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In this paper, we propose a mobility-based multiple-access interference (MAI) cancellation scheme for block spread-interleaved frequency-division multiple-access (BS-IFDMA) systems. We optimize the assignment of spreading codes to users to maximize the performance of the cancellation scheme. The optimization is formulated as an NP-hard quadratic assignment problem, which is solved using the branch-and-bound approach. We propose an efficient search algorithm that significantly reduces the number of branches to be searched and hence speeds up the computation by a factor equal to the number of users in the system. Through analysis and simulations, we demonstrate that the proposed cancellation scheme is able to mitigate the MAI in BS-IFDMA systems due to the presence of high-mobility users. We show that without the proposed scheme, a single high-mobility user in the system will cause significant system performance degradation in terms of high bit error floor for most users in the system. In comparison, the proposed cancellation scheme with proper code assignment allows half of the users in BS-IFDMA systems to have high mobility without any significant performance degradation.
Date of Publication: Jun 2013