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d2193d4c18
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> git-svn-id: http://wush.net/svn/range/software/public/openbts/trunk@6736 19bc5d8c-e614-43d4-8b26-e1612bc8e597
84 lines
2.1 KiB
Matlab
84 lines
2.1 KiB
Matlab
%
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% Laurent decomposition of GMSK signals
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% Generates C0, C1, and C2 pulse shapes
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%
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% Pierre Laurent, "Exact and Approximate Construction of Digital Phase
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% Modulations by Superposition of Amplitude Modulated Pulses", IEEE
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% Transactions of Communications, Vol. 34, No. 2, Feb 1986.
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%
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% Author: Thomas Tsou <tom@tsou.cc>
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%
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% Modulation parameters
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oversamp = 16;
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L = 3;
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f = 270.83333e3;
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T = 1/f;
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h = 0.5;
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BT = 0.30;
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B = BT / T;
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% Generate sampling points for L symbol periods
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t = -(L*T/2):T/oversamp:(L*T/2);
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t = t(1:end-1) + (T/oversamp/2);
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% Generate Gaussian pulse
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g = qfunc(2*pi*B*(t - T/2)/(log(2)^.5)) - qfunc(2*pi*B*(t + T/2)/(log(2)^.5));
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g = g / sum(g) * pi/2;
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g = [0 g];
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% Integrate phase
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q = 0;
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for i = 1:size(g,2);
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q(i) = sum(g(1:i));
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end
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% Compute two sided "generalized phase pulse" function
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s = 0;
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for i = 1:size(g,2);
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s(i) = sin(q(i)) / sin(pi*h);
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end
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for i = (size(g,2) + 1):(2 * size(g,2) - 1);
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s(i) = sin(pi*h - q(i - (size(g,2) - 1))) / sin(pi*h);
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end
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% Compute C0 pulse: valid for all L values
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c0 = s(1:end-(oversamp*(L-1)));
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for i = 1:L-1;
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c0 = c0 .* s((1 + i*oversamp):end-(oversamp*(L - 1 - i)));
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end
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% Compute C1 pulse: valid for L = 3 only!
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% C1 = S0 * S4 * S2
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c1 = s(1:end-(oversamp*(4)));
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c1 = c1 .* s((1 + 4*oversamp):end-(oversamp*(4 - 1 - 3)));
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c1 = c1 .* s((1 + 2*oversamp):end-(oversamp*(4 - 1 - 1)));
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% Compute C2 pulse: valid for L = 3 only!
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% C2 = S0 * S1 * S5
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c2 = s(1:end-(oversamp*(5)));
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c2 = c2 .* s((1 + 1*oversamp):end-(oversamp*(5 - 1 - 0)));
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c2 = c2 .* s((1 + 5*oversamp):end-(oversamp*(5 - 1 - 4)));
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% Plot C0, C1, C2 Laurent pulse series
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figure(1);
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hold off;
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plot((0:size(c0,2)-1)/oversamp - 2,c0, 'b');
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hold on;
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plot((0:size(c1,2)-1)/oversamp - 2,c1, 'r');
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plot((0:size(c2,2)-1)/oversamp - 2,c2, 'g');
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% Generate OpenBTS pulse
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numSamples = size(c0,2);
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centerPoint = (numSamples - 1)/2;
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i = ((0:numSamples) - centerPoint) / oversamp;
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xP = .96*exp(-1.1380*i.^2 - 0.527*i.^4);
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xP = xP / max(xP) * max(c0);
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% Plot C0 pulse compared to OpenBTS pulse
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figure(2);
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hold off;
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plot((0:size(c0,2)-1)/oversamp, c0, 'b');
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hold on;
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plot((0:size(xP,2)-1)/oversamp, xP, 'r');
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