一种优化WLAN上下行公平性的MAC机制

IKKK 802.11 DCF主要基于载波检测/碰撞退避多址接人协议和时隙制退避算法.因此,往往在基础架构模式中,AP与用户站在随机竞争信道过程中并无优势,并且DCF应用于多速率WLAN,各站点之间的竞争并不均匀(时间不均),存在吞吐量异常和严重不公平性问题.通过建立三维Markov链模型,对非饱和状态下802.11 MAC协议上下行带宽公平性进行了分析,由此提出基于下行链路优先的DPMAC协议,其核心思想是:AP享有优先访问信道的权利,并且可以根据下行链路状态动态调整占有信道的时间,各用户站竞争信道仍然采用原有的DCF访问方式.通过仿真证实DPMAC协议能很好地实现上下行链路公平性,达到满意的效果.     

一种采用区分队列的DCF机制

在IEEE 802.11b标准的基础结构操作模式下,DCF的竞争信道访问机制造成上下行链路之间负载不均,限制了VoIP容量的增加.提出一种改进的DCF机制:PDQ-DCF,通过在AP中设置两个不同优先级的区分队列,对语音流量和数据流量区分不同的信道服务参数,均衡AP和STA的语音负载,保证下行链路中语音流量的实时传输,提高语音传输几率.仿真结果表明在保证语音质量的前提下PDQ-DCF机制能够提高语音通信容量.     

无线局域网中PCF机制的实时性能研究与改进

针对MAC层公平性问题,本文对原有DCF机制进行了改进,根据节点所处的信道状态和其转发数据比例,动态调整竞争窗口大小,进行自适应二进制指数退避。就点协调功能(PCF)状态下点协调器(PC)对各个联入PC的站点分配媒体访问权的轮询机制进行分析并加以改进,提出一种令牌缓冲器的方法,仿真结果表明,改进PCF和DCF机制与IEEE802.11协议采用的退避算法相比,在信道公平性方面有较好的改善,并且具有较高的实时性。     

IEEE802．11MAC层性能分析及改进方案

IEEE802．11作为无线局域网的MAC层规范而被广泛研究。介绍了IEEES02．11MAC协议中最基本的功能-DCF（分布式协调功能）的访问机制。在DCF研究的基础上对MAC层的性能进行了研究和分析,并针对802．11DCF的退避机制问题提出改进方案。     

IEEE 802.11e基于竞争的信道访问机制性能分析

在回顾了IEEE802．11MAC协议的基础上，讨论并分析了即将定稿的IEEE802．11e的基于竞争的信道访问工作机制下MAC协议对QoS的支持。     

基于对数函数的Ad Hoe网络MAC退避算法

针对IEEE802．11MAC协议二进制退避算法存在的缺陷,引入随网络状态变化的参数,提出了一种基于对数函数的新退避算法。该算法使用以网络节点数为变量的对数函数,动态调整竞争窗口初始值和窗口退避增大幅度,降低了数据传输的冲突概率,利用OPNET软件实现对退避算法的网络仿真。仿真结果表明,基于对数函数的退避算法对提高AdHoe网络的吞吐量性能和公平性、降低网络延时有明显的效果,性能优于二进制退避算法。     

一种基于IEEE802.11的无线自组网MAC协议

基于IEEE802.11DCF提出了一种应用于无线自组网的媒质接入控制协议，该协议包括网络适应性退避算法和拥塞反馈两个关键机制。协议的主要思想是根据节点周围实际竞争状况和网络拥塞情况进行包调度。仿真结果表明，提出的机制有效地降低了平均端到端时延和数据丢包率,提高了信道接入公平性。     

一种自适应无线局域网协议

研究了无线局域网IEEE802.11协议的DCF(distributed coordination function)接入方式,在建模基础上进行算法改进,提出了一种新的节点自适应链路碰撞的退避算法NSAD(new self-adaptive DCF algorithm).大量的仿真实验表明,所提出的算法在吞吐量、公平性、丢包等方面较原DCF协议都有显著提高.     

在NS2中植入新的802．11接入算法的研究

IEEE802．11是目前广泛应用的无线局域网协议标准。它的MAC层协议主要解决在多个用户之间分配信道资源的问题，通过有效的介质访问机制，提高资源的使用率和网络的性能。     

一种高吞吐量的IEEE 802．11Mesh网AP选择算法

现有的IEEE802．11mesh网访问点（AP）选择算法仅仅基于对mesh用户周围链路质量的测量，无法使用户获得高性能的Internet访问。本文提出了一种基于期望传输吞吐量（Expected Transmission Throughput，EXT）的AP选择算法，该算法同时考虑了mesh路由器访问网关的能力以及用户与路由器的连接时间，目的是使用户选择能够获得最大吞吐量的路由器进行连接。同时，本文针对mesh网络特点和用户的移动方式给出了实际可行的算法实现过程。本文使用NS2对算法进行了仿真，结果证明与传统的基于接收信号强度指示（RSSI）的算法相比，基于EXT的AP选择算法可以使用户的访问吞吐量得到较大提高。     

Enhanced handoff scheme based on efficient uplink quality estimation in LTE-Advanced system

Asymmetric traffics cause downlink–uplink asymmetric interference. It can lead to a critical unbalance between a downlink and an uplink channel qualities in cell edge areas. This paper proposes an enhanced handoff scheme including an efficient uplink channel estimation method. The proposed handoff scheme determines an appropriate handoff-timing and handoff-direction according to an estimated uplink channel quality and a measured downlink channel quality. In the proposed scheme, an uplink or downlink, whose quality dominantly affects a link failure, becomes the main handoff-criterion. An efficient uplink channel estimation method is also proposed to exploit an uplink channel quality in handoff. The proposed method estimates an uplink signal strength from the measured downlink signal strength and predicts an uplink interference based on the interference-level information from neighbor base stations. We propose two different uplink estimation modes such as the simple and the precise modes. The simple estimation mode calculates the uplink channel quality for the overall bandwidth for a general handoff process. The precise estimation mode finds the best uplink band for the handoff user who wants an elaborate handoff process. In the elaborate handoff, a target base station allocates the best uplink band to a handoff user to provide the better uplink channel quality. Simulation results show that the proposed uplink estimation method can accurately compute the uplink channel quality of neighbor cells where the estimation error rate is less than 0.7% The simulation results also show that the proposed handoff scheme reduces handoff-call-dropping probability by up to 69% compared to LTE-Advanced system. In addition, the end-to-end delay of the proposed scheme can be better than that of LTE-Advanced system by 26%.     

Slot allocation algorithms in centralized scheduling scheme for IEEE 802.16 based wireless mesh networks

In IEEE 802.16 based wireless mesh networks (WMNs), TDMA (Time Division Multiple Access) is employed as the channel access method and only TDD (Time Division Duplex) is supported and there are no clearly separate downlink and uplink subframes in the physical frame structure. As the uplink and downlink traffic has different characteristics in that the uplink traffic decentralizes in each MSS (Mesh Subscriber Station) and the downlink traffic centralizes in the MBS (Mesh Base Station), different scheduling methods should be taken in the uplink and downlink. This paper presents a uniform slot allocation algorithm which is suitable for both uplinks and downlinks. To achieve higher spatial reuse and greater throughput and to avoid switching frequently between receiving and transmitting within two adjacent time slots when a relay node forwards traffic, different link selection criteria are taken into account when allocating slots for uplinks and downlinks. A combined uplink and downlink slot allocation algorithm is proposed for further improving the spatial reuse and network throughput. The proposed algorithms are evaluated by extensive simulations and the results show that it has good performance in terms of spatial reuse and network throughput. To the best of the authors’ knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.     

NOMA双连接异构网络条件解耦设计架构及性能分析

为了改善异构网络上行链路的性能及提高网络覆盖概率,提出了采用解耦上下行链路级联和双连接方案的NOMA异构网络模型,设计了一种条件解耦上下行链路级联分析框架。同时,考虑了3种可能的上行链路级联场景,在给定的下行链路级联条件下对解耦上行链路级联进行设计和分析,推导了解耦上行链路级联概率表达式,获得了主、次接入距离统计描述。数值和仿真分析首先研究了系统参数对级联概率的影响,分别研究了解耦上下行链路级联和耦合上下行链路级联方案下NOMA异构网络的条件覆盖概率和频谱效率。实验结果表明,在NOMA双连接异构网络中,所提出的DUDA方案的覆盖概率性能优于传统的CUDA方案的。此外,2种方案的频谱效率性能好坏与系统参数相关。     

TDD中继系统下行链路自适应方法

为了解决3GPP LTE-A系统中Type-Ⅱ中继不发送下行参考信号导致基站无法获得中继到用户之间的链路质量信息、无法合理选择下行数据调制编码格式的问题,提出了一种TDD中继系统中新的下行链路自适应方法.该方法利用TDD系统上下行信道互惠性和UE周期性发送的上行探测信号更准确的进行信道估计,继而选择更合适的调制编码格式.给出了工作过程及发送接收算法模型,并进而提出基站与中继协作地采用发送分集技术传输下行数据.仿真实验结果表明,新的链路自适应方法及基站与中继发送分集的传输方式,显著地提高了系统频谱利用率.     

Using traffic asymmetry to enhance TCP performance

A wealth of recent work has gone into optimizing the performance of Transmission Control Protocol (TCP) on the downlink channel of wireless networks such as for example, honing its congestion awareness mechanism so that it is minimally affected by random wireless losses, and optimizing achieved fairness of the end-to-end TCP rates. Other work has gone into balancing the allocation of a shared resource between the downlink and uplink in order to optimize TCP performance. We build on such previous research by proposing a cross-layer algorithm for resource allocation in OFDMA systems aiming not only to achieve optimal throughput for competing TCP flows but also to allocate resources appropriately between the downlink and uplink. This is important due to the increasing number of Internet applications where the mobile terminal is the TCP sender (social networking, peer-to-peer, etc.). Therefore, our scheme makes use of the asymmetry in the traffic and by defining the boundary between downlink and uplink capacity dynamically, enhance the TCP performance. Through numerical investigations we show the performance of the proposed scheme in terms of achieved fairness to the receivers and efficient allocation of downlink to uplink ratios based on the TCP traffic.     

Optimal timeslot and channel allocation considering fairness for multicell CDMA/TDD systems

The CDMA/TDD system is a highly attractive solution to support the next generation cellular mobile systems which provide unbalanced multimedia services between downlink and uplink. In this paper, we analyze the interference for downlink and uplink timeslots in a multicell CDMA/TDD system. We also mathematically formulate an optimal timeslot and channel allocation problem considering capacity fairness among cells, which is to maximize the system capacity under the given traffic unbalance, and propose an efficient algorithm based on the simulated annealing technique. Extensive experimental results show that the proposed scheme yields a good performance, and fairness among cells improves with a decrease in the system capacity.     

车联网RSU单元下行流量的性能研究

基于802.11p/WAVE的车联网是一项涉及道路交通、无线通信、自组织系统等多学科综合性的技术领域,车联网中,RSU单元可作为AP,让在行驶的汽车中的人们可以随时随地接入Internet,其中,如何建立RSU单元的下行流量模型是一个关键问题.通过引入车辆密度概率质量函数,分析车联网MAC层车辆通过RSU设备访问Internet的信道竞争情形,提出了适用于高速公路交通场景的RSU单元网络吞吐量模型,并推导出RSU上行及下行流量的性能解析式.针对RSU单元通信范围有限、相邻RSU单元之间可能存在盲区的问题,提出了一种利用同向行驶的车辆协助数据下载的VCoDS方案,以达到提高某时间段RSU的下行流量的目标.仿真实验结果证实,VCoDS方案可有效提高RSU单元的下行流量.     

Highest Urgency First (HUF): A latency and modulation aware bandwidth allocation algorithm for WiMAX base stations

The mobile WiMAX systems based on IEEE 802.16e-2005 provide high data rate for mobile wireless networks. However, the link quality is frequently unstable owing to mobility and air interference and therefore impacts the latency requirement of real-time applications. In the WiMAX standard, the modulation/coding scheme and the boundary of uplink/downlink sub-frames could be adjusted subject to channel quality and the traffic volume, respectively. This provides us a chance to design a MAC-layer uplink/downlink bandwidth allocation algorithm that is QoS/PHY-aware.This work takes into account the adaptive modulation and coding scheme (MCS), uplink and downlink traffic volume, and QoS parameters of all five defined service classes to design a bandwidth allocation algorithm that calculates the slot allocation in two phases. The first phase decides the boundary of uplink and downlink sub-frames by satisfying requests with pending latency violation and proportionating according to traffic volume, while the second phase allocates slots to mobile stations considering urgency, priority and fairness. Simulation results show our algorithm achieves zero latency violation and higher system throughput compared to existing non-QoS/PHY-aware or less-QoS/PHY-aware approaches.     

Design of efficiently encodable nonbinary LDPC codes for adaptive coded modulation

In multi-pair two-way relay networks,the control over inter-pair-interference is a challenging problem.When mobile stations(MSs)cooperate fully,to achieve the sum capacity of networks and at the same time completely remove the inter-pair-interference,a singular value decomposition(SVD)based beamforming scheme at relay station(RS)is established where SVD beamformers are applied over both uplink and downlink channels,respectively.To avoid any cooperation among users,an efcient beamforming scheme is proposed.This scheme uses the Max-SINR based receive beamformer at RS in uplink channel and the Max-SLNR based transmit precoder at RS in downlink channel.It omits interference(uplink)and leakage(donwlink)inside each pair of users due to their abilities of self-interference cancellation.This beamforming scheme provides an analytical solution like some well-known linear beamformers such as zero-forcing,block diagonalization,etc.From simulation,we find:the proposed SVD shows an excellent sum-rate performance at the expense of a large cooperation overhead at MSs and can be used as a tighter upper bound of sum-rate;the proposed scheme of Max-SINR beamformer plus Max-SLNR precoder performs much better on sum-rate than the ZF-based and the BD-based schemes by using almost the same computational amount.