problems in trying ieee 802.15.4 working from msk

Question

Hi, i took a msk code from dsplog.com and tried to modify it to test the ieee 802.15.4. There are several links on that site for ieee 802.15.4. Currently I am getting simulated ber results all approximately same for all the cases of Eb_No values. Can you help me to find why? thanks in advance!

 clear

 PN = [
       1 1 0 1 1 0 0 1 1 1 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 1 1 1 0;
       1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0;
       0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0 0 0 1 1 0 1 0 1 0 0 1 0;
       0 0 1 0 0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0 0 0 1 1 0 1 0 1;
       0 1 0 1 0 0 1 0 0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0 0 0 1 1;
       0 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0;
       1 1 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1;
       1 0 0 1 1 1 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 1 1 1 0 1 1 0 1;
       1 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1;
       1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1;
       0 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1;
       0 1 1 1 0 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0;
       0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0;
       0 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1;
       1 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0;
       1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1 1 0 1 1 1 0 0 0;
   ];

   N = 5*10^5; % number of bits or symbols

   fsHz = 1; % sampling period
   T    = 4; % symbol duration

   Eb_N0_dB = [0:10]; % multiple Eb/N0 values

   ct = cos(pi*[-T:N*T-1]/(2*T));
   st = sin(pi*[-T:N*T-1]/(2*T));

   for ii = 1:length(Eb_N0_dB)
       tx = [];

       % MSK Transmitter
       ipBit = round(rand(1,N/32)*15);

       for k=1:length(ipBit)
           sym = ipBit(k);
           tx = [tx PN((sym+1),1:end)];
       end

       ipMod =  2*tx - 1; % BPSK modulation 0 -> -1, 1 -> 1

       ai = kron(ipMod(1:2:end),ones(1,2*T));  % even bits
       aq = kron(ipMod(2:2:end),ones(1,2*T));  % odd bits

       ai = [ai zeros(1,T)  ]; % padding with zero to make the matrix dimension match 
       aq = [zeros(1,T) aq ];  % adding delay of T for Q-arm

       % MSK transmit waveform
       xt = 1/sqrt(T)*[ai.*ct + j*aq.*st];

       % Additive White Gaussian Noise
       nt = 1/sqrt(2)*[randn(1,N*T+T) + j*randn(1,N*T+T)]; % white gaussian noise, 0dB variance 

       % Noise addition
       yt = xt + 10^(-Eb_N0_dB(ii)/20)*nt; % additive white gaussian noise

       % MSK receiver 
       % multiplying with cosine and sine waveforms
       xE = conv(real(yt).*ct,ones(1,2*T));
       xO = conv(imag(yt).*st,ones(1,2*T));

       bHat = zeros(1,N);
       bHat(1:2:end) = xE(2*T+1:2*T:end-2*T); % even bits
       bHat(2:2:end) = xO(3*T+1:2*T:end-T);   % odd bits

       result=zeros(16,1);
       chiplen=32;
       seqstart=1;
       recovered = [];

       while(seqstart<length(bHat))
           A = bHat(seqstart:seqstart+(chiplen-1));
           for j=1:16
               B = PN(j,1:end);
               result(j)=sum(A.*B);
           end

           [value,index] = max(result);
           recovered = [recovered (index-1)];
           seqstart = seqstart+chiplen;
       end;


       %# create binary string - the 4 forces at least 4 bits
       bstr1 = dec2bin(ipBit,4);
       bstr2 = dec2bin(recovered,4);

       %# convert back to numbers (reshape so that zeros are preserved)
       out1 = str2num(reshape(bstr1',[],1))';
       out2 = str2num(reshape(bstr2',[],1))';

       % counting the errors
       nErr(ii) = size(find([out1 - out2]),2);
   end

   nErr/(length(ipBit)*4) % simulated ber
   theoryBer = 0.5*erfc(sqrt(10.^(Eb_N0_dB/10))) % theoretical ber