Browsing by Author "Kuo, C.-C. Jay"

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An approximately MAI-Free multiaccess OFDM system in fast time-varying channels
(2007-07-01)
Tadjpour, Layla
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Tsai, Shang-Ho
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Kuo, C.-C. Jay
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電控工程研究所
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Institute of Electrical and Control Engineering
A new multiuser orthogonal frequency division multiplexing (OFDM) transceiver with preceding, called the precoded multiuser OFDM (PMU-OFDM) system, was recently introduced by Tsai et al (2005). The PMU-OFDM system can reduce multiaccess interference (MAI) due to the carrier frequency offset (CFO) to a negligible amount by preceding the data of each user with a codeword selected from either even or odd Hadamard-Walsh codes. The performance of the PMU-OFDM system in a mobile environment, where the channel response varies within one OFDM symbol due to the Doppler effect, is evaluated in this paper. It is shown by analysis and simulation that the use of even or odd Hadamard-Walsh codewords can greatly reduce the MAI effect due to the Doppler effect as well. Generally, all Hadamard-Walsh codewords except for the all-one codeword result in considerable suppression of intercarrier interference (ICI) induced by the Doppler effect. Furthermore, we discuss codeword priority schemes to minimize ICI as much as possible. Finally, simulations results are given to demonstrate the advantage of PMU-OFDM over OFDMA with a codeword priority scheme in the Doppler environment. We show that the PMU-OFDM system with odd Hadamard-Walsh codewords outperforms a half-loaded OFDMA system in a mobile environment.
Blind recursive tracking of carrier frequency offset (CFO) vector in MC-CDMA systems
(2007-04-01)
Chien, Feng-Tsun
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Kuo, C.-C. Jay
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
A recursive algorithm for estimating and updating the effective carrier frequency offset (CFO) vector in a multicarrier code-division multiple-access (MC-CDMA) system is proposed in this work.. The recursive relation is derived based on the expectation maximization (EM) algorithm with a quadratic constraint. This new approach enables the use of linear estimation theory to tackle the CFO estimation problem with or without training data, which leads to an analytic CFO estimate in closed form. Furthermore, the multiple access interference (MAI) is mitigated using the second order statistics of the interference-plus-noise vector, which is updated in a recursive manner under the EM formulation, too. When reaching a converged estimate, a fixed-norm quadratic constraint is imposed so that the final CFO estimate is robust to an imprecise covariance matrix estimate caused by insufficient data samples. It is demonstrated by computer simulation that the performance of an MC-CDMA system without the CFO information can be restored by the proposed scheme in the sense that its bit error probability (BEP) performance is close to that with perfect CFO knowledge.
Precoded multiuser OFDM transceiver in timing asynchronous environment
(2007-10-01)
Tsai, Shang-Ho
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Lin, Yuan-Pei
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Kuo, C.-C. Jay
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電控工程研究所
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Institute of Electrical and Control Engineering
The performance of a multiuser orthogonal frequency-division multiplexing (OFDM) transceiver, called the precoded multiuser OFDM (PMU-OFDM), with time asynchronous access is investigated in this paper. It is shown that multiaccess interference (MAI) due to time asynchronism is asymptotically zero as the number of parallel input symbols N becomes large in the PMU-OFDM system. Then, we show that PMU-OFDM with even or odd Hadamard-Walsh codewords can significantly suppress the MAI effect due to time asynchronism. For the half-loaded case, no sophisticated signal processing technique is needed by PMU-OFDM for MAI, suppression. For the fully loaded situation, PMU-OFDM demands less complexity than does conventional OFDMA for interference suppression since PMU-OFDM deals with only one half of interferers.
Ultrawideband transceiver design using channel phase precoding
(2007-07-01)
Chang, Yu-Hao
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Tsai, Shang-Ho
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Yu, Xiaoli
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Kuo, C.-C. Jay
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電控工程研究所
A novel transceiver design for ultrawideband (UWB) communication systems using the channel phase precoding (CPP) technique is proposed in this work. With the CPP-UWB transceiver, we encode data symbols using the reversed order of the channel phase. A simple phase estimation algorithm is presented for the CPP-UWB implementation. Owing to its ability to coherently combine the channel magnitude of every multipath, the CPP-UWB transceiver can achieve a higher data rate by shortening its symbol duration with a tolerable interference. The performance of the CPP-UWB can be further improved using an optimal code length and/or the MMSE receiver to suppress intersymbol interference.