%  Computation and plotting of transmitter and receiver frequency responses
%  for raised cosine signaling through lowpass Butterworth filtered channel
%
A = char('-',':','--','-.');
clf
f3 = input('Enter channel filter 3-dB frequency, Hz ');
n_poles = input('Enter number of poles for Butterworth filter representing channel ');
Rb = input('Enter data rate, bits/s ');
beta00 = input('Enter starting value for beta ');
del_beta = input('Enter step size in beta (four steps used)');
for n = 1:4
   beta = beta00+(n-1)*del_beta;
   beta0(n) = beta;
	f1 = 0:.1:(1-beta)*Rb/2;
	HR1 = sqrt(1+(f1/f3).^(2*n_poles));
	f2 = (1-beta)*Rb/2:.1:(1+beta)*Rb/2;
	HR2 = sqrt(1+(f2/f3).^(2*n_poles)).*cos(pi*(f2-(1-beta)*Rb/2)./(2*beta*Rb));
	f = [f1 f2];
	HR = [HR1 HR2];
	plot(f,HR,A(n,:)),xlabel('f, Hz'),ylabel('|H_R(f)|, |H_T(f)|')
	if n == 1
   	hold on
	end
end
title(['Bit rate = ',num2str(Rb),' bps; channel filter 3-dB frequency = ',num2str(f3),' Hz; no. of poles = ',num2str(n_poles)]),...
legend(['\beta = ',num2str(beta0(1))],['\beta = ',num2str(beta0(2))],['\beta = ',num2str(beta0(3))],['\beta = ',num2str(beta0(4))],2)