Uplink Noise Complaint After BDA Commissioning
A 12-story office building installs a new 800 MHz public-safety BDA to support fireground communications. The donor antenna is on the roof, aimed at a trunked site 3 miles away; the server antenna is in the basement telecom room. After commissioning, the carrier reports that the sector serving the building has seen its uplink noise floor rise from about -116 dBm to -101 dBm, with dropped calls and busies on nearby talkgroups. The BDA's uplink gain is set to its maximum of 85 dB because the installer wanted to overcome 60 dB of coax loss. There is no oscillation alarm, and downlink coverage inside the building tests well. The AHJ requires that the system not degrade the donor network. The building has 12-hour battery backup and a manufacturer's commissioning tool.
1. What is the most likely cause of the carrier's noise-floor complaint, and why is the downlink result not evidence of uplink health?
Read the worked answer
The pattern points to uplink noise amplification, not downlink failure. The BDA uplink gain is at 85 dB to overcome 60 dB of coax loss, leaving roughly 25 dB of excess gain. A BDA cannot distinguish desired portable signals from thermal noise at its donor port; excess uplink gain raises the noise floor at the donor site. A rise from -116 to -101 dBm is 15 dB, consistent with too much uplink gain and possibly a wideband amplifier passing the entire band. No oscillation alarm does not rule out noise amplification. Downlink DAQ only proves the donor site reaches the building; it says nothing about how much noise the BDA sends back. The fix is not more filtering alone; first calculate the maximum uplink gain that keeps the BDA noise contribution at the donor receiver below the carrier's limit, often a 0.5 dB rise. Then reduce uplink gain, add a fixed attenuator if needed, and verify donor-to-server isolation. Coordinate the change with the carrier and document before/after noise measurements.
2. Describe the measurements you would make to confirm BDA-caused noise and to check isolation.
Read the worked answer
Test with the BDA off, then on, while monitoring the donor site's noise floor on a spectrum analyzer or via carrier reports. Measure at the donor receiver or at the BDA donor port with a calibrated attenuator. Also check the BDA's uplink output with a portable transmitting from several floors. If the noise floor rises only when the BDA is on, the BDA is contributing. Confirm the BDA is on the correct channels and not passing adjacent-band energy. Check isolation by injecting a test signal and measuring leakage between donor and server paths; if isolation is less than gain plus 15 dB, oscillation margin is inadequate. Record the gain setting, attenuator value, and noise rise. Do not use the downlink DAQ as proof of uplink health.
3. What corrective actions would you take, and what retest proves the problem is resolved?
Read the worked answer
Reduce uplink gain in steps until the donor noise rise is within the carrier's allowance; add attenuation on the uplink path if the BDA's minimum gain is still too high; repair or improve donor/server antenna isolation; and ensure the BDA is properly filtered and certified for the band. Re-run inbound DAQ tests in the building to confirm portable-to-donor audio still passes. Notify the carrier and AHJ, update commissioning records, and set a lockable gain setting. If the building still needs more uplink margin, improve the donor antenna gain or move it rather than running the BDA at maximum. Verify 12-hour battery and alarm reporting remain functional. A final test should show no measurable donor degradation and DAQ 3.0 inbound, with documented before/after noise-floor readings at the donor site.