Video quality based link adaptation for low latency video transmission over WLANs

Wireless Local Area Networks (WLANs) such as IEEE 802.11a/g and Hiperlan/2 utilise numerous transmission modes, each providing different throughputs and reliability levels. Many link adaptation algorithms proposed in the literature either maximise the error-free data throughput based on channel conditions or are based on the number of failed transmissions. However, these algo- rithms do not take into account the content of the data stream and strongly rely on the use of Automatic Repeat Requests (ARQs). Low latency video applications such as real-time video transmission may require no retransmission, or only a limited number of retrans- missions. Moreover, completely error-free communication is not essential, especially if robust video compression techniques are applied. In such scenarios, improved decoded video quality can be obtained with a video stream transmitted at a higher bit rate using a higher link speed but with some degree of transmission error, rather than an error-free video stream at a lower bit rate using a lower link speed. In this work, we investigate a link adaptation scheme that improves the Quality of Service (QoS) for video transmission, based on the overall received video quality (Peak Signal to Noise Ratio, PSNR), rather than by maximising the error-free throughput. We also study a practical link adaptation approach that uses PER thresholds at the PHY layer. An empirical study showed that thresholds for switching from one mode to another are much lower (almost error free) than those currently used by throughput based schemes. We show that traditional link adaptation strategies are not appropriate for real-time video transmission with no retransmis- sion. Simulation results using the H.264 video compression standard over IEEE 802.11a are presented.

D2D communication in a cellular network based on link adaptation with truncated ARQ

A device-to-device （D2D） communication mode underlaying cellular network in a single- cell environment is introduced. A practical method based on link adaptation with automatic repeat request （ARQ） is presented. Link adaptation technique, which combines adaptive modulation and coding （ AMC ） with truncated ARQ, can maximize the cellular UEs＇ data rate under prescribed delay and performance constraints. The proposed method can maximize the total transmission rate when an outage probability is determined. Numerical results show that with proper power control, the in- terference between the two links can be coordinated to increase the sum rate without overwhelming the cellular service.

A NOVEL LINK ADAPTATION SCHEME TO ENHANCE PERFORMANCE OF IEEE 802.11G WIRELESS LAN

A novel link adaptation scheme using linear Auto Regressive （AR） model channel estimation algorithm to enhance the performance of auto rate selection mechanism in IEEE 802.11g is proposed. This scheme can overcome the low efficiency caused by time interval between the time when Received Signal Strength （RSS） is measured and the time when rate is selected. The best rate is selected based on data payload length, frame retry count and the estimated RSS, which is estimated from recorded RSSs. Simulation results show that the proposed scheme enhances mean throughput performance up to 7%, in saturation state, and up to 24% in finite load state compared with those non-estimation schemes, performance enhancements in average drop rate and average number of transmission attempts per data frame delivery also validate the effectiveness of the proposed schelne.

Investigation of Adaptive Multi-Rate Link Adaptation of 3G System

In term of the features of 3G system, a novel AMR link adaptation strategy for 3G system is proposed. The impacts of AMR codec modes and power control on traffic quality of service are taken into account in the strategy at the same time. By synthetically comparing the signal-to-interference ratio value with the switching threshold and comparing the transmission power with its threshold, radio resource management can always keep each link on its proper codec mode with the corresponding optimal power level to achieve both robust speech quality and link capacity enhancement. Based on the WCDMA FDD uplink link-level simulation platform, AMR link adaptation platform is constructed. Simulation results show that the algorithm can track the fast change of channel conditions and select the most robust codec mode, thus the synthetic speech quality of AMR is better than that of signal mode during a wide range of channel conditions. The result will provide a reference strategy for AMR link adaptation of 3G system.

Application of Adaptive Coded Modulation Technology in UAV Data Link

UAV data link has been considered as an important part of UAV communication system, through which the UAV could communicate with warships. However, constant coding and modulation scheme that UAV adopts does not make full use of the channel capacity when UAV communicates with warships in a good channel environment. In order to improve channel capacity and spectral efficiency, adaptive coded modulation technology is studied. Based on maritime channel model, SNR estimation technology and adaptive threshold determination technology, the simulation of UAV data link communication is carried out in this paper. Theoretic analysis and simulation results show that according to changes in maritime channel state, UAV can dynamically adjust the adaptive coded modulation scheme on the condition of meeting target Bit-Error-Rate (BER), the maximum amount of data transfer is non-adaptive systems three times.

Adaptive Fuzzy Control of Constrained Flexible-Link Manipulators

In this paper, a novel adaptive fuzzy control scheme is presented. The controller is constructed by using a table lookup scheme and self tuning techniques, which includes the identification block, the fuzzification, the updating rule base, the defuzzification, and the crisp controller (sub controller), etc. The adaptive fuzzy controller is designed in detail by means of a triangular membership function and the center of gravity method. The control scheme addressed here is implemented to control the motion of the end effector of a two link constrained flexible manipulator. Computer simulation results show that the novel adaptive fuzzy control scheme works quite well.

ADAPTIVE PREDICTIVE CONTROL OF NEAR-SPACE VEHICLE USING FUNCTIONAL LINK NETWORK

A novel nonlinear adaptive control method is presented for a near-space hypersonic vehicle （NHV） in the presence of strong uncertainties and disturbances. The control law consists of the optimal generalized predictive controller （OGPC） and the functional link network （FLN） direct adaptive law. OGPC is a continuous-time nonlinear predictive control law. The FLN adaptive law is used to offset the unknown uncertainties and disturbances in a flight through the online learning. The learning process does not need any offline training phase. The stability analyses of the NHV close-loop system are provided and it is proved that the system error and the weight learning error are uniformly ultimately hounded. Simulation results show the satisfactory performance of the con- troller for the attitude tracking.

基于Power Extrapolation和Adaptive Method的网页评估新算法

Google的PageRank算法通过对超链接结构的分析,有效地提高了搜索结果的排序质量。PowerExtrapolation算法通过特征值直接求解马尔可夫超链接矩阵的主特征向量,但该算法的迭代次数与参数d的选择密切相关,而参数d的确定目前无明显规律可寻。另一方面,AdaptiveMethod通过将马尔可夫超链接矩阵稀疏化以达到节省迭代时间的目的。文章在PowerExtrapolation算法的基础上引入AdaptiveMethod,实验结果初步证明了新算法可以减少迭代运算的时间。

Adaptive functional link network control of near-space vehicles with dynamical uncertainties

The control law design for a near-space hypersonic vehicle（NHV） is highly challenging due to its inherent nonlinearity,plant uncertainties and sensitivity to disturbances.This paper presents a novel functional link network（FLN） control method for an NHV with dynamical thrust and parameter uncertainties.The approach devises a new partially-feedback-functional-link-network（PFFLN） adaptive law and combines it with the nonlinear generalized predictive control（NGPC） algorithm.The PFFLN is employed for approximating uncertainties in flight.Its weights are online tuned based on Lyapunov stability theorem for the first time.The learning process does not need any offline training phase.Additionally,a robust controller with an adaptive gain is designed to offset the approximation error.Finally,simulation results show a satisfactory performance for the NHV attitude tracking,and also illustrate the controller＇s robustness.

Adaptive Proportional-Derivative Sliding Mode Control Law With Improved Transient Performance for Underactuated Overhead Crane Systems

In this paper, an adaptive proportional-derivative sliding mode control(APD-SMC) law, is proposed for 2D underactuated overhead crane systems. The proposed controller has the advantages of simple structure, easy to implement of PD control, strong robustness of SMC with respect to external disturbances and uncertain system parameters, and adaptation for unknown system dynamics associated with the feedforward parts. In the proposed APD-SMC law, the PD control part is used to stabilize the controlled system, the SMC part is used to compensate the external disturbances and system uncertainties,and the adaptive control part is utilized to estimate the unknown system parameters. The coupling behavior between the trolley movement and the payload swing is enhanced and, therefore, the transient performance of the proposed controller is improved.The Lyapunov techniques and the La Salle's invariance theorem are employed in to support the theoretical derivations. Experimental results are provided to validate the superior performance of the proposed control law.

Fading Effects on the Lower Shifting of Mode Switching Thresholds in the Rate Adaptive IEEE 802.11a/g WLANs

In this paper we used the probability distribution of the average channel gain of the fading channel to analyze the degree of fading effects on both the PER (packet error rate) and the throughput in OFDM systems. Instead of solely examining the average received SNR (signal-to-noise ratio) value of a packet, considering the whole distribution of the average received SNR allows us to aggregate a better selection of the mode switching thresholds in the rate adaptive 802.11 a/g WLAN. This paper demonstrates that the set of mode switching thresholds can be determined for each individual target , so that the optimal throughput performance is obtained on a per target basis. Numerical results show that mode switching thresholds should be reduced with the lowering of target values. This conclusion could have significant implications for improving the performances of location (distance)-dependent mobile applications, since the determinations of target values are closely related to the distances between mobile devices and the access point.

Adaptive frame length algorithm for inter-satellite links

In view of the low ranging efficiency of the conventional fixed frame-length algorithm in the inter-satellite link,an adaptive frame-length algorithm is proposed. The frame length is adjusted adaptively according to the results of ranging and velocity measuring to improve ranging efficiency. Buffers which enable the frame length to be selected discretely and adaptively are introduced to avoid frequent hopping of the frame-length.Frame length marker is created to automatically identify the frame-length for frame synchronization procedures in receivers. The feasibility and the validity of the proposed algorithm to improve the efficiency of ranging are verified through both theoretic analysis and simulation,and the efficiency improves up to 88% when there are five buffers. This improvement can be further enhanced by increasing the number of buffers. Proper allocation of inter-satellite buffers is required to make a balance between the ranging efficiency and the system complexity.

基于复杂信道服务模型的高效可靠链路控制方法

面向无线通信网络化应用发展趋势,针对无线链路容错概率高、实时变化大、信道带宽窄导致链路可靠控制难的问题,提出了基于复杂信道服务模型的高效可靠链路控制方法;分析了无线传播复杂信道服务模型的多种特点,提出了基于无线信道服务模型紧密耦合高效适配的控制机制,设计了无线链路层自适应分组反馈控制流程,结合帧结构设计给出了数据分组长度、最大重数次数等关键参数演算方法;在物理层最小分组长度对应传输成功率85%~99%条件下,仿真给出最佳分组长度为最小分组的2~9倍,最大重数次数为9次缩减至4次;仿真验证表明:在业务数据包长度为最小分组长度的10~100倍时,相比无链路控制状态,传输成功率由1%~90%改进至99.99%以上,平均吞吐量可提升两倍以上。

本科层次职业教育的生态适应及其优化策略

增强本科层次职业教育的适应性,是构建适应新发展格局的现代职业教育体系的前提和基础。基于生态学理论对本科层次职业教育适应性进行分析发现,其规模效应不足,竞争优势不明显,关联效应较为分化,存在发展主体缺位、发展目标移位和发展资源不足等问题。为此,应从生态位与生态链接两个层面入手提升本科职业教育的生态适应性。一方面,科学论证培养目标,促使职业本科人才培养的生态位分离;加强师资队伍建设,推动办学质量的生态位跃迁;重塑数字化生态,推动职业本科的生态位迭代。另一方面,加强跨学科专业群建设,提升专业生态关联;加快职普融通,提升职教体系的生态关联;加强物质、信息、能量的流动,提升职业教育的产教关联。

MODEL REFERENCE ADAPTIVE CONTROL BASED ON NONLINEAR COMPENSATION FOR TURBOFAN ENGINE

The design of a turbofan rotor speed control system, using model reference adaptive control（MRAC） method with input and output measurements, is discussed for the purpose of practical application. The nonlinear compensator based on functional link neural network is used to deal with the engine nonlinearity and the hardware-in-loop simulation is also developed. The results show that the nonlinear MRAC controller has the adequate performance of compensating and adapting nonlinearity arising from the change of engine state or working environment. Such feature demonstrates potential practical applications of MRAC for aeroengine control system.

Link-16信号的盲检测接收方案设计

设计有效的高速跳频检测接收方案是Link-16信号检测接收的基础,基于多通道辐射计架构给出了一种Link-16信号的跳频检测接收方法。通过详细分析Link-16的通信体制及其信号的高速跳频特性,针对直接将多通道辐射计引入Link-16信号检测存在的不足,设计了新的自适应门限算法和判决规则,提出了简单可行的通道检测结果修正准则,最终实现了基于多通道辐射计的Link-16信号检测接收。仿真结果表明:该检测方案的门限算法对信号幅度有较强的适应能力,使得通道判决结果不受背景噪声影响,在低信噪比条件下,可有效地解决Link-16信号检测问题,从而验证了本方案的有效性。

WPAN/WLAN应用环境下基于PER的链路自适应算法

本文重点研究基于包错误率测量的链路自适应算法,分析了包错误率与信道参数的关系,提出了一种有别于固定门限值方案的优化门限值方案。仿真结果表明,优化门限值方案的网络吞吐量要大于固定门限值方案和基于信干比估计的链路自适应算法,最大增幅可达12Mbit/s。在系统引入重传机制后,网络吞吐量将有所损失,但最大不超过1Mbit/s,相比较服务质量的提高是值得的。需指出的是,虽然分析和仿真均以HIPERLAN/2为例,但不失一般性,它们同样适合于WPAN/WLAN等类似系统。

最新WLAN协议IEEE802.11g物理层性能分析

新制定的IEEE802.11g协议标准是最有发展潜力的无线局域网标准。在全面介绍了IEEE802.11g物理层的各项参数后,分析了IEEE802.11g系统性能和吞吐量性能,并给出了相应的系统性能仿真结果,其结论对今后的IEEE802.11g系统的深入研究具有重要参考作用。

WLAN时变信道下链路性能改进

在IEEE802.11n的MIMO信道模型基础上研究了时变信道下链路性能,提出了一种基于SNR动态调整调制编码方案(MCS)链路自适应方法。多普勒频移下利用基于长训练序列估计SNR动态调整MCS的方法来提高链路性能。仿真表明,该方法根据信道状况动态调整MCS,在数据吞吐率上有很大的提高。