterday, Microsoft made available a white paper about White Space that asserted that “Overall, TV white spaces technologies appear to be the optimal solution for a little more than 19 million people . . .”
Nineteen million people is a tad less than 6% of the population. Toning down the rhetoric in support of unlicensed use of the TV White Space from “transform civil society” to “provide a more cost-effective alternative for 5.8% of the market” is real progress. (The quote regarding 5.8% is my statement, not Microsoft’s.)
Still, I doubt that unlicensed operation in the TV White Space would “provide a more cost-effective alternative for 5.8% of the market.” The Microsoft white paper asserts that using TV White Spaces could provide access to 23.4 million people in rural areas for a cost in the range of $10–15 billion; it also asserts that using commercial wireless in the 700 MHz band would cost $15–25 billion to provide the same coverage. (Microsoft White Paper at pp. 12-13.) Thus, Microsoft asserts that using 700 MHz spectrum is projected to cost about 1.5 times as much as using the White Space. Microsoft cites a study by the Boston Consulting Group for these cost estimates; I could not find a copy of that study or a description of its methodology.
However, the Microsoft white paper drops some hints about the methodology. Hint number 1: they note that most TV White Space operations will be on TV channel 37 (608–614 MHz) or in the duplex gap (652–663 MHz).
Hint number 2: they state (incorrectly—more about that later) that a TV White Space signal can travel four times the distance of a 2.4 GHz Wi-Fi signal. Well four times the distance is exactly what one would calculate if one used the Friis Transmission Formula to compare signal strengths and assumed that the TV White Space system operated at 600 MHz and the antennas in both systems had equal gain. Microsoft’s assertion that signals in the TV White Space travel four times as far as 2.4 GHz Wi-Fi signals would not apply if the Wi-Fi system had an outdoor antenna as big as the antenna of the TV White Space device; here’s an example of such an antenna.
Using a big antenna like that the Wi-Fi signal received at the residence would be just as strong as the comparable White Space signal; the uplink Wi-Fi signal from the residence to the base station would be much stronger than the White Space signal.
But the Friis formula may explain the difference found in the Boston Consulting Group study between the cost of rural coverage using TV White Space and the cost of using 700 MHz LTE. If one were to apply the Friis transmission formula to compare the coverage of a 700 MHz LTE system with that of a 600 MHz TV White Space system in the same way that Microsoft applied it to Wi-Fi versus a TV White Space system, one would calculate that a base station in a TV White Space system would have about 1.4 times the coverage area of a base station in a 700 MHz LTE system.
If rural America could be covered by 10,000 TV White Space cells, it would require 14,000 700 MHz LTE cells to give the same coverage. This simple calculation leads to a conclusion (national coverage using 700 MHz systems would cost 1.4 times more than would using TV White Space systems) that is essentially the same as the Boston Consulting Group study’s conclusion that using 700 MHz systems would cost about 1.5 times more.
The analysis presented above is incorrect for at least two reasons. First, the Friis Formula is probably not the appropriate propagation model in this context. Second, even if it were the correct model, the conclusion would be wrong because of an omitted factor. Wireless coverage depends on both signal strength and bandwidth. Wireless carriers have access to 84 MHz of spectrum in the 700 MHz band—more than four times the 18 MHz of white space that Microsoft hopes will be available at all locations. In many circumstances, that extra bandwidth would more than compensate for any slight difference in signal attenuation. Consequently, systems operating in the 700 MHz band should be more effective alternatives than is suggested by Microsoft’s analysis.