Presence Of Multipath Interference Research Articles

Model Order Estimation in the Presence of Multipath Interference using Residual Convolutional Neural Networks

Model Order Estimation in the Presence of Multipath Interference using Residual Convolutional Neural Networks

Hybrid Coding and Filtering Technique for Optical IM-DD Link With Robustness to Multipath Interference and Bandwidth Limitation

In this work, a hybrid code division multiplexing (CDM) coding and digital filtering method for optical intensity modulation and direct detection (IM-DD) link is investigated to address multipath interference (MPI) and channel bandwidth limitation. CDM signaling already provides robustness to MPI through suppressing the unsynchronized paths by orthogonal (de)coding. As for digital filtering based on square root raised cosine (SRRC) waveforms, its advantage to IM-DD link includes robustness to channel bandwidth limitation due to pulse shaping. Additionally, matching filters achieve the best orthogonality only when they are synchronized well, thus robustness to MPI can be expected on the basis of CDM. We study the performance of such hybrid coding and filtering method over IM-DD link in the presence of MPI and bandwidth limitations through simulation. This is followed by experimental demonstrations of single point-to-point transmission with capacity at 112 Gbps. BER comparison between CDM only and hybrid filtering and coding method is performed by emulating MPI using a Mach-Zehnder interferometer. Furthermore, bidirectional transmission over a single fiber verifies its performance advantage and demonstrates potentials for reducing implementation costs of end-to-end optical communications.

Characterization and mitigation of multipath fading on multi-frequency GNSS signals in urban environment

The global navigation satellite systems (GNSS) receiver is likely to experience performance degradation in the presence of multipath interferences in indoor and urban canyon environments. There will be multiple reflected signals arriving at the receiver antenna from different paths and directions regarding the different locations of the nearby constructions, causing significant fluctuations and distortions on the amplitude and phase of the direct signals. This is the so-called multipath fading effects and usually exhibit with the random features of temporal, spatial, and frequency diversity across the signal bands. To have a better understanding of the multipath fading effects, a live test was conducted near Lujiazui CBD area in Shanghai. The collected data were processed to characterize the signal attenuations and phase fluctuations on multi-frequency GNSS signals via the detrend code delay, carrier phase, and navigation bit errors. The resulting scintillation index on amplitude, code delay, and carrier phase, e.g., S4, $${\sigma }_{\varphi }$$ and $${\sigma }_{\tau }$$ can be calculated. Based on these multipath features, the inter-frequency aiding strategy will be utilized for the multipath mitigations in the multi-frequency tracking loop design. The results demonstrate the effectiveness of the multi-frequency tracking algorithm in the urban environment.

Real-Time Distance Evaluation System for Wireless Localization

The paper describes the FPGA implementation of a novel position evaluation algorithm based on the time difference of arrival (TDOA) principle that combines the characteristics of an Orthogonal Frequency Division Modulation (OFDM) symbol with the properties of the Zadoff-Chu mathematical sequences. The resulting system is highly scalable and its characteristics are easily adaptable to different operating scenarios. The algorithm has been implemented using the Stratix IV-E EP4SGX70HF35C3 FPGA, requiring about 112k bit of memory and less than 44k logic elements of which about 16k are registers. The paper describes the algorithm and its FPGA implementation along with experimental results validating the system performance even in the presence of multipath interferences and showing precision in target position estimation that is better than 2 cm.

Long-haul quasi-single-mode transmissions using few-mode fiber in presence of multi-path interference.

We study long-haul Quasi-Single-mode (QSM) systems in which signals are transmitted in the fundamental modes of a few-mode fiber (FMF) while keeping other system components such as amplifiers and receivers are kept single-moded. The large-effective-area nature of the FMF fundamental modes improves system nonlinear tolerance in the expense of mode coupling along FMF transmissions which induces multi-path interference (MPI) and needs to be compensated. We analytically investigate 6-spatial-polarization mode QSM transmission systems in presence of MPI and show that in the weak coupling regime, the QSM channel is a Gaussian random process in frequency. MPI compensation filters are derived and performance penalties due to MPI and signal loss from higher-order modes are characterized. We also experimentally demonstrate 256 Gb/s polarization multiplexed (PM)-16-QAM QSM transmissions over a record distance of 2600 km with 100-km span using decision directed least mean square (DD-LMS) algorithm for MPI compensation.

Indoor localization algorithm using clustering on signal and coordination pattern

In recent years, the demand for indoor location-based services has gradually received greater attention. The presence of multipath interference has a tendency to interfere with traditional algorithms calculated based on received signal strength (RSS). The application of virtual tags can greatly reduce deployment costs and enable greater environmental adaptability. However, an excess of ineffectively filtered virtual tags will only lead to greater error in calculation. Therefore, virtual tags are combined with a two-step clustering method to replace the concept of signal hotpoint intersections due to the mutual interdependence of data in space and the influence of neighboring objects. This study improved a two-step location algorithm that combines the advantages of virtual tags and two-step clustering analysis, called clustering-based localization algorithm, offering significant improvement over most traditional localization algorithms. RSS are no longer used as a basis for clustering, and are replaced by the combination of signal and coordination pattern. Two steps of cluster analysis are performed during the filtering process. The first step utilizes the tags’ signals to perform clustering. The second step incorporates tags’ coordinate for filtering. As the clustering-based localization process considers the interactive relationship between coordinate data, it achieves superior results compared to those produced via methods that only use signal strength to select neighboring solutions. This study then constructs a wireless sensor network and assesses the effectiveness of the algorithm.

Observations of low-frequency temporal and spatial coherence in shallow water

Measurements of temporal and spatial coherence are most always confounded by the presence of multipath interference. Here, two sets of data are presented that allow separation of arrivals from individual propagating modes and then unambiguous computations of temporal and spatial coherence free of multipath effects. A consistent finding for surface reflected bottom reflected (SRBR) modes is that lower order mode are more coherent in both time and space than higher order (steeper) modes. Also, SRBR paths are much more coherent than refracted bottom reflected mode groups that cannot be separated in time and are affected by multi-mode interference.

Equivalent system model and equalization of differential impulse radio UWB systems

A discrete-time equivalent system model is derived for differential and transmitted reference (TR) ultra-wideband (UWB) impulse radio (IR) systems, operating under heavy intersymbol-interference (ISI) caused by multipath propagation. In the systems discussed, data is transmitted using differential modulation on a frame-level, i.e., among UWB pulses. Multiple pulses (frames) are used to convey a single bit. Time hopping and amplitude codes are applied for multi user communications, employing a receiver front-end that consists of a bank of pulse-pair correlators. It is shown that these UWB systems are accurately modeled by second-order discrete-time Volterra systems. This proposed nonlinear equivalent system model is the basis for developing optimal and suboptimal receivers for differential UWB communications systems under ISI. As an example, we describe a maximum likelihood sequence detector with decision feedback, to be applied at the output of the receiver front-end sampled at symbol rate, and an adaptive inverse modeling equalizer. Both methods significantly increase the robustness in presence of multipath interference at tractable complexity.

Galois field data sources for simulation of m-ary systems

Sources based on Galois fields are introduced and compared to binary ones in terms of the mean and variability of the resulting bit error rate (BER) estimate. It is found that the use of Galois field data sources for Monte-Carlo simulation of m-ary communications systems largely eliminates BER estimate variability caused by the commonly used binary data sources. Such variability is important when nonlinearities or multpath interference are present, since some of the constellation points cause higher probabilities of error than others. It has been generally assumed that enough symbols will be simulated so that the central limit theorem will cause insignificant variance in results. This is found not the case for m-ary systems in the presence of multipath interference. It is also found that it is important to map separate Galois field data sources to each sub-carrier in the case of multicarrier m-ary systems such as 16-QAM-OFDM.