Changing the center of the frequency spectrum

Hi,

I'm trying to plot this wave and have the center of the wave at zero. I was trying to do that by changing the x-axis of the graph but it did not work. The graph is on figure 2 called 'Spectrum of 2PAM signal'.

The Code is:

clear all

close all

fs =48000; %sampling frequency

Ts= 1/fs; %sampling duration

symbolrate = 1000; %transmitted pulses/second should be an integer divisor of fs

symbol_length = fs/symbolrate; %number of samples in one symbol

pulse = ones(1,symbol_length); %1 pulse

textin = input('Enter ASCII text for transmission: ' , 's'); % textin

data_input = dec2bin(double(textin)); %change txtin which is 10# to 2#

[number,length_data] = size(data_input);

data0 = reshape(data_input' , 1,number*length_data); %change the data of matrix to row vector

[number,length_data] = size(data0);

for i = 1:length_data

data(i) = bin2dec(data0(i));

end

[number, length0] = size(data);

symbol = 2*data-1;

for i = 1: length0

for j=1:symbol_length

signal((i-1)*symbol_length + j) = symbol(i) * pulse(j);

end

end

index = 0: Ts: (length0*symbol_length-1)*Ts;

% index = -Ts: Ts:0.01;

freq=fft(signal);

freq=fftshift(freq);

figure(1);

plot(index, signal);

axis([0 1.5*length0*symbol_length*Ts -2 2]);

xlabel('time');

ylabel('amplitude');

title('basband transmission signal of 2PAM');

figure(2);

plot(index, abs(freq));

xlabel('frequency');

ylabel('magnitude');

title('spectrum of 2PAM signal');