data_struct = load('TheFileName.mat');
varnames = fieldnames(datastruct);
time_domain_data = data_struct.(varnames{1});
freq_data = fft(time_domain_data);
If you know the name of the variable the data is stored in in the .mat file then you can use it directly. For example if you know the data is stored in qx83t then you can use
data_struct = load('TheFileName.mat');
time_domain_data = data_struct.qx83t;
