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This study investigates the outage probability of device-to-device communication assisted by a relay node utilising a one-way amplify-and-forward relaying strategy. The authors assume that all the terminals are equipped with a single antenna and all the users know perfect channel state information. They first derive the exact closed-form expression for characterising the outage probability performance of the system. They subsequently discuss several special scenarios and obtain the asymptotic results for each of the considered scenarios. The results can be easily computed with only the channel statistics. Based on the analysis in the high signal-to-noise ratio regime, closed-form power allocation policies are developed to improve the outage probability performance. The author's analytical results are validated via Monte Carlo computer simulations.

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This paper studies the outage probability of deviceto-device (D2D) communication aided by another D2D user using the two-way amplify-and-forward (AF) relaying protocol. We first discuss the outage behavior under strong and weak interference from the cellular network. Then the exact expressions for the outage probability under the two cases are derived. Based on these results, we give tight approximations in the high signalto-noise (SNR) regime under the two cases. Numerical results show that the outage behavior for the relay aided D2D link can be greatly enhanced without extra power. Analytical results are validated via comparisons with the Monte-Carlo simulations. I. INTRODUCTIONNext generation wireless networks are expected to provide various innovative services ubiquitously. The requirements for these services urge for more spectral efficient communications [1]. Many efforts have been spent on this and device-to-device (D2D) communication underlaying cellular networks has recently received much attention [2,3]. In D2D communications, the direct link between two users is enabled using the licensed band instead of through a base station (BS). This means that reliable communication can be obtained by very short distance communications at low transmit power [4].To avoid the harmful inter-channel interference (ICI), several approaches to suppress interference from the cellular link have been proposed. An intuitive way is to ensure orthogonal resource allocation for both D2D and the cellular links [6]. A simple transmit power control method utilizing a backoff value was proposed by the authors in [5]. According to the interference level, three reception modes for D2D users were presented in improving the outage behavior which, however, requires the use of the BS to get the information of interference [7]. In [8], beamforming and interference cancellation were considered with a multi-antenna BS in the downlink (DL).On the other hand, user cooperation has emerged as an effective means for offering significant performance benefits, e.g., harnessing diversity gains [9] and extending coverage without enhancing the source transmit powers. The performance of relay channels was first studied in [10] and by now the protocols such as amplify-and-forward (AF), decode-andforward (DF) and compress-and-forward (CF) are very well known [11,12]. Two-way relaying protocol was subsequently proposed in which two terminals exchange messages via a relay node. Due to the pre-log factor of 1/2 for half-duplex relaying protocols, two-way relay networks have attracted