About Contemporary Communications System's example

n = -20:20; x = 0.5*(sinc(n/2).^2); ts = 1/40; fs = 1/ts; t = -0.5:ts:1.5;

u05 = (t+0.5)>=0; u0 = t>=0; u1 = (t-1)>=0; h1 = 0*u05; h2 = t.*(u0-u1); h = h1 + h2; H = fft(h)*ts;

df = fs/80; f = [0:df:fs] - fs/2; H1 = fftshift(H); plot(f,abs(H1)) pause

y = x.*H1(21:61); plot(abs(H1)) pause

Hi friends , I dont understand why it is used H1(21:61) and how was H(n/2) provided(page 16)