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This paper considers two simple procedures to improve the channel frequency response estimate for uplink Zero Padded MC-CDMA systems. The first approach uses a precomputed matrix that projects the original discrete frequency response onto the subspace where the true frequency response lies. The second uses a constrained MMSE optimization that forces the estimate to belong to this subspace. Analytical expressions for the mean square error and its upper and lower bounds are presented for the conventional and the two improved channel estimators. Numerical results show significant reduction in both the multiple access interference and the noise terms of the estimation error when using the improved estimators. Simulation results indicate that MMSE receivers using each of the improved procedures achieve gain in bit error rate performance.

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