[opendtv] Can Terrestrial Broadcasting and GPS Co-exist in Adjoining Spectrum?
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 30 Jun 2011 15:55:04 -0500
http://www.tvtechnology.com/article/122176
Can Terrestrial Broadcasting and GPS Co-exist in Adjoining Spectrum?
by Charles W. Rhodes, 06.30.2011.
The FCC has granted use of space-to-earth spectrum to a firm that intends to
construct 40,000 base stations to carry broadband to the public. This sounds
like a great idea, taking some of the spectrum demands from your shoulders as
broadcasters.
But this re-purposing of the 1525-1559 MHz spectrum would come at an unforeseen
price. This spectrum is adjacent to the international GPS satellite
navigational band, 1559 - 1610 MHz. Each of these base stations would radiate
1584 watts, maybe more. These signals could jam reception of GPS signals. GPS
provides time, frequency, and location data vital to broadcasters' operations
now and even more so in the future-for example DTS transmitters must be
synchronized and the clock used for this purpose is the GPS signal.
Various federal agencies are deeply concerned over this potential jamming of
GPS. DoD, FAA, & NIST have analyzed this threat and are on record that GPS
would be endangered if the FCC proceeds with the final step authorizing
terrestrial use of this band of frequencies next to the GPS Band. Laboratory
and field testing has been carried out at the request of the FCC, which
extended the deadline to Lightsquared to deliver the final test results to July
1.
ATSC SIGNAL TESTS
My colleagues and I have learned a lot about interference between broadband
signals into DTV receivers through our experiments in my laboratory. This
knowledge led us to simulate the situation where extremely sensitive wideband
receivers are overloaded by signals such as those proposed for these 40,000
base stations.
We have carried out those tests, not with LTE signals of 5 MHz bandwidth, but
with our ATSC signal of 5.38 MHz bandwidth. As the LTE signals would be
upgraded to 10 MHz later in the implementation plan, we used pairs of our 5.38
MHz DTV signals to simulate the 10 MHz LTE signal. The test plan called for
tests with both 5 MHz and 10 MHz LTE signals, but last month, we learned that
the 10 MHz tests could not be done within the timeframe demanded by the FCC. We
have simulated all three phases and our results contradict the notion that
tests with two 5 MHz signals is the most critical and therefore tests with two
10 MHz LTE were dropped.
In our simulation, we used DTV signals in the UHF band, not LTE signals in the
L band (1525-1559 MHz. By subtracting 959.31 MHz from each frequency in the
LightSquared signals, we shifted them to the UHF band.
Fig. 1 shows one 5.38 MHz DTV signal with 3rd order distortion products falling
in both first adjacent channels. This simulated the Phase Zero LightSquared
Signal. This LTE signal would be from 1550.2 MHz to 1555.2 MHz. The 3rd order
distortions products extend from 5.38 MHz below 1550.2 MHz to 5.38 MHz above
1555.2 MHz. None of these come near the GPS Band 1570 - 1580 MHz. However there
is another 3rd order distortion product, de-sensitization and this would reduce
the GPS signal power thus causing jamming. That cannot be simulated.
"Simulated Phase Zero Signal". Add 959.31 MHz to each frequency shown to get
the LightSquared Signal frequencies.
Figure 2 shows two 5.38 MHz DTV signals simulating the LightSquared Phase One
Signal. These two signals can produce 3rd order distortion products, some of
which fall in the GPS band and could jam GPS reception. The highest frequency
3rd order distortion product is centered Marker # 4 at 1577 MHz blanketing the
GPS Band. LightSquared stated that their Phase One signal would cause the most
GPS jamming, if any occurred.
"Simulated Phase One Signal" Marker # 4 corresponds to 1577 MHz in the GPS Band.
Fig. 3 shows the Phase Two Signal. Here there are contiguous pairs of 5.38 MHz
DTV signals to simulate the 10 MHz wide LightSquared LTE signals. The spectrum
of 3rd order distortion products extends up to Marker # 4, 1584 MHz well above
the GPS band (1565 - 1580 MHz). There is a considerable noise power density at
the GPS carrier frequency of 1575.42 MHz . This high noise power density might
prevent GPS receivers from synchronizing.
"Simulated Phase Two Signal" Note that the 3rd order distortion products extend
above the GPB band in both Figs. 2 & 3.
Between markers #2 and #3 in Fig. 3, you should note that the noise floor
increased by almost 20 dB. This spectrum, 9.2 MHz wide, in the MSS band may not
be suitable for space-to-earth transmissions because of this elevation in its
noise floor due to 3rd order distortion products generated in receivers near
one or more LightSquared towers.
My simulations indicate the spectrum which may be generated in GPS receivers by
LightSquared signals. What cannot be shown by such simulations is whether
jamming of GPS receivers results from de-sensitization of the receiver or by
third order distortion products or by strong signals passing through the
receiver because of their finite selectivity. That question may not have been
answered in the LightSquared tests conducted recently and whose results were
not available at the time this was written.
Next month I intend to comment on what those tests have accomplished.
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