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CC1101: LQI differs significantly between different radios

Part Number: CC1101

Dear Team,

Our customer noticed, that when using different radios with CC1101 - i.e. Silabs or ST chips, they get different, sometimes bad, LQI values.

How exactly is LQI calculated and which RF parameters influence it? I mean - if we have two different radios vendors, which params should be set exactly the same to get best LQI value? Maybe there is some internal app note on this param?

Thanks!

Daniel

  • From the CC1101 datasheet:

    "The LQI gives an estimate of how easily a received signal can be demodulated by accumulating the magnitude of the error between ideal constellations and the received signal over the 64 symbols immediately following the sync word. LQI is best used as a relative measurement of the link quality (a low value indicates a better link than what a high value does), since the value is dependent on the modulation format."

    If they receive a signal with CC1101 the LQI should be close to equal if the incoming signal is equal, meaning that the modulation format, deviation and symbol rate is set to the same on both the TX and RX side. 

    i.e. Silabs or ST chips, they get different, sometimes bad, LQI values.

    How is this test done? Do they get low LQI values if they use a CC1101 as TX at the same position as the other radios?  

  • Hi,

    Yes, I saw that in the datasheet, but do we have a formula available for how this is determined? Is it derived from RSSI or a more complicated formula?

    Thanks,

    Daniel

  • Yes and no. As the text in the datasheet state, this is a distance from the ideal point in the constellation diagram Meaning that in the modem part of the chip this is calculated but this is basically just a number which is why we state that this should be used for relative measurement. When I have looked at the LQI it has typically been a close to zero value when the input signal is higher than ~3 dB above sensitivity limit. 

    Do you have the details on how this is tested? 

  • Thanks! I will let the customer know. I did ask about implementation details, but the customer said that they strictly wanted to know the theory behind LQI. I will get back to you with the customer's feedback