my code keeps looping indefinitely and I could not figure out why, please help

%% parameter
SNR_dB = 10;%[0:5:20];  
SNR = 10^(SNR_dB/10.);
N_iter = 100; 
n = 256;    % number of beams (transmit antennas)
%K = 32;     % number of users
L = 3;      % number of paths per user
lamada = 1; % wavelength
d = lamada/2;
%N = K;      % number of retained RF chains for conventional beamspace MIMO IA-BS
P = 32;     % total transmitted power
Imax = 30;  % iteration times of power allocation

temp_it = [];

K_list = 10:10:50; 

%%
for i_snr = 1:length(K_list) 
    
    K = K_list(i_snr);
    N = K;
    
    %SNR = SNR_linear(i_snr);
    temp = 0; temp1 = 0; temp2 = 0; temp3 = zeros(1,Imax); temp4 = zeros(1,Imax); temp5 = 0; temp6 = 0;
    temp22 = 0; temp33 = zeros(1,Imax); temp44 = zeros(1,Imax);  temp55 = 0; temp66 = 0;
    
    for iter = 1:N_iter
        H = beamspace_channel(n,K,L); % generate the signalspace channel
        U = zeros(n,n); 
        deta = 1/n;
        for i = -(n-1)/2:1:(n-1)/2
            U(:,i+(n+1)/2) = sqrt(1/n)*exp(1i*[0:n-1]*2*pi*deta*i).';
        end
        H_beam = U.'*H; % beamspace channel             
        
        [Hr1,Hr1_e] = IA_BS(H_beam,H_beam);    % IA-BS
        [Hr3,Hr3_e,F3,rf_num3,setf3] = reduce_RF(H_beam,K,H_beam); % NOMA mmWave-P1
        
        %% Full digital 
        F = H_beam*inv(H_beam'*H_beam);
        beta = sqrt(P/trace(F'*F));
        H_eq = H_beam'*F;
        for k = 1:K
            sum_inf = sum(abs(H_eq(k,:)).^2)-abs(H_eq(k,k))^2;
            temp = temp+log2(1+abs(H_eq(k,k))^2/(sum_inf+1/(SNR*beta^2)));
        end
        
        %% IA-BS
        F1 = Hr1_e*inv(Hr1_e'*Hr1_e);
        beta1 = sqrt(P/trace(F1'*F1));
        H_eq1 = Hr1'*F1;
        for k = 1:K
            sum_inf = sum(abs(H_eq1(k,:)).^2)-abs(H_eq1(k,k))^2;
            temp1 = temp1+log2(1+abs(H_eq1(k,k))^2/(sum_inf+1/(SNR*beta1^2)));
        end
        
        %% NOMA mmWave
        [SE,EE,ite,power1] = PA(Hr3,F3,setf3,K,rf_num3,1/SNR,P,Imax);
        temp3 = temp3+SE;
        temp33 = temp33+EE;
        
        %% OMA mmWave
        beta3 = P/trace(F3'*F3);
        H_eq3 = Hr3'*F3;
        for i = 1:rf_num3
            usern = setf3(i,:);
            usern(usern==0) = [];
            unum  = length(usern);
            for m = 1:unum
                sum_inf = sum(abs(H_eq3(usern(m),:)).^2)-sum(abs(H_eq3(usern(m),usern(1:end))).^2);
                temp5 = temp5+log2(1+abs(H_eq3(usern(m),usern(m)))^2/(sum_inf+1/SNR/unum/beta3))/unum;
                temp55 = temp55+log2(1+abs(H_eq3(usern(m),usern(m)))^2/(sum_inf+1/SNR/unum/beta3))/unum/(P+rf_num3*305+200)*10^3;
            end
        end
        
    end
    
    C(i_snr) = temp/N_iter;
    C1(i_snr) = temp1/N_iter;
    C3(i_snr) = temp3(end)/N_iter;  
    C5(i_snr) = temp5/N_iter;    
    
    C00(i_snr) = temp/N_iter/(P+n*305+200)*10^3;
    C11(i_snr) = temp1/N_iter/(P+K*305+200)*10^3;
    C33(i_snr) = temp33(end)/N_iter;
    C55(i_snr) = temp55/N_iter;
    
    temp_it = [temp_it;temp3];
    
end

Unrecognized function or variable 'beamspace_channel'.

%CI3 = temp_it(3,:)/N_iter;

%%
figure;
plot(K_list,C,'k-*','Linewidth',1.5);
hold on 
plot(K_list,C1,'r-*','Linewidth',1.5);
hold on 
plot(K_list,C3,'g-*','Linewidth',1.5);
hold on 
plot(K_list,C5,'c-*','Linewidth',1.5);
grid on
xlabel('The number of users');
ylabel('Spectral efficiency');
legend('Fully digital MIMO', 'The proposed beamspace MIMO-NOMA', 'MIMO-OMA', 'IA beam selection');

figure;
plot(K_list,C00,'k-','Linewidth',1.5);
hold on 
plot(K_list,C11,'r-','Linewidth',1.5);
hold on 
plot(K_list,C33,'g-','Linewidth',1.5);
hold on 
plot(K_list,C55,'c-','Linewidth',1.5);
grid on
xlabel('The number of users');
ylabel('Energy efficiency');
legend( 'The proposed beamspace MIMO-NOMA', 'MIMO-OMA', 'IA beam selection', 'Fully digital MIMO');

% figure;
% plot(ite,CI3,'g-','Linewidth',1.5);
% grid on
% xlabel('Iteration');
% ylabel('Spectral efficiency');

Answer 1

[Jan]

No, the loops do run run infinitely. They are only slow. Check this with displaying a loop counter.

Use the profile to find the bottleneck:

profile on

Then start the code. Press Ctrl-C after some time.

profile report

In which lines does Matlab spent the most time? Start optimizing the code there.

U is constant, isn't it? So avoid to create it in each iteration again, but do this once before the loop:

U = zeros(n,n); 
deta = 1/n;
for i = -(n-1)/2:1:(n-1)/2
    U(:,i+(n+1)/2) = sqrt(1/n)*exp(1i*[0:n-1]*2*pi*deta*i).';
end

Or do I see correctly that even H_beam is a constant?