Some analytical tools for the global performance evaluation of packet switching networks based on virtual paths

We present a simple model for the global performance evaluation of a packet switching network. Through a detailed analytical treatment, a set of formulae is derived which allow a compact and reliable evaluation of the most significant statistical properties of the network, such as the arrival probabilities, the queue states distribution and the mean utilizations of the links. Starting from the knowledge of these quantities, the network can be characterized also taking into account the mutual interaction between different nodes. An example is given with reference to a mesh-connected structure.     

On the performance of packet-switched cellular networks for wireless data communications

Cellular frequency reuse is known to be an efficient method to allow many wireless telephone subscribers to share the same frequency band. However, for wireless data and multi-media communications optimum cell layouts differ essentially from typical solutions for telephone systems. We argue that wireless radio systems for bursty message traffic preferably use the entire bandwidth in each cell. Packet queuing delays are derived for a network with multipath fading channels, shadowing, path loss and discontinuously transmitting base stations. Interference between cells can be reduced by appropriately scheduling transmissions or by spatial collision resolution.Portions of this paper have been presented at the IEEE International Conferences on Personal Indoor Mobile Radio Communications (PIMRC) of 1993 in Yokohama and 1994 in The Hague.     

On the performance of different node configurations in multi-fiber optical packet-switched networks

With the development of optical packet-switching (OPS) technologies, multi-fiber OPS networks will play an important role in the future data transmissions. In such networks, instead of constructing some extremely expensive node configurations with strictly non-blocking switching function, a more practical solution is multi-board switches that contain a number of small-sized switching boards. In this article, we have evaluated the performance of several different multi-board switches, based on the following two main objectives: (i) better understanding the effects of different connection schemes between switching boards and optical buffers and (ii) investigating possible schemes for achieving comparable performance to that of the ideal, strictly non-blocking switches. Extensive simulation results have shown that unlike circuit-switched net- works, multi-board OPS cannot easily perform comparably to the strictly non-blocking switch by having slightly more fibers per link. Also, such a problem can be tackled by several different approaches. The most efficient one is to equip the switch with more buffers rather than to increase the switching-board size or to enhance the buffer sharing between different switching boards.     

Improving the performance of a slotted ALOHA packet radio networkwith an adaptive array

The use of an adaptive antenna array as a means of improving the performance of a slotted ALOHA packet radio network is presented. An adaptive array creates a strong capture effect at a packet radio terminal by automatically steering the receiver antenna pattern toward one packet and nulling other contending packets in a slot. A special code preamble and randomized arrival times within each slot allow the adaptive array to lock onto one packet in each slot. The throughput and delay performance of a network with an adaptive array are computed by applying the standard Markov chain analysis of slotted ALOHA. It is shown that throughput levels comparable to carrier sense multiple access (CSMA) are attainable with an adaptive array without the need for stations to be able to hear each other. The performance depends primarily on the number of adaptive array nulls, the array resolution, and the length of the randomization interval within each slot     

Finding multi-constrained feasible paths by using depth-first search

An extended depth-first-search (EDFS) algorithm is proposed to solve the multi-constrained path (MCP) problem in Quality-of-Service (QoS) routing, which is NP-Complete when the number of independent routing constraints is more than one. EDFS solves the general k-constrained MCP problem with pseudo-polynomial time complexity O(m ² · EN + N ²), where m is the maximum number of non-dominated paths maintained for each destination, E and N are the number of links and nodes of a graph, respectively. This is achieved by deducing potential feasible paths from knowledge of previous explorations, without re-exploring finished nodes and their descendants in the process of the DFS search. One unique property of EDFS is that the tighter the constraints are, the better the performance it can achieve, w.r.t. both time complexity and routing success ratio. This is valuable to highly dynamic environment such as wireless ad hoc networks in which network topology and link state keep changing, and real-time or multimedia applications that have stringent service requirements. EDFS is an independent feasible path searching algorithm and decoupled from the underlying routing protocol, and as such can work together with either proactive or on-demand ad hoc routing protocols as long as they can provide sufficient network state information to each source node. Analysis and extensive simulation are conducted to study the performance of EDFS in finding feasible paths that satisfy multiple QoS constraints. The main results show that EDFS is insensitive to the number of constraints, and outperforms other popular MCP algorithms when the routing constraints are tight or moderate. The performance of EDFS is comparable with that of the other algorithms when the constraints are loose. This work was supported in part by the National Science Foundation under Grant CNS-0435522, by the UCOP CLC under grant SC-05-33 and by the Baskin Chair of Computer Engineering at University of California, Santa Cruz. Zhenjiang Li received the B.S. and M.S. degrees in electronic engineering from University of Science and Technology of China (USTC), Hefei, China, in 1998 and 2001, respectively. Since 2001, he has been a PhD student in the computer communication research group (CCRG) of the computer engineering department, University of California, Santa Cruz, U.S.A. His research interests include secure routing, constrained path selection, routing optimization and quality-of-service (QoS) provisioning in computer networks. He is a student member of the IEEE. J. J. Garcia-Luna-Aceves received the B.S. degree in electrical engineering from the Universidad Iberoamericana in Mexico City, Mexico in 1977, and the M.S. and Ph.D. degrees in electrical engineering from the University of Hawaii, Honolulu, HI, in 1980 and 1983, respectively. He holds the Jack Baskin Chair of Computer Engineering at the University of California, Santa Cruz (UCSC), and is a Principal Scientist at the Palo Alto Research Center (PARC). Prior to joining UCSC in 1993, he was a Center Director at SRI International (SRI) in Menlo Park, California. He has been a Visiting Professor at Sun Laboratories and a Principal of Protocol Design at Nokia. Dr. Garcia-Luna-Aceves has published a book, more than 300 papers, and nine U.S. patents. He has directed 22 Ph.D. theses and 19 M.S. theses since he joined UCSC in 1993. He has been the General Chair of the IEEE SECON 2005 Conference; Program Co-Chair of ACM MobiHoc 2002 and ACM Mobicom 2000; Chair of the ACM SIG Multimedia; General Chair of ACM Multimedia ’93 and ACM SIGCOMM ’88; and Program Chair of IEEE MULTIMEDIA ’92, ACM SIGCOMM ’87, and ACM SIGCOMM ’86. He has served in the IEEE Internet Technology Award Committee, the IEEE Richard W. Hamming Medal Committee, and the National Research Council Panel on Digitization and Communications Science of the Army Research Laboratory Technical Assessment Board. He has been on the editorial boards of the IEEE/ACM Transactions on Networking, the Multimedia Systems Journal, and the Journal of High Speed Networks. He received the SRI International Exceptional-Achievement Award in 1985 and 1989, and is a fellow of the IEEE.     

Improving PAPR performance of filtered OFDM for 5G communications using PTS

The filtered orthogonal frequency division multiplexing (F-OFDM) system has been recommended as a waveform candidate for fifth-generation (5G) communications. The suppression of out-of-band emission (OOBE) and asynchronous transmission are the distinctive features of the filtering-based waveform frameworks. Meanwhile, the high peak-to-average power ratio (PAPR) is still a challenge for the new waveform candidates. Partial transmit sequence (PTS) is an effective technique for mitigating the trend of high PAPR in multicarrier systems. In this study, the PTS technique is employed to reduce the high PAPR value of an F-OFDM system. Then, this system is compared with the OFDM system. In addition, the other related parameters such as frequency localization, bit error rate (BER), and computational complexity are evaluated and analyzed for both systems with and without PTS. The simulation results indicate that the F-OFDM based on PTS achieves higher levels of PAPR, BER, and OOBE performances compared with OFDM. Moreover, the BER performance of F-OFDM is uninfluenced by the use of the PTS technique.     

Improving the power cycling performance of the emitter contact of IGBT modules: Implementation and evaluation of stitch bond layouts

The emitter (front metallization) of IGBTs is contacted by wire bonding. In this study, the influence of the wirebond layout on the power cycling performance of IGBT modules is investigated. Stitch bonding is implemented to modify the wirebond layout of the emitter contact. The different layouts are subjected to power cycling tests. For a better understanding of the experimental results, electrical and thermo-mechanical FEM simulations are run and the current distribution and induced mechanical stress is discussed. Based on the results of this study, the emitter contact of the new HiPak module platform is designed.     

Improving GBW product on CMOS operational transconductance amplifiers by interleaved feedforward paths

This paper presents two Operational Transconductance Amplifier (OTA) compensation schemes for multistage topologies. The solutions are based on interleaved feedforward paths that cancel a non-dominant pole similarly to the zero nulling resistor technique with the advantage of avoiding resistors. Both schemes are designed in 90 nm CMOS process, the first one obtains 71 dB of DC gain, a gain bandwidth product (GBW) of 720 MHz with 360 μW of power consumption. The second proposed scheme obtains a similar DC gain and doubles the former proposed OTA GBW at the expense of 2.2 mW of power consumption for high speed applications. The compensation schemes are theoretically analyzed and the design guidelines are presented. The results of post layout simulations and corner analysis validate the new solutions.     

DA-FC法提高WCDMA系统的性能

WCDMA是第三代移动通信系统空中接口可选的主要接入技术之一。针对WCDMA系统中因功率控制不理想等因素引起的用户之间干扰的问题,推导了存在强干扰用户情况下基站接收端的信号表达式,提出了"提前判决-反馈消去法"即"DA-FC法"的解决方法;该方法提取强干扰用户的信息,经处理后再造强干扰用户的负项,并提前回送到射频接收前端处理模块,以抵消或减弱其影响,达到进一步提高WCDMA系统性能的目的。     

一种新的自适应增益盲优化算法提高自由空间光通信性能

由于大气湍流的影响,光束在大气中传输时产生 严重畸变,造成自由空间光(FSO)通信链路 性能恶化。为了提高系 统性能,构建了基于盲优化自适应光学的FSO通信系统,提出了一种新的自适应 增益随机并行梯度下降(SPGD)优化 算法。数值模拟结果表明,本文算法对大气湍流引起的波前误差进行实时补偿,能 够达到 很好的校正效果,比固定增益优化算法具有更好的收敛速度和精度。     

On the effectiveness of alternative paths in QoS routing

We propose a routing strategy in which connection requests with specific bandwidth demands can be assigned to one of several alternative paths connecting the source to the destination. The primary goal of this multiple‐path approach is to compensate for the inaccuracy of the knowledge available to routing nodes, caused by the limited frequency of link state (LS) information exchanges. We introduce a collection of K‐shortest path routing schemes and investigate their performance under a variety of traffic conditions and network configurations. We subsequently demonstrate that K‐shortest path routing offers a lower blocking probability in all scenarios and more balanced link utilization than other routing methods discussed in the literature. With our approach, it is possible to reduce the frequency of link state exchanges, and the incurred bandwidth overhead, without compromising the overall performance of the network. Based on the proposed routing scheme, we investigate different link state dissemination algorithms, which are aimed at reducing the communication overhead by prioritizing the scope and differentiating the qualitative content of LS update messages. Copyright © 2004 John Wiley & Sons, Ltd.     

Improving TCP performance in ad hoc networks using signal strength based link management

Mobility in ad hoc networks causes frequent link failures, which in turn causes packet losses. TCP attributes these packet losses to congestion. This incorrect inference results in frequent TCP re-transmission time-outs and therefore a degradation in TCP performance even at light loads. We propose mechanisms that are based on signal strength measurements to alleviate such packet losses due to mobility. Our key ideas are (a) if the signal strength measurements indicate that a link failure is most likely due to a neighbor moving out of range, in reaction, facilitate the use of temporary higher transmission power to keep the link alive and, (b) if the signal strength measurements indicate that a link is likely to fail, initiate a route re-discovery proactively before the link actually fails. We make changes at the MAC and the routing layers to predict link failures and estimate if a link failure is due to mobility. We also propose a simple mechanism at the MAC layer that can help alleviate false link failures, which occur due to congestion when the IEEE 802.11 MAC protocol is used. We compare the above proactive and reactive schemes and also demonstrate the benefits of using them together and along with our MAC layer extension. We show that, in high mobility, the goodput of a TCP session can be improved by as much as 75% at light loads (when there is only one TCP session in the network) when our methods are incorporated. When the network is heavily loaded (i.e., there are multiple TCP sessions in the network), the proposed schemes can improve the aggregate goodput of the TCP sessions by about 14–30%, on average.     

Improving reliability performance of diffusion‐based molecular communication with adaptive threshold variation algorithm

Reliability is a vital issue in the area of communication. In this paper, we particularly investigate the reliability issue for diffusion‐based molecular communication. First, we present the communication process in system model, where a classified model is further developed at the side of receiver to divide received molecules into signal, inter‐symbol interference, and noise branches. The classified model is used to prove the feasibility and necessity of improving the reliability performance. Second, an adaptive threshold variation (ATV) algorithm is designed to be able to improve the reliability performance that enables the receiver to adapt the channel condition properly through learning process. Furthermore, the complexity of ATV and the performance in various noisy conditions are discussed. An expression of signal to interference plus noise ratio is defined in numerical simulation to verify our goal with variational parameters, as well as with the adoption of ATV algorithm. The results indicate that ATV algorithm works well in general case in improving reliability performance for diffusion‐based molecular communication.     

一种提升传输机房装机空间的优化改造方案

伴随业务装机需求发展,作为传送网网络基础节点的传输机房承担的设备装机压力不断增大。通过对传输机房相关设备进行归类和分析,特别是动力设备和业务设备对于传输机房装机需求差异,获得将单层传输机房改造为双层的优化方案,进而实现提升传输机房可装机空。     

Improving the performance of linear inverse solutions by inverting the resolution matrix

This paper proposes a new strategy for improving the localization capabilities of linear inverse solutions, based on the relationship between the real solution and the estimated solution as described by the resolution matrix equation. Specifically, we present two alternatives based on either the partial or total inversion of the resolution matrix and applied them to the minimum norm solution, which is known for its poor performance in three-dimensional (3-D) localization problems. The minimum norm transformed inverse showed a clear improvement in 3-D localization. The strong dependence of localization errors with the eccentricity of the sources, characteristic of this solution, disappears after the proposed transformation. A similar effect is illustrated, using a realistic example where multiple generators at striate areas are active. While the original minimum norm incorrectly places the generators at extrastriate cortex, the transformed minimum norm localizes, for the example considered, the sources at their correct eccentricity with very low spatial blurring.     

一种利用循环前缀改善OFDM各级系统性能的方法

近年来,正交频分复用(OFDM)技术已被越来越多的人所关注,并且在IEEE 802.11无线局域网中得到实际应用.在经典的OFDM系统中,接收端恢复信号时,只是对去掉循环前缀(CP)的信号进行处理,而没有考虑循环前缀中的部分有用信息.文章提出了一种基于LS(Least Squares)准则,利用循环前缀恢复发送信号的方法,并给出这种方法的仿真结果.结果表明,当循环前缀长度较大时,文章方法得到的误码率明显低于传统方法.     

Improving the performance of a low-frequency correlator

A technique for improving the performance of a low-frequency correlator whose design is centred on a bucket-brigade delay line is presented. The technique is based on a concept which uses the unprocessed output of the delay line.     

Improving threshold performance of the IQML algorithm

In this article, improvements to the iterative quadratic maximum likelihood (IQML) algorithm are presented. The modifications are twofold: modification of the nontriviality constraint on the polynomial coefficient vector and the addition of a noise preprocessing step based on the eigendecomposition of the data covariance matrix. Simulation results are provided     

自适应性SLC的实际性能

本文论证了自适应天线旁瓣对消器(SLC)的对消性能取决于主、辅天线信号或干扰及有用信号的相干特性,而和统计相关性无直接关系。     

OFDM系统仿真与性能分析

正交频分复用（OFDM）是一种特殊的多载波方案,它既是一种调制技术,也是一种复用技术。由于其具有良好的抗频率选择性衰落或窄带干扰、频谱利用率高和成本低的优点,应用越来越广泛。采用Simulink搭建的OFDM仿真系统,既能有效测试OFDM的性能指标,也能研究和理解OFDM特点和具体应用。