Abstract-This paper proposes a full-duplex cooperative device-todevice (D2D) communication system, where the relay employed can receive and transmit signals simultaneously. We adopt such a system to assist with D2D transmission. We first derive the conditional cumulative distribution function and the probability density function (pdf) of a series of channel parameters when the interference to the base station is taken into consideration and power control is applied at the D2D transmitter and the relay node. Then, we obtain an exact expression for the outage probability as an integral and as a closed-form expression for a special case, which can be used as a good approximation to the general case when residual selfinterference is small. Additionally, we also investigate the power allocation problem between the source and the relay and formulate a suboptimal allocation problem, which we prove to be quasi-concave. Our analysis is verified by the Monte Carlo simulations, and a number of important features of full-duplex cooperative D2D communications can, thereby, be revealed. Index Terms-Cooperative device-to-device (D2D) communications, full-duplex system, outage performance, power allocation. Underlay device-to-device (D2D) communication coexisting with traditional cellular communication has been a frequent topic of research in both academia and industry for years because of its high power efficiency, high spectral efficiency, and low transmission delay [1]- [3] . Meanwhile, cooperative communication has also gained interest, since it can effectively enhance network reliability and performance [4] . Recently, researchers have tried to combine the merits of both communication systems and have proposed the concept of cooperative D2D communication [5] . However, most recent works treat only the combination of D2D communication with half-duplex relays, which will degrade the system throughput by a fraction due to the use of multiple orthogonal time or frequency slots for one complete transmission. On the other hand, full-duplex relaying is capable of overcoming this shortcoming, but at the cost of producing residual self-interference (SI) [6] . A simplified full-duplex D2D network model is proposed in [7] . The effects of residual SI are analyzed and a numerical optimization of the total transmit power in this full-duplex D2D network model is carried out without presenting analytical results in [8] . Power allocation problems in full-duplex D2D networks are analyzed in [9] . However, all aforementioned works have not considered cooperative relaying between the D2D transmitter and receiver, which restricts the reliability and effectiveness of D2D communication. A cooperative D2D network with a half-duplex relay is analyzed in [10] , which exhibits undesirable performance characteristics. A two-pair case in which a transmitter in one pair can assist as a full-duplex relay for the other pair when idle is analyzed in [11] . However, the model considered in that paper is oversimplified and the interference between two pairs is not considered. The most relevant network model related to full-duplex cooperative D2D communication is proposed and analyzed in [12] . However, that paper makes a number of assumptions, e.g., the authors suppose that a relay node is able to transmit the separated signals to two destinations simultaneously by different powers without considering mutual interference. These assumptions can be viewed as impractical in some circumstances. To provide a comprehensive study of a full-duplex cooperative D2D system, we analyze the outage performance of a novel full-duplex cooperative D2D communication system in which a relay is able to assist the D2D pair only. Our analysis is verified by Monte Carlo simulations. The contributions of this paper can be summarized as follows. 1) We propose a full-duplex relay-assisted D2D communication system, in which power control and the interference from the cellular user equipment (CUE) to the relay and the D2D receiver are considered. 2) We obtain a single integral expression for the end-to-end outage probability of the proposed system, as well as a closed-form approximation to the outage probability when residual SI is small. 3) We formulate a suboptimal power allocation method that is easily implemented due to its quasi-concave nature. The rest of this paper is organized as follows. In Section II, we present the system model. Then, we analyze the outage performance and power allocation problem of the proposed system in Section III and verify the analysis by simulations in Section IV. Finally, the paper is concluded in Section V. The model of the proposed full-duplex cooperative D2D system is given in Fig. 1 , where one base station (BS), one CUE, 1 one D2D user equipment (DUE) transmitter, one DUE receiver, and one fullduplex relay 2 are considered. They are denoted as B, C, S, D, and R, respectively, and are organized in the set Θ = {B, C, S, D, R}. Therefore, ∀i = j and i, j ∈ Θ 3 , the channel gain denoted as G ij is 1 This one-CUE assumption is validated by the scenario in which multiple CUEs are assigned resource blocks in modern cellular systems, and thus we would only expect to receive interference from at most one user in a cell [13] . 2 The full-duplex relay is capable of transmitting and receiving simultaneously, while other nodes are assumed to be half-duplex in this paper. 3 An exception is given by i = j = R, and G R R is employed to denote the instantaneous loop channel gain, leading to residual SI. 0018-9545
In REF , an outage performance analysis of FD relay-assisted D2D systems in a cellular network was provided.
Outage Performance Analysis of Full-Duplex Relay-Assisted Device-to-Device Systems in Uplink Cellular Networks
IEEE Transactions on Vehicular Technology
IEEE Transactions on Vehicular Technology
Computer Science