Time-varying Doppler Shift Research Articles

Comb-Search and Hierarchical-Clustering Based Doppler-Shift Acquisition for Space Communications

In space communications, the signal quality generally suffers due to long-distance transmission and high dynamic movement. Long-distance transmission causes a very low signal-to-noise ratio (SNR) and high dynamic movement causes a dynamic Doppler-shift on carrier frequency. To boost the signal component in SNR, multiple symbols over a long duration of time are accumulated, which is termed as long-term accumulation. However, during the long-term accumulation, the large acceleration and jerk result in a time-varying Doppler shift, thus dispersing the energy over a wide frequency range. In this paper, we propose a comb-search and hierarchical-clustering (CSHC) based Doppler-shift acquisition scheme, including the Generic CSHC algorithm and Iterative CSHC algorithm, to address the energy dispersion problem for space communications. In the Generic CSHC algorithm, the comb-search (CS) decreases the noise energy by a narrower CS-range while the hierarchical-clustering (HC) increases the signal energy by clustering multiple signal elements into one signal cluster. Then, the Iterative CSHC algorithm, which builds on the iterations of CS and HC, further decreases the noise energy and increases the signal energy. Simulations results verify that our proposed CSHC scheme significantly improves the acquisition probability, as compared with the existing schemes.

An n-order hold approach for fractional-delay interpolation in auralization

During the signal processing chain of an auralization simulating the propagation of a sound from a moving source to a stationary receiver, it is often necessary to interpolate between the samples of the source signal in order to arrive at uniformly-spaced samples at the receiver. In some cases, this interpolation is done in the receiver time frame—where the “input” samples of the source have become irregularly spaced due to time dilation effects. Canonical band-limited interpolation methods (i.e., sinc and sinc-derived approaches) cannot be applied in this case as they rely on having a uniformly-spaced input. The use of geometric interpolation methods that can handle irregularly-spaced input may not be grounded in signal processing principles and may produce unwanted artifacts and noise. This presentation outlines the possibility of embedding an irregularly spaced n-order hold signal within a highly over-sampled uniformly-spaced signal and then processing down to the desired sampling rate through successive decimations. Initial distortion and noise characteristics of the approach are shown for some basic propagation geometries. The possible benefits of using such an approach in an auralization scheme with a time-varying Doppler shift are discussed including: the prevention of aliasing, processing time advantages, and real-time processing.

Joint Estimation of Frequency Offset and Doppler Shift in High Mobility Environments Based on Orthogonal Angle Domain Subspace Projection

Conventional frequency preshift technology cannot distinguish between the large Doppler shift and the frequency offset aroused by crystal oscillator, which significantly hinders the system performance improvement in high mobility environments when the frequency offset is present. In this paper, a joint frequency offset and Doppler shift estimation algorithm in high mobility environment based on orthogonal angle domain subspace projection is proposed, which can track the fast time-varying Doppler shift and separate the Doppler shift from the frequency offset. First, an orthogonal angle domain subspace projection algorithm is proposed to project the Doppler spread caused by high mobility into equivalent Doppler shifts in each orthogonal subspace. Second, it utilizes equivalent Doppler shift of all orthogonal subspaces to jointly estimate the maximal Doppler shift and frequency offset. Finally, the estimates are used to compensate the frequency error in the received signal. Moreover, the maximum Doppler shift estimation, together with the projection vectors and the received power on each orthogonal subspace, is fed back and exploited in the subsequent transmitted signal. Simulation results show that the proposed method can estimate the two parameters accurately and eliminate almost all the errors on the Doppler rapidly changing area when SNR is up to 15 dB. Compared with the conventional receiver with automatic frequency control, the receiver with our proposed method achieves better system performance even on the Doppler rapidly changing area.

Impact Response Measurement of a Bamboo Sword “Shinai”

Impact response of a bamboo sword “Shinai” which is a practice sword used in a Japanese traditional fencing and has an unique and simple structure of light-weight bamboo plates for moderating the impact force to the human body, is accurately measured by means of modifying the Levitation Mass Method (LMM). In the method, a mass, which is levitated with an aerostatic linear bearing and hence encounters negligible friction, is made to collide with a bamboo sword under test. During the collision, the Doppler shift frequency of a laser beam reflecting from the mass is accurately measured using an optical interferometer. The velocity, position, acceleration, and inertial force of the mass are calculated from the measured time-varying Doppler shift frequency. The measurement results show that “Shinai” considerably moderates the impact force and its unique structure might be effective for robotics and automobiles to moderate the impact force acting to the human body in the case of accidents.

Statistics of near surface underwater scattered sound including wave induced Doppler

The interaction of underwater sound with the ocean surface is explored. Analytic expressions for the Doppler shift and Doppler spread of surface reflected signals are developed. Statistical models for surface scattered intensity, arrival time, and Doppler fluctuations are developed in relation to wave shape, transducer position, and transmission frequency. Measurements from two experiments are considered. Data collected from AUTEC explore surface scattering effects in the frequency regime of 8 kHz–10 kHz, while data collected in a variable speed wind wave channel at Scripps Institution of Oceanography explore scattering statistics in the 200 kHz–400 kHz (VHF) frequency regime. Physics based analytic expressions for the equations of motion of specular reflection points (SRPs) along a time-evolving wave surface are derived. The time-varying Doppler shift of VHF underwater acoustic signals is attributed to SRP motion associated with surface wave propagation. Model predicted values are compared to experimenta...

An Acoustic OFDM System with Symbol-by-Symbol Doppler Compensation for Underwater Communication.

We propose an acoustic OFDM system for underwater communication, specifically for vertical link communications such as between a robot in the sea bottom and a mother ship in the surface. The main contributions are (1) estimation of time varying Doppler shift using continual pilots in conjunction with monitoring the drift of Power Delay Profile and (2) symbol-by-symbol Doppler compensation in frequency domain by an ICI matrix representing nonuniform Doppler. In addition, we compare our proposal against a resampling method. Simulation and experimental results confirm that our system outperforms the resampling method when the velocity changes roughly over OFDM symbols. Overall, experimental results taken in Shizuoka, Japan, show our system using 16QAM, and 64QAM achieved a data throughput of 7.5 Kbit/sec with a transmitter moving at maximum 2 m/s, in a complicated trajectory, over 30 m vertically.

Adaptive time varying doppler shift compensation algorithm for OFDM-based underwater acoustic communication systems

This paper presents an adaptive approach to address the two main problems associated with the time varying Doppler shift, the first being the acceleration effects on the cyclic-prefix (CP) correlation and the second, the effect of a sudden change in the velocity direction between packets on the entire orthogonal frequency division multiplexing (OFDM) symbols. In addition, this paper considers the residual Doppler shift or carrier frequency offset (CFO) that was estimated iteratively within a range according to a design based on the sub-carrier spacing using pilots, which are basically utilized for the purpose of channel estimation. Furthermore, the proposed algorithm adopts three estimations of the symbol timing offset. These estimations are centroid-based localization over an anticipated CP window, first order expectation and autocorrelation of the received CP with its replica. Subsequently, a penalization algorithm is applied in order to drop the anomalous parameter among them. Therefore, the consequences of the inflection point that accompanies the abrupt change in the velocity are mitigated and a reliable time varying Doppler shift is obtained. This Doppler shift is fine tuned in an iterative manner. Compared with the block-based Doppler compensation approach, the proposed technique works with variable speed during packet duration. In addition, it exploits the available bandwidth more efficiently by utilizing a single preamble of linear-frequency-modulation (LFM) to detect the start of the packet. The proposed receiver was evaluated through simulations and sea trials conducted over 500m and 1000m channel ranges. In simulations, a model was designed to imitate the time varying Doppler shift with two scenarios (expansion/compression) in combination with a various multipath delay spread. The simulation results confirm that the proposed system accommodates an acceleration up to ± 1m/s2 during the packet time.

Frictional Characteristics of a Small Aerostatic Linear Bearing

Frictional characteristics of a small aerostatic linear bearing are accurately evaluated by means of a method, in which the force acting on the moving part of the bearing is measured as the inertial force. An optical interferometer is newly developed to measure the Doppler shift frequency of the laser light reflected on the small moving part. From the measured time-varying Doppler shift frequency, the velocity, the position, the acceleration and the inertial force of the moving part are numerically calculated. It is confirmed that the dynamic frictional force acting inside the bearing is almost proportional to the velocity of the moving part and is similar to the theoretical value calculated under the assumption that the flow inside the bearing is the Couette flow.

Doppler-based motion compensation algorithm for focusing the signature of a rotorcraft

A computationally efficient algorithm was developed and tested to compensate for the effects of motion on the acoustic signature of a rotorcraft. For target signatures with large spectral peaks that vary slowly in amplitude and have near constant frequency, the time-varying Doppler shift can be tracked and then removed from the data. The algorithm can be used to preprocess data for classification, tracking, and nulling algorithms. The algorithm was tested on rotorcraft data. The average instantaneous frequency of the first harmonic of a rotorcraft was tracked with a fixed-lag smoother. Then, state space estimates of the frequency were used to calculate a time warping that removed the effect of a time-varying Doppler shift from the data. The algorithm was evaluated by analyzing the increase in the amplitude of the harmonics in the spectrum of a rotorcraft. The results depended upon the frequency of the harmonics and the processing interval duration. Under good conditions, the results for the fundamental frequency of the target (~11 Hz) almost achieved an estimated upper bound. The results for higher frequency harmonics had larger increases in the amplitude of the peaks, but significantly lower than the estimated upper bounds.

수중채널 시뮬레이터를 활용한 수중음향통신 시스템 성능 분석

The performance of underwater acoustic communication system is sensitive to the Doppler shift and ISI(Inter-Symbol Interference). Therefore, the simulation algorithm needs to consider time-spread due to multipath arrivals which cause the ISI, and time-varying Doppler shift along with moving source and receiver. For this purpose, VirTEX(Virtual Time series EXperiment) based on Ray model has been developed. In this paper, VirTEX is used to compare the characteristics of ocean waveguide from the experimental data and illustrate the performance. The CIR(Channel Impulse Response) that characterizes the multipath arrivals with representative time-spread due to multipath arrivals is compared between numerically simulated and experimental probe signal. Also, the communication performance analysis for BER(Bit Error Rate) is compared between numerically simulated and experimental data signal. As a result, VirTEX can be useful as a simulation tool for evaluating the performance of underwater acoustic communication system.

Development of Material Tester Using Pendulum

The requirements for evaluating the mechanical characteristics of materials have increased in the various industrial, research and the applications such as materials testing. Therefore, the authors have proposed a method for measuring force acting a material using pendulum [1]. In the Material tester, the mechanical characteristics of material against small force are measured by means of the pendulum mechanism based on the levitation mass method (LMM) [2,3,4].In the LMM, the Doppler shift frequency is measured for measuring the inertial force. The Doppler shift frequency of a laser beam reflected from the mass is accurately measured by using optical interferometer. The velocity, position, acceleration and inertia force of the mass are calculated from the measured time-varying Doppler shift frequency. Thus, the accurate measure of frequency makes that of force.

Evaluation of Mechanical Response of Human Palm to Small Impact Force

Method of measuring mechanical characteristics of human palm against small impact force is proposed. Medical doctors qualitatively evaluate the mechanical characteristics of the human body by manipulation and percussion. In the field of bioengineering, accurate quantitative analysis of the mechanical characteristics of the human body is required. In this paper, the mechanical characteristics of a human palm against small impact force are determined by means of the Levitation Mass Method (LMM). The LMM is a method for accurately measuring varying forces without the use of force transducers. Mechanical characteristics of a human palm were measured with high accuracy by means of an instrument based on the LMM. In the measurement, the mass, that is a moving part of an aerostatic linear bearing (a hydrostatic linear air bearing, an externally pressurized air bearing), is made to collide with a human palm under test. During the collision measurement, only the Doppler shift frequency of the laser light reflecting on the mass is highly accurately measured using an optical interferometer. The velocity, position, acceleration and inertial force of the mass are calculated from the measured time-varying Doppler shift. The mechanical response of a human palm against small impact loading is also characterized from the measured data.

Method of Evaluating the Force Controllability of the Human Finger

The ability of the human finger to control force against an impact load is determined by means of a novel practical method, which is a variation of the levitation mass method. In the method, a mass that is levitated with a pneumatic linear bearing and hence encounters negligible friction is made to collide with an object under test. During the collision, the Doppler frequency shift of a laser beam reflecting from the mass is accurately measured using an optical interferometer. The velocity, position, acceleration, and inertial force of the mass are calculated from the measured time-varying Doppler shift. The method is characterized by the fact that preparation for the test is easy, and measurement accuracy is high. The importance of visual information is also determined by means of conducting the tests under conditions in which the subject's eyes are either opened or closed

Analysis of Non-WSSUS Fading Dispersive Channel Model

The Wide-Sense Stationary Uncorrelated Scattering ( WSSUS) model has long been viewed as a basic channel model to describe the fading dispersive channel. But non- WSSUS models have more universal applicability when the wireless mobile channel with broadband is studied with finer and more detailed knowledge of propagation environments. So the four-Dimension (4- D) characteristics of channel, namely time, lag, frequency, and Doppler, should be studied together. In this paper, Wigner-Ville distribution of Time-Frequency ( TF) domain is introduced to analyze channel in which the incidence rays are non-stationary and correlated with each other. Several channel models, according to different move modes of incidence rays, with time-varying Doppler shift are designed and 4-D Local Scattering Function ( LSF) are computed and simulated respectively. Our simulation results show the LSF present asymmetric and non-periodic TF distribution for some symmetric and periodic move modes of incidence rays.

Proposal for material viscoelasticity evaluation method under impact load

A novel practical method for evaluating the viscoelasticity of materials under impact load is proposed. The method is characterized by the fact that preparation of the test specimen is very easy and the testing time is very short. In the method, a mass that is levitated with a pneumatic linear bearing, and hence encounters negligible friction, is made to collide with a material under test. During the collision the Doppler frequency shift of a laser beam reflecting from the mass is accurately measured using an optical interferometer. The velocity, position, acceleration and inertial force of the mass are calculated from the measured time-varying Doppler shift. To demonstrate the high performance of the proposed method, the impact response of a gel block is highly accurately determined by means of the proposed method.

Optical method for strength test for general industrial products

A method for evaluating the strength of general industrial products is proposed. In the method, a levitated mass encountering negligible friction is made to collide with a general industrial product under test. During the collision, the frequency Doppler shift of a laser beam reflecting from the mass is accurately measured using an optical interferometer. The velocity, position, acceleration and inertial force of the mass are calculated from the measured time-varying Doppler shift. The mechanical response of general industrial products is accurately determined with this method.

Nearfield performance of passive correlation processing sonars

Passive correlation processing sonars in which the distance of a moving source from the receivers is not appreciably greater than the inter-receiver spacing are considered. In the simplified case, where two receivers are employed, the processor chooses one of the received signals as a reference and adds a corrective time delay and Doppler shift to the other signal before the correlation operation is performed. When the processor selects a time delay and Doppler shift corresponding to that present in the signals, the correlation function has a peak. The degree of correlation between the two signals is measured with the use of a passive uncertainty function. An approximation is made for narrow-band signals which is computationally advantageous. The effects of time delay and constant Doppler shift are discussed, and the degradation resulting from time varying Doppler shift is derived and plotted. Results are presented for both sinusoidal and narrow-band type signals. A technique of plotting contours of maximum data integration time is developed and shown to significantly reduce the complex problem of studying scenario geometries.