Modeling of customer retrial phenomenon in cellular mobile networks

In the planning of modern cellular mobile communication systems, the impact of customer behavior has to be carefully taken into account. Two models dealing with the call retrial phenomenon are presented. The first model considers a base station with a finite customer population and repeated attempts. A Markov chain modeling is proposed, and an efficient recursive solution of the state probabilities is presented. The second model focuses on the use of the guard channel concept to prioritize the handover traffic. Again, the retrial phenomenon plays an important role. The influence of the repeated attempt effect on the quality of service experienced by the mobile customers is discussed by means of numerical results     

Performance Analysis of Dedicated Signalling Channels in GERAN by Retrial Queues

GERAN (GSM-EDGE Radio Access Network) operators have traditionally used the Erlang B formula to estimate the number of signalling channels on a per-cell basis. Thus, it is assumed that the network behaves as a loss system with Poisson arrivals. However, the presence of automatic retrial mechanisms and correlated arrivals in these channels suggests that these assumptions might not be valid. This paper presents a performance analysis of the Stand-alone Dedicated Control CHannel in GERAN. Preliminary analysis shows that the Erlang B formula underestimates congestion and blocking on this channel. To address this issue, a queueing model with retrials and correlated arrivals is proposed, where correlation between arrivals is modelled by a simple Markov-Modulated Poisson Process. The proposed model can be tuned on a per-cell basis by statistics in the Network Management System. Model assessment is based on performance statistics from a live GERAN system. Results show that a simple retrial queueing model fails to explain blocking in cells with a large number of channels. These limitations are overcome by adding correlated arrivals in the retrial model.     

A call quality performance measure for handoff algorithms

This paper proposes a new approach for performance evaluation and comparison between existing handoff algorithms taking into consideration signal levels, call dropping, and handoff cost. Using the new approach, existing handoff algorithms are then compared in terms of signal quality and number of handoffs required to achieve a desired overall signal quality. We also provide in this paper a method to estimate handoff cost and to optimize handoff sequences for retrial (where repeated call attempts are made after a call is lost) and non‐retrial models based on the proposed approach. We observe that the Threshold with Hysteresis method performs better than other known methods including the one used in the GSM standard. Our results indicate that the Threshold with 4 dB Hysteresis method performs well for urban areas although with a high dropping probability, whereas the Threshold with 6 dB Hysteresis method suits for suburban areas with a low dropping probability. We find that handoff sequences obtained by existing handoff methods are less efficient than the optimal handoff sequence given in the paper by a margin of 29–45% for retrial model, and by 34–77% for non‐retrial model. The paper also suggests some specific parameter values to improve the performance of currently used handoff methods based on our findings. Copyright © 2010 John Wiley & Sons, Ltd.     

Multichannel random access in OFDMA wireless networks

Orthogonal frequency-division multiple access (OFDMA) systems are considered promising candidates for implementing next-generation wireless communication systems. They provide multiple channels that can be accessed via random access schemes. However, traditional random access schemes could result in an excessive amount of access delay. To address this issue, we develop a fast retrial scheme that is based on slotted Aloha and exploits the structure of OFDMA. A salient feature of this scheme is that when collisions occur instead of retrials occuring randomly in time, they occur randomly in frequency, i.e., the scheme randomly selects the subchannels for retrial. To further achieve fast access, retrials are designed to follow the 1-persistent type, i.e., no exponential backoff. To achieve the maximum throughput, we limit the maximum number of allowed retrials according to the load condition. We also consider the issue of designing for an appropriate reuse factor for random access channels in order to overcome the intercell interference problem in OFDMA multicell environments. Our finding is that full sharing, i.e., a reuse factor of one, performs best for given random access channels. Through analysis and simulation, we confirm that our fast retrial algorithm has the advantage of high throughput and low access delay, and the full sharing policy for random access channels shows high throughput as well as low collision.     

Modelling communication systems with phase type service and retrial times

The retrial phenomenon and its significant effect on network performance have been taking into account in the literature (e.g., random access protocols in computer networks, cellular mobile telephony). Most studies assume exponential distributions to guarantee the tractability of the mathematical model. However, this is a serious drawback in practice where the exponentiality is not a realistic assumption. The objective is to find a good balance between realistic assumptions and tractability. In this paper, the objective is achieved by proposing a multichannel queueing model with quasi-random input and retrial and service times of phase type.     

Efficient and accurate methodology for solving multiserver retrial systems

Proposed is a novel methodology for solving retrial systems which is based on the aggregation of levels of the Markov model beyond a given one. Its evaluation concludes that it is more accurate than previous approximations while requiring a low computational cost.     

Performance aspects of data broadcast in wireless networks with user retrials

The user retrial phenomenon and its significant impact on network performance in unicast wireless systems are known and relatively well studied in the literature. However, there have been no previous studies on the impact of the user retrial phenomenon on other types of wireless networks. The objective of this paper is to extend the analysis of the user retrial phenomenon to wireless systems which, in addition to unicast service, also support a data broadcast service. This objective is realized by defining several performance measures appropriate for the analysis of hybrid unicast-broadcast systems in the presence of users' retrials. Subsequently, we derive the exact mathematical expression for each of the measures. Based on these expressions, we prove the existence of a single broadcast scheduling scheme, which ensures optimal system performance, with respect to the given set of proposed measures, the system's throughput, and the grade and quality of service. We also take a closer look at the class of hybrid unicast-broadcast systems with autonomous estimation of data item popularities, and we elaborate on the major challenges associated with such systems. Finally, we evaluate our theoretical expressions through simulation, and we discuss their robustness with respect to moderate deviations in the underlying model.     

Transient analysis of the successful and blocked events in retrial queues

The phenomenon of repeated attempts arises naturally in telecommunication and computer networks. One aspect of the retrial feature is considered in this paper, where we investigate the transient behavior of the successful and blocked events made by the primary customers and the retrial customers.     

Nanopatterned platinum electrodes by focused ion beam in single palladium nanowire based devices

In this paper we present different devices, working as hydrogen sensors, based on single palladium nanowire and realized combining focused ion beam and dielectrophoresis techniques. The combination of these two techniques consists in the employ of focused ion beam for electrode patterning onto silicon/silicon dioxide substrates, followed by the assembly of a single palladium nanowire applying between electrodes an electric field at fixed frequency (dielectrophoresis) to a palladium nanoparticles solution. The nanowire morphology can be branched or not, depending on the applied frequency value. The devices are characterized in hydrogen environment at room temperature and their responses are compared.     

A probe-based SEU detection method for SRAM-based FPGAs

SRAM-based FPGAs are becoming increasingly suitable for avionic and space applications due to their flexibility, reconfigurability and capacity as well as their signal processing capabilities. Unfortunately, commercial-of-the-shelf (COTS) SRAM-based FPGAs are highly sensitive to ionizing radiation environment such as space or avionic, making them extremely sensitive to radiation-induced Single Event Upsets (SEUs). In this paper, we propose a detection solution able to detect SEU-effects before they affect the circuit functionalities. The developed solution overcome state-of-the-art techniques since it is able to anticipate the detection of SEU-effects with an improvement of the latency of more than 70% than traditional redundancy based mitigation techniques. Besides, the proposed solution has a negligible impact on the circuit timing, since it does not introduce any performance degradation and it has a limited cost in terms of area usage. Experimental results performed on three benchmark circuits with traditional Duplication with Comparison (DEC) error detection technique demonstrate the feasibility of the proposed method showing an improvement of the detection capability of around 98% on the average.     

一种面向嵌入式SoC设计的混合级硬/软件协同验证技术

SoC已经成为嵌入式系统必不可少的解决方案。验证如此复杂的嵌入式SoC是非常困难的,系统设计需要新的验证技术更快更好地完成系统功能验证任务。通过比较当前3种主要的嵌入式系统验证技术:软件仿真技术、硬件模拟技术、硬/软件协同验证仿真技术,介绍基于指令集仿真器和FPGA相结合的、面向IP核复用的混合级硬/软件协同验证环境,并提出混合级协同验证总线功能模块的构成。该技术不仅可以提高设计的可信性和验证速度,而且能够继承当前大多数硬件模拟验证方法。     

一种基于H.264的智能手机监控系统设计与实现

文章介绍了基于H．264编解码压缩标准的视频系统在智能手机上的实现与应用。提出了一种基于手机视频的远程监控系统的设计与实现方法，介绍了该系统的体系结构和基本原理；并分别针对局域网络和无线网络进、行测试，得出系统实施的实验结果。     

IP网络服务质量保证技术

现有的IP网络本身没有特别考虑对业务服务质量的保证技术,无法满足综合业务的应用。几年来,业界提出了一些针对IP网络的服务质量保证方法,但是并没有真正在现有网络中普及应用。随着下一代网络的提出,网络服务质量保证技术已经列为其核心技术,只有该技术得到解决,基于IP的下一代网络才能够满足电信级网络的要求。简要分析了服务质量的要素,梳理了服务质量保证的几个关键技术,并对这些技术的原理和特点进行了简要分析。     

A fast and efficient technique to apply Selective TMR through optimization

Fault tolerance is an important factor for circuits in critical applications, especially those working in harsh environments. There are many techniques to increase reliability of circuits, being those based on redundancy very popular. In this way, Triple Modular Redundancy (TMR) is frequently used, but it usually incurs high area costs. That is why other alternative techniques, as Selective TMR, are used in order to reduce this cost. In this technique, only a subset of registers is tripled, those that are more sensitive and produce a higher error rate in the circuit. However, the problem of these methodologies is the complexity of finding the optimal set of registers to triple, what usually leads to very high computation times. In this paper, a novel solution that improves Selective TMR is presented, based on the automatic and fast calculation of an initial partition prior to the optimization process. The solution has been tested on a real communication circuit, a Feed-Forward Equalizer.     

基于IMS的VCC业务实现

从语音呼叫连续性（VCC）采用的关键技术、基于IMS的VCC具体实现方案以及VCC现存问题3个方面介绍了VCC主要处理过程和VCC系统架构,并提出了一种具体基于IMS的VCC实现方案。     

Comparison of local radiation boundary conditions for the scalarHelmholtz equation with general boundary shapes

The relative accuracy of several local radiation boundary conditions based on the second-order Bayliss-Turkel (1980) condition are evaluated. These boundary conditions permit the approximate solution of the scalar Helmholtz equation in an infinite domain using traditional finite element and finite difference techniques. Unlike the standard Bayliss-Turkel condition, the generalizations considered here are applicable to noncircular solution domains. The accuracy of these conditions are investigated for elliptical and linear/circular boundaries     

Scan predictive vector quantization of multispectral images

Conventional vector quantization (VQ)-based techniques partition an image into nonoverlapping blocks that are then raster scanned and quantized. Image blocks that contain an edge result in high-frequency vectors. The coarse representation of such vectors leads to visually annoying degradations in the reconstructed image. The authors present a solution to the edge-degradation problem based on some earlier work on scan models. The approach reduces the number of vectors with abrupt intensity variations by using an appropriate scan to partition an image into vectors. They show how their techniques can be used to enhance the performance of VQ of multispectral data sets. Comparisons with standard techniques are presented and shown to give substantial improvements.     

Analysis of resource splitting scheme with cognitive based admission control for femto-WiFi wireless networks

The coexistence of femtocell and WiFi networks in a heterogeneous spectrum environment with licensed and unlicensed bands will support multi-mode femtocell users (FUs) to simultaneously transmit on both licensed and unlicensed bands. The efficient integration of both femtocell and WiFi technologies is seen as crucial for supporting the offloading of femtocell traffic to WiFi networks. To successfully deploy these integrated technologies, the overall licensed and unlicensed spectrum usage must be efficiently managed. Thus, we propose a new cognitive-based connection-level admission control with access retrial for a femtocell network that operates under a mixed spectrum of unlicensed and licensed bands. By deploying cognitive radio concepts, the FUs will utilize the unused spectrum of the existing unlicensed spectrum of the WiFi network in an opportunistic manner in addition to using the licensed spectrum. By using the retrial phenomenon policy, the blocked FUs can retry the access, which can reduce their loss probability. An analytical model using a three-dimensional continuous time Markov chain with a level-dependent quasi birth-and-death structure was developed to evaluate and study the performance of the proposed scheme. The matrix analytic method was used to obtain the steady state probability and performance measures. The result shows that the performance of FUs for integrated femtocell-WiFi networks using the proposed scheme significantly improved in terms of FUs throughput and loss probability. The results also show that the retrial phenomenon of FUs enhances their performance.     

A general time model for the specification and design of embedded real-time systems

Design of complex embedded systems feasible with current and upcoming semiconductor technologies necessitates consideration of real-time from the beginning. However, the commonly used specification techniques do not consider temporal aspects in general like fulfillment of high level timing requirements or dynamic reactions on timing violations. In this paper, we discuss the restrictions of current specification techniques for embedded real-time systems and present a general time model that solves this issue. The time model contains the progress of time, the measurement of time and the specification of timing requirements based on event traces. In contrast to earlier techniques, preconditions determine the actual relevance of a specific timing bound. Exemplified for SDL, a solution for the specification of temporal aspects is shown. The advantages of this solution are discussed in a hardware/software co-design case study from the mobile communication area.     

Pseudowhitening of weather Radar signals to improve spectral moment and polarimetric variable estimates at low signal-to-noise ratios

Pseudowhitening of oversampled signals in range is proposed as a method to improve the performance of spectral moment and polarimetric variable estimators on weather surveillance radars. In an attempt to overcome the noise sensitivity of the whitening transformation, a solution based on the minimum mean-square-error criterion is considered first; however, this transformation is less practical than whitening because it requires knowledge of the signal-to-noise ratio at every range location. Pseudowhitening techniques are introduced as practical solutions that achieve a suboptimal compromise between variance reduction and noise sensitivity. Based on regularization methods for the solution of ill-conditioned problems, two pseudowhitening schemes are proposed: the clipped singular value decomposition transformation and the sharpening filter. By comparing their statistical performance with theoretical minimum bounds, it is shown that pseudowhitening-based estimators are almost optimal under practical conditions. Estimators based on pseudowhitening techniques avoid the pitfalls of their whitening-transformation-based counterparts and lead to more accurate radar products and/or rapid data acquisition for a much wider range of signal-to-noise ratios.