Radio-over-fiber (RoF) is a technology in which a radio system seeking to access a wider bandwidth, uses optical methods and tools to distribute information. This research is an attempt to assess and compensate for the nonlinearity of an optical subsystem in a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based RoF system with a radio subsystem in an additive white Gaussian noise (AWGN) channel. By employing Alamouti code as a kind of orthogonal space-time block coding (OSTBC), we demonstrated improvement some systems performance.