How to convert JPG to bits for simulink transmission?

I am trying to design a system where I can upload a JPG file, and then run it through a matlab function block which converts the jpg file to bits so that i can use it for an OFDM IEEE 802.11a simulink model. The reference model I am using is the following link attached:

https://www.mathworks.com/matlabcentral/fileexchange/173945-simulink-simulation-of-an-ofdm-ieee-std-802-11a-1999

I am trying to replace the bernoulli generator with a Matlab Function block which converts the jpg file into bits so i can use that as for transmission, and after the simulation is over, I intend to collect those bits that were transmitted and reconstruct the image again. The OFDM system sends in 144 bits at a time, so that needs to be noted when designing the matlab code for jpg to bits. Any and all help will be greatly appreciated!

Answer 1

[Nithin]

Hi @Nisab,

To substitute the Bernoulli Binary Generator with a custom "MATLAB Function" block that provides a bitstream from an image, you’ll first need to convert the image into bits as a preprocessing step. Then, you can access these bits within the "MATLAB Function" block in Simulink. Refer to the steps below to understand the workflow:

• Pre-process the image in MALTAB to store the image as a bitstream

% Read the image file as bytes
fid = fopen('input.jpg', 'rb');
imgBytes = fread(fid, inf, 'uint8');
fclose(fid);

% Convert bytes to bits
imgBits = reshape(de2bi(imgBytes, 8, 'left-msb')', [], 1); % Column vector

% Pad bits to be a multiple of 144
padLength = mod(144 - mod(length(imgBits), 144), 144);
imgBitsPadded = [imgBits; zeros(padLength, 1)];

% Save for use in Simulink (optional)
save('imgBitsPadded.mat', 'imgBitsPadded');

• Use a "MATLAB Function" block inside Simulink to output 144 bits per call by reading from the preprocessed bitstream.

function bitsOut = getImageBits()
persistent imgBits idx

if isempty(imgBits)
    temp = load('imgBitsPadded.mat');
    imgBits = temp.imgBitsPadded;
    idx = 1;
end

% Output 144 bits per call
if idx + 143 <= length(imgBits)
    bitsOut = imgBits(idx:idx+143);
    idx = idx + 144;
else
    bitsOut = zeros(144,1); % Or wrap around
end

• Finally if you are looking to reconstruct the image, after the simulation, collect the received bits (rxBits) in the workspace and reconstruct the image as follows:

% Remove padding if added
rxBits = rxBits(1:end-padLength);

% Convert bits back to bytes
rxBytes = bi2de(reshape(rxBits, 8, [])', 'left-msb');

% Write bytes to file
fid = fopen('output.jpg', 'wb');
fwrite(fid, rxBytes, 'uint8');
fclose(fid);

• Refer to the image below which shows the results after simulation

Kindly refer to the following MathWorks documentations to know more about the functions used:

• de2bi: https://www.mathworks.com/help/comm/ref/de2bi.html
• bi2de: https://www.mathworks.com/help/comm/ref/bi2de.html
• mod: https://www.mathworks.com/help/matlab/ref/double.mod.html
• Using Persistent Variables in MATLAB Function Blocks: https://www.mathworks.com/help/hdlcoder/ug/using-persistent-variables-inside-matlab-function-blocks-for-hdl-code-generation.html

Hope this resolves your issue.

[Nisab]

Thank you so much! I was wondering if you had any parameters you used while running the simulation, for example the sample time for the image to bits matlab block, or the stop time for the simulation as well? While I was using my own image, i had gotten these results attached. There is clearly something wrong with the spectrum when I tried to run it, and the output jpg file doesnt open as well (unsupported file format from Windows 11)

Any help in troubleshooting this is greatly appreciated, thank you so much for your help so far!

[Nisab]

Adding onto my last confusion, the rxBits that i receive using my picture is 144x1x27779 double , which is way more than the txBits, 24288x1 double. Are there any parameters you used to limit the sample time so that only the txBits amount is sent instead of the 144x1x27779 double?

Your help is greatly appreciated!

Answer 2

[Abhiram]

Hi @Nisab,

To convert a JPG file to bits, you can refer to the MATLAB Answers page linked below:

https://www.mathworks.com/matlabcentral/answers/527513-converting-image-to-bits-and-vice-versa

Additionally, you can refer to the MATLAB Documentation for “Image Transmission and Reception Using 802.11 Waveform and SDR” for further guidance on how to encode and pack an image file into WLAN packets for transmission and subsequently decode the packets to retrieve the image.

https://www.mathworks.com/help/wlan/ug/image-transmission-reception-using-802-11-waveform-sdr.html

Hope this helps!

[Nisab]

Thank you so much! I'll look into the pages you've sent