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With the aim of improving the spectrum efficiency, we propose a two-phase cellular non-orthogonal and device-to-device (D2D) non-orthogonal (CNDN) transmission scheme, wherein a base station (BS) can transmit data to a near-user and a far-user simultaneously assisted by two D2D users. Either the D2D transmitter or the D2D receiver can adaptively forward the BS data to the far-user according to their decoding statuses. When the D2D transmitter forwards the cellular data, it also transmits its own data to the D2D receiver simultaneously. The superposition coding technique is adopted for the non-orthogonal transmission and the opportunistic interference cancelation is performed for the data decoding. Taking into account the adaptive operations of cellular and D2D users, we analyze the throughput of both cellular and D2D systems. Extensive simulations show that the proposed CNDN scheme greatly outperforms the cellular orthogonal and D2D non-orthogonal (CODN) transmission scheme in terms of throughput. 1.Related worksDevice-to-device (D2D) communications enable adjacent devices to interchange data directly via bypassing the BS so as to reduce the cellular traffic load and provide more connection opportunities for more neighboring devices. 4 The short-distance D2D communications can lower down the transmit power and reduce the interference to other users. Furthermore, much higher spectrum efficiency (SE) can be achieved by avoiding the two-hop cellular transmission with the

Relay-assisted transmission could effectively enhance the performance of Device-to-Device (D2D) communications when D2D user equipments (UEs) are too far away from each other or the quality of D2D channel is not good enough for direct communications. Meanwhile, security is one of the major concerns for proximity services. The secure relay selection problem for D2D communications underlaying cellular network is studied in this paper. Firstly, we define a relay selection area and derive the closed-form of outage probability in D2D links using a Poisson Point Process (PPP) method. Next, in the defined relay selection area, we propose a secure relay selection scheme for the relay-assisted D2D communication system by exploiting the social relation as a security factor. Simulation results show that the scheme based on social relation can greatly improve the security performance of relay-assisted D2D communications.

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