How can I calculate and plot the spectral power density of Square Root Raised Cosine Pulse using fft?

I was given the following Matlab function which takes 4 parameters as arguments and returns a Square Root Raised Cosine pulse.

function [phi, t] = srrc_pulse(T, Ts, A, a)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% phi = srrc_pulse(T, Ts, A, a)                                                 %
% OUTPUT                                                                        %    
%      phi: truncated SRRC pulse, with parameter T,                             %
%                 roll-off factor a, and duration 2*A*T                         %
%      t:   time axis of the truncated pulse                                    %
% INPUT                                                                         %      
%      T:  Nyquist parameter or symbol period  (real number)                    %       
%      Ts: sampling period  (Ts=T/over)                                         %      
%                where over is a positive INTEGER called oversampling factor    %      
%      A:  half duration of the pulse in symbol periods (positive INTEGER)      %        
%      a:  roll-off factor (real number between 0 and 1)                        %
%                                                                               %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

t = [-A*T:Ts:A*T] + 10^(-8); % in order to avoid division by zero problems at t=0.

if (a>0 && a<=1)
   num = cos((1+a)*pi*t/T) + sin((1-a)*pi*t/T) ./ (4*a*t/T);
   denom = 1-(4*a*t./T).^2;
   phi = 4*a/(pi*sqrt(T)) * num ./ denom;
elseif (a==0)
   phi = 1/(sqrt(T)) * sin(pi*t/T)./(pi*t/T);
else
    phi = zeros(length(t),1);
    disp('Illegal value of roll-off factor')
    return
end

For example let's suppose that T=1, Ts=1/10, A=4 and a=0. So I invoke the method like that:

phi1 = srrc_pulse(1, 1/10, 4, 0)

What I want to do next is to find the Fourier Transform of this pulse at L equally spaced points (for example L=1000) across the frequency axis from -(Fs/2) to Fs/2 where Fs is the sampling frequency, using the fft function and then plot what I get so I can have a visual approach of the spectral power density of the pulse. My problem is that I can see the plot only for the positive values of frequency, and I cannot see the negative ones. I have also attached an image of what I get on the plot. My code is the following. Any help would be greatly appreciated!

%

[Nishitha Ayyalapu]

attachment missing. Try again.

[Konstantinos]

I'm sorry. I have just attached the image. Thank you!

Answer 1

[Nishitha Ayyalapu]

I edited your inital code to go from single-sided spectrum to two-sided.

Fs = 10; %Sampling frequency
T = 1/Fs; %Symbol period
L = 1000; %Number of points
t = (0:L-1)*T; %Time vector
phi1 = srrc_pulse(1, 1/10, 4, 0); %SRRC pulse
figure
plot(Fs*t(1:50), phi1(1:50));
NFFT = 2^nextpow2(L);
Y = fft(phi1, NFFT)./L;
Y2 = fftshift(fft(phi1, NFFT)./L);
f = Fs/2*linspace(0,1,NFFT/2+1);
f2 = ((0:NFFT-1) -ceil((NFFT-1)/2))/NFFT/T;
figure
plot(f, 2*abs(Y(1:NFFT/2+1))); %one sided
figure
plot(f2, 2*abs(Y2)); %two sided

[Konstantinos]

Thank you very much! Your answer really solved my problem!

Answer 2

[Nishitha Ayyalapu]

However to answer how to get negative frequency axis:

1.) Do fftshit

2.) change frequency axis accordingly.

Below is the link which answered how to do that

http://www.mathworks.com/matlabcentral/answers/90457-extracting-frequency-from-fft-command