ofdm.cpp

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#include <itpp/comm/ofdm.h>
#include <itpp/base/specmat.h>
#include <itpp/base/operators.h>
#include <itpp/signal/transforms.h>


namespace itpp
{

OFDM::OFDM(int inNfft, int inNcp, int inNupsample)
{
  set_parameters(inNfft, inNcp, inNupsample);
}

void OFDM::set_parameters(const int inNfft, const int inNcp, const int inNupsample)
{
  it_assert(inNfft >= 2, "OFDM: Nfft must be >=2.");
  it_assert(inNcp >= 0 && inNcp <= inNfft, "OFDM: Ncp must be >=0 and <=Nfft.");
  it_assert(inNupsample >= 1 && inNupsample <= 100, "OFDM: Ncp must be >=1 and <=100.");
  Nfft = inNfft;
  Ncp = inNcp;
  Nupsample = inNupsample;
  norm_factor = std::sqrt(static_cast<double>(Nupsample * Nfft * Nfft) / (Nfft + Ncp));
  setup_done = true;
}

void OFDM::modulate(const cvec &input, cvec &output)
{
  it_assert(setup_done == true, "OFDM::modulate: You must set the length of the FFT and the cyclic prefix!");
  const int N = input.length() / Nfft;
  it_assert(N*Nfft == input.length(), "OFDM::modulate: Length of input vector is not a multiple of Nfft.");

  output.set_length(Nupsample*N*(Nfft + Ncp));
  cvec outtemp(Nfft);

  for (int i = 0; i < N; i++) {
    outtemp = ifft(concat(input.mid(i * Nfft, Nfft / 2), zeros_c(Nfft * (Nupsample - 1)),
                          input.mid(i * Nfft + Nfft / 2, Nfft / 2))) * norm_factor;
    output.replace_mid(Nupsample*(Nfft + Ncp)*i, concat(outtemp.right(Nupsample*Ncp), outtemp));
  }
}

cvec OFDM::modulate(const cvec &input)
{
  cvec output;
  modulate(input, output);
  return output;
}

void OFDM::demodulate(const cvec& input, cvec &output)
{
  it_assert(setup_done == true, "OFDM::demodulate: You must set the length of the FFT and the cyclic prefix!");
  const int N = input.length() / (Nfft + Ncp) / Nupsample;
  it_assert(Nupsample*N*(Nfft + Ncp) == input.length(), "OFDM: Length of input vector is not a multiple of Nfft+Ncp.");

  output.set_length(N*Nfft);
  // normalize also taking the energy loss into the cyclic prefix into account
  for (int i = 0; i < N; i++) {
    cvec x = fft(input.mid(Nupsample * (i * (Nfft + Ncp) + Ncp), Nupsample * Nfft));
    output.replace_mid(Nfft*i, concat(x.left(Nfft / 2), x.right(Nfft / 2)) / norm_factor);
  }
}

cvec OFDM::demodulate(const cvec &input)
{
  cvec output;
  demodulate(input, output);
  return output;
}

} //namespace itpp