%	Bit error probability for DSSS in multi-users
%
N = input('Enter processing gain (chips per bit) ');
K = input('Enter vector of number of users ');
clf
z_dB = 0:.1:30;
z = 10.^(z_dB/10);
LK = length(K);
for n = 1:LK
   KK = K(n);
	SNR_1 = sqrt((KK-1)/(3*N)+1./(2*z));
	SNR = 1./SNR_1;
	Pdsss=.5*erfc(sqrt(SNR));
	semilogy(z_dB,Pdsss),axis([0 30 10^(-6) 1]),xlabel('E_b/N_0, dB'),ylabel('P_b_i_t'),...
      text(z_dB(180), 1.3*Pdsss(180), ['No. users = ',num2str(KK),'; PG = ', num2str(N)])
   if n == 1
      grid on
      hold on
   end
end
title(['Bit error probability for DSSS; number of chips per bit = ',num2str(N)])