Performance prediction of passive time reversal communications

A recent paper [M. Stojanovic J. Acoust. Soc. Am. 117, 1173-1185 (2005)] presented theoretical performance bounds of time reversal communications. In this letter, the performance of time reversal communications is evaluated using at-sea experimental data and compared to the theory for two different approaches; (1) time reversal alone and (2) time reversal combined with channel equalization. It is found that approach (1) shows a good agreement between theory and data. On the other hand, approach (2) indicates that the data is about 3-5 dB below the theory which assumes perfect knowledge of the channel, an infinite number of taps to remove the intersymbol interference, and no need for phase tracking. Taking these into consideration, the theoretical performance can provide a useful upper bound for predicting performance of time reversal communications.     

Shannon information capacity of time reversal wideband multiple-input multiple-output system based on correlated statistical channels

Utilizing channel reciprocity, time reversal(TR) technique increases the signal-to-noise ratio(SNR) at the receiver with very low transmitter complexity in complex multipath environment. Present research works about TR multiple-input multiple-output(MIMO) communication all focus on the system implementation and network building. The aim of this work is to analyze the influence of antenna coupling on the capacity of wideband TR MIMO system, which is a realistic question in designing a practical communication system. It turns out that antenna coupling stabilizes the capacity in a small variation range with statistical wideband channel response. Meanwhile, antenna coupling only causes a slight detriment to the channel capacity in a wideband TR MIMO system. Comparatively, uncorrelated stochastic channels without coupling exhibit a wider range of random capacity distribution which greatly depends on the statistical channel. The conclusions drawn from information difference entropy theory provide a guideline for designing better high-performance wideband TR MIMO communication systems.     

Time-reversal-symmetry-broken quantum spin Hall effect

The quantum spin Hall (QSH) state of matter is usually considered to be protected by time-reversal (TR) symmetry. We investigate the fate of the QSH effect in the presence of the Rashba spin-orbit coupling and an exchange field, which break both inversion and TR symmetries. It is found that the QSH state characterized by nonzero spin Chern numbers C(±) = ±1 persists when the TR symmetry is broken. A topological phase transition from the TR-symmetry-broken QSH phase to a quantum anomalous Hall phase occurs at a critical exchange field, where the bulk band gap just closes. It is also shown that the transition from the TR-symmetry-broken QSH phase to an ordinary insulator state cannot happen without closing the band gap.     

Retrofocusing techniques in a waveguide for acoustic communications

In the paper "Retrofocusing technique for high rate acoustic communications" [M. Stojanovic, J. Acoust. Soc. Am. 117, 1173-1185 (2005)], it is suggested that the time reversal approach requires a large number of array elements to compete with other approaches. Here the analysis of that paper is extended with a modified example to compare the performance of various approaches in three respects: (1) array element distribution across the water column, (2) channel normalization, and (3) phase delay across the array. In contrast, our results show that the time reversal approach combined with channel equalization can offer nearly optimal performance with a very small four element array.     

Spatial diversity and combination technology using amplitude and phase weighting method for phase-coherent underwater acoustic communications

The UWA channel is characterized as a time-dispersive rapidly fading channel,which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate intersymbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channels need to be normalized. It adopts fractional-decision feedback diversity equalizer(FDFDE) and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can improve communication system's ability in tracking the change of underwater acoustic channel,offset the impact of multipath and noise and improve the performance of communication system.The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate(BER) reduces 1.8%.     

相干水声通信幅相加权空间分集均衡算法

水声信道的典型特点为多普勒频移严重、可利用带宽窄以及强多径干扰。空间分集均衡技术是相干水声通信中克服信道多径干扰,消除码间干扰的一种有效手段。为了极大化地输出阵增益,结合无源相位共轭方法和多通道均衡算法,本文设计了组合信噪干比的全新信道评价方法。利用改进的Sigmoid函数对各通道接收信号的幅度进行加权处理;采用二阶锁相环跟踪各通道信号的相位变化,实现各通道信号同相累加。将各通道低通滤波后的信号能量归一化,采用了分数阶-判决反馈分集均衡器,加入各通道权重系数实现了水声通信系统的分集均衡接收。仿真结果和湖试数据处理结果均表明,优化的幅相加权分集均衡接收算法能抵消多径和噪声的干扰,性能优于等增益合并接收算法。湖试数据处理结果误码率降低了1.8%。      

Weak localization and time reversal of ultrasound in a rotational flow

A one-channel time-reversal (TR) experiment is performed inside a rough reverberating cavity in the presence of a rotational flow. The amplitude of the TR wave is plotted versus the distance between the TR channel and the initial source: when they coincide, it exhibits a "time-reversal enhancement" (TRE). With no flow, the TRE is the same as the coherent backscattering enhancement (CBE). But contrary to CBE, the TRE peak is shown to be insensitive to the breaking down of reciprocity due to the flow. This new effect of weak localization is sustained by a diagrammatic approach.     

Multiple trellis-coded modulation,per-survivor processing and Reed–Solomon coding in the presence of phase noise

The aim of this paper is to consider trellis coded modulation (TCM) in the presence of phase noise. The starting point is a two-dimensional 32 point cross (32-CR) signal set that has served as a benchmark for other power and bandwidth efficient modulation schemes. Per-survivor processing (PSP) is used for carrier phase estimation, and both the phase acquisition (transient state) and phase tracking (steady state) performances are considered in an additive white Gaussian noise (AWGN) channel, in which phase offset or time varying phase noise is introduced. Based on the theoretical and simulation results it can be said that the 32-CR constellation is itself more sensitive to a phase error rather than the PSP algorithm. Due to the phase error the TCM scheme becomes a catastrophic code. Possible solutions include stringent phase noise requirements for oscillators, multiple trellis-coded modulation (MTCM) and concatenated TCM and Reed–Solomon (RS) coding. Our simulation results show that a simple combined MTCM, PSP and RS scheme has around 3 dB advantage compared to the pure TCM and PSP in the presence of phase noise. A semianalytical approach is used for performance evaluation, and the method is a particularly convenient tool to determine the rate of RS coding in detail.     

时变信道下的被动时间反转扩频水声通信

本文研究了时变信道条件下采用被动时间反转的直接序列扩频水声通信方案。多通道被动时间反转可通过对信道多径进行时间、空间聚焦实现信道匹配,但低信噪比、时变特性造成的信道特性失配对被动时间反转处理的性能造成严重影响。在垂直阵接收的基础上,本文采用码片级信道估计获取水声信道特性并进行周期性更新,并采用已判决码元产生的扩频码片作为信道估计训练序列,结合应用稀疏信道估计算法抑制零值抽头上的估计噪声,从而可有效改善时变、低信噪比条件下的被动时间反转处理的时、空多径聚焦效果,提高扩频通信性能。通过湖试实验比较了采用稀疏信道估计、传统信道估计算法的时反扩频接收机,以及经典直扩接收机的通信性能,实验结果表明：本文方案可在低信噪比获得较好的性能,并有效抑制时变信道对时反扩频通信性能的影响。     

Impact of ocean variability on coherent underwater acoustic communications during the Kauai [corrected] experiment (KauaiEx)

During the July 2003 acoustic communications experiment conducted in 100 m deep water off the western side of Kauai, Hawaii, a 10 s binary phase shift keying signal with a symbol rate of 4 kilosymbol/s was transmitted every 30 min for 27 h from a bottom moored source at 12 kHz center frequency to a 16 element vertical array spanning the water column at about 3 km range. The communications signals are demodulated by time reversal multichannel combining followed by a single channel decision feedback equalizer using two subsets of array elements whose channel characteristics appear distinct: (1) top 10 and (2) bottom 4 elements. Due to rapid channel variations, continuous channel updates along with Doppler tracking are required prior to time reversal combining. This is especially true for the top 10 elements where the received acoustic field involves significant interaction with the dynamic ocean surface. The resulting communications performance in terms of output signal-to-noise ratio exhibits significant change over the 27 h transmission duration. This is particularly evident as the water column changes from well-mixed to a downward refracting environment.     

多基地空时码探测信号设计及时反相关检测技术

为实现浅海复杂环境下的多基地声纳多源目标回波分辨,本文设计了一种适用于多入多出垂直阵信道环境下的空时码探测信号,并针对倾斜垂直阵的多途子信道差异问题,提出了信号的时反相关检测技术.空时码探测信号采用伪随机信号调制,具有良好的正交性,能在抗子信道严重衰落的同时,分辨多源目标回波.垂直阵受水流冲击,呈倾斜状态时,其多途子信道不一致性会导致各子信道传递信号无法在接收端聚焦,使阵列增益受损,同时导致时延测量能力下降和信号判决错误率上升,为此本文设计了信道训练信号用以估计多途子信道环境,通过虚拟时间反转镜获得子信道不一致条件下的最佳匹配检测信号,实现对接收信号的时反相关检测.仿真结果表明,本文所设计的探测信号和检测方法,能够克服复杂的信道条件和多途子信道不一致性引起的检测问题,满足多基地声纳探测需求,实现多源目标回波分辨.     

Geometrical optics-based ray field tracing method for complex source beam applications

Due to the fact that traditional ray field tracking approaches require a large number of geometrical optical(GO) ray tubes,they are very inefficient in many practical applications.An improved ray model scheme for a complex source beam(CSB) tracking technique is proposed in this paper.The source field can be expressed by a superposition of CSBs,then every CSB basis function has a Gaussian-type amplitude distribution and is suitable for replacing a GO ray tube in the ray tracing approach.The complex phase matching technique is adopted to find the reflected beam in the reflection point where local approximation is used to represent the curved surface in its neighborhood.A new solution to multiple reflections using the conventional right-handed reflected system is used to track the field easily.Numerical results show the accuracy of the proposed method.     

A review of phase unwrapping techniques in fringe analysis

There has been an increasing interest in the automation of fringe analysis over the last decade. In recent years a number of researchers have addressed the problem of automatic phase unwrapping in fringe analysis. The advent of phase stepping and the development of FFT techniques has moved the emphasis in automated techniques away from fringe tracking and towards fringe counting or scanning approaches, but a major difficulty with such scanning techniques has been their susceptibility to noise. However, several alternative noise-immune phase unwrapping strategies have been proposed, but these approaches have not addressed the problems caused by large scale discontinuities such as those produced by aliasing. Other methods have been developed which confront this last difficulty. This paper gives an overview of the phase unwrapping problem and describes fringe tracking, fringe scanning, noise-immune and region processing techniques that have been developed in the search for the fully automated solution.     

水平槽道内湍流变动的PTV实验研究

本文采用PIV测量技术研究充分发展水平槽道内的两相湍流的变动规律(Re=590)。首先将单相湍流的测量结果与文献中DNS的结果进行了比较,证明了PIV测量湍流脉动的可行性,并通过引入PTV算法获得了近壁对数边界层内的湍流量。对两相流动的测量结果表明,即使在1％的低颗粒质量载荷下,气体湍流已有明显的变动,并且壁面附近和槽道中心的变动规律不同。     

多径衰落信道下混沌直扩通信的可破解性

本文在文献[1]的基础上,研究多径衰落信道条件下采用无先导卡尔曼滤波混沌拟合对混沌直扩通信的可破解性.由针对混沌直扩信号的无先导卡尔曼滤波混沌拟合的状态空间方程出发,分析了多径衰落信道对于无先导卡尔曼滤波混沌拟合过程中的跟踪误差的影响,得到了信息码状态估计的值域范围,从而提出了多径衰落信道下混沌直扩信号可被破解的充分条件定理.仿真结果表明,在满足充分条件下,混沌直扩信号无论是通过时不变信道还是时变信道,都可以被成功破解,并且具有良好的误码率性能. 关键词： 混沌通信 破解 多径衰落信道 无先导卡尔曼滤波     

Performance degradation due to cross-phase modulation in optical fdm ask coherent detection systems

The performance degradation due to cross-phase modulation in optical FDM ASK coherent detection systems has been investigated in this paper. The effect of modulation statistics and phase tracking of the receivers is taken into account. Based on the theoretical treatment, power penalty and allowable launched power per channel have been estimated.     

基于LMS/SOLMS算法的时变多径水声信道估计方法

缓变信道下基于LMS算法的信道估计算法具有较好的跟踪性能,但对快变信道,LMS算法跟踪性能下降。SOLMS算法具有比LMS算法更好的跟踪性能,尤其是在快变的信道下。但由于SOLMS算法在收敛阶段的振荡性,这时收敛速度较LMS算法慢。本文提出一种收敛模式下用LMS算法获得信道的参数,收敛后则切换成SOLMS算法跟踪信道的变化的信道估计方法。新方法结合了LMS算法收敛快和SOLMS算法跟踪性能好的优点。对时变多径水声信道估计的计算机仿真实验证明了该方法的有效性。     

A modified multi-echo AFI for simultaneous B1 magnitude and phase mapping

To simultaneously acquire the B₁⁺ magnitude and B₁⁺ phase, a modified multi-echo actual flip-angle imaging (AFI) sequence is proposed. A multi-echo gradient echo sequence was integrated into every even TR of AFI to measure both magnitude and phase of B₁⁺. In addition, to increase the signal-to-noise ratio of the B₁⁺ phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is α at the odd TR and 2α at the even TR is proposed. Images were simulated to evaluate the performance of this method under various imaging and physical parameters. The performance was compared to the spin echo based B₁⁺ mapping method in phantom and in vivo studies.     

The association between upper trapezius activity and thorax movement in classical singing

This study aimed to examine in classical singing the phasing of the activity in upper trapezius (TR) to upper and lower thorax movement and to the phasing of activity in the intercostals (INT) and in the lateral abdominal (OBL) muscles. Electromyographic (EMG) activity was recorded from the TR, INT, and OBL muscles on the right side. Thorax movement (TX) was traced with two strain gauge sensors placed around the upper and lower thorax. Four professional opera singers (soprano, mezzo, tenor, and baritone) and four advanced student classical singers (three sopranos and one mezzo) participated. Three of the professional singers were 33 years, and one was 40 years. The students were between 23 and 30 years. Different arias, freely chosen by the singers from their professional repertoire, served as the singing task for the opera singers. All students sang “Summertime” from Porgy and Bess. All subjects performed their task three times with variation in vocal loudness (normal, forte, piano). Thereafter, for all subjects, a biofeedback (BF) procedure was performed on TR to lower TR activity and a repeat performance of the singing tasks was carried out. EMG activity from the three recording sites and upper and lower TX circumference were compared before and after BF. A phasing of upper TR activity to INT and OBL activity was discovered, all muscles supporting the expiration phase. During phonation, the upper TR contributes in the compression of upper TX, thus serving as an accessory muscle of expiration. Group results from both opera singers and student singers showed that EMG activity was significantly lowered after BF. The lowered TR activity resulted in an expanded upper TX circumference and less TX respiratory movement after BF.