Relay Transmission Power Research Articles

Performance Analysis of Soft-Switching FSO/THz-RF Dual-Hop AF-NOMA Link Based on Cognitive Radio

This paper presents a promising solution to address the scarcity of spectrum resources and enhance spectrum efficiency in the context of cognitive radio (CR)-based soft-switching free-space optical (FSO)/terahertz (THz) radio frequency (RF) dual-hop amplify-and-forward (AF)–non-orthogonal multiple access (ROMANO) links. The impact of maximum tolerable interference power in the primary network, transmit power in the secondary transmitter, and maximum relay transmission power on the link are thoroughly studied. The numerical results ultimately validate the effectiveness of this link in improving performance, and a comparative analysis is conducted with the without-CR scheme, highlighting the distinctive characteristics of the proposed link.

Node Scheduling for AF-Based Over-the-Air Computation

Over-the-air computation is a promising technique for efficiently aggregating data in sensor networks. This method requires that signals from all nodes arrive at the sink aligned in signal magnitude, which faces the reliability issue, especially in times of channel fading. To solve this problem, in this letter, we propose an amplify-and-forward based relay, <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Coherent</u> <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Relay</u> with <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Node</u> <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Scheduling</u> (CohR-NS), where a relay node is used to help forward signals of multiple nodes. Relay transmission power (TP) increases with the number of nodes using the relay, which is a bottleneck. We investigate how relay TP changes with relay position, and under the constraint of relay TP, study (i) how to select nodes to use relay when not all nodes requiring a relay can be supported simultaneously, (ii) how to select more nodes to use relay so as to reduce node TP, when there is a surplus in relay TP. We formulate this as an ILP (integer linear programming) problem, propose an efficient heuristic method, and confirm its effectiveness by simulation evaluation.

Energy Efficiency of Full- and Half-Duplex Decode-and-Forward Relay Channels

This article studies the energy efficiency (EE) of a three-node decode-and-forward (DF) relay channel, where the relay operates in full-duplex (FD) or half-duplex (HD) mode. In particular, for the FD mode, both the residual self-interference (RSI) and the extra circuit power consumption introduced by the self-interference cancellation (SIC) at the relay node are considered and modeled as linear functions over the relay transmission power. Under this setup, the EE maximization problems for the considered FD and HD modes subject to the total transmission power and spectral efficiency (SE) constraints are formulated as max–min problems, whose optimal power allocation is computed by fractional programming. Next, the EEs for the FD, HD, and direct transmission (DT) modes are compared under different channel conditions, and a hybrid mode selection scheme, i.e., selecting the one with the maximum EE among the above three modes, is proposed. Besides, we also prove that both the EE and the SE monotonically increase with the total transmission power under a given condition. Finally, numerical results show that the maximum EE of the FD relaying exceeds those of the HD and DT ones when the required SE is relatively high. Furthermore, the proposed hybrid mode selection scheme can provide up to 192.4% higher EE compared with the DT mode.

Energy-efficiency maximization for fixed-wing UAV-enabled relay network with circular trajectory

With the development of Unmanned Aerial Vehicles (UAVs), the applications of UAVs have been extensively explored. In the field of wireless communications, the relay nodes are often used to extend network coverage. However, traditional fixed ground relays cannot be flexibly deployed due to their low heights and fixed locations. Hence, deploying UAV as relay node is a promising solution and has become a research hotspot. In this paper, we consider an UAV-enabled relaying network in which a fixed-wing UAV is deployed between the Base Station (BS) and Ground Users (GUs). We study the energy-efficiency gap between the link “BS-UAV-GUs” and the link “BS-GUs”, and jointly optimize UAV relay transmission power and flight radius to achieve the highest energy-efficiency. Firstly, the UAV/BS-GUs channels models and the UAV energy consumption model are built. Secondly, the optimization objective function is formulated to maximize the energy-efficiency gap. Then, the solution of the optimization problem is divided into a two-step iteration process, in which the UAV relay transmission power and flight radius are adjusted to maximize the energy-efficiency gap. Finally, the experimental results under different simulation scenarios (such as cities, forests, deserts, oceans, etc.) are shown to illustrate the effectiveness of the proposed algorithm. The results show that the proposed algorithm can always find the optimal UAV relay transmission power and flight radius settings, and achieve the largest energy-efficiency gap. The convergency speed of the proposed algorithm is fast, and can obtain the optimal solution within only a few iterations.

Impacts of Imperfect CSI and Transceiver Hardware Noise on the Performance of Full-Duplex DF Relay System With Multi-Antenna Terminals Over Nakagami-m Fading Channels

In this paper, we investigate the performance of a full-duplex (FD) relay system where multi-antennas are exploited at source and destination. Unlike previous works, the impacts of imperfect channel state information (I-CSI), transceiver hardware noise (THN), and residual self-interference (RSI) are taken into account. We mathematically derive the exact closed-form expressions of the outage probability (OP), symbol error rate (SER), and ergodic capacity (EC) of the FD relay system with I-CSI, THN, and RSI over Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> fading channels. From the derived expressions, the performance of the considered system under the effects of three negative factors (I-CSI, THN, and RSI) is compared with that system in the case of all ideal factors (perfect channel state information (P-CSI), perfect transceiver hardware (P-TH) and perfect self-interference cancellation (P-SIC)), two ideal factors (P-CSI and P-TH, P-CSI and P-SIC, P-TH and P-SIC), or one ideal factor (P-CSI or P-TH or P-SIC). Numerical results show a strong impact of three negative factors on the OP, SER, and EC of the considered FD relay system, especially when the data transmission rate of the system and signal-to-noise ratio (SNR) are high. In particular, OP, SER, and EC go to the floors in the high SNR regime due to the three negative factors. Therefore, when I-CSI, THNs, and RSI exist in the FD relay system, we should use suitable source and relay transmission power to obtain excellent performance while saving energy consumption. Moreover, when two of the three negative factors are large enough, the remaining factor’s impact becomes weaker and may be neglected in certain circumstances.

Artificial Noise-Aided Secure Relay Communication With Unknown Channel Knowledge of Eavesdropper

In this article, a new relay-aided secure communication system is investigated, where a transmitter sends signals to a destination via an amplify-and-forward (AF) relay in the presence of an eavesdropper. We consider a general system configuration, where the source, relay, destination, and eavesdropper are all equipped with multiple antennas. In the practical scenarios of unknown eavesdropper's channel state information (CSI) and uncertainty of the eavesdropper's location, we aim to maximize the expected value of the system secrecy rate over the presumed distribution of the eavesdropper's channels, by exploiting the artificial noise (AN) transmitted by the source and relay nodes. The system design issue is formulated as a nonconvex stochastic optimization problem with a source transmission power constraint and a nonconvex relay transmission power constraint. A novel computational method is proposed to solve this challenging problem. The new method is developed based on an exact penalty function method together with a parallel stochastic decomposition algorithm. Numerical simulations are performed to study the effectiveness of the proposed scheme at various locations of the eavesdropper. Simulation results show that for most cases, secure communication can be achieved without the CSI knowledge of eavesdropper's channels, and the achievable secrecy rate follows the trend of a benchmark system where the eavesdropper's full CSI is available. In particular, the achievable system secrecy rate increases with the number of antennas at the legitimate users. Moreover, the optimal power allocated for the transmission of the AN increases with the system signal-to-noise ratio. The proposed computational method achieves a higher system secrecy rate than a conventional penalty function based approach.

Reliable Over-the-Air Computation by Amplify-and-Forward Based Relay

In typical sensor networks, data collection and processing are separated. A sink collects data from all nodes sequentially, which is very time consuming. Over-the-air computation, as a new diagram of sensor networks, integrates data collection and processing in one slot: all nodes transmit their signals simultaneously in the analog wave and the processing is done in the air. This method, although efficient, requires that signals from all nodes arrive at the sink, aligned in signal magnitude so as to enable an unbiased estimation. For nodes far away from the sink with a low channel gain, misalignment in signal magnitude is unavoidable. To solve this problem, in this paper, we investigate the amplify-and-forward based relay, in which a relay node amplifies signals from many nodes at the same time. We first discuss the general relay model and a simple relay policy. Then, a coherent relay policy is proposed to reduce relay transmission power. Directly minimizing the computation error tends to over-increase node transmission power. Therefore, the two relay policies are further refined with a new metric, and the transmission power is reduced while the computation error is kept low. In addition, the coherent relay policy helps to reduce the relay transmission power by half, to below the limit, which makes it one step ahead towards practical applications.

Joint power and subcarrier allocation for SWIPT in cognitive relay system

In this paper, we study power and subcarrier allocation in decode-and-forward (DF) cognitive relay network based on orthogonal frequency division multiple access (OFDMA) technology, in which power splitting (PS) is used for the secondary users (SUs) supporting simultaneous wireless information and the power transfer (SWIPT). We attempt to maximize the energy efficiency (EE) of the secondary system subject to the constraints of minimum system rate, maximum transmission power and minimum harvested energy. The optimal power allocation (OPA) algorithm is proposed to determine the optimal solution of relay transmission power, power splitting ratio and subcarrier allocation. To tackle the original non-convex optimization problem, the Dinkelbach method is used for fractional programming and the problem is decomposed into two solvable sub-problems. The Lagrangian dual algorithm is utilized to determine the optimal transmission power and the power splitting ratio. The Hungarian algorithm is then introduced to determine the optimal subcarrier allocation. Compared with some existing algorithms, the simulation results show that the proposed OPA scheme is able to achieve higher energy efficiency when certain communication quality is required.

Partial Interference Alignment Scheme for Three-Tier Downlink Heterogeneous Networks

The introduction of the relay in heterogeneous networks (HetNets) can enhance cell-edge signals, but it will also bring coexistence of multiple interference which is composed of co-tier, cross-tier and cross time slot (caused by the half-duplex mode of the relay) interference, resulting in the ineffectiveness of the existing interference alignment (IA) schemes. To solve the above problem, the partial IA scheme, which achieves IA in the case of limited antennas by rationally designing the order and method of eliminating interference, is proposed in this paper. First, we align the interference from other base stations (BSs) into the interference space generated by pico BS to reduce the interference dimension of the relay, and then solve the interference between BSs and users by null space and concatenated precoding schemes. Furthermore, the feasibility condition of antenna configuration is analyzed. The simulation results show that the proposed scheme is superior to the traditional IA scheme in sum rate, and the performance of the proposed scheme is further improved as the relay transmission power increases.

Energy Efficiency of Amplify-and-Forward Full-Duplex Relay Channels

In this letter, the energy efficiency (EE) for a three-node full-duplex (FD) relay channel with the amplify-and-forward (AF) scheme is studied. In particular, both the residual self-interference (RSI) and the circuit power consumption for imperfect self-interference cancellation (SIC) are considered and modeled as linear functions over the relay transmission power. Under this setup, the EE maximization problem for the AF FD relay channel is formulated and efficiently solved. Moreover, the EE performance of the direct transmission (DT), half-duplex (HD), and FD modes are compared under different channel conditions.

Resource allocation for wireless information and power transfer based on WBAN

In this paper, a simultaneous wireless information and power transfer (SWIPT) system with a relay in wireless body area network (WBAN) is studied, where the relay harvests energy from the radio frequency (RF) signals sent by the access point (AP), then uses RF energy to transmit energy to the user and forward the information to the AP, respectively. Firstly, we propose a planar transmission model based on the time switching (TS) and power splitting (PS) protocols. The sensor nodes are placed on the human torso in this model. In order to explore the system performance, the optimal information throughput for the two protocols is given subject to the energy harvesting and consumption balance. Then, corresponding optimal power splitting and time switching ratios which maximize the information throughputs are obtained. Besides, the effects of the relay position on system performance are also discussed. Finally, while considering the relay transmission power constraint, we solve the dynamic power allocation problem by the Lagrange multiplier iteration method. Numerical results and analysis show that our proposed optimal strategy achieves the optimal throughput. Moreover, the TS protocol has better performance than the PS protocol in dynamic model.

Short-Packet Two-Way Amplify-and-Forward Relaying

This letter investigates an amplify-and-forward two-way relay network (TWRN) for short-packet communications. We consider a classical three-node TWRN consisting of two sources and one relay. Both two time slots (2TS) scheme and three time slots (3TS) scheme are studied under the finite blocklength regime. We derive approximate closed-form expressions of sum-block error rate (BLER) for both schemes. Simple asymptotic expressions for sum-BLER at high signal-to-noise ratio (SNR) are also derived. Based on the asymptotic expressions, we analytically compare the sum-BLER performance of 2TS and 3TS schemes, and attain an expression of critical blocklength, which can determine the performance superiority of 2TS and 3TS in terms of sum-BLER. Extensive simulations are provided to validate our theoretical analysis. Our results discover that 3TS scheme is more suitable for a system with lower relay transmission power, higher differences between the average SNR of both links and relatively lower requirements on data rate and latency.

Cognitive Bidirectional Buffer-Aided DF Relay Networks: Protocol design and Power Allocation

In this paper, we consider bidirectional decode-and-forward buffer-aided relay selection and transmission power allocation schemes for underlay cognitive radio relay networks. First, a low complexity delay-constrained bidirectional relaying protocol is proposed. The proposed protocol maximizes the single-hop normalized sum of the primary network (PN) and secondary network (SN) rates and controls the maximum packet delay caused by physical layer buffering at relays. Second, optimal transmission power expressions that maximize the single-hop normalized sum rate are derived for each possible transmission mode. Simulation results are provided to evaluate the performance of the proposed relaying protocol and transmission power allocation scheme and compare their performance with that of the optimal scenario. Additionally, the impacts of several system parameters including maximum buffer size, interference threshold, maximum packet delay and number of relays on the network performance are also investigated. The results reveal that the proposed bidirectional relaying protocol and antenna transmission power allocation schemes introduce a satisfactory performance with much lower complexity compared to the optimal relay selection and power allocation schemes and provide an application dependent delay-controlling mechanism. It is also found that the network performance degrades as the delay constraint is more restricted until it matches the performance of conventional unbuffered relaying with delay constraints of three. Additionally, findings show that using buffer-aided relaying significantly enhances the SN performance while slightly weakens the performance of the PN.

Relay Selection Algorithm for Cognitive Relay Network

In the cognitive relay network, due to the different fading characteristics of communication links, not all relay nodes have constructive impacts on the system performance. A relay selection algorithm is proposed to obtain the minimum relay transmission power while satisfying the secondary user SINR requirements and interference temperature constraints. The algorithm first introduces l1 norm as the approximation of l0 norm, then constructs a minimum relay transmission power constraint problem to select appropriate relays. Simulation results show that the algorithm can efficiently conduct relay selection with mild system transmission power increase over traditional full cooperation cognitive relay network.

Energy‐efficient relaying technique for two‐hop diamond networks

Energy-efficiency (EE) is a critical metric in many types of wireless networks and is getting increasingly important for new technologies. In this paper, the authors propose an energy-efficient decode-and-forward relaying strategy for two-hop diamond networks where the source–destination communication is established by the help of two relays which have an inter-relay link. The inter-relay link gives the opportunity of correcting the data packet between relays if only one of them decodes the packet that is received from the source correctly. Hence, both relays are able to send the data packet to the destination with distributed Alamouti coding which provides multipath diversity. In addition, they introduce optimum data rate adaptation for each link separately to maximise EE for the considered network. By exploiting diversity and optimum rate adaptation, it is possible to transmit at higher rates with reasonable outage probability which has critical effects on EE and throughput. They show via computer simulations that the proposed technique is substantially more energy-efficient and has a better throughput than the successive relaying technique given in the literature; moreover, more robust to the changes in system conditions, i.e. relay location and transmission power level.

Subcarrier-Pairing-Based Resource Optimization for OFDM Wireless Powered Relay Transmissions With Time Switching Scheme

In this paper, we investigate the joint resource optimization for orthogonal frequency division multiplexing relay transmissions, where the relay uses time switching (TS) scheme for wireless information and power transfer. We aim to maximize the total rate under several constraints by adjusting the subcarrier pairing (SP), the source transmission power, the relay transmission power, and the TS factor. Different from most of the previous works that ignore the energy consumption for signal receiving and processing at relay, we take into account a realistic energy consumption model. We formulate the joint resource allocation problem as a mixed integer nonlinear programming problem, which is generally difficult to solve. First, we give the feasible condition of the considered problem. Based on this condition, we prove the ordered SP is the optimal SP, and thus SP and the rest of the resource allocation problem (RRAP) can be separately solved without loss of optimality. Then, a searching algorithm is proposed to find the global optimal solution to the RRAP. To improve the efficiency, a method is proposed to reduce the searching region to a smaller one, which is proven to contain the optimal solution. Next, to further reduce the computational complexity, we subtly transform the RRAP to an equivalent problem, where the objective function is a D.C. function (difference of concave functions). By exploiting the partial convex structure, we propose an efficient and fast algorithm. Finally, simulations verify the superior performance of the proposed algorithms comparing with related ones in terms of total rate.

Effect of low‐resolution ADCs and loop interference on multi‐user full‐duplex massive MIMO amplify‐and‐forward relaying systems

This paper focuses on a multi-user full-duplex massive multiple-input multiple-output amplify-and-forward relaying system with low-resolution analogue-to-digital convertors (ADCs). By modelling the loop-interference and quantisation noise as additive noise, we perform the analyses of the achievable spectral efficiency (SE) by considering the two canonical processing schemes, maximum ratio combining/maximal ratio transmission (MRC/MRT) and zero-forcing receive/zero-forcing transmission (ZFR/ZFT), respectively. Specially, we first obtain the tractable closed-form expressions of the achievable SE. Then, we present the asymptotic analyses under three different power-scaling scenarios. Comparison analysis shows that the low-resolution quantisers impose more performance loss on ZFR/ZFT schemes than MRC/MRT ones. At the same time, it is found that with different power-scaling schemes, the systems have different capabilities to restrict the effect of loop interference and low-resolution ADCs. Specially, the impact of both the loop-interference and quantisers on achievable SE can be restricted effectively only under the power-scaling scenario where the relay's transmission power is scaled down with the number of transmit antennas and the sources’ transmission power is fixed. For the power-scaling case where the transmission powers of both relay and sources are scaled down simultaneously with the number of antennas, only the effect of the loop interference can be restricted, but the one of low-resolution quantisers remains.

Optimization of a MIMO amplify-and- forward relay system with channel state information estimation error and feedback delay

This paper addresses the robust design of a multiple-input multiple-output amplify-and-forward relay system against channel state information (CSI) mismatch due to estimation error and feedback delay. The estimation error and feedback delay are expressed using appropriate models, from which we derive the conditional mean square error (MSE) between the desired and the received signals upon the estimated CSI. The conditional MSE is then minimized to optimize the relay beamforming matrix with relay transmission power constraint. It is shown that the proposed optimization problem reduces to the conventional minimum MSE problem when CSI mismatch vanishes. By analyzing the structure of the optimal beamforming matrix, the optimization problem is simplified so that it can be directly solved using the genetic algorithm (GA). To further reduce the computational load, we develop a relaxed version of the optimization problem. It is found that the relaxation enables us to efficiently solve the problem using water filling strategy. Computer simulations show that both GA and water filling solutions are superior to conventional solutions without CSI mismatch consideration, while the water filling is 1000 times faster than the GA.

State‐based potential power allocation game in a cooperative multiuser network

Multiuser cooperative communications represent a research area of great interest constituting a key technology for future wireless networks. In this study, the authors consider a scenario of multi-user, single-relay network, where each user employs a part of the relay's transmission power taking into account the constraint of the total transmission power of the relay. The problem of relay power allocation to individual users with a view to optimising the long-term network performance is investigated. An adequate utility function is defined, in terms of outage probability, evaluating the cooperative network's performance due to the relay's cooperation. Taking into consideration the distributed nature of cooperative networks along with the coupled constraint of relay's total transmission power, the specific problem is transformed from a network-level to a user-level optimisation problem using a modern game-theoretic tool. More specifically, the initial power allocation problem is formulated as a state-based potential game for which the corresponding Nash equilibrium exists. Finally, a gradient learning mechanism is proposed to achieve the stationary state Nash equilibrium of the game that constitutes also the global optimal solution of the initial problem. The performance of the new proposed scheme is investigated through numerical simulations for various network parameters.

Unified low‐layer power allocation and high‐layer mode control for video delivery in device‐to‐device network with multi‐antenna relays

This study proposes a quality-driven approach that jointly selects the frequency reuse mode of device-to-device (D2D) links at lower layers, the video coding mode of frames, and multiple paths at higher layers for multimedia transmission in a cellular network with multi-antenna relays placed at the intersection of adjacent cells. Based on power control of the shared relay, the user-centered mode selection problem is formulated by maximising the quality of service (QoS) with total energy consumption constrained. Underlay mode and overlay mode are both considered. For each frame, one coding mode (I, P, or B) and multiple paths (cellular links and/or D2D links with/without relays) are selected to optimise the received video quality expectation. Different retransmission polices are considered to give higher protection levels to paths with higher transmission rates. The authors’ evaluation shows that the proposed unified power allocation and mode control scheme could enhance the perceived QoS as changing the energy constraint and the distances of different paths. The underlay mode is optimal in most cases. It provides evidences that it is not always the best to code all frames as I-frames, to initialise the relay transmission power as the maximum, or to select one single path for video transmission.