Multipath Channels Research Articles

Effectiveness evaluation of FTN signaling in the presence of nonlinear distortion under multipath fading channels

Effectiveness evaluation of FTN signaling in the presence of nonlinear distortion under multipath fading channels

Impact of Common Reflecting and Absorbing Building Materials on THz Multipath Channels.

THz band communication has the potential to meet the high data rate demands of many current and future applications. However, before these networks are realized, extensive channel measurements are needed in order to characterize the wireless channel at these frequencies, in order to inform system design and deployment. In the current paper, we present a set of double‐directional channel measurements that are conducted in several relevant indoor and outdoor scenarios. Our aim is to see the effect of common building materials that might be particularly reflective or absorptive (such as energy‐saving glass, window blinds, or metallic reflectors), and how their presence changes the channel characteristics. Among other effects we find that ‐ depending on the considered dynamic range ‐ presence/absence of these materials can increase the required equalizer length by an order of magnitude.

Outlier-Resistant Localization for Smart Cities: Low-Rank Approximation-Based Approaches

For smart cities, location-based services (LBSs) are indispensable; however, the urban environment is typically a multipath channel and achieving high accuracy localization is challenging, especially in GNSS-denied environments. It is already known that there are two key factors that constrain IoT localization performance, namely, the presence of outliers in the inter-node ranging data and the difficulty to guarantee that ranging is carried out between all nodes, which means that we are dealing with an incomplete Euclidean distance matrix (EDM) contaminated with outliers. In this paper, we propose a robust localization framework, termed low-rank approximation-based localization (LRAbL). LRAbL enables network localization in a stepwise manner using partially observed EDMs that contain coarse noise (outliers). Specifically, the working process of LRAbL can be divided into three stages: the preprocessing of the observed EDM, aiming to eliminate outliers with large values, the use of low-rank approximation means to obtain the complete EDM, and finally the application of non-classical MDS to calculate the coordinates of the nodes in the network. To confirm the applicability of the proposed framework, extensive numerical experiments were conducted, which indicated that LRAbL was still able to achieve satisfactory localization results when the network links were sparse (only about 1/4 of the entries can be observed) and contained a certain percentage of large value outliers. In summary, our work provides a solution worthy of consideration for location-based services in the future Internet of Things.

Multi‐path separation and parameter estimation by single DMA in fading channel

The propagation of electromagnetic waves will produce multi-path fading of wireless channel, which degrades communication performance substantially. If multi-path can be separated, better signal transmission and reception strategies can be designed to weaken or eliminate fading and improve signal transmission performance. However, it is challenging for communication nodes with limited number of antennas. A general multi-path separation and measurement framework based on single dynamic metasurface antenna is established which rapidly changes patterns within a single pilot symbol period so that equivalent multi-dimensional received signals can be obtained. Based on the framework, a multi-path channel estimation scheme is proposed. Specifically, the parameter estimation problem is formulated and present the algorithm based on atomic norm minimisation to estimate parameters including angle of arrival and complex gain of each propagation path. Simulation results illustrate that single dynamic metasurface antenna can approach the capability of a conventional multi-antenna array, and even using single dynamic metasurface antenna with 1-bit coding can estimate angle of arrivals and complex gains of multi-path components accurately.

A new multipath channel estimation and mitigation using annihilation filter combined tracking loop implementation in software GPS receivers

The multipath effect causes severe degradation in the positioning of commercial GPS receivers. Due to multipath error, the positioning accuracy could reach a few of 10 m. If the cumulative Multipath delay is less than 0.1–0.35 chips, then it is difficult to mitigate in GPS receivers. This causes severe degradation in GPS signals and can cause a measurement bias. To alleviate this problem, the estimation of multipath parameters using annihilating filter and its mitigation in the GPS tracking loop is proposed in this work. The estimation of randomly generated multipath signals can be performed in the receiver with a lower sampling rate when compared to the larger bandwidth of the GPS baseband signal. Here, the frequency components of the multipath signal in superimposed complex exponentials have been transformed from the time delay and the amplitude of the path observables. The Rayleigh fading model in the urban scenario has been simulated in which the amplitude and the phase of the number of paths (i.e. the frequency component of superimposed complex exponentials) are set and this fading signal is convolved with GPS signal that forms the multipath faded signal. In the GPS receiver post-processing stage, with the help of the annihilation filter, the multipath components are estimated, then an inverse/adaptive filter and compensation technique are further applied to mitigate the multipath component. The mean square error with the different number of paths with noisy environments is analyzed utilizing the cadzow denoising algorithm. The simulation results of the proposed technique employed in the tracking module of the software GPS receiver under severe multipath conditions indicate a substantial enhancement in the performance of the GPS receiver with minimal code and carrier phase error when compared to the least squares and adaptive blind equalization channel techniques. Moreover, the positioning accuracy is also calculated with the inclusion of multipath components in two satellites out of six satellites used in the simulation, the results showed that the annihilation filter improved the mean position accuracy up to 9.3023 m.

A novel asymmetrically clipped optical orthogonal frequency division multiplexing system based on lifting wavelet transform for visible light communications

SummaryIn multicarrier visible light communication (VLC) systems, orthogonal frequency division multiplexing (OFDM) has attracted a lot of attention due to its high data rate, simple equalization, and resistance to intersymbol interference (ISI). However, high peak‐to‐average‐power ratio (PAPR) and Light Emitting Diodes (LED) nonlinearity have important effects on the performance of indoor VLC‐OFDM systems. Asymmetrically clipped optical (ACO)‐OFDM is a technique that waste spectral efficiency to transmit a single pole frequency division multiplexed signal over a single pole channel. In this study, a new ACO‐OFDM method based on lifting wavelet transform (LWT)‐ACO‐OFDM is proposed to restore this spectral efficiency and increase the performance of the ACO‐OFDM system. The performance of the proposed method is verified by computer simulation studies over additive white Gaussian noise (AWGN) channel and multipath optical channel for analytical and simulated bit error rate (BER) performance criterion. From the obtained simulation results, it is seen that the proposed method provides approximately 16‐dB SNR gain against the classic VLC‐OFDM, roughly 12‐dB Signal to Noise Ratio (SNR) improvement versus the Direct Current (DC)‐biased optical (DCO)‐OFDM, and nearly 6‐dB SNR enhancement against ACO‐OFDM on the additive white Gaussian noise channel. It is observed from computer simulation results that similar SNR gains are obtained in multipath optical channels.

Implementation of COFDM Reed Solomon using TMS320C6713

This sheet research presents the implementation of Coded Orthogonal Frequency Division Multiplexing (COFDM) communication system encoded using Reed Solomon (RS) Coding implemented on TMS320C6713 platform. The collaboration of MATLAB integrated development environment (IDE) and Code Composer Studio (CCS) are adopted. The real time implementation is one of the most difficult issues of OFDM implementation. The problem of real time implementation is the memory consumption and the processing time. The other problem is Synchronization of OFDM receiver. Time and frequency synchronization to distinguish the begin of the OFDM symbol. The capability of RS for encoding the error correction ability makes it the most used error correction codes algorithm, so we use it with OFDM systems to combining them make the system has excellent performance potentiality in multipath and fading channels. The bit error rate and the number of error bits are calculated during each test on the system.

Multipath Canceled RF Fingerprinting for Wireless OFDM Devices Based on Hammerstein System Parameter Separation

Radio frequency fingerprint (RFF) based authentication of wireless devices can be used in the fields of the access security at the physical-layer of wireless networks and radio spectrum management. However, the stability of RFF is easily damaged by the wireless multipath fading channel in mobile communications. An RFF fingerprinting method with the nonlinearity and in-phase and quadrature (IQ) imbalance of the transmitter is proposed for chunk-based wireless orthogonal frequency division multiplexing (OFDM) devices based on a Hammerstein system parameter separation technique, which cancels the adverse influence of the time-varying multipath channel. First, the parameters of the nonlinear model of the transmitter and finite impulse response (FIR) of the wireless multipath channel are estimated with the Hammerstein system parameter separation technique. Second, the best IQ imbalance parameter combination is obtained with the FIR estimation of the channel. Finally, the estimated parameters of the nonlinear model and IQ imbalance are used as RFFs to classify the transmitters. Theoretical analyses and numerical experiments demonstrate that the obtained RFFs are stable and the fusion authentication of transmitters with subtle differences from the same model and same series is feasible using the novel method.

Physical Layer Security in an OFDM Time Reversal SISO Communication With Imperfect Channel State Information

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\epsilon }-$ </tex-math></inline-formula> achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication.

Energy efficient OFDM mode with the ability to adapt to propagation conditions in multipath channels

Energy efficient OFDM mode with the ability to adapt to propagation conditions in multipath channels

A Method for Underwater Acoustic Key Detection Based on OFDM Pilot Sequences

The traditional physical layer key generation technology uses channel temporal variation, reciprocity, and spatial decorrelation to realize key distribution and fast update. However, due to the complex, time-varying and time-delayed features, the underwater acoustic channel environment often changes during channel detection, resulting in impaired channel reciprocity. To solve this problem, this paper proposes an orthogonal frequency division multiplexing pilot structure based on the features of underwater acoustic time-varying multipath channel. The structure can complete key detection when channel reciprocity is impaired, promotes the consistency of observation sequences, and reduces the key disagreement rate. The theoretical deduction proves the validity of the proposed method under an underwater acoustic multipath time-varying channel. Also, the simulation results verify the feasibility of the proposed method.

Experimental Analysis of Spatial Modulation Systems in Mixed LOS/NLOS Scenarios

Spatial modulation (SM) as a multiple-input multiple-output (MIMO) technique is a solution suitable which offers, with low system complexity and cost, improved spectral efficiency compared to single-input-single-output (SISO) systems. Moreover, the transmission chain is simplified which decreases energy consumption. This paper aims to analyze the SM system transmission under different line-of-sight/non-line-of-sight (LOS/NLOS) propagation scenarios. The analysis of the SM system performance is based on both simulations and experimental results and the bit error rate (BER) is tackled. Concerning the experimental results, two strategies are considered in order to configure the propagation channel. One method consists in constructing a controlled propagation environment. More precisely, the multipath channel’s Rician K-factor is imposed by configuring the power levels of the LOS and of the NLOS components. The second method consists in performing over-the-air transmissions on a realistic propagation channel. A channel sounding method allows us to measure the channel characteristics. The experimental results are confronted with the system-level simulation results, good agreement between experimental and simulation results are obtained. The results show that the SM system can maintain the performance in multipath propagation in the presence of indirect paths for both two proposed propagation environments.

Корреляционный прием частотно-манипулированных сигналов в режиме с псевдослучайной перестройкой рабочей частоты

Proposals are presented on the use of signals generated on the basis of the three-element Barker sequence for the mode of frequency-hopping spread spectrum (FHSS). The conditions for the application of the frequency hopping mode are considered. An algorithm for synthesizing signals with a simultaneous change in their frequency and phase structure has been developed. An analysis of the properties of the developed signals was carried out. The positive and negative aspects of the implementation of the developed approach are shown. Proposals for their practical application are formulated. The frequency-hopping spread spectrum is actively used in many telecommunication standards and technologies in order to increase the noise immunity of transmissions in a multipath channel. As a result, due to the short stay of the signal at the carrier frequency, the probability of long-term operation in a channel with deep fading is reduced. Since the structure of the signal and frequency-hopping spread spectrum parameters are determined by known protocols and standards, this makes it possible to evaluate its capabilities in practical applications. In order to reduce the negative consequences caused by the considered collisions, it is proposed to generate signals for the slow frequency-hopping spread spectrum based on the three-element Barker sequence. Features of the implementation of this approach are considered in this article.

Low Complexity Non-Uniform FFT for Doppler Compensation in OFDM-Based Underwater Acoustic Communication Systems

The Doppler effect critically degrades the performance of orthogonal frequency division multiplexing (OFDM) systems in general. This problem is significantly worse for underwater acoustic (UWA) communication systems due to the distinct characteristics of the underwater channel, resulting in the loss of orthogonality among sub-carriers. In order to compensate Doppler shifts, including phase noise and multipath channels in realistic communication scenarios, the joint of channel estimation and ICI reduction is often performed. However, the accuracy depends on the channel estimation and the FFT size, while this leads to increased computational complexity at the receiver. To achieve this dual goal in the actual underwater communication environment, a novel pilot structure in the frequency domain has been applied to overcome the channel impulse response (CIR) variation in a block period. The coarse Doppler shift is firstly estimated by using the received pilot signal. Afterward, the study takes advantage of the flexibility provided by non-uniform fast Fourier transform (NFFT) in choosing the sampling points to construct a fast and stable Doppler frequency Compensation Matrix-based NFFT (DCMN) to fine compensate the Doppler phase shift. Finally, this study shows the improvement of the proposed method&#x2019;s performance by actual experimental measurements and simulations.

A Joint Sonar-Communication System Based on Multicarrier Waveforms

Jointdetection and communication systems demonstrate their unique efficiencies in both spectrum and cost. In this letter, we propose a sonar-communication (SonarCom) system for underwater scenarios based on two multicarrier (MC) waveforms, which are orthogonal frequency division multiplexing (OFDM) and orthogonal chirp division multiplexing (OCDM) waveforms. Different types of waveforms provide flexibilities to deal with various underwater environments. Furthermore, a generalized likelihood ratio test (GLRT) is proposed for detection to deal with multipath channels. Time aliasing techniques are applied to leverage the signal structure. Moreover, a minimum mean square error (MMSE) equalizer is developed for communication to counter frequency-selective fading. Simulation results show that the GLRT detector for the SonarCom system outperforms the existing matched filter (MF). The OCDM scheme maintains better communication performance than the OFDM one.

Localization of Mixed Far-field and Near-field Incoherently Distributed Sources Using Two-stage RARE Estimator

In this paper, a mixed source localization method utilizing a two-stage rank-reduction (RARE) estimator is investigated. Different from the existing methods, the proposed one is built on the incoherently distributed (ID) source model, which is more appropriate for multipath and fast time-varying channels. Firstly, a general array manifold (GAM) model is established, where nominal DOAs and nominal ranges are extracted from the initial array manifold. By exploiting the shift invariance property of the far-field (FF) GAM and combining virtual source enumeration result, nominal FF DOA estimation is achieved by a one-dimensional (1-D) RARE spectral search. Secondly, the oblique projection operation is adopted to separate near-field (NF) sources, and the nominal DOA and range parameters of NF sources are subsequently obtained by jointly utilizing the manifold separation technique (MST) and another two 1-D spectral searches. With the estimated nominal DOA and range parameters, the angular spread and range spread are then successfully estimated. Moreover, the Cram&#x00E9;r-Rao bound (CRB) for the considered case is also derived. Simulation results are presented to validate the effectiveness of the proposed method.

Power Domain Multiplexing Waveform for 5G Wireless Networks

Power domain non-orthogonal multiple access combined with a universal filtered multi-carrier (NOMA-UFMC) has the potential to cope with fifth generation (5G) unprecedented challenges. NOMA employs power-domain multiplexing to support several users, whereas UFMC is robust to timing and frequency misalignments. Unfortunately, NOMA-UFMC waveform has a high peak-to-average power (PAPR) issue that creates a negative affect due to multicarrier modulations, rendering it is inefficient for the impending 5G mobile and wireless networks. Therefore, this article seeks to presents a discrete Hartley transform (DHT) pre-coding-based NOMA enabled universal filter multicarrier (UFMC) (DHT-NOMA-UFMC) waveform design for lowering the high PAPR. Additionally, DHT precoding also takes frequency advantage variations in the multipath wireless channel to get significant bit error rate (BER) gain. In the recommended arrangement, the throughput of the system is improved by multiplexing the users in the power domain and permitting the users with good and bad channel conditions to concurrently access the apportioned resources. The simulation outcomes divulge that the projected algorithm accomplished a gain of 5.8 dB as related to the conventional framework. Hence, it is established that the proposed DHT-NOMA-UFMC outperforms the existing NOMA-UFMC waveform. The key benefit of the proposed method over the other waveforms proposed for 5G is content gain due to the power domain multiplexing at the transmitting side. Thus, a huge count of mobile devices could be supported under specific restrictions. DHT-NOMA-UFMC can be regarded as the most effective applications for 5G Mobile and Wireless Networks. However, the main drawback of the proposed method is that the Fourier peak and phase signal is not easily estimated.

Outage and Error-Rate Analysis of NOMA-Inspired Multicarrier Underlay Cognitive Radio System

In this paper, the performance of a multicarrier underlay cognitive radio system is analyzed considering that the successive interference cancellation (SIC) method is used for detection. In the proposed system model, a multicarrier-based secondary user (SU) uses all the subcarriers of the primary user (PU) in underlay mode, guaranteeing the quality of service (QoS) to the primary system. The power allocation strategy is modified accordingly. With a fixed PU transmit power, power is allocated to the SU subcarriers to maximize its throughput while ensuring minimum throughput to PU. The system is analyzed for its performance considering an L-tap multipath Rayleigh fading channel. Analytical expression for outage probability at SU is derived in closed-form. Also, analytical expressions and closed-form approximations for average symbol error rates (SER) per subcarrier of PU and SU are obtained. The derived expressions are validated with the Monte-Carlo (MC) simulations. Further, the proposed scheme was implemented in an orthogonal frequency division multiplexing (OFDM)-based system with different detection schemes used at the receiver. SERs were compared through extensive simulations and it was noticed that SIC with channel estimation based on minimum-mean-squared-error (MMSE) often performed better than that based on least squares (LS) estimation. Observations reveal that the nominal error rates were achieved on the optimal choice of the parameters. The proposed system ensures sufficient throughput to SU, meanwhile eliminating the need for opportunistic sensing.

High-Rate Underwater Acoustic Localization Based on the Decision Tree

Underwater acoustic localization (UWAL) is widely applied in ocean exploration, and a key issue for the high-rate time-of-arrival (TOA)-based UWAL method is to select the direct signal in the multipath acoustic channel. However, the existing direct signal selection methods suffer from a deterioration of the selection accuracy since the received signals from adjacent periods are mixed with each other. Addressing the problem, we propose a high-rate direct signal selection algorithm in this article. First, we choose the amplitude, TOA, bandwidth, and Doppler frequency of the detected pulses as the input features. Second, a direct signal classifier is established utilizing the decision tree. Finally, the localization model is built based on the TDOA information of the direct signal, and the localization precision analysis evaluates the performance of the UWAL method. Sufficient experiments, including simulation, tank experiment, and sea trial, are implemented to verify the effectiveness of the proposed method.

Small-sized surface antenna for multipath radio channels

Small-sized surface antenna for multipath radio channels