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/***************************************************************************
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qpskdemodulator.cpp - description
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copyright : (C) 2001 by Volker Schroer
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***************************************************************************/
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/***************************************************************************
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* This program is free software; you can redistribute it and/or modify *
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* it under the terms of the GNU General Public License as published by *
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* the Free Software Foundation; either version 2 of the License, or *
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* (at your option) any later version. *
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* based on the work of Moe Wheatly, AE4JY *
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***************************************************************************/
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#include "qpskdemodulator.h"
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// phase wraparound correction table for viterbi decoder
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const double AngleTbl1[4] = { M_PI_3_2, 0.0, M_PI/2.0, M_PI};
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const double AngleTbl2[4] = { M_PI_3_2, PI2, M_PI/2.0, M_PI};
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QPskDemodulator::QPskDemodulator():CPskDemodulator()
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ViterbiDecode( M_PI_3_2); // init the Viterbi decoder
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QPskDemodulator::~QPskDemodulator()
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void QPskDemodulator::DecodeSymbol(double_complex newsamp)
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//unsigned char ch = 0;
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m_I1 = m_I0; //form the multi delayed symbol samples
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m_I0 = newsamp.real();
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m_Q0 = newsamp.imag();
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// calculates difference angle for QPSK, BPSK, and IQPSK decoding
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//create vector whose angle is the difference angle by multiplying the
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// current smaple by the complex conjugate of the previous sample.
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//swap I and Q axis to keep away from the +/-Pi discontinuity and
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// add Pi to make make range from 0 to 2Pi.
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// 180 deg phase changes center at Pi/4
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// 0 deg phase changes center at 3Pi/2
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// +90 deg phase changes center at 2Pi or 0
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// -90 deg phase changes center at Pi
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angle = M_PI + atan2( (m_I1*m_I0 + m_Q1*m_Q0),
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(m_I1*m_Q0 - m_I0*m_Q1)); //QPSK or BPSK
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bit = ViterbiDecode( angle);
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if( (bit==0) && m_LastBitZero ) //if character delimiter
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m_BitAcc >>= 2; //get rid of last zero and one
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ch = m_VaricodeDecTbl[m_BitAcc];
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m_LastBitZero = false;
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if(GotChar && (ch!=0) )
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emit setFastSquelch(true);
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if (m_OffCount++ > 20)
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emit setFastSquelch(false);
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///////////////////////////////////////////////////////////////////////
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// Soft-decision Viterbi decoder function.
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///////////////////////////////////////////////////////////////////////
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bool QPskDemodulator::ViterbiDecode( double newangle)
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int bitestimates[32];
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const double* pAngleTbl;
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min = 1.0e100; // make sure can find a minimum value
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//alternative soft decision method using angle differences
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if( newangle >= PI2/2 ) //deal with ambiguity at +/- 2PI
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pAngleTbl = AngleTbl2; // by using two different tables
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pAngleTbl = AngleTbl1;
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for(i = 0; i < 32; i++) // calculate all possible distances
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{ //lsb of 'i' is newest bit estimate
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pathdist[i] = m_SurvivorStates[i / 2].Pathdistance +
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fabs(newangle - pAngleTbl[ ConvolutionCodeTable[i] ]);
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if(pathdist[i] < min) // keep track of minimum distance
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// shift in newest bit estimates
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bitestimates[i] = ((m_SurvivorStates[i / 2].BitEstimates) << 1) + (i & 1);
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for(i = 0; i < 16; i++) //compare path lengths with the same end state
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// and keep only the smallest path in m_SurvivorStates[].
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if(pathdist[i] < pathdist[16 + i])
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m_SurvivorStates[i].Pathdistance = pathdist[i] - min;
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m_SurvivorStates[i].BitEstimates = bitestimates[i];
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m_SurvivorStates[i].Pathdistance = pathdist[16 + i] - min;
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m_SurvivorStates[i].BitEstimates = bitestimates[16 + i];
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for(i = 0; i < 16; i++) // find if more ones than zeros at bit 20 position
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ones += (m_SurvivorStates[i].BitEstimates&(1L << 20));
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if( ones == (8L << 20 ) )
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return ( rand() & 0x1000 ); //if a tie then guess
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return(ones > (8L << 20) ); //else return most likely bit value
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//////////////////////////////////////////////////////////////////////
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// Calculate signal quality based on the statistics of the phase
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// difference angle. The more dispersion of the "0" and "180" degree
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// phase shifts, the worse the signal quality. This information is used
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// to activate the squelch control. If 20 consecutive "180" degree shifts
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// occur, the squelch is forced on, and if 20 consecutive "0" degree
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// shifts occur, the squelch is forced off quickly.
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//////////////////////////////////////////////////////////////////////
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void QPskDemodulator::CalcQuality( double angle )
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angle = angle - M_PI_2;
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temp = fmod(angle,M_PI_2);
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m_DevAve =0.47 * ave1 + 0.46 * ave2 + 0.03 *temp;
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temp=100.-250.0*m_DevAve;
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emit setSquelchValue((int)temp);
added
removed
linpsk/qpskdemodulator.cpp
