Browsing by Author "Sheen, WH"

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A low-power ASIC design for cell search in the W-CDMA system
(2004-05-01)
Li, CF
;
Chu, YS
;
Sheen, WH
;
Tian, FC
;
Ho, JS
This paper presents a low-power ASIC design for cell search in the wideband code-division multiple-access (W-CDMA) system. A low-complexity algorithm that is able to work satisfactorily under the effect of large frequency and clock errors is designed first. Then, a set of low-power measures are employed in the design of hardware architecture and circuits. Finally, through power analysis, critical blocks are identified and redesigned so as to further reduce the power consumption. The final design shows that the power is reduced by 51 % from the original design of 133.6 mW to 65.49 mW, and its core area is also reduced by 31.9% from 3.4 x 3.4 mm(2) to 2.8 x 2.8 mm(2). The design is implemented and verified in a 3.3-V 0.35-mum CMOS technology with clock rate 1 36 MHz.
A new analysis of direct-sequence pseudonoise code acquisition on Rayleigh fading channels
(2001-11-01)
Sheen, WH
;
Wang, HC
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電信工程研究所
;
Institute of Communications Engineering
Accurate performance evaluation of direct-sequence pseudonoise code acquisition on Rayleigh fading channels is investigated in this paper. For fading channels the homogeneous Markov chain model, used to characterize the acquisition process over additive white Gaussian noise channels, is no longer valid due to the correlations between cell detections incurred by fading. Hence, the traditional direct and now-graph approaches for performance evaluation of the code acquisition are not applicable to fading channels. In this paper, a new analysis is proposed for accurate evaluation of the acquisition performance on Rayleigh fading channels. The analysis is quite general and can include various search strategies, types of correlators, and test methods with different performance measures: probability mass function and/or moments of acquisition time. Analytical and simulation results show that the new method predicts the acquisition performance very accurately.
Accurate performance analysis of maximum likelihood frequency estimator for OFDM systems
(2003)
Fang, KY
;
Sheen, WH
;
Sheu, JS
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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.
Burst synchronization of slotted random access with preamble power ramping in the reverse link of CDMA systems
(2003-09-01)
Sheen, WH
;
Tseng, CC
;
Ho, JS
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電信工程研究所
;
Institute of Communications Engineering
Efficient random access is one of the key designs in code-division multiple-access (CDMA) cellular systems. Random access is used by user equipment for initial access, requesting dedicated channels and transmitting short packet data to a base station. In random access, a preamble part is usually devised for fast and reliable burst synchronization, which is essential in order to avoid excessive access delay and/or repeated transmissions that may reduce the overall system capacity. This paper investigates the issue of burst synchronization for the slotted random access with preamble power ramping in the reverse link of CDMA systems. A flexible burst synchronizer based on a parallel-serial code-phase detector is proposed, which can be easily configured to achieve different complexity/performance tradeoffs. A general analysis is also presented with important design parameters being taken into account, including the number of correlators, power control error, power ramping step, diversity order, frequency offset, multipath combining, and others. The analysis is verified by computer simulations.
Comparison among the Filter Steered robust compressed domain object detection
(2004)
Ahmad, AMA
;
Talat, ST
;
Sheen, WH
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資訊工程學系
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Department of Computer Science
Object detection in video images is an interesting subject but often heavy task due to the huge volume of available approaches. The comparisons made in this paper between the different categories of algorithms are attained through literary research and are not based on direct quantitative examination. The focus of this work is on algorithms that are useful in real-time applications, realized in compressed domain and Filter Steered. This paper will provide some illumination into the area of object detection by classifying some of the current and more popular Filter Steered techniques.
New load-based resource allocation algorithms for packet scheduling in CDMA uplink
(2004)
Sheen, WH
;
Fu, IK
;
Lin, KY
;
電信工程研究所
;
Institute of Communications Engineering
Packet scheduling plays a key role in radio resource management of a wireless access network to best utilize the available radio resource. A radio-link packet scheduling consists of two steps: prioritization and resource allocation. Prioritization is a procedure that prioritizes users' order of service so as to guarantee QoS (quality of services), user's fairness and achieve as high as possible system throughput. Resource allocation, on the other hand, executes the "real" resource allocation under the constraints of physical limitations imposed by the radio segment of the access system. This paper proposes two new load-based resource allocation methods for packet scheduling in CDMA (code division multiple access) uplink with multiple services. One is called maximum-load based power allocation (MLBPA), and the other real-load based power allocation (RLBPA). MLBPA is easier to implement, but RLBPA has the better performance. By taking into consideration of the realistic channel conditions such as imperfect power control and transmit power variation, both of our methods outperform the current methods in terms of system throughput, frame loss rate and transmission delay. In addition, by monitoring the system load at all times, the planned coverage of the system can always be guaranteed.
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.
Performance analysis of noncoherent digital delay locked loops for direct sequence spread spectrum systems with Doppler shift and quantized adaptation
(2004-11-01)
Sheen, WH
;
Chang, MJ
;
Wu, CS
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電信工程研究所
;
Institute of Communications Engineering
The noncoherent second-order digital delay locked loop (DLL) with the presence of Doppler shift is investigated. The loop performance such as lock-in range, transient response, mean time-to-lose lock (MTLL), and mean square tracking error (MSE) are analyzed. The analysis is unique in two respects. First, MTLL and MSE are evaluated more accurately than by previous methods. Second, the impact of quantized adaptation on the loop performance is examined for digital DLL that was neglected previously. Numerical results show that: 1) the quantized adaptation may significantly alter the loop behaviors, including the lock-in range, transient responses, MTLL, and MSE; and 2) the traditional analysis based on Gaussian approximation may result in a large error in performance evaluation when quantized adaptation is taken into account.