In multiple access OFDM systems (OFDMA), the separation of different users is achieved by exploiting the orthogonality of the subcarriers. However, a well-known problem in OFDM-based systems is the vulnerability to synchronization errors which leads to a loss of orthogonality among the subcarriers and, hence, among the users. This problem becomes more severe in the uplink channel where the users are potentially asynchronous and have different frequency offsets. The correction of the users’ frequency offsets can not be accomplished at the base station receiver, as in the downlink. In this paper, a method is proposed to counteract the effect of different frequency offsets among users in an OFDMA uplink system with a frequency selective Rayleigh fading channel. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. This is performed by means of a Successive or Selective Cancellation method. In particular, the performance of these cancellation schemes is analyzed by assuming either an ideal or estimated frequency offset. Simulation results show that it is possible to achieve, with the interference cancellation, a performance close to the ideal case, i.e., without frequency offset among users.
Multiuser interference cancellation receivers for OFDMA uplink communications with carrier frequency offset / Fantacci R.; Marabissi D.; Papini, S. - STAMPA. - (2004), pp. 2808-2812. (Intervento presentato al convegno Global Telecommunications Conference, 2004. GLOBECOM '04. IEEE) [10.1109/GLOCOM.2004.1378866].