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Orthogonal frequency division multiplexing based multiple input multiple output techniques (MIMO-OFDM) tend to achieve high data rate transmission targets for 4G wireless communication systems. However, due to its inherent drawback of high peak-to-average power ratio (PAPR), the OFDM based transmission may severely affect the power efficiency of RF power amplifiers. In this paper, we propose a simple approach to reduce the PAPR of MIMO-OFDM signals based on the use of unused subcarriers. In order to avoid processing the signals at each transmitter separately, a peak cancelling signal is only generated at one antenna and then employed to all the others with only simple modifications. Simulation has shown that by using this proposed approach, a minimum 2 dB reduction in PAPR can be achieved for all transmit signals. Since the generation of the peak cancellation signal for each transmitter separately is avoid, the overall cost of the system can be significantly reduced.