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/* -*- 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>
class digital_ofdm_equalizer_simpledfe;
typedef boost::shared_ptr<digital_ofdm_equalizer_simpledfe> digital_ofdm_equalizer_simpledfe_sptr;
/*
* \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!
*/
DIGITAL_API digital_ofdm_equalizer_simpledfe_sptr
digital_make_ofdm_equalizer_simpledfe(
int fft_len,
const 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);
/* \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 digital_ofdm_equalizer_simpledfe : public digital_ofdm_equalizer_1d_pilots
{
public:
digital_ofdm_equalizer_simpledfe(
int fft_len,
const 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);
~digital_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>());
private:
digital_constellation_sptr d_constellation;
//! Averaging coefficient
float d_alpha;
};
#endif /* INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H */
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