Can anyone explain what is happening in the AGWN channel section to the R section of this code for me? I am really confused

clc

clear

close all

% ofdm transceiver

N = 128; % number of subcarriers

M = 20; % number of ofdm blocks

CP1 = N/8; % CP length system 1

CP2 = N/16; % CP length system 2

sigma = 0;

% Input data

data_in_1 = sign(randn(N,M));

data_in_1(N/2+1:end,:) = 0;

data_in_2 = sign(randn(N,M));

data_in_2(1:N/2,:) = 0;

% OFDM Tx

ofdm_sig1 = ifft(data_in_1);

ofdm_sig2 = ifft(data_in_2);

% CP insertion

cp_ofdm_sig1 = [ofdm_sig1(N-CP1+1:N,:); ofdm_sig1];

cp_ofdm_sig2 = [ofdm_sig2(N-CP2+1:N,:); ofdm_sig2];

% AWGN Channel

noise = sigma*(randn(size(cp_ofdm_sig1))+1i*randn(size(cp_ofdm_sig1)));

r = cp_ofdm_sig1(:).'+ [cp_ofdm_sig2(:).' zeros(1,M*(CP1-CP2))];

r = reshape(r,CP1+N,M);

% OFDM Rx

% CP removal

ofdm_sig_rx = r(CP1+1:end,:);

data_out = sign(real(fft(ofdm_sig_rx)));

% Errors

z = (data_out(1:N/2,:)~=data_in_1(1:N/2,:));

error_number = sum(z(:));

error_rate = error_number / ((N/2)*M)

% hist(real(noise(:)),100)