| In order to meet the demand for high-rate multimedia communication, the nextgeneration wireless communication system is going to attain a high-speed and reliablebroadband wireless transmission. Orthogonal frequency division multiplexing(OFDM) is one such technology. It has high spectral efficiency and the ability to resistthe strong frequency selective fading, so nowadays many communications standardshave set OFDM as one of the key technologies on physical layer. Such as, wirelessLAN standard based on IEEE802.11n, IEEE802.16e-based WiMAX technology,Long Term Evolution which is described as3.9G technology and so on. The problemof high peaks is the bottlenecks of OFDM system. It will bring nonlinear distortionand lead to higher requirements for the system’s hardware. So the problem of highPAPR results in the increase of the costs for implementation. In order to solve thisproblem, the thesis has researched on high PAPR by optimizing and improving thestructure of transmission signal.Firstly, the problems which need to be solved in OFDM system are analyzed inthis thesis. Based on the theory of OFDM technology, a system block diagram isgiven. Also, the position of PAPR module is pointed out according to the data flow.The definition and distribution of PAPR are introduced，and various techniques onPAPR reduction are summarized. The core thought of algorithms is been studied.Focused on the partial transmission sequence algorithm, linear seeking by iterativeshift algorithm is proposed to reduce the search complexity of searching process.Secondly, the constellation extension algorithm is introduced in detail. Theessence of constellation extension algorithm is to set additional offset on modulationsymbols. An algorithm called projection onto convex sets is introduced to solve thisnonlinear programming problem. A convergence-improved algorithm, gradientprojection, is proposed to magnify the clipped vector as times than origin in thisthesis. The clipped vector which consists of lots of zeros and few data reduces thecomputation of FFT greatly. Compared with the slow convergence rate of originalgorithm, the improved algorithm using gradient projection algorithm is much fasterand has better performance on PAPR reduction. Finally, an optimized gradient projection algorithm based on the idea of partialtransmit sequence algorithm and constellation extension algorithm is proposed forsignal optimization in this thesis. Optimized Gradient projection algorithm makes anoptimization of the signal by improved PTS algorithm. The output signal of improvedPTS algorithm is set as the input signal of constellation extension. Then, gradientprojection algorithm gets suboptimal solution instead of optimal solution to improvethe convergence speed. The thesis analyzed the algorithm complexity and implement-ation principle. Also, the system block diagram is introduced in this thesis. Simulationresults demonstrate that the optimized gradient projection algorithm can bring betterPAPR reduction performance with lower complexity. |
