mpsk_snr_est.h

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/* -*- c++ -*- */
/*
 * Copyright 2011,2012 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
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 * any later version.
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#ifndef INCLUDED_DIGITAL_MPSK_SNR_EST_H
#define INCLUDED_DIGITAL_MPSK_SNR_EST_H

#include <gnuradio/digital/api.h>
#include <gnuradio/gr_complex.h>

namespace gr {
  namespace digital {

    /*!
     * \brief A block for computing SNR of a signal.
     * \ingroup measurement_tools_blk
     *
     * \details
     * Below are some ROUGH estimates of what values of SNR each of
     * these types of estimators is good for. In general, these offer
     * a trade-off between accuracy and performance.
     *
     * \li SNR_EST_SIMPLE:  Simple estimator (>= 7 dB)
     * \li SNR_EST_SKEW:    Skewness-base est (>= 5 dB)
     * \li SNR_EST_M2M4:    2nd & 4th moment est (>= 1 dB)
     * \li SNR_EST_SVR:     SVR-based est (>= 0dB)
     */
    typedef enum {
      SNR_EST_SIMPLE = 0,   // Simple estimator (>= 7 dB)
      SNR_EST_SKEW,         // Skewness-base est (>= 5 dB)
      SNR_EST_M2M4,         // 2nd & 4th moment est (>= 1 dB)
      SNR_EST_SVR           // SVR-based est (>= 0dB)
    } snr_est_type_t;

    /*! \brief A parent class for SNR estimators, specifically for
     *  M-PSK signals in AWGN channels.
     *  \ingroup snr_blk
     */
    class DIGITAL_API mpsk_snr_est
    {
    protected:
      double d_alpha, d_beta;

    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       */
      mpsk_snr_est(double alpha);
      virtual ~mpsk_snr_est();

      //! Get the running-average coefficient
      double alpha() const;

      //! Set the running-average coefficient
      void set_alpha(double alpha);

      //! Update the current registers
      virtual int update(int noutput_items,
                         const gr_complex *input);

      //! Use the register values to compute a new estimate
      virtual double snr();
    };


    //! \brief SNR Estimator using simple mean/variance estimates.
    /*! \ingroup snr_blk
     *
     *  A very simple SNR estimator that just uses mean and variance
     *  estimates of an M-PSK constellation. This esimator is quick
     *  and cheap and accurate for high SNR (above 7 dB or so) but
     *  quickly starts to overestimate the SNR at low SNR.
     */
    class DIGITAL_API mpsk_snr_est_simple :
      public mpsk_snr_est
    {
    private:
      double d_y1, d_y2;
  
    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       */
      mpsk_snr_est_simple(double alpha);
      ~mpsk_snr_est_simple() {}

      int update(int noutput_items,
                 const gr_complex *input);
      double snr();
    };


    //! \brief SNR Estimator using skewness correction.
    /*!  \ingroup snr_blk
     *
     *  This is an estimator that came from a discussion between Tom
     *  Rondeau and fred harris with no known paper reference. The
     *  idea is that at low SNR, the variance estimations will be
     *  affected because of fold-over around the decision boundaries,
     *  which results in a skewness to the samples. We estimate the
     *  skewness and use this as a correcting term.
     */
    class DIGITAL_API mpsk_snr_est_skew :
      public mpsk_snr_est
    {
    private:
      double d_y1, d_y2, d_y3;
  
    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       */
      mpsk_snr_est_skew(double alpha);
      ~mpsk_snr_est_skew() {}

      int update(int noutput_items,
                 const gr_complex *input);
      double snr();
    };


    //! \brief SNR Estimator using 2nd and 4th-order moments.
    /*! \ingroup snr_blk
     *
     *  An SNR estimator for M-PSK signals that uses 2nd (M2) and 4th
     *  (M4) order moments. This estimator uses knowledge of the
     *  kurtosis of the signal (k_a) and noise (k_w) to make its
     *  estimation. We use Beaulieu's approximations here to M-PSK
     *  signals and AWGN channels such that k_a=1 and k_w=2. These
     *  approximations significantly reduce the complexity of the
     *  calculations (and computations) required.
     *
     *  Reference:
     *  D. R. Pauluzzi and N. C. Beaulieu, "A comparison of SNR
     *  estimation techniques for the AWGN channel," IEEE
     *  Trans. Communications, Vol. 48, No. 10, pp. 1681-1691, 2000.
     */
    class DIGITAL_API mpsk_snr_est_m2m4 :
      public mpsk_snr_est
    {
    private:
      double d_y1, d_y2;
  
    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       */
      mpsk_snr_est_m2m4(double alpha);
      ~mpsk_snr_est_m2m4() {}

      int update(int noutput_items,
                 const gr_complex *input);
      double snr();
    };


    //! \brief SNR Estimator using 2nd and 4th-order moments.
    /*!  \ingroup snr_blk
     *
     *  An SNR estimator for M-PSK signals that uses 2nd (M2) and 4th
     *  (M4) order moments. This estimator uses knowledge of the
     *  kurtosis of the signal (k_a) and noise (k_w) to make its
     *  estimation. In this case, you can set your own estimations for
     *  k_a and k_w, the kurtosis of the signal and noise, to fit this
     *  estimation better to your signal and channel conditions.
     *
     *  A word of warning: this estimator has not been fully tested or
     *  proved with any amount of rigor. The estimation for M4 in
     *  particular might be ignoring effectf of when k_a and k_w are
     *  different. Use this estimator with caution and a copy of the
     *  reference on hand.
     *
     *  The digital_mpsk_snr_est_m2m4 assumes k_a and k_w to simplify
     *  the computations for M-PSK and AWGN channels. Use that
     *  estimator unless you have a way to guess or estimate these
     *  values here.
     *
     *  Original paper: 
     *  R. Matzner, "An SNR estimation algorithm for complex baseband
     *  signal using higher order statistics," Facta Universitatis
     *  (Nis), no. 6, pp. 41-52, 1993.
     *
     *  Reference used in derivation:
     *  D. R. Pauluzzi and N. C. Beaulieu, "A comparison of SNR
     *  estimation techniques for the AWGN channel," IEEE
     *  Trans. Communications, Vol. 48, No. 10, pp. 1681-1691, 2000.
     */
    class DIGITAL_API snr_est_m2m4 :
      public mpsk_snr_est
    {
    private:
      double d_y1, d_y2;
      double d_ka, d_kw;
  
    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       *  \param ka: estimate of the signal kurtosis (1 for PSK)
       *  \param kw: estimate of the channel noise kurtosis (2 for AWGN)
       */
      snr_est_m2m4(double alpha, double ka, double kw);
      ~snr_est_m2m4() {}

      int update(int noutput_items,
                 const gr_complex *input);
      double snr();
    };


    //! \brief Signal-to-Variation Ratio SNR Estimator.
    /*! \ingroup snr_blk
     *
     *  This estimator actually comes from an SNR estimator for M-PSK
     *  signals in fading channels, but this implementation is
     *  specifically for AWGN channels. The math was simplified to
     *  assume a signal and noise kurtosis (k_a and k_w) for M-PSK
     *  signals in AWGN. These approximations significantly reduce the
     *  complexity of the calculations (and computations) required.
     *
     *  Original paper:
     *  A. L. Brandao, L. B. Lopes, and D. C. McLernon, "In-service
     *  monitoring of multipath delay and cochannel interference for
     *  indoor mobile communication systems," Proc. IEEE
     *  Int. Conf. Communications, vol. 3, pp. 1458-1462, May 1994.
     *
     *  Reference:
     *  D. R. Pauluzzi and N. C. Beaulieu, "A comparison of SNR
     *  estimation techniques for the AWGN channel," IEEE
     *  Trans. Communications, Vol. 48, No. 10, pp. 1681-1691, 2000.
     */
    class DIGITAL_API mpsk_snr_est_svr :
      public mpsk_snr_est
    {
    private:
      double d_y1, d_y2;
  
    public:
      /*! Constructor
       *
       *  Parameters:
       *  \param alpha: the update rate of internal running average
       *  calculations.
       */
      mpsk_snr_est_svr(double alpha);
      ~mpsk_snr_est_svr() {}

      int update(int noutput_items,
                 const gr_complex *input);
      double snr();
    };

  } /* namespace digital */
} /* namespace gr */

#endif /* INCLUDED_DIGITAL_MPSK_SNR_EST_H */