For devices with receivers, each time they transmit an advertising packet, they listen for a brief period for a specific BLE message called a Scan_Request. This message is normally used by nearby devices that may wish to communicate with the advertiser. If the advertising device receives this Scan_Request, it will respond with a Scan_Response message. This pattern is illustrated in the power versus time plot shown in Figure 1. The advertiser sends an advertisement packet, shown in blue, which is followed by a Scan_Request message from a nearby station, (shown in orange), and the advertiser follows up with a Scan_Response message, again shown in blue.
This exchange of messages is used in normal operation of BLE devices, and the OTA test system exploits this behaviour to measure PER. During a receiver test, the DUT sends an advertisement, the test system sends a Scan_Request, and if the DUT received the packet, it acknowledges with the Scan_Response. The test system keeps track of the number of Scan_Request messages that are sent and the number of Scan_Responses received. This information is used to calculate receiver PER.
For sensitivity measurements, the test system adjusts the RF signal level to find the RF level that produces a specific PER. This RF level, known as receiver sensitivity, is a common method used to specify receiver quality. The method using the advertising packets, Scan_Request messages, and Scan_Response is implemented in the BLE OTA tester, and accurately determines receiver PER or sensitivity without any direct wired communications with the DUT.