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Transceiver52M: Add 4 samples-per-symbol Laurent pulse shape 

When 4 samples-per-symbol operation is selected, replace the
existing pulse approximation, which becomes inaccurate with
non-unit oversampling, with the primary pulse, C0, from the
Laurent linear pulse approximation.

Pierre Laurent, "Exact and Approximate Construction of Digital Phase
  Modulations by Superposition of Amplitude Modulated Pulses", IEEE
  Transactions of Communications, Vol. 34, No. 2, Feb 1986.

Octave pulse generation code for the first three pulses of the
linear approximation are included.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
This commit is contained in:
Thomas Tsou 2013-08-21 13:59:52 -04:00
parent 8181b0104a
commit 9ccd9f2c3c
2 changed files with 123 additions and 9 deletions
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83
Transceiver52M/laurent.m Normal file
View File

@ -0,0 +1,83 @@
%
% Laurent decomposition of GMSK signals
% Generates C0, C1, and C2 pulse shapes
%
% Pierre Laurent, "Exact and Approximate Construction of Digital Phase
% Modulations by Superposition of Amplitude Modulated Pulses", IEEE
% Transactions of Communications, Vol. 34, No. 2, Feb 1986.
%
% Author: Thomas Tsou <tom@tsou.cc>
%
% Modulation parameters
oversamp = 16;
L = 3;
f = 270.83333e3;
T = 1/f;
h = 0.5;
BT = 0.30;
B = BT / T;
% Generate sampling points for L symbol periods
t = -(L*T/2):T/oversamp:(L*T/2);
t = t(1:end-1) + (T/oversamp/2);
% Generate Gaussian pulse
g = qfunc(2*pi*B*(t - T/2)/(log(2)^.5)) - qfunc(2*pi*B*(t + T/2)/(log(2)^.5));
g = g / sum(g) * pi/2;
g = [0 g];
% Integrate phase
q = 0;
for i = 1:size(g,2);
q(i) = sum(g(1:i));
end
% Compute two sided "generalized phase pulse" function
s = 0;
for i = 1:size(g,2);
s(i) = sin(q(i)) / sin(pi*h);
end
for i = (size(g,2) + 1):(2 * size(g,2) - 1);
s(i) = sin(pi*h - q(i - (size(g,2) - 1))) / sin(pi*h);
end
% Compute C0 pulse: valid for all L values
c0 = s(1:end-(oversamp*(L-1)));
for i = 1:L-1;
c0 = c0 .* s((1 + i*oversamp):end-(oversamp*(L - 1 - i)));
end
% Compute C1 pulse: valid for L = 3 only!
% C1 = S0 * S4 * S2
c1 = s(1:end-(oversamp*(4)));
c1 = c1 .* s((1 + 4*oversamp):end-(oversamp*(4 - 1 - 3)));
c1 = c1 .* s((1 + 2*oversamp):end-(oversamp*(4 - 1 - 1)));
% Compute C2 pulse: valid for L = 3 only!
% C2 = S0 * S1 * S5
c2 = s(1:end-(oversamp*(5)));
c2 = c2 .* s((1 + 1*oversamp):end-(oversamp*(5 - 1 - 0)));
c2 = c2 .* s((1 + 5*oversamp):end-(oversamp*(5 - 1 - 4)));
% Plot C0, C1, C2 Laurent pulse series
figure(1);
hold off;
plot((0:size(c0,2)-1)/oversamp - 2,c0, 'b');
hold on;
plot((0:size(c1,2)-1)/oversamp - 2,c1, 'r');
plot((0:size(c2,2)-1)/oversamp - 2,c2, 'g');
% Generate OpenBTS pulse
numSamples = size(c0,2);
centerPoint = (numSamples - 1)/2;
i = ((0:numSamples) - centerPoint) / oversamp;
xP = .96*exp(-1.1380*i.^2 - 0.527*i.^4);
xP = xP / max(xP) * max(c0);
% Plot C0 pulse compared to OpenBTS pulse
figure(2);
hold off;
plot((0:size(c0,2)-1)/oversamp, c0, 'b');
hold on;
plot((0:size(xP,2)-1)/oversamp, xP, 'r');

49
Transceiver52M/sigProcLib.cpp
View File

@ -422,11 +422,22 @@ void generateGSMPulse(int sps, int symbolLength)
GSMPulse->empty->isRealOnly(true);
*(GSMPulse->empty->begin()) = 1.0f;
len = sps * symbolLength;
if (len < 4)
len = 4;
/*
* For 4 samples-per-symbol use a precomputed single pulse Laurent
* approximation. This should yields below 2 degrees of phase error at
* the modulator output. Use the existing pulse approximation for all
* other oversampling factors.
*/
switch (sps) {
case 4:
len = 16;
break;
default:
len = sps * symbolLength;
if (len < 4)
len = 4;
}
/* GSM pulse approximation */
GSMPulse->buffer = convolve_h_alloc(len);
GSMPulse->gaussian = new signalVector((complex *)
GSMPulse->buffer, 0, len);
@ -435,12 +446,32 @@ void generateGSMPulse(int sps, int symbolLength)
signalVector::iterator xP = GSMPulse->gaussian->begin();
center = (float) (len - 1.0) / 2.0;
if (sps == 4) {
*xP++ = 4.46348606e-03;
*xP++ = 2.84385729e-02;
*xP++ = 1.03184855e-01;
*xP++ = 2.56065552e-01;
*xP++ = 4.76375085e-01;
*xP++ = 7.05961177e-01;
*xP++ = 8.71291644e-01;
*xP++ = 9.29453645e-01;
*xP++ = 8.71291644e-01;
*xP++ = 7.05961177e-01;
*xP++ = 4.76375085e-01;
*xP++ = 2.56065552e-01;
*xP++ = 1.03184855e-01;
*xP++ = 2.84385729e-02;
*xP++ = 4.46348606e-03;
*xP++ = 0.0;
} else {
center = (float) (len - 1.0) / 2.0;
for (int i = 0; i < len; i++) {
arg = ((float) i - center) / (float) sps;
*xP++ = 0.96 * exp(-1.1380 * arg * arg -
0.527 * arg * arg * arg * arg);
/* GSM pulse approximation */
for (int i = 0; i < len; i++) {
arg = ((float) i - center) / (float) sps;
*xP++ = 0.96 * exp(-1.1380 * arg * arg -
0.527 * arg * arg * arg * arg);
}
}
float avgAbsval = sqrtf(vectorNorm2(*GSMPulse->gaussian)/sps);