Study Research on Precoder for OFDM Systems
| Autor: | Ching-cheng Chiang, 江錦政 |
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| Rok vydání: | 2007 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 95 In this thesis, precoding techniques are used to increase frequency diversity in orthogonal frequency division multiplexing (OFDM) systems. Several orthogonal precoding matrices have been employed, and simulation results are shown for uncoded and coded systems (including block, convolutional and turbo codes). In recent years turbo codes have been an important research topic since they can achieve a performance very close to the Shannon limit, and exploit the concept of iteration. Although the iteration process is a suboptimal solution, it provides an acceptable performance. Another advantage of the iteration process is its low complexity, which allows hardware implementation. The channel model used in this research is a Rayleigh Fading channel. It has been assumed that the channel parameters are known to the receiver. The iterative receiver is based on the concept of turbo codes. The receiver’s estimator and decoder exploit and exchange soft information in order to achieve the maximum-a-posteriori (MAP) probability. However, since obtaining the MAP probability implies including every bit in the computations, complexity is increased considerably. In order to reduce the receiver’s complexity, sphere decoding is used for estimation. In this thesis, this estimator is named list sphere decoder (LSD). Using LSD or optimal estimation becomes a tradeoff issue. Amongst the several precoding techniques, the discrete Fourier transform (DFT) precoding matrix is the most effective in OFDM systems. The DFT precoding matrix provides a diversity gain which increases with the number of (signal?) dimensions. If a set of multidimensional signals is included in the system design, the DFT precoding matrix can also improve performance when low dimensions are used. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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