Spare node cooperative method for IEEE 802.11 networks

Most of the existing cooperation methods select relay node(s) mainly based on the channel state information, but do not consider whether the selected relay nodes work or not. If the selected relays are invalidated, the performance of cooperative communication will deteriorate. To resolve the above problem, this paper investigates cooperative communication in IEEE 802.11 networks, and proposes a novel Spare Cooperative Method (SCM). SCM chooses a spare cooperation node to enhance the reliability of communication, and uses an enhanced handshaking mechanism to coordinate the access of source nodes and cooperation nodes to the wireless channel. The performance of SCM is comprehensively analyzed in terms of outage probability and saturated throughput. The analysis shows that SCM improves the performance of IEEE 802.11.     

Ad Hoc网络中一种具有优先控制的自适应协同MAC协议

ARQ(Auto Repeat reQuest)机制通常用来在传输失败后重传数据,但是在深衰落信道下,源节点多次重传有可能导致系统性能持续变坏,作为未来无线通信的重要技术,协同通信能够大大提高链路的可靠性,可以有效解决上述问题。本文针对Ad Hoc网络,提出了一种基于优先控制的自适应协同MAC(Media Access Control)协议。此协议采用ARQ机制,根据瞬时信道情况,自适应地选择信道质量更好的中继节点来重传数据,并且在优化的退避算法和中继选择算法的控制下,赋予中继节点相对源节点更高的优先级,协议中还在传统RTS/CTS分组交互基础上加入了新的握手交互机制,使得协议的性能进一步提升。本文还对经典二维Markov模型进行了改进,加入了协同机制并提出中继节点冲突模型,对所提协议进行了建模并推导出了吞吐量性能表达式。仿真结果表明,本文所提协议相对IEEE 80211 DCF协议和传统采用ARQ机制的协同MAC协议有着更好的性能。
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An efficient MAC protocol for cooperative diversity in mobile ad hoc networks

Cooperative diversity is proposed to combat the detrimental effects of channel fading. In this paper, we investigate the effectiveness of cooperative diversity in interference limited ad hoc networks. The negative effects due to relay blocking on the network throughput are investigated. We show that the relay blocking problem is mainly dependent on the relay selection criterion. To overcome this problem, we propose a new cooperative diversity technique based on a modified IEEE 802.11 Medium Access Control (MAC) protocol. The throughput performance of the proposed MAC protocol is analyzed using a random structured network where nodes are assumed to be equipped with multiple antennas. In our simulations, we consider both single‐ and multiple‐relay scenarios over fading channels. Copyright © 2007 John Wiley & Sons, Ltd.     

无线Ad Hoc网络中基于业务感知的多信道MAC协议(英文)

Existing multi-channel Medium Access Control (MAC) protocols have been demonstrated to significantly increase wireless network performance compared to single channel MAC protocols. Traditionally, the channelization structure in IEEE 802.11 based wireless networks is pre-configured, and the entire available spectrum is divided into subchannels and equal channel widths. In contrast, this paper presents a Traffic-Aware Channelization MAC (TAC-MAC) protocol for wireless ad hoc networks, where each node is equipped with a single half duplex transceiver. TAC-MAC works in a distributed, fine-grai-ned manner, which dynamically divides variable-width subchannels and allocates subchannel width based on the Orthogonal Frequency Division Multiplexing (OFDM) technique according to the traffic demands of nodes. Simulations show that the TAC-MAC can significantly improve network throughput and reduce packet delay compared with both fixed-width multi-channel MAC and single channel 802.11 protocols, which illustrates a new paradigm for high-efficient multi-channel MAC design in wireless ad hoc networks.     

Analysis and determination of cooperative MAC strategies from throughput perspectives

In recent years, cooperative communication has been developed as a new communication strategy that incorporates a relay node to assist direct point-to-point transmission. By exploiting cooperative diversity, different types of techniques have been proposed to improve transmission reliability from the physical layer perspective. However, owing to the longer transmission time resulting from the cooperative schemes, there is no guarantee to enhance network throughput in view of the medium access control (MAC) performance. In this paper, system throughput of combined direct/cooperative communication is evaluated by exploiting the proposed analytical model based on the IEEE 802.11 MAC protocol. The feasibility of adopting either cooperative or direct communication is also studied in the analytical model. In terms of network throughput, whether to adopt cooperative schemes depends on the tradeoff between cooperative transmission delay and channel quality of direct communication. Moreover, two cooperative MAC protocols are proposed to determine the circumstances to activate cooperative communication according to the channel quality. The full-channel quality indicator based cooperative (FCC) MAC protocol is introduced to choose both the transmission scheme and the relay node according to the full channel quality information. However, the overhead caused by the FCC scheme can degrade the throughput performance as the number of available relays is significantly increased. Therefore, the bitwise competition based cooperative (BCC) MAC protocol is utilized to efficiently determine a feasible relay node for data transmission. Simulations are performed to validate the effectiveness of proposed analytical models and cooperative MAC protocols. It is observed that the proposed BCC scheme can outperform both the FCC protocol and conventional direct transmission with enhanced system throughput.     

Improving Spatial Reuse in Multihop Wireless Networks - A Survey

In multihop wireless ad-hoc networks, the medium access control (MAC) protocol plays a key role in coordinating the access to the shared medium among wireless nodes. Currently, the distributed coordination function (DCF) of the IEEE 802.11 is the dominant MAC protocol for both wireless LANs and wireless multihop ad hoc environment due to its simple implementation and distributed nature. The current access method of the IEEE 802.11 does not make efficient use of the shared channel due to its conservative approach in assessing the level of interference; this in turn affects the spatial reuse of the limited radio resources and highly affect the achieved throughput of a multihop wireless network. This paper surveys various methods that have been proposed in order to enhance the channel utilization by improving the spatial reuse.     

Simulation and analysis of node throughput using smart antenna in wireless mesh networks

Smart antenna technology is introduced to wireless mesh networks. Smart antennas based wider-range access medium access control (MAC) protocol (SWAMP) is used as MAC protocol for IEEE 802.11 mesh networks in this study. The calculation method of node throughput in chain and arbitrary topology is proposed under nodes fairness guarantee. Network scale and interference among nodes are key factors that influence node throughput. Node distribution pattern near the gateway also affects the node throughput. Experiment based on network simulator-2 (NS-2) simulation platform compares node throughput between smart antenna scenario and omni-antenna scenario. As smart antenna technology reduces the bottle collision domain, node throughput increases observably.     

Partition and Cooperation for Crowded Multi-rate WLANs

Recently, growth in smart devices has increased along with the number of wireless nodes in wireless local area networks (WLANs). The increased number of nodes causes throughput degradation due to frequent collisions between nodes. In multi-rate environments, transmission at low data rates limits the throughput of WLANs, and we call it performance anomaly. The time fairness approach has been proposed to resolve the performance anomaly. However, in the time fairness approach, nodes, which transmit at low data rates, experience lower throughput than in the IEEE 802.11 standard. In this paper, we propose a new medium access control (MAC) protocol, called partition and cooperation based MAC (PCMAC), to simultaneously solve these problems including frequent collisions, performance anomaly, and low throughput under time fairness scheme. First, we calculate the optimum number of nodes, which maximizes the data transmission time, for each data rate. Second, for resolving performance anomaly and reducing collisions, we divide the nodes into several partitions based on the data rate and the optimum number of nodes. Each partition has a different transmission time slot that is proportional to the number of nodes. Finally, we propose a cooperative communication scheme using a relay partition to improve the throughput of nodes with low data rates and to achieve robust cooperative transmission. PCMAC is evaluated through extensive simulation and simulation with measured data. The results show that our scheme effectively improves WLAN performance.     

Throughput analysis of IEEE 802.15.4 beacon-enabled MAC protocol in the presence of hidden nodes

The presence of hidden nodes degrades the performance of wireless networks due to an excessive amount of data frame collisions. The IEEE 802.15.4 medium access control (MAC) protocol, which is widely used in current wireless sensor networks, does not provide any hidden node avoidance mechanisms and consequently could lead to severe performance degradation in networks with hidden nodes. This paper presents a simple technique based on discrete-time Markov chain analysis to approximate the throughput of IEEE 802.15.4 MAC protocol in the presence of hidden nodes. Using different network configurations, we validate the applicability of the proposed analysis for generic star-topology networks. Based on the analysis, the effects of network size, topology, frame length and frame arrival rate on the throughput of the system are investigated.     

Trust establishment in cooperative wireless relaying networks

In cooperative wireless networks, relay nodes are employed to improve the performance of the network in terms of throughput and reliability. However, the presence of malicious relay nodes in the network may severely degrade the performance of the system. When a relay node behaves maliciously, there exists a possibility that such a node refuses to cooperate when it is selected for cooperation or deliberately drops the received packets. Trust establishment is a mechanism to detect misbehaving nodes in a network. In this paper, we propose a trust establishment method for cooperative wireless networks by using Bayesian framework. In contrast with the previous schemes proposed in wireless networks, this approach takes the channel state information and the relay selection decisions into account to derive a pure trust value for each relay node. The proposed method can be applied to any cooperative system with a general relay selection policy whose decisions in each cooperative transmission are independent of the previous ones. Moreover, it does not impose additional communication overhead on the system as it uses the available information in relay selection procedure. Copyright © 2012 John Wiley & Sons, Ltd.     

BTAC: A Busy Tone Based Cooperative MAC Protocol for Wireless Local Area Networks

Cooperative communications has been actively studied as an effective approach to achieve multi-user/spatial diversity gains and better overall system performance by coordinating multiple users in a dynamic wireless network to share their resources and capabilities. Based on the concept of cooperative communications, this paper proposes and analyzes a Busy Tone based cooperative Medium Access Control (MAC) protocol, namely BTAC, for multi-rate Wireless Local Area Networks (WLANs). A cross-layer Markov chain model is then developed to evaluate the performance of BTAC under dynamic wireless channel conditions. Analytical and simulation results show our BTAC protocol is simple, robust, fully compatible with the IEEE 802.11b standard and can achieve better throughput and delay performance than the standard Distributed Coordination Function (DCF) protocol and the recently-proposed CoopMAC protocol.     

一种自组网功率控制算法及分析

无线自组网中的移动节点大多依靠电池提供能量,因此能量是影响无线自组网性能的一个很大的瓶颈,作为事实上的无线自组网媒体接入协议,802.11并没有动态调整传输功率的能力,大大限制了网络的生存时间。采用功率控制可以提高节点的功率使用效率,减少相邻节点间的干扰,改善网络的性能。在802.11基础上提出一种基于信噪比的动态传输功率控制算法。通过进行计算机仿真,与802.11协议相比,在保持吞吐量性能的前提下,大大减少了节点的功率消耗,提高了节点的能量利用率。     

基于IEEE 802.11高速无线局域网的速率自适应MAC协议研究

目前的IEEE 802.11标准在物理层提供了对多种发送速率的支持,然而在MAC层却没有规定速率自适应的方法。该文研究了高速IEEE 802.11 无线局域网中的速率自适应方案。首先,提出了EACK协议,EACK使用基本速率发送MAC头,并在ACK帧中携带信道信息,因而能够较快速地响应信道的变化,同时具有少的开销;其次,在EACK基础上,提出了一种恒定发送时间(CEACK)的策略,CEACK能够克服传统IEEE 802.11 DCF MAC协议的理论吞吐量上限,并且具有更好的时间公平性能,能够应用于高速的无线局域网。     

Cooperative relay to improve diversity in cognitive radio networks

Recent studies demonstrated that dynamic spectrum access can improve spectrum utilization significantly by allowing secondary unlicensed users to dynamically share the spectrum that is not used by the primary licensed users. Cognitive radio was proposed to promote the spectrum utilization by opportunistically exploiting the existence of spectrum ?holes.? Meanwhile, cooperative relay technology is regarded widely as a key technology for increasing transmission diversity gain in various types of wireless networks, including cognitive radio networks. In this article, we first give a brief overview of the envisioned applications of: cooperative relay technology to CRNs, cooperative transmission of primary traffic by secondary users, cooperative transmission between secondary nodes to improve spatial diversity, and cooperative relay between secondary nodes to improve spectrum diversity. As the latter is a new direction, in this article we focus on this scenario and investigate a simple wireless network, where a spectrum-rich node is selected as the relay node to improve the performance between the source and the destination. With the introduction of cooperative relay, many unique problems should be considered, especially the issue for relay selection and spectrum allocation. To demonstrate the feasibility and performance of cooperative relay for cognitive radio, a new MAC protocol was proposed and implemented in a universal software radio peripheral-based testbed. Experimental results show that the throughput of the whole system is greatly increased by exploiting the benefit of cooperative relay.     

Mobility impact in IEEE 802.11p infrastructureless vehicular networks

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. In this paper, we provide a comprehensive evaluation of mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay. An unfairness problem due to the relative speed is identified for both broadcast and unicast scenarios. We propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. The first scheme provides dynamic level of service priority via adaptation to the number of neighboring nodes, while the second scheme provides service priority based on node relative speed. Extensive simulation results demonstrate a significant impact of mobility on the IEEE 802.11p MAC performance, the unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.     

Relay-volunteered multi-rate cooperative MAC protocol for IEEE 802.11 WLANs

In IEEE 802.11, the rate of a station (STA) is dynamically determined by link adaptation. Low-rate STAs tend to hog more channel time than high-rate STAs due to fair characteristics of carrier sense multiple access/collision avoidance, leading to overall throughput degradation. It can be improved by limiting the transmission opportunities of low-rate STAs by backoff parameters. This, however, may cause unfair transmission opportunities to low-rate STAs. In an attempt to increase overall throughput by volunteer high-rate relay STAs while maintaining fairness, we propose a new cooperative medium access control (MAC) protocol, relay-volunteered multi-rate cooperative MAC (RM-CMAC) based on ready to send/clear to send in multi-rate IEEE 802.11. In the RM-CMAC protocol, we show that the effect of hogging channel time by low-rate STAs can be remedied by controlling the initial backoff window size of low-rate STAs and the reduced transmission opportunity of low-rate STAs can be compensated by the help of volunteer high-rate relay STAs. We analyze the performance of RM-CMAC, i.e., throughput and MAC delay, by a multi-rate embedded Markov chain model. We demonstrate that our analysis is accurate and the RM-CMAC protocol enhances the network throughput and MAC delay while maintaining the fairness of low-rate STAs.     

High-performance architectures for IP-based multihop 802.11 networks

The concept of a forwarding node, which receives packets from upstream nodes and then transmits these packets to downstream nodes, is a key element of any multihop network, wired or wireless. While high-speed IP router architectures have been extensively studied for wired networks, the concept of a "wireless IP router" has not been addressed so far. We examine the limitations of the IEEE 802.11 MAC protocol in supporting a low-latency and high-throughput IP datapath comprising multiple wireless LAN hops. We first propose a wireless IP forwarding architecture that uses MPLS with modifications to 802.11 MAC to significantly improve packet forwarding efficiency. We then study further enhancements to 802.11 MAC that improve system throughput by allowing a larger number of concurrent packet transmissions in multihop 802.11-based IP networks. With 802.11 poised to be the dominant technology for wireless LANs, we believe a combined approach to MAC, packet forwarding, and transport layer protocols is needed to make high-performance multihop 802.11 networks practically viable.     

Distributed cooperative MAC for multihop wireless networks

This article investigates distributed cooperative medium access control protocol design for multihop wireless networks. Cooperative communication has been proposed recently as an effective way to mitigate channel impairments. With cooperation, single-antenna mobile terminals in a multi-user environment share antennas from other mobiles to generate a virtual multipleantenna system that achieves more reliable communication with a higher diversity gain. However, more mobiles conscribed for one communication inevitably induces complex medium access interactions, especially in multihop wireless ad hoc networks. To improve the network throughput and diversity gain simultaneously, we investigate the issues and challenges in designing an efficient MAC scheme for such networks. Furthermore, based on the IEEE 802.11 DCF, a cross-layer designed cooperative MAC protocol is proposed. The MAC scheme adapts to the channel condition and payload length.     

A novel IEEE 802.11‐based MAC protocol supporting cooperative communications

Cooperative diversity is a transmission technique, where multiple terminals form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. The concept of cooperation has already been introduced to MAC layer to design MAC protocol. But it does not take advantage of physical layer's cooperation. In this paper, we present a novel MAC protocol based on IEEE 802.11, called C‐MAC, which is able to support the basic building block of cooperative system. In other words, in C‐MAC, a source would invite a relay node into data transmission if there exits an available one. During data transmission, the source sends the signal to destination in the first time slot. The relay node will retransmit the overheard information to the destination in the second time slot. The destination combines two signals from the source and the helper to create the spatial diversity and robustness against channel fading. The C‐MAC is backward compatible to the legacy IEEE 802.11 system. The performance of C‐MAC mainly depends on physical layer's performance as it just provides the support for cooperation at the MAC layer. If the physical layer works well, C‐MAC would outperform IEEE 802.11 when considering packet error rate (PER). We also perform extensive simulation using ns‐2 with assumptive physical parameters. The results show that C‐MAC would outperform 802.11 if PER is over some threshold, e.g. when PER is 0.4, C‐MAC can achieve up to 11.5% higher throughput than IEEE 802.11. Copyright © 2011 John Wiley & Sons, Ltd.     

A utility-based resource allocation scheme for IEEE 802.11 WLANs via a machine-learning approach

The problem of allocating resources in IEEE 802.11 wireless local area networks (WLAN) is challenging due to limited bandwidth, time-varying channel conditions, and especially the distributed channel-access manner. In this paper we propose an intelligent resource allocation scheme that dynamically adjusts medium-access-control (MAC) parameters to tune channel-access opportunities and maximize the total utility. “Intelligent” refers to the capability of our approach to regulate each 802.11 node’s parameters automatically according to the changes of surrounding situations, e.g. channel conditions and number of nodes. Our intelligent allocation scheme uses neural networks to on-line learn the nonlinear function between the adopted MAC parameters and allocated throughput. Based on the learned knowledge, MAC parameters can therefore be dynamically adjusted toward the desired throughput allocation and consequently the maximal WLAN utility. Simulations results demonstrate the effectiveness of our allocation scheme in maximizing the system utility in a varying 802.11 WLAN environment.