OFDM based transmitter and receiver

Question

Jose Iglesias 2023-4-18

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https://ww2.mathworks.cn/matlabcentral/answers/1949373-ofdm-based-transmitter-and-receiver

评论： Jose Iglesias 2023-4-20

I am trying to correct an error in my Matlab code. I am debugging my OFDM based transmitter and receiver with BPSK modulation Matlab code shown below. I tried a suggestion to remove an error I was getting when I would run my code. The suggestion was to change the following code instruction:

cp_len=floor(0.1*block_size); %Length of cyclic prefix

into the following code in order to convert 'cp_len' to integer

cp_len=round(0.1*block_size); %Length of cyclic prefix

I am still getting the following error.

Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in ofdm_bpsk_ji_2 (line 74)

cyclic_prefix = ifft_Subcarrier(end-cp_len+1:end, :);

Any suggestions on how I can remove this issue in my Matlab code? I am perusing the Mathworks forum and other sources to find a solution.The complete code is shown below. Thank you in advance!

clc;

clear all;

close all;

% Initiation

no_of_data_bits = 64;

M = 64; %Number of subcarrier channel

n = 256; %Total number of bits to be transmitted at transmitter

block_size=64; %Size of each OFDM block to add cyclic prefix

cp_len=round(0.1*block_size); %Length of cyclic prefix

% Read the video file

video = VideoReader('pexels-pixabay-856027-960x540-25fps.mp4');

% Loop through each frame of the video and convert it to a binary data stream

binary_data = [];

while hasFrame(video)

frame = readFrame(video);

% Convert the image to grayscale

gray_frame = rgb2gray(frame);

% Threshold the grayscale image to create a binary image

binary_frame = imbinarize(gray_frame);

% Reshape the binary image into a 1D array and append it to the binary data

binary_data = [binary_data reshape(binary_frame, 1, [])];

end

% Calculate the total number of bits in the video

total_bits = numel(binary_data);

% Use the binary data as the input source data

data = binary_data(1:no_of_data_bits);

figure(1)

stem(data);

grid on;

xlabel('DataPoints');

ylabel('Amplitude');

title('Original Data')

% Perform BPSK modulation on the input source data

psk_modulated_data = pskmod(data,2);

figure(2)

stem(psk_modulated_data);

title('PSK Modulation')

% Converting the series data stream into parallel data stream to form

% subcarriers

S2P = reshape(psk_modulated_data,no_of_data_bits/M,M);

figure(3)

stem(S2P)

title('Subcarriers')

grid on;

% Generate channel

channel = (randn(1, 10) + 1i * randn(1, 10)) / sqrt(2)

figure(4)

stem(channel);

grid on;

% IFFT of subcarriers and add cyclic prefix

number_of_subcarriers = M;

ifft_Subcarrier = ifft(S2P, block_size);

figure(5)

plot(real(ifft_Subcarrier),'r'),

title('IFFT on all the subcarriers')

% Add cyclic prefix

cyclic_prefix = ifft_Subcarrier(end-cp_len+1:end, :);

Append_prefix = [cyclic_prefix; ifft_Subcarrier];

figure(6)

plot(real(Append_prefix),'r')

% Initiate SNR range and BER array

SNR_range = -10:1:10;

BER = zeros(size(SNR_range));

% Loop over SNR range

for i = 1:length(SNR_range)

% Calculate noise variance for current SNR

snr = SNR_range(i);

noise_var = 1 / (10^(snr/10));

% Generate noisy channel coefficients

noisy_channel = channel + sqrt(noise_var/2)*(randn(1,10)+1i*randn(1,10));

% Convert to serial stream for transmission

ofdm_signal = Append_prefix(:).';

figure(7)

plot(real(ofdm_signal));

xlabel('Time');

ylabel('Amplitude');

title('OFDM Signal')

grid on

% Apply the channel

rx_signal = conv(ofdm_signal, channel, 'same');

% Remove cyclic prefix at the receiver

rx_no_cp = reshape(rx_signal, block_size+cp_len, M);

rx_no_cp(1:cp_len, :) = [];

% Perform FFT on received data

rx_fft = fft(rx_no_cp, block_size);

% Equalize channel effect

rx_equalized = rx_fft ./ channel(1);

% Perform BPSK demodulation

rx_demod = pskdemod(rx_equalized(:), 2);

% Calculate bit error rate

ber = sum(rx_demod ~= data.') / numel(data);

end

% Plot BER vs SNR

figure;

semilogy(SNR_range, BER);

grid on;

xlabel('SNR (dB)');

ylabel('Bit Error Rate');

title('Probability of Error vs SNR');

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Daniel 2023-4-19

It's a little tricky to give good information on what's going wrong, as I can't replicate the error for a few reasons. After pasting your provided code into a temporary script, my line numbers don't match yours; and also, I'm having difficulty replicating the video file parsing stuff without that video file. Would you be able to attach a script that produces the error when run in a clean MATLAB instance in an otherwise empty directory?

Jose Iglesias 2023-4-20

Daniel,

Thanbk you for your response. I was able to resolve the issue by using this code:

cyclic_prefix = ifft_Subcarrier(:,end-cp_len+1:end);

Append_prefix = [cyclic_prefix ifft_Subcarrier];

At the momemnt I am still doing some trouble shooting and am currenlty trying add a zero padding to the channel of length

channel = [channel zeros(1, 64)];

I am doing this to equalize the channel since the OFDM sysytem has 64 subcarriers but I am getting an eror within the for loop for the "Loop over SNR range" section . This error states:

Arrays have incompatible sizes for this operation.

Error in ofdm_bpsk_ji_3 (line 102)

noisy_channel = channel + sqrt(noise_var/2)*(randn(1,10)+1i*randn(1,10));

I am going to post this new error and hopefully someone can give me some guidande on how to resolve this issue.

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OFDM based transmitter and receiver

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