You have to calculate autocorrelation using its definition :
clear all; close all; clc; syms yt(t) tau t;
% our signal y(t) = piecewise((1<t) & (t<3), 1, (3<t) & (t<4), -1, 0)
%autocorrelation R(t) syms R(t) R(tau)=int(y(t)*y(t+tau), t, -20, 20)
% plot results figure subplot(2,1,1) fplot(y, 'LineWidth', 3); grid on xlabel('t'); ylabel('y(t)'); title('Signal'); subplot(2,1,2) fplot(R, 'LineWidth', 3, 'color', [0 .5 0]); grid on; xlabel('tau'); ylabel('R(tau)'); title('Autocorrelation');
...and you will get this:
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