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IWR1642: Maximum Velocity limitation of BPM MIMO

S. Bae
Intellectual 560 points
Part Number: IWR1642

Hello, TI employees.

I am trying to redesign the antenna configuration of IWR1642Boost in order to measure the elevation and azimuth angles simultaneously using IWR1642 chip.

To this end, I moved one tx antenna element vertically by lambda/2. I initially thought using BPM MIMO technique can measure much higher velocity than TDM MIMO.

Because shorter the chirp duration, higher the maximum velocity. In TDM, chirp duration cannot be shorter than 2Tc, where Tc is the shortest chirp duration that IWR1642 can work.

If what I know is wrong, please correct me.

In BPM MIMO, decoding with 2 Tx, 4 Rx is performed during 2 chirp intervals S1 = Sa + Sb, S2 = Sa - Sb, where Sa = S1 + S2  received during t0, Sb = S1 - S2 received during t1.

And Tx1 transmits S1 Tx2 transmits S2 at every interval.

I have question from here. I think S1 at t0 and S1 at t1 are different with an object with a high velocity.

I think the above equations S1=Sa+Sb and S2=Sa-Sb assume that the phase difference of two intervals t0 and t1 is zero. Am I right?

If I am right, I think the maximum velocity when using BPM MIMO technique is limited to a certain value. 

Do we have no difference between using BPM and TDM MIMOs? Which one have better maximum velocity?

Please let know the better solution for measuring the higher velocity with simultaneous elevation and azimuth angles.

Thanks.

  • 0
    TI__Prodigy 565 points

    Hi,

     You are right on the following:

    1) TDM-MIMO results in a decrease in the native maximum velocity measurable from the range-Doppler FFT.

    2) BPM-MIMO (atleast the variant  described in Section 4.2 of the  MIMO radar app-note) also has the same maximum velocity as TDM-MIMO.  So the TDM-MIMO technique (Sec 4.1) and BPM-MIMO (Sec 4.2) have the same maximum velocity. The advantage of the BPM-MIMO is that it allows simultaneous transmission on both TX's and hence greater transmit power (=>better SNR) for the same dwell time.

    With respect to your questions (from 4.2 of the app note) : Yes, the equations S1=Sa+Sb and S2=Sa-Sb gloss over the fact that there will be a  velocity induced phase change  between the two chirp transmissions. This  phase change will be higher , the higher the velocity of the object. There is a "doppler correction" step which has to be done prior to computing S1 and S2.  Doppler correction works as follows:

     a) First the velocity of the object is first estimated from the range-Doppler FFT. 

    b) The estimated velocity is used to compute the velocity induced phase difference between adjacent chirps. (So the phase of Sb is corrected to account for this).

    c) After this correction, the calculation S1=Sa+Sb; S2=Sa-Sb is performed.

  • 0 in reply to Sandeep Rao30
    Intellectual 560 points

    Thank you a lot. I understood it more deeply. 

    Have a nice weekend.

    Best regards

    S. Bae