64-QAM Modulation and Demodulation

Here's example of Quadrature Amplitude Modulation with Bit Inputs:

M = 64;
bitsPerSym = log2(M);
x = randi([0 1],10*bitsPerSym,1); % x is input variable
% modulation
y = qammod(x,M,'bin','InputType','bit','OutputDataType', ...
    numerictype(1,16,10));
% demodulation
z = qamdemod(y,M,'bin','OutputType','bit');
s = isequal(x,double(z))

The theory of 64 QAM is to transmit data with 8 bits per symbol.

My question, if the data I generate is not a multiple of 8 (like 61 bits), how can I do 64 QAM transmission?

Answer 1

[Shashi Kiran]

Hi @Ibnu Darajat,

I understand that you are interested in how transmission is handled when the data length is not a multiple of the bits per symbol in qammod.

I have reviewed your issue, and here is how we can proceed to resolve it:

• When using 64-QAM, we transmit bits per symbol.
• If we generate data consisting of only 61 bits, we need to apply padding to ensure the data length is a multiple of 6. This is done by calculating the necessary padding as follows:

bitsPerSym = log2(M); 
numBits = length(x);
paddingBits = bitsPerSym - mod(numBits, bitsPerSym);

• In this scenario, the required padding is calculated as We add these 5 padding bits as zeros, proceed with the transmission as usual, and then remove the padding bits at the end.

Here is the MATLAB implementation of the scenario described above.

M = 64;
bitsPerSym = log2(M);
x = randi([0 1], 61, 1);

% Calculate padding required
numBits = length(x);
paddingBits =bitsPerSym - mod(numBits, bitsPerSym);

% Pad the data
x_padded = [x; zeros(paddingBits, 1)];

% Modulation & Demodulation
y = qammod(x_padded, M, 'bin', 'InputType', 'bit', 'OutputDataType', numerictype(1,16,10));
z_padded = qamdemod(y, M, 'bin', 'OutputType', 'bit');

% Remove padding
z = z_padded(1:numBits);

s = isequal(x, double(z))

s = logical
   1

Refer to the following MathWorks documentation for qammod and qamdemod respectively.

1. https://www.mathworks.com/help/comm/ref/qammod.html
2. https://www.mathworks.com/help/comm/ref/qamdemod.html

I hope this addresses your question.