Browsing by Author "Sheu, JS"

Now showing 1 - 2 of 2

Accurate performance analysis of maximum likelihood frequency estimator for OFDM systems
(2003)
Fang, KY
;
Sheen, WH
;
Sheu, JS
;
電信工程研究所
;
Institute of Communications Engineering
In orthogonal frequency-division multiplexing (OFDM) technique, system performance is degraded greatly by the inter-carrier interference (ICI) caused by the loss of orthogonality between sub-carriers. One of such causes is due to the mismatch of the carrier frequency between the transmitter and the receiver. Various maximum likelihood (ML) methods have been proposed in the literature for carrier frequency estimation in OFDM systems. In theory, the performance of the ML estimators is asymtotically close to the Cramer-Rao lower bound (CRLB). However, it might be significantly different from it at the SNRs of practical interest. In this paper, a very accurate method is proposed for analyzing the performance of the ML estimators in terms of the mean and variance of the estimators. Simulation results show, that the proposed method is much more accurate than the previous ones at low SNRs.
On the adaptability of OFDM-CDMA forward link with time-frequency spreading in multi-path fading channels
(2005)
Sheen, WH
;
Sheu, JS
;
電信工程研究所
;
Institute of Communications Engineering
OFDM-CDMA (orthogonal frequency division multiplexing, code division multiple access) with joint time and frequency spreading has a great potential to achieve very high spectrum efficiency by exploiting both the temporal and spectral characteristics of the multi-path fading channels. A full realization of this potential however is contingent on the system's ability to adapt to the changing operating environments. In this paper, we explore this adaptability for the forward link of such a system by adapting the receiver detector, spreading pattern, spreading format and spreading code to the channel and traffic conditions. Numerical results show that significant performance improvement can be achieved as compared to the non-adaptive counterparts.