基于多信道预约可冲突避免的多址接入协议

本文为多跳分布多无线网络提出了一套灵活而有效的自适应获取冲突避免（AACA）的多址接入协议。在该协议中，各节点竞争公共信道，利用RTS/CTS对话形式来预约各业务信道，成功预约后的分组传输不会受到其他节点的干扰。它使用任意确定数目信道，以异步方式工作，并且使得各节点利用半双工无线电台就可以灵活、简便地实现资源预约。分析和比较结果说明，所提出的多信道预约协议可以有效地解决隐藏终端、暴露终端以及侵入终端的问题。     

A k-Round Elimination Contention Scheme for WLANs

The contention resolution scheme is a key component in carrier-sense-based wireless MAC protocols. It has a major impact on MAC'S performance metrics such as throughput, delay, and jitter. The IEEE 802.11 DCF adopts a simple contention resolution scheme, namely, the binary exponential backoff (BEB) scheme. The BEB scheme achieves a reasonable performance for transmitting best-effort packets in small-sized wireless networks. However, as the network size increases, it suffers from inefficiency because of the medium contention, which leads to reduced performance. The main reason is that the BEB mechanism incurs an ever- increasing collision rate as the number of contending nodes increases. We devise a novel contention resolution scheme, a k-round elimination contention (k-EC) scheme. The k-EC scheme exhibits high efficiency and robustness during the collision resolution. More importantly, it is insensitive to the number of contending nodes. This feature makes it feasible for use in networks of different sizes. Simulation results show that the k-EC scheme offers a powerful remedy to medium contention resolution. It significantly outperforms the IEEE 802.11 DCF scheme in all the MAC'S performance metrics and also exhibits better fairness.     

A Limited Deflection Routing Algorithm Based on Burst Loss Threshold in OBS Networks

1 Introduction The increase in the demand of transport capacity duetothe explosive growth of the Internet IP-based traffichas fueled the development of high-speed transmissionsystems andthe emergence of Wavelength Division Mul-tiplexing ( WDM) technology[1].Soin the near future ,it will be possible to support hundreds of wavelengths ofseveral Gigabit/s each. A new approach called OpticalBurst Switching (OBS)[2]that combines the best of op-tical circuit switching and optical packet switch…     

无线传感器网络中基于NDMA的跨层协作多包接收

本文研究了协作分集下的NDMA(网络辅助分集多址接入)机制,针对无线传感器网络特点和信道矩阵满秩性要求设计中继选择准则,提出了一个新的跨层协作多包接收机制.该机制在抗信道衰落的同时可有效限制数据包重传次数,从而大大提高多包接收的效率.对新机制的性能仿真以及该机制与NDMA、联合NDMA、时隙ALOHA之间的性能对比证实了新机制的有效性.     

Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs

Many rate-adaptive MAC protocols have been proposed in the past for wireless local area networks (LANs) to enhance the throughput based on channel information. Most of these protocols are receiver based and employ the RTS/CTS collision avoidance handshake specified in the IEEE 802.11 standard. However, these protocols have not considered the possibility of bursty transmission of fragments in the corresponding rate adaptation schemes. In this article, a rate-adaptive protocol with dynamic fragmentation is proposed to enhance the throughput based on fragment transmission bursts and channel information. Instead of using one fragmentation threshold in the IEEE 802.11 standard, we propose to use multiple thresholds for different data rates so more data can be transmitted at higher data rates when the channel is good. In our proposed scheme, whenever the rate for the next transmission is chosen based on the channel information from the previous fragment transmission, a new fragment is then generated using the fragment threshold for the new rate. In this way, the channel condition can be more effectively used to squeeze more bits into the medium. We evaluate this scheme under a time-correlated fading channel model and show that the proposed scheme achieves much higher throughput than other rate-adaptive protocols.     

Enhancing IEEE 802.11 MAC via a Sender-Initiated Reservation

The IEEE 802.11 MAC adopted a collision avoidance mechanism in which contending stations should wait a random backoff time before sending a frame. While the algorithm reduces the collision probability in general, a large number of stations may still experience heavy collisions thus decrease the throughput. In this paper, we propose a simple reservation scheme for enhancing the performance of multiple access in 802.11 MAC: when a transmitter sends a frame, if it has another frame to send in its output queue, it may reserve an additional time that is needed to send the next frame and receive an ACK for the frame. Thus a sender can occupy the medium for two data frames, while reducing the collision probability and improving channel utilization via the reservation. We develop a mathematical model to analyze the performance of proposed scheme, and perform simulations to evaluate its performance compared with the original MAC.     

Collision-aware design of rate adaptation for multi-rate 802.11 WLANs

One of the key challenges in designing a rate adaptation scheme for IEEE 802.11 wireless LANs (WLANs) is to differentiate bit errors from link-layer collisions. Many recent rate adaptation schemes adopt the RTS/CTS mechanism to prevent collision losses from triggering unnecessary rate decrease. However, the RTS/CTS handshake incurs significant overhead and is rarely activated in today's infrastructure WLANs. In this paper we propose a new rate adaptation scheme that mitigates the collision effect on the operation of rate adaptation. In contrast to previous approaches adopting fixed rate-increasing and decreasing thresholds, our scheme varies threshold values based on the measured network status. Using the "retry" information in 802.11 MAC headers as feedback, we enable the transmitter to gauge current network state. The proposed rate adaptation scheme does not require additional probing overhead incurred by RTS/CTS exchanges and can be easily deployed without changes in firmware. We demonstrate the effectiveness of our solution by comparing with existing approaches through extensive simulations.     

An Efficient Backoff Algorithm for IEEE 802.15.4 Wireless Sensor Networks

IEEE 802.15.4 is one of the most prominent MAC protocol standard designed to achieve low-power, low-cost, and low-rate wireless personal area networks. The contention access period of IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA) algorithm. A long random backoff time causes longer average delay, while a small one gives a high collision rate. In this paper, we propose an efficient backoff algorithm, called EBA-15.4MAC that enhances the performance of slotted CSMA/CA algorithm. EBA-15.4MAC is designed based on two new techniques; firstly, it updates the contention window size based on the probability of collision parameter. Secondly, EBA-15.4MAC resolves the problem of access collision via the deployment of a novel Temporary Backoff (TB) and Next Temporary Backoff (NTB). In this case, the nodes not choose backoff exponent randomly as mentioned in the standard but they select TB and NTB values which can be 10–50 % of the actual backoff delay selected by the node randomly. By using these two new methods, EBA-15.4MAC minimizes the level of collision since the probability of two nodes selecting the same backoff period will be low. To evaluate the performance of EBA-15.4MAC mechanism, the network simulator has been conducted. Simulation results demonstrate that the proposed scheme significantly improves the throughput, delivery ratio, power consumption and average delay.     

A smart exponential‐threshold‐linear backoff mechanism for IEEE 802.11 WLANs

Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff mechanism, called Smart Exponential‐Threshold‐Linear (SETL) Backoff Mechanism, to enhance the system performance of contention‐based wireless networks. In the IEEE 802.11 DCF scheme, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the proposed SETL scheme, a threshold is set to determine the behavior of CW after each transmission. When the CW is smaller than the threshold, the CW of a competing station is exponentially adjusted to lower collision probability. Conversely, if the CW is larger than the threshold, the CW size is tuned linearly to prevent large transmission delay. Through extensive simulations, the results show that the proposed SETL scheme provides a better system throughput and lower collision rate in both light and heavy network loads than the related backoff algorithm schemes, including Binary Exponential Backoff (BEB), Exponential Increase Exponential Decrease (EIED) and Linear Increase Linear Decrease (LILD). Copyright © 2011 John Wiley & Sons, Ltd.     

Estimation Algorithm of Contending Stations Based on Improved DCF Model in IEEE 802.11

The fundamental access method of IEEE 802.11 is a DCF known as carrier sense multiple access with collision avoidance (CSMA/CA) scheme with exponential back-off. RTS_threshold is used to determine whether to deploy RTS/CTS access method. This threshold should vary with the number of contending stations which contend wireless media to get better throughput. The paper proposes an algorithm which estimates the number of contending stations in BSS. The algorithm is shown to be accurate which is verified by elaborate simulations.     

Achieving performance enhancement in IEEE 802.11 WLANs by using the DIDD backoff mechanism

Wireless local area networks (WLANs) based on the IEEE 802.11 standards have been widely implemented mainly because of their easy deployment and low cost. The IEEE 802.11 collision avoidance procedures utilize the binary exponential backoff (BEB) scheme that reduces the collision probability by doubling the contention window after a packet collision. In this paper, we propose an easy‐to‐implement and effective contention window‐resetting scheme, called double increment double decrement (DIDD), in order to enhance the performance of IEEE 802.11 WLANs. DIDD is simple, fully compatible with IEEE 802.11 and does not require any estimation of the number of contending wireless stations. We develop an alternative mathematical analysis for the proposed DIDD scheme that is based on elementary conditional probability arguments rather than bi‐dimensional Markov chains that have been extensively utilized in the literature. We carry out a detailed performance study and we identify the improvement of DIDD comparing to the legacy BEB for both basic access and request‐to‐send/clear‐to‐send (RTS/CTS) medium access mechanisms. Copyright © 2006 John Wiley & Sons, Ltd.     

Design and performance analysis on adaptive reservation-assisted collision resolution protocol for WLANs

In conventional IEEE 802.11 medium access control protocol, the distributed coordination function is designed for the wireless stations (WSs) to perform channel contention within the wireless local area networks (WLANs). Packet collision is considered one of the major issues within this type of contention-based scheme, which can severely degrade network performance for the WLANs. Research work has been conducted to modify the random backoff mechanism in order to alleviate the packet collision problem while the WSs are contending for channel access. However, most of the existing work can only provide limited throughput enhancement under specific number of WSs within the network. In this paper, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed to improve packet collision resulting from the random access schemes. With its adaptable reservation period, the contention-based channel access can be adaptively transformed into a reservation-based system if there are pending packets required to be transmitted between the WSs and the access point. Analytical model is derived for the proposed ARCR scheme in order to evaluate and validate its throughput performance. It can be observed from both analytical and simulation results that the proposed protocol outperforms existing schemes with enhanced channel utilization and network throughput.     

Carrier sense multiple access with improvised collision avoidance and short-term fairness

In this paper, we present a simple method to simultaneously enhance collision avoidance efficiency and short-term fairness of a most popular contention based medium access control protocol, carrier sense multiple access with collision avoidance. The key idea here is to adaptively tune the shape of contention slot selection distribution over the temporal contention window during ongoing collision resolution process which, in the legacy scheme, used to be flat throughout. The tuning mechanism is such designed that it not only maximizes the selection likelihood of relatively less collision prone contention slots over the contention window but also compensates the idle delay that the contending stations have suffered in their recent access attempt. Through rigorous numerical and simulation based analysis, the proposed scheme is shown to enhance the performance of a IEEE 802.11 based distributed wireless network in terms of network throughput efficiency and packet transmission delay while allowing individual stations to share the channel fairly even in short time scale.     

Avoiding collision with hidden nodes in IEEE 802.11 wireless networks

In wireless networks, collision is a major factor of performance degradation. In this letter, we propose a scheme for reducing collision in IEEE 802.11 networks. Each node can avoid collision by maintaining a disjoint set of time slots for transmission. Through simulation, we show that the proposed scheme is effective to reduce collision even in the presence of hidden nodes.     

一种基于IEEE 802.11的多速率自适应MAC协议

提出了一种新颖的基于连续ACK帧统计信息的IEEE 802.11多速率自适应MAC协议EARF(EnhancedARF),其主要思想是:每一个速率有各自的成功阈值——速率升高的门限值,并且该值根据信道状况(用延时因子量化)动态地变化。协议不需对现有的IEEE 802.11标准做任何修改,因此易于通过编写驱动程序实现。仿真表明在大多数信道条件下,该协议性能较现有的基于ACK帧统计的速率自适应协议如ARF,ARF3-10都有较大的提高。     

Mitigation of Packet Loss Using Data Rate Adaptation Scheme in MANETs

Node’s mobility, bursty data traffic, and dynamic nature of the network make congestion avoidance and control a challenging task in Mobile Adhoc Networks (MANETs). Congestion results in high packet loss rate, increased delays, and wastage of network resources due to re-transmissions. In this paper, we propose In-route data rate adaptation to avoid packet loss. Proposed scheme is based on the analysis of queue length of the forwarding nodes, number of data source nodes, and rate of link changes. In proposed technique, queue length of forwarding nodes is communicated periodically to the neighbor nodes using existing control messages of the underlying routing protocol. Keeping in view the queue length of forwarding nodes, number of data source nodes, and rate of link changes, initially the intermediate nodes buffer the incoming data packets upto some threshold and then, gradually shift the effect of congestion to the data source nodes. Then, the source node adapts its sending data rate to avoid congestion and to ensure reliable data communication. We have performed simulations in NS-2 simulator by varying different network metrics such as data rate, number of source nodes, and node speed. Results show that proposed technique improves network performance in terms of packet delivery ratio upto 15 %, reduction of average end-to-end delay and packet loss due to interface queue overflow upto 25 % and 14 % respectively, as compared to the static rate adaptation scheme.     

Efficient Back-off Mechanism for Multimedia Support in 802.11e EDCA Wireless Ad-Hoc Networks

This paper presents an effective back-off mechanism to improve quality of service (QoS) for multimedia applications over the IEEE 802.11e enhanced distribution channel access (EDCA) standard. It can be done with proposed algorithm called dynamic fast adaptation for contention-based EDCA (DFA-EDCA) mechanism. The main concept of proposed DFA-EDCA mechanism is to use the exponential functions to adaptively tune the back-off parameters in IEEE 802.11e EDCA according to the changes on a network load within a short period. In this proposed mechanism an intra-access category (intra-AC) differentiation mechanism is provided to increase its back-off time randomly and achieve discrimination of same priority traffic on different stations. The performance evaluations have been conducted by using Network Simulator (NS-2). The simulation results show that the proposed DFA-EDCA mechanism has greatly outperformed the previous mechanisms such as non-linear dynamic adaptation scheme of the minimum contention window (CW_min-HA), dynamic adaptation algorithm of the maximum contention window (CW_max-adaptation), adaptive enhanced distributed coordination function and the conventional EDCA in terms of goodput, gain of goodput, packet delay, collision rate and channel utilization ratio (CUR). It has significantly reduced both packet delay and collision rate simultaneously together with an obviously increment in both goodput and CUR, which lead to the improvement in QoS for multimedia applications.     

Decentralized multiuser diversity with opportunistic packet transmission in MIMO wireless sensor networks

In this study, we consider a single-hop wireless sensor network where both the sensor nodes and the controller node have multiple antennas. We focus on single beam opportunistic communication and propose a threshold-based medium access control (MAC) scheme for uplink packet transmission which exploits multiuser diversity gain without feedback in a decentralized manner. Packet transfer from sensor nodes to the controller node is initiated when the channel quality of any node exceeds the predefined threshold based on the effective signal-to-noise ratio (ESNR) measurements at the sensor nodes through linear combining techniques. The optimum threshold is determined to maximize the probability of successful packet transmission where only one sensor node transmits its packet in one time-slot. The proposed scheme trades the successful packet rate to increase the SNR of the successful packets assuming Rayleigh fading and collision-based reception model. Computer simulations confirm that proposed scheme has higher successful packet SNR compared to the simple time division multiple access (TDMA)-based MAC scheme with round-robin fashion. The use of multiple antennas at the sensor nodes can also improve the throughput of proposed scheme compared with our previous scheme without implementing the spatial diversity at the SNs.     

NOMA系统中基于协作干扰的物理层安全方案

针对NOMA系统中的信息安全传输问题,提出基于协作干扰的物理层安全方案,该方案利用外部协作节点发送干扰信号恶化窃听者的接收性能。本文首先利用泊松点过程对协作节点和窃听者的位置进行安全建模,然后推导了随机协作干扰方案下系统保密中断概率的闭式解,但随机协作干扰方案会同时恶化合法用户的接收信号,性能有限,因此提出一种信道增益受限的协作干扰方案,该方案选择到近端用户和远端用户的瑞利信道因子均小于一定阈值的协作节点发送干扰信号,会对窃听者造成更严重的影响。仿真结果表明,在NOMA系统中采用受限的协作干扰方案能够有效提升系统安全性能。      

Generic stationary backoff distributions for distributed multiple access control

In this paper, we study the characteristics of two different backoff schemes: one that maximizes the channel utilization and one that maximizes the probability of a successful transmission. Our results indicate that while the latter provides slightly lower channel utilization, its shape is much less sensitive to the number of contending nodes. That is, the channel utilization is kept almost constant for a very wide range of node densities if the backoff distribution has increasing shape. This motivates us to propose a simple heuristic backoff scheme called the Truncated Geometric Backoff Distribution (TGBD). We provide simple analytical expressions for the probability of a successful transmission and the channel utilization. We also show that the TGBD can easily be extended to support service differentiation by adjusting the window lengths, and we provide a simple approximation that gives the relative share of the capacity for a node in a priority class compared to nodes in other classes. This extended backoff scheme easily outperforms the much more complex Quality of Service (QoS) standard, IEEE802.11e EDCA. Finally, a two-stage backoff model, based on the TGBD, is proposed that further increases the probability of a successful transmission. Results indicate that the channel utilization is almost independent of the number of contending nodes.