A locally optimal handoff algorithm for cellular communications

The design of handoff algorithms for cellular communication systems based on mobile signal strength measurements is considered. The design problem is posed as an optimization to obtain the best tradeoff between the expected number of service failures and expected number of handoffs, where a service failure is defined to be the event that the signal strength falls below a level required for satisfactory service to the subscriber. Based on dynamic programming arguments, an optimal solution is obtained, which, though impractical, can be used as a benchmark in the comparison of suboptimal schemes. A simple locally optimal handoff algorithm is derived from the optimal solution. Like the standard hysteresis algorithm, the locally optimal algorithm is characterized by a single threshold. A systematic method for the comparison of various handoff algorithms that are akin to the receiver operating characteristic (ROC) curves of radar detection is presented. Simulation results show that the locally optimal algorithm outperforms the hysteresis algorithm, especially in situations where accurate prediction of signal strength is possible. A straightforward technique for adapting the locally optimal algorithm to changing environments is suggested. That natural adaptability is the algorithm's principle advantage over current approaches     

An Adaptive Hard Handoff Algorithm for Mobile Cellular Communication Systems

In this letter, we propose an adaptive hard handoff algorithm with a dynamic hysteresis value based on the received signal strength from the serving base station, for mobile cellular communication systems. A discrete‐time method is presented to evaluate handoff algorithms analytically. Performance is evaluated in terms of the average number of handoffs, the probability of link degradation, and the average handoff delay. Numerical results and simulations demonstrate that the proposed algorithm outperforms the handoff algorithm with fixed hysteresis values and the handoff algorithm using both threshold and hysteresis.     

Two-state pattern-recognition handoffs for corner-turningsituations

Handoff algorithms are used in wireless cellular systems to decide when and to which base station to handoff. Traditional handoff algorithms generally cannot keep both the average number of unnecessary handoffs and the handoff decision delay low. They do not exploit the relative constancy of path loss and shadow fading effects at any given location around a base station. However, handoff algorithms with both a negligible number of unnecessary handoffs and a negligible decision delay can be realized by exploiting this information. One example is the set of handoff algorithms using pattern-recognition introduced in previous work. In this paper, we describe how pattern-recognition handoff algorithms can be applied to the problem of turning a corner. This can be used as part of an integrated pattern-recognition handoff algorithm or together with a traditional handoff algorithm, in which case the pattern-recognition handles only the special cases like turning a corner     

Microcellular handoff using fuzzy techniques

In order to manage the high call density expected in future cellular systems, microcell must be used. The size of the microcell will cause a dramatic increase in the number of handoffs. In addition, the small size of the microcell will require handoff algorithms to respond faster than those in todays systems. The problems are further exacerbated by the corner effect phenomenon which causes the signal level to drop by 20–30 dB in 10–20 m. Thus, in order to maintain reliable communication in a microcellular system new and better handoff algorithms must be developed. The use of hysteresis and averaging window in classical handoff techniques reduces unnecessary handoffs, but causes delays which may result in the calls being dropped. A fuzzy based handoff algorithm is proposed in this paper as a solution to this problem. The performance of fuzzy based handoff algorithm was also compared to that obtained using the classical handoff algorithms.     

A model for analyzing handoff algorithms [cellular radio]

A model for analyzing the performance of handoff algorithms based on signal strength measurements made by mobile stations in a lognormal fading environment is presented. This model enables one to evaluate the effect of averaging the hysteresis on the handoff process. Handoffs are related to level crossings of the difference between the received signal strengths from two base stations. The algorithm performance is derived by modeling the level crossings as Poisson processes with time-varying rate functions. The model is seen to yield results that agree with simulations over the range of algorithm parameters of practical interest. These results can be used to determine the averaging interval and hysteresis level that achieve the optimum tradeoff between the number of unnecessary handoffs and the delay in handing off     

A pattern recognition system for handoff algorithms

In wireless cellular systems, handoff algorithms decide when and to which base station to handoff. Traditional handoff algorithms generally cannot keep both the average number of unnecessary handoffs and the handoff decision delay low. They do not exploit the relative constancy of path loss and shadow fading effects at any given location around a base station. This information can in fact be used to improve the efficiency of handoff algorithms, as we do in our new handoff algorithms using statistical pattern recognition. Handoff algorithms with both a negligible number of unnecessary handoffs and a negligible decision delay can therefore be realized     

An efficient handoff algorithm based on received signal strength and wireless transmission loss in hierarchical cell networks

Compared with the macrocell systems, the femtocell systems allow users to obtain broadband service with high data rate by using lower costs of transmit power, operation and capital expenditure. Traditional handoff algorithms used in macrocells cannot well satisfy the mobility of users efficiently in hierarchical macro/femto cell networks. In this paper based on the received signal strength (RSS) and wireless transmission loss, a new handoff algorithm in hierarchical cell networks called RWTL-HO is proposed, which considers the discrepancy in transmit power between macrocell and femtocell base stations. The simulation results show that compared with the conventional algorithm, the proposed algorithm improves the utilization of femtocells by doubling the number of handoffs; and in comparison with the handoff algorithm based on combining the RSSs from both macro and femto cell base stations, reduces half the number of redundant handoffs.     

Signal-based evaluation of handoff algorithms

We propose a new framework, based on signal quality, for performance evaluation and comparison between existing handoff algorithms. It includes new call quality measures and an off-line cluster-based computationally-simple heuristic algorithm to find a near optimal handoff sequence used as a benchmark. We then compare existing handoff algorithms and identify the trade-off between signal quality and number of handoffs.     

Direction biased handoff algorithms for urban microcells

A novel class of direction biased handoff algorithms are proposed for improving the handoff performance in urban microcells. Multi-cell handoff properties are obtained for a cell layout consisting of base stations at every other intersection of a Manhattan street grid. The direction biased handoff algorithms are shown to improve cell membership properties by simultaneously reducing the mean number of handoffs and handoff delay. A signal strength based direction estimator is shown improve handoff performance for line-of-sight handoffs.This research was supported by Nortel and completed while Dr. Austin was a Ph.D. student at Georgia Tech. Portions of this paper were presented in the Vehicular Technology Conference, Stockholm, Sweden, June 7–10, 1994.     

MDP-Based Network Selection Scheme by Genetic Algorithm and Simulated Annealing for Vertical-Handover in Heterogeneous Wireless Networks

The hybrid algorithm for real-time vertical handover using different objective functions has been presented to find the optimal network to connect with a good quality of service in accordance with the user’s preferences. Markov processes are widely used in performance modelling of wireless and mobile communication systems. We address the problem of optimal wireless network selection during vertical handover, based on the received information, by embedding the decision problem in a Markov decision process (MDP) with genetic algorithm (GA), we use GA to find a set of optimal decisions that ensures the best trade-off between QoS based on their priority level. Then, we emerge improved genetic algorithm (IGA) with simulated annealing (SA) as leading methods for search and optimization problems in heterogeneous wireless networks. We formulate the vertical handoff decision problem as a MDP, with the objectives of maximizing the expected total reward and minimizing average number of handoffs. A reward function is constructed to assess the QoS during each connection, and the AHP method are applied in an iterative way, by which we can work out a stationary deterministic handoff decision policy. As it is, the characteristics of the current mobile devices recommend using fast and efficient algorithms to provide solutions near to real-time. These constraints have moved us to develop intelligent algorithm that avoid the slow and massive computations. This paper compares the formulation and results of five recent optimization algorithms: artificial bee colony, GA, differential evolution, particle swarm optimization and hybrid of (GA–SA). Simulation results indicated that choosing the SA rules would minimize the cost function, and also that, the IGA–SA algorithm could decrease the number of unnecessary handovers, and thereby prevent the ‘Ping-Pong’ effect.     

Optimal scheduling of handoffs in cellular networks

The phenomenon of hard handoffs (as applicable to FDMA- and TDMA-based networks) as well as soft handoffs (as applicable to DS/CDMA-based networks) is formulated as stochastic optimization problems. The signals received by a mobile user are treated as stochastic processes with associated rewards, which are functions of some measurable characteristics of the received signals, while the handoff is associated with a switching penalty. This formulation captures the trade-offs involved in handoffs in a flexible manner and captures many facets of popular cellular communication systems in use currently. Using dynamic programming, necessary and sufficient conditions for determining the optimal base station(s) the mobile should be associated with during each decision epoch are derived. For the cases where the above-mentioned necessary and sufficient conditions fail to determine an optimal decision, “limited lookahead” arguments are used for determining handoff decisions. The decisions are taken in a decentralized manner, which makes its implementation easier compared to centralized algorithms. Simulation results show that for the hard handoffs, performance gain by the proposed algorithm over the simpler threshold algorithms proposed in the literature is small; however, for the case of soft handoffs, the proposed algorithm offers considerable improvement over the algorithm proposed in the IS-95 standard     

A trajectory-aware handoff algorithm based on GPS information

In this paper, we propose a trajectory-aware handoff algorithm based on position, velocity, signaling delay, and receive signal strength (RSS) of mobile terminal (MT). In order to provide seamless service in modern heterogeneous networks, handoff of the MT should be initiated with correct timing. In our algorithm, velocity of MT is divided into two parts as radial velocity and tangential velocity. For more precise handoff initiation, tangential velocity of MT is neglected, and only radial velocity of MT is considered in handoff decision making. Moreover, before handoff decision, least square line method is applied to RSS of MT to avoid unnecessary back-and-forth handoffs (ping-pong handoffs) between different services. A simulation is provided to compare the proposed algorithm with other various handoff methods, and the results prove this algorithm to have outperformed others.     

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.     

Signal Strength Ratio Based Vertical Handoff Decision Algorithms in Integrated Heterogeneous Networks

In this paper, we propose the signal strength ratio (SSR) based vertical handoff (VHO) algorithms and the averaged received signal strength (ARSS) based VHO algorithms for integrated networks of wireless local area network (WLAN) and 3G network. The performance of our proposed VHO algorithms with hysteresis and dwell timer approaches has been investigated for two different topologies of integrated heterogeneous network. Two different network topologies consider two different types of radio cell arrangement. A mobile terminal (MT) is moving within the coverage area of WLAN and 3G with constant velocity. The received signal strength at the MT of interest is measured at different sampling instant. The received signal strength with other additional parameters is used to determine the condition of VHO. We evaluate the decision delay, and the number of vertical handoffs between WLAN and 3G network using the proposed VHO algorithms. We compare the performance of our proposed VHO algorithms with existing RSS based VHO algorithm. Our proposed SSR and ARSS based VHO algorithms provide less decision delay as compared to existing RSS based VHO algorithm. Further, ARSS with dwell timer based VHO algorithm provides better performance than other VHO algorithms.     

Route optimization in mobile ATM networks

This paper presents an algorithm for optimizing the route of a connection that becomes suboptimal due to operations such as handoffs and location-based reroutes, and applies this algorithm to the handoff management problem in mobile ATM (Asynchronous Transfer Mode) networks based on the PNNI (Private Network-to-Network Interface) standard. The route optimization algorithm uses hierarchical route information of the connection and summarized topology and loading information of the network to determine a “crossover node” such that adjusting the connection from that crossover node results in an optimally routed connection. Handoff management schemes that perform local rerouting of connections have been proposed in order to support fast handoffs. These methods result in suboptimally routed connections. In this paper, we demonstrate how this route optimization algorithm can be used to optimize the route of a connection after such a handoff is executed, as the second phase of a two-phase handoff scheme. This route optimization procedure can also be executed as part of the handoff procedure resulting in a one-phase handoff scheme. Applying this route optimization algorithm, we propose two one-phase schemes, the one-phase optimal scheme and the one-phase minimal scheme. A comparative performance analysis of one- and two-phase handoff schemes is presented. Measures of comparison are handoff latency and the amount of network resources used by a connection. Handoff latency in the one-phase optimal scheme is greater than that in the two-phase schemes, and handoff latency in the one-phase minimal scheme is smaller than that in the two-phase schemes. The one-phase methods show a significant increase in efficiency of the connection compared to the two-phase methods. This revised version was published online in June 2006 with corrections to the Cover Date.     

Stochastic control of path optimization for inter-switch handoffsin wireless ATM networks

One of the major design issues in wireless ATM networks is the support of inter-switch handoffs. An inter-switch handoff occurs when a mobile terminal moves to a new base station connecting to a different switch. Apart from resource allocation at the new base station, inter-switch handoff also requires connection rerouting. With the aim of minimizing the handoff delay while using the network resources efficiently, the two-phase handoff protocol uses path extension for each inter-switch handoff, followed by path optimization if necessary. The objective of this paper is to determine when and how often path optimization should be performed. The problem is formulated as a semi-Markov decision process. Link cost and signaling cost functions are introduced to capture the tradeoff between the network resources utilized by a connection and the signaling and processing load incurred on the network. The time between inter-switch handoffs follows a general distribution. A stationary optimal policy is obtained when the call termination time is exponentially distributed. Numerical results show significant improvement over four other heuristics     

Hierarchical optimization of microcellular call handoffs

In this paper, we present a novel hierarchical optimization handoff algorithm (HOHA) which has superior performance over the conventional one- and two-threshold handoff algorithms. Our proposed algorithm scans through layers of call handoff options and determines the best candidate based on a heuristic that synchronizes reciprocal handoffs in adjacent cells and maximizes the spare channel capacity in the destination cells. Computer simulation results show that under a unity normalized load condition, the HOHA achieves a gain of about 55% and 11% in the probability of handoff failure as compared to the one- and two-threshold handoff algorithms, respectively. In addition, the total effective load carried is also improved by about 13% and 4%, respectively     

Performance of handoff algorithm based on distance and RSSI measurements

The performance of a proposed handoff algorithm based on both the distance of a mobile station to neighboring base stations and the relative signal strength measurements is evaluated. The algorithm performs handoff when the measured distance from the serving base station exceeds that from the candidate base station by a given threshold and if the measured signal strength of the adjacent base station exceeds that of the serving base station by a given hysteresis level. The average handoff delay and average number of handoffs are used as criteria for performance. Numerical results are presented to demonstrate the feasibility of the distance-based handoff algorithm, including results for an additional criterion based on relative signal strength. The proposed algorithm is compared with an algorithm based on absolute and relative signal strength measurements and with a solely distance-based algorithm. It is found that the proposed handoff algorithm performs well in a log-normal fading environment when the distance estimate error is modeled by wide-sense stationary additive white Gaussian noise.     

一种基于模糊逻辑的预判决辅助垂直切换算法

针对全IP核心网络的不同无线接入技术,提出一种基于预判决模糊逻辑的垂直切换算法.引入预判决方法有效减少不必要的数据量和系统开销.采用前向差分预测能够较准确地预测出下一时刻的信号强度,提高了切换触发速度和切换判决精度.采用模糊逻辑的方法,以接收信号强度、预测信号强度及可用带宽作为网络参量设计模糊逻辑控制器,进而采用归一量化的方法改进模糊逻辑系统,得到网络的综合性能值进行垂直切换判决.仿真分析表明,本文提出的算法能够较准确做出切换判决,有效消除乒乓效应,提高系统性能.     

基于模糊逻辑的多终端协同的垂直切换决策算法

针对异构网下多终端协同的垂直切换决策问题,提出一种基于模糊逻辑和层次分析法的垂直切换决策算法,它分为2部分:切换时机的判断和虚拟终端的构建.首先采用模糊逻辑判断切换的时机,然后采用层次分析算法与简单加权求和结合的方法选择虚拟终端的最优构成方式.仿真结果表明,算法能够降低切换次数和不必要切换率,并且能够充分体现应用需求和用户偏好.