Deep Fading Research Articles

Outage Probability of Intelligent Reflecting Surface Assisted Full Duplex Two-Way Communications

In this letter, we study the outage probability (OP) of intelligent reflecting surface (IRS) assisted full duplex two-way (TW) communication systems, which characterizes the performance of overcoming the transmitted data loss caused by long deep fades. To this end, we first derive the probability distribution of the cascaded end-to-end equivalent channel with an arbitrarily given IRS beamformer. Our analysis shows that deriving such probability distribution in the considered case is more challenging than the case with the phase-matched IRS beamformer. Then, with the derived probability distribution of the equivalent channel, we obtain the closed-form expression of the OP. It theoretically shows that the number of reflecting elements has a conspicuous effect on the improvement of the system reliability. Extensive numerical results verify the correctness of the derived results and confirm the superiority of the considered IRS assisted TW communication system comparing to the one-way counterpart.

Performance Analysis of CC-8PSSK System with SC Diversity Technology through T2T Channel

For performance improvements of T2T (train to train) communication, it is demanded to adapt a hybrid technique such as coding and diversity reception to overcome double fading channel. To ensure performance improvements of transmitted data generated by modulator, a proposed method adopts PSSK (phase-silence-shift-keying) modulation which is the hybrid method of PPM and PSK. In this paper, the performance of CC(convolutional coded) 8 PSSK system with SC(selective combining) diversity reception technique is analyzed under T2T channel which is described by double Rician fading channel using Simulink in the matlab. A core of proposed method is hybrid of convolutional coding which is securing the hamming distance between symbols and SC diversity technique which is selecting more power sequences. Therefore, avoid a deep fading by properly combining multipath signals. As result, using proposed method it makes possible to improve BER performance under T2T channel environments. By adapting proposed method, it is expected that perfprmance is improved for T2T communication applications.

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.

Two-Stage ML Detector Using Absolute Value of IQ Components and SVM for Adaptive OFDM-IM

Among various index modulation technology, adaptive orthogonal frequency division multiplexing with index modulation (A-OFDM-IM) improves reliability by solving the problem of deep fading. However, the maximal likelihood (ML) detector of A-OFDM-IM has high complexity by simultaneously performing active subcarrier detection and quadrature amplitude modulation (QAM) symbol demodulation. A simpler type of ML detector can be applied since the A-OFDM-IM has independent active states of each subcarrier and uses a single QAM constellation. In this paper, we propose two low-complexity detectors for the A-OFDM-IM. The proposed detectors have a two-stage receiving process, active subcarrier detection, and QAM demodulation. In active subcarrier detection, the presence or absence of the QAM symbol is more important than its information. Therefore we derive the thresholds using the absolute value of the in-phase and quadrature components of the received signal in the frequency domain. Using the absolute value, the distribution of noise added to inactive subcarrier is half normal. This is different from noise distribution of the ML detector, and the proposed detectors select the threshold considering the corresponding noise distribution. The first proposed detector uses a threshold derived by the ML estimation method. The second detector estimates the active subcarriers via a support vector machine. The proposed detectors have lower complexity than the ML detector because of the divided receiving process. Moreover, the theoretical analysis and simulation results show that the proposed detectors have better reliability than the ML detector due to different noise distribution and thresholds.

Performance analysis of space time coded generalized frequency division multiplexing system over generalized fading channels

AbstractDue to its low out‐of‐band emission and high spectral efficiency, the generalized frequency division multiplexing (GFDM) has been adopted as one of the most prominent waveform techniques for the 5G networks. To further combat the deep fading, the space time coding (STC) can be integrated with the GFDM system which provide high diversity gain that results into low symbol error rate at the receiver. In this article, a generalized model of space time coded GFDM system (STC‐GFDM) has been proposed. More precisely, a comprehensive analytical framework to compute the average symbol error rate (ASER) is introduced, which can be used for any MIMO configuration, code rate, and arbitrary fading channel. The proposed framework has wider applicability over generalized fading channels and having more accurate exact solution along with easy to evaluate approximate solution. Thus, the proposed framework extends the horizon of performance analysis of STC‐GFDM system over any fading scenario, that is not covered so far in the literature. In general, the closed from expressions of exact and approximate along with asymptotic ASER are derived for different fading channels using moment generating function (MGF) approach. Particularly, the ASER analysis has been performed analytically using derived expression over Rayleigh, TWDP, Nakagami‐q, and Nakagami‐m fading channels for ‐ary quadrature amplitude modulation (‐QAM). This analysis confirms that with the increase in transmit and receive antenna and decrease in modulation order, the ASER performance improves for all fading channels. Also, the improvement in ASER is observed as the power of specular components (ie, LOS paths) increases. Furthermore, these analytical results are validated using Monte‐Carlo simulation.

CONNECTION OF THE INTENSITY OF THE FLUX OF SCR PROTONS WITH THE VELOCITY OF THE CME AND WITH THE FADING OF THE RADIO EMISSION OF THE SUN IN THE DECAMETER RANGE

The relationship between SCR and CME and with fading of the continuum of noise storms and typeIV radio bursts in the decameter range is investigated. It was shown earlier that about 60% of CMEs associated with solar proton events are accompanied by deep fading of the solar radio emission in the decameter range, which coin-cides in time with CME registration. It has also been shown that fading is characterized by fading depth, the frequency bandwidth in which the fading occurs, as well as the duration of the fading and the frequency at which the maximum fading depth is observed. Further detailed studies have shown that for proton events accompanied by fading of the solar radio emission in the decameter range, the relationship between the intensity of the SCR proton flux and the CME velocity is much worse than for events without fading of the solar radio emission in the decameter range. However, it was foundthat for such events, the relationship between the flux of SCR protons and the CME velocity significantly increases if we take into account the fading depth of the solar radio emission in the decameter range.Earlier in (Isaeva, 2019), the results of a study of the relationship between the intensity of fading of the continuum of noise storms with the parameters of X-ray bursts, with the CME velocity and the velocity of coronal shock waves, as well as with the intensity of the SCR proton flux were presented. This paper presents the results of studying the relationship between the intensity of the SCR proton flux withthe parameters of type II and IV radio bursts, as well as with the CME velocity and with the velocity of coronal shock waves, depending on the intensity of fading of the solar radio emission in the decameter range at a frequency of 27 MHz. The frequency of 27 MHz was chosen because in the region of this frequency the maximum fading depth of the solar radio emission in the decameter range is observed.

MOMO. IV. The Complete Swift X-Ray and UV/Optical Light Curve and Characteristic Variability of the Blazar OJ 287 during the Last Two Decades

Abstract We have been carrying out a dense monitoring of the blazar OJ 287 with Swift since late 2015 as part of our project MOMO (Multiwavelength Observations and Modeling of OJ 287). This is the densest existing monitoring of OJ 287 involving X-ray/UV data. In this latest publication of a sequence, we characterize the multiwavelength variability of OJ 287 based on >4000 Swift single-wave-band data sets including archival data since 2005. A structure function analysis reveals a characteristic timescale of ∼5 days in the optical–UV at epochs of low-level activity and larger during outbursts. The discrete correlation function shows zero lag between optical and UV, with τ = 0 ± 1 day at the epoch of densest cadence. During outbursts (in 2016/17 and 2020) the X-rays follow the UV with near-zero lags. However, during quiescence, the delay is 7–18 days with X-rays leading or lagging, interpreted as due to a different X-ray component dominated by inverse Compton emission. Scaling relations are used to derive the characteristic length scales of the broad-line region and torus in OJ 287. A remarkable, symmetric UV–optical deep fade is identified in late 2017, lasting 2 months. We rule out occultation from the passage of a dusty cloud and a model where the secondary black hole deflects the jet between the primary and observer. We speculate about a temporary dispersion or jet swing event in the core or in a bright quasi-stationary jet feature. The deep fade reveals an additional, spatially distinct X-ray component. The epoch 2020.9–2021.1 was searched for precursor flare activity predicted by the binary black hole model of OJ 287.

Exploiting Reconfigurable Intelligent Surfaces in Edge Caching: Joint Hybrid Beamforming and Content Placement Optimization

Edge caching can effectively reduce backhaul burden at core network and increase quality-of-service at wireless edge nodes. However, the beneficial role of edge caching cannot be fully realized when the offloading link is in deep fade. Fortunately, the impairments induced by wireless propagation environments could be renovated by a reconfigurable intelligent surface (RIS). In this paper, a new RIS-aided edge caching system is proposed, where a network cost minimization problem is formulated to optimize content placement at cache units, active beamforming at base station and passive phase shifting at RIS. After decoupling the content placement subproblem with the hybrid beamforming design, we propose an alternating optimization algorithm to tackle the active beamforming and passive phase shifting. For active beamforming, we transfer the problem into a semidefinite programming (SDP) and prove that the optimal solution of SDP is always rank-one. For passive phase shifting, we introduce the block coordinate descent method to alternately optimize the auxiliary variables and the RIS phase shifts. Further, a conjugate gradient algorithm based on manifold optimization is proposed to deal with the non-convex unit-modulus constraints. Numerical results show that our RIS-aided edge caching design can effectively decrease the network cost by improving the quality of offloading links.

Towards a fast and stable filter for RSSI-based handoff algorithms in dense indoor WLANs

In dense indoor wireless networks, handoffs occur frequently. The criteria to trigger handoffs are not defined by the IEEE 802.11 standard, thus being specific to each manufacturer’s implementation. Current handoff implementations typically use RSSI (Received Signal Strength Indicator) as a performance metric and commonly cause association instability in dense environments, a well-known problem referred to as the ping-pong effect. In this paper, we present a deep analysis of RSSI traces collected in dense indoor environments using the FIBRE testbed. Based on that, we conclude that the RSSI time series presents deep fast fades that occur frequently in bursts of small sizes, which can cause ping-pongs. Motivated by this behavior, we propose a new and simple filtering mechanism called Maximum which targets to eliminate these valleys in the RSSI time series. In a nutshell, this filter chooses the maximum RSSI value from a sliding window containing the last few RSSI samples of the series. We conduct simulations based on real RSSI traces from static and mobile scenarios to evaluate Maximum with respect to other filtering mechanisms found in the literature. Additionally, we present a simplified model of the behavior of Maximum that allows us to study the probability of unwanted handoffs as a function of the RSSI of the available access points. Our analysis reveals that Maximum is able to offer a better tradeoff between handoff triggering delay and stability in mobile scenarios, while also performing well in static scenarios, effectively avoiding the occurrence of ping-pongs in most cases.

Satellite forward VDES channel modeling and impact on higher‐layer performance

SummaryVHF data exchange system (VDES) is an emerging maritime communications standard that is meant to provide ship‐to‐shore, shore‐to‐ship, and ship‐to‐ship information services. VDES also includes a satellite component to extend the service coverage also to the high sea. Ships can receive important information via VDES and access this service via satellite when they are far from the shore. This paper uses data taken during a measurement campaign to develop an ON‐OFF model for the VDE‐SAT downlink channel taking the multipath effects caused by sea reflections into account. This model has been used to evaluate the impact of packet‐level forward error correction (FEC) schemes (i.e., ideal codes, RaptorQ codes, and 2‐Dimension Reed‐Solomon codes) to further protect transmissions from deep fading events and to evaluate the impact at the transport level considering the possibility to adopt the transmission control protocol (TCP) for a file map delivery service.

Markov Chain‐Based Stochastic Modeling of Deep Signal Fading: Availability Assessment of Dual‐Frequency GNSS‐Based Aviation Under Ionospheric Scintillation

AbstractDeep signal fading due to ionospheric scintillation severely impacts global navigation satellite system (GNSS)‐based applications. GNSS receivers run the risk of signal loss under deep fading, which directly leads to a significant decrease in navigation availability. The impact of scintillation on GNSS‐based applications can be mitigated via dual‐frequency signals which provide a backup channel. However, the benefit of dual‐frequency diversity highly depends on the correlation of fading processes between signals at different frequencies. This paper proposes a Markov chain‐based model that simulates the actual behavior of correlated fading processes in dual‐frequency channels. A set of recorded scintillation data was used to capture transitions among all fading states based on the fading and recovery of each signal frequency. A statistical study of deep fading characteristics in this data revealed that the Markov chain‐based model accurately generates realistic correlated fading processes. Using the proposed model, aviation availability of localizer performance with vertical guidance down to a 200‐foot decision height (“LPV‐200”) under a strong scintillation scenario is analyzed by considering the effects of signal outages due to deep fading. A parametric analysis of the availability resulting from variations in mean time to loss of lock, mean time to reacquisition, and ionospheric delay uncertainty was conducted to investigate the performance standards on GNSS‐based aviation under scintillation. The analysis results demonstrate a significant benefit of frequency diversity on aviation availability during scintillation. This model will further enable the assessment of GNSS‐based availability for aviation and other applications under a full range of scintillation conditions.

Time Switching Based Wireless Powered Relay Transmission with Uplink NOMA.

Non-orthogonal multiple access (NOMA) utilizes power domain multiplexing to improve spectrum efficiency compared with orthogonal multiple access (OMA). In the Internet of Things (IoT) uplink NOMA networks, if the channel between the far-end node and the base station is in deep fading, allocating larger transmitting power for this node cannot achieve higher spectrum efficiency and overall system throughput. Relay cooperative communication reduces the transmitting power at the far-end node but leads to extra energy expenditure at the relay node. Fortunately, simultaneous wireless information and power transfer (SWIPT) is advocated in energy-constrained IoT networks to save energy consumption. However, early works all focus on energy harvesting (EH) from one source node or one dedicated power supply station. In this paper, we propose a time switching based wireless powered relay transmission model with uplink NOMA where our EH technique can harvest energy from two simultaneously transmitting nodes. More importantly, by optimizing relay position more energy is harvested from the near-end node at the relay and relay signal attenuation to the destination is reduced as well. Furthermore, the closed-form expressions of outage probability and overall system throughput are derived, and numerical results prove that NOMA in our EH scheme achieves better performance compared to the traditional EH scheme and OMA by optimizing the position of the relay node, time switching factor and so on.

Comparison of BCH Code Performance in LTE-Advanced Cooperative System in Suburban, Urban and Rural Environment Based on WINNER II Channel Model

Multi-path fading is a severe issue that causes fluctuations in the amplitudes of the received signals. For combating this issue, several researchers have used cooperative communication in the Long-Term Evolution (LTE)-advanced system. To further decrease the degradative effects of transmitted signals, various error correction techniques can be used to recover the erroneous components, especially when the channel has been deep fading. The paper investigates the general performance of the Convolutional Codes (CC) in the LTE-advanced cooperative system and compare it with the Bose-Chaudhuri-Hocquenghem (BCH) codes. The comparison is based on WINNER II project as the channel model between the nodes in the two coding processes. To validate the advantages of BCH code, the process were tested on three different environments; a suburban macro-cell, urban macro-cell, and the rural micro-cell with prior study to understand the appropriate relay height in every environment. Path loss models for each environment were derived and proposed as part of the proposed simulation framework. Conducted simulation studies based on bit error rate and direct connection showed that the use of the CC and the BCH codes in a relay technologies could significantly increase the system performance. It was noted that the performance of the system which used BCH nodes was better than the system using CC nodes in an LTE-advanced cooperative system.

Evaluation of Time Diversity Technique for Satellite Mitigation in Tropical Climate

Time diversity is one of the mitigation techniques for satellite communications (SatCom) systems that aims to combat deep fades cause by propagation impairments. This study presents the performance of time diversity technique for rain attenuation at Ku- and Ka-band SatCom systems in Johor Bahru, Malaysia. The Synthetic-Storm Technique (SST) has been utilized to obtain rain attenuation time series based on rain rate data measured by local rain gauge. The time diversity has been quantified through diversity gain as a function of retransmission time delay, in which shows the effectiveness of time diversity technique to improve the Quality of Service (QoS) and link availability. The results demonstrated it is feasible to use time diversity as mitigation technique at higher frequencies in a mission to support future SatCom operation in tropical region.

A deep fading assessment of the modernized L2C and L5 signals for low-latitude regions

A deep fading assessment of the modernized L2C and L5 signals for low-latitude regions

Examining the Tolerance of GNSS Receiver Phase Tracking Loop Under the Effects of Severe Ionospheric Scintillation Conditions Based on Its Bandwidth

AbstractThis study aims to evaluate the performance of phase‐locked loop (PLL) under the presence of distinct scintillation patterns in the signal. Scintillation is very common in low‐latitude regions due to the ionospheric dynamics. Under strong scintillation scenario the occurrence of deep fading events is usually registered and may cause severe degradation in the communication. The investigation conducted in this study uses the amplitude scintillation index S4, the decorrelation time, and the bandwidthas main parameters. The study evaluates 54 different combinations of these parameters. The results indicate that in order to minimize the occurrence of cycle slips in the output phase of the PLL, the most appropriate tracking loop bandwidthdepends on the values of both S4and, which characterize the scintillation fading pattern. Simulations showed that as the S4index increases, the automatic increment in the PLL bandwidth may not be the best choice as the performance depends on the temporal characteristics of the scintillating signal. The analysis showed that, among the configurations tested, the equivalent noise bandwidth of 10 Hz achieved the best performance overall. The investigation also showed that choosing the wrong parameter regarding the PLL bandwidth may increase the probability of cycle slip by up to 10 times during fading events.

Reconfigurable Intelligent Surfaces for Doppler Effect and Multipath Fading Mitigation

Owing to the envisioned new use-cases, such as immersive virtual reality and high-fidelity mobile hologram, and their potential challenging new requirements for future wireless networks, extensive research has already started on 6G and beyond wireless technologies. Despite the fact that several modern physical layer solutions have been introduced in the past decade, a level of saturation has been reached in terms of the available spectrum and adapted modulation/coding solutions, which accordingly limits the maximum capacity and reliability. Within this respective, reconfigurable intelligent surface (RIS)-empowered communication appears as a potential candidate to overcome the inherent drawbacks of legacy wireless systems. The core idea of RIS-assisted communication is the transformation of the random and uncontrollable wireless propagation environment into a reconfigurable communication system entity that plays an active role in conveying information and improving system performance. In this paper, the well-known multipath fading phenomenon is revisited in mobile wireless communication systems, and novel and unique solutions are introduced from the perspective of RISs. The feasibility of eliminating or mitigating the multipath fading effect stemming from the movement of mobile receivers is also investigated by utilizing RISs. It is shown that rapid fluctuations in the received signal strength due to the Doppler effect can be effectively reduced by using real-time tunable RISs. It is also proven that for a hypothetical propagation environment where all reflectors are coated with RISs, the multipath fading effect can be totally eliminated. Furthermore, we show that for more general propagation environments with several interacting objects, even a few real-time tunable RISs can remarkably reduce the Doppler spread and the deep fades in the received signal.

Spatial Fading in Retrodirective Channels: An Experimental Study

Simple, low-power, reliable backscatter communication systems must overcome the severe, double-fading nature of the backscatter channel. In this article, we show experimentally that when a conventional transponder is replaced by a retrodirective transponder in backscatter systems, those deep fading nulls are reduced by as much as two orders-of-magnitude. We consider three multipath scenarios all of which show a pronounced link-reliability improvement with a retrodirective transponder compared to its standard non-retrodirective counterpart. The measurements also reveal that by comparing the best case scenario of the two transponder designs under 0.001 outage probability, the retrodirective transponder requires only a 13-dB fade margin while the conventional tag requires a 34-dB fade margin—a significant improvement that would be a key enabler for next-generation backscatter systems.

CSMA/CA channel hopping in IoT environment toward intelligent multi-user diversity

In massive IoT (Internet of Things) environment, which constitutes a goal of the 5G standard, the communication channel of an MTC (machine-type-communication) device with narrowband transmission has problems that are deep fading and interference with the transmission performance. For improved communications in IoT environment, researchers aimed to increase the transmission throughput by using larger contention windows between channel hops when the probability of successful data communication drops during the previous time step to prevent the rapid increase in the number of connected devices. Hence, researchers propose a model to change the interframe spacing according to the number of contention windows for hopping and data prioritization by quickly determining network traffic variations in a massive network environment. Specifically, researchers propose CSMA/CA (carrier-sense multiple access with collision avoidance) channel hopping to prevent deep fading and obtain multiuser diversity in multichannel environments as well as improve the channel quality. Researchers confirmed that the proposed CSMA/CA channel hopping provides a data transfer rate about twice that of conventional TDMA (time-division multiple access) channel hopping for 60 connected devices. As a result of analyzing data transfer rates in channel hopping, it was shown that the collision probability is not high when the number of nodes is small, so the number of channels and contention window size are not significantly affected, but as the number of nodes increases, the number of channels and contention window size are greatly affected.

Unified Performance Analysis of MIMO Mixed RF/FSO Relaying System

This paper investigates the asymmetric dual–hop multiple input multiple output (MIMO) mixed radio frequency (RF)/free space optical (FSO) decode–and–forward (DF) relaying system. This kind of system can utilize two different fading characteristic channels to reduce the possibility of the system falling into deep fading. In addition, each link of the system adopts MIMO technology to mitigate the disadvantages of fading. In this paper, the closed form expressions of the outage probability, bit error rate (BER) and average ergodic capacity are derived. The approximate expression of the system outage probability considering the pointing error is also derived. Additionally, asymptotic performance for diversity order and diversity–multiplexing tradeoff (DMT) of the system is analyzed and discussed, which provides direct theoretical basis for practical engineering design.