GNU Radio 3.5.3.2 C++ API
digital_ofdm_frame_acquisition.h
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1 /* -*- c++ -*- */
2 /*
3  * Copyright 2006,2007,2011 Free Software Foundation, Inc.
4  *
5  * This file is part of GNU Radio
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22 
23 #ifndef INCLUDED_DIGITAL_OFDM_FRAME_ACQUISITION_H
24 #define INCLUDED_DIGITAL_OFDM_FRAME_ACQUISITION_H
25 
26 #include <digital_api.h>
27 #include <gr_block.h>
28 #include <vector>
29 
32 
34 DIGITAL_API digital_make_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
35  unsigned int cplen,
36  const std::vector<gr_complex> &known_symbol,
37  unsigned int max_fft_shift_len=10);
38 
39 /*!
40  * \brief take a vector of complex constellation points in from an FFT
41  * and performs a correlation and equalization.
42  * \ingroup demodulation_blk
43  * \ingroup ofdm_blk
44  *
45  * This block takes the output of an FFT of a received OFDM symbol and finds the
46  * start of a frame based on two known symbols. It also looks at the surrounding
47  * bins in the FFT output for the correlation in case there is a large frequency
48  * shift in the data. This block assumes that the fine frequency shift has already
49  * been corrected and that the samples fall in the middle of one FFT bin.
50  *
51  * It then uses one of those known
52  * symbols to estimate the channel response over all subcarriers and does a simple
53  * 1-tap equalization on all subcarriers. This corrects for the phase and amplitude
54  * distortion caused by the channel.
55  */
56 
58 {
59  /*!
60  * \brief Build an OFDM correlator and equalizer.
61  * \param occupied_carriers The number of subcarriers with data in the received symbol
62  * \param fft_length The size of the FFT vector (occupied_carriers + unused carriers)
63  * \param cplen The length of the cycle prefix
64  * \param known_symbol A vector of complex numbers representing a known symbol at the
65  * start of a frame (usually a BPSK PN sequence)
66  * \param max_fft_shift_len Set's the maximum distance you can look between bins for correlation
67  */
69  digital_make_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
70  unsigned int cplen,
71  const std::vector<gr_complex> &known_symbol,
72  unsigned int max_fft_shift_len);
73 
74 protected:
75  digital_ofdm_frame_acquisition (unsigned int occupied_carriers, unsigned int fft_length,
76  unsigned int cplen,
77  const std::vector<gr_complex> &known_symbol,
78  unsigned int max_fft_shift_len);
79 
80  private:
81  unsigned char slicer(gr_complex x);
82  void correlate(const gr_complex *symbol, int zeros_on_left);
83  void calculate_equalizer(const gr_complex *symbol, int zeros_on_left);
84  gr_complex coarse_freq_comp(int freq_delta, int count);
85 
86  unsigned int d_occupied_carriers; // !< \brief number of subcarriers with data
87  unsigned int d_fft_length; // !< \brief length of FFT vector
88  unsigned int d_cplen; // !< \brief length of cyclic prefix in samples
89  unsigned int d_freq_shift_len; // !< \brief number of surrounding bins to look at for correlation
90  std::vector<gr_complex> d_known_symbol; // !< \brief known symbols at start of frame
91  std::vector<float> d_known_phase_diff; // !< \brief factor used in correlation from known symbol
92  std::vector<float> d_symbol_phase_diff; // !< \brief factor used in correlation from received symbol
93  std::vector<gr_complex> d_hestimate; // !< channel estimate
94  int d_coarse_freq; // !< \brief search distance in number of bins
95  unsigned int d_phase_count; // !< \brief accumulator for coarse freq correction
96  float d_snr_est; // !< an estimation of the signal to noise ratio
97 
98  gr_complex *d_phase_lut; // !< look-up table for coarse frequency compensation
99 
100  void forecast(int noutput_items, gr_vector_int &ninput_items_required);
101 
102  public:
103  /*!
104  * \brief Return an estimate of the SNR of the channel
105  */
106  float snr() { return d_snr_est; }
107 
109  int general_work(int noutput_items,
110  gr_vector_int &ninput_items,
111  gr_vector_const_void_star &input_items,
112  gr_vector_void_star &output_items);
113 };
114 
115 
116 #endif