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ABSTRACT In this paper, an outdated channel state information‐based heterogeneous wireless communication system is analyzed, which involves an underwater multiuser acoustic sensor network, decode‐and‐forward relay, and terrestrial radio frequency destination with multiple antennas. The outdated channel state information is an utmost parameter to deal with practical conditions of underwater environment as well as of the terrestrial environment. Due to this fact, each channel is constrained to this factor in our investigation. The underwater acoustic network takes advantage of multiuser diversity based on generalized selection user scheduling, where the Lth$$ {L}_{th} $$ best user is chosen out of several active acoustic sources to continue transmitting data via relay to the destination. To boost system performance, the destination makes the use of the maximal ratio combining strategy. Moreover, each channel follows Rayleigh fading model. The closed form expressions of outage probability and average bit error rate are obtained along with their asymptotic results under the practical scenarios. The ergodic capacity and its upper bound expressions are also derived. Further, analytical results are verified with Monte‐Carlo simulation. [ABSTRACT FROM AUTHOR]

In this paper, we present analytical results on the effective capacity (EC) for a dual-hop cooperative network employing non-orthogonal multiple access and the decode-and-forward half-duplex relaying protocol. Assuming that wireless propagation is modelled by the Nakagami-m distribution, novel analytical results for the EC are deduced. These analytical results are further extended to the generalized fading channels, modeled by a mixture gamma distribution. Our proposed analysis is validated by numerical results and Monte-Carlo simulations. Besides, the numerically evaluated results have demonstrated the impact of various system-level parameters on the performance of the considered system, including the impact of fading, the power allocation coefficient and the delay constraints. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of hybrid dual hop underwater system which has been proposed for monitoring vast region under shallow sea environment. In comparison to conventional acoustic underwater systems, this hybrid underwater opto-acoustic sensor network (UOASN) is designed to offer higher data rates and shorter propagation delays by including optical carrier, which is a basic requirement of real-time monitoring application. An acoustic sensor and an optical sensor are mounted on shallow water floor at different locations that acquire the surrounding information and transmit it continuously to the underwater vehicle (UV) based on round-robin scheduling. UV is comprised of decode-and-forward relaying mechanism. The underwater acoustic link is determined by α - F fading distribution and underwater optical link follows the mixture Exponential-Generalized Gamma (EGG) model to characterize channel irradiance fluctuations. The novel closed form expressions for various end to end (E2E) signal-to-noise ratio (SNR) statistics such as equivalent probability density function (PDF), cumulative distribution function (CDF) and moments have been derived. Also, the closed form expressions are obtained for outage probability, average bit error rate, ergodic capacity and outage capacity. In order to demonstrate how different acoustic parameter settings and optical turbulence affect the system's performance, the closed form asymptotic expressions are derived for outage probability and average bit error rate to gain insights of the models. Such analysis proves the effectiveness and feasibility of the proposed system. [ABSTRACT FROM AUTHOR]

Intelligent reflecting surface (IRS) is a promising and emerging technology for futuristic networks to realize a software‐controlled wireless environment. In this work, we analyzed a dual IRS‐aided communication system's performance under the non‐ideal transceiver scenario. Specifically, we evaluate the considered scenario's spectral efficiency (SE) and the outage probability (OP). The analytical derivation of the SE and OP is corroborated through the simulations. The result demonstrates that dual IRS outperforms the single IRS scenario for the ideal transceiver case by mitigating the multiplicative pathloss effect by increasing the number of REs. However, for the non‐ideal transceiver case, the distortion noise limits the performance. Further, we have also compared the proposed dual IRS‐aided system with the baseline decode‐and‐forward (DF) relay‐based system. [ABSTRACT FROM AUTHOR]

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding.

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) is a recently emerged promising technology for beyond-5G (B5G)/6G wireless networks. In this letter, we propose to employ a signal space diversity (SSD) technique to improve the performance of distributed RIS-aided dual-hop mixed radio frequency (RF)-free-space optical (FSO) communication systems. The source-relay, source-RIS, and RIS-relay links are assumed to undergo independent but not identically distributed (i.n.i.d.) Nakagami- $m$ fading. The destination node is equipped with multiple FSO apertures and the relay-destination links follow Gamma-Gamma (GG) distributed atmospheric turbulence (AT) model with pointing errors (PEs). Novel approximate closed-form expressions for the system’s symbol error rate (SER) are derived for both the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) configurations. The numerical results show that using the SSD technique at both the RF and FSO links significantly improves the spectral efficiency and the diversity order of the system considered. [ABSTRACT FROM AUTHOR]

In this paper, we study a multi-pair two-way large-scale multiple-input multiple-output (MIMO) decode-and-forward relay system. Multiple single-antenna user pairs exchange information via a shared relay working at half-duplex. The proposed scenario considers a practical case where an increasing number of antennas is deployed in a fixed physical space, giving rise to a trade-off between antenna gain and spatial correlation. The channel is assumed imperfectly known, and the relay employs linear processing methods. We study the large-scale approximations of the sum spectral efficiency (SE) and investigate the energy efficiency (EE) with a practical power consumption model when the number of relay antennas becomes large. We demonstrate the impact of the relay antenna number and spatial correlation with reducing inter-antenna distance on the EE performance. We exploit the increasing spatial correlation to allow an incomplete channel state information (CSI) acquisition where explicit CSI is acquired only for a subset of antennas. Our analytical derivations and numerical results show that applying the incomplete CSI strategy in the proposed system can improve the EE against complete CSI systems while maintaining the average SE performance.

Intelligent reflecting surface (IRS) is an emerging key technology for the fifth-generation (5G) and beyond wireless communication systems to provide more robust and reliable communication links. In this letter, we propose a mixed dual-hop free-space optical (FSO)-radio frequency (RF) communication system that serves the end user via a decode-and-forward (DF) relay employing hybrid automatic repeat request (H-ARQ) protocols on both hops. Novel closed-form expressions of the probability density function (PDF) and cumulative density function (CDF) of the equivalent end-to-end signal-to-noise ratio (SNR) are computed for the considered system. Utilizing the obtained statistics, we derive the outage probability (OP) and packet error rate (PER) of the proposed system by considering generalized detection techniques on the source-to-relay (S-R) link with H-ARQ protocol and IRS having phase error. We obtain useful insights into the system performance through the asymptotic analysis which aids to compute the diversity gain. The derived analytical results are validated using Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

Abstract This paper analyzes the outage probability (OP) and the average symbol error rate (SER) of decode-and-forward (DF) relaying. The paper derives closed-form expressions for the OP and the average SER with optimum combining (OC) considering fast-fading multiple correlated CCIs, the correlated source-relay, and thermal noise. It is shown that the performance of the large distance between the source and the relay is better than that of the small distance, regardless of interference fading speed at the destination. We also show that given the source-relay distance, the performance of slow-fading interference is basically better than that of fast fading, except in the low signal-to-noise-ratio (SNR) regime for the distance being small. In result, the source-relay distance is generally a more dominating factor for the performance than fading CCIs.

The performance of a free space optics (FSO) transmission suffers from the atmospheric turbulence and the attenuation in foggy environment. By employing relay nodes, the error rate and the coverage area of the FSO communication system can be significantly improved. However, the pointing errors, generated because of the building sway, have the potential to eradicate the benefits of the relay-based FSO communication system. The effect of pointing errors in the presence of Gamma Gamma atmospheric fading together with path loss attenuation is considered in this paper. To counteract the adverse affects of the FSO link, a reliable millimeter-wave radio frequency (MMW RF) link is used as a backup. In this context, this paper proposes a cooperative decode-and-forward (DF)-relaying-based hybrid FSO/RF system with maximal-ratio-combining (MRC) at the destination. The system consists of FSO and RF sub-systems, where FSO sub-system has the priority to transmit and RF sub-system serves as a back up when the FSO sub-system is in outage. The exact and asymptotic outage probability and average symbol error rate (SER) expressions for the proposed system are derived in closed-form and the diversity order is determined. The effect of pointing errors on the system performance is analyzed extensively. The optimum values of transmit beam waist and radius of receiver aperture are determined. The theoretical results, which are validated by Monte-Carlo simulations, show that the proposed cooperative hybrid FSO/RF system drastically improves the system performance compared to single hop (SH) hybrid FSO/RF and cooperative FSO systems especially for large pointing errors scenario.

Performance analysis is presented for a decode-and-forward protocol based mixed radiofrequency (RF) and free-space optical (FSO) dual-hop transmission system with digital coherent detection. The RF path is modeled by Beaulieu-Xie fading, while the FSO hop is characterised by the Málaga (M) distributed turbulence with pointing errors. We first derive novel and exact analytical expressions for the cumulative distribution function (CDF), the probability density function and the moment generating function (MGF) of the overall signal-to-noise ratio by means of Meijer's G function, followed by the accurate infinite series expressions of the performance criterions, such as the outage probability, the average bit-error rate (BER) and the ergodic capacity (average channel capacity). Asymptotic analysis for the CDF, the MGF, the outage probability, the average BER, and the ergodic capacity is also provided. Monte Carlo simulations are performed to verify these derived expressions.

By incorporating the merits of satellite, aerial, and terrestrial communications, the space-air-ground integrated network (SAGIN) emerges in recent years as a promising solution to support seamless, high-rate, and reliable transmission with an extremely larger coverage than a classic terrestrial network. In essence, SAGIN is a cooperative relay network, in which high-altitude platforms (HAPs) and terrestrial base stations (BSs) serve as intermediates relaying signals between end device and satellite. In this article, we thereby view the SAGIN from the perspective of cooperative communications and introduce relay networking technologies to model and construct the framework of SAGIN. Meanwhile, we take the realistic propagation environment, HAP mobility and mathematical tractability into account and reconstruct the cooperative channel models for SAGIN, including the space-air, space-ground and air-ground links. Based on the constructed framework of SAGIN, we analyze the outage performance and approximate the outage probability as well as asymptotic outage probability in closed form. Numerical results generated by computer simulations verify our analysis and provide insight into the applicability of SAGIN. Although the relaying scenarios considered in this work are simplistic, the good tractability and expandability of the constructed framework provide a solid foundation for further research of advanced systems with complex configurations. [ABSTRACT FROM AUTHOR]

In this article, we investigate the performance of a multiuser integrated satellite-terrestrial relay network (ISTRN) with the threshold-based decode-and-forward protocol, where the satellite-relay link undergoes shadowed-Rician (SR) fading, while the relay-user links experience Nakagami-m fading. We first formulate a constrained optimization problem with an objective to maximize the system capacity in order to determine the beamforming weight vectors at the relay and thereby develop two multiuser scheduling schemes, namely, best user scheduling and user fairness scheduling. Then, we present a closed-form expression for the probability density function (PDF) of the square sum of independent and identically distributed SR random variables, which is more accurate and concise than the existing solutions. Based on the new PDF results, we derive closed-form expressions for the outage probability (OP) and ergodic capacity of the considered network using the two scheduling schemes. Furthermore, the asymptotic OP expressions at high signal-to-noise ratio are also deduced to reveal the asymptotic behavior of the considered ISTRN. Finally, simulation results are provided to validate our theoretical analysis and show the impact of various parameters on the system performance. [ABSTRACT FROM AUTHOR]

Summary: This paper investigates secure transmission of an integrated satellite‐aerial‐terrestrial network (ISATN), where multiple eavesdroppers (Eves) attempt to overhear the satellite signals cooperatively. The ISATN adopts an unmanned aerial vehicle (UAV) equipped with multiple antennas as a relay with threshold‐based decode‐and‐forward (DF) protocol. By assuming that perfect instantaneous channel state information (CSI) of the satellite‐UAV link and the statistical CSI of the UAV‐user link are available, we first propose a beamforming (BF) scheme for maximizing the achievable secrecy rate (ASR) of the considered network. Then, we derive the analytical expressions of the secrecy outage probability (SOP) and ergodic secrecy rate (ESR) of the considered system with the BF strategy under an assumption that the satellite‐UAV link undergoes the shadowed‐Rician fading, while the UAV‐user link experiences the correlated Rayleigh fading. Finally, numerical results are given to demonstrate the superiority of the proposed BF scheme against zero forcing (ZF) and maximal ratio transmission (MRT) schemes and the validity of the secrecy performance analysis. [ABSTRACT FROM AUTHOR]

In this paper, we study a multi-pair two-way half-duplex decode-and-forward (DF) massive multiple-input multiple-output (MIMO) relaying system, in which multiple single-antenna user pairs can exchange information through a massive MIMO relay. For low-complexity processing, zero-forcing reception/zero-forcing transmission (ZFR/ZFT) is employed at the relay. First, we analytically study the large-scale approximations of the sum spectral efficiency (SE). Furthermore, we focus on three specific power scaling laws to study the trade-off between the transmit powers of each pilot symbol, each user and the relay, and also focus on how the transmit powers scale with the number of relay antennas, $M$ , to maintain a finite SE performance. Additionally, we consider a practical power consumption model to investigate the energy efficiency (EE), and illustrate the impact of $M$ and the interplay between the power scaling laws and the EE performance. Finally, we consider the system fairness via maximizing the minimum achievable SE among all user pairs. [ABSTRACT FROM AUTHOR]

This article considers the problem of throughput maximization for a hybrid energy harvesting and simultaneous wireless information and power transfer (EH-SWIPT) relay system subject to energy/temperature constraints at the transmitting/receiving nodes. An EH-based single antenna source sends information to a single-antenna destination via a cooperative half-duplex multi-antenna relay, wherein the relay applies power splitting (PS) technique for obtaining energy to forward information to the destination. It is also assumed that, the receiving nodes are subject to a peak temperature constraint. Two transmission scenarios are considered; Single time slot, and multiple ones. For both scenarios, achievable rate/throughput of the system is derived and optimal resource allocations, i.e. transmission powers of the source and the relay and the optimal PS ratios at the relay, are obtained. First the multiple time slots scenario is presented, and non-convexity of the optimization problem is tackled by proposing an iterative algorithm based on closed form expressions. Then, for the single time slot one, globally optimal resource allocations are obtained. For both scenarios, some special cases are considered to better reflect the performance characteristics of the system. Finally, numerical results are presented to highlight the effect of optimal resource allocation on system performance. [ABSTRACT FROM AUTHOR]

We propose an indoor lightwave downlink wireless communication network with the non-orthogonal multiple access (NOMA) technology, that consists of one visible light communication (VLC) access point (AP) and a pair of randomly located users. Although both users can directly receive information from the AP, the performance of the far user is degraded compared to the near one due to the asymmetrical channel gains. Thus, the user cooperation strategy is proposed to improve the performance of the far user by using mixed VLC/RF relaying technique in parallel with the wireless optical direct link. To efficiently exploit the available heterogeneous links, the concept of cross-band selection combining (CBSC) is introduced, according to which the far user is continuously served by either the mixed VLC/RF or the direct VLC link. Meanwhile, the performance of the proposed scheme is thoroughly investigated and compared to appropriate baselines. To this end, we derive closed-form expressions for the outage probability of each user as well as the system sum throughput. Finally, simulation results are provided to verify the effectiveness of the proposed scheme and the accuracy of the corresponding analysis. [ABSTRACT FROM AUTHOR]

In this paper, we propose a multi-hop index modulation-aided orthogonal frequency-division multiplexing (IM-aided OFDM) system, which employs decode-and-forward (DF) relaying techniques to enable the transmission of information from source to destination via the use of multiple relays. Through the use of such a cooperative transmission system, the broadcasting coverage can be enlarged and the energy efficiency enhanced accordingly. To reduce the system complexity and provide a unified analysis without the need to employ a dual-mode transmission protocol, we also present a tailored version of the conventional subcarrier activation method, which we have modified to mitigate the zero-active subcarrier dilemma. We provide a detailed comparison of the data transmission rates of both the standard method and the tailored method presented here and give proof that the method we propose has a transmission rate, which matches or exceeds the original method. We investigate the performance of the method by analyzing and presenting the average outage probability, block error rate and network capacity in closed forms. All of the analyses presented are verified through the use of numerical simulations. The results provided in this paper present the promising vision of the IM-aided OFDM in both multi-hop networks and in their potential use in the next generation networks.

This paper studies the application of cooperative techniques for pattern division multiple access (PDMA) in downlink (DL) network. More particularly, the performance of joint decode-and-forward (DF) relaying and PDMA (DF-PDMA) is investigated over Rayleigh channel, where the direct links are existent between the base station and users. To characterize the performance of the considered scenarios above, the closed-form expressions for outage probability and sum data rate are derived. Based on the derived results, the diversity orders achieved by users are obtained. The analysis results manifest that those users with DF relaying can achieve higher diversity order, in addition, the users selecting the PDMA pattern with higher column weight can further obtain higher diversity order. Simulation results unveil that DF-PDMA outperforms PDMA and orthogonal multiple access with DF relaying (DF-OMA) in terms of outage probability and system throughput. It is also worth noting that DF-PDMA can provide better fairness compared to DF-OMA.

Bilayer low-density parity-check (LDPC) codes are an effective coding technique for decode-and-forward relaying, where the relay forwards extra parity bits to help the destination to decode the source bits correctly. In the existing bilayer coding scheme, these parity bits are protected by an error correcting code and assumed reliably available at the receiver. We propose an uncoded relaying scheme, where the extra parity bits are forwarded to the destination without any protection. Through density evolution analysis and simulation results, we show that our proposed scheme achieves better performance in terms of bit erasure probability than the existing relaying scheme. In addition, our proposed scheme results in lower complexity at the relay.

Abstract The dramatic increase of mobile devices and mobile subscriptions for delay-sensitive high-rate multimedia services has caused rapidly rising energy consumption, which is a big problem for the next generation wireless networks. It is urgent to find solutions that can satisfy the high data rate quality of service (QoS) requirements from users and obtain high energy-efficiency (EE) for the network at the same time. In this paper, energy-efficient communication is investigated for the orthogonal frequency-division multiple-access (OFDMA) based user-relay aided cellular networks that is envisioned as a promising technology to unlock the full potential of 5G networks. The EE maximization problem is formulated for dual-hop user-relay aided downlink cellular networks. Decode-and-forward (DF) relaying strategy is used by considering the QoS constraints. Since the formulated problem is a mixed-integer non-linear programming problem (MINLP) that is difficult to solve for multi-carrier, multi-user, multi-relay networks, a two-stage radio resource management solution is proposed to tackle the problem. The advantages of the proposed scheme, which not only considers the EE but also meets the data rate requirements of the users, are revealed by performing extensive simulations. [ABSTRACT FROM AUTHOR]

In the future wireless systems, non-orthogonal multiple-access (NOMA) with partial relay selection scheme is considered as developing research topic. In this paper, dual-hop relaying systems is deployed for NOMA, in which the signals are communicated with the assistance of decode-and-forward (DF) scheme. This paper presents exact expressions for outage probability over independent Rayleigh fading channels, and two partial relay selection schemes are provided. Using analytical result and Monte- Carlo method, we introduce forwarding strategy selection for fixed user allocation and exactness of derived formula is checked. The presented simulations confirm the the advantage of such proposed NOMA, and the effectiveness of the proposed forwarding strategy. [ABSTRACT FROM AUTHOR]

In this correspondence, we analyze the effects of radio frequency impairments and channel estimation error in a cellular decode-and-forward relay network, where multiple antennas are deployed at the base station (BS). Here, a single relay that has the best end-to-end signal-to-noise-plus-distortion-and-error ratio is selected among the available relays, which decode the BS information correctly in each transmission session. Furthermore, we derive an exact closed-form expression for outage probability and expected spectral efficiency in multiuser and multirelay scenario. In addition, simple asymptotic expression at high signal-to-noise ratio (SNR) regime is also obtained, which facilitates the characterization of the achievable diversity order of the practical cellular relay network. To validate the derived analytical expressions, we have presented simulation results, which are sufficiently tight across the entire range of SNRs. Finally, we examine the diversity order for the ideal as well as practical relaying systems. [ABSTRACT FROM AUTHOR]

In this correspondence paper, we study a cooperative cognitive radio network where the secondary user-transmitter (SU-Tx) assists bidirectional communication between a pair of primary users (PUs) following the principle of two-way relaying. In return, it gets access to the spectrum of the PUs to enable its own transmission to SU-receiver (SU-Rx). Furthermore, in order to support sustainable operation of the network, SU-Tx is assumed to harvest energy from the RF signals received from the PUs by using the technique of simultaneous wireless information and power transfer. Assuming a decode-and-forward behavior and power-splitting based relaying protocol at SU-Tx, closed-form expressions for outage probability of PU and SU are obtained. Simulation results validate our analytical results and illustrate spectrum-efficiency and energy-efficiency advantages of the proposed system over one-way relaying. [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]

The benefits of Decode and Forward (DF) relaying span from deployment flexibility to coverage extension and improved capacity distribution in the network. Accordingly, relaying has become an integral part of the modern radio access network standards. Besides the evident benefits of DF relaying, the fact is that it uses part of the valuable radio access resources for the communication between network elements. Thus, the amount of radio resources allocated to the Relay Link (RL) between serving Donor Base Station (DBS) and the Relay Node (RN) should be minimised. This easily makes the RL a bottleneck in conventional two-hop infrastructure relaying. Furthermore, current mobile systems apply universal frequency reuse in macrocell deployments where macrocell BS’s serve RN’s as well. As a result, RL’s may suffer from severe interference. We propose a practical limited feedback approach that can be used to simultaneously improve the signal strength and to suppress the interference in the RL. To that end, we deduce analytical formulae for the distribution of the Signal to Interference and Noise Ratio (SINR) in the RL by assuming Rice and Rayleigh fading combinations in desired and interfering links. Furthermore, we compute two-hop end-to-end (e2e) outage probabilities for different interference and limited feedback scenarios. Results show that especially in Rice fading environment, large performance gains can be obtained with only few bits feedback in case where RL has scarce radio resources and suffers from dominant interferer. This makes the proposed method very feasible for the infrastructure relaying where RN’s can be placed either in the rooftop level or other locations that provide a line of sight towards macro BS’s. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple-access (NOMA) with perfect successive interference cancellation is an emerging research topic in the future wireless communication systems. In this work, NOMA is applied to dual-hop relaying systems, where the signals are transmitted with the aid of either decode-and-forward (DF) or amplify-and-forward (AF) relay. Exact expressions for outage probability and ergodic capacity are derived over independent Rayleigh fading channels, and a tight upper bound for the outage probability of AF scheme is provided. Based on the analysis, we study forwarding strategy selection for both fixed and optimized power allocation. The presented simulations verify the accuracy of the analysis, and the efficiency of the proposed forwarding strategy. [ABSTRACT FROM AUTHOR]

Anytime codes are known to be useful for tracking and controlling unstable systems over noisy channels. Although practical anytime codes have been developed for point-to-point communication, to date no practical code constructions and tractable analysis of anytime codes are known to exist for relay networks. In this paper, we study the design and theoretical analysis of anytime codes for a three-terminal relay network. We propose bilayer codes for anytime transmission over the relay network and consider two transmission schemes for the relay-destination link. For both schemes, we analytically derive the delay-exponents and show that they closely match the corresponding numerical results obtained from density evolution. Through simulation, we also show the finite-length performance of the proposed bilayer anytime code. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a contract theory based cooperative spectrum sharing scheme with joint power and bandwidth optimization under asymmetric information, where the primary user (PU) does not know the secondary users' (SUs) private information. To improve performance, PU needs to provide incentives to stimulate nearby SUs to help forward its signal. By using contract theory, PU and SUs' negotiations are modeled as a labor market. PU and SUs act as the employer and employees, respectively. Specifically, SUs provide labor (i.e. the relay power, which can be used for forwarding PU's signal) in exchange for the reward (i.e. the spectrum access bandwidth which can be used for transmitting their own signals). PU needs to overcome a challenge how to balance the relationship between contributions and incentives for the SUs. We study the optimal contract design which consists of relay power and spectrum access bandwidth allocation. We show that the most efficient SUs will be hired by the PU to attend the cooperative communication. PU can achieve the same maximum utility as in the symmetric information scenario. Simulation results confirm that the utility of PU is significantly enhanced with our proposed cooperative spectrum sharing scheme. [ABSTRACT FROM AUTHOR]

This paper illustrates the performance of bit error rate based selection combining (BER-SC) protocol for adaptive cooperative cognitive radios. In the proposed framework, the unlicensed (i.e. secondary) system utilizes an adaptive mode of transmission to help the licensed (i.e. primary) system to achieve the desired quality of service in exchange for opportunistic spectrum access. The total transmission is divided in two phases. In Phase I, the primary transmitter (PT) broadcasts the data to the primary receiver (PR), which is overheard by the secondary transmitter (ST) and secondary receiver (SR). In Phase II, ST decodes the primary data and linearly combines its own data with the primary data. Using M-QAM the combined data is adaptively modulated, where M = 4, 16 or 64 depending on the received channel feedback, and relayed to PR and SR. At PR, BER-SC is employed to retrieve the primary data, and at SR interference cancellation is used to retrieve the secondary data. The analytical expressions are derived for the BER and the outage probability. The obtained results demonstrate the higher performance gains for both primary and secondary system by using adaptive mode of transmission at ST and BER-SC at PR. [ABSTRACT FROM AUTHOR]

The authors consider a relay network in which the source-destination pair is augmented by a set of parallel relays. In this relay network, the source transmits to the destination and the relays over multiple orthogonal channel components. Practically, orthogonal channel components is motivated by the fact that a relay cannot receive and then transmit by using the same frequency band. For this relay network, lower bound on the capacity is shown to match the upper bound, and thus establish the capacity of the discrete memoryless physically degraded relay channel. Then, the capacity result is extended to the case of additive white Gaussian noise channel. Finally, rigorous simulation results are carried out to verify the analytical results. [ABSTRACT FROM AUTHOR]

In this paper, we propose the opportunistic non-orthogonal multiple access (NOMA)-based cooperative relaying system (CRS) with channel state information (CSI) available at the source, where CSI for the sourceto-destination and source-to-relay links is used for opportunistic transmission. Using the CSI, for opportunistic transmission, the source instantaneously chooses between the direct transmission and the cooperative NOMA transmission. We provide an asymptotic expression for the average achievable rate of the opportunistic NOMA-based CRS under Rayleigh fading channels. We verify the asymptotic analysis through Monte Carlo simulations, and compare the average achievable rates of the opportunistic NOMA-based CRS and the conventional one for various channel powers and power allocation coefficients used for NOMA. [ABSTRACT FROM AUTHOR]

Ergodic capacity of dual-hop orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay system may be significantly enhanced if ordered subcarrier mapping (SCM) is implemented at relay station (R). We proved earlier that this SCM scheme also brings certain bit error rate (BER) performance improvement to assumed system. However, the level of the achieved BER reduction is very small, with no reduction at all for the high signal-to-noise ratios (SNRs) on both hops. Therefore, in this paper we propose a modification of the SCM scheme where one or more subcarriers with the lowest SNRs from both hops are omitted before ordered SCM is performed at the relay station of an OFDM DF relay system. For the scenario with Rayleigh fading statistics on both hops, and no direct communication between the source of information (S) and destination (D), we derived a closed form BER relation of the assumed uncoded BPSK modulated relay system. The analytically obtained BER results, which are verified through simulations, show that the proposed modified SCM scheme brings significant BER performance improvements for all SNR values on both hops. Additionally, in order to provide analytical tool for the assessment of ergodic capacity enhancement achieved through ordered SCM, we derived ergodic capacity expression of the dual-hop OFDM DF relay system with ordered SCM. [ABSTRACT FROM AUTHOR]

In this paper, the performance improvement of a free space optical communication system with multi-hop and beam optimization has been studied. Novel closed-form expressions for the end-to-end average bit error rate, outage probability and average capacity of multi-hop free space communication system with decode-and-forward relaying are derived. Beam optimization is done using Nelder-Mead simplex algorithm. A composite channel model using atmospheric attenuation, turbulence and pointing errors is used to model the atmospheric impairments. The saturated atmospheric conditions are modeled by negative exponential channel model, and pointing error effects are considered by normalized jitter standard deviation parameter. The performance of the free space optical communication system in terms of end-to-end average bit error rate, outage probability and average channel capacity in saturated atmospheric turbulence conditions and pointing errors is significantly enhanced by using beam-optimized serial relaying with decode-and-forward transmission. The simulation results obtained by Monte Carlo simulation are in agreement with the results obtained from the closed-form expressions. [ABSTRACT FROM AUTHOR]

In this paper, we investigate relay-assisted visible light communications (VLC) where a mobile user acts as a relay and forwards data from a transmitter to the end mobile user. We analyse the utilization of the amplify-and-forward (AF) and decode-and-forward (DF) relaying schemes. The focus of the paper is on analysis of the behavior of the mobile user acting as a relay while considering a realistic locations of the receivers and transmitters on a standard mobile phone, more specifically with two photodetectors on both sides of a mobile phone and a transmitting LED array located upright. We also investigate dependency of the bit error rate (BER) performance on the azimuth and elevation angles of the mobile relay device within a typical office environment. We provide a new analytical description of BER for AF and DF-based relays in VLC. In addition we compare AF and DF-based systems and show that DF offers a marginal improvement in the coverage area with a BER < 10–3 and a data rate of 100 Mb/s. Numerical results also illustrate that relay-based systems offer a significant improvement in terms of the coverage compared to direct non-line of sight VLC links.

This paper studies code design for the half-duplex relay channel when the transmissions take place over binary input additive white Gaussian noise (BIAWGN) channels. Using the decode-forward relay protocol, we design the relay code based on a spatially coupled low-density parity-check (SC-LDPC) code. We show a low complexity density evolution analysis for the proposed relay code. From the density evolution results, we observe that the proposed spatially coupled relay code achieves a capacity approaching performance. We also observe that the proposed code outperforms existing optimized LDPC relay codes. Through simulation results, we evaluate the finite-length performance of the proposed code. [ABSTRACT FROM AUTHOR]

In this letter, the joint optimization problem of power allocation and subcarrier-relay assignment in orthogonal-frequency-division multiplexing-based decode-and-forward relaying network is addressed. The objective is to allocate the resources fairly to maximize the signal-to-noise ratio of the poorest channel. We propose a simple optimal algorithm with polynomial time complexity to solve this nonconvex optimization problem. [ABSTRACT FROM PUBLISHER]

In this paper, the throughput of relay networks with multiple source–destination pairs under queueing constraints has been investigated for both variable-rate and fixed-rate schemes. When channel side information (CSI) is available at the transmitter side, transmitters can adapt their transmission rates according to the channel conditions, and achieve the instantaneous channel capacities. In this case, the departure rates at each node have been characterized for different system parameters, which control the power allocation, time allocation, and decoding order. In the other case of no CSI at the transmitters, a simple automatic repeat request (ARQ) protocol with fixed rate transmission is used to provide reliable communication. Under this ARQ assumption, the instantaneous departure rates at each node can be modeled as an ON–OFF process, and the probabilities of ON and OFF states are identified. With the characterization of the arrival and departure rates at each buffer, stability conditions are identified, and an effective capacity analysis is conducted for both cases to determine the system throughput under statistical queueing constraints. In addition, for the variable-rate scheme, the concavity of the sum rate is shown for certain parameters, helping to improve the efficiency of parameter optimization. Finally, through numerical results, the influence of system parameters and the behavior of the system throughput are identified. [ABSTRACT FROM PUBLISHER]

In this paper, we examine orthogonal space–time block coding with receiver maximal ratio combining (OSTBC/MRC) and selection combining (OSTBC/SC) in multiple-input–multiple-output (MIMO) decode-and-forward (DF) relay networks with underlay spectrum sharing, considering optimal and suboptimal cases. The secondary network under consideration is equipped with multiple correlated antennas at the source, the relay, and the destination. On the other hand, the primary network is composed of $L$ primary users (PUs), each of which is equipped with multiple correlated antennas. For the considered underlay spectrum sharing, the transmit power condition is related by an interference limit on the primary network and the maximum transmission power in the secondary network. In particular, new exact expressions for the outage probability of the adopted system models are obtained. Moreover, simple asymptotic expressions for the outage probability are provided to enable the characterization of the achievable diversity orders and coding gains. To enhance the secondary network performance, optimal power allocation (PA) between the secondary source and the secondary relay is obtained based on the asymptotic outage probability under the constraints of total allowable transmit power at the secondary user (SU) and maximum allowable interference power limit at the PU. The derived analytical formulas herein are supported by numerical results to clarify the main contributions. [ABSTRACT FROM AUTHOR]

In this paper, we consider the complex field network coded relay assisted communication (CFNC-RAC) channel. Although CFNC-RAC is spectrally efficient, its bit error rate performance is degraded by multi-access interference, which can be improved by appropriately allocating the user and relay powers. Since the fairness is an important factor for a practical multi-user communication system, we have proposed a rate-optimal fair power adaptation (ROFPA) technique in this work. The proposed ROFPA policy not only aims to maximize the average achievable sum-rate of CFNC-RAC under the use of the decode and forward relaying but also intends to satisfy the average rate-fairness restriction while taking the total power constraint and the network topology into account. We formulate the ROFPA as a non-convex optimization program and then derive an analytical solution for it. Extensive performance evaluation and numerical simulations validate that ROFPA method can provide significant sum-rate with considerable user fairness when compared to symbol-error-rate optimized (SER-OPT) policy proposed by Eritmen et al. (Wirel Netw, . doi:10.1007/s11276-015-0924-1). [ABSTRACT FROM AUTHOR]

In this paper, we propose a sum transmission rate maximization based cooperative spectrum sharing protocol with quality-of-service (QoS) support for both of the primary and secondary systems, which exploits the situation when the primary system experiences a weak channel. The secondary transmitter STb which provides the best performance for the primary and secondary systems is selected to forward the primary signal. Specifically, STb helps the primary system achieve the target rate by using a fraction of its power to forward the primary signal. As a reward, it can gain spectrum access by using the remaining power to transmit its own signal. We study the secondary user selection and optimal power allocation such that the sum transmission rate of primary and secondary systems is maximized, while the QoS of both primary and secondary systems can be guaranteed. Simulation results demonstrate the efficiency of the proposed spectrum sharing protocol and its benefit to both primary and secondary systems. [ABSTRACT FROM AUTHOR]

In this paper we propose a novel solution for performance enhancement of dual-hop orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay system, which assumes that relay station uses more than one antenna for downlink communications and implements transmit antenna selection (TAS) on subcarrier basis jointly with ordered subcarrier mapping (SCM). Ordered SCM is technique where subcarriers from the first hop are mapped to corresponding subcarriers on the second hop in accordance to their instantaneous signal-to-noise ratios (SNRs). It is proven to be a mapping scheme that maximizes the achievable ergodic capacity in OFDM based relay systems, while enabling bit error rate (BER) improvement at the same time. In order to assess the level of BER performance improvement in the system proposed in this paper, we analytically derive closed form BER expression of binary phase shift keying modulated OFDM DF relay system with TAS and SCM, assuming a scenario with Rayleigh fading statistics on both hops. Additionally, for further BER reduction, we analyze the combination of TAS with a modified SCM scheme, in which the subcarriers having the lowest SNRs on both hops are omitted. All derived analytical results are completely verified through simulations, showing that significant BER performance improvements are achieved with the novel proposed OFDM relay system, compared to the systems implementing only SCM, or only TAS, which approves that it can be considered as an interesting solution for the next generation of mobile cellular systems. [ABSTRACT FROM AUTHOR]

In this letter, we propose the cooperative relaying system using non-orthogonal multiple access (NOMA) to improve the spectral efficiency. The achievable average rate of the proposed system is analyzed for independent Rayleigh fading channels, and also its asymptotic expression is provided. In addition, a suboptimal power allocation scheme for NOMA used at the source is proposed. [ABSTRACT FROM PUBLISHER]

In this note, we propose a full-duplex decode-and-forward cooperative diversity scheme based on code division with orthogonal complementary codes. We consider a simple case in which a single user is assisted by a single relay to achieve cooperative diversity. When the cooperation is full-duplex, the operational delay of the relay makes the channel asynchronous and this may cause multiple-access interference (MAI) between the user and relay. The orthogonal complementary codes have perfect autocorrelation and cross-correlation properties that provide MAI-free operation in asynchronous channels. In the proposed scheme, the user and relay terminals spread their messages with complementary codes and the spread messages can reach the destination without interfering with each other. The numerical results reveal that the proposed scheme reaches the same bit-error-rate performance in both synchronous and asynchronous channels. The results are also compared with the conventional orthogonal code sets, such as Hadamard sets, which fail to provide diversity. [ABSTRACT FROM AUTHOR]