%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%生成調制信號
fs=100;
%采樣頻率
fc=40;
%載頻
fo=fs/20;
%碼率
L=300;
%信號樣本
t = (0:L-1)*1/fs;
xn=cos(8*pi*fc*t);         
y = xn + randn(size(t));    
figure(1)
plot(t,y)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%進行能量檢測
NFFT = 2^nextpow2(L);
Y = fft(y,NFFT)/L;
f = fs/2*linspace(0,1,NFFT/2);
figure(2)
plot(f,2*abs(Y(1:NFFT/2)))