wireless communication system simulation

Hello all, I had implemented a optimal detector algorithm in matlab. The complete program is running but the bit error rate curve is not coming properly. The bit error rates values should go on decreasing but in my case it decreases at start and later increases. I am attaching the code developed by me along with the algorithm. Any help in this regard would be highly appreciated.

clc;
clear all;
close all;
% Initialization
Nw = 1; % noise variance
M = 50 ; % symbol period of the tag
Mt = 4; % training symbols 
al = 0.5 ; % alpha is tag coefficient
L = 1000;
N_mc = 10^6;
N = 50; % comsecutive samples of s(n)
RCD = 0.5;% Relative channel difference.
SNRdB = 0:5:20; 
%%% initialisation complete%%%%%%%%%%%%

% Radio frequency source transmitting signal s(n)
sn = sqrt(1/2)*(randn(1,L)+1i*randn(1,L));
Ps = 1; % RF source signal power

% channels and AWGN noise
hsr = sqrt(1/2)*(randn(1,L)+1i*randn(1,L)); % channel between source and reader
hst = sqrt(1/2)*(randn(1,L)+1i*randn(1,L));% channel between source and tag
vn = 10;
htr = sqrt(vn/2)*(randn(1,L)+1i*randn(1,L)); % channel between tag and reader

wn =  sqrt(1/2)*(randn(1,L)+1i*randn(1,L)); % AWGN noise


% Thresholds
ho = hsr;
ho_abs = sqrt(sum(abs(ho).^2));
so2 = (((ho_abs)^2)*Ps)+Nw;% sigma_o square

h1 = (hsr)+ (al*(hst.*htr));
h1_abs = sqrt(sum(abs(h1).^2));
s12 = (((h1_abs)^2)*Ps)+Nw; % sigma_1 square

Nid  = 10^2 ;% number of incorrect bit decisions

% Reader / receiver

for n = 1:length (SNRdB)
    errcnt = 0;
    bitcnt = 0;
    
    while errcnt < Nid 
    
    d = (rand > 1/2); % tag
    
    if d == 0
     
     y = ((hsr + al*(hst.*htr)*d).*sn)+ 10^(-SNRdB(n)/20)* wn;
     
     ybar = y(1:N);
     z = norm(ybar);
     Z = z^2; 
    end
    
    if d == 1
     y = ((hsr + al*(hst.*htr)*d).*sn)+ 10^(-SNRdB(n)/20)* wn; 
     ybar = y(1:N);
     z = norm(ybar);    
     Z = z^2; 
    end
    
ho = hsr;
ho_abs = sqrt(sum(abs(ho).^2));
so2 = (((ho_abs)^2)*Ps)+Nw;% sigmao square

h1 = (hsr)+ (al*(hst.*htr));
h1_abs = sqrt(sum(abs(h1).^2));
s12 = (((h1_abs)^2)*Ps)+Nw; % sigma1 square
   
ThCG = ((N*so2*s12)/(s12-so2))*log(s12/so2); % threshold for complex gaussian optimal detector.
    
 if so2 > s12
     if Z > ThCG
         dhat = 0;
     else
         dhat = 1;
     end
    end
    if so2 < s12
     if Z < ThCG
         dhat = 0;
     else
         dhat = 1;
     end
 end
  if dhat ~= d
    errcnt = errcnt+1;
 end
    bitcnt = bitcnt+1;
 
    end
       BER(n) = errcnt/bitcnt;   
 
    end  
    
figure
semilogy(SNRdB,BER);
grid on;
xlabel('SNRdB');
ylabel('Bit Error Rate');