The Research On PAPR Reduction In MIMO-OFDM Systems

As the combination of MIMO and OFDM technologys, MIMO-OFDM technology can not only enhance the channel capacity and transmission rate highly, but also deal with the multi-path fading efficiently. Therefore, it becomes a key technique of the Beyond3G/4G communication system, and has very broad development prospect. The thesis mainly researches the PAPR reduction schemes of MIMO-OFDM systems. The primary contents of the thesis are as following:Selective Mapping (SLM) is an efficient PAPR reduction scheme without signal distortion in OFDM systems, but there is a disadvantage of large computational complexity. Based on the traditional Individual Selective Mapping (ISLM) of MIMO-OFDM systems, the multi-antenna collaborative working Concurrent Selective Mapping (CSLM) is investigated and the side information is lessened greatly. Meanwhile, the CSLM is improved by utilizing the orthogonality of space-time block code and as a result, the computation quantity is reduced enormously. Besides, in order to cut down the enormous Inverse Fast Fourier Transforms (IFFTs) of CSLM, a new scheme that replaces the IFFT operations with matrix multiplications is presented. The new scheme has a lower computational complexity, while keeping the same PAPR performance of CSLM.Cross-Antenna Rotation and Inversion (CARI) is a new PAPR reduction scheme in MIMO-OFDM systems, and it fully utilizes additional degrees of freedom provided by employing multiple transmit antennas. In this method, the datas from different transmit antennas are partitioned into the sub-blocks with equal sizes and then these sub-blocks are made to rotate and inverse across all antennas. So, an amount of the permutated sequence sets are obtained and then, a set with the best PAPR performance is chosen for transmission in order to reduce the PAPR of signals. However, CARI scheme requires an exhaustive search to solve a difficult combinatorial optimization problem whose complexity grows exponentially with the number of subblocks. So, two suboptimal schemes, termed Successive Suboptimal CARI (SS-CARI) and Random Suboptimal CARI (RS-CARI) are studied, and the computational complexity of CARI scheme is effectively decreased.The computational complexity and sideband information can be lowered greatly by using Subblock Successive Inversion (SSI), which is a simplified form of SS-CARI scheme. By utilizing the linear property of IFFT, SSI scheme is improved and the computational complexity of Improved Subblock Successive Inversion (ISSI) is reduced further. But ISSI has a worse PAPR performance than SS-CARI scheme. Clipping technique can remove the signals with high peak value fundamentally, so it has very good PAPR performance. After a few times of circle clipping filter, the out-of-band noise of the system is eliminated. Therefore, a combined scheme by combining the ISSI scheme and Circle Clipping Filter technique is researched, and it achieves significantly good PAPR performance. The simulation results reveal that a good tradeoff between the system complexity and the PAPR reduction capability can be obtained through choosing the appropriate times of cycle.