Term Fading Research Articles

Exploiting SWIPT-Enabled IoT-Based Cognitive Nonorthogonal Multiple Access With Coordinated Direct and Relay Transmission

This article proposes a simultaneous wireless information and power transfer (SWIPT)-enabled Internet of Things (IoT)-based cognitive nonorthogonal multiple access (NOMA) with coordinated direct and relay transmission (CDRT) system. It incorporates overlay cognitive radio (CR) and time switching (TS)-based SWIPT technology to enhance spectrum utilization and energy efficiency (EE). The proposed system comprises a primary network having a primary transmitter and its intended NOMA receivers (UE1 and UE2), accompanied by an energy-constrained secondary transmitter (ST) and its designated receiver (IoT-U). The primary transmitter communicates directly with its strong user UE1 and exploits the ST as an IoT relay to communicate with a weak user UE2. The IoT-relay node employs a TS-based receiver architecture and a decode-and-forward (DF) protocol to convey the weak user’s information along with its own information by following the NOMA principle. We evaluate the performance of the proposed system by considering both the perfect and imperfect successive interference cancellation (SIC) at the legitimate users over Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${m}$ </tex-math></inline-formula> fading in terms of outage probability (OP), system throughput, and EE. Moreover, we propose an iterative algorithm to minimize the OP by optimizing the TS factor. Furthermore, the impact of key parameters is also highlighted, which lays the guidelines for the practical design of energy-efficient and spectrum-efficient futuristic wireless communication networks.

Pilot Contamination Mitigation in Massive MIMO Cloud Radio Access Networks

Massive multiple-input multiple-output (MIMO) technology is expected to achieve significant gains in both signal-to-noise-plus-interference ratio (SINR) and throughput for 5G cellular wireless networks. Efficient and highly accurate channel state information (CSI) acquisition at the base stations (BS) is essential to achieve the potential benefits of massive MIMO systems. However, the inadequate number of orthogonal pilot sequences used for CSI estimation leads to erroneous channel estimation as it causes interference between pilot sequences. This phenomenon is coined pilot contamination and severely limits the system performance. Therefore, we address in this paper the pilot contamination problem in massive MIMO Cloud-Radio Access Networks systems. Leveraging on a real telecom operator data delivered by Call Detail Records (CDR), our objective is to maximize the average uplink achievable rate by reducing the effect of pilot contamination in massive MIMO Cloud-Radio Access Networks (C-RAN). As such a problem is a non-linear integer problem that has no solution in polynomial time, we develop a two-stage solution to solve it. First, a coalition game is proposed where Remote Radio Heads (RRHs) gather into clusters with random user-pilot allocation. Then, two greedy heuristics are applied to match each user in the cluster with a given pilot. The goal is to further improve the average uplink rate achieved in the first stage. Simulation results show that our heuristic solutions for pilot contamination mitigation outperform the traditional pilot allocation solution and a state-of-the-art pilot allocation scheme based on large-scale fading in terms of average uplink achievable rate and SINR.

New Results and Applications about the Level Crossing Rate of SC Receiver output Signal in the Presence of Gamma Shadowing and k-μ or Rician Multipath Fading

In this paper,the wireless communication system with dual SC receiver operating over shadowed multipath fading channel is considered. The multipath fadingis k-μor Rician.The received signal experiences the short term fading which is resulting in SC receiver envelope variation and Gamma long term fading resulting in SC receiver envelope power variation. The closed form expressions for joint probability density functions of SC receiver output signal envelope and their first derivative of SC receiver output signal envelope are calculated for both, k-μand Rician fading. These expressions are used for evaluation of average level crossing rate of SC receiver output signal envelopes. The numerical expressions are plotted to show the effect of Rician fading severity parameter and Gamma shadowing severity parameter on the average level crossing rate of SC receiver output signal envelope.

Delay Violation Probability and Effective Rate of Downlink NOMA Over $\alpha$-$\mu$ Fading Channels

Non-orthogonal multiple access (NOMA) is a potential candidate to further enhance the spectrum utilization efficiency in beyond fifth-generation (B5G) standards. However, there has been little attention on the quantification of the delay-limited performance of downlink NOMA systems. In this paper, we analyze the performance of a two-user downlink NOMA system over generalized $\alpha$ - $\mu$ fading in terms of delay violation probability and effective rate (ER). In particular, we derive an analytical expression for an upper bound on the delay violation probability and we derive the exact sum ER of the downlink NOMA system. We also derive analytical expressions for high and low-SNR approximations to the sum ER, as well as a fundamental upper bound on the sum ER which represents the ergodic sum-rate for the downlink NOMA system. We also analyze the sum ER of a corresponding time-division-multiplexed orthogonal multiple access (OMA) system. Our results show that while NOMA consistently outperforms OMA over the practical SNR range, the relative gain becomes smaller in more severe fading conditions, and is also smaller in the presence a more strict delay QoS constraint.

Level crossing rate of macro diversity reception in composite Nakagami-m and Gamma fading environment with interference

In this paper we consider a wireless receiver model that employs the macro diversity (MD) technique, consisting of a selection combining (SC) receiver fed by two micro diversity (mD) SC branches, which is used to simultaneously reduce the effects of Nakagami-m short term fading, Gamma long term fading, and co-channel interference on the system performances. The interfering signal is also affected by the same propagation environment as is the case for the desired signal. We derive, closed form expressions for level crossing rate (LCR) of signal-to-interference ratio at the outputs of the mD SC with n branches, as well as for the signal-to-interference ratio at the output of the whole MD SC receiver. By using the obtained expression for LCR of signal-to-interference ratio at the output of the MD SC receiver, we can further determine average fade duration of proposed system configuration. We investigate and analyze the influence of Nakagami parameter m, the average value of desired signal envelope, and the average value of co-channel interference envelope on the LCR. This work is of great importance in the design of envisioned 5G wireless communication systems in the presence of the combined effects of fading and interference. Also, the LCR is important to system performance because, for example, the LCR influences the error correction code's performance.

Hierarchical NaxCoO2 microspheres with low surface area toward high performance sodium ion batteries

Traditional NaxCoO2 (0 < x ≤ 1) generally suffers from severe capacity fading in terms of complicated phase transitions. On account of this, we synthesize P2-type NaxCoO2 microspheres (s-NCO) by the novel sodiation and calcination strategy. This s-NCO specimen occupies low surface area (~2.82 m2 g−1) and hierarchical construction. Owing to this unique microsphere architecture, s-NCO electrode materials show competitive electrochemical performance. The s-NCO electrode demonstrates excellent electrochemical stability with the specific capacity of 82.2 mAh g−1 undergoing 300 cycles at the current rate of 6 C.

Qualification and Calibration of Single-Mode Phosphosilicate Optical Fiber for Dosimetry at CERN

We report the results of several complementary radiation tests to qualify and calibrate a highly radiation sensitive Single-Mode (SM) Optical Fiber (OF) for distributed dosimetry application at CERN. The Radiation Induced Attenuation (RIA) is the physical phenomenon used to evaluate the Total Ionizing Dose (TID) received by the OF sensor. The radiation response tests comprise online RIA in the Near InfraRed (NIR) domain under 60Co γ-rays, X-rays and proton irradiations, as well as post mortem Optical Attenuation (OA) measurement to evaluate long term fading effects of the RIA. In this paper, we focus on the effect of successive irradiations, dose rate dependence and effects related to the change of the irradiation temperature up to 45 °C. On the basis of our investigation, we conclude that the studied OF is a suitable TID sensor and that it is well adapted for distributed OF radiation dose sensing. It is fully compatible with commercially available SM Optical Time Domain Reflectometers, with some noticeable advantages with respect to previously developed systems based on multimode OFs. Finally, the OF we investigated has been selected for the distributed monitoring of low radiation dose levels at CERN in the injector chain (Proton Synchrotron Booster, Proton Synchrotron, Super Proton Synchrotron) and parts of the Large Hadron Collider (LHC).

A Tractable Fading Channel Model With Two-Sided Bimodality

This paper presents the alternate Rician shadowed (ARS) fading model, a new statistical channel model that consists of two fluctuating specular components of which only one is active at a time. When a diffuse component is added, it can be regarded as a mixture of two Rician shadowed fading models. The ARS model has the advantage of providing either left-sided or right-sided bimodality (i.e., two-sided bimodality), depending on its shape parameters. This characteristic is not found in classical fading models, or even in more recent ones such as the two wave with diffuse power (TWDP) and the fluctuating two ray (FTR) models. This makes it suitable for fitting the bimodal fading distributions measured in systems, such as body-centric wireless links or land mobile satellite. To yield a mathematically tractable model, this research derives exact closed-form expressions for the following chief probability functions: 1) probability density function (PDF); 2) cumulative density function (CDF); and 3) moment generating function (MGF). The results obtained are used to analyze the performance of wireless communication systems subject to the ARS fading in terms of outage probability, ergodic capacity, and bit error rate. In addition, we show that the model provides a better fit for experimental measurements in body-centric scenarios than those reported in the literature.

RADIO RELAY SYSTEM WITH TWO-SECTION PERFORMANCE IN THE PRESENCE OF NAKAGAMI-M FADING AND RICIAN CO-CHANNEL INTERFERENCE

Wireless mobile relay communication system with two sections operating over short term fading channel in the presence of co-channel interference subjected to small scale fading has been considered. Desired signal experiences Nakagami-m fading and co- channel interference experiences Rician fading. Desired signal at output of the second section has Nakagami-m*Nakagami-m distribution. The signal-to-interference ratio of product of two Nakagami-m random variables and Rician random variable has been presented. In this paper, closed form expresions for moments of signal- to-interference ratio of considered system are calculated. The influence of Nakagami-m short term fading severity parameter and Rician κ factor on presence is analysed and discussed. Mobile relay communication system with two sections can lead to a more reliable and efficient transmission, which is of great importance in the risky environments. Key words: Nakagami-m fading, Rician fading, moment, mobile relay communications system

Approximate Best-Response Dynamics in Random Interference Games

In this paper we develop a novel approach to the convergence of Best-Response Dynamics for the family of interference games. Interference games represent the fundamental resource allocation conflict between users of the radio spectrum. In contrast to congestion games, interference games are generally not potential games. Therefore, proving the convergence of the best-response dynamics to a Nash equilibrium in these games requires new techniques. We suggest a model for random interference games, based on the long term fading governed by the players' geometry. Our goal is to prove convergence of the approximate best-response dynamics with high probability with respect to the randomized game. We embrace the asynchronous model in which the acting player is chosen at each stage at random. In our approximate best-response dynamics, the action of a deviating player is chosen at random among all the approximately best ones. We show that with high probability, with respect to the players' geometry and asymptotically with the number of players, each action increases the expected social-welfare (sum of achievable rates). Hence, the induced sum-rate process is a submartingale. Based on the Martingale Convergence Theorem, we prove convergence of the strategy profile to an approximate Nash equilibrium with good performance for asymptotically almost all interference games. We use the Markovity of the induced sum-rate process to provide probabilistic bounds on the convergence time. Finally, we demonstrate our results in simulated examples.

Performance Analysis of Interference-Limited Mobile-to-Mobile κ–μ Fading Channel

In this paper, a wireless cooperative network, with two sections, operating over multipath mobile-to-mobile interference-limited fading channel is considered. The desired signal and the cochannel interference experience κ–μ short term fading. The source and destination mobile stations are connected by an amplify-and-forward relay mobile station. The destination station signal-to-interference ratio probability density function and the outage probability closed-form expressions are derived. The closed form approximate expressions for the average fade duration and the level crossing rate are also derived. The influence of the fading channel parameters on these statistical measures are studied. The Monte-Carlo simulations are used to validate the theoretical results.

A Tale of Two Fridges: Temperature Implications on Long Term Fading, Material Degradation, and Performance Recovery in Lithium-Ion Batteries

A high capacity retention, along with a long battery life, are two aspects paramount towards the economic viability of large-scale Lithium-Ion battery (LIB) applications. As such, it is important to understand the mechanisms underlying capacity decay, as to mitigate them, if not outright reverse them. To that end, there have been many studies addressing this issue. However, most of these studies have relied on varying only a limited amount of parameters, thus limiting the scope, and depth of their conclusions: if two variables have a covariant relationship, yet the experiment only accounts for one, the presence of that relationship will be obscured. Herein, the current research explores the long-term fading in lithium-batteries, and how it is affected by a host of different factors, ranging from cycling temperature, and current rate, to initial formation, and restive period at the top of charge. These studies go on to reveal that there exists a very strong relationship between current rate, and temperature, as it pertains to fading, with higher temperatures showing surprisingly high capacity retention over the long term (Figure 1). Furthermore, we are further able to demonstrate that under certain conditions, capacity losses are recoverably, which thus alludes to the possibility of being able to prolong, if not recycle, LIBs currently in use. Lastly, this work also explores one of the big divides that still exists within the battery community: the testing of long-term cycling in Li-metal half cells, as opposed to full cells. Our experimental results show that the nature of fading mechanisms in full-cells versus half-cells are not analogous to one another. Certain mechanisms that dominate in one medium, have a greatly diminished presence in the other. As such, this work concludes by stressing the question: what is the proper way, if any, to look at long term fading mechanism in LIBs? Figure 1

LEVEL CROSSING RATE OF MACRODIVERSITY OUTPUT PROCESS IN THE PRESENCE OF η-μ SHORT TERM FADING AND GAMMA LONG TERM FADING

In this paper macrodiversity reception with macrodiversity selection combining (SC) receiver and two microdiversity MRC receivers operating over shadowed multipath fading channel is studied. Received signal experiences short term fading and correlated Gamma long term fading resulting in system performance degradation. Level crossing rate (LCR) of η-μ random process and level crossing rate of signals at outputs of microdiversity MRC receivers are efficiently calculated. By using these derived formulas, level crossing rate of macrodiversity SC receiver output signal process is calculated. By using this result, average fade duration (AFD) of the proposed wireless communication system can be calculated. The influence of η-μ short term fading severity parameter, Gamma long term fading severity parameter and Gamma long term correlation coefficient on level crossing rate is analysed and studied.

Performance of macro diversity wireless communication system operating in Weibull multipath fading environment

In this paper, we consider wireless mobile radio communication system with macro diversity reception. Signal is subject to Weibull small scale fading and Gamma large scale fading resulting in system performance degradation. Receiver uses macro diversity selection combining (SC) technique in order to reduce the impact of long term fading effects, and two micro diversity SC branches are used to mitigate Weibull short term fading effects on system performance. Probability density function (PDF), and cumulative distribution function (CDF), as well as level crossing rate (LCR) and average fade duration (AFD) of the SC receiver output signal envelope are evaluated. The obtained expressions converge rapidly for all considered values of Weibull fading parameter and Gamma shadowing severity parameter. Mathematical results are studied in order to analyze the influence of Weibull fading parameter and Gamma shadowing severity parameter on statistical properties of the SC receiver output signal.

Performance of Diversity System Output Signal in Mobile Cellular System in the Presence of α-μ Short Term Fading and Gamma Long Term Fading

Performance of Diversity System Output Signal in Mobile Cellular System in the Presence of α-μ Short Term Fading and Gamma Long Term Fading

A Spatially Aware Channel Model for Body-to-Body Communications

This paper presents the results of an experimental-based body-to-body (B2B) channel characterization in an indoor environment at 2.4 GHz in the Industrial, Scientific and Medical band. The peculiarity of B2B channels suggests the extension of classical stochastic models toward one that is aware of the reciprocal position and orientation of the bodies wearing the terminals to include the effects of body shadowing. To reach this goal, three on-body antenna positions (head, belt, and wrist) and multiple human mobility scenarios have been investigated in the measurements. The characterization of these channels shows that not only channel gain is affected by mutual body orientation, but also short and long term fading are. Exploiting this characterization, we are able to derive a comprehensive channel model whose components and parameters directly depend on the mutual body position in terms of distance and orientation.

Outage probability of correlated SIR-based SSC diversity systems over composite KG fading/shadowing channels

In this paper, the effects of multipath fading and shadowing over the propagation channel are observed through the performance analysis of switch and stay selection combining (SSC) technique. The short term fading (multipath fading) in conjunction with the long term fading (shadowing) are both modeled by Generalized-K (KG) distribution. The proposed system is considered as interference-limited system in correlated fading environment. The probability density function (PDF) of signal-to-interference ratio (SIR) at the output of SSC receiver is derived in form of Meijer G functions. According to this new expression, the outage probability is considered and the effects of fading and shadowing parameters and correlation coefficients on the performance gain are analyzed.

LCR of SC receiver output signal over α-κ-μ multipath fading channels

Wireless mobile communication system with selection combining (SC) diversity receiver is investigated in this paper. Received signal envelope experiences ?-?-? short term fading resulting in system performance degradation. Level crossing rate (LCR) and average fade duration (AFD) of SC receiver output signal envelope are obtained as rapidly converging infinite series expressions. Numerically evaluated results are presented graphically, in order to discuss the effects of transmission parameters: multipath fading severity, dominant component power and nonlinearity propagation parameter on observed LCR performance of dual SC.

Cross-Layer Design of Wireless Multihop Networks Over Stochastic Channels With Time-Varying Statistics

Network Utility Maximization (NUM) is often applied for the cross-layer design of wireless networks considering known wireless channels. However, realistic wireless channel capacities are stochastic bearing time-varying statistics, necessitating the redesign and solution of NUM problems to capture such effects. Based on NUM theory we develop a framework for scheduling, routing, congestion and power control in wireless multihop networks that considers stochastic Long or Short Term Fading wireless channels. Specifically, the wireless channel is modeled via stochastic differential equations alleviating several assumptions that exist in state-of-the-art channel modeling within the NUM framework such as the finite number of states or the stationarity. Our consideration of wireless channel modeling leads to a NUM problem formulation that accommodates non-convex and time-varying utilities. We consider both cases of non orthogonal and orthogonal access of users to the medium. In the first case, scheduling is performed via power control, while the latter separates scheduling and power control and the role of power control is to further increase users' optimal utility by exploiting random reductions of the stochastic channel power loss while also considering energy efficiency. Finally, numerical results evaluate the performance and operation of the proposed approach and study the impact of several involved parameters on convergence.

On the Performance of Semiconductor Optical Amplifier-Assisted Outdoor Optical Wireless Links

We present an analytical framework for estimating the benefits that arise from the utilization of semiconductor optical amplifiers at the receiver of fade impaired outdoor optical wireless systems. We first investigate the impact of fading on both the signal optical power and the amplified spontaneous emission, and derive analytical relations that accurately associate the system bit-error-rate with the channel state. We then utilize the analytical relations to assess the performance of the amplified system under moderate-to-strong (γ - γ) fading in terms of first and second order signal statistics. Our results show that the receiver sensitivity improvement, which is imparted by the amplification process, can drastically reduce the average bit-error-rate, link outage probability and average fade duration, provided that the link length remains within certain limits that are determined by the turbulence intensity scenario under consideration.