A truck is parked in front of 140 Broadway in lower Manhattan. Some cables coming out of the truck connect to antennas on (one could hardly call them towers) supports rising no higher than the roof of a small suburban house. One block from Ground Zero, that truck and its antennas are helping to provide wireless communications in the area.
The attack on the World Trade Center knocked out most of New York's analog and digital TV stations. It also destroyed a major fiber-optic node, hundreds of thousands of wired telephone connections, and some wireless communications sites. That was a problem not only for subscribers but also for emergency teams.
Even though it was bankrupt, former wireless Internet service provider Metricom, with the help of some laid-off employees, re-established its Internet access in the area to help. And then there was the plan of the National Imagery and Mapping Agency (NIMA) to provide communication services to FEMA.
WNYE-TV, the broadcasting arm of the New York City Board of Education, operates analog and digital TV stations and an FM radio station, as well as webcasts educational materials. Right after the attack, WNYE's analog TV station, located on the Empire State Building, began carrying commercial-free news from WABC-TV to help keep New Yorkers informed. Their FM station began carrying commercial-free news from NPR- affiliated WNYC-FM, one of the radio stations knocked out.
As for WNYE's DTV station, with FCC permission and the help of NIMA and a number of companies, it switched from 8-VSB to a very robust form of COFDM: 8k, QPSK, 1/2 FEC, 1/8 guard, for a usable data rate of about 4.15 Mbps. WNYE-DT broadcast at 1.25 kW from an antenna 483 feet above average terrain in Brooklyn on channel 24. Meanwhile, on the next channel up, WNYE-TV was transmitting at about 2,450 kW at 1,295 feet—just four miles away.
The adjacent-channel interference was in excess of 65 dB in some receiving locations. So it's not surprising that, even with the robust transmission characteristics, there were places where the low-altitude, low-power COFDM signal was completely swamped by the NTSC, and reception was not possible.
At Ground Zero, in other parts of lower Manhattan, and in Brooklyn, reception was reportedly rock solid, even in vehicles traveling at high speed. The roughly 4 Mbps data rate was divided into 2.5 Mbps for SDTV and 1.5 Mbps for data, including streamed media. WNYE provided the necessary encoders.
Although a great deal of useful data was gathered about reception, there's not much learned from this experiment that can be used in the debate over 8-VSB versus COFDM. The low-power COFDM signal was receivable in moving vehicles even with a high- power, adjacent-channel NTSC breathing down its neck, but 4.15 Mbps is clearly not the same as 19.39 Mbps. There could have been some low-data-rate version of VSB that would also have been received.
That, however, is a lesson in itself. At the moment, modulation technique aside, U.S. DTV has a single, fixed data rate. It's 19.39 Mbps no matter what. DVB-T, the European system, is much more flexible.
Suppose the attack had taken place after the DTV transition was completed. The DTV stations on the Empire State Building could have offered to carry—on multicast subchannels—the programming of those stations lost on the World Trade Center (as indeed, WCBS-DT did offer to do).
Could anything have been done with the DTV stations to provide critical information to emergency crews? Perhaps. But the data would have been only as reliable as reception of 19.39 Mbps in a 6-MHz channel in an urban area in a moving vehicle can be—in other words, not very.
When the FCC began looking into advanced television, the idea was simply to provide HDTV. Digital transmission techniques made possible multicasting, datacasting, and the like. A more flexible transmission structure could be incompatible with existing receivers, but so far, there are so few of those that replacing them all remains conceivable. A/53 may be modified anyway for "enhanced" VSB.
Of course, adding flexibility could require flexible thinking.
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