Soft handoff algorithms for CDMA cellular networks

This paper discusses the design of soft handoff algorithms for cellular communication systems. The handoff process is modeled as a hybrid system and handoff design is cast as an optimization problem based on such a model. Performance is evaluated in terms of call quality, average number of active base stations, average number of active set updates, and average amount of interference. A soft handoff algorithm, which achieves a tradeoff between these performance criteria, is obtained using principles of dynamic programming. One key feature of the algorithm is that it incorporates the effects of mobility and shadow fading in the handoff decision. Different diversity combining schemes are considered including selective combining, equal gain combining (EGC), and various optimized combining (OC) methods in the soft handoff mode. For EGC and OC, Wilkinson's and Schwartz and Yeh's methods are used to compute the statistics for the power sum of the signals. Simulation results indicate that the performance of the handoff algorithm is a function of the different combining schemes and of the different methods used to compute the statistics of the power sum. Moreover, it is observed that interference cancellation is important in order for the algorithm to be viable for cellular systems which experience interference due to using nonorthogonal multiple access.     

Adaptation of CDMA Soft Handoff Thresholds Using Fuzzy Inference System

This paper proposes a new procedure to adjust soft handoff thresholds dynamically by using fuzzy inference system. This algorithm is compared with IS-95A and IS-95B/cdma2000 soft handoffs. The aims are to increase the thresholds at high traffic loads in order to release the traffic channel for supporting more carried traffic, and to decrease the thresholds at low traffic loads in order to give high quality of traffic channel. The inputs of the proposed algorithm are the number of remaining channels of each base station and the number of active pilots in active set of each mobile station. The output is the new soft handoff thresholds. In the fuzzy inference module, the triangular membership function, the max-min composition, and the weighted average formula defuzzification are selected. By comparison of all performance indicators among three algorithms, soft handoff using fuzzy inference tends to give higher performance than those of IS-95A and IS-95B/cdma2000 soft handoffs at high traffic loads and at lower soft handoff thresholds while the quality of traffic channel is still acceptable. Moreover, the wider soft handoff window size of the proposed algorithm gives high carried traffic and low blocking probability but lower quality of traffic channels. In addition, the adaptive soft handoff window size can give lower blocking probability while still keep acceptable quality of traffic channels.     

Locally optimal soft handoff algorithms

The design of soft handoff algorithms for cellular radio systems is considered. The design problem is posed as a tradeoff between three metrics: the rate of handoffs, the mean size of the active set, and the link quality. It is argued that the algorithm that optimizes the tradeoff among these metrics is impractical. Hence, a locally optimal (LO) handoff algorithm is derived as a practical approximation to the optimal handoff algorithm. The LO algorithm is shown to yield a significantly better tradeoff than the static threshold handoff algorithm used in second-generation code-division multiple-access (CDMA) systems. It is also shown that the dynamic threshold algorithm, which is an ad hoc algorithm proposed for third-generation CDMA systems, achieves nearly the same performance as the LO algorithm. Thus, an analytical justification is developed for the dynamic threshold algorithm. Further, the handoff algorithm design is separated into independent design problems on the forward and reverse links. The forward link LO algorithm is shown to be computationally intensive but is also shown to be closely approximated by the simpler reverse link LO algorithm.     

Analysis of soft handoff algorithm for multi‐cellular systems: A finite integral approach

In this paper the performance of soft handoff algorithm based on pilot signal strength measurements has been studied. In connection with the soft handoff it has been observed that the existing analytic expressions often involve integration in infinite limits. The expressions containing such kind of integrals have been meticulously transformed into finite range equations. Probability of outage, number of base stations (BS) in the active set, and number of active set updates are some of the performance metrics commonly considered in soft handoff. Accurate closed forms for all these performance indicators have been computed which, in turn, simplifies the calculation of several other metrics entailing them. Moreover, the underlying system model takes into account of more than two BS (specifically three) extending the traditional two BS model toward more complex but realistic characterization of soft handoff performance analysis framework. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance Evaluation and Resource Management of Hierarchical MACRO-/MICRO Cellular Networks using MOSEL-2

The paper presents a performance evaluation and resource management of hierarchical MACRO-/MICRO cellular networks using the new Modeling and Evaluation Language (MOSEL-2). MOSEL-2 with new constructs has the ability to find the performance and reliability modeling and evaluation of systems with exponential and non-exponential distributions. A MACRO/MICRO cell structure is solved numerically and mathematically in this paper to handle the handoff calls. Additionally, a simulation program is written to validate these results. In order to reduce the loss probability, a guard channels are introduced at the MICRO cell and channel reservation at the MACRO cell. Additionally, the concept of queuing is introduced where there is a possibility for the handoff calls from both MACRO and MICRO layers to be queued when all the resources are occupied. MOSEL-2 is used to find the numerical solution for this problem with both exponential and general exponential (GE) distribution. The performance analysis show the efficiency of the proposed scheme to manage the handoff calls and the ability of the suggested scheme to reduce the blocking probability of handover calls and the loss probability as the main objective is to block the new connection rather than terminating the ongoing connection as well as balancing the load all over the whole network. It is shown in this paper that there are a set of important factors that affect the performance, such as: reservation policy, channel allocation, handover ratio, capacity of the queue and the variation of the inter-arrival times. These factors are discussed via some important performance measures, such as: new call blocking probability, blocking probability of handover calls, loss probability, utilization and the average delay of the queue.     

基于运动趋势的模糊逻辑垂直切换算法

随着无线异构网络的融合,移动性管理技术成为其关键问题,而切换管理又是移动性管理的重要部分。针对垂直切换管理提出了一种基于运动趋势的模糊逻辑垂直切换算法。算法分为预判定、模糊逻辑控制及切换判决3个过程。首先,在预判定阶段根据MN的运动趋势及接收信号强度滤除掉不适宜接入WLAN的网络信息,从而有效减少不必要的数据量和系统开销;其次,将接收信号强度、网络的可用带宽和网络开销送入模糊逻辑控制器,通过参数的归一量化最终得到网络综合性能值(VCPN);最后,通过综合考虑VCPN和驻留时间来进行网络切换判决。仿真结果显示,该算法能够有针对性地做出切换判决,有效消除乒乓效应,提高网络切换性能。     

基于TDD系统的高速磁悬浮软切换技术研究

磁悬浮列车是一种运行速度达600 km/h的陆地交通运输工具,对分区切换机制设计带来更大挑战。为保证列车运行过程中的可靠通信,首先对高速磁浮车地通信系统方案进行设计,面向磁悬浮车地通信软切换需求优化设计了MAC层帧结构,同时提出基于地理位置信息的软切换机制,包括分区内切换和跨分区切换;通过仿真得出列车在速度为600 km/h的情形下,切换重叠区距离设置为264 m,可以实现切换成功概率为99.6%。     

Soft handoff algorithm with variable thresholds in CDMA cellularsystems

Traffic nonuniformity degrades the performance of CDMA cellular systems. The authors present a new algorithm called the variable threshold soft handoff (VTSH) which reduces the performance degradation due to traffic nonuniformity in CDMA cellular systems. Unlike the conventional fixed handoff thresholds, the proposed algorithm allows the handoff thresholds (T ADD and T DROP) to vary dynamically according to the traffic density of each cell. This algorithm has been implemented by means of computer simulation and the results show that VTSH improves the overall CDMA system performance in terms of outage probability. The VTSH algorithm can easily be applied to CDMA cellular systems without any modification     

一种适合于第四代公网集群系统的切换算法

提出了一种适合于公网集群系统的小区切换算法.在公网集群系统中,多个群组用户同时进行越区切换,可能造成信令风暴,导致更高的切换时延,降低了小区切换成功率.通过引入桥梁群组用户这一概念,通过桥梁群组用户同时连接2个网络,利用2次垂直切换代替一次水平切换的方法,避免了大量群组用户同时发起切换测量.仿真结果表明,利用移动自组织网络为准备切换的集群用户组成一个小网络,桥梁群组用户带领其他群组用户切换的方法比传统切换算法的切换成功率高,切换时延更短.     

Cellular Network Performance Analysis: Handoff Algorithms Based on Mobile Location and Area Information

Mobile world's rapid growth has spurred development of new protocols and new algorithms to meet changing operation requirements such as mobile networking, and quality-of-service support. A key requirement in the bearer capabilities is the handover. The freedom to be able to make and receive calls anywhere, at any time, creating a totally new dimension in human communications has frequently been advertised as the main advantage of new wireless systems. Handovers are a key concept in providing this mobility. It makes it possible for a user to travel from one cell to another while having a seamless connection. Network operators give emphasis to optimize handover, since it is strongly related to dropped calls, network overload and subsequently users' criticism. The ability of a cellular network to perform efficient handovers is crucial to offer attractive services as real-time applications or streaming media as planned in third generation networks. Since signal propagation and pathloss are complex in nature, we can expect unnecessary and wrong handoff executions. Both UMTS and those of the second generation (GSM) systems will require redefined handoff algorithms of active connections as the smooth mobility support and continuous connection are essential issues for obtaining high performance and increasing user satisfaction. In this paper we present a set of intelligent algorithms using the mobile terminal (MT) location information and area awareness to assist safe handoff decisions. The implemented algorithms are validated by means of cellular network simulators that clearly show the impact of these techniques to major system performance metrics.     

Teletraffic performance evaluation of microcellular personalcommunication networks (PCN's) with prioritized handoff procedures

Evaluates four handoff priority-oriented channel allocation schemes. These give priority to handoff calls by reserving channels exclusively for handoff calls. The measurement-based handover channel adaptive reassignment scheme (MHAR-A) exclusively uses reserved handover channels for newly originated calls if a certain criterion is satisfied. All four schemes studied differ from the conventional guard channel-based handover priority-oriented channel allocation scheme. To study the schemes, a personal communication network (PCN) based on city street microcells is considered. A teletraffic simulation model accommodating a fast moving vehicle is developed, and the performance parameters are obtained. The performances of all four schemes are compared with the nonpriority scheme and the conventional guard channel-based handover priority-oriented channel allocation scheme. It was found that some of the channel allocation algorithms studied improved the teletraffic capacity over the nonpriority and the conventional guard case. Also, the probability of new call blocking and carried traffic was improved for three of the schemes when compared to the conventional guard scheme. The MHAR-A scheme did not perform up to expectation. Nevertheless, it can be used to finely control the communication service quality equivalent to the control obtained by varying the number of handoff channels in a fraction of one. Increasing the number of reserved handover channels in fraction of one can never be achieved in the conventional guard channel-based handover priority-oriented channel allocation scheme     

An Optimum Vertical Handoff Decision Algorithm Based on Adaptive Fuzzy Logic and Genetic Algorithm

Handoff decision making is one of the most important topics in wireless heterogeneous networks architecture as there are many parameters which have to be considered when triggering handoff and selecting suitable access point. More intelligent approaches which reckon user profiles, application requirements, and network conditions must be improved so that desired performance results for both user and network could be provided. In this paper we introduce a new adaptive vertical handoff decision making algorithm in which fuzzy membership functions are optimized by means of genetic algorithm. Genetic algorithm is an adaptive search technique based on natural selection and genetic rules. In addition to that, it takes places in various scientific applications and can be used to adjust the membership functions in fuzzy systems. The purpose of the study is to adjust the shape of fuzzy membership functions, properly, using genetic algorithm in order to achieve optimum handoff performance. The results show that, compared to the several different algorithms performance of the proposed approach with genetic algorithm is significantly improved for both user and network in terms of number of handoff while the other requirements are still satisfied.     

基于信道借用和信号预测的切换方法

随着业务需求的日益增长,蜂窝也越变越小,移动蜂窝通信环境中的切换成为日益重要的问题。在CDMA蜂窝系统中使用软切换,为了解决切换时的信道短缺问题,提出了许多解决方法。该文中提出了称为基于信道借用和信号预测的切换算法。当切换请求到达蜂窝时,如果没有空闲信道,就将从参与软切换的静止呼叫借用一信道,并将借用的信道分配给移动呼叫的切换请求.如果没有信道可借用,就将切换请求放入队列中,使用信号预测的方法来确定队列中的优先级。并将此切换方式的性能与其它切换方式进行了比较。     

Adaptive channel reservation scheme for soft handoff in DS-CDMAcellular systems

Soft handoff techniques in direct-sequence code-division multiple-access (DS-CDMA) systems provide mobile calls with seamless connections between adjacent cells. Channel reservation schemes are used to give high priority to more important soft handoff attempts over new call attempts. However, since the number of soft handoff attempts varies according to environmental conditions, fixed reservation schemes for handoff attempts can be inefficient. An adaptive channel reservation scheme is herein proposed to control the size of reservation capacity according to varying the number of soft handoff attempts. The proposed scheme also includes a balancing procedure between soft handoff failure and new call blocking to maximize the system capacity. To evaluate the performance of the proposed scheme, a Markovian model is developed that considers the interference-limited capacity effect of DS-CDMA systems. The analytical result shows that the proposed scheme yields a considerable enhancement in terms of new call blocking and soft handoff failure probabilities when compared with the conventional fixed channel reservation scheme     

Heterogeneous Wireless Networks: Configuration and Vertical Handoff Management

Vertical handoff is one of the most important issues for the next generation heterogeneous wireless networks. However, in many situations, unbeneficial vertical handoffs occur across intersystem heterogeneous networks cause network performance degradation. Therefore, we propose a novel configuration architecture that can be deployed in the next generation of wireless networks. Second, we propose a predictive and adaptive Vertical Handoff Decision Scheme that optimizes the handoff initiation time as well as selection of the most optimal network. The proposed vertical handoff decision algorithm considers the technology type as well as the Signal to Interference Ratio (SIR), the Mobile Station (MS) velocity, the user preferences, the applications requirements and the terminal capabilities as the most important factors to make vertical handoff decision. In order to minimize handoff costs, the proposed decision algorithm uses the dwell timer concept. The handoff costs are analyzed in terms of unnecessary and unbeneficial handoffs rate.The simulation results show that the reduction of unnecessary handoffs proposed in our vertical handoff decision scheme reduces the handoff blocking probability, the packets loss rate and the handoff overhead     

基于高频段卫星动态信道预约的自适应QoS切换算法

未来高频段卫星网络将为固定和移动用户提供多媒体通信业务,而为移动用户提供无线多媒体业务的一个最关键的问题是保证端到端连接的业务质量。该文提出了一种基于高频段卫星动态信道预约的自适应QoS切换算法,并与其它切换方式进行了性能比较。     

Modeling and performance analysis for soft handoff schemes in CDMA cellular systems

This paper investigates the features of a cellular geometry in code-division multiple-access (CDMA) systems with soft handoff and distinguishes controlling area of a cell from coverage area of a cell. Some important characteristics of the cellular configuration in soft handoff systems are used to propose a new design of efficient call admission control (CAC) in CDMA systems. Then, the paper constructs a continuous-time Markov chain (CTMC) model for CAC in CDMA with a soft handoff queue, obtains closed-form solutions, and thus develops loss formulas as performance indices such as the new blocking probability and the handoff dropping probability. In order to determine handoff traffic arrival rate, a fixed-point strategy is developed. Algorithms are also provided to stably compute loss probabilities and to determine the optimal number of guard channels. A new soft handoff scheme-eliminating pseudo handoff calls (EPHC)-is proposed to improve channel utilization efficiency based on mobility information. As an application of the loss formulas, the proposed modeling techniques are used to evaluate and compare the performance of conventional and proposed EPHC soft handoff schemes. Numerical results show the effectiveness of the proposed Markov chain models and the benefits of the new soft handoff scheme.     

Design and Performance Analysis of a New Soft Handoff Scheme for CDMA Cellular Systems

In this paper, a new soft handoff scheme for CDMA cellular systems is proposed and investigated. It is pointed out that some handoff calls unnecessarily occupy multiple channels with little contribution to the performance of handoffs in IS95/CDMA2000-based handoff schemes or systems. To alleviate performance degradation due to channel resource shortage during soft handoff, a new concept of channel convertible set (CCS), which contains several types of handoff calls that unnecessarily occupy extra channels by considering the relative mobility of the calls in the handoff area is introduced. A new scheme that reallocates those extra channels in the CCS to new handoff calls when there is no available free channel in the system is proposed. Furthermore, according to the variation of the CCS, the proposed scheme dynamically adjusts the number of guard channels reserved exclusively for handoff. Then, the feasibility and implementation issues of the proposed scheme are discussed. To evaluate and compare performance indexes of different soft handoff schemes, continuous-time Markov chain models are constructed. Automated generation and solution of the underlying Markov chains are facilitated by stochastic reward net models, which are specified and solved by stochastic Petri net package. Numerical results show that this scheme can significantly decrease both the number of dropped handoff calls and the number of blocked calls without degrading the quality of communication service and the soft handoff process.     

宽带码分多址系统下行链路软切换技术的研究

本文对宽带码分多址(WCDMA)系统下行链路的软切换技术进行了定性分析,引入了新的软切换参数:切换边际值,并与硬切换技术进行了比较,此外还对软切换宏蜂窝分集效应进行了研究,得出了宏蜂窝分集增益与信道多径特性和小区用户密度两个重要参数之间的关系式.     

基于速度触发的提前切换算法在LTE-R中的应用研究

随着高铁的快速发展,铁路移动通信系统对切换时延、切换成功率等要求更为苛刻。该文针对铁路长期演进(LTE-R)系统提出基于速度触发的提前切换算法,通过提前进行信令交互、设置切换预承载点切换的方式抑制过早或过迟切换所带来的通信中断、掉话等问题。通过信令流程图进行理论分析,并对长期演进(LTE)各结构类型数据计算可知,该算法均可缩短切换时延,其中TDD帧结构类型2时效果最为显著。最后通过仿真对比得到提前切换算法相比传统切换算法有着更高且更稳定的切换成功率,验证算法的有效性,为LTE-R在未来铁路专网的应用提供技术支持。