RE: Bluetooth Direct Test Mode

Adrian Eggenberger — Wed, 27 Aug 2025 14:48:41 GMT

Hi

In the DTM measurement the transmitter is put in a mode where he periodically sends the configured test pattern (usually PRBS9) in a fix interval. The transmit interval depends on the packet length and the used phy.

The formula for the interval is defined as follows:
I(L) = ceil((L + 249) / 625) * 625 µs

L in the formula is length of the packet in us. It's calculation depends on the used phy and uses the number of payload bytes (=> b):
1Mbit: L = 80 + (b * 8)
2MBit: L = (88 + (b * 8)) / 2
Coded S2: L = 462 + ((b * 8) * 2)
Coded S8: L = 720 + ((b * 8) * 8)

Afterwards the number of expected packets Pexp for the test duration can be calculated. For this the duration of the reception in us is required (=> t).
Pexp = t / I(L)

For PER caluclation the missing number of packets is divided by the expected packet number. Lets say the number of receive packets is c.
PER = Pexp - c / Pexp

Example with typical values:
Phy: 1MBit
Length: 37 Bytes
Reception Duration: 1s
Received Packet Count: 1570

L = 80 + (37 * 8) = 376
I(L) = ceil((376 + 249) / 625) * 625us = 625us
Expected Packet Count: Pexp = 1000000 / 625 = 1600
PER = (1600 - 1570) / 1600 = 0.01875 = 1.875%

We have written our own DTM tool that can be used with devices compatible to the bluetooth specification and our own Bluetooth Tester product. There we calculate the PER for any measurement. I think that I've seen other DTM tools that do that the same way.

Regards
Adrian