Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The analysis computed is based on the Error Vector Magnitude to check the performance of the optical OFDM system. is mostly measured by Vector Signal Analyzers (VSAs), Real Time Oscilloscopes (RTOs) measuring instruments, capturing real time data and performing frequency domain analysis using Fast Fourier Transform (FFT) [9].
Category: