- 牛志升;刘静;
This paper describes a two-level cross-talked admission control mechanism that guarantees quality of service (QoS) requirements for multimedia applications over wireless local area networks (WLANs). An enhanced distributed channel access analytical model is used to compute the maximum number of admitted users according to the QoS requirements and the packet arrival characters. Then, some channel resources are reserved for handoff calls based on the maximum number of admitted users and the call-level traffic model. The channel utilization ratio is also measured to reflect the current system traffic load. The maximum number of admitted users and the channel utilization ratio are used for admission control for applications with QoS requirements in the call level and for rate control of best effort applications in the packet level using the p-nonacknowledgement scheme. Thus, the QoS requirements are statistically guaranteed while the system is efficiently utilized. Simulations validate the effectiveness of this mechanism to guarantee the QoS and bandwidth utilization.
2008年06期 741-746页 [查看摘要][在线阅读][下载 572K] [下载次数：65 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 周宁玉;赵东风;丁洪伟;
A higher quality of service (QoS) is provided for ad hoc networks through a multi-channel and slotted random multi-access (MSRM) protocol with two-dimensional probability. For this protocol, the system time is slotted into a time slot with high channel utilization realized by the choice of two parameters p1 and p2, and the channel load equilibrium. The protocol analyzes the throughput of the MSRM protocol for a load equilibrium state and the throughput based on priority. Simulations agree with the theoretical analysis. The simulations also show that the slotted-time system is better than the continuous-time system.
2008年06期 747-753页 [查看摘要][在线阅读][下载 710K] [下载次数：61 ] |[网刊下载次数：0 ] |[引用频次：11 ] |[阅读次数：0 ] - 杨志军;赵东风;
Quality of service (QoS) support is a key attribute for multimedia traffic including video, voice, and data in wireless local area networks (LANs) but is limited in 802.11-based wireless LANs. A polling-based scheme called the point coordination function (PCF) was developed for 802.11 LANs to support the transmission of multimedia traffic. However, the PCF is not able to meet the desired practical traffic differentiation requirements for real-time data. This paper describes a QoS support polling scheme based on the IEEE 802.11 medium access control (MAC) protocol. The scheme uses a two-level polling mechanism with the QoS classes differentiated by two different access policies. Stations with higher priority traffic such as key or real-time data form the first level and can access the common channel through an exhaustive access policy. Other stations with lower priority traffic form the second level and can access the channel through a gated access policy. A system model based on imbedded Markov chain theory and a generation function were setup to explicitly analyze the mean information packet waiting time of the two-level polling scheme. Theoretical and simulation results show that the new scheme efficiently differentiates services to guarantee better QoS and system stability.
2008年06期 754-758页 [查看摘要][在线阅读][下载 469K] [下载次数：95 ] |[网刊下载次数：0 ] |[引用频次：23 ] |[阅读次数：0 ] - 甘剑松;李云洲;周世东;王京;
The distributed antenna system (DAS) is considered as a promising architecture for future wireless access. This paper describes the uplink of a power-controlled circular-layout DAS (CL-DAS) with minimum mean-square error (MMSE) receivers. Results from random matrix theory are used to show that for such a DAS, the per-user sum rate and the total transmit power both converge as the number of users and antennas goes to infinity with a constant ratio of antennas to users. The relationship between the asymptotic per-user sum rate and the asymptotic total transmit power is given for all possible values of the radius of the circle on which antennas are placed. This rate-power relationship is then used to find the optimal radius. With this optimal radius, the CL-DAS is proved to offer a significant gain compared with a traditional co-located antenna system. Simulation results demonstrate the validity of the analysis and the superiority of the DAS.
2008年06期 759-768页 [查看摘要][在线阅读][下载 752K] [下载次数：30 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 王红梅;许希斌;赵明;周世东;姚彦;
The performance of multiuser multiple-input-multiple-output (MIMO) downlink systems with block diagonalization (BD) depends on the accuracy of the channel state information (CSI) available at the transmitter and the receiver. In time-varying channels, the CSI available at the transmitter (CSIT) is always out-dated due to an inherent time delay between the uplink channel estimation and the downlink data transmission in time division duplexing (TDD) systems. This leads to a drastic degradation of system capacity. This paper first analyzes the effect of the outdated CSIT on multiuser MIMO downlink systems using the BD method and then proposes two linear processing methods, BD precoding with user selection and scheduling at the transmitter and total minimum mean squared error (MMSE) decoding at the receiver (TBDUSS-RTMMSE) and BD precoding at the transmitter with partial MMSE decoding at the receiver (TBD-RPMMSE), to mitigate the interference among data streams and users. Analysis and simulation results show that these methods can effectively reduce the impairment of the outdated CSIT to increase the system sum capacity in a suitable time delay region of the CSIT.
2008年06期 769-777页 [查看摘要][在线阅读][下载 580K] [下载次数：49 ] |[网刊下载次数：0 ] |[引用频次：4 ] |[阅读次数：0 ] - 彭聪;赵明;姚彦;
A cooperative communication scheme was developed in which the relay node performs hybrid forwarding, where the relay node will not forward data for a transmission pair unless the corresponding direct transmission fails, and the relay node will not forward data for a transmission pair unless the quality of the channel between the corresponding source and the relay is good. A performance analysis of the outage probability and the diversity-multiplexing tradeoff for the scheme shows that the forwarding mechanism efficiently reduces fading in wireless channels by achieving full diversity order of magnitude improvement. A considerable signal-to-noise ratio gain is provided by this scheme compared with existing cooperation schemes with fixed forwarding. This flexible forwarding mechanism enables the scheme to provide the optimal diversity-multiplexing tradeoff performance.
2008年06期 778-783页 [查看摘要][在线阅读][下载 486K] [下载次数：43 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - 宋健;吴青;潘长勇;杨知行;刘海涛;赵丙镇;李晓;
This paper describes field trials of two-way digital video transmissions over a 700-m long medium-voltage power cable using a frequency division duplex scheme. The purpose is to check the feasibility of using time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) technology in powerline communication (PLC). TDS-OFDM is the core technology in digital multimedia broadcasting-terrestrial (DMB-T), developed by Tsinghua University for digital television terrestrial multimedia broadcast applications and successfully adopted in the Chinese Digital Terrestrial Television Broadcasting Standard. PLC systems are widely believed to be bandwidth or data throughput limited. However, the use of known pseudo random sequences as guard intervals for synchronization and channel estimation in TDS-OFDM greatly reduces the system overhead and increases the spectrum efficiency. These experiments show that TDS-OFDM is appropriate not only for broadcasting but also for PLC applications with appropriate modifications.
2008年06期 784-789页 [查看摘要][在线阅读][下载 452K] [下载次数：88 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - 潘赟;葛宁;董在望;
A transform method was used to model a discrete time multi-tap direct sampling mixer. The method transforms the mixed filtering and down-sampling stages to separate cascade filtering and sampling stages to determine the unfolded frequency response which shows the anti-aliasing ability of the mixer. The transformation can also be applied to other mixed signal and multi-rate receiver systems to analyze their unfolded frequency responses. The transformed system architecture was used to calculate the unfolded frequency response of the multi-tap direct sampling mixer and compared with the mixer model without noise in the advanced design system 2005A environment to further evaluate the frequency response. The simulations show that the-3dB bandwidth is 3.0MHz and the voltage gain is attenuated by 1.5 dB within a 1-MHz baseband bandwidth.
2008年06期 790-795页 [查看摘要][在线阅读][下载 678K] [下载次数：50 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 孟大伟;冯振明;陆明泉;
The convergence rate of the power inversion (PI) algorithm is quite sensitive to the power of the interference with the used fixed parameters in the PI algorithm leading to degradation of its ability to handle interference. This paper presents a normalized PI algorithm that traces the stochastic characteristics of the interference. The algorithm adaptively adjusts the recursive step size to determine the constrained optimized parameters for the lowpass filter. Simulations show that the normalized PI algorithm achieves faster convergence and produces deeper nulls. The algorithm makes GPS receivers more robust in environments with large variations in the interference strength.
2008年06期 796-799页 [查看摘要][在线阅读][下载 548K] [下载次数：94 ] |[网刊下载次数：0 ] |[引用频次：29 ] |[阅读次数：0 ] - 王鹏;孟华东;王希勤;
Modulations and diversities, including the Costas-ordered stepped-frequency and nonlinear stepped-frequency waveforms are widely used in linear frequency modulation (LFM) pulse trains to reduce the relatively high autocorrelation function (ACF) sidelobes. An efficient method was developed to optimize the interpulse frequency modulation to remove most of the ACF sidelobes about the mainlobe peak, with only a small increase in the mainlobe width. The genetic algorithm is used to solve the nonlinear optimization problem to find the interpulse frequency modulation sequence. The effects on the ACF sidelobes suppression and mainlobe widening are studied. The results show that the new design is superior to the corresponding stepped-frequency LFM signal and weighted stepped-frequency LFM signal in the terms of the ACF sidelobes reduction and mainlobe spread.
2008年06期 800-806页 [查看摘要][在线阅读][下载 751K] [下载次数：37 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 王经典;陆秀洪;张利;
Automatic gain control (AGC) has been used in many applications. The key features of AGC, including a steady state output and static/dynamic timing response, depend mainly on key parameters such as the reference and the filter coefficients. A simple model developed to describe AGC systems based on several simple assumptions shows that AGC always converges to the reference and that the timing constant depends on the filter coefficients. Measures are given to prevent oscillations and limit cycle effects. The simple AGC system is adapted to a multiple AGC system for a TV tuner in a much more efficient model. Simulations using the C language are 16 times faster than those with MATLAB, and 10 times faster than those with a mixed register transfer level (RTL)-simulation program with integrated circuit emphasis (SPICE) model.
2008年06期 807-811页 [查看摘要][在线阅读][下载 529K] [下载次数：73 ] |[网刊下载次数：0 ] |[引用频次：4 ] |[阅读次数：0 ] - 肖勇;周润德;
This paper describes a circular first in first out (FIFO) and its protocols which have a very low latency while still maintaining high throughput. Unlike the existing serial FIFOs based on asynchronous micro-pipelines, this FIFO's cells communicate directly with the input and output ports through a common bus, which effectively eliminates the data movement from the input port to the output port, thereby reducing the latency and the power consumption. Furthermore, the latency does not increase with the number of FIFO stages. Single-track asynchronous protocols are used to simplify the FIFO controller design, with only three C-gates needed in each cell controller, which substantially reduces the area. Simulations with the TSMC 0.25 μm CMOS logic process show that the latency of the 4-stage FIFO is less than 581 ps and the throughput is higher than 2.2 GHz.
2008年06期 812-816页 [查看摘要][在线阅读][下载 571K] [下载次数：56 ] |[网刊下载次数：0 ] |[引用频次：6 ] |[阅读次数：0 ] - 董名垂;郑德信;SEKAR Booma Devi;陈思亮;
Internal faults in three phase induction motors can result in serious performance degradation and eventual system failures if not properly detected and treated in time. Artificial intelligence techniques, the core of soft-computing, have numerous advantages over conventional fault diagnostic approaches; therefore, a soft-computing system was developed to detect and diagnose electric motor faults. The fault diagnostic system for three-phase induction motors samples the fault symptoms and then uses a fuzzy-expert forward inference model to identify the fault. This paper describes how to define the membership functions and fuzzy sets based on the fault symptoms and how to construct the hierarchical fuzzy inference nets with the propagation of probabilities concerning the uncertainty of faults. The designed hierarchical fuzzy inference nets efficiently detect and diagnose the fault type and exact location in a three phase induction motor. The validity and effectiveness of this approach is clearly shown from obtained testing results.
2008年06期 817-822页 [查看摘要][在线阅读][下载 793K] [下载次数：46 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 董名垂;卢展宏;黄志刚;
Under-frequency load shedding (UFLS) is used in the power industry to rescue systems facing extreme disturbances to avoid system collapse. Traditionally, many computations are repeated to seek the proper power system settings such that the UFLS provides the desired good performance for selected scenarios. An adaptive UFLS method based on the genetic algorithm was developed to automate the finding of optimal parameters to minimize the repetitive trial-error calculations. Simulations demonstrate that the method has better performance than previous schemes and reduces the time and effort of the repetitive simulations.
2008年06期 823-828页 [查看摘要][在线阅读][下载 476K] [下载次数：47 ] |[网刊下载次数：0 ] |[引用频次：4 ] |[阅读次数：0 ] - 靳雪翔;张毅;李力;胡坚明;
One key function of intelligent transportation systems is to automatically detect abnormal traffic phenomena and to help further investigations of the cause of the abnormality. This paper describes a robust principal components analysis (RPCA)-based abnormal traffic flow pattern isolation and loop detector fault detection method. The results show that RPCA is a useful tool to distinguish regular traffic flow from abnormal traffic flow patterns caused by accidents and loop detector faults. This approach gives an effective traffic flow data pre-processing method to reduce the human effort in finding potential loop detector faults. The method can also be used to further investigate the causes of the abnormality.
2008年06期 829-835页 [查看摘要][在线阅读][下载 1250K] [下载次数：103 ] |[网刊下载次数：0 ] |[引用频次：10 ] |[阅读次数：0 ] - 成本茂;王红;杨士元;牛道恒;靳洋;
Register allocation in high-level circuit synthesis is important not only for reducing area, delay, and power overheads, but also for improving the testability of the synthesized circuits. This paper presents an improved register allocation algorithm that improves the testability called weighted graph-based balanced register allocation for high-level circuit synthesis. The controllability and observability of the registers and the self-loop elimination are analyzed to form a weighted conflict graph, where the weight of the edge between two nodes denotes the tendency of the two variables to share the same register. Then the modified desaturation algorithm is used to dynamically modify the weights to obtain a final balanced register allocation which improves the testability of the synthesized circuits. Tests on some benchmarks show that the algorithm gives a higher fault coverage than other algorithms with less area overhead and even less time delay.
2008年06期 836-842页 [查看摘要][在线阅读][下载 504K] [下载次数：51 ] |[网刊下载次数：0 ] |[引用频次：3 ] |[阅读次数：0 ] - 姚海龙;蔡懿慈;洪先龙;
A W-shaped multilevel full-chip routing framework using W-shaped optimization flow is used to find the final routing solution. The W-shaped flow consists of two sequential V-shaped optimization flows. The first V-shaped flow optimizes the global routing solution. The probabilistic congestion prediction technique is used to guide the global routing decision to find the routing solution that evenly distributes the nets. Then, the second V-shaped flow improves the quality of the routing result. Tests on a set of commonly used benchmark circuits and comparisons with other multilevel routing systems show that the routability, total wire length, total number of vias, and the runtime are all improved.
2008年06期 843-849页 [查看摘要][在线阅读][下载 384K] [下载次数：21 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 唐章宏;袁建生;
A method was developed to solve the combined system of the current field and the circuit. The "super-node" was used to transform the matrix for conventional nodal analyses of a circuit system from non-positive definite to positive definite. Then, a positive definite matrix for the overall system was obtained by combining the matrix from the circuit nodal analysis method and the matrix resulted from finite element method (FEM) formulation to solve the FEM fields. This approach has been successfully applied to simulate the electrical potential and current distributions on each metal layer of printed circuit boards (PCBs) and integrated circuit (IC) packages for a given power supply. The simulation results can then be used to analyze the properties of the PCBs and IC packages such as the port resistances and IR drops. The results can also be used to optimize PCB and IC package designs, such as by adjusting the power/ground distribution networks.
2008年06期 850-857页 [查看摘要][在线阅读][下载 816K] [下载次数：57 ] |[网刊下载次数：0 ] |[引用频次：3 ] |[阅读次数：0 ] -
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2008年06期 858-862页 [查看摘要][在线阅读][下载 250K] [下载次数：30 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ] <正>Tsinghua Science and Technology (Tsinghua Sci Technol), a comprehensive academic journal sponsored by Tsinghua University, is published bimonthly. This journal aims at presenting the up-to-date scientific achievements
2008年06期 863页 [查看摘要][在线阅读][下载 1782K] [下载次数：10 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ]