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This letter presents the principles of Coded Orthogonal Frequency Division Multiplexing (COFDM) system and focuses on the effect of the Guard Interval (GI) on the systemperformance. The role of the GI parameter period to solve the problem of interferences that occur at the receiver is discussed. A new COFDM concept with a Variable GI (VGI) is proposed in order to improve the Bit Error Rate (BER) performance in the presence of multipaths with variable delays. A series of simulations have been carried out on the BER performance as a function of GI, maximum delay spread, and Signal-to-Noise Ratio (SNR), with QPSK as modulation scheme over multipath fading and Additive White Gaussian Noise (AWGN). The results show that the optimum values of the GI parameter approximately equal to the maximum delay spread, and the proposed system with VGI provides a better performance compared with the fixed length GI.