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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. [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]

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]

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]

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]

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]

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]

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]

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]

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 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]

In this paper, we investigate the end-to-end performance of dual-hop proactive decode-and-forward relaying networks with $N$th best relay selection in the presence of two practical deleterious effects: i) hardware impairment and ii) co-channel interference. In particular, we derive new exact and asymptotic closed-form expressions for the outage probability and average channel capacity of $N$th best partial and opportunistic relay selection schemes over Rayleigh fading channels. Insightful discussions are provided. It is shown that, when the system cannot select the best relay for cooperation, the partial relay selection scheme outperforms the opportunistic method under the impact of the same co-channel interference (CCI) . In addition, without CCI but under the effect of hardware impairment, it is shown that both selection strategies have the same asymptotic channel capacity. Monte Carlo simulations are presented to corroborate our analysis. [ABSTRACT FROM PUBLISHER]

In this paper we apply frequency planning to the resource allocation of multi-cell and multi-user relay enhanced orthogonal frequency division multiplexing systems and propose a low-complexity algorithm taking into account of interference coordination, subcarrier and power allocation. We divide each cell into three sectors and allow different subcarrier set that can be used by the users of one sector. Such a method can help to increase the distance among users that use the same subcarriers of adjacent cells, which can reduce the impact of co-channel interference to a certain extent. Therefore, the original problem can be decoupled into three independent sub-problems by means of frequency dividing and adaptive power allocation at base station nodes which can reduce the computing complexity greatly. In the process of resource allocation for single sector, the relationship of transmission power between base station and relay node is used to transform the max-min problem into standard closed expression. With the help of dual decomposition approach, water-filling theorem and iterative power allocation algorithm, the suboptimal solution of the primal problem can be achieved finally. Simulation results illustrate that our proposed algorithm achieves almost the same performance as the optimal resource allocation and reduce the computing complexity greatly. In addition, the proposed algorithm can ensure the users fairness of different sectors. [ABSTRACT FROM AUTHOR]

This letter investigates the effect of cochannel interference (CCI) on a hybrid satellite–terrestrial cooperative relay network (HSTCN), where both the satellite–destination and satellite–relay links undergo the shadowed Rician fading, whereas the relay–destination link follows the Rayleigh fading. By assuming that decode-and-forward (DF) protocol is adopted at the terrestrial relay, and the destination is corrupted by multiple CCI, we first derive the analytical expression for the moment generating function (MGF) of the output signal-to-interference-plus-noise ratio (SINR). Then, based on the Meijer-G functions, we present an approximated yet accurate method to evaluate the average symbol error rate (ASER) of the cooperative network. Moreover, the asymptotic ASER at high signal-to-noise ratio (SNR) with respect to diversity order and array gain are also given to gain further insights. Finally, numerical results are provided to demonstrate the validity of the performance analysis as well as the impact of CCI on the HSTCN. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the capacity gain obtained by putting a multiple-antenna relay in an original single-hop multiple-input multiple-output system using an orthogonal space-time block code over uncorrelated Rayleigh fading channels. As relaying techniques, we consider decode-and-forward relaying as well as decouple-and-forward relaying in which no decoding is performed at the relay. Focusing on the low signal-to-noise ratio regime, closed-form expressions of the probability of the capacity gain are provided for the respective relaying schemes. The probability results in an explicit function of an average power ratio of the per-hop channel in dual-hop relaying to the single-hop channel. Numerical examples show the impact of the power ratio, the relaying strategy, the number of antennas and the relay location on the capacity gain, respectively. [ABSTRACT FROM AUTHOR]

The performance of dual-hop mixed radio frequency (RF)/free space optical (FSO) communication system with partially coherent beams (PCBs) is investigated on the basis of decode-and-forward (DF) protocol. Specifically, both the RF and FSO channels are assumed to follow F-distribution with proper selection of parameters. Taking the path loss, atmospheric turbulence, and pointing errors into consideration, the closed-form expressions for the probability density function (PDF) and cumulative distribution function of the received signal-to-noise ratio are derived in terms of hypergeometric function in RF and FSO links, respectively. Subsequently, the closed-form expressions for outage probability, average bit error rate, and ergodic channel capacity of this system with M-ary phase-shift keying and M-ary quadrature amplitude modulation schemes are obtained on the basis of DF protocol. The results show that shadowing in RF link as well as the pointing error and atmospheric turbulence in FSO link would result in significant performance deterioration in dual-hop mixed RF/FSO system, while PCB can bring significant performance improvement of mixed RF/FSO systems with pointing errors over weak to moderate turbulence. Furthermore, the correctness of the analytical expressions is verified by Monte Carlo simulations. This work would benefit the design and development of the mixed RF/FSO systems. [ABSTRACT FROM AUTHOR]

Physical-layer (PHY) cooperation is a technique for achieving multiple-input–multiple-output (MIMO)-like performance improvements on small devices that cannot support antenna arrays. Devices in a network transmit on behalf of their neighbors to act as “virtual MIMO” antennas. Since small devices are typically battery constrained, PHY cooperation immediately leads to the following question related to the energy efficiency (bits per joule) of devices: Is the performance improvement worth the extra energy costs of transmitting for others? Through an in-depth hardware test-bed study, we find that PHY cooperation can improve energy efficiency by as much as 320%, or it can reduce energy efficiency by as much as 25%, depending upon topology. With this performance gap in mind, we propose the distributed energy-conserving cooperation (DECC) protocol. DECC tunes the amount of effort that each device dedicates to providing cooperative assistance for others so that the energy that each device spends on cooperation is commensurate with the personal benefits that are received by that device. With DECC, users can tune their level of cooperation with completely node-localized decision-making. Thus, DECC allows nodes to tap into a large energy-efficiency benefit, suffering only a bounded preset loss when this benefit is not available. [ABSTRACT FROM PUBLISHER]

\boldmath This letter provides an optimal power allocation that minimizes source-sum-power consumption under outage constraints in a wireless multiple-sensor single-relay network. We consider decode-and-forward (DF) relaying and assume that direct-link communication does not exist. We provide an exact expression of an optimal power allocation by using Karush-Kuhn-Tucker (KKT) conditions. Constructive details used in finding the optimal power allocation as well as numerical investigation is given to verify the optimality of the proposed solution. Numerical results also show that the proposed power allocation reduces the source-sum-power consumption by nearly 34-69% compared to a non-optimal allocation. The power saving depends on the number of sources and becomes large when the number increases. [ABSTRACT FROM PUBLISHER]

We address in this paper the challenge of coping with asymmetric data rates in two-way relay channels. We consider a relay network comprising two sources and one relay. The sources communicate at different rates through the relay. That is, we assume that one source uses M1-QAM (quadrature amplitude modulation) and the other uses M1/M2-QAM hierarchical modulation where M1≠ M2. For the underlying network, we consider two decode-and-forward (DF) relaying schemes. One scheme combines hierarchical zero padding and network coding (HZPNC) at the relay. The novelty of this scheme lies in the way the two signals (that have different lengths) are network-coded at the relay. The other scheme is referred to as opportunistic user selection (OUS) where the user with a better end-to-end channel quality is given priority for transmission. We analyze both schemes where we derive closed-form expressions for the end-to-end (E2E) bit error rate (BER). Since the two schemes offer a trade-off between performance and throughput, we analyze and compare both schemes in terms of channel access probability and average throughput. We show that HZPNC offers better throughput and fairness for both users, whereas OUS offers better performance. We also compare the performance of HZPNC with existing schemes including the original zero padding, nesting constellation modulation and superposition modulation. We show through examples the superiority of the proposed HZPNC scheme in terms of performance and/or reduced complexity. [ABSTRACT FROM AUTHOR]

In this letter, we derive the probability density function (PDF) and cumulative distribution function (CDF) of the minimum of two non-central Chi-square random variables with two degrees of freedom in terms of power series. With the help of the derived PDF and CDF, we obtain the exact ergodic capacity of the following adaptive protocols in a decode-and-forward (DF) cooperative system over dissimilar Rician fading channels: (i) constant power with optimal rate adaptation; (ii) optimal simultaneous power and rate adaptation; (iii) channel inversion with fixed rate. By using the analytical expressions of the capacity, it is observed that the optimal power and rate adaptation provides better capacity than the optimal rate adaptation with constant power from low to moderate signal-to-noise ratio values over dissimilar Rician fading channels. Despite low complexity, the channel inversion based adaptive transmission is shown to suffer from significant loss in capacity as compared to the other adaptive transmission based techniques over DF Rician channels. [ABSTRACT FROM PUBLISHER]

In this letter, we present the outage performance of decode-and-forward based cooperative multicast systems with the best relay selection (CM-RS) over Rayleigh fading channels. Especially, we provide an exact and closed-form expression for the outage probability of CM-RS and its diversity order. Through numerical results, the analysis is verified, and the outage performance of CM-RS is compared with that of a single-hop multicast system. [ABSTRACT FROM PUBLISHER]

Superposition modulation makes it possible for transmitting a node's own data along with its neighbors' relay data in a single time-slot and frequency, which can improve spectral efficiency of cooperative wireless communication system. First, we consider a two-user scenario and propose a design criterion for two-dimensional superposition modulation based cooperative transmission where the users cooperate to transmit their data to a base station. We investigate M-PSK superposition modulation based cooperative scenario for Rayleigh fading channels and thoroughly derive the average symbol error rate (SER) of such system with decode-and-forward relaying. Furthermore, we introduce a design parameter called power allocation ratio (PAR) which can be adaptively changed within the derived limits. We derive SER expressions in terms of the PAR and demonstrate its effect on average SER. Based on analytical and simulation results, we show that the proposed scheme provides reduced SER and increased spectral efficiency per link compared to existing schemes in practical scenarios. Finally, we investigate multi-user scenario, propose an adaptive power allocation scheme for superposition modulation, and confirm that SER is decreased with our proposed scheme. [ABSTRACT FROM AUTHOR]

We analyze the symbol error rate (SER) of selective decode-and-forward (DF) relaying for the practical case of independent but not necessarily identically distributed two-wave with diffuse power (TWDP) fading. We first derive new exact SER expressions with M-ary phase-shift keying and M-ary quadrature amplitude modulation. We then derive tight upper bounds on the exact SER in closed form to avoid the numerical integration involved in the exact results. Furthermore, we derive concise yet valuable expressions for the asymptotic SER, which characterizes the system behavior in the high signal-to-noise ratio (SNR) regime in terms of the diversity order and the coding gain. Based on the asymptotic SER, we confirm that TWDP fading parameters have no impact on the diversity order, but affect the coding gain. We also demonstrate that the contribution of the source—destination link to the SER is higher than that of the source—relay link or the relay—destination link. Our analytical results are valid for general operating scenarios with arbitrary average SNRs and distinct TWDP fading parameters in each hop. [ABSTRACT FROM PUBLISHER]

In this paper, we explore the advantages of network coding and space-time coding in improving the performance of two-way-relayed communications where two terminals absent of direct links exchange information through a single relay in between. Network coding allows embracing the interference from other terminals thereby turning it into a capacity boost. The application of space-time codes yields higher capacity by exploiting the spatial diversity. The joint performance of both techniques is studied in this paper. Specifically, we consider the class of decode-and-forward (DF) relaying strategy, evaluated in terms of symbol error rate using BPSK and QPSK modulations by both theoretical analysis and simulation. Based on our results, DF outperforms the amplify-and-decode and partial-decode-and-forward protocols. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Cooperative communications is a class of techniques that seek to improve reliability and throughput in wireless systems by pooling the resources of distributed nodes. Although cooperation can occur at different network layers and time scales, physical-layer cooperation at symbol time scales offers the largest benefit in combating losses due to fading. However, symbol-level cooperation poses significant implementation challenges, particularly in synchronizing the behavior and carrier frequency of distributed nodes. We present the implementation and characterization of a complete real-time cooperative physical-layer transceiver built on the Rice University Wireless Open-Access Research Platform (WARP). In our implementation, autonomous nodes employ physical-layer cooperation without a central synchronization source and can select between non-cooperative and cooperative communications per packet. Cooperative transmissions use a distributed Alamouti space–time block code (STBC) and employ either amplify-and-forward (AF) or decode-and-forward (DF) relaying. We also present experimental results of our transceiver's real-time performance under various topologies and propagation conditions. Our results clearly demonstrate significant performance gains (more than 40\times improvement in packet error rate in some topologies) provided by physical-layer cooperation, even when subject to the constraints of a real-time implementation. Finally, we present methodologies for isolating and understanding the sources of performance bottlenecks in our design. As with all our work on WARP, our transceiver design and experimental framework are available through the open-source WARP repository for use by other wireless researchers. [ABSTRACT FROM PUBLISHER]

The ergodic capacities in Rayleigh fading of various wireless relaying systems with an arbitrary number of half duplex relays are analyzed, assuming channel state information is only known at the receivers. Closed-form analytical expressions for calculation of the ergodic capacities of these systems are derived. It is shown that systems with nonregenerative fixed gain relays achieve higher ergodic capacities than the corresponding systems with nonregenerative variable gain relays. A modified fixed gain relay, which incorporates the power constraint at the relays, is proposed. It is shown that systems with modified fixed gain relays slightly outperform the corresponding systems with nonregenerative variable gain relays and fixed gain relays at small signal-to-noise ratios, but attain almost the same ergodic capacities as systems with nonregenerative variable gain relays as the signal-to-noise ratio increases. In addition, the ergodic capacity of a hybrid system with both regenerative and nonregenerative relays is studied. Systems with regenerative relays employing decode-and-forward relaying offer higher ergodic capacities than the corresponding systems with any classes of nonregenerative relays or hybrid relays. [ABSTRACT FROM PUBLISHER]