>...assume the symbol T=1 
clear all 
close all 


bit_in = randint(1e3, 1, [0 1]); 

bit_I = bit_in(1:2:1e3); 
bit_Q = bit_in(2:2:1e3); 

data_I = -2*bit_I+1; 
data_Q = -2*bit_Q+1; 

data_I1=repmat(data_I',20,1); 
data_Q1=repmat(data_Q',20,1); 

for i=1:1e4 
data_I2(i)=data_I1(i); 
data_Q2(i)=data_Q1(i); 
end; 

> t=0:0.1:1e3-0.1; 
> plot(t,data_I2);axis([0 1e3 -2 2]); 

f=0:0.1:1; 
xrc=0.5+0.5*cos(pi*f); 

data_I2_rc=conv(data_I2,xrc)/5.5; 
data_Q2_rc=conv(data_Q2,xrc)/5.5; 

f1=1; 
t1=0:0.1:1e3+0.9; 
n0=randn(size(t1)); 
I_rc=data_I2_rc.*cos(2*pi*f1*t1); 
Q_rc=data_Q2_rc.*sin(2*pi*f1*t1); 
QPSK_rc_n0=(sqrt(1/2).*I_rc+sqrt(1/2).*Q_rc)+n0; 

>>..demodulation 
I_demo=QPSK_rc_n0.*cos(2*pi*f1*t1); 
Q_demo=QPSK_rc_n0.*sin(2*pi*f1*t1); 

I_recover=conv(I_demo,xrc); 
Q_recover=conv(Q_demo,xrc); 

I=I_recover(11:10010); 
Q=Q_recover(11:10010); 
t2=0:0.05:1e3-0.05; 
t3=0:0.1:1e3-0.1; 


data_recover=[]; 
for i=1:20:10000 
data_recover=[data_recover I(i:1:i+19) Q(i:1:i+19)]; 
end; 




> figure(1) 
> subplot(3,1,1);plot(t3,I);axis([0 10 -6 6]); 
> subplot(3,1,2);plot(t3,Q);axis([0 10 -6 6]); 
> subplot(3,1,3);plot(t2,data_recover);axis([0 10 -6 6]); 
> 
> figure(2) 
> subplot(2,1,1);plot(I_rc);axis([0 10000 -3 3]); 
> subplot(2,1,2);plot(Q_rc);axis([0 10000 -3 3]); 
> 
> figure(3) 
> subplot(2,1,1);plot(data_I2_rc);axis([0 10000 -3 3]); 
> subplot(2,1,2);plot(data_Q2_rc);axis([0 10000 -3 3]); 

bit_recover=[]; 
for i=1:20:20000 
if sum(data_recover(i:i+19))>0 
data_recover_a(i:i+19)=1; 
bit_recover=[bit_recover 1]; 
else 
data_recover_a(i:i+19)=-1; 
bit_recover=[bit_recover -1]; 
end 
end 

error=0; 

dd = -2*bit_in+1; 
ddd=[dd']; 
ddd1=repmat(ddd,20,1); 
for i=1:2e4 
ddd2(i)=ddd1(i); 
end 
for i=1:1e3 
if bit_recover(i)~=ddd(i) 
error=error+1 
end 
end 

p=error/1000 



subplot(2,1,1);plot(t2,ddd2);axis([0 100 -2 2]); 
subplot(2,1,2);plot(t2,data_recover_a);axis([0 100 -2 2]);