NICET ITWBS Level III cheat sheet

In-Building Public Safety Communications

• Coverage acceptance is measured as Delivered Audio Quality (DAQ): DAQ 3.0 is the minimum passing score for most public safety in-building systems, DAQ 3.4 is often specified for voice paging or critical areas; DAQ 1 is unintelligible, DAQ 5 is near-toll quality.

• Signal strength alone does not prove coverage: a system can show adequate RSSI (commonly -95 dBm or better at the antenna) while failing DAQ because of interference, delay spread, or simulcast overlap; always test with the actual subscriber radio on the actual talkgroup.

• The 95%/90% rule: a common acceptance criterion is DAQ 3.0 or better in 95% of grid test locations on each floor, with no location below DAQ 2.0; confirm the AHJ's specific percentages before testing.

• Grid testing is performed on a uniform grid (typically 20 to 25 test points per floor, spaced roughly 10 to 20 feet apart) with the donor site disabled where feasible so the result reflects the in-building system only.

• Leakage is verified outside the building envelope: measured signal at the property line or at a specified offset must stay below the AHJ threshold (often -95 to -100 dBm on the donor's downlink) to prevent interference with the macro network.

• BDA versus fiber DAS: a BDA (bi-directional amplifier) amplifies off-air donor signals and is limited by donor isolation and available signal; a fiber DAS converts RF to optical, distributes over fiber, and supports multiple bands and sectors with centralized head-end control.

• Donor isolation must exceed system gain by at least 15 dB (many designs target 20 dB) to prevent oscillation; isolation is measured as the path loss between donor antenna and service antennas, including building attenuation and antenna front-to-back ratio.

• Uplink noise contribution to the donor site must be controlled: excessive uplink gain raises the donor site's noise floor and can desensitize the macro cell; the noise rise budget is set by the carrier or AHJ and verified by measurement at the donor site.

• Required documentation for acceptance typically includes design drawings, RF path analysis, equipment data sheets, factory and field test results, grid test data with floor plans, leakage test data, and as-built records signed by the responsible technician.