obtain s parameter using matlab through GPIB-USB from network analyzer

I have some problems with the matlab code for using VNA to measure S parameter using USB-GPIB.

VNA = instrfind('Type', 'gpib', 'BoardIndex', 8, 'PrimaryAddress', 16, 'Tag', '');
if isempty(VNA)
  VNA = gpib('agilent', 8, 16);
else fclose(VNA);
  VNA = VNA(1);
end
frequency_start=2e9;
frequency_stop=3e9;

I am now doing a project involving measuring the s parameter. I will appreciate anyone who can provide me a sample matlab code. Right now, I found all the matlab programs shared by others online are not working.