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IWR1443: Sensing Estimator: Change in sampling frequency when increase in TX

Santhana Raj
Expert 1765 points
Part Number: IWR1443

Hi,

In mmWave Sensing estimator, When I increase the no of TX from 1 to 2 in the configuration, I see that the below changes,

1. The bandwidth increases by almost 2.3 times

2. The sampling frequency increases by almost 3 times.

3. The IF frequency also increases by 3 times.

I would like to know whether this increase is because it assumes the TX to be operating at the same time?? Otherwise, I see no reason for this increased values. The no of samples per chirp in these cases remain the same.

Since I am not planning to use the TX at the same time, and will be operating in time multiplexed format, I dont see the requirement to increase the bandwidth, sampling rate or IF frequency.  The increase in frame time and radar cube size is acceptable.

If there any mistake in my assumption??

Thanks in Advance,

Santhana Raj

  • 0
    TI__Mastermind 30405 points

    Hi Santhana,

    The Sensing Estimator does not assume transmitters to be operating at the same time.

    1. Let's assume that with a certain chirp time Tc, you can sample a certain Max Velocity Vmax in non-MIMO case.
    2. Velocity is a function of chirp time Tc, and if you want to maintain the same max velocity with 2 TDM TXs, then each TX needs to complete it's chirp in half the time to maintain the same effective chirp time Tc. In other words, to maintain the same Max velocity in 2TDM MIMO, 2TXs need to chirp in time Tc, thus reducing the chirp time for each transmitter to Tc/2 (or Tc/3 in 3 TX TDM MIMO). 
    3. Keeping everything else the same (Range, Range Resolution, and Velocity), this naturally means that the chirp slope is higher which in turns results in faster sampling frequency and thus higher IF bandwidth.
    4. If you want to keep the same sampling frequency and IF bandwidth as in the non-MIMO case, you need to divide the maximum velocity by the number of transmitters being used in TDM MIMO (2 or 3). For example, if your max velocity with a single TX is 150 Kmph, it'll be 75 Kmph in 2TDM MIMO. 
    5. There are techniques (known as Vmax extension algorithms) which can be applied in higher layer processing to increase the maximum velocity in MIMO case.

    Hope that answers your question.

    Regards

    -Nitin

  • 0 in reply to Nitin Sakhuja
    Expert 1765 points
    Hi Nitin,

    A short follow up question.
    Does this mean that, If I have the multiple TX operating in simultaneous mode, then my sampling rate wont increase.??


    Thank you for the explanation. I made a mistake in considering which region for a chirp. thank you for pointing out my mistake.

    Thanks again.
    Santhana Raj
  • 0 in reply to Santhana Raj
    TI__Mastermind 30405 points
    Hi Santhana,

    It was not clear if you need further support on this question. Can you please clarify?

    Thanks
    -Nitin
  • 0 in reply to Nitin Sakhuja
    Expert 1765 points

    HI Nitin,

    I want to know whether I can use simultaneous TX mode in MIMO to achieve better range resolution and velocity than what I would achieve with TD MIMO.

    Thanks again.

    Santhana Raj

  • 0 in reply to Santhana Raj
    TI__Mastermind 30405 points
    Hi Santhana,

    Yes, simultaneous MIMO does not reduce the maximum velocity as TDM MIMO does (keeping everything else the same). However, you need to consider the following:

    1. When using simultaneous MIMO, you will need to use BPM (Binary Phase Modulation) functionality in order to separate TX1 from TX2 from the reflected signal. The other option is to use simultaneous MIMO without BPM but then you sacrifice angular resolution as you don't have the virtual RX antennas in that case.
    2. With BPM, the transmit signal from each TX is coded with a PN sequence and decoding is required in the receiver to get the information from each TX. Customers are required to design their own PN sequence.
    3. BPM MIMO should not limit the max velocity as TDM MIMO. However, this assumption is valid only with a good PN sequence. If you use [+1 -1 +1 -1 +1 -1 …] as the PN sequence, it still requires two chirps to decode the two TX, so the max velocity is still half that of non-MIMO.

    Hope that answers your questions.

    Regards
    -Nitin
  • 0 in reply to Nitin Sakhuja
    Expert 1765 points
    Thank you Nitin.

    I understand the implications of BPM MIMO now more clearly, that when i have read the TI's app note on the same topic.

    I have one more small question, if you can.
    Can you provide me any technical detailed materials that would help me on deciding the PN sequence?

    Thanks & Regards,
    Santhana Raj
  • 0 in reply to Santhana Raj
    TI__Mastermind 30405 points
    Hi Santhana,

    Right now, we don't have any material related to selection of PN sequences.

    Regards
    -Nitin