User Separation for OFDMA Uplink

Separating user signals in the uplink (UL) of an orthogonal frequency division multiple access (OFDMA) system without access to the subcarrier assignment scheme (SAS) is a challenging task, but it can have a number of useful applications. In this paper, a semi-blind user separation algorithm is proposed, which assumes time synchronization and availability of information on the type of SAS used. The proposed algorithm estimates the carrier frequency offsets and time delays of each frequency allocation block by exploiting the cross-correlations over pilot subcarriers. A clustering method to group the blocks is employed, where each group belongs to a different user. Mathematical model of the proposed algorithm is presented, and possible user separation applications in OFDMA-based femtocells are discussed. Feasibility of the algorithm is proved through practical simulations.