认知无线电中认知用户的重新建模与性能分析

针对认知网络频谱接入问题,讨论了以往建模和理论分析中存在的不足.为了真实地模拟授权用户和认知用户的行为,对频谱接入策略重新进行建模,并利用矩阵几何法和离散事件系统仿真的事件调度法重新评估和比较了不同接入策略下的认知系统性能,针对不同业务类型选取最佳接入策略.理论分析和仿真实验结果一致,验证了所提模型的可靠性.频谱切换和信道预留接入策略增大了阻塞率并有效地降低了掉话率,后者通过调整预留信道数目使认知系统吞吐量最大.     

感知不确定下卫星认知无线网络多频谱接入优化策略

针对卫星认知无线网络频谱感知不确定性较大导致传统频谱接入机制效率降低的问题,该文提出一种基于动态多频谱感知的信道接入优化策略。认知LEO卫星根据频谱检测概率与授权用户干扰门限之间的关系,实时调整不同频谱感知结果下的信道接入概率。在此基础上以系统吞吐量最大化为目标,设计了一种基于频谱检测概率和虚警概率联合优化的判决门限选取策略,并推导了最佳感知频谱数量。仿真结果表明,认知用户能够在不大于授权用户最大干扰门限的前提下,根据授权信道空闲状态动态选择最佳频谱感知策略,且在检测信号信噪比较低时以更加积极的方式接入授权频谱,降低了频谱感知不确定性对信道接入效率的影响,提高了认知系统吞吐量。     

一种结合信道性能的频谱分配算法

在认知无线电网络中,由于授权信道的时变性,各可用信道的性能相差较大。为避免过多使用忙闲交替频繁的信道,首先用ON—OFF模型建模授权信道的使用情况,并推导了认知用户切换概率,提出了一种用信道状态变化率衡量信道性能的方法。在此基础上提出了一种结合信道性能的频谱分配算法,在网络吞吐量相同或相近情况下优先分配性能较好的信道。仿真分析表明,这种结合信道性能的频谱分配算法在降低切换概率的同时提高了实际总效益。     

认知 mesh 网络服务区分的动态频谱接入策略

动态频谱接入策略是实现认知无线电网络高效利用频谱的关键。与传统认知无线电网络不同,认知mesh网络中不同QoS需求的多类型业务共同接入,为适应这一特点,提出服务区分的动态频谱接入策略。策略依据业务的QoS需求确立优先级,针对不同优先级业务采取不同的信道接入方案,实时业务依据最优传输延迟期望选择接入信道集合,在减小传输延迟的同时降低数据传输过程授权用户出现的概率,普通业务选择最优理想传输成功概率的信道,降低信道切换概率。理论与实验结果表明,与传统的认知网络频谱接入策略相比,提出的策略能提供不同业务的服务区分,满足实时业务的低延迟需求,降低数据传输的中断率,同时在授权信道空闲率与网络负载较大时吞吐量性能较优。     

概率反馈动态频谱分配策略及性能分析

为了折衷认知用户吞吐量与平均时延,并适应多种网络业务需求,在认知无线电网络中引入概率反馈机制和能量检测阈值,提出一种新的动态频谱分配策略。针对认知用户的非理想感知结果,建立一种2类用户可能相互干扰的优先级排队模型,并构造状态转移概率矩阵。采用矩阵几何解方法求出系统的稳态分布,给出信道利用率、认知用户吞吐量、认知用户平均延迟及授权用户干扰率等性能指标的表达式。通过数值实验和系统仿真验证所提动态频谱分配策略性的有效性,并给出能量检测阈值的优化设置方案。     

基于认知无线电系统的新型合作功率控制博弈算法

以改进的空时分集OFDM-CDMA系统为基础，研究已认知频谱资源的公平分配问题，根据信道状况，吞吐量最大化和功率限制需求，提出了一种合作功率控制纳什议价博弈算法，以有效地降低认知用户（secondary users）的发射功率，提高网络吞吐量。仿真结果表明该算法满足了认知用户公平共享频谱资源的需求，在相同功率消耗情况下，网络吞吐量显著提高。     

混合频谱共享方式下多用户动态频谱分配算法

为解决混合overlay/underlay频谱共享方式下多用户动态频谱分配问题,构建了混合频谱共享方式下动态频谱分配模型,提出了基于Q学习的多用户动态频谱分配算法. 该算法在不对主用户产生有害干扰的前提下,以最大化次用户总吞吐量为目标,构建了与次用户相对应的虚拟次用户作为智能体. 通过与环境交互学习,进行信道和共享方式初选;频谱分配系统根据冲突情况和各智能体的学习结果调整信道分配策略直至次用户间无冲突. 仿真结果表明,该算法在无信道检测和信道先验知识的条件下,能根据前一时隙信道状态和次用户传输速率需求,实现动态信道分配和频谱共享方式确定,避免次用户间冲突,减少主次用户间冲突,有效提升次用户总吞吐量.     

应急通信系统的动态频谱接入及性能分析

本文将传统的应急通信系统与认知无线电技术相结合,在授权用户、应急用户和一般认知用户这三类用户共享频谱池的前提下,提出了两种不同接入等级的认知无线电接入策略.在这两种接入策略下,对系统动态频谱接入过程,构建了基于三维马尔可夫链的分析模型.在该模型基础上推导了应急用户和其它认知用户的阻塞率及强制终止概率,给出了它们的数学闭合表达式,并对系统性能进行了分析.仿真结果表明:三级优先级接入策略,使得应急用户的阻塞率和强制终止概率得到了降低,该策略的采用很大程度上提高了应急用户的性能.     

一种基于切换失败概率和认知用户信道容量联合优化的访问策略

现有频谱切换信道访问策略主要考虑切换失败概率,而较少考虑认知用户的信道容量。针对这一问题,提出了一种基于切换失败概率和认知用户信道容量联合优化的频谱切换信道访问策略。首先证明了按信噪比降序访问可以得到最大的认知用户信道容量,而按信道平均空闲时间降序访问可以得到最低切换失败概率。接着提出对信道平均空闲时间和信噪比进行排序并加权合并的非参数化处理。最后,按非参数化处理结果降序进行信道访问。结果表明,所提方法能够在切换失败概率接近最低的情况下,使认知用户信道容量提升50%左右。     

认知无线电频谱切换目标信道访问机制

针对认知无线电先应式决策频谱切换,该文提出按照信道平均空闲时间递减的顺序对目标信道进行访问的机制,证明了该机制在主用户信道空闲时间服从均匀分布、Rayleigh分布以及Weibull分布下均能够保证频谱切换失败概率最小。仿真结果表明所提机制得到的频谱切换失败概率要远远小于随机访问机制的切换失败概率。另外,当能够获得数十次信道空闲时间观测样本情况下,虽然存在估计误差,但所得频谱切换失败概率与理想最优频谱切换失败概率非常接近。     

Performance Analysis of Cognitive Radio Spectrum Access with Different Primary User Access Schemes

The cognitive access process based on different access schemes of the primary user in the spectrum sharing system of cognitive radio is studied. In the cognitive system with finite user population, the primary users access the licensed channels through random access, real-reservation access, pseudo-reservation access and non-random access. The cognitive users access the licensed channels without interfering the primary users. Their access process is modeled using the three-dimensional continuous Markov model. The performance analysis is presented with variation of the number of cognitive users. It is known from the simulated results that the handoff probability, the blocking probability and the forced termination probability of the cognitive users increase with the increase of the number of cognitive users. It is also found that the random access scheme has the highest handoff probability. And the real-reservation access scheme has the highest blocking probability and forced termination probability. In addition, the non-random access scheme has the maximum channel utilization.     

Analysis of Cognitive Radio Spectrum Access with Optimal Channel Reservation

A Markov chain analysis for spectrum access in licensed bands for cognitive radios is presented and forced termination probability, blocking probability and traffic throughput are derived. In addition, a channel reservation scheme for cognitive radio spectrum handoff is proposed. This scheme allows the tradeoff between forced termination and blocking according to QoS requirements. Numerical results show that the proposed scheme can greatly reduce forced termination probability at a slight increase in blocking probability     

Performance analysis of dynamic spectrum handoff scheme with variable bandwidth demand of secondary users for cognitive radio networks

Cognitive radio (CR) has attracted considerable attention as a promising technology for solving the current inefficient use of spectrum. In CR networks, available sub-channels are dynamically assigned to secondary users (SUs). However, when a primary user accesses a primary channel consisting of multiple sub-channels, data transmissions of the SUs already using the sub-channels may be terminated. In this paper, we analyze the performance of dynamic spectrum handoff scheme with channel bonding, in which the number of sub-channels used by an SU are variable. We model the multichannel CR network as a multiserver priority queueing system without waiting facility, deriving the blocking probability, the forced termination probability and the throughput for SUs. In terms of the way of forced termination, we consider two policies; one is that SUs using the largest number of sub-channels are forced to terminate their transmissions, and the other is that SUs using the smallest number of sub-channels are chosen for termination. The analysis is also validated by simulation. Numerical examples show that in both forced-termination policies, the throughput of SUs that are forced to terminate their transmissions degrades as the offered load to the system increases.     

Performance evaluation of hierarchical cellular CDMA networks with soft handoff queueing

We consider hierarchical cellular code-division multiple-access networks supporting soft handoff, where users with different mobility are assigned to different layers, i.e., microcells in the lower layer are used to carry slow users, whereas macrocells in the upper layer are for fast users, and handoff queues are provided for handoff calls that cannot obtain the required channel immediately, so that forced termination probability can be reduced. According to whether handoff queues are provided in microcells and/or macrocells, four different call admission control schemes are proposed and studied. We derive the mathematical model of the considered system with multidimensional birth-death process and utilize Gauss-Seidel iterative method to find the steady-state probability distribution and thus the performance measures of interest: new call blocking probability, handoff failure probability, and forced termination probability. Analytical results show that providing handoff queues in both microcells and macrocells can achieve largest performance improvement. Furthermore, handoff queue size greater than a threshold is shown to have little effect on performance measures of interest. Last but not least, the studied two-tier system is compared with a one-tier counterpart. It is shown that the two-tier system performs better in terms of average number of handoffs per fast call.     

Analysis of a hierarchical cellular system with reneging anddropping for waiting new and handoff calls

We analyze a hierarchical cellular system with finite queues for new and handoff calls. Both the effect of the reneging of waiting new calls because of the callers' impatience and the effect of the dropping of queued handoff calls as the callers move out of the handoff area are considered, besides the effect of the guard channel scheme. We successfully solve the system by adopting the multidimensional Markovian chain and using the transition-probability matrix and the signal-flow graph to obtain the average new-call blocking probability, the forced termination probability, and the average waiting time of queued new and handoff calls. We further investigate how the design parameters of the buffer sizes and guard channel numbers in macrocell and microcells affect the performance of the hierarchical cellular system. The results show that provision of a buffering scheme and guard channel scheme can effectively reduce the new call blocking probability and the forced termination probability in the hierarchical cellular system, and the effectiveness is more significant in the macrocell than in the microcells     

机会频谱接入系统的切换请求排队机制及性能分析

针对非实时业务的特点,该文提出了机会频谱接入系统的两种切换请求排队方案,并采用二维Markov模型对其性能进行了建模分析,得到了呼叫阻塞率、强制中断率、信道利用率等系统性能关键参数的解析结果。数值结果分析表明,提出的两种切换请求排队方案是降低系统强制中断率的有效方法,且可克服切换信道预留机制阻塞率高、信道利用率低的缺点,信道利用率最多可提高26%。仿真实验验证了理论分析的正确性。     

Stochastic spectrum handoff protocols for partially observable cognitive radio networks

Recent studies have been conducted to indicate the ineffective usage of licensed bands due to static spectrum allocation. In order to improve spectrum utilization, cognitive radio (CR) is therefore suggested to dynamically exploit the opportunistic primary frequency spectrums. How to provide efficient spectrum handoff has been considered a crucial issue in the CR networks. Existing spectrum handoff algorithms assume that all the channels can be correctly sensed by the CR users in order to perform appropriate spectrum handoff process. However, this assumption is impractical since excessive time will be required for the CR user to sense the entire spectrum space. In this paper, the partially observable Markov decision process (POMDP) is applied to estimate the network information by partially sensing the frequency spectrums. A POMDP-based spectrum handoff (POSH) scheme is proposed to determine the optimal target channel for spectrum handoff according to the partially observable channel state information. Moreover, a POMDP-based multi-user spectrum handoff (M-POSH) protocol is proposed to exploit the POMDP policy into multi-user CR networks by distributing CR users to frequency spectrum bins opportunistically. By adopting the policies resulted from the POSH and M-POSH algorithms for target channel selection, minimal waiting time at each occurrence of spectrum handoff can be achieved which will be feasible for multimedia applications. Numerical results illustrate that the proposed spectrum handoff protocols can effectively minimize the required waiting time for spectrum handoff in the CR networks.     

Performance Analysis of Channel Assignment with Prioritized Handoff Procedures in GSM

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Modeling and efficient handling of handoffs in integrated wireless mobile networks

We propose and analyze two handoff schemes without and with preemptive priority procedures for integrated wireless mobile networks. We categorize the service calls into four different types, namely, originating voice calls, originating data calls, voice handoff request calls, and data handoff request calls and we assume two separate queues for two handoff services. A number of channels in each cell are reserved exclusively for handoff request calls. Out of these channels, few are reserved exclusively for voice handoff request calls. The remaining channels are shared by both originating and handoff request calls. In the preemptive priority scheme, higher priority is given to voice handoff request calls over data handoff request calls and can preempt data service to the queue if, upon arrival, a voice handoff request finds no free channels. We model the system by a three-dimensional Markov chain and compute the system performance in terms of blocking probability of originating calls, forced termination probability of voice handoff request calls, and average transmission delay of data calls. It is observed that forced termination probability of voice handoff request calls can be decreased by increasing the number of reserved channels. On the other hand, as a data handoff request can be transferred from a queue of one base station to another, there is no packet loss of data handoff except for a negligibly small blocking probability.     

Artificial bee colony–based spectrum handoff algorithm in wireless cognitive radio networks

In this work, we proposed a new artificial bee colony–based spectrum handoff algorithm for wireless cognitive radio systems. In our wireless cognitive radio system, primary users, secondary users, and related base stations exist in the same communication environment. For our artificial bee colony–based algorithm, secondary users that always struggle to discover an idle channel have a leading role. While honey bees work hard to find the best‐quality nectar source for foraging, secondary users try to find idle channels for making communication. In this way, secondary users are organized for different missions such as sensing and handoff similar to honey bees to minimize spectrum handoff delay by working together. In the spectrum handoff stage, some secondary users must sense the spectrum so that the interrupted secondary user may perform the spectrum handoff process. In our developed spectrum handoff algorithm, the spectrum availability characteristic is observed on the basis of the missions of the bees in the artificial bee colony algorithm with the aim of minimizing the spectrum handoff delay and maximizing probability of finding an idle channel. With the help of the algorithm that is developed using the artificial bee colony, spectrum handoff delay of secondary users is considerably decreased for different number of users without reducing probability of finding an available channel.