Multipath Simulator Research Articles

Low-latency GNSS multipath simulation and building wall detection in urban environments

Precise navigation for fully autonomous driving—especially in dense urban areas—requires periodic precise position estimates. Global Navigation Satellite System (GNSS) technology has the potential to provide absolute positioning accuracy at a centimeter level. However, buildings in urban environments cause signal distortions and signal reflections—the so-called multipath—which are the most challenging parts in the GNSS error budget. Hence, we developed a scalable real-time multipath simulator for mitigating potential multipath receptions. The simulator uses three-dimensional (3D) building information, satellite, and user positions. The key drivers of latency are the calculation of reflection, diffraction, and line-of-sight, as well as the response time of the 3D building model database. The memory manager of the graphic processing units (GPUs) in combination with a dedicated load balancer enables fast and efficient multipath analysis. Selected case studies demonstrate the simulator’s potential to significantly improve the position accuracy of the processing engine. The use of the multipath simulator reduces the error in 61% of the error measurements in a stress test scenario to less than half of the non-multipath processing. The scalability of the simulator is demonstrated by combining the multipath simulator with a traffic simulator. Furthermore, we present a novel methodology for the detection of walls using GNSS signals to better account for incomplete or erroneous 3D building information in GNSS signal processing.

Anti-Multipath Orthogonal Chirp Division Multiplexing for Underwater Acoustic Communication

Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new anti-multipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of $10^{-6}$ , while the OFDM has a BER floor of $10^{-3}$ . The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

Experimental results of multipath behavior for GPS L1-L2 and Galileo E1-E5b in static and kinematic scenarios

Abstract Current trends like Autonomous Driving (AD) increase the need for a precise, reliable, and continuous position at high velocities. In both natural and man-made environments, Global Navigation Satellite System (GNSS) signals suffer challenges such as multipath, attenuation, or loss-of-lock. As Highway Assist and Highway Pilot are AD next steps, multipath knowledge is necessary for this typical user-case and kinematic situations. This paper presents a multipath performance analysis for GPS and Galileo satellites in static, slow, and high kinematic scenarios. The data is provided from car test-drives in both controlled and unrestricted, near-natural environments. The Code-Minus-Carrier (CMC) and cycle-slip implementations are validated with measurement data from consecutive days. Multipath statistical models based on satellite elevation are evaluated for the three investigated scenarios. Static models derived from the car setup measurements for GPS L1, L2 and Galileo E1 and E5b show a good agreement with a state-of-the-art model as well as the enhanced Galileo signals performance. Slow kinematic multipath results in a controlled environment showed an improvement for both navigation systems compared to the static measurements at the same place. This result is confirmed by static and slow kinematic multipath simulations with the same GNSS receiver. Post-processing analysis on highway measurements revealed a bigger multipath bias, compared to the open-sky static and slow kinematic measurement campaigns. Although less critical as urban or rural, this indicates the presence of multipath in this kind of environment as well. The impact of different parameters, including receiver architecture and Signal-to-noise ratio (SNR) are analyzed and discussed. Differential position (DGNSS) based on code is computed for each epoch and compared against GNSS/INS integrated position for all three measurement campaigns. The most significant 3D absolute error occurs where the greatest multipath envelope is found.

Occurrence and detection of impact multipath simulations of bending angle

In the moist tropical lower troposphere, atmospheric refractivity fields are strongly affected by water vapour and have complicated, non‐spherically symmetric structures. Strong horizontal gradients of refractivity could make the simulated bending angle by raytracing a multivalued function of the impact parameter, which is called impact multipath for brevity. In this study, we first show such occurrences of impact multipath in the tropical lower troposphere using the National Centers for Environmental Prediction/Global Forecast System analysis as input to a raytracing operator for COSMIC radio occultations (ROs) in March and April 2017. An up‐to‐600 m lift in altitude for the impact parameter is observed for simulated RO rays in the presence of a strong horizontal gradient of refractivity over 250 km distances from the perigee, rendering the simulation bending angles multivalued functions of impact parameter. A quality‐control procedure is then developed to effectively identify the large variations of parameter along the simulated rays and impact multipath simulations while keeping the simulated rays below and above the multivalued rays. Many more RO data in the boundary layer could be used for data assimilation in numerical weather prediction by not removing the RO data below the impact multipath levels in the moist tropical lower troposphere.

Monte-Carlo simulations of Ising-like materials: Multipath Metropolis vs cluster update approach

Multipath Metropolis simulation of classical Heisenberg model has been proposed in Physica A 441, 69 (2016). In contrast to the original single–path algorithm, multipath simulation approach is inherently parallel because it uses different random–walk paths that are mutually independent. To further test the reliability of multipath approach, single flip multipath simulation is tested against standard cluster update algorithm implemented with ALPS libraries in case of exactly solvable 2D Ising model.

An EM model for radar multipath simulation and HRRP analysis of low altitude target above electrically large composite scale rough surface

ABSTRACTThe present paper deals with a new electromagnetic model in evaluating the radar multipath scattering and its influence on radar High Resolution Range Profile (HRRP) characteristics of a low altitude target above the composite scale rough surface. The model is underpinned by the theory of facet-based rough surface description at high radar frequencies. It involves the coupling mechanisms by employing an extended image method on locally mirror-like facets with two-scale formulas. It has the advantage to reflect both the specular and diffuse features in description of the multipath scattering mechanisms in the real scene. Meanwhile, it can accomplish electromagnetic simulations over the electrically large scale and complex environments effectively and efficiently. The model is validated by the exact computational electromagnetic numerical method. Numerical simulations investigate the Normalized Radar Cross Section (NRCS) of a missile-like target above a soil land area at different heights and observe the multipath impacts on the HRRP characteristics with different simulating parameters. The conclusions of this work are valuable for the radar detection and remote sensing.

Monitoring Bare Soil Freeze–Thaw Process Using GPS-Interferometric Reflectometry: Simulation and Validation

Frozen soil and permafrost affect ecosystem diversity and productivity as well as global energy and water cycles. Although some space-based Radar techniques or ground-based sensors can monitor frozen soil and permafrost variations, there are some shortcomings and challenges. For the first time, we use GPS-Interferometric Reflectometry (GPS-IR) to monitor and investigate the bare soil freeze–thaw process as a new remote sensing tool. The mixed-texture permittivity models are employed to calculate the frozen and thawed soil permittivities. When the soil freeze/thaw process occurs, there is an abrupt change in the soil permittivity, which will result in soil scattering variations. The corresponding theoretical simulation results from the forward GPS multipath simulator show variations of GPS multipath observables. As for the in-situ measurements, virtual bistatic radar is employed to simplify the analysis. Within the GPS-IR spatial resolution, one SNOTEL site (ID 958) and one corresponding PBO (plate boundary observatory) GPS site (AB33) are used for analysis. In 2011, two representative days (frozen soil on Doy of Year (DOY) 318 and thawed soil on DOY 322) show the SNR changes of phase and amplitude. The GPS site and the corresponding SNOTEL site in four different years are analyzed for comparisons. When the soil freeze/thaw process occurred and no confounding snow depth and soil moisture effects existed, it exhibited a good absolute correlation (|R| = 0.72 in 2009, |R| = 0.902 in 2012, |R| = 0.646 in 2013, and |R| = 0.7017 in 2014) with the average detrended SNR data. Our theoretical simulation and experimental results demonstrate that GPS-IR has potential for monitoring the bare soil temperature during the soil freeze–thaw process, while more test works should be done in the future. GNSS-R polarimetry is also discussed as an option for detection. More retrieval work about elevation and polarization combinations are the focus of future development.

A Forward GPS Multipath Simulator Based on the Vegetation Radiative Transfer Equation Model.

Global Navigation Satellite Systems (GNSS) have been widely used in navigation, positioning and timing. Nowadays, the multipath errors may be re-utilized for the remote sensing of geophysical parameters (soil moisture, vegetation and snow depth), i.e., GPS-Multipath Reflectometry (GPS-MR). However, bistatic scattering properties and the relation between GPS observables and geophysical parameters are not clear, e.g., vegetation. In this paper, a new element on bistatic scattering properties of vegetation is incorporated into the traditional GPS-MR model. This new element is the first-order radiative transfer equation model. The new forward GPS multipath simulator is able to explicitly link the vegetation parameters with GPS multipath observables (signal-to-noise-ratio (SNR), code pseudorange and carrier phase observables). The trunk layer and its corresponding scattering mechanisms are ignored since GPS-MR is not suitable for high forest monitoring due to the coherence of direct and reflected signals. Based on this new model, the developed simulator can present how the GPS signals (L1 and L2 carrier frequencies, C/A, P(Y) and L2C modulations) are transmitted (scattered and absorbed) through vegetation medium and received by GPS receivers. Simulation results show that the wheat will decrease the amplitudes of GPS multipath observables (SNR, phase and code), if we increase the vegetation moisture contents or the scatters sizes (stem or leaf). Although the Specular-Ground component dominates the total specular scattering, vegetation covered ground soil moisture has almost no effects on the final multipath signatures. Our simulated results are consistent with previous results for environmental parameter detections by GPS-MR.

Assessment of modernized GPS L5 SNR for ground-based multipath reflectometry applications

Global Positioning System (GPS) signal-to-noise ratio (SNR) measurements can be employed to retrieve environmental variables in multipath reception conditions, whereby direct or line-of-sight transmission is received simultaneously with coherent reflections thereof. Previous GPS SNR multipath studies of soil moisture and snow depth have focused on the legacy GPS L1 and L2 bands. In the present paper, short-delay, near-grazing incidence multipath from the L5-band GPS SNR is assessed for its value in detecting soil moisture and snow depth. The L5 signal will become more important in the future because of compatibility and interoperability among the different global satellite navigation systems. The L5 results are compared with L2C estimates to determine whether the L2C–L5 differences (given their differing power budgets and their modulation properties) are significant. To address these questions, measurements and simulations were employed. A physically-based multipath simulator was enhanced to investigate the differences between parameter retrievals for the L2C and the L5 GPS signals. Parameter retrievals from synthetic observations for different scenarios were compared. Comparisons included varying reflector height, surface material, and surface roughness. Measurements from two GPS stations in Colorado, USA, were used to retrieve soil moisture and snow depth conditions. Over a 153-day period that encompassed the catastrophic 2013 Colorado flooding event, L2-derived volumetric soil moisture had an RMS difference of 0.042cm3/cm3 while the L5 RMS difference was 0.034cm3/cm3 with respect to in-situ data (values of volumetric soil moisture range between 0.04 and 0.34cm3/cm3). In a separate 483-day campaign, L5-derived snow depth estimates were compared to L2C-derived values and found strongly correlated, deviating from a one-to-one relationship by only 0.00001±0.0064cm/cm. These results indicate the absence of any detectable biases in L5 as compared to L2C for retrieving soil moisture and snow depth from GPS SNR multipath observations.

A Novel Prediction Model for WBAN Frequency Nonselective Fading Channel

The prediction of the frequency non-selective WBAN fading channel behaviors is investigated. An novel WBAN prediction channel model is implemented based on Suzuki fading channel and Rice method and the parameters of the proposed model has been adjusted through MEA, MEDS, Jakes and Monte Carlo multipath simulation method. The performance evaluation for the adaptive WBAN channel model based on simulations is given. The validity of the proposed prediction model is confirmed by comparison of the first order statistics of the measured data.

Impact of Scattering Within a Multipath Simulator Antenna Array on the Ricean Fading Distribution Parameters in OTA Testing

In this paper, we investigate the unwanted scattering that exists within the multipath simulator (MPS) array antennas employed in over the air (OTA) testing of mobile terminals. The impact of scattering is evaluated in terms of the measurement uncertainty of the average received power and the Ricean <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$K$</tex> </formula> -factor. The maximum ratio combining diversity is investigated for a generic device under test comprising two half-wavelength dipole antennas. We provide closed-form expressions for the uncertainties of the average received power and the Ricean <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$K$</tex></formula> -factor for a uniform circular array of MPS antennas and a 2-D uniformly distributed angle-of-arrival spectrum. We also derive the maximum number of MPS antennas and the minimum ring radius of the MPS system as a function of the separation between the most distant antenna elements if the device under test employs a uniform linear array. As a result, we provide design guidelines for MPS array in terms of the number antennas, the radius of the MPS array and the wavelength of the carrier frequency.

Multipath Simulator Measurements of Handset Dual Antenna Performance With Limited Number of Signal Paths

Antenna pairs for diversity or MIMO functionality are characterized under the assumption of a certain statistical distribution of the incident signals over angle and polarization, but also assuming a signal environment with a large number of signal paths. In many real-life environments, however, only a few signal paths contain most of the transferred power. A multipath simulator can be used to realize signal environments with a controlled number of signal paths. This paper presents measurements of dual antenna performance using a multipath simulator with 2-16 signal paths. The results are analyzed in terms of statistical power distributions, power imbalance, correlation coefficient, multiplexing efficiency, and diversity gain. Differences in performance depending on the number of signal paths are noted, illustrating the value of considering the number of signal paths in characterization.

Measurement Uncertainty in Multipath Simulators Due to Scattering Within the Antenna Array—Theoretical Model Based on Mutual Coupling

Multipath simulators are attracting much interest as tools for mobile terminal tests. The equipment uses an array of antenna elements placed around the test object, and signals to the test object are distributed over the array in various ways. The array antennas cause unwanted scattering, resulting in increased disturbance level in the room, with consequences to the measurement uncertainty. In this letter, a quantification of the measurement uncertainty is found through a study of the mutual coupling between the array elements. A statistical approach is applied, and an expression for the standard deviation in transferred power is derived. Measurements are presented that verify the expression.

Multipath simulator for mobile terminal antenna characterisation

A measurement equipment for terminal antennas is presented. The equipment simulates a 2-D multipath signal environment in a lab, enabling fast measurements of weighted radiation properties. It consists of 16 antenna elements encircling the antenna under test, and a 1-to-16 feed network including Doppler generators on the 16 branches. In-the-air combining at the terminal under test gives fast fading. Averaging the measured power over the fading gives a value proportional to the antenna mean effective gain. The signal environment at the test object corresponds to a realistic 2-D scattering environment. The equipment can thereby also be used for simulated drive tests. In the presented work, a multipath simulator for 2–2.6 GHz is designed, built and verified in different types of rooms.

Travelling‐wave multipath simulation of two‐conductor HF signalling over indoor power‐line networks and RMS‐delay‐spread dependence

AbstractAn efficient, travelling‐wave based multipath‐simulation technique is presented, suitable for the estimation of Power‐Line Communication channels' response in terms of both transfer function and power delay profile. The cable transmission parameters are thereupon requisite, as well as the vicinal network topology. The algorithm applies directly to narrowband single‐phase HF signalling over cabling compliant with the two‐conductor transmission‐line model. Experimental verification throughout the 1.6–30 MHz band on the basis of the quantitative path‐incorporation criterion proposed, reveals resultant amplitude‐response accuracy of no less than 5 dB. Dependency of the RMS delay spread on the multitude of network's impedance‐mismatch points, the overall network length, as well as the carrier frequency, is in addition investigated. Quite clear functional interrelations ensue thereupon, whereat proper analytical descriptions are proposed. Applicability of the multipath approach introduced is indicated for narrowband and wideband channel capacity estimations, and relevant hints towards the treatment of multiconductor line structures are suggested. Copyright © 2006 AEIT

Experiments on real-time 1-Gb/s packet transmission using MLD-based signal detection in MIMO-OFDM broadband radio access

This paper presents experimental results on real-time packet transmission of greater than 1 Gb/s using 4-by-4 multiple-input-multiple-output (MIMO) multiplexing and maximum-likelihood detection (MLD)-based signal detection with a decreased level of computational complexity in orthogonal frequency-division multiplexing (OFDM) radio access. We apply our previous algorithm called adaptive selection of surviving symbol replica candidates (ASESS) based on the maximum reliability in MLD employing QR decomposition and the M-algorithm (QRM-MLD) to reduce the extremely high level of computational complexity in the conventional MLD. The experimental results using multipath fading simulators are in good agreement with the computer simulation results. The loss in the required received signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) is suppressed to within approximately 1-2 dB. Therefore, through experiments, we demonstrate that the QRM-MLD employing ASESS is very beneficial in reducing the influence of hardware implementation loss, as well as in decreasing the required received Eb/N0. We further show that the extremely high-speed real-time packet transmission of greater than 1 Gb/s in a 100-MHz channel bandwidth (i.e., 10 bit/s/Hz) is achieved at the average received Eb/N0 per receiver antenna of approximately 12 dB using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing

Effects of Multipath and Signal Blockage on GPS Navigation in the Vicinity of the International Space Station (ISS)

Previous studies examining GPS relative navigation for spacecraft performing rendezvous with the International Space Station (ISS) have not accounted for degradation in GPS navigation performance due to multipath or blockage of GPS signals by the ISS. This study analyzes these effects on GPS navigation in the vicinity of the ISS. A simulation of a spacecraft GPS receiver operating near the ISS has been developed. This simulation includes orbit models for the GPS constellation, the ISS, and the spacecraft, as well as models for GPS signal blockage and multipath. The blockage simulation shows that aiding of GPS is required when the spacecraft approaches within 60 m of the ISS. The multipath simulation shows the expected trends in range errors as a function of GPS satellite elevation angle, distance from the ISS, number of multipath rays, and the radar cross-sectional area of the ISS.

Simulation of Simultaneous Events in Regular Expressions for Run-Time Verification

When specifying system requirements, we want a language that can express the requirements in the simplest and most intuitive form. Although the MaC system provides an expressive language, called MEDL, it is generally awkward to express certain features like temporal ordering of complex events, timing constraints, and frequencies of events which are inherent in safety properties. MEDL-RE extends the MEDL language to include regular expressions to easily specify timing dependencies and timing constraints. Due to simultaneous events generated by the MaC system, monitoring regular expressions by simulating DFAs would result in a potential problem. The DFA simulations would involve concurrent multi-path simulations and result in exponential running time. To handle simultaneous events inexpensively, we generate a dependency graph to identify possible simultaneous events. Further, we augment the original DFAs with alternative transitions, which will substitute for multi-path simulations.

Development and application of GPS signal multipath simulator

A ray tracing GPS signal multipath simulator which takes into account the signal reflection and diffraction from surrounding objects has been developed. By properly modeling the environment around an antenna and the GPS receiver's tracking loop, this simulator can assess the GPS signal multipath error. Thus, it can be used in the early design phase of an experiment to foretell hazardous environmental configurations that can cause severe multipath. It can also aid in finding the best antenna type, location, and orientation within a given environment, and provides a quantitative estimate of multipath errors on GPS measurements. The capability of the simulator is demonstrated by using the International Space Station (ISS) environment.

Experiments on pilot symbol-assisted coherent multistage interference canceller for DS-CDMA mobile radio

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10/sup -3/ is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link.