WHAT IS THE USE OF THIS FUNCTION IN THIS OFDMA SIMULATION CODE "ofdma_mapping"

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PLEASE TELL ME THE USE AND HOW TO BUILd THE FUNCTION, "ofdma_mapping",in this code, THAT CAN DO THE WORK OF "q_out=ofdma_mapping(y,ylen)"

Here is the code. Please help and So many thanks in advance.

% Number of Subcarriers
NS=512;
% Input Generation
x=rand(1,NS)>0.5;
fftlength=512;
nd=6;
BW=5e6;
FS=2*BW;%Sampling Frequency
% Conversion of data from serial to parallel
p=series2parallel(x,NS);
% M-ary Modulaton of PSK & QAM
M=2;
X=0;
for Count1=2:1:7;
    if(M==2||M==4||M==16||M==64)
        M=M+X
        %M-ary modulation for producing y
        if(M<=8)
            %Modulation for PSK
            y=modulate(modem.pskmod(M),p);
        else
            %Moodulation for QAM
        end
        ylen=length(y);
        %Applying Mapping   
        q_out=ofdma_mapping(y,ylen);
        %Apply IFFT operation
        outifft=ifft(q_out);
        %Cyclic Prefix Addition
        cp(count1,:)=cyclicpad(outifft,64);
        %Length of CP
        cplength=length(cp);
        %Conversion of data from parallel to serial
        out=reshape(cp(count1,:),1,cplength);
        %Signal transmits through AWGN channel
        ynoisy=awgn(out,100,'measured');
        %Addition of relay fading
        c=rayleighchan(1/1000,100,[0 2e-5],[0 -9]);
        rf=filter(c,ynoisy);
        %Conversion of data from serial to parallel
        p2=series2parallel(rf,cplength);
        re_par=real(p2);
        %Remove cyclic prefix
        rcp(count1,:)=decyclicpad(p2,64);
        rcplength=length(rcp);
        %FFT
        zzfft=fft(rcp(count1,:),fftlength);
        %Apply Demapping
        qq_out=ofdma_demapping(zzfft,fftlength);
        outfft=qq_out;
        %%Applung IFFT
        %zfft=ifft(qq_out);
        if(M<=8)
            %Demodulation of PSK at Reciever
            z=demodulate(modem.pskdemod(M),outfft);
        else
            %Demodulation of QAM at reciever
            z=demodulate(modem.qamdemod(M),outfft);
        end
        %conversion of data from parallel to serial
        xdash=reshape(z,1,NS)
        berr=0;
        for a=1:1:NS;
            if(xdash(:,a)==x(:,a))
                berr=0;
            else
                berr=berr+1;
            end
        end
        tberr(count1,:)=berr;
        Eb_No=0:1:NS-1;
        Eb_No=0.4*Eb_No;
          if(M<=8)
              ber(count1,:)=berawgn(Eb_No,'psk',M,'nondiff');
              Pe(count1,:)=erfc(sqrt(0.9*Eb_No)*sin(pi/M));
          else
              ber1(count1,:)=berawgn(0.9*Eb_No,'qam',M);
              Pe(count1,:)=2*((1-(1/sqrt(M)))*erfc(sqrt((1.5*Eb_No)/(M-1))));
          end
          for init=1:1:32
              switch M
              end
          end
      end
      M=2^count1;
  end
  figure()
  %Plot SNR and BER
  semilogy(Eb_No,ber(2,:),'k',Eb_No,ber(3,:),'g',Eb_No,ber1(5,:),'b',Eb_No,ber1(7,:),'r');
  axis([ 25 0.0001 1]);
  xlabel('SNR[dB]')
  ylabel('BER')
  legend('BPSK','QPSK','16-QAM','64-QAM')
  figure()
  %Plot Error Probability
  semilogy(Eb_No,Pe(2,:),'k',Eb_No,Pe(3,:),'r',Eb_No,Pe(5,:),'b',Eb_No,Pe(7,:),'g');
  axis([0 50 0.0001 1]);
  xlabel('SNR[dB]')
  ylabel('Probability of Error')
  legend('BPSK','QPSK','16-QAM','64-QAM')
  title('OFDMA')
  h=spectrum.periodogram;
  figure()
  HS=psd(h,outifft,'SpectrumType','twosided','NFFT',NS,'FS',FS);
  plot(HS)
  xlabel('Sampling Frequency (2 * BW)in MHz')
  ylabel('Power Spectral Density [dBm/Hz]')
  title('OFDMA')
  grid off;