Multipath Channels Research Articles

Channel-Adaptive Wireless Image Transmission With OFDM

We present a learning-based channel-adaptive joint source and channel coding (CA-JSCC) scheme for wireless image transmission over multipath fading channels. The proposed method is an end-to-end autoencoder architecture with a dual-attention mechanism employing orthogonal frequency division multiplexing (OFDM) transmission. Unlike the previous works, our approach is adaptive to channel-gain and noise-power variations by exploiting the estimated channel state information (CSI). Specifically, with the proposed dual-attention mechanism, our model can learn to map the features and allocate transmission-power resources judiciously based on the estimated CSI. Extensive numerical experiments verify that CA-JSCC achieves state-of-the-art performance among existing JSCC schemes. In addition, CA-JSCC is robust to varying channel conditions and can better exploit the limited channel resources by transmitting critical features over better subchannels.

Directional Modulation for Secure IoT Networks via Accurate Phase Response Control

This article presents an accurate phase response control (APRC) algorithm to realize directional modulation (DM), improving the physical layer security of Internet of Things (IoT) networks. Specifically, the concept of normalized array response control is developed to realize the phase response control, and the designed weight vector is constructed as the sum of an initial weight vector and an appended weight vector. The precondition of precise phase response control is theoretically derived, and a geometric interpretation of phase response adjustment is given. The APRC algorithm can derive an analytical solution to realize the precise phase response control and normalized magnitude response control jointly, and the computational complexity of the APRC algorithm is much lower than the existing approaches. Moreover, a flexible DM scheme based on the APRC algorithm is presented to realize the specific PSK symbol synthesis. Different from the existing works that are only feasible for the single direction PSK modulation design, the APRC algorithm is applicable for the more general multipath and quasistatic block Rayleigh channels. The synthesis results with fixed and random initial weights are illustrated to show the wide applicabilities of the APRC algorithm, and the bit error rate simulations are given to validate the APRC algorithm.

A Non-Line-of-Sight Wireless Indoor Localization System Using Custom-Designed Radio Relay and Uniform Circular Array

Indoor localization using WiFi signals, though getting studied for many years, is not adopted extensively in real indoor environments yet. One possible impediment is that many prior systems perform worse in non-line-of-sight (NLoS) indoor scenarios than in line-of-sight (LoS) scenarios or even fail to work. Nevertheless, realizing localization in both LoS and NLoS scenarios is indispensable for a viable system working in indoor environments. Since existing state-of-the-art solutions mainly aim to localize a target in LoS indoor scenarios, we present a system that uses multipath channel parameters to achieve localization with a single AP in indoor NLoS scenarios. The proposed introduces deployable and simple custom-designed radio relays to assist localization, which can receive signals from the target and forward them to the AP in NLoS scenarios. Field trials conducted in a real NLoS scenario show that our system achieves a median 3-D localization accuracy of 86 cm with a uniform circular array (UCA) in most NLoS conditions, i.e., the direct path from a target to AP is not completely blocked. More importantly, even if in a challenging NLoS condition, i.e., the direct path does not exist, our system can still achieve a median 2-D localization accuracy of 85 cm.

PAPR Reduction Techniques and Their BIT Error Rate Measurement at OFDM in LTE System

Orthogonal frequency division multiplexing (OFDM) has become an indispensable part of waveform generation in wideband digital communication since its first appearance in digital audio broadcasting (DAB)and it is indeed in use. These descriptions are simplified version of the detailed descriptions provided by 3gpp. It is a superior technology for the high-speed data rate of wire-line and wireless communication systems. The OFDM has many advantages over other techniques such as its high capacity and immunity against multipath fading channels. However ,one of the main drawbacks of the OFDM system is the high-peak-to-average power ratio (PAPR) that leads the system to produce in-band distortion and out-of-band radiation because of the non-linearity and reduces its efficiency is the distortion of the signal caused at the High Power Amplifier (HPA) of a transmitter . Therefore, it is highly desirable to reduce the PAPR of an OFDM signal. For this, numerous techniques have been proposed to overcome the PAPR problem such as i) selective mapping(SLM) ii) partial transmit sequence (PTS), iii) clipping, iv) clipping and filtering. In this paper, the PTS technique was analytically reviewed as one of the important methods to reduce the high PAPR problem. Simulations are used to analyze the efficiency of the techniques used which signifies OFDM to be providing much better PAPR reduction and a better Bit Error Rate (BER) . From simulation results clipping method shows good PAPR reduction with significant amount of BER degradation. Clipping and filtering method shows slight increase in PAPR with small degradation in BER performance than Clipping method and both methods are computationally less complex. PTS provides good PAPR reduction with high computational complexity.

Machine learning-enabled MIMO-FBMC communication channel parameter estimation in IIoT: A distributed CS approach

Compressed Sensing (CS) is a Machine Learning (ML) method, which can be regarded as a single-layer unsupervised learning method. It mainly emphasizes the sparsity of the model. In this paper, we study an ML-based CS Channel Estimation (CE) method for wireless communications, which plays an important role in Industrial Internet of Things (IIoT) applications. For the sparse correlation between channels in Multiple Input Multiple Output Filter Bank MultiCarrier with Offset Quadrature Amplitude Modulation (MIMO-FBMC/OQAM) systems, a Distributed Compressed Sensing (DCS)-based CE approach is studied. A distributed sparse adaptive weak selection threshold method is proposed for CE. Firstly, the correlation between MIMO channels is utilized to represent a joint sparse model, and CE is transformed into a joint sparse signal reconstruction problem. Then, the number of correlation atoms for inner product operation is optimized by weak selection threshold, and sparse signal reconstruction is realized by sparse adaptation. The experiment results show that the proposed DCS-based method not only estimates the multipath channel components accurately but also achieves higher CE performance than classical Orthogonal Matching Pursuit (OMP) method and other traditional DCS methods in the time-frequency dual selective channels.

Compact UWB MIMO antenna with dual band-notch characteristics

ABSTRACT This article describes a quad-port ultra-wideband multiple-input multiple-output (MIMO) antenna with dual-band notch characteristics that is very compact and highly isolated. The compactness and high isolation of this design are achieved by etching a modified Minkowski fractal structure and an inverted L-shaped stub made available by the partial ground plane. Simply by incorporating ‘C’- and ‘U’-shaped slots in the radiating patch, it is possible to obtain dual-band rejection characteristics that are very suitable for wireless local area network band and X-band downlink satellite communication of 5.0–5.9 GHz and 7.1–7.9 GHz, respectively, in this proposed structure. The entire design is housed in a very compact size of 30 mm × 30 mm. A few key parameters, such as channel capacity loss (CCL), envelope correlation coefficient (ECC), diversity gain (DG) and total active reflection coefficient (TARC), are examined to ensure that the MIMO system meets its requirements. Furthermore, a mutual coupling value of less than −22 dB and an operating frequency band of 3.1–12 GHz allow the design to be very effective in a multipath channel environment.

Security Assessment of Phase-Based Ranging Systems in a Multipath Environment

Phase-based ranging has been widely deployed in proximity detection scenarios including security-critical applications due to their low implementation complexity on existing transceivers. In this work, the security of multi-carrier phase-based ranging systems in a multipath propagation environment is investigated. We present a threat model that can successfully target any decreasing distance in different multipath environmental conditions rendering the phase-based ranging method insecure. We assess the feasibility of attacks in various attack scenarios through simulations using a multipath channel and demonstrate a simplified version of the attacker model implemented in hardware. We show that the attacker can spoof the measured distance to less than one meter when the devices are separated by 30 meters. The evaluation of possible countermeasures and their limitations for different threat models is performed.

A Novel Combined Double Binary Turbo Coding and Color Shift Keying Technique for Flicker Mitigation in Multi Carrier Visible Light Communications

Although radio frequency (RF) systems have been developed at a level that can operate almost within theoretical limits in terms of spectral efficiency due to increasing demands, the RF spectrum allocated for wireless mobile communication has started to fill up rapidly and become insufficient. Visible light communication (VLC), one of the optical wireless communication systems, stands out as a promising technology that can be an alternative to RF technology in some application areas and can help meet the demands by playing a complementary role in some application areas. However, VLC systems face the flickering problem. In this study, it is proposed to combine the double binary Turbo coding (DBTC) technique with color shift keying (CSK) modulation, which does not have a flickering problem, as a solution to the flicker problem of VLC systems. The proposed combined DBTC-CSK method is tested on optical OFDM systems from multi-carrier wireless communication systems. Monte-Carlo simulations are performed to verify the performance of the proposed DBTC-CSK-OFDM system in terms of bit-error-rate (BER) over AWGN channel, single-path optical LOS channel and multi-path optical channel. According to the simulation results it is observed that, DBTC-CSK-OFDM system has better BER performance than that of RS–CC coded CSK-OFDM and un-coded CSK-OFDM systems, providing 5.5 dB and 14 dB SNR gain at multi-path optical channel environments, respectively.

Performance evaluation of 4-quadrature amplitude modulation over orthogonal frequency division multiplexing system in different fading channels scenarios

<p>Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation (MCM) technique that divides the wide bandwidth into parallel narrow bands, each of which is modulated by orthogonal subcarriers. Currently, OFDM is a high-spectral efficiency modulation technique that is used in a variety of wired and wireless applications. The transmitted signal in a wireless communication channel spreads from transmitter to receiver through multiple reflective paths. This triggers multipath fading, which causes variations in the received signal's amplitude and phase. Slow/fast and frequency-selective/frequency-nonselective are the main types of multipath fading channels. Therefore, in this paper, we proposed new models for modeling multipath fading channels, such as the exponential fading channel and the Gamma fading channel. In addition, new bit-error-rate (BER) derivations have been derived. The performance of the OFDM system over proposed channel models has been evaluated using Monte-Carlo simulation and compared to the Rayleigh fading channel model. The obtained results via simulations show that the exponential fading channel at a rate parameter (λ=0.5) outperforms the Rayleigh fading channel by 6 dB for all values of Eb/No, while the Gamma fading channel at (α=2) outperforms the Rayleigh fading channel by 3 dB for all values of Eb/No.</p>

Assessment of Cellular Signals of Opportunity for High-Altitude Aircraft Navigation

Opportunistic navigation with cellular signals is studied and demonstrated on a high-altitude aircraft. An extensive campaign was conducted by the Autonomous Systems Perception, Intelligence, and Navigation Laboratory (ASPIN) in collaboration with the U.S. Air Force (USAF) to sample ambient cellular signals of opportunity (SOPs) at different altitudes in different regions in Southern California, USA. The carrier-to-noise ratio is characterized as a function of altitude and horizontal distance. It was found that signals from cellular transmitters can be reliably acquired and tracked for navigation purposes by receivers mounted on high dynamics aircraft flying at altitudes as high as 23,000 ft above ground level and horizontal distances as far as 100 km. The multipath channel is also characterized as a function of altitude, showing clean channels with a dominant line-of-sight component for altitudes up to 23,000 ft. To further assess the potential of cellular SOPs for aircraft navigation, a 51-km trajectory traversed over a period of 9 minutes was estimated exclusively using cellular SOPs (i.e., with no GPS or other sensors, except for barometric altimeter measurements), resulting in a three-dimensional position (3-D) 10.5-m position root-mean-squared error.

Modeling and Analysis of MIMO Multipath Channels With Aerial Intelligent Reflecting Surface

Recently, intelligent reflecting surface (IRS) has become a research focus for its capability of controlling the radio propagation environments. Compared to the conventional terrestrial IRS, aerial IRS (AIRS) exploiting unmanned aerial vehicle (UAV)/high-altitude platform (HAP) can provide better deployment flexibility. To this end, a three-dimensional (3D) one-cylinder model is first developed for AIRS-assisted multiple-input multiple-output (MIMO) narrowband channels. In order to change the wireless channel with AIRS and create a favorable propagation environment, we propose a novel method of designing the phase-shifts for the IRS elements. Based on the model, channel impulse response (CIR), space-time correlation function, and channel capacity are derived and thoroughly investigated. A key observation in this paper is that multipath and Doppler effects in radio propagation environments can be effectively mitigated via adjusting the phase-shifts of IRS. More specifically, for the special propagation environments in the absence of any scatterers, it is found that the effects of multipath fading can be completely eliminated by IRSs. While for the general propagation environments with multiple scatterers, a small number of IRS elements can also significantly reduce the Doppler spread and the deep fades of the channels. Based on the numerical investigation of channel correlations, it is shown that channel non-stationarity is not introduced into the time domain when the phase shift of IRS is linear related to the time. Moreover, the channel capacity can also be improved by the proposed methods. Finally, the model with non-ideal IRSs is considered and it is found that using non-ideal IRSs results in poor performances compared with using ideal IRSs. These conclusions will provide a fundamental support for developing intelligent and controllable propagation environments of the future sixth-generation (6G) wireless networks.

Design of a New Non-Coherent Cross-QAM-Based M-ary DCSK Communication System

In this paper, a new non-coherent cross-quadrature amplitude modulation (XQAM)-based M-ary differential chaos shift keying (XQAM-MDCSK) system is proposed. In such a system, an autocorrelator is adopted at the receiver to obtain the channel compensation value. This framework can be extended to various amplitude phase shift keying-based MDCSK systems, such as star QAM-based MDCSK (star QAM-MDCSK) and square QAM-based MDCSK (SQAM-MDCSK) systems. Moreover, the bit error rate (BER) expression of the proposed XQAM-MDCSK system is derived over a multipath Rayleigh fading channel. Results show that the proposed XQAM-MDCSK system can achieve better BER performance and a lower peak-to-average power ratio (PAPR) compared to the star QAM-MDCSK system. Furthermore, we also show that the performance of the proposed system can be close to that of a system with perfect channel state information (CSI).

The Statistical Analysis of the Security for a Wireless Communication System with a Beaulieu-Xie Shadowed Fading Model Channel

The paper considers the problem of a wireless communication system’s physical level security for a multipath signal propagation channel and the presence of a wiretap channel. To generalize the propagation effects, the Beaulieu-Xie shadowed channel model was assumed. To describe the security of the information transfer process, such a metric as the secure outage probability of was considered. An analytical expression of the secure outage probability was obtained and analyzed depending on the characteristics of the channel and the communication system: the average value of the signal-to-noise ratio in the main channel and the wiretap channel, the effective path-loss exponent, the relative distance between the legitimate receiver and the wiretap and the threshold bandwidth, normalized to the bandwidth of a smooth Gaussian channel. The analysis considers the sets of parameters that cover all practically important scenarios for the functioning of a wireless communication systems: both deep fading (corresponding to the hyper-Rayleigh scenario) and small fading, both in the case of a significant line-of-sight component and a significant number of multipath clusters, and with significant shadowing of the dominant component and a small number of multipath waves, as well as all intermediate options. It is found out that the value of the energy requirements for guaranteed secure communication at a given speed is determined primarily by the power of multipath components, as well as the existence of an irreducible secure outage probability of a communication session with an increase for channels with strong overall shadowing of the signal components, which from a practical point of view is important to take into account when imposing requirements for the values of the signal-to-noise ratio and the data transfer rate in the direct channel, providing the desired degree of security of the wireless communication session.

Multi-User Joint Detection Using Bi-Directional Deep Neural Network Framework in NOMA-OFDM System.

Non-orthogonal multiple access (NOMA) has great potential to implement the fifth-generation (5G) requirements of wireless communication. For a NOMA traditional detection method, successive interference cancellation (SIC) plays a vital role at the receiver side for both uplink and downlink transmission. Due to the complex multipath channel environment and prorogation of error problems, the traditional SIC method has a limited performance. To overcome the limitation of traditional detection methods, the deep-learning method has an advantage for the highly efficient tool. In this paper, a deep neural network which has bi-directional long short-term memory (Bi-LSTM) for multiuser uplink channel estimation (CE) and signal detection of the originally transmitted signal is proposed. Unlike the traditional CE schemes, the proposed Bi-LSTM model can directly recover multiuser transmission signals suffering from channel distortion. In the offline training stage, the Bi-LTSM model is trained using simulation data based on channel statistics. Then, the trained model is used to recover the transmitted symbols in the online deployment stage. In the simulation results, the performance of the proposed model is compared with the convolutional neural network model and traditional CE schemes such as MMSE and LS. It is shown that the proposed method provides feasible improvements in performance in terms of symbol-error rate and signal-to-noise ratio, making it suitable for 5G wireless communication and beyond.

Simple and Efficient LoRa Receiver Scheme for Multipath Channel

This paper presents a novel LoRa (Long Range) receiver operating in frequency selective Multi-Path Channel (MPC). The dechirped received LoRa wave-forms under MPC allows us to derive a simple and efficient LoRa receiver scheme by using a MF (Matched Filter) approach that aims to maximize the SNR (Signal-to-Noise Ratio) at the symbol index frequency of the DFT output. We show that the MF receiver can be seen as RAKE structure where interference peaks related to multipath, exhibited at DFT output, are recombined in a constructive way. Detection performance is driven by channel energy and the benefit of this novel MF/RAKE receiver over original coherent and non-coherent receiver appears only for MPC that exhibits significant paths energy. These two MF and RAKE receivers have however different implementation complexities that are studied in details. We provide in that sense recommendations on which receiver variant to use for real operations, depending on complexity constraints. Finally, the proposed MF/RAKE receiver outperforms previous results on TDEL (Time Delay Estimation LoRa) receiver over MPC, especially at low SNR and higher LoRa Spreading Factor (SF) parameter, at the cost of higher but reasonable complexity.

Interference immunity and energy efficiency of digital communications systems in multipath channel with fading

In the study of measures to improve channel efficiency, we developed and presented a performance analysis of a digital communication system for image transmission in a multipath channel with fading, in the MATLAB/Simulink environment, one of the main advan tages being that it provides high interference immunity in the presence of noise and interference and signal delays. The scheme has been investigated for bit error rate (BER) on additive white Gaussian noise (AWGN) channel and multipath Rice and Rayleigh fading channel operating under the influence of inter-symbol interference (hereinafter ISI) and different parameters for the channel, where we considered cases of signal delay similar for EPA (extended pedestrian a model (EPA) standards). In this paper, we estimate the effectiveness of compensation for signal distortion in radio channels with fading on the basis coded orthogonal frequency-division multiplexing (COFDM) technology, an adaptive equalizer, amplifiers on the transmitter side and a normalizer on t he receiver side, which is an acute problem in the field of wireless communication. This system can be applied to transmit better quality high-resolution radiological images over a wireless network inside a building and for a model analogous of pedestrian traffic.

Análisis de la técnica NOMA-OFDM en un canal multitrayecto y usando estimación de canal

NOMA (Non-Orthogonal Multiple Access) is a non-orthogonal access technique that can increase spectral efficiency and is considered a candidate technology for 5G. NOMA-OFDM combines the non-orthogonal access of NOMA with OFDM, which is widely used due to several advantages such as its high spectral efficiency. Channel estimation is an essential process in a NOMA-OFDM system, so in this paper a channel estimation scheme with preamble for the downlink is proposed considering a multipath channel and the LS (Least Square) technique. The analysis of the results is performed based on graphs of BER (Bit Error Rate) vs Eb/N0 (Energy per bit to noise power spectral density ratio) of the simulation implemented in Matlab and shows that channel estimation is possible with the LS method with the advantage that it is a simple technique although it produces a degradation of approximately 3 dB in the BER compared to the ideal case of perfect estimation. In addition, the results show that the selection of the injection factor is very important for the performance of the users to be adequate.

Impact of Human Blockage on Dynamic Indoor Multipath Channels at 27 GHz

Human blockage and its dynamics are potential challenges for millimeter-wave (mm-wave) mobile communication. This article presents the results of wideband measurements at 27 GHz with one human blocker close by a dynamic mobile terminal (MT) as well as one or multiple dynamic human blockers further away from an MT. The measured human blockage loss is largest when the direct path (DP) in a line-of-sight (LOS) is blocked, but this loss is limited by other multipath components (MPCs). For nonline-of-sight (NLOS) channels, it is shown that human blockage loss is typically negligible. The presented measurement results show that human blockage loss in multipath channels is much smaller than that reported in diffraction- and measurement-based models, which neglect or minimize the contribution of all MPCs other than the DP. This suggests that the multipath nature of the indoor wireless channel highly limits the impact of human blockage.

Reference-Modulated PI-DCSK: A New Efficient Chaotic Permutation Index Modulation Scheme

In this paper, we propose a reference-modulated permutation index differential chaos shift keying (RM-PI-DCSK) over multipath fading channels. Due to quasi-orthogonal property between different permutated chaotic signals, the proposed scheme can remarkably suppress the inter-sequence interference to recover the desired information. Moreover, the proposed RM-PI-DCSK has one more modulation bit in the reference segment than the conventional permutation index differential chaos shift keying (PI-DCSK). Both the reference signal in the first time-slot and a permuted replica of the reference in the second time-slot are modulated for efficient transmission. The theoretical bit-error-rate (BER) performance and the highly consistent simulation results demonstrate that the proposed RM-PI-DCSK can achieve a performance gain 3 dB over the conventional PI-DCSK at a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-4}$</tex-math></inline-formula> , and this gain becomes larger in a high-delay-spread channel. In particular, the performance superiority of the RM-PI-DCSK is further demonstrated in a typical ultra-wideband (UWB) channel with narrow-band interference. Therefore, this proposed scheme can be deemed as an efficient transmission alternative for chaotic-based wireless communications, such as in wireless local area network (WLAN) applications and indoor applications of the transmitted-reference UWB.

TACAN: The Shaping of Delay Distribution Under Multipath Fading Channel for Industrial IoT Systems

The wireless-enabled Industrial Internet of Things (IIoT) system is promising due to its flexibility and cable-free deployment. The varying fading channel will lead to the random transmission delays and jitters, which are the major challenges hindering the adoption of wireless communication in mission-critical industrial systems. The performance or even the stability of closed-loop feedback control system will degrade severely with such delays and jitters. As a result, the maximum delay margin should be met to guarantee the performance of the IIoT system. Aiming to better satisfy this requirement, a novel concept to shape the delay distribution under the industrial multipath fading channel is proposed in this article. Consequently, a two-layer closed feedback control algorithm, referred as TACAN in this article, is designed through the decoupling of original optimization function, by which the variance of the delay distribution is minimized to improve the reliability and stability of the IIoT systems. The performance of proposed delay shape control method is verified by both the classical Rician channel model and the field measured industrial fading channel responses.