Analysis of a contention-based opportunistic spectrum access under general channel activity model

We investigate a distributed contention-based spectrum access scheme in cognitive radio networks where ON/OFF periods of the channel by primary users follow discrete phase (PH) type distributions. The main motivation for ON/OFF having PH distributions is that the channel activity has a more general behavior depending on the primary users’ traffic. In the past most other researchers assumed that ON/OFF periods of a channel follow a geometric distribution for the purpose of mathematical tractability even though this assumption is restrictive.We propose a distributed medium access control (MAC) scheme for the secondary users (SUs) which is characterized by a constant contention window size and a method to decide whether for each SU to participate in competition or not depending on the queueing delay of a head-of-line (HoL) packet. In order to investigate the performance of our proposed MAC protocol, we construct a two-dimensional Markov chain which incorporates both the proposed MAC scheme and the general channel activity. The resulting one-step transition probability matrix of the Markov chain has a very special structure. With the help of the censored Markov chain method, we provide a computationally efficient method to obtain the stationary distribution of the Markov chain. We then obtain the system capacity, which is defined as the maximum number of SUs that can be accommodated with a quality of service (QoS) guarantee on the packet dropping probability and the packet delay. Numerical examples show that the system capacity considerably depends on the distributions of ON/OFF periods and our proposed MAC scheme achieves a higher capacity than the existing one.     

适用于飞行自组网的闲置时隙预约TDMA协议

在飞行自组网中,固定时隙分配时分多址接入（TDMA）协议存在闲置时隙无法成功使用的问题。通过对TDMA协议引入闲置时隙预约机制,提出一种支持业务优先级传输机制的闲置时隙预约TDMA协议。采用短帧长的方式满足协同与控制业务的低时延传输需求,并利用闲置时隙预约机制允许节点使用闲置时隙传输感知业务,从而满足感知业务的高吞吐量传输性能要求。仿真结果表明,与CF-MAC和CTMAC协议相比,该协议能够在降低传输时延的同时,有效提高信道利用率和网络吞吐量。     

Packet delay analysis on IEEE 802.11 DCF under finite load traffic in multi-hop ad hoc networks

In this paper, the average packet delay on IEEE 802.11 DCF under finite load traffic in multi-hop ad hoc networks is analyzed. We employ a Markov chain model to analyze the probability of transmission at each node in an arbitrary slot and derive the channel access delay. We model each node using an M/G/1 queue and derive the queueing delay. The model is extended from analyzing the single-hop average packet delay to evaluating the end-to-end packet delay in multi-hop ad hoc networks without assuming the traffic to be in a saturation state. To validate our analytic results, we have done extensive simulation. The analytic and the simulation results match very well.     

基于中断概率的星地网络鲁棒功率控制算法

相较于传统的地面认知网络,星地认知网络链路传输时延较长,因此基于实时信道感知的认知用户中断概率较高。以离散时间马尔科夫链描述授权用户的动态及衰落信道,同时考虑信道转移概率的不确定性,建立基于似然不确定性模型的信道状态马尔科夫链,利用鲁棒的向后递归方法得到离线存储的功率分配矩阵,并对网络中多个用户采用博弈论的方法,进而提出基于中断概率的功率控制算法。仿真结果表明,与RRAP和TS-RS-PA算法相比,该算法在网络延迟存在的情况下能够更有效地降低中断概率并节约能耗。     

基于分组到达率的802.11 DCF性能分析

针对WLAN(wireless local area network)基础结构模式中的IEEE 802.11 DCF(distributed coordination function)机制,提出了一种基于分组到达率的性能分析模型.模型不仅考虑了终端数量、传输负载、二进制指数回退机制等因素,而且分析了MAC(mediunl access control)层有限队列对系统性能的影响.在每个终端被模型化为M/M/1/K队列的基础上,进一步利用虚拟时隙在时间上离散化终端MAC层队列状态,并采用离散时间的三维马尔可夫链对系统性能建模.基于该模型得到了归一化吞吐量、分组时延和丢包率.仿真分析结果表明,该模型能够有效地预测变化的分组到达率情形下DCF机制的性能.     

OFDM符号特性对DCF性能影响研究

认知无线网络由于使用信道频段的长时延特性,OFDM符号时间及Slottime均较大,使得CSMA/CA协议中的SIFS等参数值增大。另外,随着无线局域网络物理层速率的提高,每个OFDM符号携带的数据比特数加大。为评估物理层长时延及高速环境下的DCF性能,基于二维Markov模型得出了CSMA/CA系统吞吐量表达式,数学分析表明系统吞吐量主要由网络节点数、数据包长度、OFDM携带信息比特数、Slottime及OFDM符号时间长度决定。理论计算及仿真测试结果显示,OFDM携带信息比特数越多,Slottime越大,OFDM符号时间长度越长,CSMA/CA的吞吐量性能越低。     

基于Markov延迟特性的闭环网络控制系统研究

针对控制网络中固有的随机传输延迟 ,提出了一种新颖的控制模式 ,实现了对存在多步随机传输延迟的网络控制系统的数学建模 .基于该模型 ,利用Markov链理论 ,得到了满足给定性能指标的随机最优控制律 ,同时给出了求取相应的Markov链状态转移矩阵的方法 .文末通过实验研究 ,验证了所提理论的正确性和有效性     

面向最小延迟的无线mesh网络时隙分配问题研究

在基于IEEE802.15.4的工业无线传感器网络中,通过时分复用技术来实现节点间并行通信,以提高网络的吞吐量.通常,网络中的管理器根据全网拓扑对网络中的每个连接分配一个超帧中相应的时隙.不同的时隙分配方案中,连接在超帧中活动的顺序也不同,对网络性能的影响也不同.本文研究了不同时隙分配方案在网络吞吐量和数据延迟方面对网络的影响,定性分析了时隙分配方案与数据延迟的关系,并根据mesh结构的特点提出一种使用粒子群优化的算法,用于在网络管理器上对时隙分配进行优化,使得数据延迟最小化,以满足工业无线网络实时性应用的要求.     

改进的VANET分布式自适应时分多址分配机制

在车载自组织网（Vehicular Ad-hoc Network,VANET）环境下,针对多节点同时接入信道时的竞争冲突问题,将分布式时分多址（Time Division Multiple Access,TDMA）和空分多址（Space Division Multiple Access,SDMA）思想相结合,提出改进的分布式自适应时分多址分配机制（Modified Decentralized Adaptive TDMA Scheduling mechanism,MDATS）.MDATS协议的节点通过帧信息（Frame Information,FI）的交互,获取两跳范围内其他节点时隙使用情况,从而筛选出空闲时隙集.竞争区域将根据空闲时隙数在空间上均分成多个逻辑区段,节点则根据所处的逻辑区段与空闲时隙的对应关系确定竞争使用的时隙.该协议通过对空闲时隙在空间上的分散化,降低同时接入节点之间的竞争冲突.仿真结果表明,MDATS协议与其他类似MAC协议相比,能够达到更高的信道接入成功率、更低的接入时延性能以及更高的时隙利用率.     

基于分簇的无线传感器网络MAC节能算法

为减少节点能耗和提高信道利用率,提出一种基于分簇结构的无线传感器网络MAC节能算法(EEC-MAC)。在TDMA机制的基础上,采用时隙系数动态调整簇内节点的时隙大小,降低数据的传输时延。对于部分不需要数据传输的节点不分配时隙,使其拥有较长的睡眠时间来节约能量。簇内节点按其剩余能量系数形成时隙分配顺序,减少状态转换的能耗。簇间节点采用基于CSMA/CA机制的随机分配策略实现通信。仿真结果表明,EEC-MAC节能效果较好,具有较小的平均通信时延和较长的网络生命周期。     

一种竞争窗口自适应调整算法设计

在传统的增强型分布式信道接入机制中,每个接入类别队列的主要参数都是静态的,并没有考虑到无线网络的状态。为此,利用时隙利用率来判断网络负载程度,参考每个AC队列的冲突率来动态调整最小竞争窗口和最大竞争窗口。仿真实验结果表明,该算法在保证实时业务要求的同时,能降低延迟,提高无线局域网吞吐量。     

CSMA/CA在CSMA/TDMA混合网络中的性能分析

CSMA/CA机制与TDMA在同频段共存时会存在相互干扰,为评估混合网络中CSMA/CA的性能表现,将TDMA视为周期性的干扰提出改进的二维Markov分析模型,能够计算不同条件下的CSMA/CA饱和吞吐量,并通过仿真实验证明了模型的有效性。同时讨论了TDMA时隙分配方式对系统性能产生的影响,仿真结果表明时隙均匀分配方式能保证较低的时延和时延抖动,而连续分配方式能使CSMA/CA获得较高的吞吐量。提出将基于最低信道需求时间的阈值计算方法与吞吐量分析模型相结合,用于在不同应用场景下进行时隙分配方式的选择。     

基于网络效用最大化与冲突避免的无线传感器网络MAC协议

针对周期汇报型无线传感器网络(WSN)中的无线信号冲突和能量利用效率问题,提出了一种基于网络效用最大化与冲突避免的媒体访问控制(UM-MAC)协议。该协议基于时分多路复用(TDMA)调度机制,将效用模型引入无冲突的节点工作时隙分配过程中,把链路可靠性、网络能耗归纳到一个统一的效用优化框架中;进而提出了一个启发式算法,使网络能够快速找到一个基于网络效用最大化与冲突避免的节点工作时隙调度方案。将UM-MAC协议与S-MAC协议和冲突避免MAC(CA-MAC)协议进行比较,在不同节点数量的网络环境中,UM-MAC获得的网络效用较大,平均数据包成功发送率较高,生命周期介于S-MAC与CA-MAC之间,在不同的网络负载下所有节点发数据包到汇聚节点的平均时延有所增加。仿真实验结果表明:UM-MAC协议较好地解决了冲突干扰问题,提高了网络的数据包成功发送率和能量利用效率等性能;在低网络负载时,TDMA类协议的性能并不比竞争类协议好。     

Effects of access phases lengths on performance of IEEE 802.15.6 CSMA/CA

In this paper, we study interplay of prioritizing mechanisms in recently released IEEE 802.15.6 standard for Wireless Body Area Networks (WBANs). Among eight User Priorities (UPs) we study the impact of access phases lengths and CSMA/CA mechanism on the network performance. We consider a star-topology single-hop WBAN in which the hub operates in beacon mode with superframe boundaries. Our study is conducted in saturation regime and an error prone channel in order to indicate performance bounds for all UPs. We have developed an analytical model for the CSMA/CA-based Medium Access Control (MAC) layer of IEEE 802.15.6 which includes Markov chain and probabilistic models for all UPs. In order to validate the analytical model we have developed a simulation model in OPNET. We investigate the effectiveness of deploying exclusive access phases and second random access phase on the network performance. We also examine the medium utilization and normalized throughputs of all UPs. Obtained performance bounds indicate potential tradeoff between the prioritizing parameters which affect the saturation throughputs.     

EPA网络控制系统丢包分析

针对基于时钟同步技术的时分调度网络控制系统工作方式,分析了基于EPA协议的网络丢包原理.建立在不同实时性要求情况下EPA网络系统的丢包模型,并且分析了时钟偏差抖动为马尔科夫链时,通信周期、时间片长度以及时钟同步精度对系统稳定性的影响.提出了提升EPA系统实时性的方法.最后对EPA系统的稳定性进行了分析.     

Structural Properties of Optimal Transmission Policies Over a Randomly Varying Channel

We consider the problem of transmitting packets over a randomly varying point to point channel with the objective of minimizing the expected power consumption subject to a constraint on the average packet delay. By casting it as a constrained Markov decision process in discrete time with time-averaged costs, we prove structural results about the dependence of the optimal policy on buffer occupancy, number of packet arrivals in the previous slot and the channel fading state for both i.i.d. and Markov arrivals and channel fading. The techniques we use to establish such results: convexity, stochastic dominance, decreasing-differences, are among the standard ones for the purpose. Our main contribution, however, is the passage to the average cost case, a notoriously difficult problem for which rather limited results are available. The novel proof techniques used here are likely to have utility in other stochastic control problems well beyond their immediate application considered here.      

WLAN中MAC协议性能分析研究

对媒体接入控制(Media Access Control,MAC)协议进行性能分析,可以深入理解MAC协议的操作特点,发现吞吐量、数据包时延与丢失等性能遭受影响的本质原因。对当前马尔可夫链性能分析法加以详细地分析与总结,分别阐述了无线局域网络中分布式协调功能和增强的分布式信道接入两种MAC协议的性能分析研究工作。对进一步的研究工作进行了分析与展望。     

Throughput analysis of the IEEE 802.11 distributed coordination function considering erroneous channel and capture effects

This paper presents a performance study of the distributed coordination function (DCF) of 802.11 networks considering erroneous channel and capture effects under non-saturated traffic conditions employing a basic access method. The aggregate throughput of a practical wireless local area network (WLAN) strongly depends on the channel conditions. In a real radio environment, the received signal power at the access point from a station is subjected to deterministic path loss, shadowing, and fast multipath fading. The binary exponential backoff (BEB) mechanism of IEEE 802.11 DCF severely suffers from more channel idle time under high bit error rate (BER). To alleviate the low performance of IEEE 802.11 DCF, a new mechanism is introduced, which greatly outperforms the existing methods under a high BER. A multidimensional Markov chain model is used to characterize the behavior of DCF in order to account both non-ideal channel conditions and capture effects.     

Performance analysis of CSMA/CA protocols with multi-packet transmission

Wireless objects equipped with multiple antennas are able to simultaneously transmit multiple packets by exploiting the channel’s spatial dimensions. In this paper, we study the benefits of such Multiple Packet Transmission (MPT) approach, when it is used in combination with a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol for fully interconnected networks, addressing the interactions between the two mechanisms and showing the performance gains that can be achieved. To this end, a very simple Media Access Control (MAC) protocol that captures the fundamental properties and tradeoffs of a CSMA/CA channel access protocol supporting MPT is introduced. Using this protocol as a reference, a new analytical model is presented for the case of non-saturated traffic sources with finite buffer space. Simulation results show that the analytical model is able to accurately characterize the steady-state behavior of the reference protocol for different number of antennas and different traffic loads, providing a useful tool for understanding the performance gains achieved by MAC protocols supporting MPT.     

IEEE802.11e无线局域网中的接入延迟分析模型

IEEE802.11e标准为各个服务类别制定了不同的信道竞争参数数值,使得无线局域网具备了业务区分能力。标准中所提出的增强型分布式接入方式主要包括虚拟碰撞、节点回退、最小竞争窗口以及仲裁帧间隔等机制。本文建立三维离散马尔可夫模型分析队列在接入过程中的竞争行为,进而准确估计各个队列的接入延迟。仿真结果表明,该模型能够准确地描述资源竞争情况,接入延迟计算法方法可根据不同系统参数精确地计算接入延迟性能。