sdrangel/sdrbase/dsp/rootraisedcosine.h

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5.5 KiB
C++

///////////////////////////////////////////////////////////////////////////////////
// Copyright (C) 2015 Edouard Griffiths, F4EXB //
// Copyright (C) 2021 Jon Beniston, M7RCE //
// //
// This program 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 as version 3 of the License, or //
// (at your option) any later version. //
// //
// This program 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 V3 for more details. //
// //
// You should have received a copy of the GNU General Public License //
// along with this program. If not, see <http://www.gnu.org/licenses/>. //
///////////////////////////////////////////////////////////////////////////////////
#ifndef INCLUDE_ROOTRAISEDCOSINE_H
#define INCLUDE_ROOTRAISEDCOSINE_H
#include <cmath>
#include <vector>
#include "dsp/dsptypes.h"
// Root-raised-cosine low-pass filter for pulse shaping, without intersymbol interference (ISI)
// https://en.wikipedia.org/wiki/Root-raised-cosine_filter
// This could be optimised in to a polyphase filter, as samplesPerSymbol-1 inputs
// to filter() should be zero, as the data is upsampled to the sample rate
template <class Type> class RootRaisedCosine {
public:
RootRaisedCosine() : m_ptr(0) { }
// beta - roll-off factor
// symbolSpan - number of symbols over which the filter is spread
// samplesPerSymbol - number of samples per symbol
// normaliseUpsampledAmplitude - when true, scale the filter such that an upsampled
// (by samplesPerSymbol) bipolar sequence (E.g. [1 0 0 -1 0 0..]) has maximum
// output values close to (1,-1)
void create(double beta, int symbolSpan, int samplesPerSymbol, bool normaliseUpsampledAmplitude = false)
{
int nTaps = symbolSpan * samplesPerSymbol + 1;
int i, j;
// check constraints
if(!(nTaps & 1)) {
qDebug("Root raised cosine filter has to have an odd number of taps");
nTaps++;
}
// make room
m_samples.resize(nTaps);
for(int i = 0; i < nTaps; i++)
m_samples[i] = 0;
m_ptr = 0;
m_taps.resize(nTaps / 2 + 1);
// calculate filter taps
for(i = 0; i < nTaps / 2 + 1; i++)
{
double t = (i - (nTaps / 2)) / (double)samplesPerSymbol;
double Ts = 1.0;
double numerator = 1.0/Ts * (sin(M_PI * t / Ts * (1.0-beta)) + 4.0*beta*t/Ts*cos(M_PI*t/Ts*(1+beta)));
double b = (4.0 * beta * t / Ts);
double denominator = M_PI * t / Ts * (1-b*b);
if ((numerator == 0.0) && (denominator == 0.0))
m_taps[i] = 1.0/Ts * (1.0+beta*(4.0/M_PI-1.0));
else if (denominator == 0.0)
m_taps[i] = beta/(Ts*sqrt(2.0)) * ((1+2.0/M_PI)*sin(M_PI/(4.0*beta)) + (1.0-2.0/M_PI)*cos(M_PI/(4.0*beta)));
else
m_taps[i] = numerator/denominator;
}
// normalize
if (!normaliseUpsampledAmplitude)
{
// normalize energy
double sum = 0;
for(i = 0; i < (int)m_taps.size() - 1; i++)
sum += std::pow(m_taps[i], 2.0) * 2;
sum += std::pow(m_taps[i], 2.0);
sum = std::sqrt(sum);
for(i = 0; i < (int)m_taps.size(); i++)
m_taps[i] /= sum;
}
else
{
// Calculate maximum output of filter, assuming upsampled bipolar input E.g. [1 0 0 -1 0 0..]
// This doesn't necessarily include the centre tap, so we try each offset
double maxGain = 0.0;
for (i = 0; i < samplesPerSymbol; i++)
{
double g = 0.0;
for (j = 0; j < (int)m_taps.size() - 1; j += samplesPerSymbol)
g += std::fabs(2.0 * m_taps[j]);
if ((i & 1) == 0)
g += std::fabs(m_taps[j]);
if (g > maxGain)
maxGain = g;
}
// Scale up so maximum out is 1
for(i = 0; i < (int)m_taps.size(); i++)
m_taps[i] /= maxGain;
}
}
Type filter(Type sample)
{
Type acc = 0;
unsigned int n_samples = m_samples.size();
unsigned int n_taps = m_taps.size() - 1;
unsigned int a = m_ptr;
unsigned int b = a == n_samples - 1 ? 0 : a + 1;
m_samples[m_ptr] = sample;
for (unsigned int i = 0; i < n_taps; ++i)
{
acc += (m_samples[a] + m_samples[b]) * m_taps[i];
a = (a == 0) ? n_samples - 1 : a - 1;
b = (b == n_samples - 1) ? 0 : b + 1;
}
acc += m_samples[a] * m_taps[n_taps];
m_ptr = (m_ptr == n_samples - 1) ? 0 : m_ptr + 1;
return acc;
}
private:
std::vector<Real> m_taps;
std::vector<Type> m_samples;
unsigned int m_ptr;
};
#endif // INCLUDE_ROOTRAISEDCOSINE_H