One Remote Unit Dead in a Fiber DAS
A convention center uses a fiber-fed DAS for 700/800 MHz public safety. Eight remote units serve halls and concourse. During pre-acceptance testing, Hall C reads -102 dBm and DAQ 1.0, while adjacent Hall B reads -85 dBm and DAQ 4.0. The head-end shows normal RF input from the donor, and the fiber span to Hall C is about 800 m with two splitters. The RU in Hall C powers up and reports no alarm, but its optical receive level is 3 dB lower than the design value. The AHJ requires 95% general-area coverage at -95 dBm and DAQ 3.0. The contractor wants to increase the head-end gain for all RUs to compensate. The building has a mix of single-mode fiber and LC connectors, and the RU is in a ceiling space above a service corridor.
1. Outline a troubleshooting sequence from the head-end to the Hall C RU. What measurements matter most?
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Start at the head-end and work toward the affected RU. Confirm the RF input to the fiber master is within design and that all RUs are not being overdriven. At the Hall C RU, measure optical receive power with a calibrated power meter at 1310/1550 nm and compare to the design value; a 3 dB deficit is significant. Use an OTDR or optical return-loss test on the 800 m span and inspect LC connectors for contamination, tight bends, and damaged polish. Check the two splitters for correct ratio, port assignment, and excess loss. Then measure RF output at the RU antenna port with a known input; if optical power is low but RF input is normal, the fiber/connector path is suspect. Swap the RU with a known-good unit only after optical tests. Do not raise head-end gain for all RUs; that would overdrive Hall B and other sectors.
2. Evaluate the contractor's proposal to increase head-end gain for all RUs.
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Reject the proposal. Increasing head-end gain raises the optical modulation index and RF level to every RU, including Hall B, which already reads -85 dBm and DAQ 4.0. Overdrive can cause distortion, AGC action, and interference. The Hall C deficit is isolated to one RU and one fiber path, so a global gain change treats the symptom and may create new failures. If the RU optical input is 3 dB low, the correct fix is in the fiber plant: connector cleaning, re-termination, splitter repair, or replacing a damaged patch cord. Only after the optical loss is corrected should the RU RF gain be set per design. If the RU itself has failed, replace it. A global increase also risks violating the maximum ERP and uplink noise limits.
3. What repair and retest steps would you document before AHJ acceptance?
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Clean and inspect all LC connectors on the Hall C span, then re-test optical power. If loss remains, use an OTDR to locate a bend or bad splice; replace the damaged patch cord or re-terminate. Verify splitter ratios and that the correct port feeds Hall C. After optical power is within design, set the RU RF gain to the commissioning value and measure signal and DAQ in Hall C. Retest adjacent halls to confirm no change. Document optical power, RF input/output, and DAQ. If the RU is faulty, swap it and repeat. The acceptance test should show Hall C at or above -95 dBm and DAQ 3.0, with 95% general-area coverage and no degradation in other sectors. Update as-built fiber assignments.