Comparison of CDMA-OFDM and CP-OFDM Using Alamouti Scheme in MIMO Systems

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This paper deals with a combination of OFDM and the Alamouti scheme for transmission of digital information. First the OFDM modulation scheme is presented. The Alamouti decoding scheme cannot be directly applied to OFDM modulation. The Alamouti coding scheme requires a complex orthogonality property; whereas OFDM only provides real orthogonality. However, under special conditions, a transmission scheme combining CDMA and OFDM can satisfy the complex orthogonality condition. Adding a CDMA component can thus be seen as a solution to apply the Alamouti scheme in combination with OFDM. However, a detailed analysis shows that the CDMA-OFDM combination has to be built taking into account particular features of the transmission channel.