Analysis of Potential Interference Issues Related to FCC Order 20-48 (2023) / Chapter Skim
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The order authorizes Ligado to provide terrestrial base station to mobile earth station downlink transmissions in the 1526–1536 MHz band within the mobile satellite services (MSS) -allocated 1525–1559 MHz band and mobile earth station to terrestrial base station uplink transmissions in the 1627.5–1637.5 and 1646.5–1656.5 MHz bands of the 1626.5–1660.5 MHz band.
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• Task 3: Assess the feasibility, practicality, and effectiveness of the measures in the FCC order to mitigate harmful interference effects on DoD devices, opera tions, and activities. In providing responses to these tasks, the committee stresses three important points: • Throughout this report, the term "Harmful Interference" -- when capitalized -- is a defined term used by the FCC and is specifically not the same thing as the general term "interference," which describes what happens in a receiver when some other signal affects the intended received signal in a way that 2 A N A LY S I S O F P OT E N T I A L I N T E R F E R E N C E I S S U E S R E L AT E D TO F C C O R D E R 2 0 - 4 8
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ANSWERS TO THE THREE ELEMENTS OF THE STUDY TASK Task 1: Approaches to Evaluating Harmful Interference Concerns Task 1 asks which of the two prevailing proposed approaches to evaluating harmful interference concerns -- one based on a signal-to-noise interference protection criterion (IPC) and the other based on a device-by-device measurement of the GPS position er ror -- most effectively mitigates risks of harmful interference with GPS services and DoD operations and activities.
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As such, these approaches impede progress in making more efficient and effective use of the spectrum. A new ap plicant for emissions in a new adjacent channel will have great difficulty in determining the emitter power levels and stand-off distances that will be guaranteed not to cause Harmful Interference to the installed base of GPS receivers.
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The committee also reached the following conclusion regarding the state of the art in GPS receivers: Conclusion 3: It is within the state-of-the-practice of current technology to build a receiver that is robust to Ligado signals for any GPS application, and all GPS receiver manufacturers could field new designs that could co-exist with the authorized Ligado signals and achieve good performance even if their existing designs cannot. Turning to impacts on MSS, the committee concluded that the Globalstar system is unlikely to experience harmful interference, because only its uplink is in the L-band and it uses code-division multiple access (CDMA)
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The FCC order enumerates several potential mitigations when a receiver experiences harmful interference, including but not limited to enacting exclusion zones for Ligado emitters; replacing components (e.g., antennas or filters) or full receivers; enabling a "kill switch" mechanism for Ligado to turn off emitters in some geographic locations; and additional negotiated mitigations between Ligado and the affected government agency for Ligado to reduce emissions to an acceptable received power level over certain installations.
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Such actions are highly application specific. These include replacing antenna subsystems; full-scale replacement of older commercial GPS receivers with newer models; and negotiated extended exclusion zones in which no Ligado emitters are placed.
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Some form of more definitive receiver standards and establishment of set time periods where adherence to those receiver standards will ensure successful operation for a fre quency band's incumbents and new entrants seem to be important tools in this regard.
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