Subscripted assignment dimension mismatch

function [power_alloc, sum_rate] = power_allocation_noma(channel_gains, target_rates, total_power)
% channel_gains: Matrix of channel gains for each user and subchannel
% target_rates: Target rates for each user
% total_power: Total available power

[num_users, num_subchannels] = size(channel_gains);

% Initialize power allocation matrix
power_alloc = zeros(num_users, num_subchannels);

for u = 1:num_users
    % Calculate interference from other users on each subchannel
    interference = sum(power_alloc(:, setdiff(1:num_subchannels, u)) .* channel_gains(:, setdiff(1:num_subchannels, u)), 2);
    
    % Calculate power allocation using water-filling algorithm
    lagrange_multiplier = (target_rates(u) - interference) / channel_gains(u, u);
    allocated_power = max(lagrange_multiplier, 0);
    
    % Allocate the calculated power to all subchannels for the user
    power_alloc(u, :) = allocated_power * ones(1, num_subchannels);
    
end

% Calculate sum rate
sum_rate = sum(log2(1 + power_alloc .* channel_gains), 'all');

I get this error:

subscripted assignment dimension mismatch.

Error in power_allocation_noma (line 20)

power_alloc(u, :) = allocated_power * ones(1, num_subchannels);

Error in main2 (line 13)

[power_alloc, sum_rate] = power_allocation_noma(channel_gains, target_rates, total_power);

Answer 1

[Florian Bidaud]

This is because your interference variable is a 1*num_users vector, leading to lagrange_multiplier also being a 1*num_users vector. Thus allocated_power * ones(1, num_subchannels) is a num_subchannels*num_users matrix.

In the end you try to affect a num_subchannels*num_users matrix to a 1*num_subchannels vector.

The dimensions of the left and right side of the "=" are different

I think you can have what you seek with changing allocated power definiton to :

allocated_power = max(max(lagrange_multiplier, 0));