/* -*- c++ -*- */ /* Copyright 2012 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_EQUALIZER_SIMPLEDFE_H #define INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H #include <digital/api.h> #include <digital/constellation.h> #include <digital/ofdm_equalizer_base.h> namespace gr { namespace digital { /* \brief Simple decision feedback equalizer for OFDM. * \ingroup ofdm_blk * \ingroup eq_blk * * Equalizes an OFDM signal symbol by symbol using knowledge of the * complex modulations symbols. * For every symbol, the following steps are performed: * - On every sub-carrier, decode the modulation symbol * - Use the difference between the decoded symbol and the received symbol * to update the channel state on this carrier * - Whenever a pilot symbol is found, it uses the known pilot symbol to * update the channel state. * * This equalizer makes a lot of assumptions: * - The initial channel state is good enough to decode the first * symbol without error (unless the first symbol only consists of pilot * tones) * - The channel changes only very slowly, such that the channel state * from one symbol is enough to decode the next * - SNR low enough that equalization will always suffice to correctly * decode a symbol * If these assumptions are not met, the most common error is that the * channel state is estimated incorrectly during equalization; after that, * all subsequent symbols will be completely wrong. * * Note that the equalized symbols are *exact points* on the constellation. * This means soft information of the modulation symbols is lost after the * equalization, which is suboptimal for channel codes that use soft decision. * */ class DIGITAL_API ofdm_equalizer_simpledfe : public ofdm_equalizer_1d_pilots { public: typedef boost::shared_ptr<ofdm_equalizer_simpledfe> sptr; ofdm_equalizer_simpledfe( int fft_len, const gr::digital::constellation_sptr &constellation, const std::vector<std::vector<int> > &occupied_carriers = std::vector<std::vector<int> >(), const std::vector<std::vector<int> > &pilot_carriers = std::vector<std::vector<int> >(), const std::vector<std::vector<gr_complex> > &pilot_symbols = std::vector<std::vector<gr_complex> >(), int symbols_skipped = 0, float alpha = 0.1, bool input_is_shifted = true); ~ofdm_equalizer_simpledfe(); void equalize(gr_complex *frame, int n_sym, const std::vector<gr_complex> &initial_taps = std::vector<gr_complex>(), const std::vector<gr_tag_t> &tags = std::vector<gr_tag_t>()); /* * \param fft_len FFT length * \param constellation The constellation object describing the modulation used * on the subcarriers (e.g. QPSK). This is used to decode * the individual symbols. * \param occupied_carriers List of occupied carriers, see ofdm_carrier_allocator * for a description. * \param pilot_carriers Position of pilot symbols, see ofdm_carrier_allocator * for a description. * \param pilot_symbols Value of pilot symbols, see ofdm_carrier_allocator * for a description. * \param alpha Averaging coefficient (in a nutshell, if \f$H_{i,k}\f$ is the channel * state for carrier i and symbol k, * \f$H_{i,k+1} = \alpha H_{i,k} + (1 - \alpha) H_{i,k+1}\f$. Make this * larger if there's more noise, but keep in mind that larger values * of alpha mean slower response to channel changes). * \param symbols_skipped Starting position within occupied_carriers and pilot_carriers. * If the first symbol of the frame was removed (e.g. to decode the * header), set this make sure the pilot symbols are correctly * identified. * \param input_is_shifted Set this to false if the input signal is not shifted, i.e. * the first input items is on the DC carrier. * Note that a lot of the OFDM receiver blocks operate on shifted * signals! */ static sptr make( int fft_len, const gr::digital::constellation_sptr &constellation, const std::vector<std::vector<int> > &occupied_carriers = std::vector<std::vector<int> >(), const std::vector<std::vector<int> > &pilot_carriers = std::vector<std::vector<int> >(), const std::vector<std::vector<gr_complex> > &pilot_symbols = std::vector<std::vector<gr_complex> >(), int symbols_skipped=0, float alpha=0.1, bool input_is_shifted=true ); private: gr::digital::constellation_sptr d_constellation; //! Averaging coefficient float d_alpha; }; } /* namespace digital */ } /* namespace gr */ #endif /* INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H */