This paper investigates the image communications in two-hop wireless relay networks (TH-WRNs) where a source node sends images to a destination node with the assistance of a relay node via two hops, i.e. source-to-relay and relay-to-destination links. Due to the broadcast nature of wireless media, there exists an eavesdropper who tries to overhear and recover the images. Aiming to enhance the security and also to save transmission bandwidth of the image communications, we propose a secure relaying transmission (SRT) protocol by exploiting both random linear network coding (RLNC) and image super-resolution (ISR) techniques. In the proposed protocol, the original high-resolution (HR) images are downscaled at the source node and the RLNC is employed at both the source and relay nodes to conceal the original images from the eavesdropper. The RLNC decoding and ISR are adopted at the destination node to decode and recover the HR images, while the eavesdropper cannot decode the images due to the unawareness of the coefficient matrices and the reference images in the RLNC. It is shown that the proposed SRT protocol achieves a significantly higher performance at the destination than at the eavesdropper. Furthermore, with high-quality relaying hops, the SRT protocol outperforms the secure direct transmission (SDT) protocol with only a direct link between the source and the destination nodes. Finally, simulation results are provided to verify the findings.

In this study, the authors first propose an analytical model for investigating the impacts of power allocation (PA) and cell density allocation (CDA) on coverage efficiency (CE) of heterogeneous wireless cellular networks (HWCNs) under limited resources. It is shown that the interference among cells that belong to different tiers is reduced significantly in higher path loss environment and results in a higher coverage. The network coverage of the HWCN can be further extended with the deployment of a higher cell density in a more lossy environment. This accordingly leads us to develop an optimisation problem (OP) to maximise the CE by optimising the PA and CDA under the constraint of limited cell power and total power available in the downlink HWCN. In particular, they proposed a two‐stage approach for solving the OP to sequentially obtain the heuristic value of the CDA and PA due to complicated objective function along with various involved parameters. Numerical results reveal that the coverage obtained by the heuristic solution at the first‐stage is significantly improved with lower power than the conventional approach. Furthermore, an enhanced overall CE is achieved for all cases of the power constraint when applying fully two stages in their proposed algorithm.

Non‐orthogonal multiple access (NOMA) along with cooperative communications have been recognised as promising candidates for the fifth generation (5G) wireless networks and have attracted many researchers. Every networked device however has its own limited power supply. To this extent, this study investigates a power‐splitting relaying (PSR) protocol for wireless energy harvesting (EH) and information processing in the NOMA systems to prolong the lifetime of the energy constrained relay nodes in wireless networks so as to avail the ambient radio‐frequency (RF) signal as well as to simultaneously harvest the energy and process the information. Decode‐and‐forward relaying is employed at the relay node where the energy from the received RF signal is harvested and exploited to forward the information to the destination. Specifically, the outage probability and ergodic rate of the PSR protocol are derived to realise the impacts of EH time, EH efficiency, power splitting ratio, source data rate, and the distance between nodes. It is shown that an outperformance in terms of the energy efficiency is achieved with an increased EH efficiency and the employment of the NOMA when compared to the conventional orthogonal multiple access.

Key update and residence management have been investigated as an effective solution to cope with desynchronization attacks in mobility management entity (MME) handovers. In this paper, we first analyze the impacts of the key update interval (KUI) and MME residence interval (MRI) on handover processes and their secrecy performance in terms of the number of exposed packets (NEP), signaling overhead rate (SOR), and outage probability of vulnerability (OPV). Specifically, the bounds of the derived NEP and SOR not only capture their behaviors at the boundary of the KUI and MRI, but also show the tradeoff between the NEP and SOR. Additionally, through the analysis of the OPV, it is shown that the handover security can be enhanced by shortening the KUI and the desynchronization attacks can be avoided with high-mobility users. The above facts accordingly motivate us to propose a multi-objective optimization (MO) problem to find the optimal KUI and MRI that minimize both the NEP and SOR subject to the constraint on the OPV. To this end, two scalarization techniques are adapted to transform the proposed MO problem into single-objective optimization problems, i.e., an achievement-function method via fractional programming (FP) and a weighted-sum method. Based on the derived bounds on NEP and SOR, the FP approach can be optimally solved via a simple numerical method. For the weighted-sum method, the firefly algorithm (FA) is utilized to find the optimal solution. The results show that both techniques can solve the proposed MO problem with a significantly reduced searching complexity compared to the conventional heuristic iterative search technique.

This paper investigates the security at the physical layer in cooperative wireless networks (CWNs) where the data transmission between nodes can be realised via either direct transmission (DT) or relaying transmission (RT) schemes. Inspired by the concept of physical-layer network coding (PNC), a secure PNC-based modify-and-forward (SPMF) is developed to cope with the imperfect shared knowledge of the message modification between relay and destination in the conventional modify-and-forward (MF). In this paper, we first derive the secrecy outage probability (SOP) of the SPMF scheme, which is shown to be a general expression for deriving the SOP of any MF schemes. By comparing the SOPs of various schemes, the usage of the relay is shown to be not always necessary and even causes a poorer performance depending on target secrecy rate and quality of channel links. To this extent, we then propose an opportunistic secure transmission protocol to minimise the SOP of the CWNs. In particular, an optimisation problem is developed in which secrecy rate thresholds (SRTs) are determined to find an optimal scheme among various DT and RT schemes for achieving the lowest SOP. Furthermore, the conditions for the existence of SRTs are derived with respect to various channel conditions to determine if the relay could be relied on in practice.

In LTE networks, key update and residence management have been investigated as an effective solution to cope with desynchronization attacks in mobility management entity (MME) handovers. In this paper, we first analyse the impacts of the key update interval (KUI) and MME residence interval (MRI) on the handover performance in terms of the number of exposed packets (NEP) and signaling overhead rate (SOR). By deriving the bounds of the NEP and SOR over the KUI and MRI, it is shown that there exists a tradeoff between the NEP and the SOR, while our aim is to minimise both of them simultaneously. This accordingly motivates us to propose a multiobjective optimisation problem to find the optimal KUI and MRI that minimise both the NEP and SOR. By introducing a relative importance factor between the SOR and NEP along with their derived bounds, we further transform the proposed optimisation problem into a single-objective optimisation problem which can be solved via a simple numerical method. In particular, the results show that a higher accuracy of up to 1 second is achieved with the proposed approach while requiring a lower complexity compared to the conventional approach employing iterative searches.

Non‐orthogonal multiple access (NOMA) in amplify‐and‐forward relay systems with partial relay selection (PRS) is investigated. More specifically, new exact closed‐form expressions for the outage probabilities at two users are derived, based on which an asymptotic analysis at high signal‐to‐noise ratio (SNR) is carried out. Additionally, to investigate the performance gap between the NOMA and orthogonal multiple access (OMA) schemes, a closed‐form approximate expression at high SNR for the sum rate is derived. Furthermore, relying on the results, the impact of the PRS on the sum rate and outage probability of the proposed NOMA scheme is examined. In particular, the derived asymptotic expressions show that the proposed scheme can improve over the traditional OMA not only the sum rate but also the user fairness. Finally, simulation results are presented to corroborate the analytical results.

Hybrid wired-wireless Network-on-Chip (WiNoC) has emerged as an alternative solution to the poor scalability and performance issues of conventional wireline NoC design for future System-on-Chip (SoC). Existing feasible wireless solution for WiNoCs in the form of millimeter wave (mm-Wave) relies on free space signal radiation which has high power dissipation with high degradation rate in the signal strength per transmission distance. Moreover, over the lossy wireless medium, combining wireless and wireline channels drastically reduces the total reliability of the communication fabric. Surface wave has been proposed as an alternative wireless technology for low power on-chip communication. With the right design considerations, the reliability and performance benefits of the surface wave channel could be extended. In this paper, we propose a surface wave communication fabric for emerging WiNoCs that is able to match the reliability of traditional wireline NoCs. First, we propose a realistic channel model which demonstrates that existing mm-Wave WiNoCs suffers from not only free-space spreading loss (FSSL) but also molecular absorption attenuation (MAA), especially at high frequency band, which reduces the reliability of the system. Consequently, we employ a carefully designed transducer and commercially available thin metal conductor coated with a low cost dielectric material to generate surface wave signals with improved transmission gain. Our experimental results demonstrate that the proposed communication fabric can achieve a 5 dB operational bandwidth of about 60 GHz around the center frequency (60 GHz). By improving the transmission reliability of wireless layer, the proposed communication fabric can improve maximum sustainable load of NoCs by an average of 20:9 and 133:3 percent compared to existing WiNoCs and wireline NoCs, respectively.

In this paper, we propose beamforming schemes to simultaneously transmit data to multiple information receivers (IRs) while transfering power wirelessly to multiple energy-harvesting receivers (ERs). Taking into account the imperfection of the instantaneous channel state information, we introduce a probabilistic-constrained optimization problem to minimize the total transmit power while guaranteeing data transmission reliability, secure data transmission, and power transfer reliability. As the proposed optimization problem is non-convex and has an infinite number of constraints, we propose two robust reformulations of the original problem adopting safe-convex-approximation techniques. The derived robust formulations are in semidefinite programming forms, hence, they can be effectively solved by standard convex optimization packages. Simulation results confirm the superiority of the proposed approaches to a baseline scheme in guaranteeing transmission security.

This paper considers an underlay access strategy for coexisting wireless networks where the secondary system utilizes the primary spectrum to serve its users. We focus on the practical cases where there is uncertainty in the estimation of channel state information (CSI). Here the throughput performance of each system is limited by the interference imposed by the other, resulting in conflicting objectives. We first analyze the fundamental tradeoff between the tolerance interference level at the primary system and the total achievable throughput of the secondary users. We then introduce a beamforming design problem as a multiobjective optimization to minimize the interference imposed on each of the primary users while maximizing the intended signal received at every secondary user, taking into account the CSI uncertainty. We then map the proposed optimization problem to a robust counterpart under the maximum CSI estimation error. The robust counterpart is then transformed into a standard convex semi-definite programming. Simulation results confirm the effectiveness of the proposed scheme against various levels of CSI estimation error. We further show that in the proposed approach, the trade-off in the two systems modelled by Pareto frontier can be engineered by adjusting system parameters. For instance, the simulations show that at the primary system interference thresholds of -10 dBm (-5 dBm) by increasing number of antennas from 4 to 12, the secondary system throughput is increased by 3.3 bits/s/channel-use (5.3 bits/s/channel-use).

This paper investigates the security at the physical layer of cooperative relay communications. Inspired by the principle of physical-layer network coding (PNC), we propose a new secure relaying scheme, namely secure PNC-based modify-and-forward (SPMF). In the proposed scheme, the relay node linearly combines the decoded data from the source node with an encrypted key before conveying the mixed data to the destination node. As both the linear PNC operation and encrypted key at the relay are unknown to the eavesdropper, the SPMF scheme provides a double security level in the system. Particularly, taking into account the practical scenario of the imperfect knowledge shared between the relay and destination, the secrecy outage probability (SOP) of the proposed SPMF scheme is analysed and evaluated in comparison with modify-and-forward, cooperative jamming, decode-and- forward and direct transmission schemes. The proposed scheme is shown to achieve a performance improvement of up to 3 dB when compared to the conventional schemes under imperfect knowledge of shared information between the nodes.

Device to Device (D2D) communication has been proposed as an underlay to Long-Term evolution (LTE) network as a means of harvesting the proximity, reuse and hop gains. However, D2D communication can also serve as a technology for providing public safety and disaster relief services. In this article, the basic concepts of D2D communications are first introduced and then existing fundamental works on disaster communication are discussed. We focus on the performance of the network architecture by utilizing the relay assisted transmission which can effectively enhance the capacity and power saving of the network. We also propose the distance based strategy to reduce the computational complexity and power transmission. Finally, simulation results are provided to verify the proposed architecture.

The downlink problem of a cloud‐based central station (CCS) to multiple base stations (BSs) in a heterogeneous cloud radio access network (HCRAN) sharing the same time and frequency resources is studied. The authors adopt nonorthogonal multiple access (NOMA) and propose power allocation for the wireless downlink in the HCRAN. Taking into account practical channel modelling with power consumptions at BSs of different cell types (e.g. macrocell, micro‐cell etc.) and backhauling power, they analyse the energy efficiency (EE) of the practical HCRAN utilising NOMA. Simulation results indicate that the proposed NOMA for the HCRAN outperforms the conventional orthogonal frequency division multiple access scheme in terms of providing higher EE of up to four times. Interestingly, the results reveal a fact that the EE of the NOMA varies as a quasi‐concave function. An optimisation problem is accordingly introduced to find the optimal number of BSs that maximises the EE of the HCRAN. It is shown that, with a low power supply at the CCS, a double number of micro BSs can be served providing an improved EE of up to 1.6 times compared with the macro BSs and remote radio heads, while they achieve the same EE performance with high‐power CCS.

This study investigates cooperative spectrum sensing (CSS) in cognitive wireless radio networks (CWRNs). A practical system is considered where all channels experience Nakagami‐ m fading and suffers from background noise. The realisation of the CSS can follow two approaches where the final spectrum decision is based on either only the global decision at fusion centre (FC) or both decisions from the FC and secondary user (SU). By deriving closed‐form expressions and bounds of missed detection probability (MDP) and false alarm probability (FAP), the authors are able to not only demonstrate the impacts of the m ‐parameter on the sensing performance, but also evaluate and compare the effectiveness of the two CSS schemes with respect to various fading parameters and the number of SUs. It is interestingly noticed that a smaller number of SUs could be selected to achieve the lower bound of the MDP rather using all the available SUs while still maintaining a low FAP. As a second contribution, they propose a SU selection algorithm for the CSS to find the optimised number of SUs for lower complexity and reduced power consumption. Finally, numerical results are provided to demonstrate the findings.

This paper investigates double-threshold-based energy detector for cooperative spectrum sensing mechanisms in cognitive wireless radio networks. We first propose a hybrid double-threshold-based energy detector (HDTED) to improve the sensing performance at secondary users (SUs) by exploiting both the local binary/energy decisions and global binary decisions feedback from the fusion center (FC). Significantly, we derive closed-form expressions and bounds for the probabilities of missed detection and false alarm considering a practical scenario where all channel links suffer from Rayleigh fading and background noise. The derived expressions not only show the improved performance achieved with the HDTED scheme but also enable us to analyze the impacts of the number of the SUs and the fading channels on the cooperative spectrum sensing performance. Furthermore, based on the derived bounds, we propose an optimal SU selection algorithm for forwarding the local decisions to the FC, which helps reduce the number of forwarding bits for a lower-complexity signaling. Finally, numerical results are provided to demonstrate the validity of the analytical findings.

In this paper, we study simultaneous data transmission and power transfer for multiple information receivers (IRs) and energy-harvesting receivers (ERs) in cellular networks. We propose an optimization problem to maximize a linear combination of the useful signal power at each IR and total received power at each ER subject to the following three sets of constraints: i) data reliability by maintaining the required level of signal to interference plus noise ratio (SINR) for all IRs; ii) information security by keeping leakage SINR levels of all IRs at every ER below a predefined value, which helps prevent possible eavesdroppers, i.e., ERs, from detecting information aimed for the IRs; and iii) power consumption by guaranteeing that the transmit power at each base station is within its available budget. Using semidefinite relaxation (SDR) technique, we then transform the proposed problem into a convex form. We further prove that the transformed problem always yields rank-one optimal solution. Hence, our approach does not require any computationally expensive randomization procedure, as usually utilized in a typical SDR method, in obtaining optimal solutions. Simulation results indicate that the proposed approach prevails conventional scheme in terms of providing higher IRs' total throughput, higher received power level at each ER and more secure for the transmission of information.

The rapid advances in technology lead to a mass-market adoption of the multi-radio high-end smartphone devices and an exponential increase in the mobile broadband data traffic. In the search for a solution to cope with this explosion of data traffic, Device-to-Device (D2D) communications become an attractive approach for enhancing the performance of cellular networks. The emerging of WiFi Direct framework introduced new opportunities for mobile-to-mobile opportunistic offloading. In this context, this paper proposes ECO-M, an Energy-efficient Cluster-Oriented solution for Multimedia delivery in a LTE D2D environment using WiFi Direct. Experimental test-bed measurements and numerical results show the efficiency of such a solution in terms of energy efficiency, battery discharge and battery lifetime when compared with a standard LTE environment. Subjective tests were carried out to study not only the impact of user perceived quality on the various multimedia quality levels choices but also the impact of user preferences in terms of energy conservation versus multimedia quality.

This paper investigates downlink performance of a heterogeneous wireless cellular network (HWCN) consisting of multi-tier cells operating within various indoor and outdoor propagation environment models. The coverage probability of the practical HWCN is derived using stochastic geometry modeling. The derived expressions not only characterise the effects of building infrastructures and transmission environment on the coverage region in practice but also enable us to propose an optimal power allocation at cells in different tiers. Specifically, a heuristic algorithm is developed for the power allocation at the tiered cells within various propagation models to minimise the total power consumption. The proposed scheme is shown to achieve a lower total power consumption saving up to 45% for a given target coverage probability of a specific region when compared to the equal power allocation technique.

Due to the ever growing mobile broadband data traffic over the cellular networks, the small cell deployment is seen as a promising solution for the network operators to increase their network capacity at low cost. This in turn would lead to an increase number of handovers (HOs) for the mobile users, which could affect the device power consumption. In this context, this paper investigates the impact of the HO process on the device energy consumption while performing VoD over a real LTE Small Cell experimental environment. Subjective tests are carried out to study the impact of the video quality on the user perceived QoE. The results show that by changing the quality level of the multimedia stream the energy can be greatly conserved while the user perceived QoE is still acceptable. Furthermore, by adapting to a lower quality level during the HO process, up to 56% energy savings could be achieved.

This paper proposes a hybrid double-threshold based energy detector (HDTED) for cooperative spectrum sensing over imperfect channels in cognitive wireless radio networks (CWRNs). The proposed HDTED improves the sensing reliability in CWRNs by exploiting both local decisions at secondary users (SUs) in either binary or energy values and global binary decisions feedback from fusion centre. Particularly, we consider a practical scenario where all channel links suffer from fading and noise. By deriving closed-form expressions and bounds for the probabilities of missed detection and false alarm, we not only show the improved performance achieved with the HDTED scheme but also validate the effects of the number of the SUs and the fading channels on the overall sensing performance.

This paper investigates power allocation for reliable downlink transmission in cellular networks subject to quality-of-service (QoS) constraints. The reliability of data transmissions is assured by hybrid automatic repeat request protocol with incremental redundancy (HARQ-IR). By developing and evaluating the effective throughput of the HARQ-IR protocol for the downlink transmission under buffer-limited QoS constraints, we propose a novel adaptive power allocation algorithm to efficiently exploit the available resources when only limited channel state information (CSI) is available. The proposed algorithm, based on monotonic optimisation, allows near-optimal performance to be achieved with reasonably low complexity.

This paper investigates power allocation for reliable downlink transmission in cellular networks subject to quality-of-service (QoS) constraints. The reliability of data transmissions is assured by hybrid automatic repeat request protocol with incremental redundancy (HARQ-IR). By evaluating the effective throughput of the HARQ-IR protocol for the downlink transmission under QoS constraints, we propose a novel power allocation policy to efficiently exploit the available resources. Furthermore, based on the derived effective throughput, optimization problem is formulated and solved, in which all channels are effectively allocated to all users. Finally, numerical results are provided to support theoretical findings.

One of the main challenges towards reliable multicast transmissions over wireless networks is the dynamics of the wireless links (e.g. wireless errors, fading, interference, collisions, etc.) that can cause retransmissions overhead over the limited available bandwidth. To this end this paper considers the scenario of wireless multicast networks where network coding is applied to improve network throughput. We first propose a novel cross-layer optimisation framework for network topology design in order to optimise the wireless multicast rate, data flow of the wireless links, energy supply and node lifetime. The performance of the proposed solution is evaluated and compared against other solutions from the literature in terms of system throughput, total energy, and network lifetime. The results show that the proposed cross-layer design outperforms the other schemes involved, reaching up to 50% increase in the system throughput.

This paper is concerned with cooperative spectrum sensing (CSS) in cognitive wireless radio networks (CWRNs). A practical scenario is investigated where all channels suffer from Nakagami-m fading. Specifically, we analyse the probabilities of missed detection and false alarm for two CSS schemes where the collaboration is carried out either at fusion centre (FC) only or at both the FC and secondary user (SU). By deriving closed-form expressions and bounds of these probabilities, we not only show that there are significant impacts of the m-parameter of Nakagami fading realisation for different channel links on the sensing performance but also evaluate and compare the effectiveness of the two CSS schemes with respect to various fading parameters and the number of SUs. Finally, numerical results are provided to validate the theoretical analysis and findings.

This paper investigates double-threshold based energy detection for cooperative spectrum sensing in cognitive wireless radio networks. We first propose a hybrid double-threshold based energy detector (HDTED) to improve sensing performance at the secondary users (SUs) by exploiting both their local decisions and the global binary decisions at the fusion centre (FC). We then derive closed- form expressions for the probability of missed detection and the probability of false alarm considering a practical scenario where all channel links suffer from Rayleigh fading. The derived expressions not only show the improved performance but also enable us to develop an optimal SU selection algorithm for forwarding the local decisions to the FC. Finally, numerical results are provided to demonstrate the validity of the analytical findings.

This paper investigates relay selection for reliable data transmission in relay-assisted multisource multicast networks (RMMNs) where multiple source nodes distribute information to a set of destination nodes with the assistance of multiple relay nodes. Hybrid automatic repeat request with incremental redundancy (HARQ-IR) is used and supported by either a physical-layer network coding (PNC) or an analog network coding (ANC) technique employed at the relays. By deriving efficiency metrics of the HARQ-IR protocols, we propose relay selection schemes for RMMNs to minimize the transmission delay and energy consumption. Simulation results are provided to analyse each relay selection scheme.

In this study, the authors consider cooperative transmission in cognitive wireless relay networks (CWRNs) over frequency‐selective fading channels. They propose a new distributed space‐time–frequency block code (DSTFBC) for a two‐hop non‐regenerative CWRN, where a primary source node and multiple secondary source nodes convey information data to their desired primary destination node and multiple secondary destination nodes via multiple cognitive relay nodes with dynamic spectrum access. The proposed DSTFBC is designed to achieve spatial diversity gain as well as allow for low‐complexity decoupling detection at the receiver. Pairwise error probability is then analysed to study the achievable diversity gain of the proposed DSTFBC for different channel models including Rician fading and mixed Rayleigh–Rician fading.

This paper considers channel quality indicator (CQI) reporting for data exchange in a two-way multi-relay network. We first propose an efficient CQI reporting scheme based on network coding, where two terminals are allowed to simultaneously estimate the CQI of the distant terminal-relay link without suffering from additional overhead. In addition, the transmission time for CQI feedback at the relays is reduced by half while the increase in complexity and the loss of performance are negligible. This results in a system throughput improvement of 16.7% with our proposed CQI reporting. Upper and lower bounds of the mean square error (MSE) of the estimated CQI are derived to study performance behaviour of our proposed scheme. It is found that the MSE of the estimated CQI increases proportionally with the square of the cardinality of CQI level sets although an increased number of CQI levels would eventually lead to a higher data rate transmission. On the basis of the derived bounds, a low-complexity relay selection (RS) scheme is then proposed. Simulation results show that, in comparison with optimal methods, our suboptimal bound-based RS scheme achieves satisfactory performance while reducing the complexity at least three times in case of large number of relays.

In this paper, we consider cooperative transmission in cognitive wireless relay networks (CWRNs) over frequency-selective fading channels. We propose a new distributed space-time-frequency block code (DSTFBC) for a two-hop nonregenerative CWRN, where a primary source node and multiple secondary source nodes convey information data to their desired primary destination node and multiple secondary destination nodes via multiple cognitive relay nodes with dynamic spectrum access. The proposed DSTFBC is designed to achieve spatial diversity gain as well as allow for low-complexity decoupling detection at the receiver. Pairwise error probability is then analysed to study the achievable diversity gain of the proposed DSTFBC for different channel models including Rician fading and mixed Rayleigh-Rician fading.

This paper considers relay positioning in wireless butterfly networks consisting of two source nodes, a relay node and two destination nodes. The relay-assisted data transmission from two source nodes to two destination nodes is carried out based on either physical-layer network coding (PLNC) or analog network coding (ANC) techniques. The reliability of communications in wireless network-coded butterfly networks (WNCBNs) is assured by applying hybrid automatic repeat request protocol with incremental redundancy (HARQ-IR). In this paper, we first evaluate the total energy consumption (EC) of both PLNC and ANC schemes in WNCBNs. Based on the derived EC, we develop algorithms that optimize the location of the relay node for PLNC and ANC schemes in order to minimize the total EC for a green communication system subject to node location and power allocation constraints. Finally, simulation results are provided which identify the optimized relay locations for various scenarios of node location and power allocation in the WNCBN.

In this paper, we investigate the energy-delay tradeoff (EDT) of data transmissions in wireless butterfly networks consisting of two source nodes, a relay node and two destination nodes. Two network coding (NC) techniques, namely physical-layer NC (PLNC) and analog NC (ANC), are exploited to reduce the number of transmissions at the relay by half. Additionally, hybrid automatic repeat request with incremental redundancy (HARQ-IR) is applied to assure the reliability of data transmissions in wireless network-coded butterfly networks (WNCBNs). By deriving the EDT of both PLNC and ANC schemes in WNCBNs, we evaluate not only their energy efficiency, but also the effectiveness of the relay positions in WNCBNs. It is found that the PLNC scheme is more energy efficient than both the ANC and direct transmission (DT) schemes when the relay is located either at the centre or close to the destinations. In addition, it is shown that both the PLNC and ANC schemes are shown to be less energy efficient than the DT scheme when the relay is located near the sources.

This study is concerned with cooperative spectrum sensing (CSS) mechanisms in three‐hop cognitive wireless relay networks (CWRNs). The data transmission from a source to a destination is realised with the aid of two layers of cognitive radio (CR) users which are in the transmission coverage of two primary users. The authors first propose a new CSS scheme for a layer of CR users to improve the spectrum‐sensing performance by exploiting both local decisions at the CR users and global decisions at the fusion centre. Particularly, the probabilities of missed detection and false alarm for a practical scenario where all sensing, reporting and backward channels suffer from Rayleigh fading are derived. The derived expressions not only show that the proposed CSS achieves a better sensing performance than the conventional scheme but also characterise the effects of the fading channels on the sensing reliability. Furthermore, the authors propose a CSS scheme for two CR layers in a three‐hop CWRN using binary operations to help reduce one phase of sensing for a higher system throughput.

This paper considers retransmission mechanisms in wireless regenerative relay networks. The retransmission can be realized in a cooperative manner with the assistance of all available relays. This may result in a high overall power consumption due to a considerable number of overlapped packets in the retransmission, especially when the number of relays is large. In this paper, we propose a cooperative retransmission (CR) scheme based on relay cooperation and binary XOR operation to significantly reduce the number of packets to be retransmitted for a more power efficient system with non-overlapped retransmission. Particularly, we derive the error probability of the retransmission decision at the source and relays, which shows that the proposed CR scheme also improves the reliability of the retransmission. Finally, simulation results are presented to verify the analytical findings and show that the proposed CR scheme reduces the number of retransmissions and removes overlapped retransmitted packets.

This paper is concerned with cooperative spectrum sensing (CSS) mechanisms in cognitive wireless relay networks (CWRNs). Conventionally, the decision of available spectrum for cognitive radio (CR) users is carried out based on the global decision at a fusion center (FC). In this paper, we propose a new CSS scheme to improve the spectrum sensing performance by exploiting both local decisions at the CR users and global decisions at the FC. Particularly, by deriving the probabilities of missed detection and false alarm for a practical scenario where all sensing, reporting, and backward channels suffer from Rayleigh fading, we show that our proposed CSS scheme achieves an improved sensing performance over the conventional scheme. Furthermore, we propose a CSS scheme based on network coding (NC) for a specific CWRN where the transmission from a source node to a destination node is realized with the aid of two groups of CR users located in the transmission coverage of two primary users (PUs). The proposed NC-based CSS scheme helps reduce one phase of sensing for a higher system throughput compared to the conventional scheme which requires eight phases in total to monitor all available spectrums of both PUs.

This paper considers data exchange between two terminals in a nonregenerative two-way relay network. We first propose two efficient channel quality indicator (CQI) reporting schemes based on XOR and superposition coding for single-relay networks. These schemes allow two terminals to simultaneously estimate the CQI of the distant link without incurring additional overhead. In addition, the transmission time for CQI feedback is reduced by half while the loss of performance is negligible. Upper and lower bounds of the mean square error (MSE) of the estimated CQI are derived to analyze various effects on the performance of the proposed schemes. We then extend our MSE analysis to multi-relay networks where a low-complexity relay selection scheme is proposed based on the derived bounds. Simulation results show that, in comparison with conventional methods, this suboptimal bound-based scheme achieves satisfactory performance while reducing the complexity at least three times in case of large number of relays.

Distributed space-time block codes (DSTBCs) have been developed to exploit diversity gains in cooperative communications systems. In this paper, we propose a new DSTBC based on unique word (UW) extension, referred to as the D-UW-STBC. The D-UW-STBC is devised for wireless relay networks with the amplify-and-forwarcl protocol and frequency selective fading channels. The data is transmitted from the source to the destination by a block-wise manner, resembling the block level of the Alamouti scheme. At the source, a UW is padded to the tail of each data block, which plays as a cyclic prefix, and thus allows for low-complexity implementation in the frequency domain. The additional purpose of using UW in this paper is to estimate the channels. However, the introduction of UWs makes it difficult to decouple the detection of data blocks. To achieve the orthogonality of the equivalent space-time channel, we carefully cancel the interference induced by the UWs. Also, a least square (LS) channel estimation approach is proposed, in which, the relay is transparent to the source and destination. The optimal UWs arc designed to minimise the effect of additive noise. Bit-crror-ratc (BER) performance comparison of various receiver schemes is carried out by computer simulations.

This paper is concerned with block acknowledgement (ACK) mechanisms in wireless regenerative relay networks. In an N-relay network, a total of (2N+1) block ACK packets is required to acknowledge the data transmission between source and destination nodes via the N-relay nodes. In this paper, the authors propose a block ACK scheme based on network coding (NC) to significantly reduce the ACK overheads by N block ACK packets. In addition, this achieves a reduction of N(N–1) computational operators. Particularly, we derive the error probability of the determination of the packets to be retransmitted at the source and relays, which shows that the NC-based scheme also improves the reliability of block ACK transmissions. Furthermore, asymptotic signal-to-noise (SNR) scenarios for forward links are considered and a general expression of error probability in multi-relay networks is derived for each SNR scenario. Finally, simulation results are presented to verify the analytical findings and demonstrate a lower number of data retransmissions for a higher system throughput.

This paper considers block acknowledgement (BACK) mechanisms in wireless regenerative relay networks. Conventionally, a total of (2N+1) BACK packets are required in wireless regenerative N-relay networks to acknowledge the data transmission between the source and destination via the relays. In this paper, based on network coding (NC) we propose an efficient BACK scheme to significantly reduce the required acknowledgement load by N BACK packets. Importantly, by deriving the error probability of the determination of the packets to be retransmitted at the source or relays, it is found that the utilised NC helps improve the reliability of BACK transmissions. Furthermore, some asymptotic scenarios of the forward links are considered and a general expression of error probability in multi-relay networks is derived for each scenario. Finally, simulation results are shown to verify the analytical findings.

This paper proposes the distributed space-time block code (D-STBC) over frequency selective fading channel that achieves both spatial diversity gain and low decoding complexity with decoupling property in amplify-and-forward (AF) relay networks. These two goals are simultaneously achieved by our proposed code design, where the source permutates and conjugates the transmitted data before sending to the second relay. The diversity gain is investigated by analyzing the pairwise error probability (PEP) of the proposed scheme. In the analysis, we assume that one hop is line-of-sight (LOS) transmission modeled by Rician fading and the other hop is non-line-of-sight (NLOS) transmission experienced by Rayleigh fading. For each case, the PEP is derived based on the corresponding fading channel model.

This paper proposes a new distributed space-time block code (DSTBC) for single-carrier communications systems based on training sequence (TS). The proposed DSTBC is devised for amplify-and-forward (AF) relay networks over frequency selective fading channels. The data is transmitted from the source to the destination by a block-wise manner, resembling the block level of Alamouti scheme. A TS is padded to the tail of each information-bearing data block. The use of TS in this paper is twofold. First, it plays as a cyclic extension, thereby allowing us to construct a low-complexity receiver in the frequency domain. Second, it can be used for channel estimation. In contrast to zero padding technique, the introduction of TS makes it difficult to decouple the detection of data blocks. To recover the orthogonality of the equivalent space-time channel, we carefully treat the interference induced by the training sequences. As the second contribution, a least square (LS) channel estimation approach is also proposed for the proposed DSTBC. The optimal training sequences are designed to minimize the effect of additive noise. Bit-error-rate (BER) performance comparison is carried out by computer simulation.