Optimal power‐aware relay placement for cooperative wireless sensor networks

We study the problem of optimizing the symbol error probability (SEP) performance of cluster‐based cooperative wireless sensor networks. Recent studies in literature show that an efficient relay selection protocol based on simple geographical information of the nodes to execute cooperative diversity can significantly improve the SEP performance at the destination of such networks. As well, similar line of research on optimal power allocation (for the source and relay nodes) can be found in literature. However, to achieve the best SEP performance at the destination of a cooperative wireless sensor network, joint optimization of power allocation and relay placement should be accomplished. To this aim, we reformulate the SEP of a multi‐hop cooperative communication in a general form and optimize transmitted power level and relay placement simultaneously. This analysis is developed for both amplify‐and‐forward and decode‐and‐forward relaying protocols. Simulation results demonstrate that the proposed joint optimization can effectively improve the SEP performance of the network. Copyright © 2013 John Wiley & Sons, Ltd.     

Joint improvements for capacity and power‐efficiency of mobile WiMAX by relaying schemes

The mobile WiMAX standard promises to enable low‐cost mobile Internet applications over extensive areas and to meet the capacity requirements by combining advanced multiple input multiple output (MIMO) and relay transmission techniques. In this paper, we propose a solution to increase the channel capacity between wireless links and to conserve the average required uplink transmission power consumption simultaneously through deploying relay stations' (RSs) locations judiciously. Two relaying schemes, analogue (amplify and forward) relaying and digital (decode and forward) relaying from a mobile device to the base station (BS) through a relay node, are adopted with weighting filters to increase the channel capacity. Based on the enhanced channel capacity, a new manipulation way to save power is introduced by deploying RSs strategically where the branch‐and‐bound (BB) algorithm is applied to determine the placements of RSs. Our simulation results demonstrate the significant improvement of network capacity by applying the weighting filter techniques and the great power saving of the average total network power by utilizing the BB algorithm to arrange RSs locations. Copyright © 2011 John Wiley & Sons, Ltd.     

Nelder‐Mead–based power optimization for secrecy enhancement in amplify‐and‐forward cooperative relay networks

Cooperative communication based on relaying nodes has been considered as a promising technique to increase the physical layer security (PLS) performance in wireless communications. In this paper, an optimal power allocation (OPA) scheme based on Nelder‐Mead (NM) algorithm is proposed for improving the secrecy rate of amplify‐and‐forward (AF) cooperative relay networks employing cooperative jamming (CJ) scheme. The proposed hybrid jamming scheme allows the source and selected relay to transmit the jamming signal along with the information to confound the eavesdropper. The path selection probability of ant colony optimization (ACO) algorithm is used for selecting the relay for transmission. The performance based on secrecy rate is evaluated for “n” trusted relays distributed dispersedly between the source and destination. Gradient‐based optimization and three‐dimensional exhaustive search methods are used as benchmark schemes for comparison of the proposed power optimization algorithm. The secrecy performance is also compared with conventional AF scheme and CJ scheme without power optimization (EPA). The impact of single and multiple relays on secrecy performance is also evaluated. Numerical results reveal that, compared with the gradient method and exhaustive search algorithm, the proposed power allocation strategy achieves optimal performance. Also, the derived OPA results show a significantly higher secrecy rate than the EPA strategy for both CJ and AF schemes.     

Game‐theoretic modeling and analysis of relay selection in cooperative wireless networks

In this paper, distributed single relay selection in cooperative wireless networks is modeled as a Chinese restaurant game (CRG). Specifically, the CRG is used to model strategic relay selection decisions of source nodes, taking into account negative network externality due to the potential sharing of relay nodes among source nodes. Two cases are studied as follows: (i) perfect relay transmit power (RTP) knowledge and (ii) imperfect RTP knowledge. Under the first case, a distributed relay selection algorithm is proposed and shown to converge to a Nash equilibrium grouping. Under the second case, a reinforcement learning algorithm is proposed and combined with the distributed relay selection algorithm to allow network nodes to select rate‐maximizing relays. Simulation results verify the efficiency of the proposed distributed relay selection algorithm when compared with other relay selection schemes and demonstrate that it yields a network sum‐rate that is comparable with that of centralized relay selection. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance analysis of spectrally efficient amplify‐and‐forward opportunistic relaying scheme for adaptive cooperative wireless systems

In this paper, we propose an adaptive amplify‐and‐forward (AF) relaying scheme that selects the best relay among the available relay nodes opportunistically to cooperate with a source node for improvement of the spectral efficiency. This improvement can be achieved by introducing a policy that gives the useful cooperative regions and defines a switching threshold signal‐to‐noise ratio that guarantees the bit error rate (BER) of cooperative transmission is below the target. We model all links as independent non‐identically distributed Rayleigh fading channels. We then derive closed‐form expressions for the average spectral efficiency, average BER, and outage probability when an upper bound for the signal‐to‐noise ratio of the end‐to‐end relay path is applied and adaptive discrete rate is considered. Numerical and simulation results show that the proposed scheme, compared with the outage‐based AF incremental relaying, AF fixed relaying, and the conventional direct transmission, can achieve the maximum average spectral efficiency while maintaining the average BER and outage probability. Copyright © 2013 John Wiley & Sons, Ltd.     

Power allocation and relay selection for AF two‐path successive relaying networks

Two‐path or successive relaying, which aims to establish two relay links transmitting different information symbols in adjacent time slots, has recently emerged as an attractive wireless communication protocol to improve the spectral efficiency in half‐duplex cooperative systems. In this paper, we investigate power allocation and relay selection techniques for amplify‐and‐forward two‐path successive relaying networks. Our approach is based on the maximization of the received SNR subject to a total power budget consumed by the source and the relay assisting this specific transmission. Two scenarios including with and without direct link are considered here. We show that the main problem has a closed‐form solution and only requires a few amounts of feedback bits to be broadcasted. Numerical results reveal that the proposed approaches are more insensitive to the inter‐relay interference and robust to channel estimation errors; meanwhile, they perform better than the existing schemes. Copyright © 2013 John Wiley & Sons, Ltd.     

Achievable sum‐rate maximization using relay/antenna selection for MIMO two‐way AF relaying with channel uncertainties

This paper focuses on the layered relay‐and‐antenna selection (LRAS) for achievable sum‐rate (ASR) maximization while considering the impacts of residual self‐interference due to channel estimation errors in multiple‐input multiple‐output two‐way amplify‐and‐forward relaying systems. Two LRAS algorithms, namely, the Gram–Schmidt and the adaptive discrete stochastic approximation selection techniques, are investigated based on the ASR maximization under an equal power allocation. To alleviate the complexity burden of the LRAS strategies, the optimal relay and the subset of transmit‐and‐receive antenna pairs are determined by a two‐stage selection mechanism. By taking two LRAS strategies and correlated channel uncertainties into account, the development of a two‐way multiple‐input multiple‐output multi‐amplify‐and‐forward‐relay system is able to provide improved robustness against the channel state information mismatch and the residual self‐interference. Copyright © 2016 John Wiley & Sons, Ltd.     

A two‐hop equalize‐and‐forward relay scheme in OFDM‐based wireless networks over multipath channels

Relay communications have attracted increasing research attentions as a cost‐effective technique to improve spatial diversity, service coverage, and energy efficiency in wireless networks. However, existing relay schemes (e.g., amplify‐and‐forward and decode‐and‐forward (DF) schemes) still face several major challenges, particularly the accumulation of multipath channels effect in AF and long processing latency in DF. To address these issues, we propose a novel equalize‐and‐forward (EF) relay scheme to enhance the retransmission reliability while maintaining low processing delay at the relay node. In particular, the proposed EF relay estimates and equalizes the channel between source and relay to eliminate the channel accumulation effect without signal regeneration. To further reduce the relay processing time, the channel estimation and equalization in the proposed EF design are performed in parallel. The proposed equalization is realized by presetting the equalizer coefficients with the current channel response that is predicted in parallel using multiple past channel responses. Numerical results show that the proposed EF relay scheme can achieve comparable symbol error rate performance as the DF relay with much less relay latency. In addition, the EF relay exhibits low outage probability at the same data rate as compared with traditional amplify‐and‐forward and DF schemes. schemes. Copyright © 2015 John Wiley & Sons, Ltd     

Design of Distributed Beamforming for Dual‐Hop Multiple‐Access Relay Networks

This paper studies a dual‐hop multiple‐access relay network where two independent source nodes transmit information to a common destination node with the aid of multiple single‐antenna amplify‐and‐forward relays. Each relay node is subject to an individual power constraint. We focus on the design of distributed beamforming schemes for the relays to support the transmission rate requirements of the two sources. To this end, we first characterize the achievable rate region for this network via solving a sequence of corner point optimization problems proposed in this paper. We also develop several low‐complexity suboptimal schemes in closed form. Two inner bounds of the achievable rate region are theoretically shown to be approximately optimal in two special scenarios. Finally, numerical results demonstrate the effectiveness of our proposed approaches.     

Transmission scheduling in a multi‐channel wireless network with bidirectional relaying links

Using network coding in a wireless network can potentially improve the network throughput. On the other hand, it increases the complexity of resource allocations as the quality of one transmission is affected by the link conditions of the transmitter to multiple receivers. In this work, we study time slot scheduling and channel allocations jointly for a network with bidirectional relaying links, where the two end nodes of each link can exchange data through a relay node. Two scenarios are considered when the relay node forwards packets to the end nodes. In the first scenario, the relay node always forwards network‐coded packets to both end nodes simultaneously; in the second scenario, the relay node opportunistically uses network coding for two‐way relaying and traditional one‐way relaying. For each scenario, an optimization problem is first formulated for maximizing the total network throughput. The optimum scheduling is not causal because it requires future information of channel conditions. We then propose heuristic scheduling schemes. The slot‐based scheduling maximizes the total transmission rate of all the nodes at each time slot, and the node‐based scheduling schedules transmissions based on achievable transmission rates of individual nodes at different channels. The node‐based one has lower complexity than the slot‐based one. Our results indicate that although the node‐based scheduling achieves slightly lower throughput than the slot‐based one, both the proposed scheduling schemes are very effective in the sense that the difference between their throughput and the optimum scheduling is relatively small in different network settings. Copyright © 2015 John Wiley & Sons, Ltd.     

Outage analysis of opportunistic‐based hybrid decode‐amplify‐forward relaying in polar code environment

Outage analysis plays a vital role in wireless systems to determine reliable transmission and effective communication. Incremental hybrid decode‐amplify‐forward (IHDAF) relay offers a way of meeting the challenges of capacity and coverage improvement with great potential in cooperative communication networks. Therefore, opportunistic incremental hybrid relaying must be integrated with coding schemes to achieve full diversity. In this paper, the outage behavior of polar coded and distributed coded cooperative relaying schemes is analyzed. Simulation results show that opportunistic incremental HDAF using polar code offers an outage capacity of 17 b/s/Hz for 4 × 4 multiantenna and 45 b/s/Hz in 8 × 8 multiantenna systems with an outage of 10^?8 and 10^?13, respectively. Moreover, the polar coded opportunistic IHDAF system in 8 × 8 MIMO achieves 2 and 6 dB higher gains compared with amplify‐and‐forward (AF) and decode‐and‐forward (DF) relaying schemes. The closed‐form expression for outage probability has been derived through Marcum‐Q approximations and processed through Monte Carlo simulations.     

Performance evaluation of multiuser diversity in multiuser two‐hop cooperative multi‐relay wireless networks using maximal ratio combining over Rayleigh fading channels

Multiuser diversity (MUD) cooperative wireless networks combine the features of the MIMO systems without confronting the physical layer constraints by providing multiple copies of the transmitted signal from the source to the destination with the help of the relay node. Cooperative wireless networks have attracted the full attention in the last few years and are implemented widely in many wireless communication systems to adapt for the fading impairments, provide higher data rates, and improve the performance of the wireless communication systems. In this paper, we present an informative study for the reason of evaluating the performance of the MUD in the multiuser two‐hop cooperative multi‐relay networks using maximal ratio combining. Furthermore, we derive tight closed‐form expressions of outage probability and symbol error probability for the amplify‐and‐forward and fixed decode‐and‐forward protocols with the MUD. Additionally, we conduct a simulation study to show to what extent our analytical and simulation results agree with each other. It is worthy to mention that our analytical and simulation results agree fairly with each other under high average signal‐to‐noise ratio, whereas they show that our proposed system with multiple relays provides significant improvements over those previously proposed systems having only one relay. Copyright © 2013 John Wiley & Sons, Ltd.     

Energy‐efficient partner selection in cooperative wireless networks: a matching‐theoretic approach

In this paper, the problem of stable energy‐efficient partner selection in cooperative wireless networks is studied. Each node aims to be paired with another node so as to minimize the total energy consumption required to meet a target end‐to‐end signal‐to‐noise ratio requirement and thus maintain quality of service. Specifically, each node ranks every other node in the network according to their energy saving achievable through cooperation. Two polynomial time complexity algorithms based on the stable roommates matching problem are proposed through which nodes are paired according to their preference lists. The first algorithm, denoted Irving's stable matching, may not always have a stable solution. Therefore, the second algorithm—which is a modified version of Irving's algorithm and denoted maximum stable matching—is proposed to find the maximum number of stable disjoint pairs. Simulation results are provided to validate the efficiency of the proposed algorithms in comparison with centralized energy‐efficient partner selection as well as other matching algorithms, yielding a trade‐off between stability and total energy consumption, but comparable symbol error rate performance and network sum rate. Copyright © 2016 John Wiley & Sons, Ltd.     

Exact SEP and performance bounds for orthogonal space‐time block codes in cooperative multiantenna AF relaying networks

In this paper, we derive a moment generating function (MGF) for dual‐hop (DH) amplify‐and‐forward (AF) relaying networks, in which all nodes have an arbitrary number of antennas, with orthogonal space‐time block code (OSTBC) transmissions over Rayleigh fading channels. We present an exact error rate expression based on the derived MGF and another analytical approach to derive achievable performance bounds as closed‐forms of symbol error rate, outage probability, and normalized channel capacity. Furthermore, we derive the asymptotic behavior of symbol error rate and outage probability. From this asymptotic behavior, it is shown that the diversity order and its dependence on antenna configurations can be explicitly determined. Simulation results are also presented to verify their accuracy by comparing with numerical results and to provide an insight to the relationship between relaying networks' antenna configuration and diversity order. It is confirmed that the transmit antenna gain of the source node and the receive antenna gain of the relay node can be obtained only when the relay is close to the destination, and then, the transmit antenna gain of the relay node and the receive antenna gain of the destination node can be obtained only when the relay is close to the source.     

Exact analysis of the multihop multibranch hybrid AF/DF relaying networks

In this paper, the exact analysis of a multihop multibranch (MHMB) relaying network is investigated wherein each relay can operate in amplify‐and‐forward (AF) or decode‐and‐forward (DF) modes depending upon the decoding result of its received signal. If a relay decodes the received signal correctly, it works in DF mode; otherwise, the relay operates in AF mode. Therefore, we name such relaying network as hybrid amplify‐and‐forward and decode‐and‐forward (hybrid AF/DF) relaying network. We first investigate the signal transmission from source to destination node via n number of relays in a hybrid AF/DF MHMB mode. Then, we obtained the statistical features and analyze the end‐to‐end signal‐to‐noise ratio (SNR). Finally, a comprehensive performance analysis is conducted by using maximal ratio combining (MRC) scheme at the destination node. For comparison, we also obtained the results using selection combining (SC) scheme at the destination node. To the best of our knowledge, very few works in the literature have considered a general system model of MHMB relaying network wherein each relay can operate in AF or DF modes, that is, a hybrid AF/DF relaying network. Accordingly, the analysis of our system model is not only novel and exact, but also is comprehensive and can be employed in the future works.     

Resource allocations in relay‐assisted cellular networks

In a relay‐assisted cellular network, the transmission mode (either direct transmission or relaying) and the transmit power of the source and relay nodes affect not only transmission rates of individual links but also the rates of other links sharing the same channel. In this paper, we propose a cross‐layer design that jointly considers the transmission mode/relay node selection (MRS) with power allocation (PA) to optimize the system rate. We first formulate an optimization problem for a cellular system, where the same frequency channel can be reused in different cells. A low complexity heuristic MRS scheme is proposed on the basis of the link and interference conditions of the source and potential relay nodes. Given the transmission mode and relay node (if the relaying mode is chosen) of each link, the transmit power of the source and relay nodes can be solved by geometric programming. This method for MRS and PA can achieve a close‐to‐optimum performance, but implementing the PA requires heavy signalling exchanged among cells. To reduce the signalling overheads, we finally proposed a heuristic and distributed method for MRS and PA inspired by some asymptotic analysis. Numerical results are conducted to demonstrate the rate performance of the proposed methods.Copyright © 2013 John Wiley & Sons, Ltd.     

MMSE‐based transceiver design for distributed MIMO amplify‐and‐forward cooperative networks in correlated channels

In this paper, the source‐precoder, multiple‐relay amplifying matrices, and destination‐equalizer joint optimization is investigated in distributed MIMO amplify‐and‐forward multiple‐relay networks with direct source–destination transmission in correlated fading channels. With the use of taking both the direct link and spatial correlation between antenna elements into account, the cooperative transceiver joint design is developed based on the minimum mean‐squared error criterion under individual power constraints at the source and multiple‐relay nodes. Simulation results demonstrate that the cooperative transceiver joint design architecture for an amplify‐and‐forward MIMO multiple‐relay system outperforms substantially the noncooperative transceiver design techniques on the BER performance under the spatial‐correlation channels.Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal Amplify‐and‐Forward Scheme for Parallel Relay Networks with Correlated Relay Noise

This paper studies a parallel relay network where the relays employ an amplify‐and‐forward (AF) relaying scheme and are subjected to individual power constraints. We consider correlated effective relay noise arising from practical scenarios when the relays are exposed to common interferers. Assuming that the noise covariance and the full channel state information are available, we investigate the problem of finding the optimal AF scheme in terms of maximum end‐to‐end transmission rate. It is shown that the maximization problem can be equivalently transformed to a convex semi‐definite program, which can be efficiently solved. Then an upper bound on the maximum achievable AF rate of this network is provided to further evaluate the performance of the optimal AF scheme. It is proved that the upper bound can be asymptotically achieved in two special regimes when the transmit power of the source node or the relays is sufficiently large. Finally, both theoretical and numerical results are given to show that, on average, noise correlation is beneficial to the transmission rate — whether the relays know the noise covariance matrix or not.     

Performance study of hybrid decode–amplify–forward (HDAF) relaying scheme for physical layer security in wireless cooperative network

In this paper, the secrecy performance and power allocation of the signal‐to‐noise ratio‐based hybrid decode–amplify–forward (HDAF) relaying protocol in wireless cooperative network are investigated to get security at physical layer. The performance metrics considered are secrecy rate and intercept probability. The Ergodic secrecy rate is approximated theoretically. The effect of relay and eavesdropper locations on the secrecy performance of the system is analyzed. It is found that maximum secrecy rate is obtained for the relay close‐to‐destination case and minimum for the relay close‐to‐eavesdropper case. Jamming schemes are superior in secrecy rate performance than without jamming schemes. To enhance the secrecy rate further with the optimized relay and jammer powers, invasive weed optimization (IWO) algorithm‐based power allocation is proposed. Here, maximizing the secrecy rate is defined as the cost function for the proposed IWO algorithm‐based power allocation. Comparative study is done over the conventional equal and proposed power allocation schemes for validation. The proposed power allocation scheme proved to be superior. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimal resource allocation for multiple network‐coded two‐way relay in orthogonal frequency division multiplexing systems

This paper studies optimal resource allocation for multiple network‐coded two‐way relay in orthogonal frequency division multiplexing systems. All the two‐way relay nodes adopt amplify‐and‐forward and operate with analog network coding protocol. A joint optimization problem considering power allocation, relay selection, and subcarrier pairing to maximize the sum capacity under individual power constraints at each transmitter or total network power constraint is first formulated. By applying dual method, we provide a unified optimization framework to solve this problem. With this framework, we further propose three low‐complexity suboptimal algorithms. The complexity of the proposed optimal resource allocation (ORA) algorithm and three suboptimal algorithms are analyzed, and it is shown that the complexity of ORA is only a polynomial function of the number of subcarriers and relay nodes under both individual and total power constraints. Simulation results demonstrate that the proposed ORA scheme yields substantial performance improvement over a baseline scheme, and suboptimal algorithms can achieve a trade‐off between performance and complexity. The results also indicate that with the same total network transmit power, the performance of ORA under total power constraint can outperform that under individual power constraints. Copyright © 2012 John Wiley & Sons, Ltd.