Digital Communication System - Decoding

Question

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I have an error in this code but I can not tell why?!

%DECODING
dec_data=reshape(dec_data,4,length(bits)/4)';
mq_rece=zeros(size(mq));
mq_rece(1:1/(fs*sampling_rate):length(mq))=bi2de(dec_data,'left-msb')'+min(mq);   
%DAC - Extracting sampled quantized data from decoded binary sequence

The full code (from mathwork):

%**************************************************************************
%                           DIGITAL COMMUNICATION SYSTEM
%                                    BPSK MODEL
%**************************************************************************
%IMORTANT VARIABLES USED
%t                    - Modeled system time  
%fm                   - Main frequency of message signal
%harm                 - Frequency components of the message signal
%amp                  - Amplitude of the corresonding harmonic frequency component
%sampling_rate        - System sampling rate . Default rate is 20 samples per cycle of maximum freqency component
%range                - Range of the time sampling is done . Default range is 2 cycles of minimum frequncy component
%msg                  - Message signal to be trasnmitted
%minamp               - Minimum amplitude that can be used in this model 
%n_sample             - Number of samples per cycle of the message signal
%fs                   - Sampling frequency for sampling the message signal
%msamp                - Sampled version of the message signale with sampling frequency fs
%no_of_levels         - Number of quantization levels                        
%quantile             - Quantile interval
%code                 - Representation levels 
%mq                   - Quantization output of sampled message signal 
%mq1                  - Sampled version of quantized sample message signal
%bits                 - Binary sequence of sampled quantized signal (output of ADC)
%fc                   - Carrier signal frequency 
%nsamp                - System Samples per cycle of carrier signal  
%ncyc                 - Number of cycles of carrier signal for one bit interval                                                           
%tb                   - Bit interval 
%t_tran               - Total transmission time , in order to find diffrence between already existing time variable 't' see Note-1 
%mod_sig              - BPSK modulated carrier signal
%tran_sig             - Siganl after transmission through AWGN channel , so that Awg noise is added with the original transmitted signal
%f_freq               - Frequency range used for visualising FFT of the signal
%f_tran               - Noise added transmitted signal representation in frequency domain
%f_rece               - Received signal after removing noise ,in frequency domain
%dec_data             - Binaray data extracted from BPSK modulated carrier signal
%mq_rece              - Decoded sampled quantized message signal data calculated from extracted binary sequence
%f_out                - Reconstruted signal in frequency domain after filtering
%out                  - Reconstruted signal from the received signal ,in time domain i.e, output of the receiver
%gain                 - gain of the amplifier expressed in ratio not in db
%
%ABBREVATIONS
%ADC    -    Analog to Digital Convertor 
%               Converts analog signal to digital signal 
%AWGN   -    Anti-White Gaussian Noise
%               A channel model with zero mean noise , commonly used channel model .    
%DAC    -    Digital to Analog Convertor
%               Converts digital signal to analog signal
%Rx     -    Receiver
%Tx     -    Transmitter
%*****************************-TRANSMITTER-********************************
clear all;
clc;
%MESSAGE SIGNAL PARAMETERS
fm=1e3;                                  %Main frequency of message signal
harm=[ 1 0.5 2 1 ];                      %Frequency components of the message signal
                                         %In this model the message signal is represented by fourier sine series   
amp=[ 1 2 3 1 ];                         %Amplitude of the corresonding harmonic frequency component
sampling_rate=1/(20*max(fm*harm));       %System sampling rate . Default rate is 20 samples per cycle of maximum freqency component
range=2/min(fm*harm);                    %Range of the time sampling is done . Default range is 2 cycles of minimum frequncy component
t=0:sampling_rate:range;                 %System timing
%MESSAGE SIGNAL 
msg=zeros(size(t));
for k=1:length(harm)
    msg=msg+amp(k)*sin(2*pi*harm(k)*fm*t);
end
minamp=min(msg);                         %Minimum amplitude that can be used in this model . Normally ,it need to be kept as a global constant
                                         %But for flexibility of program it is made here as variable depending on message signal value  
%SAMPLING
n_sample=5;
fs=n_sample*max(harm*fm);                                                       %Sampling ferqency .
msamp=zeros(size(msg));
msamp(1:1/(fs*sampling_rate):length(t))=msg(1:1/(sampling_rate*fs):length(t));  %Sampled output signal
figure(1);plot(t,msg); grid on 
hold on
stem(t,msamp);
xlabel('time');ylabel('Message signal,Sampled signal and Quantized signal');
title('MESSAGE SIGNAL , SAMPLED MESSAGE SIGNAL AND QUANTIZED SIGNAL');
legend('Message signal','Sampled Signal','Quantized Signal');
%QUANTIZATION
no_of_levels=4;                                                            %Number of quantization levels                        
quantile=(max(msamp)-min(msamp))/(no_of_levels);                           %Quantile interval
code=min(msamp):quantile:max(msamp);                                       %Representation levels
mq=zeros(size(msamp));                                                          
for k=1:length(code)
    values=(( msamp>(code(k)-quantile/2) & msamp<(code(k)+quantile/2)));   
    mq(values)=round(code(k));                                             %Quantization output of sampled message signal
end
clear values;
stem(t,mq,'r*');grid on
legend('Message signal','Sampled Signal','Quantized Signal');
%ENCODING
if min(mq)>=0
    minamp=0;                                                        
end
mq1=mq-round(minamp);                                                      %Shifting negative values to postive side for conversion to binary and sampling 
                                                                           %quantized message signal
bits=de2bi(mq1(1:1/(fs*sampling_rate):length(mq)),4,'left-msb')';          
bits=bits(:)';                                                             %ADC - Generating binary sequence of sampled quantized signal 
figure(5);stem(bits,'*r');hold on;
legend('Bit sequence in Transmitter','Bit sequence in Receiver');
%PASS BAND MODULATION (BPSK)
fc=1e6;                                                                    %Carrier signal frequency 
nsamp=10;                                                                  %System Samples per cycle of carrier signal  
ncyc=2;                                                                    %Number of cycles of carrier signal for one bit interval                                                           
tb=0:1/(nsamp*fc):ncyc/fc;                                                 %Bit interval 
t_tran=0:1/(nsamp*fc):(ncyc*length(bits))/fc+(length(bits)-1)/(nsamp*fc);  %Total transmission time   
mod_sig=zeros(size(t_tran));                                               
l=1;
for k=1:length(tb):length(mod_sig)
    if(bits(l)==1)
        mod_sig(k:k+length(tb)-1)=cos(2*pi*fc*tb);                         %Phase Modulation of carrier for repesenting binary symbol one
    else
        mod_sig(k:k+length(tb)-1)=-cos(2*pi*fc*tb);                        %Phase Modulation of carrier for repesenting binary symbol zero
    end
    l=l+1;
end
%**********************AWGN-CHANNEL****************************************
tran_sig=awgn(mod_sig,10);                                                  %Transmisson of modulated carrier signal through AWGN channel 
figure(2);plot(t_tran,mod_sig,'.-b',t_tran,tran_sig,'r');
axis([0 3*ncyc/fc -2 2]);
xlabel('time');ylabel('Tx signal and Tx signal with noise');
title('TRANSMITTED SIGNAL AND NOISE ADDED TRANSMITTED SIGNAL');
legend('Transmitted signal','Transmitted signal with noise added');
%*************************RECEIVER*****************************************
%Filter
f_freq=-(nsamp*fc)/2:(nsamp*fc)/length(t_tran):(nsamp*fc)/2-(nsamp*fc)/length(t_tran);                                   %Frequency range used for 
                                                                                                                         %visualising FFT of the signal
f_tran=fft(tran_sig);                                                                                                    %FFT of f                              
figure(3);plot(f_freq,fftshift(f_tran),f_freq,fftshift(fft(mod_sig)),'g');grid on;
xlabel('Frequency');ylabel('Signal Amplitude');
legend('Modulated Signal','Received Signal');
title('MODULATED SIGNAL VS RECEIVED SIGNAL IN FREQUENCY DOMAIN');
f_rece=zeros(size(f_tran));               
fir=(f_freq < -3*fc | f_freq>3*fc);
f_rece(fir)=f_tran(fir);                                                                                                 %Filtering noisy signal in 
f_rece(~fir)=0.5*f_tran(~fir);                                                                                           %frequnecy domain to remove noise
                                                                                                                          
                                                                                                                          
t_rece=ifft(f_rece);                                                                                                      %Noise removed signal 
figure(4);plot(t_tran,t_rece);grid on;
xlabel('time');ylabel('Received Signal');
title('RECEIVED SIGNAL AFTER NOISE IS FILTERED');
delete f_freq f_tran f_rece;
clear f_freq f_tran  f_rece;
%Demodulation
dec_data=zeros(size(bits));
l=1;
for k=1:length(tb):length(t_tran)                                          %Extracting binary data from carrier using correlation method                                            
        a=corrcoef(cos(2*pi*fc*tb),t_rece(k:k+length(tb)-1));                                                              
        b=mean(real(a(:)));
        if b>0.5
            dec_data(l)=1;
        else
            dec_data(l)=0;
        end
        l=l+1;
end
figure(5);stem(dec_data);grid on;
xlabel('Bit position');ylabel('Bit sequence in Recevier and Transmitter');
title('BIT SEQUENCE IN RECEIVER Vs TRANSMITTER');
legend('Bit sequence in Transmitter','Bit sequence in Receiver');
%DECODING
dec_data=reshape(dec_data,4,length(bits)/4)';
mq_rece=zeros(size(mq));
mq_rece(1:1/(fs*sampling_rate):length(mq))=bi2de(dec_data,'left-msb')'+min(mq);    %DAC - Extracting sampled quantized data from decoded binary sequence
%SIGNAL RECONSTRUCTION
f_freq=-1/(2*sampling_rate):1/(sampling_rate*length(t)):1/(2*sampling_rate)-1/(sampling_rate*length(t));                                  
f_rece=fft(mq_rece);                                                               %FFT of received extracted sampled quantized signal      
f_out=zeros(size(f_rece));                                            
figure(6);plot(f_freq,fftshift(f_rece),f_freq,fftshift((fft(msg))));grid on;
xlabel('frequency');ylabel('FT-msg and FT-Received signal');
title('ORIGINAL SIGNAL Vs RECEIVER OUTPUT IN FREQUENCY DOMAIN');
legend('Receiver output','Original signal');
f_out((f_freq < -17000 | f_freq > 17000))=f_rece((f_freq < -17000 | f_freq > 17000));  %Filtering in frequency domain for reconstruction of signal from sampled
                                                                                   %quantized data     
out=ifft(f_out);                                                                   %Reconstructed output signal 
figure(7);plot(t,4*out,t,msg,'r');grid on;
xlabel('time');ylabel('msg-sig and Rx-output');
title('ORIGINAL SIGNAL Vs RECEIVER OUTPUT');
legend('Receiver output','Original signal');
gain=4;                                                                            %Gain of amplifier (simple ratio not in db)
out=out*gain;                                                                      %Output after amplification
figure(8);plot(t,out);grid on;
xlabel('time');ylabel('Receiver output');
title('RECEIVER OUTPUT');