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In this paper we investigate the impact of the pilot density (and associated pilot distances for both frequency and time domains) in the performance of the channel estimation for MC-CDMA systems, when using dedicated sub-carriers to transmit pilots with 2-D rectangular patterns. The analytical formulation of the channel estimation MSE for the single-user downlink scenario with a generic channel estimation scheme for a given power efficiency is presented. The attained results go beyond the commonly accepted rule of thumb that suggests that an oversampling factor of two will result in a good trade-off between performance and pilot overhead. The analytical formulation was validated by system simulations. A scenario using the 2-D LS-DFT channel estimation scheme confirmed the analytical result. The lowest channel estimation MSE for a given value of power efficiency is achieved when the pilot density is the one that closely fulfils the 2-D sampling theorem.