Satellite selection scheme for reducing handover attempts in LEO satellite communication systems

Low earth orbit (LEO) satellite communication systems perform frequent intersatellite handovers for both fixed and mobile users. This paper proposes a satellite selection scheme for new/handover call requests when two or more satellites can be seen simultaneously. Each satellite in this scheme has a non-uniform transmitter antenna gain according to its relative position inside the coverage area. The antenna gain is proportional to the residual distance in the satellite's direction of movement and it compensates for the difference in path losses between satellite links. The residual distance distribution of the selected satellite and the mean number of intersatellite handovers during a call connection are calculated and compared with the results based on conventional methods. The proposed scheme can reduce the intersatellite handover call attempt rate without increasing system load and terminal complexity. Furthermore, this scheme can be extended to reduce both intersatellite and interbeam handover call attempt rates in a multiple spot beam environment. Especially, the average number of intersatellite and interbeam handovers during a call can be significantly reduced by using a hybrid algorithm with the proposed non-uniform power transmission scheme. © 1998 John Wiley & Sons, Ltd.     

Interbeam handover in LEO satellite systems

This paper evaluates the performance of interbeam handover in LEO satellite systems. The evaluation is based on an analytical model that is specific for interbeam handover. There are three performance parameters concerned: handover probability, forced termination probability and time availability. From the results it can be seen that the interbeam handover probability depends strongly on the roll-off characteristics of the beam pattern. Forced termination and time availability depend on the quality of the received signal and the selected criterion. A comparison between the use of signal level and position as handover criteria is presented. © 1998 John Wiley & Sons, Ltd.     

Satellite handover techniques for LEO networks

Low earth orbit satellite constellations could play an important role in future mobile communication networks due to their characteristics, such as global coverage and low propagation delays. However, because of the non‐stationarity of the satellites, a call may be subjected to handovers, which can be cell or satellite handovers. Quite many techniques have been proposed in the literature dealing with the cell handover issue. In this paper, a satellite handover procedure is proposed, that investigates and exploits the partial satellite diversity (namely, the existing common coverage area between contiguous satellites) in order to provide an efficient handover strategy, based always on a tradeoff of blocking and forced termination probabilities for a fair treatment of new and handover calls. Three different criteria were examined for the selection of a satellite. Each one of them could be applied either to new or handover calls, therefore we investigated nine different service schemes. A simulation tool was implemented in order to compare the different service schemes and simulation results are presented at the end of the paper. Copyright © 2004 John Wiley & Sons, Ltd.     

Evaluation of handover mechanisms in shadowed low earth orbit land mobile satellite systems

This paper presents a quantitative analysis by computer simulation of the active set update (ASU) handover algorithm for a shadowed low earth orbit (LEO) land mobile satellite (LMS) environment. As a precursor to the handover analysis, the mutual visibility statistics for a 66 satellite polar and 48 satellite rosette-type constellation are presented. These results show the statistical nature of the levels of satellite diversity and mobile-to-satellite elevation angles (to the highest satellite) within each network and also indicate the influence of the channel characteristics on the handover strategy. A two-state Markov modulated channel model is assumed in the handover analysis, and this enables the assessment of increased levels of power and time hysteresis on the quality of service and network signalling load in a shadowed land mobile satellite environment. In particular, attention is given to the different modes of ASU operation for hard handover, switch diversity and soft handover.     

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.     

低轨卫星星座网的切换研究

提出了一种适用于装备有星际链路的低轨卫星星座网的切换策略——最小跳数切换策略。该策略以端到端连接的跳数为基础,充分利用了低轨星座网的特点,将切换过程与路由有机地结合为一体,有效解决了不同卫星之间的切换问题。仿真结果表明,同已有的切换策略相比,该策略能够提供一定程度的QoS保障,获得较好的系统性能,如较低的传播延时和较小的切换频率,具有很好的实用价值。     

QoS handover management for multimedia LEO satellite networks

Low earth orbit (LEO) satellite systems gained considerable interest towards the end of the previous decade by virtue of some of the appealing features that are endowed with, such as low propagation delay and the ability to communicate with handheld terminals. However, after the limited commercial success of the first networks of this kind, future satellite networks are now conceived as complementary rather than competitive to terrestrial networks. In this paper, we focus on one of the most influential factors in system performance, that is, the handover of a call. First, we provide a succinct review of the handover strategies that have been proposed in the literature. Then we propose two different satellite handover techniques for broadband LEO satellite systems that capitalize upon the satellite diversity that a system may provide. The proposed schemes cater for multimedia traffic and are based on the queuing of handover requests. Moreover, a deallocation scheme is also proposed according to which capacity reservation requests are countermanded when the capacity that they strive to reserve is unlikely to be used. Simulation studies further document and confirm the positive characteristics of the proposed handover schemes.     

低轨卫星通信系统中快速终端的切换技术

提出了低轨卫星通信系统（LEO）中两种支持快速终端的切换算法,详细介绍了系统模型的结构,以全球星系统和铱系统为例,通过仿真,分别对“最短路径优先”和“最长覆盖时间”2种算法下快速终端的切换时延情况进行统计和比较,得到了有一定参考价值的结果.     

A seamless soft handover management scheme in NGEO satellite networks

Satellite networks can provide extensive geographic coverage to diverse user population, but handovers of active communications should be controlled considering the satellites' high speed. Mobile IP and many other handover algorithms have been proposed for terrestrial wireless networks. However, the satellite network exhibits several unique features compared with the terrestrial wireless networks, such as the long delay. This paper proposes a seamless soft handover scheme (S2H) for satellite networks. S2H designs an identity locator split addressing to solve the multihoming problem and applies the network coding scheme during handover. This manner is able to reduce the interference opportunity, save transmissions, and achieve low handover delay, short queue length, and high throughput. Relying on the simulation results, S2H is able to achieve better performance compared with the proposed handover algorithms in the satellite environment when there are multiple traffic flows. Copyright © 2013 John Wiley & Sons, Ltd.     

Real-time prediction and updating method for LEO satellite handover based on time evolving graph

In order to solve the problem of low handover efficiency raised by the interwined impacts of time-varying topology of LEO satellites and terminal movement,a real-time satellite handover prediction framework based on time evolving graph and a shortest path dynamic updating method was proposed.This framework dynamically updated the handover prediction results from the temporal and spatial dimensions according to the terminal movement.The simulation results show that the framework can effectively improve the accuracy of the forecast and avoid the unnecessary handover.     

卫星移动通信系统星间链路空间参数分析

通过对现有的卫星移动通信系统星间链路的分析发现,伴随着卫星座高度的增加,卫星移动通信系统星间链路指向的动态变化范围不断扩大,星间链路的相对距离也不断增大。由此引出对最优化的极轨道卫星移动通信系统星暗链路主要参数的模拟与分析,从中得到了一些规律性的结论,为星间链路的设计与建立提供了理论基础;同昱,卫星移动通信系统星间链路指向性的周期性变化,也为星间链路建立过程中最优化搜索算法的建立提供了探索的方向和     

阴影衰落信道下多波束卫星移动通信系统的动态信道分配策略

针对已有动态信道分配策略在阴影衰落环境中性能损失较大的问题,对卫星移动通信系统的阴影衰落信道模型进行分析,提出了一种动态信道分配策略。该策略结合用户的运动状态,通过在用户的预测运动轨迹上选取抽样点,将这些抽样点的平均干扰作为信道分配的指标,并采用链路质量约束避免了流量较大时的性能恶化。仿真结果表明提出的动态信道策略可在不增大阻塞率的前提下,在中低流量时将用户平均信噪比提高约0.5 dB。该策略可以应用于阴影衰落信道下的地面移动卫星通信系统,以提高用户的平均链路性能。     

切换保留信道与新呼叫排队相结合的LEO星座通信系统信道分配方案研究

提出一种适用于LEO(低轨道)星座通信系统的信道分配方案。该方案为切换呼叫提供了保留信道,降低了切换呼叫的阻塞概率。同时,采取新呼叫排队策略抑制保留信道引起的新呼叫阻塞概率的恶化,如果正在进行的呼叫离开,队列中的新呼叫可以按照次序获得分配信道。结果表明,该方案可以显著降低切换呼叫阻塞概率,并使新呼叫阻塞概率得到改善。     

低轨卫星通信网络中改进的综合加权接入算法

针对用户终端有效与可靠接入低轨卫星通信网络的需求,基于现有的接入算法,综合考虑了卫星对用户终端的覆盖时间、卫星空闲信道数以及卫星接收信号的信噪比因素,提出了一种改进的综合加权接入算法,对该算法的目标函数和参数进行了设计与计算.仿真结果表明,采用改进的综合加权接入算法,新呼叫阻塞率和强制中断率等性能都得到了明显的改善.     

面向Massive MIMO低轨卫星通信系统的鲁棒高效波束成形设计

以低轨卫星通信系统能量效率最大化为优化目标,重点研究鲁棒波束成形设计。假设卫星侧配备大规模多输入多输出技术,综合考虑传播延迟和多普勒频移造成的信道状态信息误差影响,在发射功率受限和用户服务质量约束下,提出了一种鲁棒高效的波束成形方案。考虑到遍历用户速率和遍历信干噪比都没有闭合表达式,采用具备闭合表达式的近似值代替,同时,采用半正定规划算法将非凸二次约束二次规划形式问题进行等价转换,并提出了一种联合二次变换分式规划和凹凸过程的内外两层嵌套迭代算法。最后,采用一种惩罚函数算法解决半正定规划算法中存在的秩1约束问题。仿真结果表明,所提算法性能优于传统算法,且鲁棒性较高。     

Topological design,routing and capacity dimensioning for ISL networks in broadband LEO satellite systems

This paper considers the network design of intersatellite link (ISL) networks in broadband LEO satellite systems, where the major challenge is the topology dynamics. First, a general method to design convenient ISL topologies for connection‐oriented operation is presented, and a reference topology for numerical studies is derived. A permanent virtual topology is then defined on top of the orbiting physical one, thus forming a framework for discrete‐time dynamic traffic routing. On this basis, heuristic and optimization approaches for the combined routing and dimensioning task, operating on discrete time steps, are presented and their performance is numerically compared. It is shown that minimizing the worst‐case link capacity is an appropriate target function, which can be formulated as linear optimization problem with linear constraints. Using linear programming (LP) techniques, the dimensioning results are clearly better than with simple heuristic approaches. Copyright © 2001 John Wiley & Sons, Ltd.     

Variable-target admission control for nonstationary handover traffic in LEO satellite networks

Handover management is of great importance in a low Earth orbit (LEO) satellite system. However, the blocking performance of handover traffic in an LEO satellite system is not easily formulated with an acceptable level of accuracy because of the nonstationary nature of handover traffic, which causes difficulty in quantitatively making optimal decisions for channel assignment to meet a given quality-of-service requirement. Even though many challenging schemes have been proposed to dynamically determine the number of guard channels, most either use heuristic methods or are based on simplifying assumptions, causing invalid decision results. As an alternative, we suggest a quantified method to minimize the fraction of the number of blocked calls out of the number of total calls under nonstationary handover traffic. We develop new mathematical formulations of those fraction and optimization models with efficient exact solution algorithms. Performance analysis shows that the proposed method improves the blocking probabilities of handovers and new calls; this improvement results from a highly adaptive and reactive characteristic to the fluctuating handover traffic condition.     

Adaptive distribution‐based handover scheme for cellular communication networks

In this paper, we propose a new Generalized Distribution‐Based Handover (DBHO) to deal with the inefficient utilization of spectral resources due to the non‐uniform cell loads. The DBHO scheme is different from the existing adaptive schemes since it uses a new criterion to initiate handover when moving from/to a congested cell. Two risk factors are used to dynamically change handover boundaries according to the distribution of traffic loads. This controls the handover initiation process such that a user in a congested cell that is moving to a free cell is allowed to initiate a handover to a new cell earlier, as long as the signal received from the target cell is higher than a certain threshold. While delaying the handover initiation process for a user moving in the opposite direction, as long as the signal received from the serving cell is not lower than a certain threshold. Our results show a substantial reduction in the handover and call dropouts rates. Our scheme is complementary to the existing adaptive schemes proposed in the literature. The proposed scheme also gives cellular system designers a new tool to optimize the overall network performance by initiating handovers based on the traffic intensities. Frequent handovers increase the load on switching networks, which consequently degrades the Quality of Service (QoS). Existing handover schemes usually use parameters such as the received signal strength for initiating a handover with some additional measurements to reduce unnecessary handovers and call dropouts. These schemes perform well when cell loads are somewhat evenly distributed, but fail to account for nonuniform traffic, as is often the case in microcells. Hence, it is desirable to design efficient handover schemes to avoid unnecessary handovers, reduce call dropouts and yet dynamically adapt to the variation of traffic among cells. In this paper, we present a new adaptive handover scheme that dynamically changes the handover boundaries to balance cell loads and to effectively reduce the average number of handovers. Copyright © 2004 John Wiley & Sons, Ltd.     

低轨卫星通信系统下行链路设计

针对低轨卫星通信系统特点,结合当前世界上应用最广泛的数字卫星电视广播( Digital Video Broadcasting,DVB)协议,对卫星通信下行链路进行设计与验证.介绍了协议整体架构的设计和链路模型的搭建,重点描述了同步模块中的预频偏估计过程的设计.频偏预估计实现了在进行精细的频偏估计和补偿之前对大频偏的估计...     

低轨卫星网络的航点分段路由及业务性能分析

提出一种基于卫星航点的分段路由(waypoint-segment routing,WSR)算法,WSR算法以可预测的卫星网络拓扑运动周期为基础,根据卫星节点链路状态确定卫星航点的位置;利用分段路由灵活规划分组传输路径的机制,提前响应网络拓扑变化,计算得到一条不受网络拓扑快照切换影响的传输路径。基于NS-3仿真平台进行仿真实验,设置源节点与目标节点在反向缝同侧与不同侧两种场景,选取优化链路状态路由(optimized link state routing,OLSR)算法和最短路径算法与WSR进行时延抖动与分组丢失率的对比分析。实验证明WSR与OLSR相比,两种场景下最大时延抖动分别降低46 ms与126 ms,分组丢失率分别降低30%和21%,并且能够解决拓扑快照切换导致分组传输路径中断的问题。