Search

This article establishes an analytical framework for the upper bound on the average Age-of-Information (AoI) in first-come-first-served (FCFS) wireless communications where a certain level of outage probability is unavoidable. To begin with, we analyze the average AoI and derive a general upper bound for G/G/1 systems with a certain outage probability. Subsequently, for an M/M/1 system with the FCFS scheme, we obtain a concise closed-form expression of the upper bound, and further refine the upper bound after analyzing the relative error. Interestingly, it is found by the analysis that the relative error is independent of the service rate, and the upper bound becomes tighter as the outage probability increases. Based on the refined upper bound, we minimize the average AoI for the communications suffering from block Rayleigh fading. We derive a closed-form expression of the outage probability over a fading channel, and then prove that the refined upper bound is a convex function with respect to the average update generating rate. Consequently, we optimize the AoI performance by solving a convex optimization problem formulated utilizing the refined upper bound expression. The numerical results indicate that the minimum average AoI can be reduced by either increasing the service rate or the transmission power. [ABSTRACT FROM AUTHOR]

Optical wireless communication (OWC) over atmospheric turbulence and pointing errors is a well-studied topic. Still, there is limited research on signal fading due to random fog in an outdoor environment for terrestrial wireless communications. In this paper, we analyze the performance of a decode-and-forward (DF) relaying under the combined effect of random fog, pointing errors, and atmospheric turbulence with a negligible line-of-sight (LOS) direct link. We consider a generalized model for the end-to-end channel with independent and not identically distributed (i.ni.d.) pointing errors, random fog with Gamma distributed attenuation coefficient, double generalized gamma (DGG) atmospheric turbulence, and asymmetrical distance between the source and destination. We develop density and distribution functions of signal-to-noise ratio (SNR) under the combined effect of random fog, pointing errors, and atmospheric turbulence (FPT) channel and distribution function for the combined channel with random fog and pointing errors (FP). Using the derived statistical results, we present analytical expressions of the outage probability, average SNR, ergodic rate, and average bit error rate (BER) for both FP and FPT channels in terms of OWC system parameters. We also develop simplified and asymptotic performance analysis to provide insight on the system behavior analytically under various practically relevant scenarios. We demonstrate the mutual effects of channel impairments and pointing errors on the OWC performance, and show that the relaying system provides significant performance improvement compared with the direct transmissions, especially when pointing errors and fog becomes more pronounced. [ABSTRACT FROM AUTHOR]

A hybrid satellite-terrestrial relay network (HSTRN) has been envisioned as a promising communication architecture for future wireless communications, which can prominently reduce the impact of shadow fading and expand service coverage. In this paper, we conduct theoretical performance analysis for a dual-hop communication system, consisting of multiple relays and multiple users in the HSTRN with the amplify-and-forward protocol. For the shadowing effect and the multipath effect, we introduce the shadowed-Rician distribution and Nakagami- $m$ fading to characterize the channel models for the satellite-relay and the relay-user links, respectively. Additionally, the opportunistic scheduling scheme is employed, where the optimal relay and the optimal user are selected from the alternative nodes with the maximum instantaneous signal-to-noise ratio. Thereafter, the analytical expressions including ergodic capacity and average symbol error rate (SER) are derived. More specifically, we also present the asymptotic behavior of the average SER at the high signal-to-noise ratio (SRN) regime. The theoretical analysis is validated by the numerical results. Moreover, the results also reveal that the performance of the system can be improved by increasing the number of relays and users under different channel parameters and various modulation schemes. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) and ambient backscatter communication (AmBC) have gained a lot of interest as key enabling technologies in wireless communications systems. NOMA is a key player for enhancing spectrum utilization either by multiplexing multiple messages for the same user or allowing multi-users access, while AmBC shows great potential for enhancing both spectrum and energy efficiency for battery-limited devices. In this paper, the performance of a downlink NOMA multiplexing based symbiotic-radio ($SR$) AmBC system is analyzed over Nakagami-m fading channels. New closed-form expressions for the exact and asymptotic outage probabilities are derived. Moreover, we analyze the diversity order and the influence of the system parameters on the outage performances. Besides, we proposed a power allocation optimization technique to achieve an outage-optimal performance. Through representative Monte-Carlo simulations, we have verified the analytical results. Finally, we compared the performance of the proposed system against a benchmark OMA-based system. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a secure mobile edge computing (MEC) network in the presence of multiple eavesdroppers, where multiple users can offload parts of their tasks to the computational access point (CAP). The multiple eavesdroppers may overhear the confidential task offloading, which leads to information leakage. In order to address this issue, we present the minimization problem of the secrecy outage probability (SOP), by jointly taking into account the constraints from the latency and energy consumption. With the aim to improve the system secrecy performance, we then introduce three user selection criteria to choose the best user among multiple ones. Specifically, criterion I maximizes the locally computational capacity, while criterion II and III maximize the secrecy capacity and data rate of main links, respectively. For these criteria, we further analyze the system secrecy performance by deriving analytical and asymptotic expressions for the SOP, from which we can conclude important insights for the system design. Finally, simulation and analytical results are provided to verify the proposed analysis. The results show that the three criteria can efficiently safeguard the MEC networks, compared to the traditional local computing and fully offloading, especially with a large value of user number. [ABSTRACT FROM AUTHOR]

The development of unmanned aerial vehicles (UAVs) is one of the driving forces for revolutionizing wireless communications in the modern era. Although there are many unique features of UAV networks, their widespread use is still hampered by the short battery life. As a result, most of the research efforts in this domain mainly have focused on the efficient use of energy through trajectory planning and cooperative communication. While there have been a few proposals to use energy harvesting techniques for UAV networks, a complete understanding of the performance limits of such networks is still missing in the literature and needs further investigations. Within this context, our paper provides a comprehensive performance evaluation of UAV relay networks (UAVRNs) employing non-orthogonal multiple access (NOMA) technique. Specifically, a cooperative communication system, where a UAV serves as a mobile relay, is fully considered. The UAV relay is considered to be wirelessly-powered and harvests energy from the radio signals received from a nearby base station. For the sake of comparison, both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols are considered. Subsequently, the closed-form expressions of outage probabilities and ergodic capacities are provided for each UAV relaying protocol. Extensive simulations are performed to verify the accuracy of the derived closed-form expressions. The results provided in this work characterize the outage and ergodic capacity performance of NOMA-enabled UAVRNs. [ABSTRACT FROM AUTHOR]

A UAV-aided wireless power transfer and data collection network is studied, where it is assumed that when the harvested energy at the sensor node (SN) cannot surpass its circuit activation threshold or the received data rate at UAV falls below a minimal required rate threshold, the information outage occurs. The closed-form expressions of energy outage probability and rate outage probability are derived at first, and then the overall outage probability and coverage performance of the system are analyzed. Based on which, an optimization problem is formulated to minimize the overall outage probability by optimizing UAV’s elevation angle and the time splitting (TS) factor. Since the problem is non-convex and has no known solution, an alternating optimization (AO)-based algorithm with Golden-section (GS) based linear search method is designed to find the global optimal solution. In order to explore the maximum coverage area of the UAV for a given tolerable outage probability, another optimization problem is also formulated to maximize the coverage range by optimizing UAV’s elevation angle. By using Karush-Kuhn-Tucker (KKT) conditions, the closed-form solution of the optimal elevation angle for maximizing the coverage area is derived. Monte Carlo simulations verify the accuracy of the derived closed-form expression of the overall outage probability and the semi-closed-form expressions of the optimum UAV’s elevation angle and TS factor. It shows that there exist a unique optimum elevation angle and the TS factor to achieve the minimum overall outage probability, and significant performance gain can be obtained by using our proposed optimization scheme. The developed theoretical results can be useful to the design of UAV-aided wireless communication systems with wireless power transfer. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) communication has emerged as a promising solution for providing non-line-of-sight (NLOS) wireless connectivity due to strong molecular and aerosol scattering at UV wavelength. However, performance of UV based communication systems is severely impaired due to nonlinear transfer-characteristics of light emitting diode (LED), which degrades the overall symbol error rate (SER) performance. In addition, UV based communication systems are also impaired by multiplicative distortion due to turbulence, that causes detrimental instantaneous outages. Hence, in this work, first an expression for the outage probability is derived for a nonlinear UV communication system via analytical characterization of the statistics of the additive distortion. Further, utilizing the proposed analytical model for additive distortion, the error rate of reproducing kernel Hilbert space (RKHS) based detectors is quantified for the nonlinear outdoor NLOS UV channel. Additionally, using the derived expression for error-rate, an RKHS based minimum symbol error rate (MSER) equalizer is formulated to mitigate the distortion due to LED nonlinearity, and to enhance the error-rate performance of the considered nonlinear NLOS UV link. Convergence of the proposed MSER equalizer is analyzed, and improvements in error-rate promised by the proposed equalizer are validated by computer simulations over typical NLOS UV channels. [ABSTRACT FROM AUTHOR]

In supporting the new 6G standard traffic services-massive ultra-reliable low-latency communications (mURLLC), several advanced techniques, including statistical delay-bounded quality-of-service (QoS) provisioning theory and finite blocklength coding (FBC), have been developed to upper-bound both delay and error-rate for time-sensitive multimedia applications. On the other hand, cell-free (CF) massive multi-input multioutput (m-MIMO), where a large number of distributed access points (APs) jointly serve a massive number of mobile devices using the same time-frequency resources, has emerged as one of the 6G key promising techniques to significantly improve various QoS performances for supporting mURLLC. However, it is challenging to statistically guarantee stringent mURLLC QoS-requirements for transmitting multimedia traffics over CF m-MIMO and FBC based 6G wireless networks. To overcome these problems, we develop analytical models to precisely characterize the delay and error-rate bounded QoS performances while considering non-vanishing decode-error probability for CF m-MIMO based schemes. In particular, we develop FBC based system models and apply the Mellin transform to characterize arrival/service processes for our proposed CF m-MIMO modeling schemes. Then, we formulate and solve the delay violation probability minimization problem and obtain the closed-form solution of the optimal rate adaptation policy for each mobile user over 6G CF m-MIMO mobile wireless networks in the finite blocklength regime. Our simulation results validate and evaluate our proposed schemes for statistical delay and error-rate bounded QoS provisioning. [ABSTRACT FROM AUTHOR]

It is very challenging to detect an unmanned-aerial-vehicle (UAV) when it is applied to launch an attack by the enemy’s country, due to its feature of mobility and flexibility. Against this background, in this paper, from wireless communication point of view, we study the detection probability of home country’s ground vehicle (GV) to enemy’s ground-station (GS)-to-UAV transmission system. We assume that the location of the GV is randomly distributed inside the space confined by the largest detection distance. Moreover, we assume the enemy’s UAV is randomly distributed in the coverage space of the GS-to-UAV (G2A) transmission link but also keep the security distance from the GS. To this end, we first characterize the statistical features of the signal-to-noise-ratio (SNR) over the G2V and G2A links. Then, we define detection outage probability (DOP) and average detection capacity (ADC), and show their approximations. Finally, Monte-Carlo simulations are conducted to verify the correctness of our proposed analytical models. [ABSTRACT FROM AUTHOR]

With the rapid development of wireless communications, the signals may be transmitted in different media, such as visible light and millimeter-wave along the communication path. In this case, there will be different devices working in different media, which cannot directly communicate to each other. For alleviating this problem, we first conceive a cross-media communication scenario where two devices for different media could communicate with the aid of a relay working in both media. Furthermore, we derive the average outage probability of the uplink and downlink communications. Finally, aiming at minimizing the derived average outage probability, the optimal power allocation (PA) scheme at the relay side is also derived and verified by simulation results. [ABSTRACT FROM AUTHOR]

The primary objective of this paper is to investigate the performance improvement provided by a helper, with the aim of applications to a lossy communication system in wireless sensor networks (WSNs) and/or Internet of Things (IoT). Initially, we formulate the system model as a problem of multiterminal source coding with a helper, and determine an inner bound on the achievable rate-distortion region for binary sources. Based on Shannon's lossy source-channel separation theorem, we further derive the outage probability of the system over block Rayleigh fading channels. The numerical results indicate that a helper can obviously expand the achievable rate-distortion region and provide diversity gains which in turn decrease the outage probability. Finally, we evaluate the practical performance of outage probability through simulations with exclusive-or (XOR) as an example of the helper structure. Both the simulation and theoretical results show second order diversity by introducing a helper, and lower outage probability by increasing the acceptable distortions. [ABSTRACT FROM AUTHOR]

One of the major goals of emerging wireless systems is to prolong the lifetime of wireless communication devices. To this end, this contribution evaluates and optimizes the performance of simultaneous wireless information and power transfer (SWIPT) with an integrated energy and information receiver, which has the advantage of low complexity and energy cost. A tractable expression for the achievable rate is first derived, which is subsequently used to quantify the achievable harvested energy–rate region for the two fundamental SWIPT protocols, namely, power-splitting (PS) and time-switching (TS). In this context, the joint harvested energy–rate outage probability is then defined and minimized for a point-to-point and multicasting system, determining the optimal PS and TS factors for both linear and nonlinear energy harvesting models. In addition, a TS-based broadcasting system is dynamically optimized by maximizing the energy harvested by all users under an achievable rate threshold for each user. The formulated optimization problem is, in fact, particularly challenging due to the non-convex form of the expression for the achievable rate. Yet, an effective solution is ultimately achieved by converting this problem into a convex one. Also, respective computer simulation results corroborate the effectiveness of the proposed framework. Overall, it is shown that the offered results provide meaningful theoretical and practical insights that will be useful in the design and efficient operation of wireless powered systems. Indicatively, unlike the trend in common separated receivers, a region has been identified, where TS outperforms PS. [ABSTRACT FROM AUTHOR]

Mobile cloud computing systems, or simply mobile clouds, have attracted tremendous attention because they allow mobile devices with limited computational resources to offload complex computations. However, due to the channel uncertainty and the complexity of a computation task, mobile computation offloading may suffer from poor outage performance that the offloaded task cannot be completed within the desired delay constraint. Thus, how to efficiently identify and overcome the outage bottleneck, which could be used to optimize resource allocation schemes and improve the system performance effectively is an open problem. In this paper, we shall develop a unified framework that minimizes the overall outage probability in various mobile computation offloading scenarios. More specifically, the outage bottleneck is defined and identified by adopting asymptotic analysis, without any need of the accurate outage probabilities in both transmissions and computations. To overcome the outage bottleneck, resource pairing, matching, and allocation policies are investigated. Both theoretical analysis and numerical results show that the outage bottleneck relies on not only the availability of spectrum and computation resources but also the probability distributions of computation complexities of the computation tasks. [ABSTRACT FROM AUTHOR]

This paper investigates an integrated wireless communication system including non-orthogonal multiple access, full-duplex relaying, and energy harvesting techniques (named as EH-FD-NOMA). In this scheme, an energy-limited full-duplex relay harvests energy from a source at the first stage. Then, the relay detects the superimposed signal from the source and transmits the decoded signal to destination. Closed-form outage probabilities and ergodic rates at the relay and destination are derived. Numerical results verify the analytical results and show the superior performance of the EH-FD-NOMA if compared to its counterparts. [ABSTRACT FROM AUTHOR]

In this paper, we study the outage performance of simultaneous wireless information and power transfer based three-step two-way decode-and-forward relay networks, where both power-splitting (PS) and “harvest-then-forward” are employed. In particular, we derive the expressions of terminal-to-terminal (T2T) and system outage probabilities based on a Gaussian–Chebyshev quadrature approximation, and obtain the T2T and system outage capacities. The effects of various system parameters, e.g., the static power allocation ratio at the relay, symmetric PS, as well as asymmetric PS, on the outage performance of the investigated network are examined. It is shown that our derived expression for T2T outage capacity is more accurate than existing analytical results, and that the asymmetric PS achieves a higher system outage capacity than the symmetric one when the channels between the relay node and the terminal nodes have different statistic gains. [ABSTRACT FROM AUTHOR]

This paper studies a specific low-power wireless technology capable of reaching a long range, namely long range (LoRa). Such a technology can be used by different applications in cities involving many transmitting devices while requiring loose communication constrains. We focus on electricity grids, where LoRa end-devices are smart meters that send the average power demanded by their respective households during a given period. The successfully decoded data by the LoRa gateway are used by an aggregator to reconstruct the daily households’ profiles. We show how the interference from concurrent transmissions from both LoRa and non-LoRa devices negatively affect the communication outage probability and the link effective bit-rate. Besides, we use actual electricity consumption data to compare time-based and event-based sampling strategies, showing the advantages of the latter. We then employ this analysis to assess the gateway range that achieves an average outage probability that leads to a signal reconstruction with a given requirement. We also discuss that, although the proposed analysis focuses on electricity metering, it can be easily extended to any other smart city application with similar requirements, such as water metering or traffic monitoring. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) is currently considered as a promising technology for the next-generation wireless networks. In this paper, the error rate performance of NOMA systems is investigated over Nakagami- $m$ fading channels, while considering imperfect successive interference cancelation. In particular, this paper focuses on the pairwise error probability (PEP) analysis, where exact PEP expressions are derived to characterize the performance of all users under different fading conditions. The obtained PEP expressions are then used to derive an exact union bound on the bit error rate (BER). Through the derived PEP expressions, the asymptotic PEP analysis is presented to investigate the maximum achievable diversity gain of NOMA users. Moreover, using the derived BER bound, the power allocation problem for all users in NOMA systems is considered under average power and users BER constraints, which allows realizing the full potential of NOMA. Monte Carlo simulation and numerical results are presented to corroborate the derived analytical expressions and give valuable insights into the error rate performance of each user and the achievable diversity gain. [ABSTRACT FROM AUTHOR]

This paper proposes a novel design of cooperative non-orthogonal layered multicast multiple access in a heterogeneous network, where the information is encoded into the messages of high priority (HP) and low priority (LP). Two types of multicast users coexist in the network: 1) regular users (RUs), which are located far away from the base station (BS) and expect to decode only the HP message (due to the weak channels), and 2) advanced users (AUs), which are located close to the BS and expect to decode both HP and LP messages. To improve the reliability of layered multicast, we consider that the successful AUs (those AUs who successfully decode the HP and LP messages) serve as potential relays to assist other AUs/RUs. Based on this idea, two novel cooperation strategies are proposed for different cases of channel information availability. For each proposed strategy, we derive closed-form exact outage probabilities of AUs and RUs and then further analyze their diversity orders. Moreover, considering that the layered multicast is outage-constrained, we theoretically evaluate the energy consumption of both strategies and demonstrate their energy saving gains over the direct non-orthogonal multiple access for layered multicast. Finally, our theoretical analysis is verified by numerical results, and the advantages of the proposed strategies are also demonstrated. [ABSTRACT FROM AUTHOR]

In this paper, the advantages of joint detection (JD) in a hybrid-duplex unmanned aerial vehicle (UAV) communication system (HBD-UCS) are investigated as a step toward addressing spectrum scarcity in UAV communications. Through extensive outage probability and finite signal-to-noise-ratio diversity gain analysis, we showed that the performance of JD is independent of the strength and the data rate of the inter-UAV interference signal. On the contrary, the successive interference cancellation (SIC) detector requires the data rate of the interfering UAV to be less than the ground station before meaningful performance can be seen. At the system level, it is observed that the half-duplex UAV communication system outperforms the HBD-UCS with JD at moderate and high SNR regimes, as the latter is constrained by self-interference at the full-duplex ground station. Finally, we investigated the multiplexing gain region and showed that the joint detector offers higher diversity gain over a wide range of multiplexing gains over the interference ignorant and the SIC detector. [ABSTRACT FROM AUTHOR]

Relay assisted communication helps to extend the transmission coverage area. In more realistic scenarios, the available relays are untrusted, which try to decode the confidential signals during their main relaying function. Thus, these untrusted relays are regarded as potential eavesdroppers, and the confidential signals are vulnerable. A positive secrecy rate can be achieved at the physical layer without any cryptography, in particular, the mutual interference achieved from two-way relaying network can be used to secure the confidential signals against eavesdroppers attack. In this paper, the relay selection of two-way untrusted relays network is presented, and a directed beamforming is aligned to zero force the transmission at all other non-selected relays. Particularly, we propose a selection scheme based on max–min criterion, which requires an exhaustive search. Then, the lower bound (LB) of the secrecy rate is introduced to minimize the complexity of the selection criterion. The closed forms of the secure outage probability (SOP) for the max–min LB scheme is formulated and compared with other schemes. The simulations results validate the proposed analytical forms, and the performance of max–min LB scheme appears to be quite close to that of the max–min scheme with limited complexity. [ABSTRACT FROM AUTHOR]

We present a novel framework for analyzing and optimizing the outage performance of a multiple amplify-and-forward relay network, where $N$ relays, each with finite cache capacity, are deployed to assist the transmission from a base station equipped with $L$ antennas to the destination. In this network, if the requested file is cached at the relays and the associated relay channels are in good condition, then the data are directly transmitted from the relays to the destination; otherwise, traditional two-hop transmission is performed. Based on this concept, we propose a relay-selection criterion to choose the best relay, which maximizes the received signal-to-noise ratio at the destination. For this criterion, we derive exact and asymptotic analytical expressions for the system outage probability in Nakagami- $m$ fading. To further improve the network outage performance, we optimize the cache placement, which minimizes the outage probability. Numerical and simulation experiments are performed to validate the analysis and demonstrate the conditions where the proposed cache placement strategy outperforms the existing ones. [ABSTRACT FROM AUTHOR]

In this study, we deal with a cooperative wireless communications system of security constraints in which there is no knowledge about channel state information (CSI) of the eavesdropper. In order to improve the tradeoff between security and reliability, we opportunistically employ relays via an incremental and selective cooperation with respect to the CSI of the main link. In this cooperation, named as opportunistic cooperative transmission (OCT), the first priority is communication through the direct link as far as possible. However, a relay that has the best link to the main destination is employed for retransmitting the source information when the direct link is so weak which cannot provide a specific quality. In fact, since improving the reliability of the favorable communications by use of the relays increases the risk of successful interception by the eavesdropper at the same time, the OCT scheme tries to avoid excess of rebroadcasting the secret information by the relays whenever the direct link is strong enough; otherwise it employs an opportunistically selected relayed link. Hence, it can improve the security for a specific level of reliability and vice versa. We derive closed-form expressions for the security-reliability tradeoff (SRT) and secrecy outage probability (SOP) of the OCT scheme. Moreover, we provide asymptotic analyses which are illustrative on the security behavior of the proposed scheme. Finally, simulation results are presented which confirm validity of the analysis and reveal the desired performance of OCT in terms of both SRT and SOP. [ABSTRACT FROM AUTHOR]

It is challenging to derive closed-form performance expressions for composite channels considering both the large-scale lognormal fading and small-scale fading, thus most existing analyses on the composite channels are based on numerical approaches such as the Gauss–Hermite quadrature. In this paper, we develop a simple framework to achieve closed-form asymptotic expressions for the outage probabilities and the error rates of the composite lognormal-X fading channels, where the “X” can be Rayleigh, Rician, Nakagami- $m$ or any other fading channels that have finite diversity orders. It is proved that the lognormal fading contributes an exponential factor to the asymptotic expressions, and the exponential factor is related to the diversity order of the X channel and the lognormal parameters. The new analytical tool is shown to be versatile in evaluating the performance of radio-frequency communications suffering both the large-scale and small-scale fading, including diversity reception systems, relaying systems, and distributed antenna systems. It is also shown to be useful in evaluating free-space optical communication systems with pointing error, and in deriving asymptotic signal-to-noise ratio gaps between different modulation formats over lognormal fading channels. The elegant asymptotic expressions reveal insights into the composite fading channels and can be a criterion for various system designs. [ABSTRACT FROM AUTHOR]

We propose a power-and rate-adaptation scheme for cloud radio access networks (C-RANs), where each radio remote head (RRH) is connected to the baseband unit pool through optical links. The RRHs jointly support the users by efficiently exploiting the enhanced spatial degrees of freedom. Our proposed scheme aims at maximizing the effective capacity (EC) of the user subject to both per-RRH average- and peak-power constraints, where the EC is defined as the maximum arrival rate that can be supported by the C-RAN under the statistical delay requirement. We first transform the EC maximization problem into an equivalent convex optimization problem. By using the Lagrange dual decomposition method and solving the Karush–Kuhn–Tucker equations, the optimal transmission power of each RRH can be obtained in the closed form. Furthermore, an online tracking method is provided for approximating the average power of each RRH. For the special case of two RRHs, the expression of the average power of each RRH can be calculated in the explicit form. Hence, the Lagrange dual variables can be computed in advance in this special case. Furthermore, we derive the power allocation for two important extreme cases: first, no delay constraint and, second, extremely stringent delay requirements. Our simulation results show that the proposed scheme significantly outperforms the conventional algorithm without considering the delay requirements. Furthermore, when appropriately tuning the value of the delay exponent, our proposed algorithm is capable of guaranteeing a delay outage probability below $10^{-9}$ when the maximum tolerable delay is 1 ms. This is suitable for the future ultra-reliable low-latency communications. [ABSTRACT FROM AUTHOR]

This paper studies physical-layer security for a cache-enabled heterogeneous cellular network comprised of a macro base station and multiple small base stations (SBSs). We investigate a joint design on caching placement and file delivery for realizing secure and energy-efficient transmissions against randomly distributed eavesdroppers. We propose a novel hybrid “most popular content” and “largest content diversity” caching placement policy to distribute the files of different popularities. Depending on the availability and placement of the requested file, we employ three cooperative transmission schemes, namely, distributed beamforming, frequency-domain orthogonal transmission, and best SBS relaying. We derive analytical expressions for the connection outage probability and secrecy outage probability for each transmission scheme. Afterward, we design the optimal transmission rates and caching allocation successively to achieve a maximal overall secrecy throughput and secrecy energy efficiency, respectively. Numerical results verify the theoretical analyses and demonstrate the superiority of the proposed hybrid caching policy. [ABSTRACT FROM AUTHOR]

One promising approach to expand the coverage and enhance the performance of cellular networks is to employ multitier cellular networks. In this paper, an uplink two-tier communication network is studied, in which macrousers, femtousers, and femto access points are geometrically located inside the coverage area of a macro base station according to a Poisson point process. Each femtocell is assumed to have a fixed backhaul constraint that puts a limit on the maximum number of femto and macrousers it can service. Under this backhaul constraint, the network adopts a special open-access policy in which each macrouser is either assigned to its closest femto access point or to the macro base station. The choice is made based on the ratio between the distance of the macrouser from the macro base station and its distance from its closest femto access point. Under this model, upper and lower bounds on the outage probabilities experienced by users serviced by femto access points are derived as functions of the distance between the macro base station and the femto access point serving them. Similarly, upper and lower bounds on the outage probabilities of the users serviced by the macro base station are obtained. The bounds in both cases are confirmed via simulation results. [ABSTRACT FROM AUTHOR]

As a promising non-orthogonal multiple access scheme, sparse code multiple access (SCMA) technology has attracted much attention. Because inter-user interference is present in code domain and multi-user iterative detection is required, user capacity and outage probability analysis for uplink SCMA systems are challenging and have not been presented in the literature. In this paper, the capacity region for uplink SCMA systems is analyzed, based on which the common and individual outage probability regions are calculated. Optimizing the outage probability within the outage probability region can be casted as an Lagrangian duality problem and solved by an iterative descent algorithm, which however imposes high complexity since the expectation operation is required in each iteration. To reduce the computational complexity of solving this Lagrangian duality problem, an adaptive algorithm is developed, which is capable of providing the optimal outage probability and adaptively updating it. Furthermore, a power allocation policy is naturally obtained to achieve the optimized outage probability in the outage probability region. [ABSTRACT FROM AUTHOR]

This paper analyzes an emerging architecture of cellular network utilizing both planar base stations uniformly distributed in the Euclidean plane and base stations located on roads. An example of this architecture is that where, in addition to conventional planar cellular base stations and users, vehicles also play the role of both base stations and users. A Poisson line process is used to model the road network and, conditionally on the lines, linear Poisson point processes are used to model the vehicles on the roads. The conventional planar base stations and users are modeled by the independent planar Poisson point processes. We use Palm calculus to investigate the statistical properties of a typical user in such a network. Specifically, this paper discusses two different Palm distributions, with respect to the user point processes depending on its type: planar or vehicular. We derive the distance to the nearest base station, the association of the typical users, and the coverage probability of the typical user. Furthermore, we provide a comprehensive characterization of coverage of all possible cellular transmissions in this setting, namely, vehicle-to-vehicle, vehicle-to-infrastructure, infrastructure-to-vehicle, and infrastructure-to-infrastructure. [ABSTRACT FROM AUTHOR]

Millimeter wave (mmWave) technology has been advocated as a promising infrastructure to provide reliable communications, both in indoor and outdoor environments. In this paper, we extend the application of mmWave to the uplink communication between smart meters (SMs) and a gateway. Such a communication is subject to interference from SMs belonging to adjacent networks and blockage caused by human bodies. Using a 3-D stochastic blockage model, we derive the outage probability. When human-body blockage is neglected, the high signal-to-noise-ratio (SNR) analysis shows a diversity gain of $(m_{L}M)$ , where $m_{L}$ is the Nakagami-fading parameter of the line of sight (LOS) of the reference transmitter’s channel, and $M$ is the number of receive antennas at the gateway. Accounting for human-body blockage, the diversity gain reduces to $(m_{N}M)$ , where $m_{N}$ is the Nakagami-fading parameter of the non LOS of the reference transmitter’s channel. Our analysis shows that the probability that an SM is in LOS decays exponentially with the link length and the density of blockages. Although at high SNR blockage reduces the diversity gain, our numerical results show that blockage may decrease the outage probability at finite SNR. [ABSTRACT FROM AUTHOR]

Multiantenna full-duplex (FD) relaying, as a promising solution for future wireless communications, has received many attentions. In this paper, focusing on FD decode-and-forward relaying with only partial channel state information available, we are interested in the relay beamforming optimization from an outage probability minimization perspective. Due to the implicit outage probability objective function, it is rather hard to find an optimal solution directly. To overcome this obstacle, we first obtain an explicit form of the outage probability, which involves intricate Marcum Q-functions. Based on the derived objective, we find out the optimal relay receive and transmit beamforming directions, which turns out to be the maximum-ratio combining/maximum-ratio transmission strategy. Then, we rigourously prove that the optimal relay transmit power is the unique solution to a strictly quasi-concave problem and, hence, can be efficiently determined. The performance advantage of the proposed optimal design over conventional schemes is validated via simulations. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the issues of joint user pairing and subchannel allocation (JUP-SA) in a multisubchannel multiuser nonorthogonal multiple access (NOMA) system. We first investigate the uplink and propose a JUP-SA scheme so that the minimum achievable diversity order of users is maximized. By deriving the upper and lower bounds of the outage probability of the worst-performance user, the achievable diversity order of the scheme is obtained. Following this, the downlink of the system is considered and a JUP-SA scheme that maximizes the minimum diversity order is presented. The closed-form expression of the worst-case outage probability is then derived and validated by simulation results. Numerical results show that both schemes in uplink and downlink NOMA systems can achieve the same diversity order as that of exhaustive search. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of a spectrum sharing decode-and-forward multiple-relay network (SSDMN) with direct link under the joint impact of two practical and detrimental imperfections, called transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Here, we consider the hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. By taking these imperfections into account, we derive an exact closed-form expression for the outage probability of the considered SSDMN employing selection cooperation over independent and non-identical Rayleigh fading channels. We identify three important ceiling effects invoked by HIs, namely relay cooperation ceiling (RCC), direct link ceiling (DLC), and overall system ceiling. Specifically, we highlight the impact of these prevailing effects on the system’s performance and provide some useful insights about the tolerable level of HIs while designing the practical systems for a given rate requirement. We also illustrate that a potential direct link and a relaying link can, respectively, partially compensate for the incurred performance losses due to RCC and DLC effects. Moreover, we investigate the asymptotic behavior of outage probability to highlight the impact of HIs and CEEs on the achievable diversity and coding gains. [ABSTRACT FROM AUTHOR]

In this paper, a two-hop decode-and-forward wireless powered relaying cognitive radio network with interference alignment over Rayleigh fading channels is investigated. The energy-constrained secondary relay node harvests energy from both the information and interference signals. Then, the harvested energy is used by the relay to forward the information signal from the source to the destination node. By applying beamforming matrices to primary and secondary networks, the performance metrics, such as outage probability, capacity, and bit error rate are studied under perfect and imperfect channel state information scenarios for both power-splitting relaying (PSR) and time-switching relaying (TSR) protocols. In addition, the optimal network performance is achieved by calculating the optimal energy harvesting time-switching and power-splitting coefficients. Finally, closed-form expressions for the outage probability of primary and secondary users are derived. Monte Carlo simulation results corroborate the analytical ones and show that the PSR technique outperforms TSR in the considered system model. [ABSTRACT FROM AUTHOR]

A two-way relay non-orthogonal multiple access (TWR-NOMA) system is investigated, where two groups of NOMA users exchange messages with the aid of one half-duplex decode-and-forward relay. Since the signal-plus-interference-to-noise ratios of NOMA signals mainly depend on effective successive interference cancellation (SIC) schemes, imperfect SIC (ipSIC), and perfect SIC (pSIC) are taken into account. In order to characterize the performance of TWR-NOMA systems, we first derive closed-form expressions for both exact and asymptotic outage probabilities of NOMA users’ signals with ipSIC/pSIC. Based on the derived results, the diversity order and throughput of the system are examined. Then, we study the ergodic rates of users’ signals by providing the asymptotic analysis in high signal-to-noise ratio (SNR) regimes. Finally, numerical simulations are provided to verify the analytical results and show that: 1) TWR-NOMA is superior to TWR-OMA in terms of outage probability in low SNR regimes; 2) due to the impact of interference signal at the relay, error floors and throughput ceilings exist in outage probabilities, and ergodic rates for TWR-NOMA, respectively; and 3) in delay-limited transmission mode, TWR-NOMA with ipSIC and pSIC have almost the same energy efficiency. However, in delay-tolerant transmission mode, TWR-NOMA with pSIC is capable of achieving larger energy efficiency compared with TWR-NOMA with ipSIC. [ABSTRACT FROM AUTHOR]

In this correspondence paper, we propose a secure user scheduling scheme, which selects a user for data transmission and an extra user for artificial noise generation in a multiuser single-input and multiple-output wiretap network. The proposed scheme does not require the instantaneous channel state information of the eavesdropper but effectively reduce the secrecy outage probability. Specifically, the secrecy outage probability of the proposed scheme is mathematically derived in closed form when we consider maximum-ratio combining at both desired receiver and eavesdropper. Through simulations, we verify our analytical results and investigate the effect of unknown location of the eavesdropper on the secrecy outage probability when we consider the notion of a secrecy protected zone. [ABSTRACT FROM AUTHOR]

Simultaneous wireless information and power transfer (SWIPT) is a promising technology to mitigate the power constraint problem in wireless sensor networks by harvesting energy from received signals. A two-hop communication system is studied, where a relay node uses SWIPT to harvest energy and forward data. An eavesdropper exists and receives information both from the source and the relay. We considered two energy harvesting models, linear and nonlinear ones. The closed-form expression of the system's outage probability under information-theoretic physical layer security is derived for both models. We also analyze the optimal power splitting ratio and optimal relay distance with the aim to minimize the secrecy outage probability. For the nonlinear model, we derived the closed-form expression of the optimal power splitting ratio after the circuit becomes saturated. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA) is emerging as a promising multiple access technology for the fifth-generation wireless networks. By leveraging user-specific SCMA codebooks, SCMA possesses the potential to enable non-orthogonal resource access, and thus leading to the overloading gain. Benefiting from this, SCMA can accommodate more users and achieve a higher throughput than orthogonal frequency division multiple access. In this correspondence, we propose a tractable analytical framework using tools from stochastic geometry to investigate the performance of SCMA wireless networks. In particular, we obtain explicit closed-form expressions for the success probability and the area spectral efficiency (ASE). Based on the derived results, we prove that the ASE is a quasi-concave function of the transmitter density. Moreover, we shed light on the impact of SCMA codeword dimension $K$ and the number of non-zero entries in SCMA codeword $N$ on the network performance. Specifically, we demonstrate that the optimal transmitter density and the maximum ASE scale super-linearly with rate $K^{N}$. Simulation results validate the accuracy of the presented analysis and exhibit the superiority of SCMA. [ABSTRACT FROM AUTHOR]

In this paper, we consider secure transmission of a private file, which comprises multiple packets. A network-coding assisted automatic-repeat-request (ARQ) scheme is proposed to improve the security at both file level and packet level. Specially, the ARQ protocol degrades the intercept probability of the whole file at the eavesdropper, while the network coding guarantees the per-packet security. The closed-form intercept probability and outage probability of the whole file and each packet are analytically presented, and we then investigate the tradeoff between the security performance and the outage probability. The proposed scheme is further extended to the cooperative ARQ scenario, and simulation results are provided to corroborate our theoretical findings. [ABSTRACT FROM AUTHOR]

This paper considers a simultaneous wireless information and power transfer (SWIPT) based decode-and-forward relaying network, where the “harvest-then-forward” strategy is employed. We focus on designing optimal static and dynamic transmission schemes of joint time allocation and power splitting to conduct the relay in terms of outage performance and ergodic performance, respectively. In particular, the analytical expressions for the outage probability and ergodic capacity are derived to determine the optimal static power splitting (PS) and time allocation (TA) ratios. Moreover, we study the dynamic transmission scheme and formulate two joint optimization problems to minimize the outage probability and maximize the instantaneous channel capacity. Considering that the two optimization problems are nonconvex, a split-step iterative method is proposed to obtain the optimal dynamic PS ratio and TA ratio for minimizing the outage probability, and an alternate convex optimization method is employed to solve the problem of maximizing the instantaneous channel capacity. Simulation results verify the advantages of the proposed schemes over three peer schemes. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new relaying scheme, referred to as priority-based max-link relay selection, for buffer-aided decode-and-forward cooperative networks. We give the first priority to the relay buffers having status full, the second priority to the relay buffers having status empty, and the third priority to the relay buffers having status neither full nor empty. The best relay node is selected corresponding to the link having the highest channel gain among the links within a priority class. By adopting a Markov chain approach to analyze the state transition matrix that models the evolution of the buffers status, we derive analytical expressions for the outage probability and the average bit error rate. Analytical expressions for the steady-state probability vector are also obtained, and through these expressions, it is shown that states with the same probabilities can be grouped, thus reducing the size of the state transition matrix. We propose a general state-grouping-based method to obtain the reduced state transition matrix, which in turn reduces the computational complexity in obtaining the steady-state distribution. Our analytical and simulation results demonstrate that the proposed relaying scheme has better performance gain over the conventional max-link scheme. [ABSTRACT FROM AUTHOR]

This paper investigates the performance analysis of dual selection (i.e., combining user selection and transmit antenna selection) with maximal ratio combining (MRC) over nonidentical imperfect channel estimation in a multiple-input multiple-output system. This paper derives the exact and approximate cumulative density function and probability density function (PDF) for the dual selection with MRC over nonidentical imperfect channel estimation. Using the distribution, the exact and approximate closed-form expressions of the outage probability are derived with quantification of diversity order. Further, this paper derived the exact ergodic capacity and approximate outage capacity of the the dual selection via Gaussian and Gamma approximations. The numerical results show that the capacity of the dual selection with MRC is enhanced by the number of receive antennas as well as the number of user terminals and sum of channel correlation coefficients, while the order of the receive spatial diversity for outage probability is eliminated by the nonidentical channel estimation error. [ABSTRACT FROM AUTHOR]

In this paper, a novel cooperative non-orthogonal multiple access (NOMA) system is proposed, where one near user is employed as decode-and-forward relaying switching between full-duplex (FD) and half-duplex (HD) mode to help a far user. Two representative cooperative relaying scenarios are investigated insightfully. The first scenario is that no direct link exists between the base station (BS) and far user. The second scenario is that the direct link exists between the BS and far user. To characterize the performance of potential gains brought by the FD NOMA in two considered scenarios, three performance metrics outage probability, ergodic rate, and energy efficiency are discussed. More particularly, we derive new closed-form expressions for both exact and asymptotic outage probabilities as well as delay-limited throughput for two NOMA users. Based on the derived results, the diversity orders achieved by users are obtained. We confirm that the use of direct link overcomes zero diversity order of far NOMA user inherent to FD relaying. In addition, we derive new closed-form expressions for asymptotic ergodic rates. Based on these, the high signal-to-noise ratio (SNR) slopes of two users for FD NOMA are obtained. Simulation results demonstrate that: 1) the FD NOMA is superior to the HD NOMA in terms of outage probability and ergodic sum rate in the low SNR region; and 2) in delay-limited transmission mode, the FD NOMA has higher energy efficiency than the HD NOMA in the low SNR region; However, in delay-tolerant transmission mode, the system energy efficiency of the HD NOMA exceeds the FD NOMA in the high SNR region. [ABSTRACT FROM PUBLISHER]

BATS codes were proposed for communication through networks with packet loss. A BATS code consists of an outer code and an inner code. The outer code is a matrix generation of a fountain code, which works with the inner code that comprises random linear coding at the intermediate network nodes. In this paper, the performance of finite-length BATS codes is analyzed with respect to both belief propagation (BP) decoding and inactivation decoding. Our results enable us to evaluate efficiently the finite-length performance in terms of the number of batches used for decoding ranging from 1 to a given maximum number, and provide new insights on the decoding performance. Specifically, for a fixed number of input symbols and a range of the number of batches used for decoding, we obtain recursive formulae to calculate the stopping time distribution of BP decoding and the inactivation probability in inactivation decoding. We also find that both the failure probability of BP decoding and the expected number of inactivations in inactivation decoding can be expressed in a power-sum form where the number of batches appears only as the exponent. This power-sum expression reveals clearly how the decoding failure probability and the expected number of inactivation decrease with the number of batches. When the number of batches used for decoding follows a Poisson distribution, we further derive recursive formulas with potentially lower computational complexity for both decoding algorithms. For the BP decoder that consumes batches one by one, three formulae are provided to characterize the expected number of consumed batches until all the input symbols are decoded. [ABSTRACT FROM PUBLISHER]

In this paper, we study the physical-layer security in a 3-D multiuser visible light communication (VLC) network. The locations of access points (APs) and mobile users are modeled as two 2-D, independent and homogeneous Poisson point processes at distinct heights. Using mathematical tools from stochastic geometry, we provide a new analytical framework to characterize the secrecy performance in multiuser VLC networks. Closed-form results for the outage probability and the ergodic secrecy rate are derived for networks without AP cooperation. Considering the cooperation among APs, we give tight lower and upper bounds on the secrecy outage probability and the ergodic secrecy rate. To further enhance the secrecy performance at the legitimate user, a disk-shaped secrecy protected zone is implemented in the vicinity of the transmit AP. Based on the obtained results, it is shown that cooperating neighboring APs in a multiuser VLC network can bring performance gains on the secrecy rate, but only to a limited extent. We also show that building an eavesdropper-free protected zone around the AP significantly improves the secrecy performance of legitimate users, which appears to be a promising solution for the design of multiuser VLC networks with high security requirements. [ABSTRACT FROM PUBLISHER]

We study the coverage performance of multiantenna [multiple-input multiple-output (MIMO)] communications in heterogeneous networks (HetNets). Our main focus is on open-loop and multistream MIMO zero-forcing beamforming at the receiver. Network coverage is evaluated adopting tools from stochastic geometry. Besides fixed-rate transmission (FRT), we also consider adaptive-rate transmission (ART) while its coverage performance, despite its high relevance, has so far been overlooked. On the other hand, while the focus of the existing literature has solely been on the evaluation of coverage probability per stream, we target coverage probability per communication link&#x2014.comprising multiple streams—which is shown to be a more conclusive performance metric in multistream MIMO systems. This, however, renders various analytical complexities rooted in statistical dependence among streams in each link. Using a rigorous analysis, we provide closed-form bounds on the coverage performance for FRT and ART. These bounds explicitly capture impacts of various system parameters including densities of BSs, SIR thresholds, and multiplexing gains. Our analytical results are further shown to cover popular closed-loop MIMO systems, such as eigen-beamforming and space-division multiple access. The accuracy of our analysis is confirmed by extensive simulations. The findings in this paper shed light on several important aspects of dense MIMO HetNets: first, increasing the multiplexing gains yields lower coverage performance; second, densifying network by installing an excessive number of low-power femto BSs allows the growth of the multiplexing gain of high-power, low-density macro-BSs without compromising the coverage performance; and third, for dense HetNets, the coverage probability does not increase with the increase of deployment densities. [ABSTRACT FROM AUTHOR]

In spite of the significant advancements in wireless connectivity, the static form of the network infrastructure cannot guarantee an uninterrupted operation of the ever-growing wireless consumer electronics in emergency situations such as natural disasters. In such occasions, employing flexible aerial nodes can tackle this issue by recovering the communication rapidly, when the need for connectivity is of utmost importance. In this paper, we study the use of aerial nodes for communication recovery after a communication breakdown. We provide an analytical model of the recovery probability that demonstrates the capabilities of such networks. In the performance evaluation, we show the effects of the altitude and the distance between the aerial nodes on the recovery probability and verify them with simulations. Moreover, we introduce our testbed and preliminary experimental work that shows promising results for aerial networks. Finally, we discuss useful insights for the network design and present some open issues that exist in this field. [ABSTRACT FROM AUTHOR]

We investigate the problem of data synchronization in which a sender has a set of packets to be distributed to all the receivers via a broadcast channel. Initially, each receiver has some fraction of the packets. At each time slot, the sender might broadcast a packet to all the receivers. The goal is to find a broadcast scheme that minimizes the number of time slots until all the receivers successfully obtain all the packets. We propose two probabilistic models on how the initial fractions of packets at receivers are distributed. These models arise naturally in many large-scale systems, such as peer-to-peer networks, data centers, and distributed storage systems. Based on these models, we establish probabilistic bounds and asymptotic results on the minimum number of time slots to successfully transmit all the packets to all the receivers. Next, we propose and analyze a number of random network coding algorithms for finding the approximately optimal solution. Theoretical analysis and simulations are provided to verify the probabilistic bounds and the proposed algorithms. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the throughput of wireless systems in the presence of random data arrivals and quality-of-service (QoS) requirements, which are statistically characterized by the queueing-bound violation probability. By combining the concepts of effective capacity and effective bandwidth, we propose a unified analytical framework to investigate the achievable throughput. Employing the proposed unified framework, we further acquire the explicit expressions of the throughput of QoS guaranteed wireless systems with the channel state information (CSI) known/unknown at the transmitter. Specifically, the acquired throughput is characterized by the first- and second-order statistics of the random data arrivals, and it is shown that the QoS requirements affect the throughput by the second-order statistics of the random data arrivals and the random data transmissions. In addition, our theoretical analysis demonstrates that the throughput under QoS constraints is tighter than the conventional stable throughput, and that the first-order statistic of the random data arrivals is sufficient to characterize the throughput when the system can tolerate an arbitrarily long queueing delay. In particular, we prove that the queueing-bound violation probability decays exponentially with the queueing bound. Finally, simulation results corroborate the theoretical analysis. [ABSTRACT FROM PUBLISHER]

As the base of quality-of-service (QoS) guarantee and system optimizing, performance analysis is very important for interference-limited networks. The performance of a user is closely related to its service rate. However, in interference-limited networks, the transmissions of users interfere with each other, and the power control and unsaturated traffic make the interference relationship among users more complicated. Hence, the service rates of users are correlated with each other and are difficult to derive. According to the relation between the queue length and the service rate, we propose an $N$- dimensional Markov chain, which models queue states of users, to study the performance of an interference-limited system. By calculating the conditional service rate, which is only related to the channel quality given the power policy, the transition matrix is derived. By using the transition matrix, we solve the Markov chain and obtain the stationary probability of the queue length of each user, which enables the calculations of other performance metrics, i.e., delay and packet overflow probability, and so on. Furthermore, to reduce the computational complexity, we decompose the proposed $N$- dimensional Markov chain. Then, based on the decomposition, we propose an iterative algorithm. The comparisons between simulation results and analytical results under different scenarios verify that our method can provide good estimations of each user's performance. The results derived by our method can determine the condition under which the power control algorithm for the saturated system is suitable in unsaturated networks and in which the new power policy is needed. [ABSTRACT FROM PUBLISHER]