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CC1312R: What's the relationship between RSSI and LQI for CC1312?

Part Number: CC1312R

Hi TI,

We are working on the CC1312 with SDK version simplelink_cc13x2_sdk_2_20_00_71.

From SDK, we can get RSSI and LQI from rssi and mpduLinkQuality member of ApiMac_mcpsDataInd_t structure.

However, the same RSSI seems not to correspond to the same LQI.

For eample, the same RSSI is sometimes with good LQI, and other times, it is with bad LQI.

So we want to know the relationship between RSSI and LQI and how does TI 15.4 stack calculate LQI?

Thanks!

Best regards,

Vince

  • Hi,

    The formula that calculates the LQI is the following:
    LQI = (MAC_SPEC_ED_MAX * (RSSIdbm - ED_RF_POWER_MIN_DBM)) / (ED_RF_POWER_MAX_DBM - ED_RF_POWER_MIN_DBM);
    where
    MAC_SPEC_ED_MAX = 255
    ED_RF_POWER_MIN_DBM = -87
    ED_RF_POWER_MAX_DBM = 10

    Then if my math is correct, the formula for RSSI should be

    RSSI in Dbm = ((LQI*97)/255) - 87


    In theory you should always get the same LQI if you always have the same RSSI.
  • Hi Hector,

    Thanks for your response.
    The RX sensitivity of CC13x2 is –109dbm.
    It means rx side can receive packet if the RSSI of the packet is higher than –109dbm.
    However, from this equation, ED_RF_POWER_MIN_DBM is only -87dbm.
    It means if RSSI is lower than -87dbm, all LQI will be 0.

    This is unreasonable because rx side can still receive packet even when LQI is 0.
    I think the reasonable value of ED_RF_POWER_MIN_DBM is -109dbm.
    Can you help to explain?
    Thanks!
  • this is the default equation that we use, LQI is up to interpretation so in this case an LQI of 0 means a bad link quality given that if the link quality gets any worse than that we would not be able to receive the packet therefore there would not be an LQI