GNU Radio Manual and C++ API Reference 3.8.5.0
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ofdm_equalizer_simpledfe.h
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1/* -*- c++ -*- */
2/* Copyright 2012 Free Software Foundation, Inc.
3 *
4 * This file is part of GNU Radio
5 *
6 * GNU Radio is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 3, or (at your option)
9 * any later version.
10 *
11 * GNU Radio is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with GNU Radio; see the file COPYING. If not, write to
18 * the Free Software Foundation, Inc., 51 Franklin Street,
19 * Boston, MA 02110-1301, USA.
20 */
21
22#ifndef INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H
23#define INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H
24
28
29namespace gr {
30namespace digital {
31
32/*!
33 * \brief Simple decision feedback equalizer for OFDM.
34 * \ingroup ofdm_blk
35 * \ingroup equalizers_blk
36 *
37 * \details
38 * Equalizes an OFDM signal symbol by symbol using knowledge of the
39 * complex modulations symbols.
40 * For every symbol, the following steps are performed:
41 * - On every sub-carrier, decode the modulation symbol
42 * - Use the difference between the decoded symbol and the received symbol
43 * to update the channel state on this carrier
44 * - Whenever a pilot symbol is found, it uses the known pilot symbol to
45 * update the channel state.
46 *
47 * This equalizer makes a lot of assumptions:
48 * - The initial channel state is good enough to decode the first
49 * symbol without error (unless the first symbol only consists of pilot
50 * tones)
51 * - The channel changes only very slowly, such that the channel state
52 * from one symbol is enough to decode the next
53 * - SNR low enough that equalization will always suffice to correctly
54 * decode a symbol
55 * If these assumptions are not met, the most common error is that the
56 * channel state is estimated incorrectly during equalization; after that,
57 * all subsequent symbols will be completely wrong.
58 *
59 * Note that the equalized symbols are *exact points* on the constellation.
60 * This means soft information of the modulation symbols is lost after the
61 * equalization, which is suboptimal for channel codes that use soft decision.
62 *
63 */
65{
66public:
67 typedef boost::shared_ptr<ofdm_equalizer_simpledfe> sptr;
68
70 const gr::digital::constellation_sptr& constellation,
71 const std::vector<std::vector<int>>& occupied_carriers =
72 std::vector<std::vector<int>>(),
73 const std::vector<std::vector<int>>& pilot_carriers =
74 std::vector<std::vector<int>>(),
75 const std::vector<std::vector<gr_complex>>& pilot_symbols =
76 std::vector<std::vector<gr_complex>>(),
77 int symbols_skipped = 0,
78 float alpha = 0.1,
79 bool input_is_shifted = true);
80
82
83 void equalize(gr_complex* frame,
84 int n_sym,
85 const std::vector<gr_complex>& initial_taps = std::vector<gr_complex>(),
86 const std::vector<tag_t>& tags = std::vector<tag_t>());
87
88 /*
89 * \param fft_len FFT length
90 * \param constellation The constellation object describing the modulation used
91 * on the subcarriers (e.g. QPSK). This is used to decode
92 * the individual symbols.
93 * \param occupied_carriers List of occupied carriers, see ofdm_carrier_allocator
94 * for a description.
95 * \param pilot_carriers Position of pilot symbols, see ofdm_carrier_allocator
96 * for a description.
97 * \param pilot_symbols Value of pilot symbols, see ofdm_carrier_allocator
98 * for a description.
99 * \param alpha Averaging coefficient (in a nutshell, if \f$H_{i,k}\f$ is the channel
100 * state for carrier i and symbol k,
101 * \f$H_{i,k+1} = \alpha H_{i,k} + (1 - \alpha) H_{i,k+1}\f$. Make this
102 * larger if there's more noise, but keep in mind that larger values
103 * of alpha mean slower response to channel changes).
104 * \param symbols_skipped Starting position within occupied_carriers and
105 * pilot_carriers. If the first symbol of the frame was removed (e.g. to decode the
106 * header), set this make sure the pilot symbols are correctly
107 * identified.
108 * \param input_is_shifted Set this to false if the input signal is not shifted, i.e.
109 * the first input items is on the DC carrier.
110 * Note that a lot of the OFDM receiver blocks operate on
111 * shifted signals!
112 */
113 static sptr make(int fft_len,
114 const gr::digital::constellation_sptr& constellation,
115 const std::vector<std::vector<int>>& occupied_carriers =
116 std::vector<std::vector<int>>(),
117 const std::vector<std::vector<int>>& pilot_carriers =
118 std::vector<std::vector<int>>(),
119 const std::vector<std::vector<gr_complex>>& pilot_symbols =
120 std::vector<std::vector<gr_complex>>(),
121 int symbols_skipped = 0,
122 float alpha = 0.1,
123 bool input_is_shifted = true);
124
125private:
126 gr::digital::constellation_sptr d_constellation;
127 //! Averaging coefficient
128 float d_alpha;
129};
130
131} /* namespace digital */
132} /* namespace gr */
133
134#endif /* INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H */
An abstracted constellation object.
Definition constellation.h:63
Definition ofdm_equalizer_base.h:70
Simple decision feedback equalizer for OFDM.
Definition ofdm_equalizer_simpledfe.h:65
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)
void equalize(gr_complex *frame, int n_sym, const std::vector< gr_complex > &initial_taps=std::vector< gr_complex >(), const std::vector< tag_t > &tags=std::vector< tag_t >())
Run the actual equalization.
boost::shared_ptr< ofdm_equalizer_simpledfe > sptr
Definition ofdm_equalizer_simpledfe.h:67
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)
#define DIGITAL_API
Definition gr-digital/include/gnuradio/digital/api.h:30
std::complex< float > gr_complex
Definition gr_complex.h:27
GNU Radio logging wrapper for log4cpp library (C++ port of log4j)
Definition basic_block.h:46