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/* -*- c++ -*- */
/*
* Copyright 2006,2007,2011 Free Software Foundation, Inc.
*
* This file is part of GNU Radio
*
* GNU Radio is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3, or (at your option)
* any later version.
*
* GNU Radio is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GNU Radio; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
#ifndef INCLUDED_DIGITAL_OFDM_FRAME_ACQUISITION_H
#define INCLUDED_DIGITAL_OFDM_FRAME_ACQUISITION_H
#include <gr_block.h>
#include <vector>
class digital_ofdm_frame_acquisition;
typedef boost::shared_ptr<digital_ofdm_frame_acquisition> digital_ofdm_frame_acquisition_sptr;
digital_ofdm_frame_acquisition_sptr
digital_make_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
unsigned int cplen,
const std::vector<gr_complex> &known_symbol,
unsigned int max_fft_shift_len=10);
/*!
* \brief take a vector of complex constellation points in from an FFT
* and performs a correlation and equalization.
* \ingroup demodulation_blk
* \ingroup ofdm_blk
*
* This block takes the output of an FFT of a received OFDM symbol and finds the
* start of a frame based on two known symbols. It also looks at the surrounding
* bins in the FFT output for the correlation in case there is a large frequency
* shift in the data. This block assumes that the fine frequency shift has already
* been corrected and that the samples fall in the middle of one FFT bin.
*
* It then uses one of those known
* symbols to estimate the channel response over all subcarriers and does a simple
* 1-tap equalization on all subcarriers. This corrects for the phase and amplitude
* distortion caused by the channel.
*/
class digital_ofdm_frame_acquisition : public gr_block
{
/*!
* \brief Build an OFDM correlator and equalizer.
* \param occupied_carriers The number of subcarriers with data in the received symbol
* \param fft_length The size of the FFT vector (occupied_carriers + unused carriers)
* \param cplen The length of the cycle prefix
* \param known_symbol A vector of complex numbers representing a known symbol at the
* start of a frame (usually a BPSK PN sequence)
* \param max_fft_shift_len Set's the maximum distance you can look between bins for correlation
*/
friend digital_ofdm_frame_acquisition_sptr
digital_make_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
unsigned int cplen,
const std::vector<gr_complex> &known_symbol,
unsigned int max_fft_shift_len);
protected:
digital_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
unsigned int cplen,
const std::vector<gr_complex> &known_symbol,
unsigned int max_fft_shift_len);
private:
unsigned char slicer(gr_complex x);
void correlate(const gr_complex *symbol, int zeros_on_left);
void calculate_equalizer(const gr_complex *symbol, int zeros_on_left);
gr_complex coarse_freq_comp(int freq_delta, int count);
unsigned int d_occupied_carriers; // !< \brief number of subcarriers with data
unsigned int d_fft_length; // !< \brief length of FFT vector
unsigned int d_cplen; // !< \brief length of cyclic prefix in samples
unsigned int d_freq_shift_len; // !< \brief number of surrounding bins to look at for correlation
std::vector<gr_complex> d_known_symbol; // !< \brief known symbols at start of frame
std::vector<float> d_known_phase_diff; // !< \brief factor used in correlation from known symbol
std::vector<float> d_symbol_phase_diff; // !< \brief factor used in correlation from received symbol
std::vector<gr_complex> d_hestimate; // !< channel estimate
int d_coarse_freq; // !< \brief search distance in number of bins
unsigned int d_phase_count; // !< \brief accumulator for coarse freq correction
float d_snr_est; // !< an estimation of the signal to noise ratio
gr_complex *d_phase_lut; // !< look-up table for coarse frequency compensation
void forecast(int noutput_items, gr_vector_int &ninput_items_required);
public:
/*!
* \brief Return an estimate of the SNR of the channel
*/
float snr() { return d_snr_est; }
~digital_ofdm_frame_acquisition(void);
int general_work(int noutput_items,
gr_vector_int &ninput_items,
gr_vector_const_void_star &input_items,
gr_vector_void_star &output_items);
};
#endif
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