CC1200 LQI and RSSI

Anders

Other Parts Discussed in Thread: CC1200

Hi,

I'm currently trying to understand how LQI is calculated and presented throughout a packet. Btw. I'm using GFSK for modulation in 434MHz band.

I understand it is a metric to see how easily it was to demodulate the data and I have read http://e2e.ti.com/support/low_power_rf/w/design_notes/calculation-and-usage-of-lqi-and-rssi.aspx

Questions :

1. If I choose to append RSSI and LQI bytes to the packet, then I get the same values in the append bytes as the register values if I would read the registers at the end of the packet, correct?

2. If I read the LQI from register while receiving the bytes e.g. at every one or two bytes, then I get different values, so how often is the LQI written to registers and based on what?

3. Do you have a formula on how the LQI is calculated, primarily interested in GFSK/FSK?

4. Sometimes I see LQI value of 0 appended as well as readout from register while receiving what does this mean?, the user guide states that LQI=0 is not valid, how can it be like this because the packet is received correctly.

5. Also how often is RSSI written to register and how long time is the RSSI based on?

Best regards,

Anders

over 10 years ago

0 TER over 10 years ago

TI__Guru**** 317180 points

1) Yes

2) I have to check that up

3) No. The LQI is the distance in the constellation between the received signal and the ideal. Hence the distance can be 0 for strong signals. What I have seen is that the LQI only gives a value different for 1 (0) when the signal is closer to the sensitivity limit than ~10 dB.

5) You can find some information about this here: http://www.ti.com/lit/zip/swra428/ http://www.ti.com/lit/an/swra413/swra413.pdf

0 Anders over 10 years ago in reply to TER

Prodigy 210 points

Hi TER,

Thanks for your answers

2) Please get back once you have clarified

3. Does the LQI have any unit and e.g what would a LQI=2 or 20 mean in terms of distance between ideal and received? Also how big can the LQI be before the receiver can not perform correct demodulation? Your observation of LQI = 0/1 is also what I see on CC1200->CC1200 above sensitivity limit However you might consider changing the register description of LQI as it states that LQI=0 is not valid and what we see is that LQI=0 means received=ideal.

/Anders

0 Anders over 10 years ago in reply to Anders

Prodigy 210 points

Hi TER,

Did you find the answers for my questions?

/Anders

0 TER over 10 years ago in reply to Anders

TI__Guru**** 317180 points

Sorry, checked it but forgot to answer. The feedback I got is that LQI is updated every 32 symbols so it's a bit strange that you read different values.

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CC1200 LQI and RSSI