Path-Loss Model
A path-loss model is a formula that links received radio power to distance. In RTLS work it is the usual bridge from an RSSI sample in dBm to a guessed range in meters (feet). The common log-distance form needs two calibrated pieces: a reference power at a known short distance (often near 1 m / 3 ft), and a fade exponent usually written as n. Free-space thinking starts near n = 2. Indoor halls with racking, stock, and people often need a higher n, and that value is not fixed for the whole roof.
The model is an assumption about the radio path, not a tape measure. When steel, bodies, antenna orientation, or multipath change loudness without a true move, the inverted distance is wrong even if the dBm number is honest. The plant explainer is RSSI is signal strength, not distance.
Engines still use path-loss models because they are cheap to run on BLE and Wi-Fi stacks. The job for operations is to know when the conversion is good enough for zone presence, and when bay identity needs denser anchors, filtering, or a different measurement such as Angle of Arrival.
Key Components
Reference power - RSSI measured at a known short distance, often near 1 m (3 ft), used as the model's zero point.
Path-loss exponent (n) - How fast power falls as distance grows. Open space sits near 2. Cluttered indoor halls often need a higher value, tuned on site.
Shadowing and noise term - Extra fade from blockers and short-term multipath that the simple distance curve does not explain.
Site calibration - Walking known points with racks loaded and movers present, so n matches the hall you will run, not an empty lab.
Applications in Manufacturing and Logistics
Classic and inverted BLE location engines convert RSSI through a path-loss model to place trucks, cages, and badges on a hall map used for asset tracking and forklift tracking. Coarse wing or gate presence can tolerate a loose conversion. Aisle and bay search for internal logistics fails first when the model treats a steel-shadowed tag as far away. How BLE builds use those samples is in BLE for RTLS.
Benefits and Challenges
The benefit is that any receiver that already reports RSSI can feed a location guess without specialised ranging hardware. The challenge is that n and the reference power drift with inventory height, parked fleet, and antenna mount. A model tuned in an empty aisle understates error on a loaded shift. Filtering smooths packet noise. It does not fix a wrong fade story.
Related Terms
RSSI is the power input. Line of sight and NLOS describe whether the direct path exists. Multipath adds reflected copies that change loudness. Indoor positioning and RTLS are the systems that consume the converted range.
Frequently Asked Questions
Is path loss the same as distance? No. Path loss is how power fades. Distance is what the engine infers after applying a model.
Why does one hall need a different n than another? Racking density, wall materials, mezzanines, and how people and trucks fill the aisle change fade rate. Copying another site's exponent is a common error.
Can more anchors fix a bad path-loss model? Density helps when coverage was the hole. It does not erase NLOS or multipath that still corrupt loudness at each receiver.
Do time-of-flight radios use the same model? Time of flight and similar ranging methods measure travel time, not RSSI loudness. They still fight NLOS and multipath, but they do not invert dBm through n as the primary ruler.