Analysis of the loss of orthogonality through Doppler spread in OFDM systems

The paper quantifies the significant effects of Doppler spread in a time variant mobile radio channel on the performance of OFDM multi-carrier systems. A suitable model for OFDM is introduced and serves as the basis for the derivation of the inter-carrier interference (ICI) due to the loss of sub-carrier orthogonality. Several Doppler spectra are analyzed and compared, such as the classical and uniform distributions, as well as the case of two distinct values for the Doppler frequency. We can thus employ the computed variance of the ICI process, together with the channel noise, in system analyses and designs, and avoid lengthy time-domain simulations. We also analyze optimal receiver frequency synchronisation in the case of Doppler spreads.