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AWR1642: range profile dB offset

Kei
Intellectual 750 points
Part Number: AWR1642

Hello, TI team,

I have a question about Y-axis of range profile.

According to the URL below, Y-axis of range profile is calculated by the following formula.
Value in dB = received value * 20*log10(2) / (256 * numTxAnt * numRxAnt) + some dB offset related to FFT size scale compensation in 1DFFT and 2DFFT

I understand the first term (received value * 20*log10(2) / (256 * numTxAnt * numRxAnt) ) but I don't understand 2nd term (some dB offset).

I referred to dspFftScalComp1() and dspFftScalComp2() in mmWave.js, so I got the formula, but I don't understand the meaning of the formula.
Could you tell me what is the some dB offset?

e2e.ti.com/.../awr1642-how-to-deal-with-raw-data-in-mmw-demo

  • +1
    TI__Guru*** 148804 points

    Hi,

    During the FFT processing the data may be scaled down to avoid overflow. This is the reason why there is a compensation for display

    thank you

    Cesar

  • 0 in reply to Cesar
    Intellectual 750 points

    Hi, 

    Thank you for your answer. I understand.

  • 0 in reply to Kei
    TI__Guru*** 148804 points

    Hi,

    Please give me some time to check with the team why the offset is negative

    thank you
    Cesar

  • 0 in reply to Cesar
    Intellectual 750 points

    Hi,

    Do you have any updates on this?

    Thank you,

  • +1
    TI__Guru*** 148804 points
    Yes,
     
    Sorry for the delay.
     
    Here is some feedback I have received from one of the team members:
     
    “As I remember the idea was for a particular target to have the same peak height in the range profile window  for any combination of range and Doppler FFT size. In a DSP the range FFT function 16x16 the radix-4 stage scaling is equal to ((½)^ceiling[log4(N)-1]), and in Doppler FFT (32 bit function) there is no scaling.
    For a ADC signal (sine wave with a constant amplitude) the range profile peak height depends on the FFT sizes. I think we picked the smallest configurable Range and smallest Doppler FFT sizes, and relative to that combination we calculate the compensation scale for any other Range and Doppler FFT size combination.
     
    The peak in range profile for a tone with amplitude A, range FFT size NRange, and Doppler FFT size NDoppler, would be:
     
    P = log2(A * NRange * ((½)^ceiling[log4(NRange)-1]) * NDoppler) * numAntennas  * 512/(20*log10(2))
     
    In a sequence we calculate range FFT, then Doppler FFT, convert to log2 magnitude in Q9 format and sum across antennas and then convert to decibels (scale up by (20log10(2)).
     
    So to see the constant peak height on the plot for any combination of FFT sizes, the scale is calculated relative to  NRangeMin and NDopplerMin  (I believe it was 64 and 16) and then applied to the received range profile.”
     
    Thank you
    cesar
     
     
  • 0 in reply to Cesar
    Intellectual 750 points

    Hi, 
    Thank you for your answer. I understand.

  • 0 in reply to Kei
    TI__Guru*** 148804 points

    great!