ofdm_sync_sc_cfb.h

Go to the documentation of this file.

/* -*- c++ -*- */
/*
 * Copyright 2013 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
 
#ifndef INCLUDED_DIGITAL_OFDM_SYNC_SC_CFB_H
#define INCLUDED_DIGITAL_OFDM_SYNC_SC_CFB_H
 
#include <gnuradio/digital/api.h>
#include <gnuradio/hier_block2.h>
 
namespace gr {
namespace digital {
 
/*!
 * \brief Schmidl & Cox synchronisation for OFDM
 * \ingroup ofdm_blk
 * \ingroup synchronizers_blk
 *
 * \details
 * Input: complex samples.
 * Output 0: Fine frequency offset, scaled by the OFDM symbol duration.
 *           This is \f$\hat{\varphi}\f$ in [1]. The normalized frequency
 *           offset is then 2.0*output0/fft_len.
 * Output 1: Beginning of the first OFDM symbol after the first (doubled) OFDM
 *           symbol. The beginning is marked with a 1 (it's 0 everywhere else).
 *
 * The evaluation of the coarse frequency offset is *not* done in this block.
 * Also, the initial equalizer taps are not calculated here.
 *
 * Note that we use a different normalization factor in the timing metric than
 * the authors do in their original work[1]. If the timing metric (8) is
 * \f[
 * M(d) = \frac{|P(d)|^2}{(R(d))^2},
 * \f]
 * we calculate the normalization as
 * \f[
 * R(d) = \frac{1}{2} \sum_{k=0}^{N-1} |r_{k+d}|^2,
 * \f]
 * i.e., we estimate the energy from *both* half-symbols. This avoids spurious detects
 * at the end of a burst, when the energy level suddenly drops.
 *
 * [1] Schmidl, T.M. and Cox, D.C., "Robust frequency and timing synchronization for
 * OFDM", Communications, IEEE Transactions on, 1997.
 */
class DIGITAL_API ofdm_sync_sc_cfb : virtual public hier_block2
{
public:
    typedef std::shared_ptr<ofdm_sync_sc_cfb> sptr;
 
    /*! \param fft_len FFT length
     *  \param cp_len Length of the guard interval (cyclic prefix) in samples
     *  \param use_even_carriers If true, the carriers in the sync preamble are occupied
     * such that the even carriers are used (0, 2, 4, ...). If you use all carriers, that
     * would include the DC carrier, so be careful. \param threshold detection threshold.
     * Default is 0.9.
     */
    static sptr
    make(int fft_len, int cp_len, bool use_even_carriers = false, float threshold = 0.9);
 
    virtual void set_threshold(float threshold) = 0;
    virtual float threshold() const = 0;
};
 
} // namespace digital
} // namespace gr
 
#endif /* INCLUDED_DIGITAL_OFDM_SYNC_SC_CFB_H */