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Altera_Forum's avatar
Altera_Forum
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17 years ago

OFDM using DSP Builder

Hi,

I'm trying to implement a simplistic (4 channel) QPSK OFDM transmitter/receiver using Altera's DSP Builder and Megacore IP in Simulink.

Let me just mention that I have very basic knowledge of OFDM and pretty much any book/reference material I read has too much detail and goes over my head. So correct me anywhere I start going off the track.

Let me explain what I have so far (implementing what I have learned so far):

Bits to Trasnmit: 11001011

Since I am using 4 channels (all QPSK), I can transmit 8 bits in parallel. These bits get split up into pairs of two and go into seperate channel busses. Now I have:

Channel 1: 11

Channel 2: 00

Channel 3: 10

Channel 4: 11

These bits get encoded into symbols (using Gray Coded QPSK). So now the channels contain:

Channel 1 I: +0.7071

Channel 1 Q: +0.7071

Channel 2 I: -0.7071

Channel 2 Q: -0.7071

Channel 3 I: +0.7071

Channel 3 Q: -0.7071

Channel 4 I: +0.7071

Channel 4 Q: +0.7071

*note: each I/Q bus is 18 bits

Now I know I need to send the data into the FFT block. The FFT block takes in a real and imag input and I'm completely lost.

Reading the FFT block documentation, I need to scale the input by (2^data_precission - 1). The output should also be scaled by (2^-exp_out / 2^data_precission - 1).

Any help or hint at the right direction would be greatly appreciated.

19 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    You simply need to go back to:

    Channel 1: 11

    Channel 2: 00

    Channel 3: 10

    Channel 4: 11

    Instead of the +0.7071 and -0.7071 values, you may transmit the 1 and 0 values to the FFT engine.

    If you don't have pilot tones and you need only to transmit your payload data, that is simply what you need to do.

    Of course the output scaling can be done by choosing the appropriate output dynamic range and pushing the maximum and minimum binary values into the FFT engine instead of 1 and 0.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    I believe you mean (ifft) if Tx.

    I don't see how you send 1 & 0 to ifft unless it is (mapper + ifft).

    You have done some mapping to I/Q then just scale it to fft input.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    I also have a question related to this, if you are using the streaming architecture for the FFT block (we would use it here wouldn't we?), should we be using the Avalon-ST interface?

    I don't quite understand the Avalon-ST, and all the examples I found on the web are from Quartus II, not DSP Builder.

    And since the FFT (streaming mode) can only take a max of an 24-bit bus for the Real and Imaginary inputs, we cannot just simply concatenate the output bits after mapping the symbols to their real and imaginary parts, how can this be solved?
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    I don't use DSP builder(& I don't know anybody using it in the industry, only saw the party here!).

    If data precision is 24 for either Im/Re then it is more than enough.

    I believe by "streaming fft" altera means a continous output though processing is block based. I can use two fft cores swapping the processing resulting in continuous output but after initial delay. You can control this double processing yourself without the need for avalon hand.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Thanks for your replies kaz, as for using DSP Builder, I don't have much of a choice since we are suppose to be learning it (Senior Project) and hopefully in the future it will become better; I hope so, I don't want to learn this and never use it!

    I think I know why most people in the industry don't bother with DSP builder, it's more expensive if you don't have a Matlab license and you have to get used to the Simulink way of doing things where you have to make sure your inputs/outputs match a certain type of signal, and I'm still not used to connecting blocks together, I feel like I'm in grade-school. :)
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    I didn't want to discourage. It is great to have design acceleration tools to keep managers happy then lose your job at the end. We all have licenses and companies will pay for such tools readily. We all use cores but there is tool race and a variety(C => HDL, C => fpga,Matlab => fpga,Labview => fpga,

    ... and vendor cores). Of these vendor cores are popular in the industry but as go up you lose the actual understanding of underlying concepts and get engaged with tool related problems and this slows it down.

    May be the new generation will make better use of it.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    Thank you all for your replies.

    *Correction about first post: I'm using the FFT block as an IFFT. As per OFDM design, the receiver contains an IFFT.

    orchestradirector:

    I don't understand what you mean when you say that I can transmit 1 and 0 values to the IFFT. In this case, where does the QPSK "constellation mapping" come in?

    kaz:

    I do I/Q mapping (QPSK). I mapped two bits into a symbol using QPSK. I and Q for each channel get the real and imaginary value. Based on my scenario, I have the following data after I/Q mapping:

    Channel 1 I: +0.7071

    Channel 1 Q: +0.7071

    Channel 2 I: -0.7071

    Channel 2 Q: -0.7071

    Channel 3 I: +0.7071

    Channel 3 Q: -0.7071

    Channel 4 I: +0.7071

    Channel 4 Q: +0.7071

    My overall question is: Based on my scenario, how do I input the data into the IFFT (Altera FFT Block) and what would the data look like?
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    I can tell you about scaling for ifft but don't know about other signals around your ifft...I leave that to you.

    for n bits scale as follows(for I or Q):

    value = round(value*2^(n-1)-1)

    connect I to real input, Q to imaginary input.(you may reverse that for test)

    As to output: the best test is to check results numerically against Matlab function:

    y=ifft(complex(I,Q)) using the same scale inputs

    compare y(real and imaginary) to your output

    Just a simple way to understand ofdm idea:

    The mapped data is assumed in frequency domain then you do ifft to go to time domain. Remember one sinusoid in time domain corresponds to one constant value in frequency domain. So instead of generating tones in time domain you generate them in frequency domain through the ifft input vector, just put your constants or values there but needs correct spacing...each value results in one tone at the ifft output at corresponding frequency. As such ofdm can generate thousands of carriers without any NCO. and it goes on....the point here is one carrier only modulates one symbol while in classic modulators one carrier modulates a lot of symbols. The mass of tones cancel each other due to orthogonality and one symbol per carrier is left out clean that doesn't spread around.

    Kaz
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi zeep25,

    I just wanted to tell you that any kind of QPSK mapping (after appropriate scaling) will lead you to send to each channel of the IFFT block a +[quantity] and a -[quantity]. In order to maximize your IFFT output (exploit the whole dynamic range) you may simple send your ones and zeros as the maximum and minimum possible values depending on the chosen resolution.

    Example: if your IFFT input is 8-bits wide, an the values are unsigned, the zero will be 0x00 and the maximum will be 0x11. Of course take care if using 2s complement notation, these values must be converted into 2s complement.

    The IFFT output can be scaled if its amplitude must be changed depending of what is around your IFFT.

    Maybe I've synthesized my idea in a too short way...

    Cheers

    OD
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    I agree with OD about the idea of full dynamic range and in that case you can scale to +127 & -127 for 8 bits.

    However, in actual systems with possibility of other modulation schemes e.g. QAM then you need to normalise all the levels to get unitary power.

    For QPSK the .707 per I/Q gives a unitary power as I^2+Q^2 = 1

    edit:

    if power levels are different in the final signal, you are likely to have a lot of head butting with the RF engineer especially so with ofdm as peak/average power is very high.

    Kaz