Modern digital communication systems take advantage of the combined applicationof orthogonal frequency-division multiplexing (OFDM) and multiple-inputmultiple-output (MIMO) systems, often denoted as MIMO-OFDM. OFDM modulation combats frequency selective fading effectively by dividing a wideband frequency selective fading channel into parallel narrowband ?at fading subchannels. MIMO methods can improve the error performance, signal quality, and system capacity. Therefore, the combination of MIMO technology and OFDM is a viable solution for future wireless communication systems. The time domain MIMO-OFDM signal is a summation of several orthogonal waveforms. So, large peaks which are much higher than the average of the signal may happen. This phenomenon is called high peak-to-average power ratio (PAPR), which is a major drawback of MIMO-OFDM systems at the transmitter side. The high PAPR increases the complexity of digital-to-analog converters (DACs). Also, it can cause non-linear distortion due to non-linear devices such as high power amplifiers (HPAs). The non-linear distortion introduces spectral spreading, inter-modulation, and changes in the signal constellation. PAPR reduction schemes based on signal modification have been proposed to deal with the PAPR problem. Various PAPR reduction methods for single-input single-output (SISO)-OFDM systems have been proposed. Recently, several articles addressed the PAPR reduction for MIMO-OFDM systems. In this thesis, three new methods have been proposed for PAPR reduction in MIMO-OFDM systems. In the proposed methods, the OFDM block at each transmit antenna is partitioned into disjoint subblocks. These frequency domain subblocks are transformed to the time domain by IFFT operation. Then, spatial and temporal processing are applied to the time domain subblocks across all transmit antennas. Finally, a candidate sequence with the best PAPR statistics is chosen for each transmit antenna. Simulation results show that the proposed methods substantially outperform the conventional PAPR reduction schemes. Also, the proposed methods induce lower complexity than that of the conventional schemes. Keywords: MIMO, OFDM, Peak-to-average power ratio, Spatial and temporal processing
