时隙ALOHA系统稳定性分析*

研究了基于非对称多包接收模型的时隙ALOHA随机接入系统的稳定性。引入了非对称多包接收(MPR)模型,计算了媒体接入控制(MAC)容量区域,得到了两用户系统ALOHA稳定区域的详细特征。结果表明,随着MPR容量的提高,稳定区域从凹形区域变化到凸形区域,ALOHA稳定区域与MAC容量区域是一致的,而且当发送概率为1时,系统不需要传输控制,即对于两用户捕获信道而言,发送概率为1的ALOHA系统稳定性是最优的。     

P坚持时隙ALOHA稳定性

动态频谱共享无线通信系统使用时隙ALOHA协议完成对数据信道的竞争,稳定性问题是时隙ALOHA协议的固有特性。通过建立p坚持时隙ALOHA的系统模型,分析了在重传概率确定情况下,新包的生成率对系统稳定性的影响,以及新包的生成率已知情况时,重传概率对系统稳定的影响;通过数值计算与仿真,给出了两种情况下系统稳定的条件。     

时隙ALOHA协议下的网络化控制系统协同设计

针对网络化控制系统中信道容量有限的问题,本文提出一种基于时隙ALOHA通信协议的控制与通信协同设计方法.将控制系统的采样周期划分为若干等长度的时隙,在每个时隙中,系统的分布式传感器通过时隙ALOHA协议来随机竞争接入网络.由于在不同的采样周期各个传感器的接入状态不同,整个状态反馈控制系统将在若干子系统之间进行切换.据此,本文建立了离散的切换系统模型,并利用分段李雅普诺夫函数方法和平均驻留时间技术得到了能够保证系统指数稳定的充分条件.然后,给出能够保证控制系统稳定所需的信道吞吐率的界限,进而得到了时隙ALOHA协议中的最大重传次数与控制系统衰减率的定量关系.通过上述方法,本文建立了控制-通信协同设计的框架结构,可将控制器的增益矩阵和时隙ALOHA通信协议进行协同设计.最后,通过仿真验证了本文所提出的协同设计方法的有效性.     

Power level selection schemes to improve throughput and stability of slotted ALOHA under heavy load

We propose and analyse three different power selection schemes for slotted ALOHA random access protocol operating under multiple power levels. Through analysis and numerical examples we demonstrate that these schemes can significantly improve the performance of slotted ALOHA, in terms of throughput and stability, under heavy load. The proposed schemes are truly distributive in nature and can be easily implemented in wireless access systems without requiring any centralised control.     

On time-triggered and event-based control of integrator systems over a shared communication system

When analyzing networked control systems, where the control loop is closed over a communication system, it is crucial to take the communication system into account. Hence, time-triggered and event-based control of an integrator system with noise over a shared communication system is analyzed. Thereby, analytical models of different communication systems are used and the analysis is focused on the effect of the communication system on the performance, as well as the interaction between control and communication. For time-triggered control, where the event times are known in advance, a deterministic communication protocol can be used. Hence, time-triggered control with the two most well-known deterministic communication protocols, time division multiple access (TDMA) and frequency division multiple access (FDMA), is analyzed. For event-based control, where the events appear at random times, a contention-based communication protocol should be used. Hence, event-based control is analyzed with different contention-based communication protocols: pure ALOHA, slotted ALOHA, a queueing system, and Erlang’s loss model. It turns out that time-triggered control with either TDMA or FDMA outperforms event-based control with pure or slotted ALOHA. However, event-based control with a properly designed queueing system gives an even better performance. Thus, we conclude that it is crucial to take the details of the communication system into account.     

基于分组的动态时隙ALOHA算法

详细介绍了基本ALOHA算法、时隙ALOHA算法、帧时隙ALOHA算法和动态帧时隙ALOHA算法基本原理和系统效能, 分析了上述4种算法各自特点. 提出了基于分组的动态帧时隙ALOHA算法, 该算法根据标签数量对标签进行分组, 并动态匹配最佳帧长. 仿真结果表明, 改进后算法系统具有吞吐率高、不受标签数量限制、节约帧时隙等优势.     

伪贝叶斯稳定性算法性能研究*

针对时隙ALOHA的稳定性控制问题,建立了时隙ALOHA系统模型,研究了伪贝叶斯稳定性调整算法的原理。研究了实际负荷与估计负荷的差异对调整过程的影响。结果表明,该算法能够保证系统的稳定性,其调整时间随系统总体输入负载的变化增大而变长。为减小调节时间,需要对该算法进行改进。     

一种改进的ALOHA防碰撞算法

针对现有动态帧时隙ALOHA防碰撞算法在射频识别系统中的标签识别效率最高只能达36.8%的问题,利用标签码元序列的唯一性,改进经典的动态帧时隙ALOHA防碰撞算法中随机选择时隙的方式,提出一种基于标签码元的碰撞序列进行时隙选择的方法,有效降低了标签碰撞的概率,从而提高系统识别效率。仿真结果表明改进的ALOHA标签防碰撞算法识别效率最低为37.5%,随着实际标签数目与碰撞位序列所能确定的标签数目越接近,识别效率越高,最高能达到100%,明显优于现有的动态帧时隙ALOHA算法。     

扩频时隙ALOHA及其在CDMA通信中的应用

文章在分析传统ALOHA随机多址协议不足的基础上对扩频时隙ALOHA的基本原理及其性能进行了研究,并对其在第三代CDMA移动通信系统中的应用作了探讨。     

Optimal design parameters in a multihop packet radio network using random access techniques

Takagi and Kleinrock studied the problem of optimal transmission ranges in a packet radio network with mobile terminals, assuming that each terminal is equipped with an omidirectional antenna. The purpose of this paper is to examine the possible improvement gained by directional antennas. The protocols considered include slotted ALOHA (with and without capture) and non-persistent CSMA. Results show throughput performance can be greatly enhanced by proper directional antennas. Two routing strategies, most-forward-routing and point-to-destination, are investigated and compared. The study shows both strategies posses roughly the same degree of performance. However the latter is attractive for its simplicity of system operation.     

A Multi-station packet-radio network

A new structure for packet-radio networks, called the multi-station network, is suggested and analyzed. We describe the salient features of this structure that consists of a large number of nodes and several stations. We then focus on one of the main problems within the suggested structure, the problem of forwarding packets from the nodes to the stations through a shared radio channel. Two basic forwarding schemes are investigated and compared. The classical slotted ALOHA is considered as an access scheme to the shared channel.     

自适应动态时隙ALOHA防碰撞算法的研究

时隙ALOHA算法是射频识别系统中通过分时复用方式的一种防碰撞算法,应答器在分配到的时隙中与阅读器进行数据交互.提出一种改进的自适应帧时隙ALOHA防碰撞算法,能够使得时隙到优化分配,进一步提高应答器的识别效率.     

The stability problem of broadcast packet switching computer networks

Summary Certain computer networks have been implemented using a radio broadcast frequency over which a large set of terminals are allowed to transmit packets of bits; one such example is the ALOHA computer network [1]. In such systems, a basic problem is that of the blocking of terminals whose transmission of a packet has overlapped in time with transmission by some other terminal. In this paper we consider a slotted ALOHA packet transmission scheme with an infinite set of terminals each transmitting at an infinitesimally small rate. We present a probabilistic model of such a system to show that the slotted ALOHA system with an infinite population is inherently unstable. This paper confirms the simulations and results of Kleinrock and Lam [8].     

RFID动态帧时隙ALOHA防冲突中的标签估计和帧长确定

为提高射频识别(Radio frequency identification, RFID)标签的识别效率, 本文针对RFID动态帧时隙ALOHA防冲突系统, 提出了新的标签估计方法和帧长确定方案. 标签估计中采用了不同的贝叶斯代价函数, 提出了几种贝叶斯标签估计方法, 它们的估计结果准确, 而且通过减小标签数取值范围可使计算复杂度得到降低. 随后, 推导出一种根据标签数确定最优帧长的方案, 它能使系统达到最大的信道利用率, 该最大信道利用率要大于帧的时隙数等于标签数时所能达到的最大利用率.     

一种超宽带MAC层信道时间分配的改进方法

针对超宽带媒体访问控制(MAC)层的信道资源分配在可变比特率业务和不定比特率业务中可能存在的丢帧或带宽利用率低的问题,提出了一种基于开槽ALOHA协议即时抢占信道时间分配的改进方法,为数据流分配空闲的信道时间。仿真结果表明,与IEEE802.15.3建议的分配方式相比,这种改进方法能够在一定程度上降低任务失败率和延迟抖动,提高有效吞吐量,从而有效地支持高QoS要求的传输。     

基于自供电的无线传感器网络MAC层协议研究

针对自供电的无线传感器网络,提出了一种新的MAC层协议DFSA（动态帧时隙ALOHA）,该协议对能量进行系统管理。仿真结果表明在具有能量收集能力的传感器网络中,DFSA能够充分利用节点的能量收集能力,实现网络能量充分利用,在保证较高的传输效率下获得最高的信道利用率,同时提高了数据传输的可靠性。     

一种新的RFID标签数目估算方法

动态帧时隙ALOHA反碰撞算法中帧长度调整的关键在于对阅读器读写范围内标签数目的估算。通过模拟帧时隙ALOHA算法得到了不同帧长度时标签数目与碰撞时隙数目的关系曲线。创建了碰撞时隙中的平均标签数目e和碰撞时隙所占比例P_C之间关系的数学表达式,该表达式对所有的帧长度都适用。在此基础上提出了一种新的RFID标签数目估算方法。该方法计算量小,不需要预先建立和存储查找表,有利于提高RFID系统的实时响应速度和降低硬件成本。 MATLAB仿真表明,提出的标签数目估算方法比现有的方法估算准确度有较大提高。     

常见的RFID标签估算方法及其统计分析

随着射频识别技术在诸多领域的应用,其反碰撞问题也得到了广泛的关注。标签估算作为动态帧时隙ALOHA算法中的关键,其估算精确度对系统识别有着直接的影响。在对常见的标签估算方法进行介绍的基础上,本文通过计算机仿真实验和统计的不同方法,在估算准确度和稳定性等方面进行了分析和对比。     

Throughput-delay performance of packet-switching multiple-access channel with power capture

Power capture is the ability of a receiver to receive correctly the strongest of several messages that arrive during overlapping intervals. The presence of power capture in multiple-access packet switching channels introduces inequality in the access conditions to the channel when packets arrive at the receiver at different power levels. Two models for such channels under slotted ALOHA protocol with power capture are considered. In the first, a packet is transmitted at one of K possible power levels, and it is received correctly if no other packet is transmitted at the same or higher level. The second model is of a ground radio network where a received packet's power is inversely proportional to the distance it propagates. In this model, a packet transmitted from a distance r is received correctly if no other packet is transmitted from a distance smaller than a · r (a ? 1). A technique developed by Lam (1974) is generalized to obtain the throughput-delay characteristics of the channel for both models. The effect of high power traffic on the lower power traffic is discussed, and it is shown that the ‘Sisyphus distance’ phenomenon predicted by Abramson (1977) for the case a = 1 in the second model does not exist in the more realistic case of a > 1.     

新型的RFID动态帧时隙ALOHA防碰撞算法

基于动态帧时隙ALOHA算法,提出一种新型的标签数目估计算法.该算法利用当前帧的时隙信息,结合贝叶斯算法获得标签数目的概率函数分布,从而更加准确地估计出下一帧的标签数目.仿真结果表明,该算法对标签数目的估计误差维持在1.4％,信道吞吐率逼近理论值36.8％,证明了其有效性.