% Define the filter taps
h_ = {[1],[1 1],[1 2 1],[1 3 3 1]};

for i = 1:length(h_)
    h = h_{i};
    
    [H, w] = freqz(h, 1, 1024, 1);
    
    figure(1);
    hold on
    plot(w, 10*log10(abs(H)), 'LineWidth', 2,'DisplayName',['$(1+D)^2$']); %todo
    xlabel('Normalized Frequency');
    ylabel('Amplitude in dB');
    grid on;
    ylim([-20,10])
end