The seeds of LightSquared's failure to win government clearance to build a 4G-LTE network can, ironically, be found in the "approval" the company received just 13 months ago.

In January 2011, the Federal Communications Commission (FCC) was clearly getting a positive vibe from LightSquared's plan to build an open-access network using both satellites and cell towers. The conditional approval issued by the agency stressed the positives of LightSquared's plan, noting that "if LightSquared successfully deploys its integrated satellite/terrestrial 4G network, it will be able to provide mobile broadband communications in areas where it is difficult or impossible to provide coverage by terrestrial base stations (such as in remote or rural areas and non-coastal maritime regions), as well as at times when coverage may be unavailable from terrestrial-based networks (such as during natural disasters)."

LightSquared chalks this up to a failure on the part of GPS makers, and this is at least partly true

The FCC detailed further "public interest benefits," such as increasing competition among mobile wireless providers, spurring innovation in the consumer device industry by providing a network not tied to any single carrier, and providing additional broadband capacity at a time "when the use of such services is increasing exponentially." The FCC would probably argue today that these needs are still just as pressing. Yet this past Tuesday the commission decided to withdraw its conditional approval.

Despite the FCC's glowing remarks about LightSquared, the conditional approval made it clear the plan would never gain final clearance unless it could be implemented without interfering with GPS devices. In a nutshell, LightSquared needed a special waiver because it is trying to use spectrum allocated for low-power space-to-ground transmissions for something it was not originally allocated for: high-power ground-only transmissions that could fuel a nationwide wireless mobile broadband network. The portion of L-Band spectrum controlled by LightSquared is adjacent to the spectrum used by GPS devices, and GPS devices, according to repeated tests, would be unable to receive the signals intended for them because the high-power LightSquared signals would overpower the GPS ones.

This doesn't mean LightSquared signals would travel outside their allotted spectrum. Rather, most GPS devices are incapable of filtering out signals from adjacent frequencies—particularly when those signals are many times stronger than the signals GPS devices are supposed to receive. LightSquared chalks this up to a failure on the part of GPS makers, and this is at least partly true. Michael Marcus, who worked at the FCC for 25 years and is now a consultant on wireless technology and spectrum policy, writes that cellular base stations have been "allowed next to the GPS band since 2003" under FCC policy, yet GPS makers "paid little attention to the fact that GPS would be having a new neighbor with much stronger signals in some places than the original MSS [mobile satellite services] signals."

Marcus, who does some consulting work for LightSquared, further writes that the "GPS industry has not pressed the filter manufacturers for the latest technology," and "as a result many GPS receivers have a lingering vulnerability to strong adjacent band signals that results from GPS manufacturers ignoring policy changes made in the US almost a decade ago."

Blame GPS makers as much as you want, but interference is still a problem

It thus seems there is good reason to criticize the GPS industry for failing to create devices that filter out signals from adjacent bands. The "Save Our GPS" industry group that opposes the LightSquared plan did not respond to an interview request from Ars this past week. But given the simple reality that most current GPS devices cannot filter out LightSquared signals, government-commissioned studies have concluded that it would be impractical to force GPS makers to retrofit all existing devices in time for the mobile network's proposed launch in 2012. If one just ignores the question of whose fault this is, the government has concluded that the GPS system is simply too important to disrupt.

Devices connecting to the LightSquared network would have two chips, one to connect to cell towers and another to connect to satellites. The spectrum that would be used for the ground towers is spectrum originally allocated for satellite-to-ground transmissions. LightSquared's initial plan called for 40,000 cellular base stations across the US.

LightSquared: Spectrum reallocation drives innovation

"Lots of spectrum is allocated to one thing and it evolves into something else. For instance, broadcast spectrum has been reallocated for wireless use," argued LightSquared spokesman Chris Stern in an interview with Ars. "The cell phone spectrum I'm talking on now was originally allocated for something else. Changing allocation is something that not only has happened for decades, but it is something that is really important for the evolution of technology."

Still, the FCC has discretion over how spectrum is allotted and can—as it ultimately did in this case—decide that certain slices of spectrum aren't suitable for a particular use.

Ideally, Marcus tells Ars, the FCC and NTIA "could have adopted a set of power flux density limits on LightSquared that would have protected all the safety-related uses of GPS and most of the non-safety related uses," with a gradual relaxation of limits as GPS technology improved to prevent interference. "However, due to resource and structural problems at FCC and NTIA this was not really feasible," he said.

LightSquared cell tower signals would be far stronger than the signals GPS devices must receive from space, but Stern says that's not the problem. For one thing, he said, LightSquared agreed to reduce its power levels by a factor of 32 after its initial plan raised a huge outcry from the GPS industry. For another, he says, "GPS devices are designed to look into spectrum that is licensed to LightSquared. That's the problem.... The commercial GPS folks... viewed the LightSquared spectrum as a vacant lot that nobody would use and they could use. It was sort of like if you lived in a subdivision and no one built in the lot next door to yours, and one day you're like 'I'm going to build my patio out into their lot,' and then one day somebody buys the lot next door to you and plans to move in and you say 'you can't move in there because I have my patio on your lot.'"

If GPS makers were to make a similar analogy, it might be that LightSquared's proposed cell towers are more like a loud neighbor. If you live in an apartment building and can't hear the sound of your own voice because the neighbors blast heavy metal music at ear-shattering levels 24 hours a day, it's not your fault for having ears that are capable of detecting sounds from ten feet away. The neighbors might be creating the noise within their own walls, but it's still loud enough to drown out the sounds made by the people next door.