LightSquared vs. GPS raises big spectrum issues
- 26 July, 2011 07:15
The argument raging over LightSquared's proposed LTE network and possible interference with GPS services is actually two arguments over two sets of frequencies, both of which the startup hopes eventually to use.
One of the bands allocated to LightSquared, called the "upper band," sits right next to spectrum set aside exclusively for GPS (Global Positioning System). Its "lower" band is farther from the GPS frequencies but has been used by satellite services that make GPS more accurate. In both cases, the signals from LightSquared's LTE (Long-Term Evolution) towers would be stronger than those of the satellite-based services. But beyond that, the issues are very different.
LightSquared plans to offer wholesale mobile capacity over both a satellite and a terrestrial LTE network, which other service providers can sell to subscribers separately or as one service. In January, the U.S. Federal Communications Commission agreed to let the company use spectrum in the MSS (Mobile Satellite Service) band for LTE. But first, it will require LightSquared to resolve potential interference with GPS.
The first step in that process was a series of tests focused on the upper band, which LightSquared had planned to use for its initial rollout. Those tests found massive disruption to GPS, so the carrier proposed starting out in the lower band and using the higher frequencies only after the problems there had been worked out. Interference in the lower band is not likely to affect as many GPS devices, though critics say the impact might still be significant for many devices.
A heated dispute
The LightSquared interference debate is among the most heated in recent years in the U.S., pitting the critical and widely used GPS service against a new mobile data entrant with a new technology and business model. The FCC and mobile industry agree that more spectrum will be needed to meet the country's mobile data needs in the coming years, but critics say it would be a mistake to allocate these particular bands to a 4G cellular network.
The argument over the upper band (1545.2MHz to 1555.2MHz) is especially notable because it doesn't involve any operator transmitting signals on anyone else's frequencies.
Both LightSquared and GPS vendors and users agree that the proposed LTE base stations would not generate signals outside LightSquared's assigned band. GPS satellites also stick to their own channels when sending information down to receivers, including personal navigation devices and cellphones. The receivers don't transmit anything at all, but only "listen" for transmissions from the GPS satellites.
But backers of LightSquared and of GPS don't agree on why the two networks ended up clashing. GPS backers say it's because the LTE network is billions of times more powerful than the distant satellites that until now have been alone in the MSS band. For its part, LightSquared faults GPS vendors for not equipping their receivers with stronger filters that can tune out the nearby signals.
Like cellphones and FM radios, GPS receivers take in analog waves and try to extract the signals that modulate those waves. To get the best possible signal, they may take in radio waves from all across the GPS band. Generally, the more of the band that a receiver can use, the more precise its location information can be. But if the receiver picks up irrelevant signals, that makes it harder to discern location data.
The challenge is to make a receiver that's sensitive to transmissions on every frequency up to the edge of the band but not to transmissions from outside it. When two different bands are right next to each other, as with GPS and LightSquared's upper band, that becomes harder. When one set of signals is stronger than the other, that adds another degree of difficulty.
LightSquared believes it's possible to solve the problem with commercially viable filters. Its opponents think that can't be done.
"There has never been, nor will there ever be, a filter than can block out signals in an immediately adjacent band that are so much more powerful," the Coalition to Save Our GPS said in a written response to LightSquared's plan.
GPS satellites broadcast a 50-watt signal from 12,000 miles up, across an entire continent, said Pete Large, a vice president at GPS vendor Trimble Navigation who works with the Coalition. By contrast, the LTE base stations used in the recent tests produced signals of up to 1,600 watts within about a mile of GPS receivers. LightSquared's signals were stronger by 1 billion times or more, Large said.
Not a brand-new problem
This isn't the first time a spectrum dispute has grown out of a situation where both sides were sticking to their frequencies. Mobile operator Nextel, which was acquired by Sprint in 2005, pieced together its network from many different bits of spectrum and subsequently faced a problem with adjacent-band interference. Its patchwork of frequencies in the 800MHz band were interleaved with spectrum assigned to public-safety and private wireless systems.
"Everyone was doing what they were allowed to do, but basically everyone was getting interference," said analyst Tim Farrar of TMF Associates.
In 2005, Nextel agreed to gradually move some of its operations to spectrum in the 1.9GHz band and allow for simplification of the 800MHz band into fewer, wider sections. Sprint Nextel spent billions of dollars to move public-safety users to other bands, and the process took several years, Farrar said.
Another so-called "adjacent interference" problem in the U.S. affected police radios that used frequencies close to analog TV channel 14, said Dale Hatfield, an adjunct professor at the University of Colorado at Boulder and a former chief of the FCC's Office of Engineering and Technology. Like the LightSquared issue, that interference problem involved one network operating at higher power than the other. The problem eventually was solved with stronger filters on the police-radio receivers, as well as other steps, Hatfield said.
It's much more common for spectrum fights to involve two types of users sharing the same frequencies, said Farpoint Group analyst Craig Mathias. In fact, in the U.S., most radio bands legally can be used for more than one type of service, he said. Exclusive licensed frequencies, such as the GPS band, LightSquared's upper and lower bands, and the channels auctioned off to mobile operators, are exceptions rather than the rule.
Sharing can lead to conflict. For example, after the IEEE (Institute of Electrical and Electronics Engineers) created the 802.11a standard that brought Wi-Fi to parts of the 5GHz band, the popular wireless LAN technology started to pose a threat to other applications in the band, such as military radar. Even though the frequencies were unlicensed and Wi-Fi use was permitted, it was classified as a "secondary" use. Radar and other applications were "primary" uses, with higher priority.
In response, the IEEE passed the 802.11h specification in 2003. It included dynamic frequency selection, which can detect other devices in a channel and shift the Wi-Fi operation to another channel, and transmission power control, which allows Wi-Fi devices to reduce the strength of their signals to limit interference with satellites.
Another band, another problem
Having two networks vying for the same frequencies is the problem in LightSquared's lower band, from 1526MHz to 1536MHz. This band has been used by GPS augmentation services, which send additional location data to GPS receivers that require more precise bearings. As a result, the receivers for those services are set to listen for signals in the band where LightSquared wants to operate its LTE network, which could cause interference with GPS augmentation.
Used for purposes including agriculture, construction, mapping and surveying, this technology can determine a receiver's location to within two centimeters, according to agricultural equipment maker John Deere, which sells augmented GPS systems. Ironically, LightSquared itself helps to provide these services. The company's satellite business, formerly called SkyTerra, is already operating and sells satellite capacity to Omnistar, a GPS augmentation provider. Omnistar was acquired in March by Trimble Navigation.
LightSquared has acknowledged there is an interference problem in the lower band but said it affects fewer than 1 percent of all GPS devices. LightSquared believes interference with GPS augmentation can be solved by providing those services on different frequencies and implementing a simple filter to keep receivers from listening for them in LightSquared's band.
But to solve the problem, LightSquared will need to learn more about how GPS augmentation receivers work, and the companies that make them have not been willing to talk, said LightSquared Executive Vice President Martin Harriman.
Trimble's Large disputed this. "We offered them everything they wanted to know," Large said.
According to some of their official rhetoric, LightSquared and its opponents are far from agreement on either the upper or the lower band. But Harriman thinks it can all be worked out after a short breather.
"What we have suggested is that we all take a little bit of time out ... or calm down a little bit and get more rational," he said. "It's going to take us multiple months to study it in detail, do more testing and determine what the options are." Those might involve changes to GPS, LightSquared's network, or both. But actually carrying out the solution could take much longer, he acknowledged.
At least one GPS vendor thinks the problem in LightSquared's upper band can be solved. GPS filters have been improved over the past several years so they can work in close proximity to the many other radios in cell phones, said Kanwar Chadha, founder of GPS silicon vendor SiRF and chief marketing officer of CSR, which now owns SiRF. Building one that could tune out the powerful LTE signals would only be a matter of time and money, Chadha said. He did not speculate on how much time and money but shared Harriman's hope for more dialog.
"Within the next several months, we should be able to figure out whether there is a cost-effective way to handle it or not," he said.
"These sorts of things can be and have been resolved in the past by engineers working together," said Hatfield, the former FCC official.
The growing demand for mobile services may lead to more adjacent interference issues as regulators and service providers try to make more efficient use of every bit of wireless spectrum, he said.
"As you jam people tighter and tighter together ... these sorts of conflicts become somewhat inevitable," Hatfield said.