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광대역 통신 시스템의 간섭 제거를 위한 반복 수신기의 설계

광대역 통신 시스템의 간섭 제거를 위한 반복 수신기의 설계
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Since the invention of turbo decoding, iterative processing (or turbo processing) was widely investigated for applications such as channel estimation, equalizationand channel decoding. The turbo decoding principle shows remarkable performance improvements over conventional methods. However, iterative processingmay still be a burden in practical communication systems due to its high computational complexity, even with the increasing hardware processing capabilities. Therefore, design of iterative receivers achieving both extremelylow error rates and low complexity is and will be an important problem in many communication systems. We first design a new scheduling algorithm for turbo equalizers combined with low-density parity-check (LDPC) decoders. We analyze the convergencebehavior of turbo equalizers combined with LDPC decoders and then derive simple metrics and criteria for monitoring the status of iterative processing. We then propose an adaptive scheduling algorithm to reduce the overall complexity of a turbo equalizer combined with an LDPC decoder. Numerical results show that the proposed adaptive scheduling has lower complexity than conventional schedulings, while maintaining almost the same error rate. Our second concern is to design an iterative receiver for cancelling intercarrier interference (ICI) arising from the phase noise in orthogonal frequencydivision multiplexing (OFDM) systems. We present a new idea of employing the time-averages of the phase noise within subblocks of a received OFDM symbol for phase noise compensation. In order to implement this idea, we develop a new iterative receiver based on subblock processing. A formula for the signalto-interference-plus-noise ratio (SINR) after phase noise compensation is derived and its values are evaluated under a variety of phase noise conditionsin order to demonstrate the efficiency of the proposed algorithm. Numerical results show that the receiver employing the proposed algorithm achieves performanceclose to that of an OFDM system without phase noise over a wide range of conditions. In the last part of this thesis, we propose a blind algorithm to compensate for the phase noise in OFDM systems. In the proposed algorithm, the phase noiseover each subblock is approximated by its time-average. Under this approximation, the squared magnitude of the channel gain multiplied by the data symbol at each subcarrier is shown to be expressed in terms of these time-averages and the discrete Fourier transform (DFT) coefficients of the received samples at each subblock with zero padding. Based on this relationship, the proposedalgorithm compensates for the phase noise without the need for pilot symbols. Numerical results show that the proposed algorithm outperforms conventional algorithms as well as it requires lower computational complexity.
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