Calibration Procedure for Bluetooth Low Energy (BLE) Received Signal Strength Indicator (RSSI)

The Exposure Notifications framework relies on a precise calibration process to estimate the distance between devices using Bluetooth Low Energy (BLE) received signal strength indicators (RSSI). This document outlines the methodology used by the RSSI Calibration Tool app, also referred to as the calibration app, to calibrate BLE transmissions.

To begin, the reference device emits packets at a given power level. The device under test (DUT), which is typically a smartphone or tablet, measures the uncalibrated RSSI of these packets. To calculate the RSSI correction, the app takes the median of 10 RSSI measurements for each power level. This process is repeated for multiple power levels, including ULTRA_LOW_POWER and LOW_POWER.

The DUT and reference device are placed one meter apart in a stationary position to minimize environmental factors that may affect the measurement. The devices are typically held back-to-back in portrait mode to ensure consistent antenna sensitivity and device orientation.

The calibration app collects data from multiple devices using different phone orientations, placements, and environmental conditions. This comprehensive approach helps to account for variations in antenna sensitivity, device orientation, and environmental conditions that can impact RSSI measurements.

Once the data is collected, it is used to estimate calibration values. The calibration values are then applied to each device's BLE transmission power level (TX_power) to ensure accurate distance estimation. The goal of this process is to minimize errors in estimating exposure risk, which relies on accurately determining how close users have been to each other.

The RSSI Calibration Tool app employs a sophisticated methodology that involves collecting data from many devices as possible, generating a per-device TX_power, and using the BLE RSSI calibration values to calibrate transmissions. By following this procedure, the Exposure Notifications framework can provide accurate distance estimates between devices, enabling effective exposure risk assessment and mitigation.

In summary, the Bluetooth Low Energy (BLE) Received Signal Strength Indicator (RSSI) Calibration Procedure uses a combination of device measurements, antenna sensitivity, device orientation, and environmental conditions to estimate calibration values. These values are then applied to each device's BLE transmission power level to ensure accurate distance estimation, enabling effective exposure risk assessment and mitigation in Exposure Notifications.

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