I have no idea what you actually want to do.
It is generally appropriate to use filtfilt rather than filter for signal processing applications, and to put the excitation signal in the centre of the vector, especaially if you want to do Fourier transforms on it later. Also, I can find no reference for either ‘delta’ or ‘unitstep’. I have substituted the functions that I can find references for.
Try this:
N = 60; n = 0:N-1;
b = [0.18 0.1 0.3 0.1 0.18];
a = [1 -1.15 1.5 -0.7 0.25];
d = dirac(1);
u = heaviside(1);
nd = n;
nu = n;
nd(N/2) = d;
nu(N/2) = u;
y1 = filter(b,a,nd);
y2 = filter(b,a,nu);
figure
plot(n, y1, n, y2)
grid
Experiment to get the results you want.
