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CC1101: Preamble Quality Threshold and Sync Word Detection

Ruth Desta
Prodigy 50 points
Part Number: CC1101


Tool/software:

Hello,

I’ve been reading the CC1101 datasheet and had a few questions about the Preamble Quality Threshold (PQT).

From what I understand, setting a >0 PQT value will delay sync word detection until the threshold is met. But if I leave PQT at 0, does that mean the device starts trying to detect the sync word right after it detects that the current bit is different from the previous one? Or does it still wait for a certain number of preamble bytes before it allows sync detection, regardless of PQT? I'd appreciate any deeper explanation of how the PQT threshold influences sync detection timing.

Lastly, are there any tradeoffs to using a higher PQT value? I get that it helps avoid false positives from noise, but does it risk missing valid packets or making detection slower in lower SNR conditions? Any tips on how to tune this threshold for noisy environments without losing sensitivity would be really helpful.

Sincerely,

Ruth

  • +1
    TI__Guru* 86731 points

    Hi Ruth

    Setting PQT to 0 simply means that there is no preamble gating of sync word detection.

    The radio starts looking for sync word immediately after going into RX, and if it finds a sync word, it starts filling up the RXFIFO, regardless of how many preamble bits it detected (does not keep track of this since PQT = 0). Even if you set PQT = 0, you still need to transmit preamble. The receiver uses this preamble for bit synchronization, and will not be able to detect a sync word if there are no preamble preceding it.

    Setting PQT to something other than 0 can be useful in cases where you for example are using a shorter sync word than the recommended 4 bytes.

    If you use for example only 2 bytes sync word, you will see that the rate of false sync detects increases (radio finding sync in random noise). Adding a PQT threshold in these cases will reduce the number of false sync detects, as it will require a certain number of preamble bits before even starting to look for sync.

    By increasing PQT, you decrease the risk of false packets, but at the same time you increase the risk of losing your real packets.

    You need to do testing yourself for your system, in the environment your system will run, to find out where you want to set the threshold, and you need to ask yourself what is most important; Not losing packets, or avoid receiving false packets?

    BR

    Siri

  • 0 in reply to Siri
    Prodigy 50 points

    Hi Siri,

    Thank you for your answer. I just have a few follow up questions.

    1. Can you explain a bit more regarding how bit synchronization works for CC1101?

    2. Does the bit synchornization continue throughout the whole packet or does the process stop after it detects the preamble?

    3. How many bits are needed to finish bit synchronization in low sensitivity environments like -110dBm or-90dBm?

    Sincerely,

    Ruth

  • 0 in reply to Ruth Desta
    TI__Guru* 86731 points

    Hi Ruth

    1) We do not provide info on how things are implemented internal in the chip to do this

    2) From the data sheet: "The bit synchronization algorithm extracts the clock from the incoming symbols. The algorithm requires that the expected data rate is programmed as described in Section 12. Re-synchronization is performed continuously to adjust for error in the incoming symbol rate."

    3) You should use the recommended preamble length from the recommended settings in SmartRF Studio. These are the settings used under characterization, to achieve the sensitivity numbers you can find in the data sheet for the different PHYs.

    BR

    siri