Joint relay selection and power allocation protocol for two-way relaying system

In this paper, relay selection, subcarrier pairing and power allocation are investigated jointly to maximize the channel capacity for OFDM-based cooperative networks under a total power constraint. The problem can be formulated as a mixed binary integer programming problem, which is NP-hard and very difficult to manipulate. In order to achieve an acceptable performance with a proper complexity, we first solve the relay selection and subcarrier pairing problem under equal power allocation. Then, power allocation is performed according to the equivalent channel power gains of the subcarrier pairs using water-filling. Simulations are carried out to validate our analysis and show that the proposed scheme can achieve an acceptable capacity with a proper complexity.

This paper investigates an unmanned aerial vehicle (UAV) enabled full-duplex relaying system. By assuming that the UAV follows a circular trajectory and applies decode-and-forward relaying strategy, we study the joint design of beamforming and power allocation to maximize the instantaneous data rate, under both the individual and the sum power constraints over the source and relay nodes. As the problem is non-convex, we propose an efficient sub-optimal solution based on block-coordinate descent method by decomposing the problem into two sub-problems: a beamforming optimization sub-problem with given power allocation and a power allocation sub-problem with fixed beamforming. For the beamforming design sub-problem, the optimal solution is obtained based on the semi-definite relaxation technique. For the power allocation sub-problem, the optimal solution is obtained in closed form. Then, the closed-form cumulative distribution function and outage probability expressions for sub-optimal beamforming with both uniform power allocation and optimal power allocation are derived. In addition, simple and informative high signal-to-noise ratio (SNR) approximations for outage probability expressions are presented to gain insights. Finally, the optimal flying altitude that minimizes the average outage probability is obtained via one-dimensional search.

A novel cross-layer joint relay selection and power allocation (CL-JRSPA) scheme over physical layer and data-link layer is proposed for cooperative relaying system in this paper. Our goal is finding the optimal relay selection and power allocation scheme to maximize system achievable rate when satisfying total transmit power constraint in physical layer and statistical delay quality-of-service (QoS) demand in data-link layer. Using the concept of effective capacity (EC), our goal can be formulated into an optimal joint relay selection and power allocation (JRSPA) problem to maximize the EC when satisfying total transmit power limitation. We first solving optimal power allocation (PA) problem with Lagrange multiplier approach, and then solving optimal relay selection (RS) problem. Simulation results demonstrate that CL-JRSPA scheme gets larger EC than other schemes when satisfying delay QoS demand. In addition, the proposed CL-JRSPA scheme achieves the maximal EC when relay located approximately halfway between source and destination, and EC becomes smaller when the QoS exponent becomes larger.

This paper studies the performance limits of two-way relay channel (TWRC) at finite signal-to-noise ratio (SNR) in Rayleigh fading environment. A two-phase decode-and-forward (DF) protocol is considered. We first derive closed-form expressions for both outage probability and diversity-multiplexing tradeoff (DMT). Our results are general and suitable for any time sharing and any rate allocation in the two-way relay protocol. It is found that DF outperforms amplify-and-forward (AF) when either multiplexing gain or SNR is small enough, otherwise, DF is inferior to AF in term of outage probability. Meanwhile, finite-SNR DMT of DF is always lower than that of AF regardless of SNR due to the additional sum-rate constraint imposed on the relay node for full decoding. Furthermore, the optimum relay location for any given combination of time sharing and rate allocation is presented.

Physical-layer network coding (PNC) promises a significant gain In overall network throughput for multi-user cooperative communications. In a multi-unicast scenario, one drawback associated with PNC is an additional noise term, coined as network coding (NC) noise, which severely degrades the system data rate. In this paper, our contribution to address this challenging problem is source and relay power control with the objective of maximization of the minimum average achievable rate among all the source-destination pairs subject to a given total power constraint. We further develop a joint power allocation and relay selection scheme, which only rely on long-term channel statistics, to extend our results to general network topologies. We show that the joint optimization problem can be divided into two problems: optimal power allocation and optimal relay selection where the former is scalable and leads to a power assignment algorithm that exhibits the same optimization complexity for any number of sources in the network and the latter can be performed in a decentralized manner. By means of simulations, we validate our theoretical developments and verify the efficiency of our algorithm in improving the average achievable rate compared to a multi-unicast system with no power control or relay selection. We conclude that the proposed algorithm largely combats the adverse effects of NC noise while achieving near optimal fairness.

A joint relay selection and power allocation scheme to maximise energy efficiency with restriction on the minimum spectral efficiency requirement for an amplify-and-forward two-hop relay network is presented. A closed-form solution of the low-complexity power allocation for such a system has been derived, which is easy to implement and has the potential to find wide application in green communications. Some numerical examples are provided to validate the analysis.

In this paper, the ergodic sum rate and the outage probability of a downlink single-antenna channel with K users are analyzed in the presence of Rayleigh flat fading, where limited channel state information (CSI) feedback is assumed. Specifically, only 1-bit feedback per fading block per user is available at the base station. We first study the ergodic sum rate of the 1-bit feedback scheme and consider the impact of feedback delay on the system. A closed-form expression for the achievable ergodic sum rate is presented as a function of the fading temporal correlation coefficient. It is proved that the sum rate scales as log log K, which is the same scaling law achieved by the optimal nondelayed full CSI feedback scheme. The sum-rate degradation due to outdated CSI is also evaluated in the asymptotic regimes of either large K or low SNR. The outage performance of the 1-bit feedback scheme for both instantaneous and outdated feedback is then investigated. Expressions for the outage probabilities are derived, along with the corresponding diversity-multiplexing tradeoffs (DMTs). It is shown that, with instantaneous feedback, a power allocation based on the feedback bits enables doubling of the DMT compared with the case with short-term power constraint in which a dynamic power allocation is not allowed. However, with outdated feedback, the advantage of power allocation is lost, and the DMT reverts to that achievable with no CSI feedback. Nevertheless, for finite SNR, improvement in terms of outage probability can still be obtained.

In view of the scarce spectrum resources and inconvenient deployment in wireless communications, this work focuses on an unmanned aerial vehicle (UAV) enabled non-orthogonal multiple access (NOMA) system where a UAV deployed as a base station serves two users by NOMA. A joint power allocation and aerial jamming (PAAJ) scheme is proposed to achieve reliable and secure communications for the system in the presence of a malicious eavesdropper. To be specific, dynamic power allocation is adopted to ensure the reliability of the system and another friendly UAV jammer is introduced to improve the security of the system. The closed-form expressions of connection outage probability (COP), secrecy outage probability (SOP), and effective secrecy throughput (EST) are derived to evaluate the performance of the security-required user in the practical condition of probabilistic line-of-sight (LoS) and non-line-of-sight (NLoS) air-to-ground (A2G) channels. Moreover, the asymptotic EST is analyzed to obtain further insights. Numerical results show the validity of theoretical derivations and the superiority of the proposed PAAJ scheme over benchmark schemes. The EST of the UAV-enabled NOMA communication system outperforms the conventional terrestrial NOMA system at the low transmitting power from the UAV base station. There exists an optimal height of the UAV base station to maximize the EST, which provides a useful framework for designing UAV-enabled communication systems with heterogeneous service requirements.

This paper investigates the outage performance of simultaneous wireless information and power transfer (SWIPT) in two-way relay channels, where two sources exchange their information via an energy harvesting (EH) relay with digital network coding (NC) employed. The relay harvests energy from the received radio-frequency (RF) signals transmitted from the two sources and then use it to forward the received signals. For such a system, we study the power splitting-based two-way relay (PS-TWR) transmission strategy and analytically derive an explicit expression of the system outage probability. In order to explore the system outage performance limit, we formulate an optimization problem to minimize the system outage probability by jointly optimizing the power splitting ratio and the time assignment ratio. Since the problem is non-convex and have no known solution method, we present a genetic algorithm (GA) to solve it. Numerical results demonstrate our theoretical analysis and show that the power splitting ratio and the time assignment ratio greatly affect the system outage performance. Moreover, it is also shown that for the SWIPT-enabled two-way relay system, the relay node should be deployed closer to the source with better channel quality and when it is positioned on the middle point between the two sources, the system gets the worst outage performance, which is very different from traditional non-SWIPT two-way relay systems.

We investigate the outage performance of a user scheduling with antenna selection (USAS) scheme for a cellular multiuser two-way relay system over a Rayleigh flat-fading environment. Here, a multi- antenna base station communicates with several mobile users via an amplify-and-forward (AF) relay. The USAS strategy opportunistically selects the best user among multiple users and the best antenna at the base station for bidirectional communication such that the overall system outage probability is minimized. We derive an exact expression for outage probability of the considered scheme that is applicable for all values of signal-to-noise ratio (SNR). To gain better insights and identify key parameters, we present an approximate expression of outage probability in closed-form under a high SNR assumption. Finally, we highlight the impact of antenna selection on the achievable diversity of the considered scheme under various channel conditions. Numerical and simulation results are presented to confirm the performance gain of the considered scheme.

We investigate the joint problem of relay selection and optimal sharing of relay power in wireless cellular networks with multicast traffic. We use two different performance metrics to maximize the network performance. We first present a mixed Boolean-convex optimization model to maximize the overall network capacity and solve this combinatorial problem optimally using branch and bound technique. We then show that obtaining the optimal solution is computationally not feasible for large network sizes and, unlike the case of unicast traffic, a water filling method does not yield near optimal solutions in multicast scenarios. We thus adopt an algorithm based on sequential fixing which substantially reduces the computation time and achieves near optimal solutions. In addition, we present a mixed integer linear programming model to maximize the capacity of the minimum capacity link and show that the model is very efficient to reach the optimal solutions.

This paper studies the case of an overlay cognitive radio network where the primary user leases spectral resources to the secondary user in exchange for cooperation, considering that both type of users have specific quality of service requirements. We investigate the problem of joint power and time allocation for the secondary access during the cooperative phase, with a view to optimizing the effective capacity of the primary user given an average energy constraint for the secondary user. Afterwards, the optimal power allocation of the secondary user for its own transmission phase is investigated in order to maximize the effective capacity of the secondary link. The proposed joint power and time allocation mechanism is compared with an optimal time/constant power allocation scheme and a less sophisticated baseline allocation scheme, i.e. power allocation under constant time and its superiority is proven for various network parameters. The reference model of one primary---one secondary user is extended to a general multi user cognitive radio network through the proposed pairing mechanism based on matching theory. Particularly, considering the remarks of the reference scenario, we propose two different matching schemes (with/without consideration of primary users' quality of service requirements) and we confirm their superiority compared to other matching mechanisms.

In this paper, a joint subcarrier and power allocation algorithm is proposed to improve the physical layer security in cooperative orthogonal frequency division multiple access (OFDMA) networks, where several source-destination pairs and one untrusted relay are involved. The relay is friendly and intends to help these pairs to enhance their communications. However, it may be overheard by a malicious eavesdropper at the same time. To optimize the joint subcarrier and power allocation with low complexity, we divide the optimization problem into two simpler subproblems. Firstly, the subcarriers are assigned to the source-destination pairs by employing the dual approach under the assumption that the relay power is equally allocated to all the subcarriers. Then, the relay power is allocated to the subcarriers based on the alternative ascending clock auction mechanism. In addition, we prove that the two subproblems can converge after a finite number of iterations. We also find that the proposed auction is cheat-proof and, thus, can avoid the cheating behaviors in the auction process. Numerical results demonstrate that our proposed algorithm can effectively improve the system sum secrecy rate, and the convergence performance is also desirable.

A new structure for a regenerative two-way relay (TWR) system, called a wired TWR (WTWR) system is introduced. The WTWR consists of two relay nodes that are connected through an optical link, and it operates as a single conventional TWR node. For the WTWR system, beamforming vectors maximising the sum rate of two source/destination nodes (i.e. users) are designed. The WTWR exploiting the proposed beamforming is applicable to various systems that can employ a TWR protocol, such as heterogeneous networks, distributed antenna systems, and coordinated multipoint transmission systems.

The explosive demands for mobile broadband service bring a major challenge to 5G wireless networks. Device-to-device communication, adopting side links for user-direct communication, is regarded as a main technical source for offloading large volume of mobile traffic from cellular base station. This article investigates the joint power and subcarrier allocation scheme for device-to-device communication in 5G time division duplex systems. In time division duplex system, instead of utilizing an exclusive portion of the precious cellular spectrum, device-to-device pairs reuse the subcarriers occupied by cellular users, thus producing harmful interference to cellular users in both uplink and downlink communication, and strongly limiting the spectrum efficiency of the system. To this end, we focus on the maximization of device-to-device throughput while guaranteeing both uplink and downlink channel quality of service of cellular users as well as device-to-device pairs. The problem is formulated as a mixed integer non-linear programming (MINLP) problem. To make it tractable, we separate the original MINLP problem into two sub problems: power allocation and sub-carrier reusing. The former is to develop optimal power allocation for each device-to-device pair and each cellular user, with the constraints of maximum power and quality of service. It is solved by geometric programming technique in convex optimization method. The latter is derived as a one-to-many matching problem for scheduling multiple subcarriers occupied by cellulars to device-to-device pairs. It is solved by Hungarian method. Simulation results show that the proposed scheme significantly improves system capacity of the device-to-device underlay network, with quality of service of both device-to-device users and cellular users guaranteed.