降雪干扰下基于演化博弈的低轨量子卫星多用户切换策略
Low-orbit quantum satellites are part of building a global secure communication network. However, as single quantum satellites move fast relative to ground terminals with limited service time and the satellite-ground quantum link is susceptible to atmospheric conditions (e.g. rain, snow, haze, etc.), ground end-users need to switch to other satellites available for service in time to meet the sustainable communication demands. In the common coverage area, if the user only chooses the currently proposed single-attribute decision strategy, such as the minimum communication elevation angle, the optimal entanglement degree or the minimum link attenuation, the optimal single attribute can be achieved with losing the advantage of other attributes. This will easily result in load imbalance and uneven resource allocation of quantum satellites, and even communications may be interrupted in serious cases. To this end, we consider the attenuation interference of snowfall on the satellite-ground link and the process of end-user-associated switching of quantum satellites, and build a multi-attribute evolutionary game switching model to achieve Nash equilibrium in the allocation of quantum satellite resources. Meanwhile, satellite resource allocation can be maximized to enhance the switching success rate of users in the low earth orbit (LEO) quantum satellite communication network.
Evolutionary game theory combines biological evolutionary properties with game theory to make the system stable through constant comparison and imitation in multiple choices. In actual quantum satellite switching, the users switching satellites at the same time do not know each other's state information in a completely rational way, which satisfies the non-rational conditions of evolutionary games. We analyze the quantum channel attenuation characteristics under snowfall and atmospheric turbulence according to the Gamma spectral distribution function of snow and obtain the variation of channel attenuation with transmission distance. The bandwidth that can be allocated to users by the quantum satellite corresponds to the current satellite load state, which means that more users result in fewer quantum satellite bandwidth resources available to each user. The longer remaining service time of the quantum satellite indicates that the selection of this strategy prolongs the service cycle of the user and reduces the user switching number. The communication elevation angle reflects the channel condition of the satellite-ground link, and the larger communication elevation angle leads to shorter communication distance, lower link attenuation, and better channel conditions. As the elevation angle is difficult to measure in real time with the terrain environment obstruction, the measured elevation angle cannot reflect the channel conditions, and thus the communication elevation angle is converted into the link attenuation characteristics which can directly represent the channel conditions. Therefore, we define a utility function based on the user's bandwidth, remaining satellite service time, and link attenuation, and define an overhead function based on the inter-satellite transmission delay and channel entanglement to obtain the user's payoff function. Finally, we derive the dynamic replication equation of the satellite to build an evolutionary game switching model.
Firstly, the effect of snowfall on the satellite-ground link attenuation is analyzed. Under certain snowfall intensity, the total attenuation suffered by the link increases as the light quantum propagation distance rises. In the case of a certain propagation distance of the light quantum signal, as the snowfall intensity increases, the total communication link attenuation grows due to the scattering and absorption effect between the light quantum signal and snow particles in the atmosphere. It results in the subsequent increase in the total communication link attenuation, and the atmospheric snowfall environment will exert a significant influence on quantum satellite communications (Fig. 1). The number of users to be switched simultaneously is 1000 and the number of available strategies is 2. In the six switching experiments, the number of users selected for Quantum LEO1 varies with the iteration number (Fig. 3), and the number of users plateaus with the increasing iteration number, which demonstrates that the proposed quantum satellite multi-user switching strategy has sound convergence stability. As the game proceeds, the gains gradually converge to the average gain of all users and reach equilibrium by the sixth iteration. Then the users revenue do not increase and stabilize to ensure the multi-user fairness during the quantum satellite switching (Fig. 4). The single-attribute judgment switching method converges faster than the multi-attribute switching decision, and the number of users connected to Quantum LEO1 increases when equilibrium is finally reached, but this is at the expense of the other attributes (Fig. 5). Experimental results show that the switching strategy based on the evolutionary game improves the switching success rate by 1.2% over the switching strategy based on the lowest link attenuation (Fig. 6) under the switching user number of 660. When the minimum entanglement threshold is set to 0.8, the switching success rate of the evolutionary game-based switching strategy improves by 1.5% over the switching strategy based on the optimal entanglement (Fig. 7) under the switching user number of 700.
An evolutionary game-based quantum satellite switching strategy is proposed for the multi-user switching scenario of quantum satellites under a snowfall environment. Various attributes affecting the quantum satellite switching decision are analyzed and combined with the attenuation characteristics between quantum star-ground links to obtain the effect function, the overhead function, and then the average gain function of users. An evolutionary game model is built by considering the transmission among users and between users and quantum satellites, and the performance of the switching strategy using this model is simulated. The results show that the proposed strategy not only has sound stability with the influence among multiple attributes considered but also can make the quantum satellite load relatively balanced. Finally, compared with the single-attribute quantum satellite switching strategy based on minimum link attenuation and optimal entanglement, the proposed strategy can also improve the success rate of user switching more effectively, providing references for future multi-user dynamic switching design of low-orbit quantum satellite networks under snowfall interference environment.
1 引言
相比于传统通信,量子通信结合量子力学与密码学等理论,具有高保密、高抗干扰性,有望彻底改变信息的传输和加密方式,并不断朝着构建可扩展量子网络的方向发展[1],如量子密钥分发(QKD),在经济、**等重要领域已经由纯粹的理论转变为实用技术[2-4],逐步建立起具有数字签名加密等技术的量子安全网络。但由于设备缺陷及探测等损耗,增加QKD的比特率和范围是一项艰巨但重要的挑战。文献[5-6]使用相位相干光信号和辅助测量站,使安全密钥速率突破了无中继密钥容量。文献[7]通过对两个干涉检测进行后匹配来实现异步双光子Bell状态测量,在没有相位跟踪和相位锁定的条件下超过了密钥容量,为可扩展的量子通信网提供了实验基础。文献[8]证明了使用激光脉冲对抗状态准备缺陷和脉冲相关性等潜在源缺陷的高效四相测量设备独立QKD的安全性,显著提高了安全密钥速率、延长了传输距离。
低轨道量子卫星网是构建高保密、高容量、低延迟的全球化量子通信链路必不可少的一部分。自“墨子号”成功发射以来,已经有诸多学者验证了低轨量子卫星与地面终端之间传输的可行性[9-11]。2017年,Takenaka等[12]在地面站与一个微型卫星之间通过四光子计数器成功区分了每脉冲约0.146个光子的量子态,验证了星地量子密钥分发的适用性。2020年,潘建伟团队[13]实现了从卫星到1200 km外的地面站之间的量子密钥分发。2022年,Trinh等[14]结合深度学习的长短期记忆递归神经网络报道了世界上第一个近地轨道微卫星与地面站之间的大气通道统计模型实验,为低轨量子卫星网络的构建提供了参考。
低轨道量子卫星的高度范围为500~1000 km,相对于地面终端移动速度快,可供服务时间有限,且用户通常需要在3~4 min之内切换下一颗卫星,因此多用户与卫星之间的切换判决策略是目前构建量子卫星通信星座亟待解决的问题。文献[15]提出了以信道链路衰减最低为阈值的切换策略,提高了灰霾环境背景下用户的切换成功率。文献[16]提出了一种基于纠缠度阈值的卫星切换策略,提升了链路通信质量和服务的连续性。文献[17]分析了通信仰角对量子卫星通信的信道误码率、生存能力等各项性能参数的影响,验证了以可变仰角作为切换判决准则的可行性。然而上述方法均仅以单属性(如纠缠度、最小链路衰减、通信仰角)作为切换判决准则,只能达到单属性性能上的最优,牺牲了其他属性的优势,同时也未考虑用户之间的相互影响,这会造成量子卫星的负载失衡和资源分配不均,严重时甚至会导致卫星切换失败或者通信中断。
演化博弈论由Smith和Price[18]于1973年首次提出。演化博弈理论将生物进化特性与博弈论结合,可以检验不同策略在环境中的生存和复制能力,通过不断地比较和模仿,在多次选择的过程中使系统最终达到稳定。在实际的量子卫星切换中,同时进行卫星切换的用户并不能完全理性地知道彼此的状态信息,这满足了演化博弈的非有理性条件。为使用户得到更加公平的卫星资源,均衡量子卫星的负载,本文考虑了降雪对量子星地链路的影响,基于演化博弈论,提出了一种多属性判决的卫星切换方法。
2 降雪背景下星地量子信号传输特性分析
由于量子保密通信中的量子信号一般为弱光信号,其对自由空间中的尘埃、雨雪等大气因素较为敏感,从而产生吸收、散射等效应,严重影响光量子信号的传输特性。考虑降雪环境与大气湍流的影响,光量子信号在传输过程中的信道总衰减[19]为
式中:
降雪会对星地量子链路产生突发干扰,为方便研究雪粒子特性,用降雪强度
式中:
式中:
式中:
式中:
图 1. 不同降雪强度下链路总衰减与光量子传输距离的关系
Fig. 1. Relationship between the total attenuation of links and the transmission distance of optical quanta under different snowfall intensities
从
对于低轨道量子卫星通信网而言,当终端用户通过量子卫星进行通信时,为避免降雪突发干扰使通信链路产生中断,用户端需要根据降雪信道特征,以及卫星仰角及卫星剩余服务时间等参数,及时搜索并切换至其他可供服务的卫星继续保持信息交换,且需满足卫星的负载均衡。本文提出一种基于演化博弈的量子卫星多用户切换策略,通过博弈策略,各用户终端在达到通信性能的前提下,同时满足量子卫星资源分配的公平性,降低了切换次数,提升了卫星的通信效率及降雪环境下通信的可靠性。
3 基于演化博弈的低轨道量子卫星多用户切换算法
3.1 多用户演化博弈切换模型
在低轨道量子卫星组成的通信网络中,某一区域的终端用户可能被多个量子卫星波束同时覆盖,网络覆盖模型如
为解决降雪等大气环境衰减因素或当前星负载过大等原因导致的当前卫星服务质量差的问题,终端用户须及时切换接入其余可供服务的量子卫星,继续保持通信。而在实际的卫星切换选择过程中,终端用户相互之间无法知道彼此的理性信息,如果仅选择单一指标(如通信仰角、信道纠缠度、卫星剩余服务时间等)进行一次选择的切换判决方法,易导致卫星资源分配不公,陷入局部最优,造成当前选择卫星负载过重或切换失败。针对上述问题,本文考虑降雪对星地链路的衰减干扰影响及终端用户关联切换量子卫星的过程,建立演化博弈模型,通过博弈及动态选择机制,实现量子卫星资源分配的纳什均衡。
假设某一区域内需要进行切换的用户数为
1)博弈方。定义量子卫星分配资源的
2)策略。博弈方所能关联的量子卫星,每一种策略对应一颗候选量子卫星
3)收益。在量子卫星通信切换网络中,用户选择卫星
4)群体比例。定义为选择某种策略
3.2 收益函数分析
效用函数用来描述用户对选择策略的满意程度。在低轨道量子卫星通信网络系统中,卫星服务质量是影响用户选择策略的主要因素。本文综合考虑终端用户与量子卫星通信时的带宽、卫星剩余服务时间及卫星仰角三个属性建立用户的效应函数。量子卫星可分配给用户的带宽对应了当前卫星的负载状态,接入卫星的用户数量
对部分参数进行归一化后,定义用户选择策略
式中:
卫星
式中:
在切换过程中,当多用户同时连接至同一个卫星时,会超出卫星负载,导致链路中断。为了平衡卫星的资源,定义开销函数为用户数
式中:
式中:
式中:
式中:
由上述分析,最终得到用户连接卫星
用户需要进行切换时,可根据
3.3 基于演化博弈的低轨量子卫星切换算法
目前的量子卫星切换策略通常只考虑卫星仰角或信道情况(如纠缠度或链路衰减等),并未考虑用户之间的影响过程。在本文的演化博弈模型中,切换时博弈方观察自己的收益与平均收益的值,当
式中:
根据上述分析,得到降雪背景下基于演化博弈的量子卫星多用户切换策略过程如下。
步骤1:有
步骤2:各用户根据
步骤3:量子卫星控制端根据
步骤4:各博弈方不断对自身收益
步骤5:博弈方在关联至新的卫星后,控制器判断卫星目前的负载情况,调整接入用户的数量,从而改变用户的开销,保证卫星的负载均衡,重新返回步骤2执行,直至各博弈方达到演化均衡,即最终满足
4 仿真
假设需要同时进行切换的多用户数量为1000,可供选择的策略数
表 1. 仿真参数
Table 1. Simulation parameter
|
图 3. 接入Quantum LEO1的用户数随迭代次数的变化
Fig. 3. Variation of the number of users connected to Quantum LEO1 with the number of iterations
从
图 4. 用户收益随迭代次数的变化关系
Fig. 4. Relationship between user revenue and the number of iterations
从
权重因子反映了切换时的准则,如在一次通信过程中,需要以最小切换次数为准则,则要求卫星提供最长的服务时间,即可以设置权重因子为
图 5. 不同准则下接入Quantum LEO1的用户数
Fig. 5. Number of users with access to Quantum LEO1 under different standards
设置轨道高度为300 km,系统呼损率为5%,切换时可供选择的卫星数为2,在多用户需要进行选择切换的区域public coverge area内,
从两种不同切换策略的成功率随该区域内同时切换至Quantum LEO1的用户数的变化情况中可以看出,随着关联该卫星的用户数增多,量子卫星的负载增大,通信可靠性下降,两种策略的切换成功率均呈减小状态。但本文所提策略由于用户之间的博弈,考虑了用户之间的切换均衡,同时以降雪为具体的传输环境,考虑了链路衰减、最长服务时间等多属性因素对切换判决的影响,通过判断收益值使量子卫星的资源均衡分配,因此可以有效提升切换过程的成功率。如当切换用户数为660时,基于演化博弈的切换策略相比于基于链路衰减最低的量子卫星切换策略的切换成功率上升了1.2%。
文献[16]提出了一种基于最优纠缠度的切换方法,通过设置纠缠度阈值,低于阈值的用户选择候选卫星中纠缠度最大的卫星进行切换,从而保证链路的正常通信。设置最小纠缠度阈值
图 7. 本文算法与最优纠缠度策略的切换成功率对比
Fig. 7. Comparison of the switching success rate between the proposed algorithm and the optimal entanglement degree strategy
从
5 结论
本文针对降雪环境下量子卫星多用户切换场景,提出了一种基于演化博弈的量子卫星切换策略。然后,分析了影响量子卫星切换判决的多种属性,并结合量子星地链路间的衰减特性,得到了用户的效应函数与开销函数,进而得到用户的平均收益函数。综合考虑了用户与用户之间、用户与量子卫星之间的传输情况,构建了演化博弈模型,并对使用该模型的切换策略性能进行仿真。结果表明,本文提出的策略考虑了多属性之间的影响,不仅具有较好的稳定性,还能够使量子卫星的负载相对均衡。最后,通过与其他单属性量子卫星切换方法相比,本文所提策略也能更有效地提升用户切换的成功率,为未来量子卫星通信的可靠性及有效性设计提供一定的参考。
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Article Outline
薛长春, 聂敏, 杨光, 张美玲, 孙爱晶, 裴昌幸. 降雪干扰下基于演化博弈的低轨量子卫星多用户切换策略[J]. 光学学报, 2023, 43(24): 2427001. Changchun Xue, Min Nie, Guang Yang, Meiling Zhang, Aijing Sun, Changxing Pei. Multi-User Switching Strategy of Evolutionary Game-Based Low-Orbit Quantum Satellite Under Snowfall Disturbance[J]. Acta Optica Sinica, 2023, 43(24): 2427001.