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In this paper, we address the physical layer security (PLS) of multi-relay non-orthogonal multiple access (NOMA) schemes with partial relay selection (PRS) using decode-and-forward (DF) and amplify-and-forward (AF) protocols. We propose a partial relay scheme aiming to select the best relay according to the highest signal-to-noise ratio (SNR) of the first link. We derive new exact and asymptotic expressions for strictly positive secrecy capacity (SPSC) and secrecy outage probability (SOP) considering Rayleigh fading channels. Numerical results demonstrate that AF and DF provide a similar system performance. They prove that partial relay selection improves SPSC and reduces SOP when the relays are near the legitimate receiver. Numerical results also reveal that the performance boosts for a high number of relays even for the high SNR of the eavesdropper channel. The secure NOMA system with PRS is capable of providing better secrecy performance than the secure NOMA system with random relay selection. Asymptotic results show that the use of multiple relays does not affect the system performance.

In this paper, we study covert communications in a hybrid decode-and-forward (DF)/ amplify-and-forward (AF) relay system. The considered relay in normal operation forwards messages from a source node to a destination node in either DF or AF mode on request. Meanwhile, the source and destination nodes also attempt to secretly exchange covert messages such as confidential or sensitive information and avoid detection by the covert message detector embedded on the relay. We first establish an optimal DF/AF mode selection criterion to maximize the covert rate based on the analyses of delay-aware achievable covert rates of individual DF and AF modes. To further reduce the time complexity, we propose a low-complexity selection criterion as well for practical use. The numerical results demonstrate the covert rate gain as high as 50% and running time gain as high as 20% for particular system parameters, which verify the effectiveness of the proposed criteria. [ABSTRACT FROM AUTHOR]

In free-space optical communication (FSOC) systems, atmospheric turbulence can bring about power fluctuations in receiver ends, restricting channel capacity. Relay techniques can divide a long FSOC link into several short links to mitigate the fading events caused by atmospheric turbulence. This paper proposes a Reinforcement Learning-based Relay Selection (RLRS) method based on Deep Q-Network (DQN) in a FSOC system with multiple transceivers, whose aim is to enhance the average channel capacity of the system. Malaga turbulence is studied in this paper. The presence of handover loss is also considered. The relay nodes serve in decode-and-forward (DF). Simulation results demonstrate that the RLRS algorithm outperforms the conventional greedy algorithm, which implies that the RLRS algorithm may be utilized in practical FSOC systems. [ABSTRACT FROM AUTHOR]

The unmanned aerial vehicle (UAV) is considered a promising auxiliary relay for enhancing network coverage due to its easy deployment, agility, and maneuverability. In order to overcome the obstruction of towering buildings and the limitation of relaying energy, this paper proposes a half-duplex decode-and-forward UAV-assisted relaying network based on simultaneous wireless information and power transfer scheme. The power splitting (PS) is implemented at the UAV relay to address the challenge of energy limitation by separating information and energy from the radio frequency signal. We also study the impact of hardware impairments on transmitters, which can actually degrade communication performance. We derive both an exact expression and asymptotic analysis for the outage probability (OP) with energy harvesting and hardware impairments. In addition, we formulate an optimization form to find the optimal PS ratio and minimize the outage probability. Due to the nonconvex nature of this optimization problem, we perform a series of transformations to reformulate it into a convex optimization form. Experimental simulations validate our theoretical results. Specifically, optimal altitude for UAV is about 300 m and relatively constant in different conditions. The OP of our work is nearly 0.33 smaller than that of AF relaying in a given situation. Moreover, we provide practical guidance for the design and deployment of the UAV-assisted communication by exploring the effects of UAV's altitude, hardware impairments level, and power splitting ratio on the outage probability. [ABSTRACT FROM AUTHOR]

In order to tackle the problem of reducing the transmission delay of multi-data acquisition source nodes in wireless sensor networks, we propose a cooperative method based on decode-and-forward (DF) and physical-layer network coding. In this proposed method, the two source nodes simultaneously transmit the respective information to the relay node and the data sink node using the orthogonal carriers. The relay node demodulates the received information separately from two source nodes and employs quadrature phase shift keying to modulate these information before forwarding them. The data sink node decodes the information of two source nodes using log likelihood ratio based on the signals received from the direct link and the relay link. Simulation results of bit error rate and system throughput show that the cooperative method can effectively improve the performance of the cooperative system compared with the traditional DF. [ABSTRACT FROM AUTHOR]

This paper proposes the height-dependent channel correlation coefficient and leverages it to produce a hybrid correlated channel. The efficacy of the drone-assisted cooperation (DAC) system has been assessed using the hybrid correlated channel coefficient, and the findings are contrasted with those obtained using the uncorrelated hybrid channel coefficient. Maximal ratio combining (MRC), a diversity combining technique, is employed at the destination node. Amplify-and-forward (AF) and decode-and-forward (DF) are utilized at the relay node (drone) to assess the overall performance of the DAC system in the proposed scenario. The paper introduces two algorithms designed to calculate the capacity and outage probability for DAC systems within a correlated hybrid channel environment. Finally, the effectiveness of the proposed model has been depicted in the simulation results. The proposal might prove helpful in a dire situation where the base station faces disruptions. [ABSTRACT FROM AUTHOR]

This paper presents the unmanned aerial vehicle (UAV) relays' assisted vehicle-to-everything (V2X) network to implement the internet of things (IoT) systems with improvement in the coverage area. Such a network benefits from many advantages of the non-orthogonal multiple access (NOMA) scheme. We have implemented a decode-and-forward (DF) scheme for these UAVs. Then, we characterize the channels as Nakagami-m fading to evaluate the performance of the system. We derive closed-form expressions of outage probability (OP), ergodic capacity (EC), and throughput. The results show that the performance of the system depends on the transmitted signal-to-noise ratio (SNR) at the base station and the heights of the UAV relays. Target rate and power allocation factors are two main parameters that can be adjusted to achieve better performance. The results also compare to the system without UAV and OMA technique that shows the advantages of deploying UAV-assisted NOMA. Therefore, the design of NOMA for UAV relay-assisted V2X systems provides sufficient demand. The simulation results verified the effectiveness of the proposed UAV network and the precision of the theoretical analysis. [ABSTRACT FROM AUTHOR]

In emergency scenarios, infrastructure of wireless networks is usually damaged and becomes out of service. Therefore, Unmanned Aerial Vehicles (UAVs) have been widely nominated as an alternative to provide prompt and efficient wireless connectivity. To this end, UAV-assisted communication systems are recently being under investigation. In this paper, the focus is on UAV-assisted dual-hop links in which the source, relay and destination can be either ground-based or aerial nodes. Specifically, the paper analyzes the performance of these links in terms of the outage probability (OP) and the bit error rate (BER) considering eight different setups of dual-hop UAV-assisted links operating over the millimeter wave (mmWave) band. Unlike others, the performance analysis is conducted considering practical models of the channel fading, path loss, antennas gain and UAVs’ fluctuations. A generalized mathematical framework is followed to obtain the cumulative distribution function (CDF) and the probability density function (pdf) of the overall signal-to-noise ratio (SNR), which are utilized to characterize closed form expressions for the average OP and BER for all proposed setups. Analytical results are validated by Monte Carlo simulations which investigate the role of many setup parameters including the number of antenna elements, the fluctuation intensity, the transmit power and the modulation order. Moreover, results indicate that increasing the number of aerial entities in the link negatively affects its performance due to the impact of the fluctuations on the directional antennas in the mmWave band.

In free-space optical communication (FSOC) systems, atmospheric turbulence can bring about power fluctuations in receiver ends, restricting channel capacity. Relay techniques can divide a long FSOC link into several short links to mitigate the fading events caused by atmospheric turbulence. This paper proposes a Reinforcement Learning-based Relay Selection (RLRS) method based on Deep Q-Network (DQN) in a FSOC system with multiple transceivers, whose aim is to enhance the average channel capacity of the system. Malaga turbulence is studied in this paper. The presence of handover loss is also considered. The relay nodes serve in decode-and-forward (DF). Simulation results demonstrate that the RLRS algorithm outperforms the conventional greedy algorithm, which implies that the RLRS algorithm may be utilized in practical FSOC systems.

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This paper investigates the performance analysis of radio frequency/free space optical-visible light communication (RF/FSO-VLC) system, where a hybrid RF/FSO link is connected to an indoor VLC subsystem using decode-and-forward (DF) relay. The selection combining (SC) technique is used at the DF relay. We have considered Nakagami-m fading, Double Generalized Gamma (DGG) distributed atmospheric turbulence with Rayleigh distributed pointing errors and Lambertian pattern of the light emitting diode (LED) for the RF, FSO, and VLC links, respectively. To analyze the performance of the considered co-operative system, different performance metrics such as outage probability, bit-error-rate (BER), and ergodic capacity of the system are taken into consideration. For deriving the performance metrics, novel expressions for cumulative distribution function and probability density function of the instantaneous signal-to-noise-ratio (SNR) of hybrid RF/FSO-VLC system in terms of Meijer-G function are obtained and used. The numerical results show that the outage probability of the system is more in the strong turbulence conditions and intensity modulation/direct detection (IM/DD) technique in comparison to weak turbulence and heterodyne detection scenario. It is also seen from the results, that the BER of the system is better in case of coherent binary modulation techniques in comparison with non-coherent binary and M-ary modulation techniques. Moreover, the capacity of the system increases as the number of LEDs increase and as the height of the LEDs reduce. The asymptotic and simulation results match with the derived analytical results. [ABSTRACT FROM AUTHOR]

In this paper, the performance analysis of a cooperative radio frequency-free space optical communication-visible-light-communication (RF–FSO–VLC) system using decode-and-forward (DF) relays is presented. In the proposed model, the RF link acts as the core network, the FSO link is used for providing last mile connectivity for the indoor cell users communicating through VLC environment. The RF link is characterized by Nakagami-m-distributed fading, while the FSO link undergoes double generalized gamma distributed atmospheric turbulence and pointing errors and the VLC link is characterized by the Lambertian pattern of the LED. The signal at each DF relay is decoded , re-encoded and again decoded before sending to the next relay. The signal to the first relay is sent by an RF link. The received signal is then decoded, re-encoded and again decoded to send to the second relay via a FSO link. From the second relay the signal is transmitted to the end users present in the indoor environment via an VLC link. The closed-form expressions of the statistical characteristics of signal-to-noise-ratio of the DF based triple-hop system are evaluated using which various performance metrics, i.e, outage probability, bit error rate (BER), and capacity of the considered system are obtained. The numerical results show that the system's performance gets severely affected by the RF fading, atmospheric turbulence, and pointing errors. The outage performance of the system decreases as field-of-view and the height of the light-emitting-diode (LED) increase. It is also seen from the results that the BER performance of the system is better in case of heterodyne detection as compared to direct-detection. It is also observed that the capacity of the system decreases when the effect of atmospheric turbulence and pointing error is high and it increases when the no. of LEDs increases. [ABSTRACT FROM AUTHOR]

This paper proposes a cooperative non-orthogonal multiple access (NOMA) based two-way transmission scheme for a two-user cellular system, in which a base station (BS) exchanges information with a directlink user and an indirect link user. With superimposition coding, physical-layer network coding (PLNC) and successive interference cancellation, the BS and the two users are able to exchange messages within three time slots. We derive the sum-rate upper bound of the proposed scheme and investigate the optimal time and power allocations to maximize the sum-rate bound. Numerical results demonstrate that the derived bound is tight and the proposed scheme can improve the sum-rates significantly as compared with existing NOMA-based schemes.

This paper studies overlay-based cognitive radio networks (ObCRNs) where transmission from a licensed transmitter (LT) to a licensed receiver (LR) is supported by an unlicensed transmitter (UT) who has its own communication to an unlicensed receiver (UR). To improve power efficiency of UT and diversity gain at UR and LR, UT operates in direct transmission (DT) or amplify-and-forward (AF) mode or decode-and-forward (DF) mode, and UR and LR combine signals with selection combining (SC) and maximum ratio combining (MRC) from LT and UT. Explicit outage probability expressions for ObCRNs with MRC/SC and DT/AF/DF are proposed to promptly rate its performance. Numerous results reveal the efficacy of the proposed solution and its flexibly-controlled performance. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of covert communications in different types of a relay system: decode-and-forward (DF), compress-and-forward (CF) and amplify-and-forward (AF). We consider a source node that attempts to send both public and covert messages to a destination node through a relay on which a covert message detector is embedded. By taking the minimum detection error probability (DEP) at the relay into account, we optimize the power distribution between the public and covert messages to achieve the maximum covert rate. We further make a delay-aware comparison among DF, CF and AF relay systems with the obtained closed-form covert rates and conduct an extensive examination on the asymptotic behaviors in different limits. Our analyses reveal that CF or AF tend to outperform DF for high source transmit power or low relay transmit power, while various system parameters such as the processing delay, minimum required quality of service for public messages and DEP threshold lead to different performance relationships among DF, CF and AF for high relay transmit power. Numerical results verify our investigation into the performance comparison in various channel models. [ABSTRACT FROM AUTHOR]

The signal quality in high-bandwidth free space optical (FSO) systems deteriorates due to atmospheric turbulence and pointing errors. Employing techniques such as adaptive transmission and relay selection (RS) can mitigate their effects. This paper analyzes the performance of a dual-hop decode-and-forward multi-relay FSO system with an adaptive M-ary phase shift keying scheme. This analysis is based on the recently proposed Fisher–Snedecor  F channel model and considers the impact of pointing errors. We propose two partial relay selection schemes based on the source-to-relay or relay-to-destination channel state information to reduce the complexity of the optimal relay selection scheme. In this investigation, we derive closed-form expressions for the outage probability, modulation level selection probability, and spectral efficiency (SE) and compare the performance of the proposed RS schemes under balanced and unbalanced link cases. We observe an improvement in the SE with an increase in the number of modulation levels and the number of relays. Moreover, it is noted that the performance of the system can be restricted by the quality of either the source-to-relay or the relay-to-destination link, even if the quality of the other link is perfect. Finally, the outcomes obtained through the derived expressions are validated using Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

The risk of information leakage increases when the relay technology is introduced to extend the communication range and improve the quality of communication services of cell-edge users in visible light communication (VLC) systems. To evaluate these security risks, physical layer security (PLS) performance for decode-and-forward (DF) relay-aided VLC system is investigated in this paper. Specifically, the theoretical expressions of the upper and lower bounds of the security outage probability (SOP) for the DF relay-aided VLC system under the amplitude constraint are obtained based on a stochastic geometry analysis method. Monte Carlo simulation validates the correctness of the derived theoretical expressions of the upper and lower bounds of the SOP. Moreover, simulation results show that the security outage performance of the DF relay-aided VLC system is deteriorated compared with the non-relay VLC system. Furthermore, the influences of the eavesdroppers' distribution density, the radiation radius of LED source, the radiation radius and deployment height of the relay on the SOP have been investigated. Last but not least, the optimal deployment height of the relay can be determined for achieving the optimum security outage performance. These parameters can effectively guide the design of the secure relay-aided VLC system.

In order to meet the demand for large-scale device access in Narrowband Internet of Things (NB-IoT) and to overcome the problem that some resources are wasted due to the use of single decode-and-forward (DF) or amplify-and-forward (AF) collaboration in traditional collaborative communication, this paper introduces a non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) hybrid access method into the NB-IoT and proposes a new Hybrid NOMA transmission protocol with relay adaptively selectable collaboration, which is represented as an algorithm. Based on this protocol, we classify the whole transmission process into five states after deriving the outage probability of each link. We then consider the system as a discrete-time Markov model, the closed expressions of the system outage probability and throughput are derived based on the system steady-state probability. In order to improve the system's reliability, we further optimize the above protocol by allowing the source node to retransmit the unsuccessful received superimposed signals a limited number of times. Numerical results and simulations show that the outage probability is lower when multiple retransmissions are possible. The proposed relay adaptive collaborative hybrid NOMA transmission protocol has advantages over the pure OMA transmission mode. [ABSTRACT FROM AUTHOR]

Summary: This paper analyzes the performance of maximal ratio transmission (MRT)‐based cooperative communication system over Nakagami‐ m fading channels in the existence of co‐channel interference which is becoming a critical factor since the limited spectrum bands are shared by growing number of devices. In particular, a dual‐hop decode‐and‐forward relaying is investigated when multiple interferers affect the relay and the destination nodes. Firstly, the cumulative distribution function (CDF) and the probability density function (PDF) of the signal‐to‐interference‐plus‐noise ratio (SINR) are derived. Then, the exact expressions for the outage probability (OP), average bit error probability (ABEP), and ergodic capacity are obtained. Furthermore, asymptotic expressions for OP and ABEP are provided to find the diversity and coding gains. Finally, simulation results are presented to validate our theoretical findings. [ABSTRACT FROM AUTHOR]

Relay-assisted wireless communications, where both the relay and the final destiny employ diversity-combining techniques, represent a compelling strategy for improving the signal-to-noise ratio (SNR) for mobile terminals, mainly at millimeter-wave (mmWave) frequency bands. In this sense, this work considers a wireless network that employs a dual-hop decode-and-forward (DF) relaying protocol, in which the receivers at the relay and at the base station (BS) use an antenna array. Moreover, it is considered that the received signals are combined at reception using equal-gain-combining (EGC). Recent works have enthusiastically employed the Weibull distribution so as to emulate the small-scale fading behavior in mmWave frequencies, which also motivates its use in the present work. For this scenario, exact and asymptotic expressions for the system's outage probability (OP) and average bit error probability (ABEP) are derived in closed form. Useful insights are gained from these expressions. More precisely, they illustrate how the system and fading parameters affect the performance of the DF-EGC system. Monte Carlo simulations corroborate the accuracy and validity of the derived expressions. Furthermore, the mean achievable rate of the considered system is also evaluated via simulations. Useful insights regarding the system performance are obtained from these numerical results. [ABSTRACT FROM AUTHOR]

This study introduces the non-orthogonal multiple access (NOMA) technique into the wireless energy harvesting K-hop relay network to increase throughput. The relays have no dedicated energy source and thus depend on energy harvested by wireless from a power beacon (PB). Recently, NOMA has been promoted as a technology with the potential to enhance connectivity, reduce latency, increase fairness amongst users, and raise spectral effectiveness compared to orthogonal multiple access (OMA) technology. For performance considerations, we derive exact throughput expressions for NOMA and OMA-assisted multi-hop relaying and compare the performance between the two. The obtained results are validated via Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) has been deemed to be one promising technique for the sixth-generation wireless communication network. However, due to the attenuation properties, the VLC ranges are usually limited. To unlock the advantage of multiple light-emitting diodes (LEDs) and extend the coverage of VLC systems, a relay-aided VLC system with non-orthogonal multiple access (NOMA) is proposed in this paper. Specifically, a decode-and-forward (DF) relay protocol is introduced to construct the relay-aided VLC system with NOMA. Moreover, the closed-form expression of the outage probability (OP) in the DF relay-aided VLC system with NOMA is derived to accurately characterize the theoretical performance of the system. Thereafter, the effects of the corresponding parameters on the OP are investigated. Simulation results verify the accuracy of the derived theoretical expression and illustrate that the DF relay-aided VLC system with NOMA can effectively improve system outage performance. Compared to the direct transmission NOMA VLC system without relay link, our scheme can obtain $\text{15}\,\text{dB}$ transmit signal-to-noise ratio gains in $10^{-3}$ OP level. Last but not least, the optimal transmit semi-angle and deployment height of the relay can be determined for achieving the optimal system outage performance.

As a promising technology, free-space optical (FSO) communication plays an important role in the next generation of wireless communication. A dual-hop FSO communication system with decode-and-forward (DF) protocol is proposed in this paper, considering the influences of the atmospheric turbulence (AT), atmospheric absorption and pointing error impairments, in which the Málaga distribution model is used to characterize the fading caused by AT in each link. The end-to-end statistical expressions of our proposed dual-hop Málaga-Málaga communication system are derived, such as the cumulative distribution function (CDF), probability density function and moment generating function. Thereafter, using the above statistical results, the accurate end-to-end expressions of outage probability, average bit error rate (ABER) and ergodic capacity are deduced under the direct detection intensity modulation and heterodyne detection techniques. Furthermore, the end-to-end asymptotic expressions for CDF and the ABER at high signal-to-noise ratio as well as simplified expressions from simple basic functions are obtained. The effects of different AT and pointing error impairments conditions on the proposed system are analyzed based on theoretical and numerical results. Finally, Monte Carlo simulation results indicate that all our novel deduced expressions are basically consistent with the numerical results.

The signal quality in high-bandwidth free space optical (FSO) systems deteriorates due to atmospheric turbulence and pointing errors. Employing techniques such as adaptive transmission and relay selection (RS) can mitigate their effects. This paper analyzes the performance of a dual-hop decode-and-forward multi-relay FSO system with an adaptive M-ary phase shift keying scheme. This analysis is based on the recently proposed Fisher–Snedecor F channel model and considers the impact of pointing errors. We propose two partial relay selection schemes based on the source-to-relay or relay-to-destination channel state information to reduce the complexity of the optimal relay selection scheme. In this investigation, we derive closed-form expressions for the outage probability, modulation level selection probability, and spectral efficiency (SE) and compare the performance of the proposed RS schemes under balanced and unbalanced link cases. We observe an improvement in the SE with an increase in the number of modulation levels and the number of relays. Moreover, it is noted that the performance of the system can be restricted by the quality of either the source-to-relay or the relay-to-destination link, even if the quality of the other link is perfect. Finally, the outcomes obtained through the derived expressions are validated using Monte Carlo simulations.

This paper proposes an overlay network wherein a pair of licensed source (LS) and licensed destination (LD) is adaptively assisted by a pair of unlicensed source (US) and unlicensed destination (UD). By processing adaptively at US and UD relied on the decoding statuses of LD and US, licensed communication is always supported with a high diversity gain from which unlicensed communication also benefits from removing licensed interference as well as transmitting with the highest available power, eventually improving reliability of both unlicensed and licensed communication. This paper also proposes closed-form formulas of outage probabilities at LD and UD for prompt reliability evaluation over κ - μ shadowed fading channels. Multiple results illustrate the superiority of the proposed adaptive processing scheme as compared to its counterpart. Further, the reliability of the suggested scheme can be flexibly controlled and optimized with setting specifications reasonably. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) has been deemed to be one promising technique for the sixth-generation wireless communication network. However, due to the attenuation properties, the VLC ranges are usually limited. To unlock the advantage of multiple light-emitting diodes (LEDs) and extend the coverage of VLC systems, a relay-aided VLC system with non-orthogonal multiple access (NOMA) is proposed in this paper. Specifically, a decode-and-forward (DF) relay protocol is introduced to construct the relay-aided VLC system with NOMA. Moreover, the closed-form expression of the outage probability (OP) in the DF relay-aided VLC system with NOMA is derived to accurately characterize the theoretical performance of the system. Thereafter, the effects of the corresponding parameters on the OP are investigated. Simulation results verify the accuracy of the derived theoretical expression and illustrate that the DF relay-aided VLC system with NOMA can effectively improve system outage performance. Compared to the direct transmission NOMA VLC system without relay link, our scheme can obtain $\text{15}\,\text{dB}$ transmit signal-to-noise ratio gains in $10^{-3}$ OP level. Last but not least, the optimal transmit semi-angle and deployment height of the relay can be determined for achieving the optimal system outage performance. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) have drawn considerable attention due to their ability to direct electromagnetic waves into desirable directions. Although RISs share some similarities with relays, the two have fundamental differences impacting their performance. To harness the benefits of both, we propose a downlink system wherein a relay and an RIS improve performance in terms of energy-efficiency. Using singular value decomposition (SVD), semidefinite programming (SDP), and function approximations, we propose different solutions for optimizing the beamforming matrices at the base-station (BS), the relay, and the phase shifts at the RIS to minimize the total power under quality-of-service (QoS) constraints. The problem is solved when the relay operates in half-duplex and full-duplex modes and when the reflecting elements have continuous and discrete phase shifts. Simulation results compare the performance of the system with and without the RIS or the relay, under different optimization solutions. The results show that the system with full-duplex relay and RIS outperforms the other scenarios, and the contribution of full-duplex relay is higher than that of the RIS. However, an RIS outperforms a half-duplex relay when the required QoS is high. The results also show that increasing the number of reflecting elements improves the performance better in the presence of a relay than in its absence. [ABSTRACT FROM AUTHOR]

Intelligent reconfigurable surfaces (IRSs) have gained much attention due to their passive behavior that can be a successor to relays in many applications. However, traditional relay systems might still be a perfect choice when reliability and throughput are the main concerns in a communication system. In this work, we use an IRS along with a decode-and-forward relay to provide a possible solution to address one of the main challenges of future wireless networks which is providing reliability. We investigate a robust transceiver design against the residual self-interference (RSI), which maximizes the throughput rate under self-interference channel uncertainty-bound constraints. The yielded problem turns out to be a non-convex optimization problem, where the non-convex objective is optimized over the cone of semidefinite matrices. We propose a novel mathematical method to find a lower bound on the performance of the IRS that can be used as a benchmark. Eventually, we show an important result in which, for the worst-case scenario, IRS can be helpful only if the number of IRS elements are at least as large as the size of the interference channel. Moreover, a novel method based on majorization theory and singular value decomposition (SVD) is proposed to find the best response of the transmitters and relay against worst-case RSI. Furthermore, we propose a multi-level water-filling algorithm to obtain a locally optimal solution iteratively. We show that our algorithm performs better that the state of the art in terms of time complexity as well as robustness. For instance, our numerical results show that the acheivable rate can be increased twofold and almost sixfold, respectively, for the case of small and large antenna array at transceivers. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we consider a wireless powered cognitive relaying system with a secondary relay (SR) capable of harvesting wireless energy. Along with an access point (AP) continuously transmitting the primary data to a primary user (PU), a secondary source (SS) can transmit the secondary data to a secondary destination (SD) with the help of SR using the decode-and-forward (DF) protocol. SR can harvest energy from both SS and AP in both time and power domains using time-splitting and power-splitting techniques. The interference from primary data transmissions can help boost the amount of harvested energy at SR. The transmit power of SS is regulated by the interference threshold at PU and the allowable peak power. Despite the above two constraints, the transmit power of SR is further constrained by the amount of harvested energy. Once SR successfully decodes the data from SS, it will forward the data to SD using a constrained power. We analyze the approximate outage probabilities for both primary and secondary systems. Simulation results are provided to verify the effectiveness of our theoretical analysis and reveal the impacts of various parameters to the outage performance.

In this paper, we present an asynchronous network coding (ANC) transmission strategy for multiuser co-operative networks. In particular, we consider a relay network comprised of multiple sources and mutiple relays. The sources all transmit simultaneously. The relay receives a sum of the signals with different delays. The decoded signals at the relay are network-coded by asynchronous delays and transmitted to the destination. We first present a decoding strategy for our ANC. We also compare our proposed ANC to complex field network coding and show the superiority of our proposed scheme in terms of decoding complexity and bit error rate (BER) performance. Next, we develop three relay selection strategies with different complexity in conjunction with the proposed ANC scheme. The proposed full selection and joint selection (JS) schemes consider both channel quality and delay effect in the selection. But the proposed individual selection scheme only considers channel quality in the selection. We derive an end-to-end (E2E) BER expression in terms of worst E2E SNR for the JS scheme. We also theoretically show that the proposed schemes achieve full diversity. In addition, we compare the performance of the three proposed relay selection schemes by simulations.

The Internet of Vehicles (IoV) has always attracted attention as the emerging communication network with the most development potential in the 5G era. However, the performance of IoV under 5G ultra-dense networks is an open issue, especially in practice the outage probability and ergodic capacity of the relay cooperative IoV network under aggregate interference are still unclear. Therefore, an opportunistic Decoding and Forwarding (DF) relay cooperative transmission algorithm was proposed in this paper when the destination node of IoV has aggregated interference. In addition, based on mathematical theoretical knowledge such as numerical analysis, the closed expressions of the outage probability and ergodic capacity of the IoV system under aggregated interference was derived. Finally, simulation experiments verify the effectiveness of the proposed scheme and the correctness of the theoretical analysis, which improves the transmission rate of the system. [ABSTRACT FROM AUTHOR]

With the further development and wide application of wireless technology in 6G, people are increasingly inseparable from wireless devices, from the ground to the air. The power consumption and service life of devices have become an urgent problem to be solved. In this paper, the energy harvesting relaying system based on the power splitting method with unidirectional DF half-duplex and full-duplex relay is proposed. The relay node is powered by itself through energy harvesting. Specifically, we analyze the system capacity, and the formula of system outage probability is also obtained. Simulation results show the performance of the half-duplex and full-duplex system. With a smaller power splitting factor, more energy is harvested, the outage probability is lower, and the capacity is higher. [ABSTRACT FROM AUTHOR]

This paper investigates the outage performance of simultaneous wireless information and power transfer (SWIPT)-enabled relay networks with the decode-and-forward relaying protocol, where the effect of the energy triggering threshold at the relay on the system performance is considered. The closed-form expressions of the system outage probability and throughput are derived in Rician channel fading. Monte Carlo Simulation method is used to verify the accuracy of the derived closed-form expressions. The effects of some system parameters on the system performances are discussed via simulations, which show that the system outage probability increases with the increase of the minimum transmission rate required by the users and also decreases with the increase of the energy conversion efficiency. Besides, the system throughput increaseswith the increment of the transmit power of the source node, as well as the energy conversion efficiency. Additionally, the outage performance of the system with the equal two-hop distance is better than that of the system with unequal two-hop distance. [ABSTRACT FROM AUTHOR]

This article investigates the outage performance of a novel two-user multi-relay non-orthogonal multiple access (NOMA)-based hybrid satellite-terrestrial relay network (HSTRN), in which one user has a direct link to the satellite (termed as the direct-link user), while the other user (termed as the relay-aided user) seeks the help of the direct-link user or the $K$ dedicated decode-and-forward (DF) relays to acquire its desired signal. A relaying protocol and a three-stage relay selection strategy are proposed for minimizing the whole system outage probability (WSOP) and providing full diversity order (DO) for both users. Exact and asymptotic outage probabilities of the considered network are derived, showing that under the proposed relay selection strategy, the relay-aided user, the direct-link user and the whole system each can achieve a DO of $K + 1$. Finally, numerical results are presented to verify the theoretical analyses, manifest the impacts of key parameters on the system performance, and demonstrate the advantages of our proposed relay selection scheme over other benchmarks. [ABSTRACT FROM AUTHOR]

We consider a two-way half-duplex decode-and-forward (DF) relaying system with multiple pairs of single-antenna users assisted by a cell-free (CF) massive multiple-input multiple-output (mMIMO) architecture with multiple-antenna access points (APs). Under the practical constraint of imperfect channel state information (CSI), we derive the achievable sum spectral efficiency (SE) for a finite number of APs with maximum ratio (MR) linear processing for both reception and transmission in closed-form. Notably, the proposed CF mMIMO relaying architecture, exploiting the spatial diversity, and providing better coverage, outperforms the conventional collocated mMIMO deployment. Moreover, we shed light on the power-scaling laws maintaining a specific SE as the number of APs grows. A thorough examination of the interplay between the transmit powers per pilot symbol and user/APs takes place, and useful conclusions are extracted. Finally, differently to the common approach for power control in CF mMIMO systems, we design a power allocation scheme maximizing the sum SE.

In this paper, we investigate the usage of a nonlinear energy harvester at a full-duplex (FD) relay, which can harvest the energy from radio frequency (RF) signals transmitted from the source. We mathematically derive the exact closed-form expressions of outage probability (OP), throughput, and average symbol error rate (ASER) of the nonlinear energy harvester-wireless information and power transfer-FD relaying (N-WIPT-FDR) system over Rayleigh fading channels. Unlike linear energy harvester, nonlinear energy harvester results in more computational complexity. Furthermore, it makes the outage floor happen faster even when self-interference cancellation (SIC) techniques are effectively applied due to the saturation power threshold of the nonlinear energy harvester. The combination of nonlinear energy harvester characteristic and residual self-interference (RSI) has significant impacts on the OP, throughput, and ASER of the considered N-WIPT-FDR system. On the other hand, there is an optimal energy harvesting (EH) time duration, which minimizes the OP. This optimal value depends on the transmission power of the source and the saturation power threshold of the nonlinear energy harvester. Monte-Carlo simulations are conducted to validate the derived mathematical expressions.

In this paper, we analyze the performance of an energy harvesting (EH)-assisted overlay cognitive non-orthogonal multiple access (NOMA) system. The underlying system consists of a primary transmitter-receiver pair accompanied by an energy-constrained secondary transmitter (ST) with its intended receiver. Accordingly, ST employs a time switching (TS) based receiver architecture to harvest energy from radio-frequency signals of the primary transmissions, and thereby uses this energy to relay the primary information and to transmit its own information simultaneously using the NOMA principle. For this, we propose two cooperative spectrum sharing (CSS) schemes based on incremental relaying (IR) protocol using amplify-and-forward (AF) and decode-and-forward (DF) strategies, viz., CSS-IAF and CSS-IDF, and compare their performance with the competitive fixed relaying based schemes. The proposed IR-based schemes adeptly avail the degrees-of-freedom to boost the system performance. Thereby, considering the realistic assumption of the NOMA-based imperfect successive interference cancellation, we derive the expressions of outage probability for the primary and secondary networks under both CSS-IAF and CSS-IDF schemes subject to the Nakagami-m fading. In addition, we quantify the throughput and energy efficiency for the considered system. The obtained theoretical findings are finally validated through numerous analytical and simulation results to reveal the advantages of the proposed CSS schemes over the baseline direct link transmission and orthogonal multiple access schemes.