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Altera_Forum
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11 years ago

cross correlation of OFDM

hi i have been trying to implement multiplierless cross correlation of OFDM for IEEE 802.16d and i am stuck on the part as to how should i represent my correlator coeffiients . as they are in complex for such as 16 correlator coeffiecients -1,-1j .... for quantization level 1 and what should be my known preamble as it should also be of 16 samples. i am currently confused about this part i have bee reading various paper on it and none specify how to represent correlator coefficients in binary form and what should it be correlated with can you please help

18 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    my correlator coefficients are gm=-1,-1j,1,1,1,-1j,-1,0,-1j,-1,1j,1j,1j,-1,-1j,0 and it needs to be correlated with 16 signal samples to generate one correlator output. if anyone can put me in the right direction it would be great

  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    --- Quote Start ---

    my correlator coefficients are gm=-1,-1j,1,1,1,-1j,-1,0,-1j,-1,1j,1j,1j,-1,-1j,0 and it needs to be correlated with 16 signal samples to generate one correlator output. if anyone can put me in the right direction it would be great

    --- Quote End ---

    Away from specifics of your case a complex correlator is based on sum of products just like complex FIR.

    every real output sample = sum of (Re1.Re2 - Im1.Im2) for all 16 elements

    every imag output sample = sum of (Re1.Im2 + R2I.m1) for all 16 elements

    where Re1 is real input and Im1 is imag input, Re2 is real coeff, Im2 is imag coeff

    since your coeffs are zeros or ones you don't need scaling but 0 means zero result and ones mean pass input
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    --- Quote Start ---

    Away from specifics of your case a complex correlator is based on sum of products just like complex FIR.

    every real output sample = sum of (Re1.Re2 - Im1.Im2) for all 16 elements

    every imag output sample = sum of (Re1.Im2 + R2I.m1) for all 16 elements

    where Re1 is real input and Im1 is imag input, Re2 is real coeff, Im2 is imag coeff

    since your coeffs are zeros or ones you don't need scaling but 0 means zero result and ones mean pass input[/QUOTE

    hey thank you for your reply one confusion as when we do cross correlation i need a fixed input too with which correlation will be done of the coefficients i read papers and it suggests as OFDM is my application i will get that input from there is it?

    and i m willing to design a multiplierless correlator using shift and add how could that be done. i am planning on designing a correlator using binary bits which gives better performance and computation and uses less area. so i am planning to do it by correlating two binary inputs and getting the putput using shift and add do you have any idea how should i proceed with it
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    hey thank you for your reply one confusion as when we do cross correlation i need a fixed input too with which correlation will be done of the coefficients i read papers and it suggests as OFDM is my application i will get that input from there is it?

    and i m willing to design a multiplierless correlator using shift and add how could that be done. i am planning on designing a correlator using binary bits which gives better performance and computation and uses less area. so i am planning to do it by correlating two binary inputs and getting the putput using shift and add do you have any idea how should i proceed with it

    and yes with your idea what should be my other input for correlation .i am quite new to this
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    --- Quote Start ---

    hey thank you for your reply one confusion as when we do cross correlation i need a fixed input too with which correlation will be done of the coefficients i read papers and it suggests as OFDM is my application i will get that input from there is it?

    and i m willing to design a multiplierless correlator using shift and add how could that be done. i am planning on designing a correlator using binary bits which gives better performance and computation and uses less area. so i am planning to do it by correlating two binary inputs and getting the putput using shift and add do you have any idea how should i proceed with it

    and yes with your idea what should be my other input for correlation .i am quite new to this

    --- Quote End ---

    Your input is the OFDM received signal (you are designing part of a working receiver hopefully). Your coeffs are those you have given and are I assume standard for your preamble. From where did you get those nice coeffs, I don't know about them and why are they zeros and ones only.

    You can add directly in parallel (leads to very wide adder so requires pipelining) or use shift_add (transposed structure). There ia plent around about either structure.
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    yes i was working on that previously but designing a working receiver of OFDM was too much work so now i am focussing on just creating a multiplierless correlator that is by using shift and add. i read about it ...is it not possible to do cross correlation on two binary numbers coz i read that only floating or fixed point input is used for that? as i want to use 8 bit numbers and then get to the correlator output using shift add and how to do i proceed with that. can you provide with some data or idea how to do it

    and about the OFDM coefficients i read a paper about multiplierless correlation of OFDM if you want i can send you that
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    8 bit data is fixed point most likely

    You can attach your ofdm doc here or to my email accessed from forum.
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    --- Quote Start ---

    you can find the paper here on this link

    http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1205462&url=http%3a%2f%2fieeexplore.ieee.org%2fiel5%2f30%2f27133%2f01205462.pdf%3farnumber%3d1205462

    and do you have ideas about reliable shift add techniques

    --- Quote End ---

    Your case is straightforward addition of 16 values. It can be done as shift add or add in parallel. Either will work.

    Look at FIR structures (direct and transposed, the direct is parallel addition of products, the transposed is shift add of products).

    There is a lot around on the internet.

    In your case:

    you have no mults and each product result is either 0 or +input value or -input value(inverted)

    you need to check orientation of received signal with that of coeffs. You either use coeffs as 1~ 16 or 16~1. One will be correct.
  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    hi thank you kaz i have been trying to understand add and shift for multiplication i have two inputs say 29 and 8 in binary and want to get the multiplied output using add and shift i tried double shift and add and many methods but having problems if i change inputs