TV Band Offers Mobile Phone Service
July 24, 2002
Technology startup SigFx taps VBI spectrum to provide personal communications
A Mississippi startup is exploring new ways of using excess DTV spectrum for sending and receiving cellphone calls.
| The SigFx Personal Base Station serves as a stationary wireless link for mobile devices such as cellphones and PDAs|
Although still in early testing, SigFx has begun using the VBI broadcast spectrum on a low-power analog station to transmit and receive cellphone and data transmissions. Cellphone communications are also possible when stations convert to digital, the company said. SigFx is testing this prototype system at Channel 46, a local 1 kW UHF station; it recently received a three-year experimental license from the FCC to begin testing the service on Channel 10, a VHF station in the Jackson, Miss., area.
Using off-the-shelf VBI insertion technology and a proprietary return-signal processor, SigFx thinks the technology will improve cellphone reception both in the U.S. and abroad, possibly offering improved mobile access to NTSC and PAL countries that do not have widespread cellular network towers.
"It's a good deal for the TV station [in the U.S. and abroad] because it gives them a revenue stream that they don't now have," said Dr. Dallas Nash, CEO of SigFx.
SPACE FOR SALE
The wireless two-way network constructed by SigFx uses vacant spectrum present in both analog and digital television transmissions to provide two-way personal wireless voice or data services.
In an analog broadcast, SigFx installs an off-the-shelf VBI inserter and taps into a station's transmit antenna. SigFx installs a proprietary 6 RU return signal processor (RSP) at the station headend to provide interface communications to the cellphone. Like a traditional telephone call, an incoming call is placed through a public-switched network and linked to a switch installed at the station, which then interfaces with the RSP. The RSP digitizes the signal, transmits the data through the unused portion of the television band and connects it to a mobile receiver. When the recipient selects "talk," a backchannel is opened to the RSP via a SigFx-installed receive antenna array collocated at the station's broadcast tower.
The system requires a SigFx mobile phone with the ability to operate on any TV channel from 54 MHz to 806 MHz, as well as on cellular and PCS frequencies.
Larger than a standard cellphone, the SigFx phone has an electronically steerable antenna as well as four on-board processing chips designed to measure the local RF environment.
Compared to a standard cellphone, which has one processor, the excess processors allow the signal to be detected by the RSP and lets callers make phone calls via GSM, CDMA and TDMA mobile phone formats. "If you want to roam on a cellular or PCS providers GSM, TDMA or CDMA network [instead of a TV band], you can do so," Nash said. "When you're in an area where it is more appropriate to use standard cell-type technology, the phone can revert [automatically] to that."
PERSONAL BASE STATION
In late June the company announced its newest communications system, the "Personal Base Station," which serves as a stationary wireless link for mobile devices such as cellphones and PDAs.
SigFx expects to have service available in the U.S. next year and may announce internationally available service before then. "Depending on what country we're operating in [and its regulatory laws], we are able to use any portion - or the entire portion - of a 6 MHz-8 MHz spectrum associated with a TV channel," said Gary Conerly, director of commercial sales for SigFx. "When it comes to digital TV, there is no VBI so it merely becomes a matter of what percentage of that TV channel is left to be used for innovative services."
The company has faced some roadblocks, including problems handling multiple calls simultaneously and the relatively weak strength of a cellphone signal in a noisy television band. The phones are also larger than standard mobile models due to a large brick-sized battery transceiver.
In short, however, SigFx believes there is a desire for cellphones that don't experience the drop-offs and dead spots of today's traditional cellular technology.
"Because television channels operate at a lower frequency, they travel a much greater distance than your standard cellular or PCS frequencies," Conerly said. A high-power UHF station may transmit from 20 to 40 miles while a standard cellular tower may only transmit between 10-15 miles. "As a result there are much fewer dead spots [on a TV coverage map] than with cellular towers," he said.