Asymptotic Closed-form Expressions Research Articles

Performance Analysis of Monostatic Multi-Tag Backscatter Systems With General Order Tag Selection

We investigate the performance of a monostatic multi-tag backscatter communication system with a general order tag selection criterion. In particular, we derive novel exact expressions for the average throughput and effective throughput. We further obtain simplified closed-form asymptotic expressions for the average and effective throughputs assuming a large number of tags. In addition, we obtain a unified exact expression for the average bit error rate, which is applicable to a wide class of binary modulation formats.

Asymptotic Analysis for Overlap in Waveform Relaxation Methods for RC Type Circuits

Waveform relaxation (WR) methods are based on partitioning large circuits into sub-circuits which then are solved separately for multiple time steps in so called time windows, and an iteration is used to converge to the global circuit solution in each time window. Classical WR converges quite slowly, especially when long time windows are used. To overcome this issue, optimized WR (OWR) was introduced which is based on optimized transmission conditions that transfer information between the sub-circuits more efficiently than classical WR. We study here for the first time the influence of overlapping sub-circuits in both WR and OWR applied to RC circuits. We give a circuit interpretation of the new transmission conditions in OWR, and derive closed form asymptotic expressions for the circuit elements representing the optimization parameter in OWR. Our analysis shows that the parameter is quite different in the overlapping case, compared to the non-overlapping one. We then show numerically that our optimized choice performs well, also for cases not covered by our analysis. This paper provides a general methodology to derive optimized parameters and can be extended to other circuits or system of differential equations or space–time PDEs.

In brief

PAGE 712 A pair of Chinese researchers propose a Selected and Refined Local Attention Module for object detection. Their framework comprises a local attention module and a feature fusion module to retain as much useful information as possible. Compared with classical object detection methods, the proposed model obtains better feature representation and discriminant power. PAGE 739 Researchers from Brazil present a new communication architecture based on the ‘Internet of Things' paradigms, the objective of the architecture is to provide a communication mechanism suitable for use in rural distribution networks, which it succeeded at. The authors also highlight the physical implementation of the network. Pipeline of proposed framework PAGE 737 Researchers from China propose a domain adversarial Siamese network which attempts to eliminate the domain influence on speech representation in Automatic Speaker Recognition applications. Results show that the network performs at least 10% better than typical methods, especially in scenarios with unknown domains and unbalanced data amounts. Time histogram with MIMO PAGE 716 Researchers representing Spain, Ecuador and Denmark have analysed the effect of correlation between forwards and backwards links on the capacity of backscatter communication systems, obtaining an analytical expression for the average capacity under a correlated Rayleigh product fading channel. Closed-form asymptotic expressions for high and low signal-to-noise ratio regimes are also obtained. Proposed network architecture PAGE 699 Professor B. Y. Kong presents a multi-touch detector architecture incorporating an efficient buffering scheme. The screen can buffer two consecutive requests simulataneously, meaning only one request is made to a buffer at a time. Implementation results show that the proposed architecture can diminish the silicon area by 36% and power consumption by 17% compared to state of the art. Average capacity as a function of average SNR at receiver for different values of the power correlation coefficient. Multi-touch detection

Effect of correlation on the capacity of backscatter communication systems

The authors analyse the effect of correlation between the forward and backward links on the capacity of backscatter communication systems. With that aim, we obtain an analytical expression for the average capacity under a correlated Rayleigh product fading channel, as well as closed-form asymptotic expressions for the high and low signal-to-noise ratio (SNR) regimes. The proposed works' results show that correlation is indeed detrimental for a fixed target SNR; contrarily to the common belief, the authors also see that correlation can be actually beneficial in some instances when a fixed power budget is considered.

Secure Outage Analysis for Cooperative NOMA Systems With Antenna Selection

The secrecy outage probability (SOP) performance is investigated for a cooperative non-orthogonal multiple access system under eavesdropping attack, where a base station transmits signals to a far user via a full-duplex relay, and to a near user directly at the same time, in the presence of an eavesdropper. All nodes are configured with multiple antennas and antenna selection is adopted by the base station and relay to enhance the security performance. The exact and asymptotic closed-form SOP expressions are derived, which incorporate the impact of imperfect channel state information and apply to general cases considering the quality of service of the far and near users, whether or not the wiretap channel is known, and the location of the eavesdropper. Monte Carlo simulations corroborate the analysis and the superiority of the proposed system over the single-antenna system and the one without antenna selection. It is also revealed that the asymptotic SOP performance of the near user is nearly independent of the power allocation.

Secure Transmission in Cognitive Wiretap Networks with Full-Duplex Receivers

This paper studies the secure transmission in the dual-hop cognitive wiretap networks, where the secondary transmitter (Alice) aims to transmit confidential information to the secondary receiver (Bob) in the face of a multi-antenna relay (Relay), while the malicious eavesdropper (Eve) is used to eavesdrop the confidential information from Alice and Relay. To improve security, we design two transmission schemes, namely maximal-ratio combining/maximal-ratio transmission-selection combining (MRC/MRT-SC) with half-duplex (HD) receiver and maximal-ratio combining-zero forcing beamforming/maximal-ratio transmission-selection combining-zero forcing beamforming (MRC-ZFB/MRT-SC-ZFB) with full-duplex (FD) receiver. To evaluate the secrecy performance obtained from the proposed schemes comprehensively, the new closed-form and simple asymptotic expressions for the secrecy outage probability (SOP) and secrecy throughput (ST) of our considered networks with MRC-ZFB/MRT-SC-ZFB and MRC/MRT-SC schemes are derived, respectively. Thus, we explore the effect of various schemes on system secrecy performance in terms of SOP and ST. Analytical results and numerical simulations demonstrate that MRC-ZFB/MRT-SC-ZFB achieves better performance in the two proposed schemes. In particular, we show that the FD receiver plays a crucial role in designing the cognitive wiretap networks for protecting the legitimate link against attack from the malicious eavesdropping.

On the asymptotic analysis of the high-order statistics of the channel capacity over generalized fading channels

In this article, we provide further asymptotic analysis to the higher-order statistics (HOS) of the channel capacity over generalized fading channels, especially by proposing simple and closed-form expressions each of which can be easily computed as a tight-bound revealing the existence of constant gap between the actual and asymptotic HOS of the channel capacity in the limit of both high- and low-signal to noise ratios (SNRs). As such, we show that these closed-form asymptotic expressions are insightful enough to comprehend the diversity gains. The mathematical formalism we followed in this article is illustrated with some selected numerical examples that validate the correctness of our newly derived asymptotic results.

Optimal Relay Selection for Secure NOMA Systems Under Untrusted Users

The key idea of non-orthogonal multiple access (NOMA) is to serve multiple users in the same frequency band to improve the spectral efficiency. Whereas a security flaw emerges in the presence of untrusted users or low security clearance users who can access the messages of trusted users due to the spectrum sharing and may work as eavesdroppers. To achieve reliable and secure communications of NOMA systems under untrusted users, we in this paper propose two relay selection (RS) schemes termed decode-and-forward (DF) and amplify-and-forward (AF) protocols based optimal relay selection (i.e., DORS and AORS) schemes, respectively. To explicitly reveal the benefits of the DORS and AORS schemes, we derive the exact and asymptotic closed-form expressions of the secrecy outage probability (SOP) and positive secrecy capacity probability (PSCP) achieved by the two schemes, and analyze the optimality of the two schemes. Analysis and simulation results indicate that both the DORS and AORS schemes significantly outperform the benchmark schemes, in addition to achieving the same SOP and desirable PSCP at high SNR.

Hardware- and Interference-Limited Cognitive IoT Relaying NOMA Networks With Imperfect SIC Over Generalized Non-Homogeneous Fading Channels

Internet-of-Things (IoT) technology has received much attention due to its great potential to interconnect billions of devices in a broad range of applications. IoT networks can provide high-quality services for a large number of users and smart objects. On the other hand, massive connectivity in IoT networks brings problems associated with spectral congestion. This issue can be solved by applying cognitive radio (CR) and non-orthogonal multiple access (NOMA) techniques. In this respect, this paper studies the performance of cooperative CR-NOMA enabled IoT networks over a generalized α - μ fading channel model. Closed-form analytical expressions of the end-to-end outage probability (OP) for the secondary NOMA users are derived using the Meijer's G-function with a consideration of the impacts of the interference temperature constraint, primary interference, residual hardware impairments and imperfect successive interference cancellation. Moreover, to acquire some useful insights on the system performance, asymptotic closed-form OP expressions are provided. Additionally, the impact of α and μ fading parameters on the outage performance is examined and, as a result, it is concluded that the system performance sufficiently improves as α and/or α increase. Furthermore, the outage performance of the proposed system model is shown to outperform that of an identical IoT network operating on orthogonal multiple access. Finally, the provided closed-form OP expressions are validated with Monte Carlo simulations.

Optimal Uplink Pilot-Data Power Allocation for Large-Scale Antenna Array-Aided OFDM Systems

The effective aggregate rate (AR) performance of large-scale multiple-input multiple-output aided orthogonal frequency division multiplexing systems based on time division duplexing protocol critically depends on uplink (UL) pilot-data power allocations for all users. We derive an asymptotic closed-form expression of the system's achievable AR for typical channel estimation schemes, which is a function of individual users’ UL power allocation factors. We prove that the effective AR is a convex function of each UL power allocation factor, when all the other factors are given. The globally optimal UL pilot-data power allocation that maximizes the system's effective AR can easily be obtained using an iterative procedure, by solving an univariate convex optimization with the golden section method for individual UL power allocation factors one by one in each iteration. Owing to the piecewise convexity of the effective AR, this iterative algorithm guarantees to find the unique globally optimal solution with only one iteration. The simulation results confirm that the achievable effective AR is significantly enhanced through the proposed optimization of UL pilot-data power allocation.

Secure Transmission Designs for NOMA Systems Against Internal and External Eavesdropping

The key idea of non-orthogonal multiple access (NOMA) is to serve multiple users in the same resource block to improve the spectral efficiency. Whereas due to the resource sharing, a security flaw of NOMA emerges in the presence of internal untrusted users, especially untrusted near users who are closer to the base station and can easily access the confidential information for paired far users. To mitigate the flaw, in this paper, we investigate the reliable and secure transmission of NOMA systems with untrusted near users, and propose joint beamforming and power allocation (JBP) scheme for the scenario. Meanwhile, from security point of view, we extend to a worse-case scenario where both untrusted near users and external eavesdroppers exist, and propose joint artificial noise aided beamforming and power allocation (JANBP) scheme to achieve a reliable and secure transmission for the scenario. The exact and asymptotic closed-form expressions of secrecy outage probability (SOP) for the two scenarios are derived to evaluate the secrecy performance achieved by the proposed schemes, respectively. The analysis and simulation results show the superiority of the proposed JBP and JANBP schemes in terms of combating internal and external eavesdropping, and also indicate the two schemes can achieve the same SOP at high SNR.

Centralized vs Decentralized Targeted Brute-Force Attacks: Guessing With Side-Information

According to recent empirical studies, a majority of users have the same, or very similar, passwords across multiple password-secured online services. This practice can have disastrous consequences, as one password being compromised puts all the other accounts at much higher risk. Generally, an adversary may use any side-information he/she possesses about the user, be it demographic information, password reuse on a previously compromised account, or any other relevant information to devise a better brute-force strategy (so called targeted attack). In this work, we consider a distributed brute-force attack scenario in which $m$ adversaries, each observing some side information, attempt breaching a password secured system. We compare two strategies: an uncoordinated attack in which the adversaries query the system based on their own side-information until they find the correct password, and a fully coordinated attack in which the adversaries pool their side-information and query the system together. For passwords X of length $n$ , generated independently and identically from a distribution $P_{X}$ , we establish an asymptotic closed-form expression for the uncoordinated and coordinated strategies when the side-information $\mathrm {Y}_{(m)}$ are generated independently from passing X through a memoryless channel $P_{Y|X}$ , as the length of the password $n$ goes to infinity. We illustrate our results for binary symmetric channels and binary erasure channels, two families of side-information channels which model password reuse. We demonstrate that two coordinated agents perform asymptotically better than any finite number of uncoordinated agents for these channels, meaning that sharing side-information is very valuable in distributed attacks.

On physical layer security in WSN over GK fading channels during intercept events

In this paper, wireless sensor network (WSN) security issue in the presence of an eavesdropper is analysed. The sensor-sink as well as sensor-eavesdropper channels are assumed to be subjected to generalized-K fading. According to the physical layer security framework, we employ round-robin, optimal sensor and cumulative distribution function-based scheduling scheme to enhance secure connection between sensor nodes and to attain low probability of intercept. Novel analytical results are given in a form of easy-to-compute analytical expressions for the intercept probability. Additionally, asymptotic closed-form expressions are derived and diversity orders of scheduling schemes are determined. Numerical results demonstrate important impacts of fading and shadowing parameters of main and wiretap links, network size and selected scheme on WSN security. Presented results are validated by independent Monte Carlo simulations.

Secrecy Rate Analysis for Millimeter-Wave Lens Antenna Array Transmission

Physical layer security is vital to millimeter-wave communications enabled by large-scale arrays, particularly the energy-efficient lens antenna arrays (LAAs). However, the broad application of LAAs can be hindered by the lack of a proper understanding of the secrecy performance. This letter derives an asymptotic closed-form expression for the secrecy rate of LAA, despite the critical challenges including the coupling of unknown lens beam responses. With the new secrecy rate analysis, the optimal power assignment for the legitimate transmission is achieved, leading to the maximization of LAA secrecy. This power assignment is unprecedentedly studied in LAA due to the previous absence of an analytical secrecy rate. Simulations validate the accuracy of the analysis over wide ranges of system parameters.

A new approach for two-way relaying networks: improving performance by successive interference cancellation, digital network coding and opportunistic relay selection

In this paper, we propose a new approach based on combination of successive interference cancellation (SIC), digital network coding (DNC), and opportunistic relay selection in a two-way cooperative scheme in which two source nodes send simultaneously their packets to each other with a help of multiple relaying nodes, called as SIC–DNC protocol. In the proposed SIC–DNC protocol, the cooperative relays use the SIC technique to decode sequentially the packets from received sum signals, and then encode these packets by the DNC technique. These relays operate in the half-duplex mode, and suffer interference signals from imperfect SIC operations. A best relay is selected by the opportunistic relay selection based on taking maximization of signal-to-interference-plus-noise ratios from a successful decoding relay set to two source nodes in the last time slot. Exact and asymptotic closed-form outage probability expressions are obtained to evaluate the proposed SIC–DNC protocol, and then are verified by performing the Monte Carlo simulations. Our results show performance improvement of the proposed SIC–DNC protocol because of increase of the number of the cooperative relays, decrease of the residual interference signal powers, and respect to optimal relay locations and optimal power allocation coefficients of the near source. In addition, insightful comparisons with a conventional two-way decode-and-forward scheme are provided to prove highlight performances in asymmetric two-way cooperative schemes whereas the proposed SIC–DNC protocol owns better spectrum utilization efficiency. Finally, the simulation results are harvested to valid the exact and asymptotic analysis values.

Performance analysis for mixed [formula omitted] shadowed and exponentiated Weibull distributed dual-hop system with multiuser diversity in C-RAN

This paper investigates the performance of mixed radio-frequency/free-space optical (RF/FSO) system with multiuser diversity over mixed fading channels suitable for the Cloud Radio Access Network (C-RAN). Specifically, the considered system consists of K users, a decode-and-forward (DF) relay and a destination. The relay receives the data from the user with best channel conditions and forwards it to the destination. Furthermore, supposing that the RF link experiences κ−μ shadowed fading which combines small-scale multipath fading with line-of-sight shadowing, whereas the FSO link is subject to Exponentiated Weibull (EW) distributed turbulence. We first derived a novel probability density function (PDF) expression for the κ−μ shadowed fading so that the κ, m and can be equal to the arbitrary positive real value. Then, analytical expressions for the end-to-end outage probability (OP) and average bit error rate (ABER) of the overall network are derived assuming an intensity modulation with direct detection (IM/DD) scheme. The closed-form asymptotic expressions at high signal-to-noise (SNR) regimes are also presented. Capitalizing on the asymptotic analysis, we can obtain the diversity gain and discuss the bottlenecks in the mixed RF/FSO systems. Additionally, the impact of several parameters, including the number of users, atmospheric turbulence conditions and pointing errors on the overall system are studied. The simulation results are provided to prove the feasibility of deployment of the system in real-life scenarios.

Asymptotic Outage Analysis on Dual-Branch Diversity Receptions Over Non-Identically Distributed Correlated Lognormal Channels

It is challenging to quantify the left tail behavior of the cumulative distribution function of a sum of lognormal random variables, especially when they are correlated and non-identically distributed. The mathematical intractability hampers the analysis on diversity receptions over lognormal fading channels. In this paper, a novel asymptotic technique is exploited to study non-identically distributed dual-branch lognormal fading channels with correlation. Closed-form asymptotic outage probability expressions are derived for maximum-ratio combining (MRC), equal-gain combining (EGC), and selection combining (SC). Three counterintuitive facts are revealed: 1) one of the two channels contributes no asymptotic performance gain to the diversity reception systems under certain conditions, implying that a link can be discarded without causing asymptotic performance loss; 2) SC outperforms EGC under certain conditions; and 3) the outage probability of SC is arbitrarily close to that of MRC under certain conditions. The new result reveals insights into the long-standing problem of asymptotic analysis for correlated lognormal fading channels, paving the way for analysis on more general channel models. This paper also provides an efficient approach to evaluate the non-identically distributed lognormal fading channels at a high signal-to-noise ratio. Practically, the analytical results prevent some redundant units that contribute no performance gain.

New approximate and asymptotic closed-form expressions for the outage probability and the average BER of MIMO-FSO system with MRC diversity technique over Gamma–Gamma fading channels with generalized pointing errors

This paper studies the outage probability (OP) and the average bit-error rate (BER) of multiple-input multiple-output (MIMO) free-space optical (FSO) system with maximal ratio combining (MRC) diversity technique over Gamma–Gamma (GG) fading channels with generalized pointing errors modeled by the Beckmann distribution. A new approximate closed-form probability density function (PDF) at high signal-to-noise ratio (SNR) for MRC scheme is derived by inverse Laplace transformation method. Based on the developed PDF, new approximate and asymptotic closed-form expressions for the OP and the average BER are derived, respectively. Then, the OP and the average BER performance of the MIMO-FSO system with MRC scheme at high SNR are analyzed with different parameters, including number of transmitters and receivers, receiver aperture sizes, nonzero boresight, and different jitters. Finally, numerical results and Monte Carlo simulations corroborate the accuracy of the derived approximate and asymptotic expressions over a wide range of SNR.

The Security of Cognitive Multiuser Networks Over Space-and-Time Correlated Channels

In this paper, we investigate the secrecy performance of cognitive multi-input multi-output multiuser (MIMO-MU) wiretap networks over space-and-time correlated fading channels. Specifically, we propose a joint transmit antenna selection (TAS) and multiuser scheduling scheme to improve the security of the considered system. To thoroughly analyze the impact of the joint TAS and multiuser scheduling scheme and the channel correlation on the physical layer security, the new exact and asymptotic closed-form expressions for the secrecy outage probability of cognitive MIMO-MU wiretap networks are derived in two distinct scenarios, i.e., space-and-time correlated fading channels and independent-and-perfect fading channels. From the theoretical and simulation results, several important remarks are concluded as follows. 1) The channel space correlation does not affect the achievable secrecy diversity order in a high main-to-eavesdropper ratio (MER) regime. While it produces different impacts on the secrecy performance with different values of the MER in a low MER regime. 2) The channel time correlation on the main channel can reduce the achievable secrecy diversity order from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N_{\rm A}N_{\rm B}$</tex-math></inline-formula> to 1. 3) The joint TAS and multiuser scheduling scheme can achieve better performance than the traditional random scheduling scheme over space-and-time correlated fading channels.

Asymptotic Analysis on Diversity Receptions Over Fading Channels With Correlated Shadowing

Ultra-dense wireless networks deploy multiple antennas within the decorrelation distance of lognormal shadowing, making it challenging to perform performance analysis. In this correspondence paper, we study the asymptotic performance of diversity receptions over composite lognormal-X fading channels, where the lognormal shadowing components are correlated. Closed-form asymptotic error rate and outage probability expressions of maximum-ratio combining, equal-gain combining, and selection combining are derived. It is revealed that the correlation between lognormal components only contributes a constant factor to the performance expressions, and does not change the diversity order.