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Help! give me a example of CYCLIC PREFIX REMOVE in FFTC

Yang Meng
Prodigy 70 points

please give me a example of CYCLIC PREFIX REMOVE in FFTC

I have a test , I  set Queue Cyclic Prefix Register FFTC_QUEUE_X_CYCLIC_PREFIX_REGISTER = 0x80010000(bit CYCLIC_PREFIX_REMOVE_EN = 1 and bit CYCLIC_PREFIX_REMOVE_OFFSET = 1),  the input datas shoud remove the first one(16bit I,16bit Q), but it does not,  why?

it does not get function of CYCLIC PREFIX REMOVE?

thanks!

  • 0 in reply to Yang Meng
    TI__Mastermind 39540 points

    Hi Yang,

     

    What is the size of the DFT you are using?.

    For example if you are doing an 16 point DFT ( on a data of 16 samples) and also if you have set the DFT size in the FFT config register as 16, then after removal of 1 sample you will be left with 15 samples. It doesnt work  this way.

    You will have to provides with 17 samples at the input to FFT co processor  , so that it can remove one of the samples ( which is the case in your exmaple) and perform the 16 point DFT on it. You will be able to see 16 samples at the output.

     

    Thanks,

    Kishore.

  • 0 in reply to Kishore
    Prodigy 30 points

    Hi Faraday

    I am also facing problem while using the CP removal functionality in the FFTC engine.

    I am using 2048-fft. and the CP removal length is 160.

    I am giving 2208 samples as input.

    I have set all the values in the local configuration registers of FFTC correctly and checked also.

    But on simulator mode the engine does not give any output.

    While running the code using EVM , the engine does not discard the first 160 samples rather take the first 2048 samples and gives its FFT (in FFT mode).

    Please suggest the necessary solution.

    Thanks & Regards

    Ravi

  • 0 in reply to Ravi Jain
    Prodigy 105 points
    Hi,
    I am facing the same problem as mentioned by Ravi. I have read some other thread on e2e forum where it was said that CP removal works with single block only. Currently, I am working with single block but still facing the issue with (2048+160) samples .
    Also , if anyone could confirm whether CP removal with FFTC works on simulator .

    Thanks and Regards,
    Harshdeep
  • 0 in reply to Harshdeep Sokhey
    Prodigy 225 points
    Hi,

    CP Removal Does Work on simulator,
    When Cyclic Prefix Removal is enabled , it removes the Last k Samples i.e from the Last kth sample, it takes N samples and performs the FFT.
    k = offset.
    N = DFT SIZE.

    According to:- www.ti.com/.../sprabf3.pdf.
    In this it says "the FFTC takes the last N samples from the CYCLIC_PREFIX_REMOVE_OFFSET when FFT size is N. Since the offset and the
    samples taken are counted from the tail of data buffer in the packet"

    eg. if number of packet size = 144;
    DFT Size (N) = 128;
    Cp Offset = 4;
    Then the last 4 samples are removed, and then the Last N, i.e 128 samples are taken and FFT operation is performed. (samples 13 to 140 is
    considered as the input samples for FFT engine in this case)

    Regards
    A Asha Kumari
  • 0 in reply to Asha Kumari
    TI__Guru 71516 points

    A Asha,

    Thank you for the verification.  We will close this thread now.

    ~Leonard