Channel Impulse Response Function Research Articles

Non-Line-of-Sight Full-Duplex Ultraviolet Communications Under Self-Interference

The full-duplex optical communications can be achieved simply by separating the transmitting link and the receiving link in the space. However, it is challenging to achieve the non-line-of-sight (NLOS) full-duplex ultraviolet (UV) communication due to the serious self-interference caused by the strong multiple scattering effects of UV signals. To explore the capacity of NLOS full-duplex UV communications, in this paper, we first quantify the self-interference using an analytical channel impulse response function. Based on the quantified self-interference, we then derive the error rate and the corresponding achievable information rate (AIR) for on-off keying modulation, 4-digital-pulse-interval modulation and 4-pulse-position modulation. We further propose a self-interference cancelling (SIC) method to mitigate the impacts of self-interferences. Simulation results show that the proposed SIC method can significantly improve the error rate and AIR performances. Besides, we find that the NLOS full-duplex UV communication will gradually lose its advantage over the NLOS half-duplex UV communication as either the communication distance or the elevation angle increases. However, using the proposed SIC method, the NLOS full-duplex UV communication can hold its advantage in a wide range of system geometries.

Ice-tethered acoustic buoys for real-time active and passive monitoring in the Western Arctic

Real-time monitoring of channel impulse response functions in the Beaufort Lens was performed by five Ice-Tethered Acoustic Buoys (ITABs) during March-September 2021. Each ITAB had an eight-element vertical line array and a 900-1000 Hz acoustic source, providing a reciprocal acoustic transmission of up to a 300 km range. Acoustic navigation of one ITAB was successfully tested using the acoustic travel-time from the other ITABs. In addition, a strong single arrival through the Beaufort Lens was observed on each ITAB. Simulations using the measured Q-functions [T. C. Yang, IEEE J. Ocean. Eng. 30(4), 865–880 (2005)] showed a reliable long-range acoustic communication through the Beaufort lens within the 900–1000 Hz frequency band. Finally, a passive ITAB was deployed in March 2022 to measure third-octave under/in-ice ambient noise in the Western Arctic. Both experiments showed that ITABs can be used for under-ice acoustic navigation and communication as well as real-time ambient noise measurements in the Western Arctic. [Work supported by the ONR.]

Channel-flow response function considering the downstream tidal effect and hydraulic characteristics

The channel impulse response function derived from the linearized Saint Venant equations has been widely applied to predict the streamflow discharge. Huang and Lee (2020) indicated that the distribution type of the impulse response function proposed by Dooge et al. (1987a) was restricted because of the constant reference parameters, therefore, they introduced the time-varying reference parameters according to the upstream inflow condition to reinforce the flexibility of adjusting the impulse response function. However, this model, called channel hydrological response function (CHARFU), was merely applicable to a channel under the assumption of a downstream open boundary. This study intended to apply the other impulse response function which was derived from the downstream disturbance and combine this function with the one derived from the upstream input to extend the model applicability in various channel states. To consider the temporal change of the downstream boundary condition, the time-varying reference parameters were also introduced in the function according to the tidal level at the estuary. A routing procedure that explained the integration of the two impulse responses for both upstream and downstream inputs was proposed as well for the runoff simulation of the entire river basin. A series of tests showed that the proposed model could even yield similar hydrographs with the comparative cases that applied the numerical model based on the Saint Venant equations owing to the revision of reference parameters. A practical case of a river basin was also provided to demonstrate the effectiveness of the proposed model to reflect the influence of the downstream tidal level on the simulated hydrograph. Although the proposed linear channel routing method still can not be applied to simulate a channel reach with flow transitions such as a sharp change of water surface elevation due to the assumption of gradually varied flow in the linearized governing equations, it can be more stable than a finite difference model and avoids the latter’s occasional numerical oscillations, especially when applied to a shallow channel with an irregular bed.

Three-Dimensional MIMO Channel Model with High-Mobility Wireless Communication Systems Using Leaky Coaxial Cable in Rectangular Tunnel

With the rapid development of high-mobility wireless communication systems, e.g., high-speed train (HST) and metro wireless communication systems, more and more attention has been paid to the wireless communication technology in tunnel-like scenarios. In this paper, we propose a three-dimensional (3D) nonstationary multiple-input multiple-output (MIMO) channel model with high-mobility wireless communication systems using leaky coaxial cable (LCX) inside a rectangular tunnel over the 1.8 GHz band. Taking into account single-bounce scattering under line-of-sight (LoS) and non-line-of-sight (NLoS) propagations condition, the analytical expressions of the channel impulse response (CIR) and temporal correlation function (T-CF) are derived. In the proposed channel model, it is assumed that a large number of scatterers are randomly distributed on the sidewall of the tunnel and the roof of the tunnel. We analyze the impact of various model parameters, including LCX spacing, time separation, movement velocity of Rx, and K-factor, on the T-CF of the MIMO channel model. For HST, the results of some further studies on the maximum speed of 360 km/h are given. By comparing the T-CF between the dipole MIMO system and the LCX-MIMO system, we can see that the performance of the LCX-MIMO system is better than that of the dipole MIMO system.

Space-Time Correlation for Three-Dimensional MIMO Channel Model Using Leaky Coaxial Cables in Rectangular Tunnel

The rapid development of high-speed train and Metro communications has provided new challenges for the application of MIMO technologies. Therefore, we propose a three-dimensional (3D) multiple-input multiple-output (MIMO) channel model using leaky coaxial cable (LCX) in a rectangular tunnel. The channel model is based on geometry-based single-bounce (GBSB) channel model and the electric field distribution of LCX in the tunnel environment. The theoretical expressions of channel impulse response (CIR) and space-time correlation function (CF) are also derived and analyzed. The CFs for different model parameters (moving velocity and moving time) and different regions of the tunnel are simulated by Monte Carlo method to verify the theoretical derivation at 1.8 GHz. In the same parametric configuration of nonstationary tunnel scenarios, the time delay of the first minimum value of CFs for LCX-MIMO is 1/5 of the time delay of the minimum value of CFs for dipole antennas MIMO when the train moving velocity is 360 km/h. It is shown that, for MIMO system, the performance of using LCXs is better than using dipole antennas.

Monte-Carlo Integration Models for Multiple Scattering Based Optical Wireless Communication

Monte-Carlo models are analyzed for multiple scattering channels in optical wireless communications. It is demonstrated that the system impulse response function can be obtained by Monte-Carlo integration model. The convergence performance for the Monte-Carlo integration model is analyzed and improved by introducing different sampling methods. The simulation results show that the gamma function model for channel impulse response function can only be applied to the cases where the common volume between the transmitted light beam and the receiving field-of-view is open. Numerical simulation suggests that for a three-order scattering case, the computation efficiency of the Monte-Carlo integration model based on partial importance sampling is about 12 times of the original Monte-Carlo integration model based on uniform sampling, and 5.6 times of the widely used Monte-Carlo simulation model. The numerical results also show that the Monte-Carlo integration model based on partial importance sampling has higher computation efficiency than the Monte-Carlo simulation model in a higher-order scattering communication scenario.

Multi-shipping sources classification method based on the line spectral components of shipping radiated noise

The passive target detection based on shipping radiated noise is the key technology for underwater operations today. It has high research value both in civil and military applications. In this paper, the feature extraction of the underwater channels in the cepstrum domain is performed by using the line spectral components of the shipping radiated noise. The channel impulse response (CIR) function is used as the classification basis for the multi-shipping sources, and the line spectral components belonging to the same shipping source in the sonar received signal are searched to solve the classification problem of the multi-shipping radiated noise sources in the complex ocean background.

Characterizing ocean acoustic channel through databases: Issues, requirements, and methods

There is a growing interest in underwater acoustic telemetry. This calls for methods to evaluate the performance of various (proposed) modulation and equalization algorithms under the same channel conditions. An ideal solution is to develop acoustic channel simulators. However, this effort has not been fruitful since acoustic channel simulations (at practical communication frequencies) so far have failed to capture the characteristics of real ocean channels. Currently, acoustic communications still rely on field experiments for evaluation of the algorithm performance. The problem with this approach is that each experiment is usually tailored to a particular modulation scheme and the cost prohibits testing of many different algorithms under the same ocean conditions. Some data-based channel simulators have been proposed where realizations of the channel impulse response (CIR) functions are generated based on the scattering function estimated from real data but it is still not adequate. A scheme to build ocean channel database is proposed in this paper, where CIR is deduced from data on a fractional symbol-by-symbol basis with negligibly small channel estimation error (using the surrogate signal predication error as an indicator) so that the developers can test the performance of different algorithms under same, realistic ocean conditions.

HiCIRF: A high‐fidelity HF channel simulation

A high‐fidelity HF channel simulation has been developed that is suitable for Over‐the‐Horizon Radar (OTHR) and HF communication system design studies and test planning. The simulation capability is called HiCIRF, for High‐frequency Channel Impulse Response Function. HiCIRF provides simulated HF signals corresponding to transmissions from individual transmitter array elements to individual receiver array elements for propagation through the naturally disturbed or undisturbed ionospheric channel. Both one‐way link geometries and two‐way radar geometries can be simulated. HiCIRF incorporates numerical ray tracing and stochastic signal structure computations to realistically simulate signal scatter by small‐scale ionization structure. Stochastic signal generation is employed to generate signal realizations that can be used for OTHR array design and advanced signal processing studies.

Improving bit-error-rate performance of the free-space optical communications system with channel estimation based on radiative transfer theory

In order to improve the performance of terrestrial free-space optical communication systems in adverse visibility conditions, we present a method for estimation of the atmospheric channel impulse response function which governs the optical intensity propagation. This method reduces run-time computational demands and system complexity in comparison to our previously proposed dual-wavelength channel estimation technique. We consider propagation of optical wavelengths in fog, where the droplet diameters are close to the wavelength and thus scattering and absorption effects are significant. A method for rapid calculation of a channel response function based on estimating the effective optical depth of the channel and curve-fitting is described. The channel response estimate can then be used to design a receiver-side equalizer (minimum mean-squared error linear equalizer) to correct the signal distortion due to propagation through the dispersive channel. The channel estimates are based on parametric curve-fitting functions which have been developed using the modified-vector radiative transfer theory to model the channel response. The optimal fit parameters are found using particle-swarm optimization to minimize the simulated bit-error rate of the received signal.

The COST259 Directional Channel Model-Part I: Overview and Methodology

This paper describes a model for mobile radio channels that includes consideration of directions of arrival and is thus suitable for simulations of the performance of wireless systems that use smart antennas. The model is specified for 13 different types of environments, covering macro- micro- and picocells. In this paper, a hierarchy of modeling concepts is described, as well as implementation aspects that are valid for all environments. The model is based on the specification of directional channel impulse response functions, from which the impulse response functions at all antenna elements can be obtained. A layered approach, which distinguishes between external (fixed), large-scale-, and small-scale- parameters allows an efficient parameterization. Different implementation methods, based on either a tapped-delay line or a geometrical model, are described. The paper also derives the transformation between those two approaches. Finally, the concepts of clusters and visibility regions are used to account for large delay and angular spreads that have been measured. In two companion papers, the environment-specific values of the model parameters are explained and justified

Universal properties of the channel Q function: Applications to underwater acoustic communications

Channel Q function is, for a time-invariant channel, the sum of autocorrelation functions of the channel impulse response functions over multiple receivers [T. C. Yang, IEEE J. Oceanic Eng. 28, 229–245 (2003)]. It can be used to characterize the performance (output SNR) of acoustic communications using either passive-phase conjugation or decision feedback equalizer [T. C. Yang, IEEE J. Oceanic Eng. 29, 472–487 (2004)]. Channel Q function also determines the channel capacity for a single source and multiple receivers (SIMO). We analyze in this paper the properties of the channel Q function and show that it possesses universal and stable properties for underwater acoustic waveguides, irrespective of the channel sound-speed profiles, source and receiver range/depths. This property is examined with real data collected in various oceans with significant temporal variations in the propagation conditions and/or source-receiver range. Applications to underwater acoustic communications are discussed. [Work supported by the Office of Naval Research.]

Field-calibrated source tracking using channel measurements from high-frequency mobile platform transmissions

Modern signal processing methods for acoustic arrays in a waveguide use acoustic propagation models to construct signal replicas that capture channel effects such as multipath and refraction. However, such model-based techniques require a detailed map of the propagation environment (often unavailable) and impose the significant computational burden of modeling the channel. We propose an alternative approach to this problem, in which mobile platforms such as AUV’s and gliders act as acoustic beacons for calibrating the field, replacing modeled signal replicas with directly measured channel impulse response functions. Key issues are whether channel estimates made in a (typically) high-frequency communications band and at a limited set of points can be extrapolated to lower frequency and to the entire field. Furthermore, the channel response may change with time, depending on the variability in the ocean waveguide, requiring periodic refresh of the calilbration. We will demonstrate this approach in simulation and with experiment data from two high-frequency experiments, KauaiEx and the Makai Experiment, in which a variety of acoustic communications waveforms were transmitted from towed and fixed systems.

Intensity and time-angle fluctuations of high frequency signal propagation in shallow water

Several high frequency experiments in recent years have been directed towards assessing signal variability as a function of channel environmental parameters. In this paper, results from one of these experiments in a shallow water region are shown. Acoustic communication coded signals were transmitted between a source and two receiver arrays separated 1 and 2 km respectively while detailed measurement of the channel environmental parameters were recorded. Channel impulse response function and the correlation time of the channel are obtained. Ray theory is used to analyze and interpret the experimental data. Arrival time-angle fluctuations were found to be directly correlated with the environmental variability due to ocean dynamics in the experiment region.

The effect of internal waves on mid-frequency underwater acoustic communications

Internal waves are abundant in coastal waters and are known to have a significant impact on acoustic signal propagation. The effect includes significant signal fading to loss of signal coherence depending on the acoustic signal frequencies. This paper presents the analysis results using binary phase-shifted keying signals for underwater acoustic communications during the ASCOT01 experiment. We report the measurements of temporal coherence time as a function of time and relate it to the performance of the decision feedback equalizer. The interpacket coherence was measured using the LFM signals in each packet which were separated by ∼2 min. We see that the intensity and arrival time of the multipath arrivals vary significantly from packet to packets. The signal coherence within a packet (intrapacket) is measured using consecutive m-sequences. We find that at mid (2–5 kHz) frequencies, the acoustic environment, while randomly changing with time on the scale of minutes, presents an instantaneous deterministic environment for each packet; the coherence time is much longer than the packet length. The environmental impact can be mitigated by adequate sampling of the channel impulse response function. [Work supported by the ONR.]

Model-based source localization in the 8–16 kHz band using the channel impulse response function

SignalEx tests have been conducted in a variety of shallow water coastal environments to relate the performance of acoustic communications systems to the prevailing oceanographic conditions. These tests have typically consisted of using a fixed receiver (one to four elements, spaced for diversity) and a transmitter drifting out to ranges beyond the minimum detectable level. During these tests, waveforms to probe the channel in the 8 to 16 kHz band were transmitted at regular intervals. These probe signals were used to test source localization algorithms at these high frequencies. Model-based source localization at these very high frequencies requires either very accurate modeling or algorithms inherently robust against model mismatch. Although the first couple of multipath arrivals can be stabilized from ping to ping, the later arrivals exhibit rapidly fluctuating amplitudes and times of arrival, due to the motion of the ocean surface, water column variability, and the varying bathymetry as the transmitter drifts in range. Measurements of the time-varying channel response and source localization results using the 8–16 kHz band SignalEx channel probes will be presented from sites at the New England Front and the Coronado Bank off San Diego.

Statistics of Multipath Transmission of Finite Bandwidth Signals

The statistical properties of a random variable E that denotes either the time-averaged power in a received band of noise or the total energy in a received transient waveform are studied for situations where the variablity in E is due to uncertainty in the detailed structure of the multipath transmission channel. The transmission channel impulse response function is modeled by a nonstationary Gaussian random process—a model that predicts the Rayleigh probability density for received amplitudes of transmitted pure tones. General expressions are derived for the mean and variance of E. These expressions are reduced to the practically important case of impulse response functions that are δ-correlated in time. Approximate expressions and bounds are derived for the relative variance of E. Special cases of the results are shown to be in essential agreement with results previously obtained in studies of the responses of rooms to bands of noise.