LightSquared Makes a Legal Move
Company, in dispute with GPS receiver makers, petitions the Federal Communications Commission confirm its right to spectrum.
Published: Dec 20, 2011
In the ongoing radio spectrum dispute between LightSquared and GPS equipment makers, LightSquared has made the next move. The Reston, Va., company has asked the U.S. Federal Communications Commission to confirm its right to use the spectrum licensed to the company by the federal government. In addition, the company says it wants the FCC to confirm that commercial GPS manufacturers have no right to interference protection from the LightSquared network.
The company has maintained for some time that GPS equipment makers are using spectrum beyond what they're allowed. This petition will push that point with the FCC. In a press statement announcing the petition, Jeff Carlisle, vice president of regulatory affairs and public policy, says: "The one inescapable conclusion from two rounds of independent testing is that the incompatibility problem is not causes by LightSquarde's network. It is clear that GPS devices are purposefully designed to look into LightSquared's licensed spectrum, and given this evidence, we believe decision-makers should consider LightSquared's legal rights as the licensee."
Carlisle says the FCC authorized the company to build its network eight years ago and that the original license was endorsed by the GPS industry. "Commercial GPS device makers have had nearly a decade to design and sell devices that do not infringe on LightSquared's licensed spectrum," he says.
The key move in the petition - to confirm that GPS manufacturers have no right to interference protection - is the greatest point of contention. The Save Our GPS coalition has long maintained that LightSquared needs to pick up the tab for any changes needed to existing receivers before it can implement its new network.
The LightSquared high-speed Internet system will use 40,000 towers placed around the country to provide a 4G LTE (for long-term evolution) backbone that other companies can use. Dozens have already signed on as partners of LightSquared in anticipation of the building of the new network. This GPS interference issue remains the key stumbling block.
In the press statement, Carlisle adds: "While we ask the FCC today to confirm our legal rights, LightSquared remains fully committed to cooperate with all parties - the GPS industry, GPS users and the federal government" to ensure the new network is deployed in a way that is compatible. He adds that the company recognizes the importance of GPS and believes "that GPS devices can peacefully co-exist adjacent to our network."
The LightSquared commitment is to have broadband available to 260 million Americans by 2015 - an effort that will cost $14 billion. The company says it will work with the government to develop a "complete solution" to the interference issues.
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Hint, the configuration might be changed to allow multiple lesser attenuation filters in series, perhaps separated by amplifiers, but no amplifier can be allowed to saturate from the off-band interference signal, and "sneak paths" for the strong interference signal around filter segments, such as in the power circuitry for the amplifiers, or the printed circuit board dielectric, will easily destroy this topology option.
Anyway you approach the problem of separating a small signal, like a flashlight on the moon, from a large signal, like a nuclear bomb detonation from 1000 feet away, you are going to have a big engineering challenge ahead of you. Good luck, because we need a PNT system, and Loran has been terminated.
Anonymous on 12/29/2011 10:23:00 AM
LightSquared, or any competitor using near frequency strong terrestrial transmission, will produce an off-band signal that will be received by the GPS receiver because of the bandwidth of the GPS receiver "patch" antenna. This is called "quality factor" or "Q" and is a measure of center frequency divided by 3db bandwidth. Unfortunately, GPS uses a high frequency, and Q diminishes with frequency (why radios and TVs all use a lower frequency IF stage, to get better transmitter station selectivity, and do not count on the antenna or RF stage for much help there).
GPS receivers can use a DSP computer to, and we will use the analogy of a noise canceling headset, make an interference signal "anti-noise" to add into the desired signal with noise, to remove the noise from the desired signal. Unfortunately, the signal must also be heard in the signal plus noise coming from the antenna to the DSP computer input.
Design assumption: the filter between the patch antenna and the first LNA RF amplifier inside the GPS antenna must reduce the interference signal amplitude down to equal to or lesser than the desired GPS signal before the DSP computer can be effective at eliminating the noise.
If you have zero loss at the GPS bandpass, how much LightSquared signal attenuation do you need over the entire noise spectrum bandpass? Let's do a "back of the envelope" engineering guesstimate:
You need to reduce the power spectral density of the noise to or below the power spectral density of the signal. Power spectral density is measured in watts per square foot.
Watts (the undesired is stronger, goes proportionally):
db = 10 log Pi/Pr
How much does LightSquared transmit - don't know, keeps changing.
How much is the effective radiated power of a GPS satellite - there is a spec for that.
db = 10 log 15,750/300
db = 17.2
Square Feet (the undesired is closer, goes inverse square proportionally):
db = 10 log Rr**2/Ri**2
How far away is LightSquared - assume 1000 feet
How far away is a GPS satellite - assume 11,000 miles x 5,280 feet per mile
Surface of a sphere is 4 x Pi x radius squared
The 4 and the Pi factor out
db = 10 log (11,000 x 5,280)**2 / 1,000,000
db = 95.3
Attenuation required = 17.2 + 95.3 = 112.5db
If you have zero loss at the GPS bandpass, how much LightSquared signal attenuation do you need over the entire noise spectrum bandpass? Let's do this another way, using "the other side of the envelope" engineering numbers:
GPS is at around -127dBm on the ground.
LightSquared's lower, re-re-revised (they're going to turn it way up later, they admit) power level is -30dBm "near" their tower. Don't expect a definition of "near", and keep in mind that LightSquare keeps changing numbers.
Attenuation required = -30dbm - (-127dbm) = 97db
But what does this mean:
db is a log scale so 10db is a factor of 10, 20db is 100, 30db is 1000, 40db is 10,000, 50db is 100,000...
97db is, everybody get, 10**9.7 = LightSquared 5,011,872,336 times stronger than GPS
112.5db is, everybody get, 10**11.25 = LightSquared 177,827,941,000 times stronger than GPS
Really really really really big! Somehow, LightSquared accusing the PNT committee of being off by 32 times doesn't seem like much (only 15db)!
You can adjust the number down as LightSquared offers new lower power limits, and up as they increase the power as they claim they will. And you can adjust the number up as you decide on lower effective range limits (how far from the LightSquared tower your GPS should not work). You get the idea how this is done.
Hint, the antenna design will buy you something, which is not accounted for here, but not that much.
Hint, the DSP ability might be better than assumed, and this is the dimension where discovery and invention might make things considerably eventually better far in the future.
Hint, the configuration might be changed to allow multiple lesser attenuation filter
Anonymous on 12/29/2011 10:19:00 AM
"It is clear that GPS devices are purposefully designed to look into LightSquared's licensed spectrum". Yes, this is correct. OmniStar's data goes through SkyTerra satellites. SkyTerra is now owned by LightSquared. The problem is not that GPS users look at that spectrum, the problem is that LightSquared wants to use that spectrum differently than it was intended for. That spectrum was supposed to for low power satellite-only broadcasting, and LightSquared wants to put high-power ground transmitters on that spectrum.
Anonymous on 12/22/2011 4:14:00 PM