Browsing by Author "Tadjpour, Layla"

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An approximately MAI-Free multiaccess OFDM system in fast time-varying channels
(2007-07-01)
Tadjpour, Layla
;
Tsai, Shang-Ho
;
Kuo, C.-C. Jay
;
電控工程研究所
;
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.
Complexity reduction of maximum likelihood multiuser detection (ML MUD) receivers with carrier interferometry codes in MC CDMA
(2008)
Tadjpour, Layla
;
Tsai, Shang Ho
;
Kuo, C. C. Jay
;
電機工程學系
;
Department of Electrical and Computer Engineering
We propose to use carrier interferometry (CI) codes to reduce the complexity of the maximum likelihood multiuser detection (ML MUD) receiver in MC CDMA systems with carrier frequency offsets (CFOs). Complexity reduction is achieved by exploiting the sparsity of the cross correlation matrix of CI codes. It is shown that, for a fully loaded MC CDMA with spread gain N over a multipath channel with length L, the complexity of the ML MUD receiver grows exponentially with 2L - 1 (instead of N), where L << N in a typical setting. Moreover, an upper bound for the minimum error probability is derived. Finally, simulation results are given to corroborate theoretical results.
Simplified Multiaccess Interference Reduction for MC-CDMA With Carrier Frequency Offsets
(2010-06-01)
Tadjpour, Layla
;
Tsai, Shang-Ho
;
Kuo, C. -C. Jay
;
電機工程學系
;
Department of Electrical and Computer Engineering
Multicarrier code-division multiple-access (MC-CDMA) system performance can severely be degraded by multiaccess interference (MAI) due to the carrier frequency offset (CFO). We argue that MAI can more easily be reduced by employing complex carrier interferometry (CI) codes. We consider the scenario with spread gain N, multipath length L, and N users, i.e., a fully loaded system. It is proved that, when CI codes are used, each user only needs to combat 2(L - 1) (rather than N - 1) interferers, even in the presence of CFO. It is shown that this property of MC-CDMA with CI codes in a CFO channel can be exploited to simplify three multiuser detectors, namely, parallel interference cancellation (PIC), maximum-likelihood, and decorrelating multiuser detectors. The bit-error probability (BEP) for MC-CDMA with binary phase-shift keying (BPSK) modulation and single-stage PIC and an upper bound for the minimum error probability are derived. Finally, simulation results are given to corroborate theoretical results.