In this paper, a novel cooperative non-orthogonal multiple access (NOMA) system is proposed, where one near user is employed as decode-and-forward relaying switching between full-duplex (FD) and half-duplex (HD) mode to help a far user. Two representative cooperative relaying scenarios are investigated insightfully. The first scenario is that no direct link exists between the base station (BS) and far user. The second scenario is that the direct link exists between the BS and far user. To characterize the performance of potential gains brought by the FD NOMA in two considered scenarios, three performance metrics outage probability, ergodic rate, and energy efficiency are discussed. More particularly, we derive new closed-form expressions for both exact and asymptotic outage probabilities as well as delay-limited throughput for two NOMA users. Based on the derived results, the diversity orders achieved by users are obtained. We confirm that the use of direct link overcomes zero diversity order of far NOMA user inherent to FD relaying. In addition, we derive new closed-form expressions for asymptotic ergodic rates. Based on these, the high signal-to-noise ratio (SNR) slopes of two users for FD NOMA are obtained. Simulation results demonstrate that: 1) the FD NOMA is superior to the HD NOMA in terms of outage probability and ergodic sum rate in the low SNR region; and 2) in delay-limited transmission mode, the FD NOMA has higher energy efficiency than the HD NOMA in the low SNR region; However, in delay-tolerant transmission mode, the system energy efficiency of the HD NOMA exceeds the FD NOMA in the high SNR region.

Exploiting Full/Half-Duplex User Relaying in NOMA Systems

Cooperative communication has used to be a hot topic and it has been studied extensively in the past 10 years, but in recent years, it becomes less likely to find substantial innovation in this field as before. In this paper, we propose a new hybrid decode-forward and amplify-forward with non-orthogonal multiple access (NOMA) (HDAF-NOMA) transmission scheme for a cellular system with multiple relays. To the best of our knowledge, this is the first work that attempts to integrate decode-forward (DF), amplify-forward, and NOMA into one strategy design to improve system performance. To verify the performance advantages, the proposed HDAF-NOMA scheme is compared with the other four traditional schemes in terms of channel capacity and average system throughput, and the optimal number of selected DF relays is also determined for the HDAF-NOMA scheme. Simulation results show that compared with the traditional schemes, the proposed HDAF-NOMA scheme can achieve larger sum channel capacity for the transmission of $x_{1}$ and $x_{2}$ , and it can also achieve larger average system throughput at high SNR region.

Hybrid Decode-Forward & Amplify-Forward Relaying With Non-Orthogonal Multiple Access

Edge computing enables data collected by Internet of Things (IoT) devices to be stored in and processed by local fog nodes as well as allows IoT users to access IoT applications via these nodes at the same time. In this case, the communications latency critically affects the response time of IoT user requests. Owing to the dynamic distribution of IoT users [i.e., user equipments (UEs)], drone base station (DBS), which can be flexibly deployed for hotspot areas, can potentially improve the wireless latency of IoT users by mitigating the heavy traffic loads of macro BSs. Drone-based communications poses two major challenges: 1) the DBS should be deployed in suitable areas with heavy traffic demands to serve more UEs and 2) the traffic loads in the network should be allocated among macro BSs and DBSs to avoid instigating traffic congestions. Therefore, we propose a traffic load balancing scheme in such drone-assisted fog network to minimize the wireless latency of IoT users. In the scheme, we divide the problem into two subproblems and design two algorithms to optimize the DBS placement and user association, respectively. Extensive simulations have been set up to validate the performance of the proposed scheme.

Towards Traffic Load Balancing in Drone-Assisted Communications for IoT

The wide application of Internet of Things (IoT) in industrial automation encourages the emergence of a new paradigm of industrial IoT systems, wireless control system (WCS), where the system and/or control information is delivered over wireless channels. In practical systems, WCSs would consist of multiple control-loops in general, the resource competition among which would seriously increase mutual interferences and transmission collisions, making it is difficult to provide the required transmission reliability for the control strategy. To address this issue, we design the control strategy together with the hybrid cooperative transmission scheme for multiloop WCSs in a proactive way. We first define the overall system cost function to explore the impacts of standard linear quadratic regulator control cost and wireless transmission reliability on the control performance. In order to further minimize the overall system cost while guaranteeing the control stability, we then propose a control performance aware cooperative transmission scheme, which is formulated as a constrained optimization problem. Decomposition method and heuristic algorithms are designed based on the feature of network structure to solve the formulated mixed integer nonlinear programming problem efficiently. Finally, simulation results demonstrate that by using the proposed strategy, the overall system cost is significantly reduced, decreasing by 78% and 82% compared to the cases without considerations of system dynamics and without cooperative transmission, respectively.

Control Performance Aware Cooperative Transmission in Multiloop Wireless Control Systems for Industrial IoT Applications

In this article, we consider an energy-harvesting (EH) relay system consisting of a source, a destination, and multiple EH decode-and-forward (DF) relays that can harvest the energy from their received radio signals. A power-splitting ratio is employed at an EH DF relay to characterize a tradeoff between the energy used for decoding the source signal received at the relay and the remaining energy harvested for retransmitting the decode outcome. We propose an optimal power-splitting-based relay selection (OPS-RS) framework and also consider the conventional equal power-splitting-based relay selection (EPS-RS) for comparison purposes. We derive closed-form expressions of outage probability for both the OPS-RS and EPS-RS schemes and characterize their diversity gains through an asymptotic outage analysis in the high signal-to-noise ratio region. We further examine an extension of our OPS-RS framework to an EH battery (EHB)-enabled cooperative relay scenario, where the EH relays are equipped with batteries used to store their harvested energies. We propose an amplify-and-forward (AF)-based EHB-OPS-RS scheme and a DF-based EHB-OPS-RS scheme for AF and DF relay networks, respectively. Numerical results show that the proposed OPS-RS always outperforms EPS-RS in terms of outage probability. Moreover, the outage probabilities of the AF- and DF-based EHB-OPS-RS schemes are much smaller than that of the OPS-RS and EPS-RS methods, demonstrating the benefit of exploiting the batteries in EH relays. Additionally, the DF-based EHB-OPS-RS substantially outperforms the AF-based EHB-OPS-RS and such an outage advantage becomes more significant, as the number of EH relays increases.

Joint Power Splitting and Relay Selection in Energy-Harvesting Communications for IoT Networks

A new relaying scheme with the combination of incremental decode-and-forward (DF) relaying and selective relaying employed hybrid decode-amplify-forward (HDAF), termed incremental HDAF relaying (IHDAF), is proposed and analyzed in this paper. Closed-form expressions of outage probability for the incremental HDAF relaying scheme is derived. Simulation results and analysis show that the incremental HDAF relaying scheme outperforms the incremental DF relaying scheme especially when the relay locates near the destination.

Performance analysis of hybrid DAF based incremental relaying cooperative system

In this paper, we propose a power allocation scheme to minimize the total power consumption in a decode-and-forward (DF) multi-relay cooperative system considering the required data rate. Based on the minimal transmit power of the source and the instantaneous channel state information (CSI) between the source and the relays, the optimal relays will be selected to reduce the total system power consumption. With the application of Lagrange multiplier technique together with relay selection, the power minimization problem can be solved. Simulation results show that the total power consumption of the proposed scheme is significantly reduced compared with that of the direct transmission and the best relay scheme in opportunistic relay selection (ORS).

Lagrange multiplier based power minimization algorithm with relay selection in cooperative system

In this paper, we introduce the opportunistic nonorthogonal amplify-and-forward (NAF) single-antenna cooperative system with blind relays over Rayleigh fading channel and the optimal power allocation is studied for this scheme. The optimal power allocation for NAF system is carried out with three steps based on the opportunistic relay selection (ORS) and its formulation is obtained to maximize the system capacity. With the help of extreme value algorithm, the power allocation factor can be assigned for the source and the selected relay node with a simple method. Simulation results show that the performance of the proposed optimal power allocation scheme outperforms the equal power allocation scheme and direct transmission as well as the orthogonal amplify-and-forward (OAF) scheme.

Optimal power allocation of opportunistic nonorthogonal amplify-and-forward cooperative systems with blind relay

In this paper, we propose and analyze a new relaying scheme for the three-node cooperative relaying system, named as incremental hybrid decode-amplify-forward relaying (IHDAF), where the relay may choose to keep silent or transmit message in decode-and-forward (DF) or amplify-and-forward (AF) mode based on the qualities of the channels among the source, relay, and destination. Closed-form expressions of the outage probability and bit error rate (BER) of the proposed IHDAF protocol are derived. The optimal relationship of the two important signal-to-noise ratio (SNR) thresholds at the relay and destination to decide whether cooperation is necessary and the cooperative mode has been achieved. Moreover, the effects of the power allocation schemes, SNR thresholds, and relay locations on the outage probability and BER of the IHDAF protocol are studied. Theoretical analysis and simulation results show that the IHDAF relaying scheme outperforms the incremental-selective DF (ISDF), incremental DF (IDF), and cooperative STBC schemes, especially when the relay is close to the destination.

Performance Analysis of SNR-Based Incremental Hybrid Decode-Amplify-Forward Cooperative Relaying Protocol

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Performance analysis of hybrid DAF based incremental relaying cooperative system

Lagrange multiplier based power minimization algorithm with relay selection in cooperative system

Optimal power allocation of opportunistic nonorthogonal amplify-and-forward cooperative systems with blind relay

Performance Analysis of SNR-Based Incremental Hybrid Decode-Amplify-Forward Cooperative Relaying Protocol