Browsing by Author "Kuo, CCJ"

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An approximately MAI-free multiaccess OFDM system in carrier frequency offset environment
(2005-11-01)
Tsai, SH
;
Lin, YP
;
Kuo, CCJ
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電控工程研究所
;
Institute of Electrical and Control Engineering
The carrier frequency offset (CFO) effect of a new multiaccess orthogonal frequency division multiplexing (OFDM) transceiver is analyzed. We show that multiaccess interference (MAI) due to CFO can be mitigated by choosing a proper set of orthogonal codes. That is, if we use M/2 symmetric or antisymmetric codewords of the M Hadamard-Walsh codewords, MAI can be greatly reduced to a negligible amount. The new OFDM system has been shown to be approximately MAI-free when there is no CFO. The new result developed here shows that the system continues to be approximately MAI-free even in the presence of CFO. Furthermore, we derive a close form for the reduced MAI, which reveals an interesting property of the system. That is, when the number of users increases, the reduced MAI and the inter carrier interference (ICI) due to self-CFO will decrease. Finally, it is demonstrated by simulation that the proposed OFDM scheme with proper code selection outperforms the OFDMA and the multicarrier code division multiplexing access (MC-CDMA) systems in a CFO environment.
Blind recursive tracking of carrier frequency offsets in MC-CDMA systems
(2005)
Chien, FT
;
Kuo, CCJ
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
A blind recursive algorithm to effectively update the carrier frequency offset (CFO) vector in a multicarrier code-division multiple-access (MC-CDMA) system is studied in this research. The recursive relation is developed based on the expectation maximization (EM) algorithm with a quadratic constraint. This new perspective enables the use of linear estimation theory to deal with the blind CFO estimation problem, leading to an analytic CFO estimate in closed-form. The multiple access interference (MAI) is mitigated using the second order statistics of the interference-plus-noise vector, which can also be updated in a recursive manner under the EM formulation. Finally, we verify the effectiveness of the proposed algorithm with computer simulation.
Performance analysis of multicarrier CDMA systems with frequency offsets and random spreading under optimum combining
(2006-04-01)
Chien, FT
;
Hwang, CH
;
Kuo, CCJ
;
傳播研究所
;
電子工程學系及電子研究所
The carrier-frequency offset effect on the performance of asynchronous multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems with aperiodic random spreading and correlated Rayleigh fading is studied in this paper. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise ratio (SINR) of the combined statistics and exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability formula, is derived by averaging several random parameters including asynchronous delays, correlated Rayleigh fading, and signature sequences. The analytic results obtained are applicable to MC-CDMA with appropriate modifications. Furthermore, we show that the MC-CDMA system with a common random signature sequence over all subcarriers for a given user outperforms that with distinct sequences over different subcarriers. Finally, the performance of MC-CDMA systems using the optimum combining technique is compared with that of different combining filters in the simulation.