nsignal = signal + noise; % transmitted signal after adding the noise
flo = 2e9;
fup = 3e9; % assuming here that freq. range of the antenna is 2~3 GHz
fc = (flo + fup)/2;
fs = 4.5e9; % stopband frequency
Wp = [flo fup]/(fs/2); % normalized passband interval
Ws = [(fc/2) (fc+(fc/2))]/(fs/2); % normalized stopband interval
Rp = 3; % 3-dB attenuation at passband
Rs = 30; % 30-dB attenuation at stopband
[n,Wn] = buttord(Wp,Ws,Rp,Rs); % Butterworth filter
[b,a] = butter(n,Wn);
channelsignal = filter(b, a, nsignal); % The resulting signal