5 GHz Coverage Gap in a Warehouse Aisle
A 40,000 sq ft warehouse with 30 ft ceilings has APs mounted on the ceiling grid in a checkerboard pattern at roughly 80 ft spacing. Handheld scanners on 5 GHz drop connections in two aisles lined with 12 ft steel racking full of metal parts. 2.4 GHz works but is congested (neighboring networks and Bluetooth). Site survey at floor level shows 5 GHz RSSI around -72 dBm in open aisles but -80 to -85 dBm inside the racked aisles, with SNR of 12-15 dB. The customer insists on 5 GHz only for scanner throughput. Channel plan uses 20 MHz channels with UNII-1 and UNII-3 only.
1. Explain why 5 GHz underperforms in the racked aisles despite acceptable open-aisle readings.
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The racked aisles are a different RF environment than the open aisles, and the survey numbers confirm it:
- Steel racking full of metal creates shadowing and multipath; 5 GHz has shorter wavelength and less penetration/diffraction than 2.4 GHz, so attenuation behind racks is much higher.
- Floor-level RSSI of -80 to -85 dBm with SNR 12-15 dB is at or below the threshold where 5 GHz rates become unreliable and retries climb; -72 dBm in open aisles is fine, so the problem is localized.
- 80 ft spacing designed for open coverage is too sparse for obstructed aisles; ceiling height (30 ft) also increases distance and puts the AP above the rack tops, so the signal path is blocked rather than grazing.
- 20 MHz channels in UNII-1/UNII-3 limit channel reuse options, so you cannot simply add APs on new channels without co-channel interference.
The judgement: this is a coverage/obstruction problem, not a channel-width or interference problem.
2. Recommend a design change that meets the 5 GHz-only requirement without a full re-cable.
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Targeted, not uniform, densification:
- Add APs specifically for the two problem aisles, mounted lower (e.g., on rack ends or side-mount brackets at 12-15 ft) so the signal path is down the aisle rather than through racks. Lower mounting is the key change - ceiling APs above rack tops cannot illuminate the aisle floor.
- Use directional antennas on those APs aimed along the aisle to concentrate coverage and reduce co-channel spill.
- Re-plan channels using 20 MHz and enable UNII-2/2e (with DFS awareness) to gain reuse slots for the added APs; verify no radar events in the area.
- Set a minimum basic rate / disable low rates on 5 GHz so scanners associate only at usable SNR and roam rather than cling to a weak AP.
- Validate with a post-survey at scanner height (not ceiling height) targeting ≥ -67 dBm and SNR ≥ 25 dB in the aisles.
Keep 2.4 GHz enabled as a fallback for non-critical devices even if scanners are 5 GHz-only.