Last June I wrote about issues of coexistence of GPS and Lightsquared’s 4G nationwide wireless broadband system. At that time I was not able to get a comment from either Lightsquared or the GPS receiver manufacturer interest group, the Coalition to Save Our GPS (CSOG).
This issue came to my attention again when LightSquared tweeted a link to a video critical of GPS manufactures' efforts to stop LightSquared from implementing their broadband network on a band adjacent to the GPS band. The misleading video, which has now been removed from their website and YouTube, was titled Find Out Who is Trying to Block Wireless Innovation in America. The video claimed “GPS commercial device manufacturers have inappropriately encroached onto the airwaves licensed by the government to Lightsquared since 2003. Despite the encroachment, LS has been working with the FCC and other federal agencies to resolve the GPS interference issue.”
To anyone unknowledgeable about the issue, it sounds like GPS “device” (they won’t say receiver) manufactures are making products that transmit on their frequency space.
Lightsquared’s PR firm was responsive to my calls and e-mails, but they could offer no engineering information whatsoever. I suspect my contacts there honestly want to provide technical information, but they are not used to dealing with technical information, have trouble getting it, and are cautious about talking about it.
The PR firm for the CSOG, was much more helpful. They tell me the GPS manufacturers’ primary concern is the terrestrial base stations called the ancillary terrestrial component (ATC). These are basestations located in less rural areas and take some of the load off the satellite system. ATC stations do not have satellite uplink; they are connected through a terrestrial backbone. They operate on the same frequencies as the satellite system and overpower the LightSquared satellite signals in the area covered by the ATC base.
CSOG claims that ATCs can develop -12dBm on a typical GPS receiver’s antenna located 500ft away from an ATC base. Free-space path loss (FSPL) at 1600MHz over 500ft is 80dB. That means their effective radiated power (EIRP) must be 68dBm because 68dBm - 80dB = -12dBm. GPS receivers need to detect signals in the -130dBm range. CSOG rightly says it’s unreasonable to design a receiver with 120dB of adjacent-band rejection.
I suspect that the ATC bases’ output power is some reasonable value like 30dBm and CSOG’s testing with a very high gain antenna to acheive 68dBm EIRP. It would not make sense for the bases to have an output power over 30dBm because the handheld units certainly have an output power less than than 30dBm and the communication needs to be two-way. An antenna with gain that high would have to be carefully aimed to cover a target 500ft away. The GPS user could simply walk a few feet out of the antennas tight radiation pattern to re-acquire the GPS satellites.
Let’s assume a more modest ATC base EIRP is 20dBm. 20dBm - 80dB = -60dBm on a GPS receiver’s antenna 500 ft away. Let’s assume a GPS receiver can tolerate up to -45dBm of adjacent-band interference. That means the interference range is only 90ft. Here is the signal strength from a 20dBm transmitter, discounting antenna gain and feedline loss and assuming unubstructed line-of-sight.
This should be considered a best-case scenario because EIRP is probably > 20dBm.
The idea of a satellite-based 4G system with a terrestrial component for more-densely-populated areas is exciting. Adjacent band conflicts are nothing new. To get around this one it might even be worthwhile to modify the whole GPS system so that it incorporates a terrestrial component in all areas served by the ATCs.
LightSquared should entertain ideas like this instead of denying the problem and running misleading ads. The ads are alienating would-be supporters. They should also give their PR firm basic technical information. From what I can tell the PR firm wants to live up to its tagline of “evidence-based communications”, but they’re not getting enough “evidence” from their client.
The very concept of a satellite projecting cell-like “spot-beams” on the earth and can communicate both ways to handheld units is amazing and worthy of an entire post. I will append this post and write another one when LightSqured gives me some technical facts.
Can anyone from the GPS receiver world comment on how much out-of-band interference is tolerable for a decent GPS receiver?
Supposing my guestimate is right and Lightsquared will disrupt GPS receivers up to 100 feet away, is that an acceptable price for having a nationwide broadband network?
Update 10/10/2011: The GPS manufacturers group has gotten back to me on total output power and antenna gain, which I speculated about in the article. CSOG claims the two antennas LightSquared plans to use have gains of 14.8dB and 16.5dB. They plan to feed 45.5dBm (that's right, 35.5W) into it, and are licensed to put a maximum of 55.5dBm (355W) into it for a EIRP of 72dBm! That cannot be simply to talk to handheld battery-power devices. This means even IF they use an omni antenna, line-of-sight interference looks bad even miles away from the ATC base:
What are they doing with all that power? I must be getting something wrong; I would love for a LightSqare supporter to set me straight by leaving a comment or sending a message through my website.
