
Why LightSquared failed: It was science, not politics - oliverdamian
http://arstechnica.com/tech-policy/news/2012/02/why-lightsquared-failed.ars
======
anrope
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.

~~~
pithon
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?

~~~
voodoomagicman
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."

~~~
gregatragenet
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).

------
skew
Have any of these articles included hard numbers?

These slides say GPS received power is around -160dBm
[http://www.ima.umn.edu/talks/workshops/8-16-18.2000/van-
dier...](http://www.ima.umn.edu/talks/workshops/8-16-18.2000/van-
dierendonck/GPSSIGSTRUCTURE.PDF)

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?

~~~
jacquesgt
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.

------
jleader
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.

------
herge
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.

~~~
pithon
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.

