Elevated Noise Floor at a 700 MHz Trunked Site
Your county operates a six-site 700 MHz P25 Phase 2 trunked system. Site 3 serves a commercial corridor and a hospital. Over three afternoons, the site's control-channel noise floor rose from -115 dBm to -101 dBm between 1300 and 1700, while mobile RSSI at the site remained normal. Portable users within 1.5 km reported failed registrations and audio gaps, and the site's BER rose to 5-7% on the voice channel. A spectrum sweep shows broadband energy across 769-775 MHz with peaks near 769.7 and 773.3 MHz. A new 700 MHz broadband LTE site began operation 250 m away last week, and a paging transmitter 1.2 km away has been on the air for years. No new public-safety transmitter was added. The system vendor suggests adding a second site to improve coverage.
1. Is this primarily a coverage problem? What measurements would distinguish interference from a coverage shortfall?
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This is not primarily a coverage problem. In a true coverage shortfall, portable and site-received signal levels fall together and the noise floor stays near baseline. Here mobile RSSI at the site is normal, but the noise floor is 14 dB higher and BER is 5-7%, which points to receiver desensitization or in-band interference.
Key measurements:
- Measure the noise floor at the receiver input or multicoupler, not at the antenna, using a calibrated spectrum analyzer. Compare the 1300-1700 window with the morning baseline.
- Key down the site's own transmitters into a dummy load and see whether the elevated energy persists; this separates self-intermodulation from external energy.
- Insert a bandpass filter and observe whether BER and noise floor improve.
- Correlate the onset with the new LTE site's commissioning, and check for out-of-band emissions, receiver blocking, and intermodulation products.
- Calculate whether the paging transmitter's harmonic or mixing product falls in 769-775 MHz; distance alone does not rule it in or out.
Normal RSSI plus elevated noise and high BER is an interference signature. Adding a site would mask the symptom and add cost without removing the source.
2. Rank the mitigation options and explain what you would do first, second, and last.
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Rank by reversibility, cost, and likelihood of addressing the measured source:
- Receiver-side filtering and gain control. Install a high-Q bandpass filter or cavity at the site receive path, verify receive gain, and reduce front-end gain if the receiver is being blocked. This is fast, reversible, and directly tests the desensitization hypothesis.
- Antenna isolation. Move or shield the receive antenna, increase vertical separation from the LTE site, or use a directional pattern that reduces energy from the interferer. Re-test noise floor and BER after each change.
- Coordinated remediation with the LTE operator. Request emission testing and site filtering or antenna adjustments. Document dates, times, and measured levels. Do not assume the new site is automatically at fault; the paging harmonic must also be measured.
- Paging transmitter remediation. If a harmonic or mixing product is confirmed, request filtering or frequency coordination action.
- New site or fill-in last. Only after the noise floor is controlled should you consider coverage additions. A second site would not fix receiver desensitization and could worsen intermodulation.
Record before-and-after noise floor, BER, and registration success so the fix is defensible in an after-action review.