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Interleave-division multiple-access (IDMA) has recently been proposed as a multiuser scheme that uses low complexity turbo-like iterative receivers and can outperform coded CDMA. Power allocation has been shown as an effective means for improving the performance of iterative receivers, and can be readily applied to multiuser systems including IDMA and CDMA. Power allocation allows the decoder to operate in a successive cancellation (or onion peeling) mode where the higher-power layers converge first, decreasing their contribution to the residual noise, and then the lower-power layers converge. In this paper, we propose an alternative optimisation approach where users are allocated different FEC codes in order to achieve onion peeling operation at the receiver and thereby improve system performance. We derive a numerical method for optimising performance based on FEC code allocation, and simulation results are presented. For small multiuser systems, results demonstrate that the performance of the proposed system approaches the theoretical single user bound.