Last week you may have seen several news reports about Google’s interest in using “TV white space” to provide wireless services. Some of the articles talked about using TV white space to provide gigabits of data bandwidth. This didn’t make sense to me, so I tracked down Google’s filing to see exactly what Google’s engineers said.
In the filing Google asserted, “This large amount of spectrum, coupled with advanced signal processing techniques made practical by the exponential growth in computing power (Moore’s law), can make data rates in the gigabits-per-second available in the not-to-distance future. As a result, we soon could see a low-cost and open infrastructure, supporting a near-unlimited bandwidth Internet service, improving every year as computer and radio technologies continue to evolve. This would be akin to a faster, longer range, higher data rate WiFi service—‘WiFi 2.0’ if you will.”
Is this fact or fiction? If the location is an isolated rural area with few over-the-air TV stations available, it may be feasible. If it is in an urban area, like New York, Philadelphia, or Los Angeles, it will be very difficult to find a TV channel that won’t be occupied by a full power or low power broadcaster in the market or in an adjacent market. While some adjacent channels may be available, tests have shown operation on channels next to a broadcast station is likely to cause interference to that station.
How much data can a 6 MHz TV channel carry? Broadcasters are able to achieve 19.39 Mbps, one-way, using 8-VSB. More robust transmission modes will provide less bandwidth. For 2.4 GHz WiFi channels, which have a 22 MHz bandwidth, almost four times that of a TV channel, the maximum data rate is 54 Mbps using the 802.11g standard, with a typical throughput of 19 Mbps. The 802.11n standard can achieve up to 248 Mbps (74 Mbps throughput). Doing the math, a direct scaling of the 802.11n data bandwidth to a 6 MHz channel shows a TV white space channel might be able to carry about 70 Mbps, best case. If Google could find 14 white space channels in a market, it might be able to achieve, at best, a 1 Gbps data rate.
There are, however, other ways to count bandwidth. If the data rate Google was talking about was per market, and you had a hundred cells in a market each covering a small area, then its claim of gigabit-per-second data rates for the whole market could be achieved with two white space channels. Each user, however, would be sharing, at best, 140 Mbps with all the other users in that cell. If the cell sizes are small, why not use existing unlicensed spectrum at 2.4 and 5.8 GHz, non-exclusive 3.6 GHz spectrum, or WiMax bands, which offer wider bandwidths and less interference?
Google proposed setting aside channels 36 and 38 for use by wireless microphones. One problem with that is both channels are currently used by high power DTV stations in Los Angeles and New York, as well as in many other large markets where alternative in-core high-VHF or UHF DTV channels for broadcasters
Google’s filing proposed powers up to 4 watts EIRP. Based on the FCC’s testing of cable TV set-top boxes, such devices are likely to cause interference to cable TV reception in the same or nearby buildings. There is no “white space” in cable TV. If anyone wonders whether digital signals can interfere with cable TV reception, a recent situation in San Diego, where KFMB-DT had to significantly reduce power on DTV Channel 7 to avoid interference to cable subscribers, shows it is an issue. In San Diego, cable interference was seen several miles from the KFMB-DT transmitter site. While the power levels are significantly higher than the 4 watts proposed by Google, field strength increases exponentially as distance decreases. A 4 watt transmitter on building next door will have the same impact as a much higher-power transmitter miles away.
Google should be commended for recognizing some of the interference problems and proposing solutions to avoid interference. Google does not depend on sensing alone to avoid interference to channels in use. In isolated rural areas, it may work. In dense urban areas, if interference to over-the-air and cable TV viewers is to be avoided, the amount of available spectrum and data capacity, at least to the end user, appears to be far less than what Google claims.
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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