Questiny Awarded a Contract for Software Modeling Support Services with the US Army Space Command

/ April 10th, 2014 / Comments Off on Questiny Awarded a Contract for Software Modeling Support Services with the US Army Space Command

Questiny has been awarded a new $100K  contract to supply software and modeling services to the US Army Space Command at Peterson Air Force Base.  This continues to update the Wireless Network Simulator (WiNS) supplied  over the past year, and continues to provide services in support of the Defense Information Systems Agency (DISA) Mix-of-Media study.  Delivery of the updated software will occur around July 1, 2014.

Audio Distortion Analyzer Added

/ January 18th, 2014 / Comments Off on Audio Distortion Analyzer Added

On a lark, I bought this audio distortion analyzer from eBay for $100 including shipping.  Similar working units on eBay run anywhere from $150 to $900, so this was a fair price for what I got.  Convenience really drove this purchase as to make FM receiver measurements (Signal plus noise and distortion, or SINAD) the distortion of the demodulated audio tone is measured at the speaker output.  Questiny has digital instruments that can do this in easy fashion, but the set-up can take some time, so I thought I would buy a used distortion analyzer and see how it worked.  This unit came last week, and was DOA; plus the feet had been damaged in shipping.  This is not altogether unexpected as the seller stated that the unit powered up, but had not been tested in any other fashion .

$_57I confirmed that the unit did light up the power indicator light, but checking the service manual showed that the light is actually right across the input mains, so it is no indication of function.  In the end, three electrolytic capacitors had failed.  One on the positive rail of the power supply, and two in the meter circuit.  Upon replacing the capacitors, the unit sprang to life.  A few hours with the manual walking through the calibration procedure, and the unit seems to be functioning as expected.  The only issue remaining is that the frequency dial is off, but the set screws have been stripped.  That’s going to take some effort to figure how to get this off without further damage.  I may have to resort to buying a sacrificial unit on eBay.  The unit is also capable of AM detection up to 65 MHz, but that doesn’t seem to working either.  Likely just a diode failure.

In the end, I have to be impressed by HP’s design.  This unit is over 40 years old, and the construction used discrete transistors.  It has probably 30 electrolytic capacitors and only three of them failed after forty years.  Not bad.  Further, during the calibration procedure I was able to measure distortion down to .03%.  That’s about a 70 dB null from 40 year-old analog circuitry!  Not bad.  Even as a just voltmeter, it has a 10 MOhm input impedance and a frequency response of nearly 3 MHz.

The bottom-line is that this is a handy addition to the lab, and repairing it really gave me a chance to check out some of the other equipment in the lab such as the in-circuit ESR  (equivalent series resistance) meter, the LCR (inductance, capacitance, resistance) meter, the variable isolation transformer, the function generator, and the digital oscilloscope.  All of these units provided invaluable contributions to the troubleshooting and justified their procurement.  I have to make special note of the LCR meter.  Typically, electrolytic caps open as the dry out, but in this case, two of the capacitors actually shorted.  The ESR meter showed a small amount of resistance indicating a good cap, but a dc continuity test showed them as shorted.  One of the capacitors in the meter circuit was in between an open and a short.  Here is where the LCR meter really shined.  The capacitor was rated at 50 uF, but it measured at 800 uF!  Not shorted, and not open, but not operating as rated.  Without the meter, I would either have not replaced this one.  In the end, I will re-cap the whole unit.  If 10% of the caps have failed, it’s likely others will fail soon.  More to the Mouser order!

 

Questiny’s New Radio Lab Commissioned

/ January 16th, 2014 / Comments Off on Questiny’s New Radio Lab Commissioned

Over the past year Questiny has been compiling a set of equipment to allow us to develop a basic radio lab development and test capability.  Through many hours on eBay, we have constructed the following.  The top-end frequency is limited to about 3 GHz, but it is a start, and can be expanded when needs arise.  Our budget was about $25K which is new the low-end for a single new piece of Agilent equipment.  Through careful purchasing, all of the equipment was purchased within the budgeted amount.  We can now do basic design testing of analog radios, and through the use of the software defined radios, we can do testing of digital communications was well.

Radio Lab

With this capability, Questiny can now start to prototype some of our ideas related to our wireless communications ideas.  Stay tuned.

Corporate value statements

/ February 4th, 2013 / Comments Off on Corporate value statements

Sometimes, a corporate value statement is just so well stated that I am compelled to share it through our corporate blog. It’s a implied working philosophy that we have embraced but never stated so elegantly to grow our company. But credit goes to Reed Hastings, the CEO of Netflix, who authored the presentation. I will forgive you for your past sins committed in devaluing your stock price from its high of approximately $300 per share to current $190 (it was down to $52.81 in 2012).

For LightSquared, “And now the end is near,…”

/ February 16th, 2012 / Comments Off on For LightSquared, “And now the end is near,…”

To quote a famous Frank Sinatra song, “the end is near.”  At least it appears that way for LightSquared.  On February 14, the NTIA released a letter to the FCC stating, “We conclude at this time that there are no mitigation strategies that both solve the interference issues and provide LightSquared with an adequate commercial network deployment.”  Responding to that letter, today the FCC released a spoken statement that said, “The International Bureau of the Commission is proposing to (1) vacate the Conditional Waiver Order, and (2) suspend indefinitely LightSquared’s Ancillary Terrestrial Component authority to an extent consistent with the NTIA letter.”  In short, LightSquared’s access to the spectrum has been denied.  It’s likely that from this point forward, the lawyers will be the primary recipient of funds.

This is a sad result, although predictable.  As LightSquared heads for bankruptcy  or buyout (the fourth in this Companies history, if I count correctly), it’s appropriate to reflect.  While nearing the end, the aftermath of LightSquared’s attempts will be long.  However, I am hopeful that some positive change might result.  While true that politics became the dominate force in this debate, I consider that actually a plus since politics often arises when so many are affected by the outcome.  As I have said many times, the outcome of this could be predicted by the number of voters that had GPS units compared to the voters with LightSquared handsets.  But in a larger context, this whole issue has raised the awareness of many to our reliance on GPS, its vulnerability; the intense battle for spectrum to support our insatiable appetite for wireless devices; and the complexities of dealing with spectrum regulation (there were at least five agencies involved in this debate excluding Congress, Dept. of Transportation, NTIA, FCC, DoD, NPEF, and the FAA to name a few).  It is my hope that the result of this will create an awareness that GPS has become a critical public service worthy of strengthened policy and legislation protecting its civilian use.  It is also my hope, that the FCC and DoD can better their spectrum management to address situations where manufacturers that ignore the technical parameters in adjacent spectrum cannot seek “protection in numbers”, and the FCC begins to monitor the circumstances that foster spectrum squatting.

I may in a minority, but I actually think the FCC got more right with this than wrong.  Nothing about the January 2011 waiver required a change in the technical parameters for LightSquared, so in my mind, it did not require a Notice for Proposed Rule Making (NPRM) by the FCC.  However, there are those such as Scott Pace, Director of Space Policy at George Washington University, that disagree believing that such a widespread deployment of terrestrial capacity was a change in the spectrum use.  It is an argument, and one that LightSquared bet on – at least one way.  However, the GPS community bet the opposite way.  Any Company deploying a Ancillary Terrestrial Component in this frequency range will create the issues raised by the LightSquared testing.  It’s just a matter of how widespread.

My last hope for a positive outcome of this drama, is the highlighting of the support for space technology and space communications.  LightSquared may have been a thinly veiled attempt at auction free wireless spectrum (hardly free by Falcon’s accounting at the moment), but it was a satellite system too; and in that it was an attempt to create a economically  viable hybrid network of satellite and terrestrial capacity.  That in itself is a noble goal in that it would bring a national service to rural communities and provide a competitive alternative to the wireless oligarchy emerging in the US.  The good news on that front is that while LightSquared may be at its end, EchoStar is coming, ViaSat has launched, and either of them have a better chance at success.  To quote Mr. Churchill, “This is not the beginning of the end, but it is the end of the beginning!”

Legislation Adds Further Delays to LightSquared

/ January 6th, 2012 / Comments Off on Legislation Adds Further Delays to LightSquared

The “National Defense Authorization Act for Fiscal Year 2012” signed by President Obama signed at the end of 2011 included a section (Title IX, Subtitle B, Section 911) that directed that ” The Federal Communications Commission shall not lift the conditions imposed on commercial terrestrial operations in the Order and Authorization adopted on January 26, 2011 (DA 11–133), or otherwise permit such perations, until the Commission has resolved concerns of widespread harmful interference by such commercial terrestrial operations to covered GPS devices.”  In other words, LightSquared is prohibited from deploying their terrestrial base stations until it is determined that there are no impacts to “covered GPS devices” (i.e., military GPS receivers).  The Act goes on to order the Secretary of Defense to report to Congress every 90 days the results of testing that t if Military GPS receivers are effected by LightSquared operations.  This report is to be delivered to Congress every 90 days for two years.  Ouch!

Such legislation has some dire consequences for LightSquared.  Some background may be needed.  First, the FCC provides no protection for commercial receivers of any kind (other than to limit their own radiation through Type A and Type B certifications).  If a manufacturer builds a poor receiver that does not function, too bad for that manufacturer.  Also, there are no protections for commercial civilian GPS receivers.   the fact is that the GPS system is a DoD satellite navigation system, designed and operated by and for the military.  As far as I know, no federal agency or legislation protects the civilian use of GPS, and no tax dollars outside of the DoD’s budget goes to operate or maintain GPS.  If the DoD saw fit to turn GPS off, they could do so, and commercial users have no recourse.  No one believes such a thing would happen, but I understand that it is fully within the DoD’s right to do so.  This is why the Defense Authorization Act can only address “covered GPS devices.”  However, this creates quite a problem for LightSquared.

LightSquared has taken several steps to mitigate the interference to commercial GPS devices.  They have offered to reduce their power levels, operate on limited carriers for a period of time for manufacturers to improve their receivers and time for these improved receivers to diffuse into the market.  In addition, LightSquared has developed new filters to assist commercial precision GPS receivers that operate within the LightSquared spectrum.  All of these are the actions of a “good spectrum neighbor”, but unfortunately, they do not apply to the Military.  The Military has, and now must, conduct their own testing of these devices.  This means that LightSquared has to protect the US Military use of GPS before Congress will allow the FCC to license LightSquared. The consequence to LightSquared is that this will take time, and time is something I believe LightSquared is running short of.

It’s been reported that LightSquared needs additional funding to continue operations, and that these financial needs are becoming immediate.  If the Military can determine that LightSquared will not interfere with their receivers, then LightSquare might be granted a license to operate, paving the way for more funding.  But how long will this take?  I speculate that it could take the DoD more than 90 days just to identify the organizations impacted and the GPS equipment that should be tested.  Once that is decided, units would have to be procured and tested under conditions the US Military users accept.  To me, this sounds like a very long process.

For planning, let’s say the results are that there are some impacts the Military GPS devices, but they can be fixed with modifications to the GPS receivers (all big ifs).  Such modifications will have to conform to military procurement.  Military procurements take much longer civilian manufacturing.  Furthermore, such procurements are much more expensive, and many of the GPS devices are within classified equipment.  In times of declining defense budgets, where would the DoD find such money?  Under this scenario, LightSquared has limited options to mitigate the impacts and reduce any delays.

LightSquared might have a trump card, and I don’t know why they have not played it so far.  The card is this.  If GPS is so critical to Military Operations, then why can a civilian system affect its performance when it is operating well within its technical parameters and on an adjacent spectrum?  Doesn’t this mean that GPS is extremely vulnerable, and needs to be fixed anyway?  That may be happening.  I understand that GPS 3/R, the latest GPS replacement satellites, offers improved protection for Military users.  However, that still doesn’t ease the delay for LightSquared.  In the end, the “tyranny of the clock” may be working against LightSquared.

I think it sad if LightSquared should succumb to the business pressure resulting from these delays.  LightSquared is the first real attempt at a hybrid satellite-terrestrial communications system.  It is my opinion that such systems are needed if a viable communications satellite industry continue in the United States.  Such a system does not have to operate on spectrum adjacent to GPS, but it will need access to spectrum amenable to dual terrestrial and satellite operation.

Interference Report Leaked to Bloomberg Indicates High Interference Levels

/ December 12th, 2011 / Comments Off on Interference Report Leaked to Bloomberg Indicates High Interference Levels

An NTIA Report leaked to Bloomberg Press stated that LighSquared’s basestations continue to disrupt GPS receivers.  The Bloomberg article specifically stated,  “The results from testing conducted Oct. 31 to
Nov. 4 show that “millions of fielded GPS units are not compatible” with  the planned nationwide wholesale service, according to the draft seen by Bloomberg News.   “LightSquared signals caused harmful interference  to majority of GPS receivers tested,” according to the draft prepared  for a meeting next week of U.S. officials reviewing the LightSquared  proposal. “No additional testing is required to confirm harmful
interference exists.””  The article went on to say that, “It [the Report] found that 69 of 92, or 75 percent, of receivers tested “experienced harmful interference” at the equivalent of 100 meters (109 yards) from a
LightSquared base station.”  Without a copy of the report and access to the test data, Questiny cannot comment on the validity of the claims.  However, our previous analysis shows that even the single carrier LightSquared spectrum plan would degrade the performance of GPS receivers that did not have sufficient attenuation at the LightSquared’s frequencies (1526-1536 MHz).  It appears that the NTIA’s direct testing has confirmed this situation — but this is not new news.

LightSquared stated at the conclusion of the FCC testing in June of 2011 that additional filtering would be needed, and contracted with the Javed corporation to construct external filters for such GPS units.  There has never been any dougt in our minds that such filters could be developed, and that they would provide sufficient filtering of the LightSquared signal to permit operation of the GPS units.  The issue has always been the conditions under which EXISTING GPS receivers would work WITHOUT MODIFICATION.  Thus it is also no surprise to to us that GPS users would frequently  find themselves in such conditions.  The report states taht 75% of the receivers would be impacted when within 100m (328 feet) of the base station.  This is fairly close to the basestation, and such close range only provides 76 dBm of attenuation (free-space) due to propagation leading to a received power at the GPS unit of (62 dBm-76 dB) -14 dBm — a strong signal level.  The GPS units must then filter that signal by more that 100 dB to minimize the effect of the LightSquared signal on its performance.  It is not surprising that standard commercial units have such filtering when they were likely designed assuming adjacent signal levels of -120 dBm not -14 dBm!

All of this is the backdrop to the FCC’s decision that it has to address the fact that even if LightSquared operates well within the technical limits of their license, such operation will still degrade the operation of existing GPS receivers not designed to accommodate such signals.  As for our prediction as to how this will turn out…as there are more voters with GPS units than voters with LightSquared units, Questiny has always believed that the FCC will eventually side with the GPS community, and revoke LightSquared’s license.  From there, LightSquared will likely sue the FCC for damages, and the FCC will find other spectrum for LightSquared.  All of this will likely be too late for LightSquared to achieve sufficient revenue to remain solvent, and once again, LightSquared will find itself at the brink of bankruptcy.

 

Am I Missing Something?

/ September 21st, 2011 / Comments Off on Am I Missing Something?

An article came out today announcing that LightSquared solves GPS interference with new device!  The article stated that  GPS device manufacturer Javad GNSS has come up with a device that “”The solution we’ve come up with dispels the myth that a product that eliminates  interference couldn’t be done,” Martin Harriman, executive vice president of  LightSquared’s Ecosystem Development and Satellite Business, said on a  conference call with reporters. “We did it. And it didn’t cost billions of  dollars or 10 years to do it. We did it.”  The article goes on to say, “Harriman said that through its partnership with Javad GNSS, which makes GPS  devices for such clients as the U.S. Geological Survey, has developed new  receivers that can be used to avoid interference between LTE and GPS.  Javad GNSS  has also completed designing and testing prototypes that can be adapted to work  with existing high-precision GPS devices, including those already being used in  the field.”

Am I missing something?  I thought the issue was not the inability to build new receivers, but the willingnesss of existing manufacturers and GPS users to accept retrofits and modifications to units already in the field?  Afterall this is not really a “Eureka” moment considering that LightSquared has had to 1) reduce the generation of intermodulation products by dropping the upper carriers and 2)consolidate GPS augmentation signals in that same part of their spectrum.  This latest announcement seems to be simply  a new receiver with better filtering, and that only has an effect AFTER the other two changes have been made.

After my shock that this is presented as such a “revelation”, the factis that with better filtering, the precision GPS receivers can coexist wtih LightSquared using the lower 10 MHz.  But we always knew this.  The issue for LightSquared is that this still sacrifices half of their capacity, and add the cost of retrofitting existing GPS receivers.  If anyone reads this article and believes that this solves the two-carrier LightSquared problem, please comment and let me know.

Analysis of LightSquared’s New Proposal

/ September 20th, 2011 / Comments Off on Analysis of LightSquared’s New Proposal

In their latest proposal, LightSquared has offered some concessions to mitigate their impacts with GPS.  They have offered the following adjustments to their technical design:

  1. Limit the received power within the region 50m to 500m to under -30 dBm
  2. Limit the degradation due to LS signals to under a 1 dB degradation in C/N0
  3. Move the satellite GPS augmentation signals to the upper portion of the MSS Spectrum (above 1,536 MHz), and offer filters with 40 dB rejection at 1,536 MHz

On the first item of their proposal, the analysis below shows that in free-space, the received power from a LightSquared tower will exceed -30 dB by as much as 21.5 dB (power received at 50m from a 62 dBW EIRP  base station at a 10m antenna height). However, if the average loss is more urban-like (loss exponent of 2.7), the the received power drops to more than 3 dB below the -30 dBm limit they have set.

The FCC dictates that free-space loss be used in all calculations.  However, urban environments are not free-space, actual attenuation is often larger than free-space.  Such facts indicate that LightSquared is betting that propagation loss in excess of free-space is more likely, and they will not have to reduce their transmit power and reduce their transmit power.  Although calculated, this is a risky bet.  Measurements on GPS receivers made by LightSquared indicate that many general purpose GPS receivers perform well when subjected to LightSquared interference below -30 dBm with the exception of the precision GPS receivers that use augmented GPS information.  This brings up the second and third aspects to their proposal.

It seems LightSquared has dropped their opposition to using a 1 dB drop in received GPS signal-to-noise ratio (or Carrier-to-noise density ratio, C/N0) as the bench mark for performance.  This is a positive step forward.  Many standards use a 1 dB criteria as the limit of harmful interference.  In fact, some satellite regulations require interference be limited to a C/N0 degradation of less than 0.25 dB, so 1 dB is quite gracious.  Furthermore, it is measurable.

The last point in their proposal is the effect of their carrier on the precision receivers.  This is a more difficult issue as the receivers are designed to receive augmented GPS signals transmitted over satellites using the LightSquared frequency band.  To solve this, LightSquared has proposed consolidating the GPS augmentation signals at the upper portion of their band, and supplying the precision GPS receivers with external filters that provide the needed rejection/attenuation of the LightSquared carrier in lower portion of the band.  The analysis they present in their FCC proposal indicates that the degradation to precision receivers is limited, but the question remains if manufacturers and system operators (such as John Deere, Inc.) would accept such additions. 

In short, LightSquared has indeed made concessions by taking on more risk relative to interference with GPS receivers, such concession will make their system roll-out and set-up much more difficult/expensive. Furthermore, while LightSquared may now have a technically workable proposal for their initial roll-out, it still does not provide access to the upper portions of their spectrum in the long-term.  If an agreement is not reached for that spectrum, LightSquared will have one-half the capacity their current business model assumes.  Not a pretty picture for investors.

Analysis of Received Power Limitation to Below -30 dBm

LightSquared has offered a maximum received power level of less than -30 dBm within a circle of 50m to 500m around its towers.  This level seems currently based upon their latest test results of GPS receivers to their current frequency plan of using only the lower 10MHz portion of their spectrum. 

LightSquared Testing of General GPS Receivers to Lower 10 MHz LTE Carrier

In addition, a sectored antenna is assumed with a downtilt of 7 degrees.Below is an analysis that examines the received power levels at GPS receivers from three different height Light Squared

The plot below shows the region around the tower where LightSquared intends to limit the received power to less than -30 dBm.  The minor ticks are every 10m showing the resolution to which LightSquared will measure the power levels.

Power Controlled Region around LightSquared ATC Tower

Starting with the power limitation on the ground, we can write an equation for the received power, Pt=PrGv/FSL.  This is the transmit power times the antenna pattern divided by the spreading loss (aka, free-space loss).  The average propagation loss is a function of the distance from the transmit source, the frequency, and the average loss exponent.  Free-space loss has a loss exponent of 2.  The formula is shown below.

The distance from the transmit source may be determined from the Pythagorean theorem assuming the tower antenna height and the distance from the tower base.


Combining these two equations shows and plotting as a function of transmit antenna height (10, 20, and 30 meters), we have the following free-space propagation loss for the three tower antenna heights.  As shown, there is little difference between the tower heights.

Free-space Loss between GPS Receiver and Tower Antennas (Antenna Heights = 10m, 20m, 30m)

The plot shows that the loss across the area surrounding a tower varies from a minimum of 71 dB to over 90 dB.  However, if the loss exponent is increased to 2.7, representing a suburban envioronment (urban environments have even higher exponents), the loss increases to 98 dB to over 120 dB (top, green line).

Propagation Loss from 20m Antenna Height to GPS Receiver for Loss Exponents 2.0, 2.4, and 2.7

The maximum effective isotropic radiated transmit power (EIRP) for LightSquared is 62 dBm, and the following plot shows the maximum received power at a GPS receiver from a 20m tower along the 50m to 500m distance LightSquared has defined.  The top curve (blue) shows the power received assuming free-space loss.  Note that since the FSL nearly the same for the three tower heights, this is what typical interference calculations for GPS receivers would use.  This shows a received power level of -8.8 dBm, or 21.2 dB higher than the -30 dBm specified by LightSquared’s proposal.  However, if the environment is more like an suburban environment, the received power is -33.5 dBm, or 3.5 dB better than the -30 dBM in the proposal.

Power Received at GPS Receiver Assuming Free-space Loss and Transmit EIRP of 62 dBm

However, the situation is  better when including the effects of a sectored cellular antenna.  The antenna pattern will further attenuate the LS signal as the GPS receivers move closer to the tower. 

A cellular antenna such as the Kathrein 724222 or 742223 has a 20 degree half-power beam width (HPBW), and the cellular operators typically point the antennas down to the ground (called a down tilt angle).  Typical downtilt angles vary from 3 degrees to 8 degrees.  For this analysis, 7 degrees is used.  If we assume a parabolic elevation antenna pattern, then along the maximum azimuth pattern, the antenna directivity is shown below (Downtilted Base Station Antennas –A Simulation Model Proposal and Impact on HSPA and LTE Performance, Fredrik Gunnarsson, Martin N Johansson, Anders Furuskär, Magnus Lundevall, Arne Simonsson, Claes Tidestav, Mats Blomgren).


This antenna model assume a side-lobe level of -20 dB. 


The geometric angle to the mobile terminal from the base station transmit antenna may be determined from simple trigonometry resulting in the following plot.


As shown, the angle to the GPS user is  is below 32 degrees.

Angle from Tower Antenna to GPS Receiver


Combining the angle calculations into a antenna equation, and calculating the the received power to a GPS receiver results in the following equation( as a function of the distance from the tower).  Plotting the equation for the various tower heights, frequency and loss exponent.


Plotting the above equation for three antenna/tower heights (10m, 20m, and 30m) shows that the received power when the antenna pattern is included.  The worst case is for a 10m tower antenna height, 50m away with a received power of -8.5 dBm.  Next is the 20m tower height with a received power of -10.6 dBm, and finally the 30m tower with -14.1 dBm.  Notice, however, that the power levels increase above this within the 50m circle for the lower tower heights.  This is also where one might expect free-space loss (exponent=2) to be more prevalent.


Increasing the loss exponent to 2.7, the received power is -33.1 dBm, -35.4 dBm, and -40.1 dBm for tower heights of 10m, 20m, and 30m, respectively.


Subtracting the -30 dBm from the above plot indicates the level of additional attenuation required for LightSquared to meet their proposed power limit.  The plot below shows the maximum attenuation they would need to achieve (for free-space) is approximately 21.5 dB (10m antenna tower).  This is a substantial reduction in power levels.  However, if the loss exponent is increased from 2 (free-space) to 2.7, LightSquared actually has margin!



Note that this risk is mitigated by the probability that a GPS receiver will be within it 50 meter radius of the tower.  If we assume that GPS receivers are uniformly distributed around the tower, we can calcuate the probability that GPS receivers are within the a particular distance from the tower.  As shown, 99% of the users are further away than 50m, and 95% are more than 110m away from the tower.


THE BOTTOM LINE.  This is the key point of LightSquared’s proposal.  By shifting to received power, typcial interference analysis would indicate that LightSquared would have to reduce their power level by 21.5 dB to 40.5 dBm.  However, by taking the risk that most areas will be experiencing something greater than free-space loss, LightSquared has a greater opportunity to operate at their current power levels.  This is not incorrect reasoning, but it does involve a calculated risk.  Flat, open regions that experience nearly free-space loss will indeed have to operate at reduced power, and in those regions, LightSquared will require more towers to cover the same area, or have to accept less capacity.

Companion Receiver Analysis

It’s insightful to examine how the received power from a LightSquared tower compares to typical GPS receiver characteristics.  From this analysis, we can determine the internal attenuation that GPS receivers likely have to the lower LightSquared carrier.  A usefull resource for GPS calculations is from National Instruments (http://zone.ni.com/devzone/cda/tut/p/id/7189).  This states that a good value to use for the received power from GPS satellites is -136 dBm, and that typical receivers have a noise figure between 2 dB and 5 dB.

The GPS receiver sensitivity may be determined from the following equation. 


Calculating the receiver noise density for a 10 dB noise figure:

N0:=Nden(10) =

This is equal to a receiver C/N0 of:

CN0:=-136-N0 =

For a 1 dB degradation in C/N0, the required interference noise density (-5.8 dB from the receiver noise density)

I0:=N0+10*log(10,10^0.1-1) =

If the interfereing LightSquared signal is received at the specified -30 dBm, or a noise density of -100 dBm (-30 dBm-10Log(10MHz), then nearly 76 dB of added attenuation is needed to achieve a 28 dB CNR. Such attenuation is possible when there are no intermodulation products created in the receiver front-end, and the receiver tests performed by LightSquared appears to support this.  However, the situation requires even more receiver attenuation when we consider the expected maximum average received power calculated from above.

When the received power is incorporated with the required interference noise density produced by LightSquared’s carrier, the required receiver attenuation exceeds 90 dB when 50m from the tower. 


 

FCC asks for more information on LightSquared, GPS

/ August 10th, 2011 / Comments Off on FCC asks for more information on LightSquared, GPS

A article published today cited that the head of the FCC’s Office of Engineering and Technology, Julius Knapp, sent a letter to LightSquared and the GPS Council asking for more information on which devices were specifically interfered with by LightSquared, ane the effects of LightSquared’s new proposal to use only the lower portion of their licensed band (1,526 MHz to 1,536 MHz).  This is a valid, and not unexpected, question from the FCC especially since the Technical Working Group Report randomized the devices so that no correlation could be made between the device, device manufacturer and the test result.  This is understandable since the GPS receiver manufacturers likely want to preserve their proprietary performance from public disclosure to their competitors.  However, as Questiny looked at the data, we could not correlate the measured performance to the device characteristics.  For example, some of the receivers use automatic gain control (AGC) and 3-bit analog to digital convertors (A/Ds) , whereas other receivers use no AGC, and a 1-bit A/D.  The performance between these two devices is predictably differerent (the 1 bit A/D will be more sensitive to overload).  The figure shows the results of static tests performed for the General Navigation Class of GPS Receivers where the LightSquared power level was measured at the device such that the GPS carrier-to-noise level was reduced by 1 dB (or 25%).  As the figure shows, the range of power needed to create the same degradation across receivers varied by more than 70 dB (10 million time).  This is a huge difference in performance.  As the TWG only provided an index for the device, their technical characteristics could not be correlated to account for this range of performance difference.  (Note gaps in the measurements indicated devices that did not suffer any loss or were not tested.)

Regardless of the previous results of the TWG.  LightSquared essentially nullified their relevance when they proposed a new frequency plan on June 15.  Now, as expected, the FCC has ordered additional testing for this new LightSquared frequency plan, and they have reassured the GPS community that they will not impact the GPS performance.  As my colleague at TMF points out, this could put severe pressure on LightSquared vis-a-vis Sprint’s ability to back out of their deal should LightSquare be unable to raise additional capital and make their required payments.

Received Power (at Device) Required for 1 dB loss in GPS in C/N0 for General Navigation Devices