function H =raised_cos_baseband_filter(f,f_data,alpha)
%function output= name(input)....this is the syntax
% have to be careful not to use an existing function name. 
%
% alpha is the excess bandwidth factor and f is the signal frequency, and f_data the data rate
%
%
%
% first create the raised-cosine transfer function....
%
y=abs(f) ;
if y < (1-alpha)*f_data/2
   H=1;
else
   if y > (1+alpha)*f_data/2
      H=1E-120; % making this zero causes numerical problems
   else
      H=(cos( pi*y/alpha/f_data/2 + pi*(1-1/alpha)/4))^2 ;
   end
end
%
%end function