Room-Level Asset Tracking in a Hospital that Refuses to Touch the Clinical Wi-Fi
A 900-bed hospital wants to track 1,200 movable assets across a nine-floor tower: 600 infusion pumps, 300 wheelchairs and 300 high-value surgical instrument trays. Nursing managers must locate a pump within two minutes and only need room-level accuracy. IT already runs 60 Wi-Fi access points on 2.4 GHz channels 1, 6 and 11 plus 5 GHz for clinical laptops, and refuses to let any new system use the clinical SSID or install agents on the APs. Radiology has two 3 T MRI suites inside shielded rooms, and 40 doors use 13.56 MHz HF RFID badges for access control. Sterile services already reads passive UHF labels at one cabinet and wants trays tracked through washer-disinfectors at 90 C. Wheelchair tags must run three years on a CR2477 coin cell; pump tags can be recharged weekly. Nothing may interfere with existing clinical wireless.
1. Select a technology for each of the three asset classes and justify each choice against the stated constraints.
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Treat the three classes differently, because their requirements are different.
Infusion pumps: active BLE tags, rechargeable, advertising at a short interval with fixed anchors in corridors and above room doors. Pumps move constantly and must be findable from a phone, so passive tags are useless and room-level is enough. BLE needs no change to the clinical SSID and no agent on the APs, which satisfies the IT constraint outright.
Wheelchairs: the same BLE infrastructure but a coin-cell-optimised tag with an accelerometer that switches between fast and slow advertising. They are low-value, high-count and rarely searched for, so a longer interval is acceptable and the three-year budget becomes reachable.
Surgical trays: passive UHF RFID read at portals, not continuous tracking. Trays only need to be known at checkpoints (issue, return, cabinet), so 300 continuously transmitting tags would be waste and would add 2.4 GHz load for no benefit. Use on-metal UHF tags rated for the washer-disinfector at 90 C. If any tray also enters a 134 C autoclave, UHF inlays will not survive; handle that step with a laser-etched code and keep RFID on the outer wrap stage.
Do not touch the 13.56 MHz access-control system: it is a separate band and a separate safety-critical function, and reusing its readers would create both regulatory and operational coupling nobody asked for.
2. How do you keep BLE from disturbing the Wi-Fi network, and what changes inside the MRI suites?
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BLE advertising deliberately uses channels 37, 38 and 39 at 2402, 2426 and 2480 MHz, which sit at the edges or outside the Wi-Fi 1/6/11 channel spans, so advertising traffic is a poor interferer by design. Three things still need care. First, connection-oriented BLE (for firmware updates or pairing) hops across all 40 channels and does overlap Wi-Fi; keep those sessions short and rare, or restrict them to maintenance windows. Second, tag density matters more than protocol: 600 pumps advertising every second in one corridor is a real airtime load. Use motion-triggered intervals and a 1 s floor only while moving. Third, anchors should be wired or PoE, never Wi-Fi backhaul, so the tracking system adds no clients to the clinical SSID.
Inside a 3 T MRI suite, nothing transmits. The shielded room is a Faraday cage, so a tag inside is invisible and an anchor inside is useless. Worse, any tag containing ferromagnetic material is a projectile hazard. Place anchors outside the suite and use a door portal to record entry and exit; the room's last-seen state is then inferred from the portal transition. Decide explicitly whether tagged pumps are allowed into the magnet room at all, and if they are, specify MR-conditional tags.
Finally, room-level means about 3 to 5 m, achieved with three or more anchors per room and a calibration walk, not trilateration from one gateway in the corridor.
3. Show the energy budget for a wheelchair tag and explain what it implies for advertising interval.
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A CR2477 is roughly 1,000 mAh. Three years is 26,280 hours, so the allowable average current is 1,000,000 uAh / 26,280 h, about 38 uA.
An advertising event is short but expensive: roughly 1 to 2 ms at 10 mA average across the three advertising channels. At a 1 s interval, that is about 1.5 ms / 1,000 ms x 10 mA = 15 uA, plus a few microamps for the accelerometer and leakage, comfortably inside 38 uA. At 100 ms it becomes 1.5 ms / 100 ms x 10 mA = 150 uA, roughly four times the budget, and life falls to well under a year. The arithmetic, not the marketing sheet, rules out fast continuous advertising on a coin cell.
The design implication is adaptive advertising: 1 s while the accelerometer detects motion, 10 s or slower when stationary, with a low-duty scan for a locate request. Wheelchairs sit still most of the day, so the weighted average lands far below the budget, and the two-minute locate requirement is met because a moving chair is exactly when the tag is fast. Verify with a two-week current-profile capture on a real unit rather than trusting the theoretical sum.