Using the Access Code Correlator to determine the start of a RX Packet

Original Post: Access Code Correlator (eGroups Msg. ) Date: 2000-09-16

The specs state that 'In the receiver of the Bluetooth unit, a sliding correlator correlates against the access code and triggers when a threshold is exceeded. This trigger signal is used to determine the receive timing'

Essentially this means that the Access Code Correlator matches the received word with the expected one, and when the match crosses a threshold, it declares start of a valid packet. This means that correlation is a continuous process which is carried over a number of received data bits. In the receiver of the Bluetooth unit, a sliding correlator correlates against the access code and triggers when a threshold is exceeded. This trigger signal is used to determine the receive timing.

When your correlator give an indication that the threshold is reached, that's where the RX packet starts. Assume during the +- 10 us uncertainty window, we have:

..........Access Code|Packet Header|Payload..............
|<---- +- 10 us ------>|

As the Receiver must do the correlation between this received data stream (with a locally generated Access Code from the start of the +-10us window), the output of your correlator should pass over a threshold when the locally generated Access Code aligns with the received Access Code, this thus, is where the packet starts.

You have to choose the threshold yourself, there is no regulation for that (and it can be adaptive).

Even if you change the correlator threshold, the start position of the packet can still be determined. The correlation is performed on all 68 bits. If for example the threshold is set for 60 bits, and a packet is received that has 8 bit errors in the access code, the correlation trigger point will only occur after receiving the 68th bit - after receiving the 67th bit, few of the bits will correlate because the whole 68 bit pattern is offset by one bit. After receiving the 68th bit then 60 of the bits will match and the threshold is reached. By lowering the threshold, it is more likely that a correlation will occur on random noise, or on other packet contents.