The title of this article has it backwards. LightSquared's technology is entirely scientifically feasible.
The problem is political-ish, and the U.S. government and GPS industry carry a good portion of the blame:
The way that spectrum is allocated in the U.S., the chunk of spectrum that LightSquared got just happens to be designated as low-power, space-ground. Or at least, that's what their neighbors (GPS) are doing.
According to LightSquared, GPS makers have been lazy in their filter design, under the assumption that they wouldn't have to deal with much noise from neighbors on the spectrum.
I can understand saving cost by not designing really aggressive filters, but it would be sad to see an innovative idea like LightSquared be shut down because the GPS people don't want to clean their act up.
I hope LightSquared can trade for another chunk of spectrum that will allow them to move forward. This is really cool technology, and a great step forward.
GPS makers have been designing their receivers under the rules the FCC has set forth for decades, that the bands surrounding GPS are for sky level signals. I wouldn't call that "lazy".
Imagine if the NTB suddenly decided that it would be OK for people to drive tanks at 1000 MPH on existing highways. Would you call carmakers "lazy" because cars currently on the road wouldn't be safe to drive anymore? Or would it be OK to tell hundreds of millions of people to replace their cars immediately at exorbitantly higher cost?
It sounds like it is not so cut and dry. From the article:
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."
The quote from the consultant for LightSquared is taken out of context or just plain wrong. i.e. this statement:
cellular base stations have been "allowed next to the GPS band since 2003"
The FCC started allowing an "Ancillary Terrestrial Component (ATC)" specifically to supplement areas where satellite signals may have dead zones due to, say, dense urban areas. This would also be a low-power signal. It was put in place for things like Sirius/XM satellite radio to have acceptable performance in urban areas.
ATC is intended to (1) be a small supplement to a satellite-based service and (2) have a comparably low-power signal to the signal provided by the satellites.
Once LightSquared came along and got waivers so that (1) their whole 'satellite' spectrum could be terrestrial, and (2) could run much higher powered terrestrial stations (power levels similar to cellular towers) the amount of power they were radiating for their network eclipsed the weak signals GPS units depend on to function. The design and filtering for GPS units is fine for the adjacent spectrum to be used for similar systems/services as allocated (until the last 18 months).
Unfortunately, for some technical reasons that are too arcane to get into here, extremely sharp filters are not suitable for GPS receivers.
It would have been better for everyone, including LightSquared, if the political appointees at the FCC had not tried to simultaneously ignore international frequency-allocation treaties, disregard the advice of their own engineering-educated colleagues, and break the laws of physics.
Still, the reasons for interference are more political (FCC rules), than scientific.
There's only so much usable electromagnetic spectrum out there, so neighbors need to play nicely together. Especially when your neighbor is trying to do something awesome.
Well, the requirement for GPS was that they be able to filter out adjacent low power level signals (signals from space).
LightSquared is now arguing that changing the requirement to be able to filter out high power (terrestrial) signals is reasonable, and it is reasonable to retroactively apply that requirement against the millions of existing GPS receivers that were designed to meet a different requirement and thus will have to be discarded.
I would have a lot less heartache over the requirements change if LightSquared would be willing to bear the cost of their proposed change of requirements, rather than forcing me to bear the costs. This means they would need to replace my GPS receivers (a car navigation unit, a boat navigation unit, three cell phones, and an iPad). A quick mental summation is about $2,500, and that is pretty cheap compared to a lot of people out there.
You think that's expensive, think of all the military gps units that would need to be replaced. Do you have any idea how much one military-grade gps unit costs? Probably more than all the civilian gps units on your whole block combined. And just about every military vehicle in production has at least one. Aircraft have 2-3. Then there are the commercial gps units in airplanes, which also have redundancy requirements.
There is no chance that LightSquared will get their way. The cost to the military (taxpayers) would be stupendous.
Did LightSquared not know about GPS noise before bidding on the spectrum they received? Maybe LightSquared got their spectrum for cheap because there was so much noise and nobody else could find a use for it.
There isn't any more noise in the spectrum they bid on than any other unused portion.
They purchased the spectrum to be used for satellite communications. There was an FCC loophole that allowed them to supplement satellite communications with terrestrial signals in certain cases. They decided to move ahead with a FULL terrestrial system. They decided to bet large - and lost.
Any EE could have shown the blasting a signal literally billions of times stronger in an adjacent band would completely kill functionality of devices that rightfully would never ever have expected to need to design for it.
I see unsourced claims LightSquared signals would be around -70dBm, which is at least roughly consistent with LTE.
If so, that's a signal 1,000,000,000 stronger than the satellite-to-ground signals LightSquared's band was allocated for, immediately adjacent to the GPS band (10 or 20 Mhz apart, near 1.5Ghz).
Is there some reason it's not totally ridiculous to expect GPS receivers to be engineered for that kind of noise?
From the 2nd page of the article, quoting from the "Technical Working Group" (which supposedly included representatives from both LightSquared and the GPS industry):
"the interfering signal at a distance of 500 feet from the [cell] tower [was] up to 800 billion times more powerful than the distant GPS signals being received from space,"
and a mile away power levels were "400 million times those of the GPS signals."
That's a difference of ~87 to 120 dB, which seems consistent with the 90 dB difference you cite.
According to the article, the interference is 86dB above the GPS signal (400mn.×). Wikipedia gives figures of 60-80dB. The LightSquared band is from 50MHz to 16MHz below the GPS L1 band, which as a percentage of 1.5GHz is from 1% to 3%. This is not very far away, and 86dB is a lot. I don't think it's reasonable to expect any equipment to have that kind of selectivity; after all, high-Q filters are not only expensive, they are often also heavy and bulky.
Also there is an upper bound on filter sharpness given by the latency constraints (GPS needs accurate timing, and a brick wall filter has infinite latency) but that bound's probably not even being approached here; I don't have hard data on GPS signal latency requirements nor a simple formula relating rejection to latency.
A first order filter will attenuate at a rate of 20 dB per decade away from its cutoff frequency. For example, at a frequency 1000 times higher than its cutoff frequency, a first order filter will attenuate by 60 dB.
An n-th order filter will attenuate at 20*n dB per decade.
Let's assume based on your numbers the GPS band is 20 MHz away from LightSquared's 1.5 GHz-ish band. That's log(1.52/1.5) = 0.00575 of a decade.
We need 180 - 70 = 110 dB of attenuation to push the LightSquared signal amplitude well below the GPS signal amplitude. Solving for n above, 1/0.000575 x 110/20 = 956.
An order 956 filter is not something you can design for a buck or two. It's certainly not something you would design if you expected to filter out other space-based -160 dBm signals. Plus, the numbers above are very aggressive. You're going to need to do even better if you don't want to distort the signal you care about.
Last December I submitted a Forbes article, "Lightscrewed: How Washington Whipped Phil Falcone"(http://news.ycombinator.com/item?id=3382580) which described the big money politics on both sides, and also mentioned the economic argument that LightSquared wouldn't have gotten such a deal on their spectrum purchase if the GPS interference problem hadn't been obvious to anyone who did their EE due diligence.
I suspect there's a lot of money being spent on PR to get these articles out there.
18 comments
[ 2.9 ms ] story [ 57.3 ms ] threadThe problem is political-ish, and the U.S. government and GPS industry carry a good portion of the blame:
The way that spectrum is allocated in the U.S., the chunk of spectrum that LightSquared got just happens to be designated as low-power, space-ground. Or at least, that's what their neighbors (GPS) are doing.
According to LightSquared, GPS makers have been lazy in their filter design, under the assumption that they wouldn't have to deal with much noise from neighbors on the spectrum.
I can understand saving cost by not designing really aggressive filters, but it would be sad to see an innovative idea like LightSquared be shut down because the GPS people don't want to clean their act up.
I hope LightSquared can trade for another chunk of spectrum that will allow them to move forward. This is really cool technology, and a great step forward.
Imagine if the NTB suddenly decided that it would be OK for people to drive tanks at 1000 MPH on existing highways. Would you call carmakers "lazy" because cars currently on the road wouldn't be safe to drive anymore? Or would it be OK to tell hundreds of millions of people to replace their cars immediately at exorbitantly higher cost?
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."
cellular base stations have been "allowed next to the GPS band since 2003"
The FCC started allowing an "Ancillary Terrestrial Component (ATC)" specifically to supplement areas where satellite signals may have dead zones due to, say, dense urban areas. This would also be a low-power signal. It was put in place for things like Sirius/XM satellite radio to have acceptable performance in urban areas.
ATC is intended to (1) be a small supplement to a satellite-based service and (2) have a comparably low-power signal to the signal provided by the satellites.
Once LightSquared came along and got waivers so that (1) their whole 'satellite' spectrum could be terrestrial, and (2) could run much higher powered terrestrial stations (power levels similar to cellular towers) the amount of power they were radiating for their network eclipsed the weak signals GPS units depend on to function. The design and filtering for GPS units is fine for the adjacent spectrum to be used for similar systems/services as allocated (until the last 18 months).
It would have been better for everyone, including LightSquared, if the political appointees at the FCC had not tried to simultaneously ignore international frequency-allocation treaties, disregard the advice of their own engineering-educated colleagues, and break the laws of physics.
Still, the reasons for interference are more political (FCC rules), than scientific.
There's only so much usable electromagnetic spectrum out there, so neighbors need to play nicely together. Especially when your neighbor is trying to do something awesome.
LightSquared is now arguing that changing the requirement to be able to filter out high power (terrestrial) signals is reasonable, and it is reasonable to retroactively apply that requirement against the millions of existing GPS receivers that were designed to meet a different requirement and thus will have to be discarded.
I would have a lot less heartache over the requirements change if LightSquared would be willing to bear the cost of their proposed change of requirements, rather than forcing me to bear the costs. This means they would need to replace my GPS receivers (a car navigation unit, a boat navigation unit, three cell phones, and an iPad). A quick mental summation is about $2,500, and that is pretty cheap compared to a lot of people out there.
There is no chance that LightSquared will get their way. The cost to the military (taxpayers) would be stupendous.
They purchased the spectrum to be used for satellite communications. There was an FCC loophole that allowed them to supplement satellite communications with terrestrial signals in certain cases. They decided to move ahead with a FULL terrestrial system. They decided to bet large - and lost.
Any EE could have shown the blasting a signal literally billions of times stronger in an adjacent band would completely kill functionality of devices that rightfully would never ever have expected to need to design for it.
These slides say GPS received power is around -160dBm http://www.ima.umn.edu/talks/workshops/8-16-18.2000/van-dier...
I see unsourced claims LightSquared signals would be around -70dBm, which is at least roughly consistent with LTE.
If so, that's a signal 1,000,000,000 stronger than the satellite-to-ground signals LightSquared's band was allocated for, immediately adjacent to the GPS band (10 or 20 Mhz apart, near 1.5Ghz).
Is there some reason it's not totally ridiculous to expect GPS receivers to be engineered for that kind of noise?
"the interfering signal at a distance of 500 feet from the [cell] tower [was] up to 800 billion times more powerful than the distant GPS signals being received from space,"
and a mile away power levels were "400 million times those of the GPS signals."
That's a difference of ~87 to 120 dB, which seems consistent with the 90 dB difference you cite.
(edited: s/LightSpeed/LightSquared/)
Also there is an upper bound on filter sharpness given by the latency constraints (GPS needs accurate timing, and a brick wall filter has infinite latency) but that bound's probably not even being approached here; I don't have hard data on GPS signal latency requirements nor a simple formula relating rejection to latency.
An n-th order filter will attenuate at 20*n dB per decade.
Let's assume based on your numbers the GPS band is 20 MHz away from LightSquared's 1.5 GHz-ish band. That's log(1.52/1.5) = 0.00575 of a decade.
We need 180 - 70 = 110 dB of attenuation to push the LightSquared signal amplitude well below the GPS signal amplitude. Solving for n above, 1/0.000575 x 110/20 = 956.
An order 956 filter is not something you can design for a buck or two. It's certainly not something you would design if you expected to filter out other space-based -160 dBm signals. Plus, the numbers above are very aggressive. You're going to need to do even better if you don't want to distort the signal you care about.
I suspect there's a lot of money being spent on PR to get these articles out there.