%實驗五  升余弦滾降濾波系統的誤碼率
global dt t f df N T %全局變量
close all
clear Eb_N0 Pe
N=2^15; 					%采樣點數
L=4; 						%每碼元的采樣點數
M=N/L						%碼元數
Rb=2;						%碼速率是2Mb/s
Ts=1/Rb; 				%碼元間隔
dt=Ts/L;					%時域采樣間隔
df=1/(N*dt)				%頻域采樣間隔
T=N*dt 					%截短時間
Bs=N*df/2 				%系統帶寬
alpha=0.5				%滾降系數=0.5

t=linspace(-T/2,T/2,N); %時域橫坐標
f=linspace(-Bs,Bs,N)+eps; %頻域橫坐標

figure(1)
set(1,'Position',[10,30,500,200])
%設定窗口位置及大小
hr1=sin(pi*t/Ts)./(pi*t/Ts);
hr2=cos(alpha*pi*t/Ts)./(1-(2*alpha*t/Ts).^2);
hr=hr1.*hr2;
HR=abs(t2f(hr));
GT=sqrt(HR);
GR=GT;

for loop1=1:10
    Eb_N0(loop1)=(loop1-1)  %分貝值變為真值
    eb_n0(loop1)=10^(Eb_N0(loop1)/10);
    Eb=1;
    n0=Eb/eb_n0(loop1);   	%信道噪聲譜密度
    sita=n0*Bs; 				%噪聲功率

    n_err=0;     				%誤碼計數

    for loop2=1:3
        a=sign(randn(1,M));	%發送碼元
        imp=zeros(1,N);
        imp(L/2:L:N)=a/dt;
        IMP=t2f(imp);

        n_ch=sqrt(sita)*randn(size(t)); %信道噪聲
        nr=real(f2t(t2f(n_ch).*GR));

        sr=real(f2t(IMP.*HR))+nr;
        y=sr(L/2:L:N);						%以L/2為起點，步長L,取樣點N
        aa=sign(y);                    %接收碼元
        n_err=n_err+length(find(aa~=a)) ;
    end
    Pe(loop1)=n_err/(M*loop2);		%誤碼率Pe'=n_err/(M*loop2)
    figure(1)
    semilogy(Eb_N0,Pe,'g');
    eb_n0=10.^(Eb_N0/10);				%還原為真值
    hold on
    semilogy(Eb_N0,0.5*erfc(sqrt(eb_n0)));
    axis([0,9,1e-4,1])
    xlabel('Eb/N0')
    ylabel('Pe')
end

end