LightSquared Las Vegas Test Towers Flawed, FCC Filing Shows
May 26, 2011 By: Alan CameronGNSS Design & Test Newsletter, May 2011
Results from the LightSquared/GPS testing just concluded in the field near Las Vegas, Nevada, may produce overly optimistic results regarding the effects of the LightSquared signal on GPS receivers. This is because the LightSquared equipment may be broadcasting at lower-than-planned levels for its initial deployment on cell towers across the United States. Further, LightSquared may not currently be prepared or equipped to broadcast according to the terms of its business plan or its conditional waiver.
On May 16, the joint LightSquared/GPS Working Group filed its latest report with the Federal Communications Commission (FCC). Appendix G of that report describes the LightSquared Live‐Sky Test Environment, with its first section titled “Field Test Methodology.”
On May 23, the Working Group filed an addendum to that report, a revised Appendix G. The first section of that Addendum is now called “Field Test Methodology and Transition Details to Reach Single Carrier Planned Power.”
From the Addendum’s first and second paragraphs; the text in bold is new with the Addendum, relative to the May 16 version:
“The base station installation is representative of actual deployment, including a 2° electrical antenna downtilt, except that the dual carrier EIRPs [effective or equivalent isotropically radiated power] in this field test are lower for some band plans than the initial deployment plan.”
“In LightSquared’s initial deployment plan, the base station will emit L‐band signals at 32dBW/carrier (29 dBW/carrier/MIMO branch) in all deployment phases. However this field test began with an EIRP of approximately 29 dBW (59 dBm) per carrier for dual-carrier tests because the base station software is not yet capable of transmitting two channels simultaneously from the same unit(see Table 2, Note 1) 2. For single-carrier tests, whether using the upper 5-MHz channel or the lower 5-MHz channels alone, the base stations in the field test from May 18 onward has been augmented to emit the originally planned 32 dBW (62 dBm) EIRP. However due to current software limitations, the two carrier emission case will reach only 59 dBm/carrier and not 62 dBm/carrier EIRP as planned for commercial deployment. In other words, the net sector-EIRP for the 2-carrier case will be 62 dBm, not 65 dBm as planned for commercial deployment.”
Let me repeat one key passage of that last paragraph, bolding certain words for emphasis:
“However due to current software limitations, the two carrier emission case will reach only 59 dBm/carrier and not 62 dBm/carrier EIRP as planned for commercial deployment. In other words, the net sector-EIRP for the 2-carrier case will be 62 dBm, not 65 dBm as planned for commercial deployment.”
Most readers of this newsletter will note that –3dB = a factor of 2, or halving of the emitted power.
Some details and limited explication follow below. The bottom line is that LightSquared does not appear to have developed the full software suite nor possess the full equipment to implement the plan the company says has been in preparation for many years. Critical testing is being conducted under conditions that do not truly replicate what may be the case if the FCC allows this to go forward.
A rush to judgment.
Why is this happening? It appears that the software in LightSquared’s base station equipment is not ready. The following passage also comes from Appendix G, this time found in a footnote to the fourth out of five pages, and again new in the Addendum:
“* Note 1: The current pre-production eNodeB software does not support two carriers per sector until a future release. For two carrier tests, the eNodeB will require that each carrier be separately input to one of the two antenna ports and will result in the MIMO gain not being present. Thus total sector EIRPs will 62.0 dBM for the two carrier tests.”
The Las Vegas cell sites being used for tests — tests that could determine the future of the GPS industry and many, many sectors, other industries, and millions of end users — are only emitting half the power they should be for a critical test (2-carrier). Therefore, the test results will be optimistic for those tests.
Because of the non-linear nature of interference problems faced in these tests, such as GPS receiver overload, saturation, and intermodulation, this deficiency cannot simply be corrected in test results analysis after the fact.
The stated purpose of the field testing is to see how devices and receivers perform in a real-world environment. It is eminently reasonable for the GPS community to expect LightSquared to show up with transmitters that properly replicate its stated plans. After all, GPS manufacturers and interested parties have shown up in good faith with equipment ready to go.
It appears that the process is being rushed. In addition to the many other critical problems created or potentially launched by this initiative, LightSquared is simply not ready, technically speaking.
The new Addendum further begs the question of how LightSquared could have provided a full-power live-sky environment in earlier New Mexico tests, if it cannot provide one currently in Las Vegas. This would mean that results of the live-sky tests conducted there, bad as they are (see First Responders Find LightSquared Interference with Emergency Services, Public Safety), would also be over optimistic.
Readers may also notice that Table 2 in the Addendum (revised) version of Appendix G, and some of the other commentary, talk about differences on the first two days of the test versus the days after that. On the first two days, the problem was worse: half-power in single carrier mode and one-quarter power in two carrier mode. The first problem has been fixed, however the two carrier mode is still half what it should be.
Note further that this has nothing to do with LightSquared’s authorized licensed power, which is 15kW. LightSquared has stated that it only intends to use about 1/10 of that power, and it is that reduced power level that cannot be replicated in the two carrier mode.
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