Questiny Group

Analysis of LightSquared™™ Terrestrial Carriers on GPS Receivers

Keith R. Barker, Questiny Group, Inc.

Monday LightSquared™ offered to move its spectrum from the current location to mitigate their impact on existing GPS receivers.  This is the first admission that no real solution existed to this problem.  Details are sketchy at this moment, but in an article published by Wireless Week, they stated that, ” LightSquared™ plans to use spectrum leased under an existing contract with Inmarsat instead of its own L-band spectrum until it can figure out how to use its own bandwidth without affecting GPS.  The company also said its base stations will transmit their signals at half-strength to further minimize interference.”  That article went on to state that, “The Inmarsat spectrum slated to be used by LightSquared™ runs from 1526 MHz to 1536 MHz and is located further away from bands used by GPS receivers, which run from about 1559 MHz to 1610 MHz, helping to reduce the likelihood that LightSquared™’s transmitters will knock out GPS service.”  The article quoted the Company as stating that even this fix would not remove the impact to all of the precision GPS receivers currently deployed.

The article suggests that LightSquared™ is giving up on a two-carrier configuration in their spectrum.  In their FCC filing, LightSquared™ proposed a phased deployment plan.  The last phase, Phase 2, used two 10 MHz wide LTE carriers located at 1,526-1,536 MHz and 1,545.2-1,555.2 MHz.  Each of these carriers were opertated at an effective radiated power of 32 dBW.  The plan suggested by LightSquared™ appears to drop plans to use the upper carrier (1,545.2-1,555.2 MHz), and to cut the power of their carriers to 32 dBW.

As a wireless communications engineer, I can say that the new frequency plan will reduce the impact of the LightSquared™ carriers on GPS receivers, but it will also have a dramatic effect on LightSquared™ capital plan and capacity.  Reducing the number of carriers and their powers will mean that LightSquared™ needs to deploy more towers to cover the same area, or LS users will have to accept less in terms of the quality of service.  How much of a loss to quality depends on the details of the LightSquared™ receiver design.

To examine the impact of the proposed spectrum change, Questiny Group performed some quick calculations based upon a notional GPS receiver design.  Figure 1 shows a notional GPS receiver design that might be typically deployed in the marketplace.  The receiver includes a front-end filter, usually a surface acoustic wave (SAW) filter, followed by a low-noise amplifier (LNA) prior to entering the GPS receiver chip.  The “interference” issue between GPS and LightSquared™ is manifested in the performance of this LNA when subject to high power signals such as LightSquared™.  How much power reaches the LNA is a function of the SAW filter, and if that power exceeds the limits of the LNA, the LNA will distort the LightSquared™ and GPS signals.  This is analogous to turning up your car radio so loud that you begin hearing the distortion.  However, GPS only uses a small fraction of the spectrum the filter and LNA allow to pass, so that not all of that distortion has an impact on the GPS signals.  How much does fall within the GPS signal bandwidth depends on the carrier frequencies of LightSquared™ and the performance of the LNA.  It is this complex set of trades between the LightSquared™ spectrum, the filter performance, and the LNA performance that makes this problem seem so confusing.  But it’s not that difficult to understand once the problem is broken down.

The GPS signal is located around the center and the LightSquared™ LTE carriers are located off to the left.  This is shown in Figure 3.  This figure shows how what the spectrum input to the LNA might look like with the LightSquared™ carriers are adjacent to the GPS band.  If the LNA was a linear device, and the GPS receiver chip had a narrow (2 MHz) filter, then there would be no issue between LightSquared™ and GPS.  However, LNAs are not entirely linear devices, and just like a car radio, LNAs will distort any strong input signals.  How much depends on the quality of the LNA.

This would be the case when the GPS receiver is far away from the LightSquared™ tower.  The top plot shows an output spectrum (dashed) that is nearly identical to the input spectrum (blue), and the distortion components (bottom, red/green/cyan) are below the fundamental signal (blue).  In this situation, there would be minimal impact on GPS receiver performance.

However, if the GPS receiver moves closer to the tower such that the LNA begins to become overloaded and distorts the input signals, we arrive at the situation in Figure 5.  Here the level of the input power to the LNA has increased to nearly -40 dBm (the 1 dB compression point), and the spectrum at the output of the LNA being compressed (top dashed line) and distorted (bottom).  One can see the distortion components increase to a level nearly identical to the noise floor.

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This will cause severe reduction in the performance of the GPS receiver. LightSquared™ now proposes to remove the carrier closest to the GPS band, and Figure 6 shows the spectrum and distortion of the “assumed” new frequency plan of LightSquared™. As shown, the output spectrum (top dashed line) is still compressed, although not quite as heavily, and the distortion components are much, much less. Even with an increase in input power of +20 dB (100x) to simulate moving closer to the LightSquared™ towers, the distortion components are 12 dB below the noise floor providing minimal impact to the GPS receiver (see Figure 7). Thus, I would say that removing the upper carrier from LS’s frequency plan does significantly reduce the impact to GPS receivers. I expect that the extension LS filed with the FCC to delay the technical report will take into account the testing of receivers against this new frequency plan.

Posted in Blog, GPS, Intermodulation, LightSquared, SatCom, Spectrum