In this work, a frequency-diversity scheme is used with an orthogonal-frequency-division-multiplexing (OFDM) system to improve bit-error-rate (BER) performance without increasing the signal bandwidth or decreasing the transmission rate. In this scheme, the same OFDM symbol is transmitted on multiple carrier frequencies, each with a different gain. The number of transmitted symbols is equal to the diversity order (N), and as a result, the transmission rate is reduced by a factor of N. This means that the system sacrifices some transmission rates to gain the benefits of diversity. To further improve the spectral efficiency of the transmitted signal, a multiple-input-multiple-output (MIMO) transmitter is used in spatial multiplexing mode. The number of antennas, known as the spatial multiplexing order, is equal to the diversity order. By employing multiple antennas, the system can transmit multiple independent data streams simultaneously, thereby increasing the overall data rate. The proposed system assumes operation in a frequency-selective channel, which means that the channel introduces different frequency-dependent effects on the transmitted signal. In the receiver, a maximal-ratio-combiner (MRC) is utilized to combine the received signals from the different carrier frequencies and antennas. The MRC combines the signals in a way that maximizes the signal-to-noise ratio (SNR) and improves the overall system performance. To evaluate the performance of the proposed system, the system is implemented using GNU Radio, a free and open-source software development toolkit for building software-defined radios.