<p>This study utilizes a frequency-diversity approach to enhance the bit-error-rate (BER) performance in orthogonal-frequency-division-multiplexing (OFDM) systems. In this system, the same OFDM symbol is sent to different subcarriers, with the spacing between these subcarriers being equal to or greater than the coherent bandwidth of the channel. The number of subcarriers utilized to transmit the same modulated symbol corresponds to the desired diversity order. Since the transmission rate will decrease by this diversity order, a multiple-input multiple-output (MIMO) system operating in spatial multiplexing mode is utilized to increase the transmitted signal's spectral efficiency. If the rank of the MIMO channel (the spatial multiplexing order) is identical to the diversity order, the bandwidth efficiency will remain consistent with systems without diversity. The power efficiency in the proposed system will increase by the used diversity order. The proposed system is appropriate for working in a frequency-selective channel. The maximal-ratio-combiner (MRC) combines the same received symbol from the different subcarriers to increase the signal-to-noise ratio of the decision symbol. The simulation results display the BER performance enhanced significantly in Rayleigh frequency selective channel without affecting the bandwidth efficiency.</p>