Feedback strategies for iterative channel estimation in OFDM underwater acoustic communications

The extremely limited bandwidth in underwater acoustic communication makes channel estimation using fewer pilot symbols more challenging. Iterative channel estimation( ICE) can be used to refine channel estimation with limited number of pilots,by coupling the channel estimator with channel decoder. In this paper,various feedback strategies in ICE are discussed. The performance of a decision feedback based on the cost function is improved by modifying the design and another four feedback strategies are summarized,including hard/soft decision feedback and their threshold-controlled versions. Simulation results show that ICE can achieve impressive gains over the non-iterative receiver and the gains are more significant with fewer pilots. Furthermore,soft decision feedback outperforms hard decision feedback; while the feedback based on the cost function and soft decision feedback have quite close performance.

CSI Feedback-based CS for Underwater Acoustic Adaptive Modulation OFDM System with Channel Prediction

In this paper, we investigate the performance of adaptive modulation （AM） orthogonal frequency division multiplexing （OFDM） system in underwater acoustic （UWA） communications. The aim is to solve the problem of large feedback overhead for channel state information （CSI） in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing （CS）. We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate （BER） and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying （QPSK） modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform （DCT） method.

A DECISION FEEDBACK EQUALIZER FOR NON-LINEAR CHANNELS

A Simple and useful decision feedback equalizer used for non-linear channels with severe linear distortion and mild non-linear distortion is proposed. It is a combination of a nonlinear channel equalizer based on connectionist model and a common decision feedback equalizer for linear channels. For a typical non-linear channel model it is shown that the equalization performances of the proposed equalizer are improved significantly.

Low-complexity transceiver design scheme based on channel null-space feedback

To reduce the negative impact of channel quantization errors, a low-complexity transceiver joint design scheme for both the transmit beamformers and receive combining vectors is proposed in the two-user multiple-input multiple-output （MIMO） system. In the scheme, the channel nullspace quantization vector is used as the transmit beamformer of the interference user directly based on channel null-space feedback. Since the interference can be determined at the receiver, interference rejection combining （IRC） is jointly utilized to cancel the inter-user interference. Simulation re- sults show that the proposed scheme can provide substantial sum-rate improvement especially at high SNR.

A New Fairness-Oriented Packet Scheduling Scheme with Reduced Channel Feedback for OFDMA Packet Radio Systems

In this paper, we propose a flexible and fairness-oriented packet scheduling approach for 3GPP UTRAN long term evolution (LTE) type packet radio systems, building on the ordinary proportional fair (PF) scheduling principle and channel quality indicator (CQI) feedback. Special emphasis is also put on practical feedback reporting mechanisms, including the effects of mobile measurement and estimation errors, reporting delays, and CQI quantization and compression. The performance of the overall scheduling and feedback re-porting process is investigated in details, in terms of cell throughput, coverage and resource allocation fairness, by using extensive quasistatic cellular system simulations in practical OFDMA system environment with frequency reuse of 1. The performance simulations show that by using the proposed modified PF ap-proach, significant coverage improvements in the order of 50% can be obtained at the expense of only 10-15% throughput loss, for all reduced feedback reporting schemes. This reflects highly improved fairness in the radio resource management (RRM) compared to other existing schedulers, without essentially com-promising the cell capacity. Furthermore, we demonstrate the improved functionality increase in radio re-source management for UE’s utilizing multi-antenna diversity receivers.

Static CSI Extraction and Application in the Tomographic Channel Model

In this paper, the statistical properties of parameters of each path in wireless channel models are analyzed to prove that there is the static part in channel state information(CSI) which can be extracted from huge amounts of CSI data. Based on the analysis, the concept of the Tomographic Channel Model(TCM) is presented. With cluster algorithms, the static CSI database can be built in an off-line manner. The static CSI database can provide prior information to help pilot design to reduce overhead and improve accuracy in channel estimation. A new CSI prediction method and a new channel estimation method between different frequency bands are introduced based on the static CSI database. Using measurement data, the performance of the new channel prediction method is compared with that of the Auto Regression(AR) predictor. The results indicate that the prediction range of the new method is better than that of the AR method and the new method can predict with fewer pilot symbols. Using measurement data, the new channel estimation method between different frequency bands can estimate the CSI of one frequency band based on known CSI of another frequency band without any feedback.

An Efficient CSI Feedback Scheme for Dual-Polarized MIMO Systems Using Layered Multi-Paths Information

Massive MIMO is one of tile enabling technologies tbr beyond 4G and 5G systems due to its ability to provide beamforming gain and reduce interference Dual-polarized antenna is widely adopted to accommodate a large number of antenna elements in limited space. However, current CSI（channel state information） feedback schemes developed in LTE for conventional MIMO systems are not efficient enough for massive MIMO systems since the overhead increases almost linearly with the number of antenna. Moreover, the codebook for massive MIMO will be huge and difficult to design with the LTE methodology. This paper proposes a novel CSI feedback scheme named layered Multi-paths Information based CSI Feedback （LMPIF）, which can achieve higher spectrum efficiency for dual-polarized antenna system with low feedback overhead. The MIMO channel is decomposed into long term components （multipath directions and amplitudes） and short term components （multipath phases）. The relationship between the two components and the optimal precoder is derived in closed form. To reduce the overhead, different granularities in feedback time have been applied for the long term components and short term components Link and system level simulation results prove that LMPIF can improve performance considerably with low CSI feedback overhead.

Effects of Acute Mechanical Stretch on the Expression of Mechanosensitive Potassium Channel TREK-1 in Rat Left Ventricle

To explore the role of mechanosensitive potassium channel TREK-1, Western blot analysis was used to investigate the expression changes of TREK-1 in left ventricle in acute mechanically stretched heart. Forty Wistar rats were randomly divided into 8 groups （n=5 in each group）, subject to single Langendorff perfusion for 0, 30, 60, 120 min and acute mechanical stretch for 0, 30, 60, 120 min respectively. With Langendorff apparatus, an acute mechanically stretched heart model was established. There was no significant difference in the expression of TREK-1 among single Langendorff perfusion groups （P〉0.05）. As compared to non-stretched Langendorff-perfused heart, only the expression of TREK-1 in acute mechanically stretched heart （120 min） was greatly increased （P〈0.05）. This result suggested that some course of mechanical stretch could up-regulate the expression of TREK-1 in left ventricle. TREK-1 might play an important role in mechanoelectric feedback, so it could reduce the occurrence of arrhythmia that was induced by extra mechanical stretch.

Enhancement in Channel Equalization Using Particle Swarm Optimization Techniques

This work proposes an improved inertia weight update method and position update method in Particle Swarm Optimization (PSO) to enhance the convergence and mean square error of channel equalizer. The search abilities of PSO are managed by the key parameter Inertia Weight (IW). A higher value leads to global search whereas a smaller value shifts the search to local which makes convergence faster. Different approaches are reported in literature to improve PSO by modifying inertia weight. This work investigates the performance of the existing PSO variants related to time varying inertia weight methods and proposes new strategies to improve the convergence and mean square error of channel equalizer. Also the position update method in PSO is modified to achieve better convergence in channel equalization. The simulation presents the enhanced performance of the proposed techniques in transversal and decision feedback models. The simulation results also analyze the superiority in linear and nonlinear channel conditions.

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。

Improved Interference Cancellation Scheme for <i>X</i>Channels with Four Antennas

Interference cancellation scheme without feedback is proposed for X channels with four antennas at each user. Space-time codeword with Alamouti structure is designed for each user. Codewords are combined according a certain rule. The unwanted codewords are cancelled by linear operation on the received signals. Then, multi-user interference is mitigated by the orthogonal property of the Alamouti code. Comparing with the existing scheme for the same scene, feedback information is not required in the proposed scheme. So the transmission efficiency is improved.

一种单反馈通道振荡射流激励器设计

为了设计一种有利于分离控制的振荡射流激励器,选取单反馈通道的形式,重点研究第二喉道与扩张段参数对射流振荡特性与偏转襟翼分离控制效果的影响,并总结几何参数设计依据,完成激励器设计并对比了其与典型激励器的分离控制效果。结果表明:反馈段参数应使射流在混合段扩张部分的偏角最大;混合段参数应使射流附着于混合段壁面;第二喉道应使射流向流场中均匀传递动量,其过小或过大均会减小射流偏角并导致射流向流场中传递动量不均匀;扩张段扩张角应增大至刚好不阻挡射流偏转,过大会使扩张段内射流发生不对称附着现象;增大出口高度可以增大射流偏移量,但其过大会延长射流在扩张段两侧的停滞时间。不同激励器的分离控制效果对比表明,采用自主设计激励器时的偏转襟翼减阻量是采用典型激励器时的3.8倍。

激光回馈双通道位移测量创新实验平台设计

针对激光回馈技术中多目标同步检测以提高测量效率的问题,提出了一种基于线性调频的测量通道拓展方法。在线性电流注入下,激光器输出的光频率近似线性变化;2个被测物体到激光器距离不同时,回馈信号的频谱中出现分别对应2个物体的独立谱峰,通过全相位频谱分析方法在2个谱峰处进行相位解算可以重构2个物体的运动曲线。基于Python语言构建的仿真模型和相应的双通道位移测量实验系统均证实了该方法的有效性,2个通道的位移测量相对误差小于5%。

多采样率双通道有源噪声反馈控制系统的研究

传统有源噪声控制系统中的音频编解码器只能使用高速率的单一采样率,因此系统中的建模滤波器和自适应滤波器需要的长度较长,而且预留给每次迭代计算的时间很短,这导致了计算量大、难以实现实时在线计算的问题。针对以上困难,首先提出一种多采样率的方法,即采用抽取和插值的方法为有源噪声控制系统提供更低的采样率,从而在减少计算量的同时增加可用于迭代计算的时间,并基于最小均方算法对次级声通道进行建模,推演多采样率双通道反馈有源噪声控制系统模型;其次搭建基于座椅头靠和数字信号处理器的实验平台,对多采样率的双通道有源噪声反馈控制系统的有效性进行实验验证。验证结果表明,针对低频噪声,在相应的频段上本文降噪系统可以达到约15 dB的降噪量,降噪效果明显。

智能反射面辅助通信中的信道估计方法

信道信息对于智能反射面(Intelligent Reflecting Surface, IRS)辅助的通信系统十分关键。由于IRS反射单元的数量十分巨大,信道估计和信道反馈的开销一直制约着IRS辅助的通信系统的性能。为解决这一难题,介绍了IRS辅助的通信系统中信道的特性。不同用户共享同一基站(Base Station, BS)-IRS信道,而BS-IRS-用户串联信道具有很强的关联性。基于这一特有的信道特性,提出了IRS辅助的通信系统上行信道估计方法、下行信道估计和信道反馈方法;理论性地刻画了以上方法所需要的最小信道估计开销和信道反馈开销,以揭示所提方法在IRS辅助的通信系统中相对于传统信道信息获取方法的巨大优势。

MIMO干扰对齐中低开销反馈拓扑设计

干扰对齐是一种解决无线通信系统的干扰问题、提升系统容量的重要技术。但实施干扰对齐需要发送端已知全局信道状态信息。为解决在多用户MIMO干扰信道场景下实施干扰对齐会产生大量信道状态信息开销的问题,提出了连续干扰对齐反馈拓扑设计。该反馈拓扑依据用户数目设置了多对干扰对齐发送端,并连续性地执行干扰信号对齐,能够有效降低信道状态信息开销。对比已有反馈拓扑,结果表明,当用户数目K≥6时,所提的连续干扰对齐反馈拓扑能够以最低的信道状态信息开销去实现干扰对齐,这更有利于干扰对齐技术的实际应用。

基于状态反馈的无人机通信接入拥塞动态控制研究

为解决复用信道内的通信接入拥塞问题,需要控制无人机通信链路中数据信息样本的实时累积量,故基于状态反馈研究无人机通信接入拥塞动态控制方法;建立信道复用模型,并借助MAC连接协议,定义通信数据包,实现对无人机集群通信节点接入行为的处理;按照状态反馈模型完善状态观测器结构,求解通信数据的离散化反馈表达式,在划分通信优先级条件的同时,确定无人机队列的实时调度关系,完成无人机通信行为控制;根据动态阈值窗口定义条件,确定通信链路长度,再联合相关通信数据样本,计算链路时延量的具体数值,完成动态控制方法的设计;实验结果表明,应用设计方法可将通信链路中数据信息样本实时累积量控制在5.7×10^(13)MB之下,能够较好解决复用信道内的通信接入拥塞问题,符合实际应用需求。

一种面向112 Gb/s PAM4接收机的自适应均衡设计方案

提出了一种适用于超短距离(Very Short Reach,VSR)信道、面向112 Gb/s PAM4(Pulse Amplitude Modulation 4)接收机的自适应均衡设计方案。在该方案中,接收机前端利用3个连续时间线性均衡器(Continuous Time Linear Equalizer,CTLE)对信号分别在高频、中频和低频进行补偿,可变增益放大器(Variable Gain Amplifier,VGA)和饱和放大器(Saturation Amplifier,SatAmp)则用于对信号幅值的缩放。除了3个数据采样器外,引入4个辅助采样器用于进一步改善阈值自适应算法性能。同时,采用符号最小均方算法,利用接收端数据采样器和辅助采样器之间的偏移推动辅助参考电压收敛到信号星座电平,从而确保PAM4接收信号的眼图在垂直方向上3个眼睛具有相等的间隔和恒定的信噪比(Signal-to-Noise Ratio,SNR)。仿真结果表明,所提出的112 Gb/s PAM4接收机能够在损耗为15 dB的信道上实现小于10~(-12)的误码率,并且具有良好的眼图性能,其最差眼高为75 mV,眼宽为0.34 UI(Unit Interval),与传统方案相比具有显著的性能提升。

无反馈通道自激扫掠喷嘴工作特性

无反馈通道自激扫掠喷嘴仅在压力驱动下,通过两股射流在耦合腔内部的相互作用,就能在出口实现液柱喷射方向的自激发高频变化。为了探索此新型喷嘴在航空发动机内燃油喷射的应用潜力,本研究综合采用高速阴影成像法和二维数值模拟方法,研究了不同工作介质、不同缩比尺寸下,无反馈通道自激扫掠喷嘴的特征流速、工作频率、扫掠张角等参数随工作压降的响应变化情况。结果表明,无反馈通道自激扫掠喷嘴在较宽的工作压力范围(0.1~6 MPa)和特征尺寸范围(0.2~7 mm)内均能够实现稳定的自激发扫掠振荡喷射,产生30°以上的扫掠张角和5 kHz以上的振荡频率。无反馈通道自激扫掠喷嘴的扫掠张角基本不随压力和等比缩放特征尺寸而变化,但是其工作频率与其喉道处的特征流速成正比,与喉道宽度成反比,且其频率响应存在双模态现象。

基于AI通信的大规模MIMO信道状态信息反馈网络

在大规模多输入多输出系统中,由于天线数量的增加导致信道状态信息反馈带宽开销增大。为了减少反馈开销,提出了一种基于深度学习的反馈网络。该网络将卷积注意力模块和快速迭代收缩阈值算法(Fast Iterative Shrinkage Thresholding Algorithm,FISTA)进行了结合。为了贴合实际应用,考虑到了噪声情况,分析了阈值敏感度。仿真结果表明,该网络在不同环境下其性能和鲁棒性可以得到进一步提高。