#### joserse46

##### Newbie level 4

In the block diagram i forgot to label x (inphase) and y(quadrature) before the AWGN channel

Oh and the output of the decoder should be d_hat1 and d_hat2

http://obrazki.elektroda.pl/3087588200_1361063670.jpg

Matlab code[%qpsk.m models baseband qpsk with N input bits

clear

clc

N = 10^4; %number of input bits

Eb_No = 1:.2:7; % In decibels

for k=1:length(Eb_No);

EB_NO = 10.^(k/10); % linear scale

%source

for i = 1:N;

d1(i) = randint;

d2(i) = randint;

end

for j = 1:N

%qpsk encoding

if [d1(j) d2(j)] == [0 0];

I(j) = sqrt(EB_NO*2) ;

Q(j) = 0 ;

elseif [d1(j) d2(j)] == [0 1];

I(j) = 0 ;

Q(j) = -sqrt(EB_NO*2);

elseif [d1(j) d2(j)] == [1 0];

I(j) = -sqrt(EB_NO*2) ;

Q(j) = 0 ;

else

I(j) = 0;

Q(j) = sqrt(EB_NO*2);

end

%pulse shaping

for i = 1:4;

s = [.5 .5 .5 .5];

x(4*(j-1)+i) = I(j)*s(i);

y(4*(j-1)+i) = Q(j)*s(i);

end

%channel

for i = 1:4;

w_i(4*(j-1)+i) = randn *(1/sqrt(2));

w_q(4*(j-1)+i) = randn *(1/sqrt(2));

x_hat(4*(j-1)+i) = x(4*(j-1)+i)+w_i(4*(j-1)+i);

y_hat(4*(j-1)+i) = y(4*(j-1)+i)+w_q(4*(j-1)+i);

end

%correlator

for i = 1:4

z_i(i) = x_hat(4*(j-1)+i)*s(i);

z_q(i) = y_hat(4*(j-1)+i)*s(i);

end

i_hat(j) = z_i(1)+z_i(2)+z_i(3)+z_i(4);

q_hat(j) = z_q(1)+z_q(2)+z_q(3)+z_q(4);

%qpsk decoder

theta(j) = atan2(q_hat(j),i_hat(j));

if -(pi/4) <= theta(j) && theta(j) < (pi/4);

d_hat1(j) = 0;

d_hat2(j) = 0;

elseif (pi/4) <= theta(j) && theta(j) < (3*pi/4);

d_hat1(j) = 1;

d_hat2(j) = 1;

elseif -(3*pi/4) <= theta(j) && theta(j) < (pi/4);

d_hat1(j) = 0;

d_hat2(j) = 1;

else

d_hat1(j) = 1;

d_hat2(j) = 0;

end

end

%probability of error

C1 = xor(d1,d_hat1);% this creates a vector where a 1 is given

C2 = xor(d2,d_hat2);% when they differ or an error has occured

Pb_simulated(k) = (sum(C1)+sum(C2))*(1/N);%overall error for each EB/NO

Pb_theoretical(k) = qfunc(sqrt(2*EB_NO));

%symbol error

Symbol_error(k) = sum(or(C1,C2));%Symbol error occurs if error in

%C1 or C2

%plots

xlim([0 7])

ylim([10^(-5) 1])

semilogy(Pb_simulated, 'red')

hold on

semilogy(Pb_theoretical,'blue')

grid on

title('QPSK Modulation');

xlabel('EB/No(dB)', 'fontsize', 12');

ylabel('Probability of Error', 'fontsize', 12');

hleg1 = legend('Pb simulated','Pb theoretical');

set(hleg1,'fontSize',12);

end

][/CODE]