diff options
Diffstat (limited to 'gr-digital/include/digital/ofdm_equalizer_simpledfe.h')
| -rw-r--r-- | gr-digital/include/digital/ofdm_equalizer_simpledfe.h | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/gr-digital/include/digital/ofdm_equalizer_simpledfe.h b/gr-digital/include/digital/ofdm_equalizer_simpledfe.h new file mode 100644 index 0000000000..9d286576bc --- /dev/null +++ b/gr-digital/include/digital/ofdm_equalizer_simpledfe.h @@ -0,0 +1,130 @@ +/* -*- 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 */ + |