TCRA: a time‐based channel reservation scheme for handover requests in LEO satellite systems

In this paper, we propose a time‐based channel reservation algorithm (TCRA) suitable for handover and call admission control procedures in future mobile satellite systems. These systems are characterized by a high rate of handover attempts which can degrade significantly their performance. Therefore, we propose TCRA, a scheme which guarantees a null handover failure probability by using a channel reservation strategy in the cells to be crossed by the user. The performance of TCRA has been compared to the guaranteed handover (GH) scheme. The TCRA reservation method has the advantage of a better channel utilization by locking the resources only for their expected time of use. A mathematical model has been developed for both schemes, and its results have been validated through simulations. Copyright © 2003 John Wiley & Sons, Ltd.     

高空平台通信系统中重叠区辅助切换保证策略

为了确保高空平台通信系统提供通信业务的连续性,提出重叠区辅助切换保证策略(GHS-OA)。该策略在连接允许控制决策过程中,除利用高空平台和用户位置信息之外,还利用蜂窝之间的重叠区来协助判定并阻塞可能引起切换失败的新呼叫。建立多业务条件下基于时间的信道预留算法(TCRA)和GHS-OA的分析模型,推导上述2种策略在2类业务条件下新呼叫阻塞性能的数值解。通过仿真验证分析模型的有效性,与TCRA相比,在2类业务条件下GHS-OA可以极大地提升新呼叫阻塞性能,同时能够获得零切换掉话率,重叠区比例越大,利用重叠区的信道资源越多,GHS-OA的新呼叫阻塞性能越好。     

Handover queuing strategies with dynamic and fixed channelallocation techniques in low Earth orbit mobile satellite systems

This paper deals with the performance evaluation of various resource management strategies that are suitable for low Earth orbit mobile satellite systems (LEO-MSSs). A user mobility model has been proposed and its statistical parameters have been derived. Both fixed channel allocation (FCA) and dynamic channel allocation (DCA) techniques have been considered. Moreover, in order to reduce the handover failure probability, we have assumed that interbeam handover requests which do not immediately obtain service can be queued. In particular, two different queuing disciplines have been compared: (a) the first input first output (FIFO) scheme and (b) a new technique called last useful instant (LUI) which is based on the knowledge of the maximum time within which the handover procedure must be accomplished. Implementation aspects for the LUI technique in a LEO-MSS have been discussed also in comparison with the measurement-based priority scheme (MBPS), previously proposed in the literature on this subject. The efficiency of the LUI queuing scheme as regards the FIFO technique has been investigated by simulations for both DCA and FCA techniques. An analytical approach has been also presented in order to allow the performance evaluation of the FCA scheme with different handover queuing disciplines     

Hybrid channel adaptive handover scheme for non-GEO satellitediversity based systems

This article presents an improved hybrid handover scheme, which is proposed for the satellite component of UMTS. The scheme is channel adaptive and is generic as it can operate in soft and hard handover. The handover algorithm uses different power thresholds to add/drop a highest and second highest satellite and uses dual satellite diversity only under critical channel conditions. The analysis includes the simulation of a LEO constellation and the elevation-dependent satellite channel. Based on the channel time series the hybrid scheme shows improved performance when compared to existing handover schemes and is less bandwidth demanding than continuous diversity     

Handover Ranging Power Adjustment Using Uplink Channel Information in IEEE 802.16e/m

This letter proposes a handover ranging power adjustment scheme to improve handover performance. Incorrect ranging power can degrade handover performance due to the increased handover latency; therefore, the proposed scheme exploits the uplink channel information to adjust the uplink handover ranging power. Simulation results demonstrate that the proposed scheme reduces call outage probability by 33% compared to that of the conventional scheme. It also improves the number of users who satisfy the system requirements for handover interruption time.     

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     

Different queuing policies for handover requests in low Earth orbitmobile satellite systems

In this paper, a mobility model suitable for low Earth orbit mobile satellite systems (LEO-MSS's) has been presented, and its statistical parameters have been derived in order to evaluate the impact of the mobility on the performance of the fixed channel allocation (FCA) strategy. Moreover, we have foreseen that interbeam handover requests, which do not immediately find service, can be queued to reduce the handover failure rate. Two different queuing disciplines have been assumed: (1) the first-input-first-output (FIFO) scheme and (2) an idealized strategy that requires knowledge of the last useful instant (LUI) within which the handover procedure must be completed in order to rank the queued handover requests. An analytical approach has been developed to compare these queuing techniques, and its results have been validated through simulations     

Handover prioritizing scheme for reducing call failure probability in cellular wireless network

This paper proposes a scheme suitable for managing handover in wireless cellular network. The main objective of the proposed scheme is to reduce the probability of forced termination of ongoing call due to handover failure. The scheme employs a queuing discipline, and the priority of queued is based on the residual time of the mobile user in the overlap region between two adjacent cells, assuming that the user's location and speed can be determined, then we applied the ascending priority; it means that the users having a shorter residual time join the head of the queue (i.e., high priority) while those having longer residual time at the end of the queue (i.e., low priority). Fixed channel allocation strategy (FCA) is employed and simulation results obtained concern: call blocking probability (CBP), handover failure probability, and average waiting time in the queue. Also simulation results are compared to those obtained by: non‐prioritized scheme (FCA), and FCA queuing with FIFO discipline. Results show that our proposed method decreases significantly handover failure probability compared with other two schemes. Copyright © 2009 John Wiley & Sons, Ltd.     

A New Connection Admission Control for Spotbeam Handover in LEO Satellite Networks

Frequent spotbeam handovers in low earth orbit (LEO) satellite networks require a technique to decrease the handover blocking probabilities. A large variety of schemes have been proposed to achieve this goal in terrestrial mobile cellular networks. Most of them focus on the notion of prioritized channel allocation algorithms. However, these schemes cannot provide the connection-level quality of service (QoS) guarantees. Due to the scarcity of resources in LEO satellite networks, a connection admission control (CAC) technique becomes important to achieve this connection-level QoS for the spotbeam handovers. In this paper, a geographical connection admission control (GCAC) algorithm is introduced, which estimates the future handover blocking performance of a new call attempt based on the user location database, in order to decrease the handover blocking. Also, for its channel allocation scheme, an adaptive dynamic channel allocation (ADCA) scheme is introduced. By simulation, it is shown that the proposed GCAC with ADCA scheme guarantees the handover blocking probability to a predefined target level of QoS. Since GCAC algorithm utilizes the user location information, performance evaluation indicates that the quality of service (QoS) is also guaranteed in the non-uniform traffic pattern.     

QoS provisioning for large-scale multi-ap WLANs

Large-scale deployment of IEEE 802.11 wireless LANs (WLANs) with a high density of access points (APs) has become commonplace due mainly to its potential for numerous benefits, such as ubiquitous service coverage, seamless handover, and improved link quality. However, the increased AP density can induce significant channel contention among neighboring cells, thus causing severe performance degradation and throughput imbalance between cells. There have been a plethora of research efforts to improve the WLAN performance, but most of them focused only on single WLAN environments without accounting for inter-cell contention. The de facto QoS-provisioning mechanism for WLANs, i.e., the Enhanced Distributed Channel Access (EDCA), is no exception to this. The EDCA focuses only on inter-flow priority distinction and has not considered the effect of inter-cell contention which significantly restricts its efficiency. This paper presents an enhanced QoS provisioning framework that takes into account inter-cell level differentiation as well as inter-flow level priority, which may be viewed as extension of QoS provisioning from a single-WLAN domain to a multi-WLAN domain. We also propose an architecture for managing multi-AP systems in which a central controller regulates the wireless channel occupancy of APs by adaptively configuring the cell-level QoS parameters. Our extensive simulation results show that the proposed inter-AP cooperative QoS scheme overcomes the limit of legacy 802.11e and provides a high level of fairness in large-scale densely-deployed WLANs.     

Cooperative directional inter-cell handover scheme in high altitude platform communications systems

A novel Cooperative Directional inter-cell Handover Scheme (CDHS) for High Altitude Platform (HAP) communications systems is proposed, in which the handover target cell and the two cells adjacent to this handover target cell work cooperatively to exploit the traffic fluctuation to improve handover performance. Users in the overlap area of the overloaded handover target cell will be forced to handover directionally before their optimal handover boundary in order to free up resources for the handover calls which would otherwise be dropped due to the shortage of resources and queue time out. Simulation results show that the handover call dropping probability is greatly reduced (at least 60%) compared with the general queue handover scheme, with little performance reduction to the call blocking probability, and the Not in the Best Cell (NBC) average time is only increased moderately. Moreover, an optimal cell radius can be achieved for a specific platform speed by minimizing the unified system performance, which is the linear combination of the handover call dropping probability and the NBC average time.     

Handover and New Call Admission Policy Optimization for G3G Systems

Handover blocking of ongoing calls due to the mobility of users is a quantity that determines at most the Quality of Service (QoS) in microcellular and picocellular G3G systems environments. In this paper we propose a call admission policy, based on the fractional guard channel scheme, which additionally considers the blocking of new calls. Simulation results show that the proposed policy gives an improved system performance, compared to the most commonly used handover algorithms.     

A Guard Code Scheme for Handover Traffic Management in WCDMA Systems

A key performance indicator of mobile wireless networks is failure probability of handover calls. In this paper, we propose a Call Admission Control policy which prioritizes handover calls over new calls in WCDMA systems. The OVSF code occupancy of the system is modeled by a Markov chain and the differentiation between handover and new calls is performed at the code level by introducing a “guard code” scheme. The scheme belongs to the well-known family of guard channel schemes and reserves some code capacity to favor the continuation of handover calls over the new calls. As the management of the general case is intractable, we solve certain numerical instances of the problem and manage to calculate several performance metrics like new call blocking and handover failure probabilities and code utilization. We complete our study with simulation results in the case of higher OVSF code tree capacity.

Traffic performance analysis of handover in GMPCS systems

This paper presents an analysis of handover process and its effect on the traffic performance in global mobile personal communications by satellite (GMPCS) systems. With the nongeostationary satellite used for the system, the handover scheme needs to be applied to make calls completed without any interruption. An analytical model is developed for the analysis of the handover process. We derive the mean number of handovers and handover delay with various satellite antenna patterns and different settings of handover parameter. A suitable traffic model of the whole system is also derived after due considerations of the handover process. The system performance measures include new call blocking probability, call dropping probability, and mean number of handovers per call. A computer simulation is developed and used. We also analyze the system performance with a number of handover priority schemes applied. Based on the study results, the handover parameters are selected to maximize the traffic performance. It is shown that we can improve the overall traffic performance of GMPCS system by setting handover parameters properly and using the handover priority scheme     

Study and enhancement on buffer management for smooth handover in MIPv6

For improving Transfer Control Protocol (TCP) performance in mobile environment,smooth handover with buffer management has been proposed to realize seamless handovers. However in our simulation, even if smooth handover in Mobile IPv6 (MIPv6) is implemented, TCP can not always achieve better performance due to packets forwarding burst. Based on the study of buffer management for smooth handover, this paper proposes an enhanced buffer management scheme for smooth handover to improve TCP performance. In this scheme, a packet-pair probing technology is adopted to estimate the available bandwidth of the new path from Previous router (Prtr) to Mobile Node (MN), which will be used by Prtr to control the buffered packets forwarding. The simulation results demonstrate that smooth handover with this scheme can achieve better TCP performance than the original scheme.     

低轨卫星网小区移动性对切换的影响

低轨卫星网络点波束覆盖布局与用户相对运动方向决定了用户在小区间的切换关系,并直接影响用户切换性能。该文针对两种典型的小区运动模式建立了移动模型,分析了驻留时间、切换概率和平均切换次数等移动性指标,并结合具体的信道分配策略分析比较了小区移动性对用户切换性能的影响。结果表明在小区低重叠度的条件下小区运动模式Ⅱ的用户切换性能优于模式Ⅰ,更适用于低轨卫星网络,为低轨卫星点波束的布局提供了参考。     

An efficient handover decision method based on frame retransmission and data rate for multi-rate WLANs

To enhance the communication performance at handover between multi-rate WLANs, we propose a new handover decision method that can be applied to our previously reported handover management scheme, which handled a handover by utilizing two WLAN interfaces (IFs) through cross-layer collaboration between layer 2 and layer 4. It should be noted that we here propose a new handover decision scheme for traversing between multi-rate WLANs, while our previous decision scheme works only in fixed-rate WLANs. In this paper, to treat a handover between multi-rate WLANs, we employ two kinds of information: (1) the most frequently used data rate (MFDR) for assessing the stable communication performance of a multi-rate WLAN, and (2) the frame retransmission ratio (FRR) for assessing its exact communication performance. The MFDR enables us to estimate the area where we should start handover. If the MFDRs of two interfaces are same in the area, the FRR allows us to compare the wireless condition on the two interfaces precisely to give an optimal handover point. Through simulation experiments, we show that our proposed scheme certainly estimates an appropriate handover point as a result of multi-path transmission (s), thereby providing handover successfully. That is, the proposed method can determine handover at an optimal point depending on the various distances between access points, the mobile node (MN) velocity, and the MN moving pattern. Moreover, our proposed scheme prevents the redundant network load caused by multi-path transmission as much as possible, thereby providing the ideal TCP communication performance.     

Seamless spectrum handover considering differential path-loss in cognitive radio systems

In cognitive radio systems, spectrum handover occurs when a secondary user changes frequency due to the appearance of a primary user. Spectrum handover may result in degraded system performance because of the different propagation loss of the different frequency. In this case, data transmission is disrupted and it is more difficult to provide seamless service. Here, we propose a seamless handover scheme based on the prediction of cell coverage. The proposed handover scheme can avoid service disruption and can reduce redundant handovers. The efficiency of the scheme is validated by simulation results.     

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.     

Distributed traffic adaptive channel allocation

Dynamic channel allocation (DCA) schemes adapt to the time variant demand for channels in cellular mobile telephony systems. In this paper we propose a DCA scheme that smoothly changes the channel allocation by solving the following problem. Given a cell structure, a collection of channels, the frequency reuse distance, an allocation of channels to cells, and the number of active connections per cell, accommodate a new call or a new handover by minimally reconfiguring the established allocation of channels to cells. First, this problem is formulated as 0–1 quadratic programming problem. Next, we present a distributed, heuristic solution to the problem, which is based on the observed behaviour of the optimal algorithm. Finally, we present some simulation results on the performance and the feasibility of the distributed algorithm. © 1997 John Wiley & Sons, Ltd.