Browsing by Author "Hsiao, Chang Lung"

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On the transmitter-based preprocessing for 2-D OFDM-CDMA forward-link systems over time-varying Rayleigh fading channels
(2008-05-01)
Kuo, Chih-Cheng
;
Sheen, Wern-Ho
;
Chang, Chung-Ju
;
Hsiao, Chang Lung
;
電信工程研究所
Transmitter-based preprocessing is investigated for 2-D orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) forward-link systems for improving performance and shifting signal processing complexity from a mobile unit to a base station. Preprocessing schemes that are based on zero forcing (ZF) with power normalization, minimum mean square error (MMSE), and ZF with multiuser water filling (ZF-MWF) criteria are jointly investigated with 2-D spreading pattern optimization and multiuser scheduling from an information-theoretic viewpoint. Numerical results show that 1) the performance of preprocessing is quite sensitive to the 2-D spreading pattern for SNRs of interest, for example, 20% degradation on the sum data rate is observed for MMSE preprocessing if the spreading pattern is not properly selected; 2) ZF-MWF may substantially outperform the other two criteria depending on the SNRs; and 3) multiuser scheduling provides a significant performance improvement on the system sum data rate.
Transmitter-Based Pre-Processing for OFDM-CDMA Forward-Link Systems with Two-Dimensional Spreading
(2006)
Kuo, Chih-Cheng
;
Sheen, Wern Ho
;
Chang, Chung-Ju
;
Hsiao, Chang Lung
;
電信工程研究所
Transmitter-based signal pre-processing is investigated for the two-dimensional (2-D) OFDM-CDMA (orthogonal frequency division multiplexing, code division multiple access) forward link systems, aiming to increase the system sum-capacity and shift the signal processing complexity of a mobile unit to the base station. For the non-selective failing channels, the optimum pre-processing that achieves the maximum ergodic sum-capacity is obtained by applying the principle of multi-user water filling under the condition of zero multiple access interference (MAI). For the selective fading channels, on the other hand, three sub-optimum methods including ZF (zero-forcing) with power normalization, MMSE (minimum mean square error), and the newly proposed ZF with multi-user water filling are investigated under the constraint of total transmit power. Numerical results show that: (i) The new pre-processing outperforms the traditional other two in terms of achievable sum-capacity. (ii) 2-D spreading pattern should be adapted to the channel conditions so as to minimize MAI and thus maximize the sum-capacity. In this regard, 2-D spreading has advantage over the 1-D one. (iii) The fewer the users scheduled at a time, the higher the system capacity; a result obtained from exploiting the multi-user diversity.