Hello there!
Do you have any guidelines for a design that includes GNSS and LTE-M to make sure that my LTE transmission will not interfere with the reception of the GNSS signal?
Thanks in advance!
Brian
Dear Brian, please find my answers below:
GNSS and LTE-M Coexistence: Mitigation Measures
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SAW Filtering
As a first mitigation measure, virtually every GPS receiver integrates one or two SAW (Surface Acoustic Wave) filters in the signal path to attenuate out-of-band signals. -
Antenna Isolation
Use a dedicated GNSS antenna and maintain physical separation between antennas — as much as possible within your device’s form factor. Good antenna isolation is the most effective protection, especially against in-band interference generated by the LTE-M modem. -
Transmission Indicator GPIO
The GM02S module includes a specific GPIO pin (#9), labeled “Transmission Indicator,” which can be used to notify the GNSS chipset when LTE transmission occurs. This requires the GNSS chipset to support an anti-jamming feature based on GPIO signaling (TX blanking). -
LTE-M Transmission Behavior
LTE-M operates in HD-FDD mode and therefore does not transmit continuously, reducing the risk of persistent interference.
Characterizing GNSS Sensitivity Impact
To evaluate the impact on GNSS sensitivity, you can easily perform tests using the GM02S EVK and a GNSS EVK. By using an RF combiner and coupler, you can measure the Carrier-to-Noise (C/N) degradation on satellite reception as a function of modem output power.
Recommended test scenarios:
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Continuous TX Tone Testing
Transmit a continuous tone at various output power levels. This represents a worst-case scenario and does not reflect the actual HD-FDD behavior. -
Realistic Uplink Testing
Test in front of a call box with a defined amount of uplink data, using several output power levels (e.g., 23 dBm, 15 dBm, 0 dBm).