The device-to-device (D2D) communications effectively reduce the load of base station and solves the problem of communication resource shortage. Thanks to the relay-assisted communication technology, D2D can expand the coverage and improve the reliability. But there may not be sufficient relay users to match each D2D user. On the other hand, in order to save the user resources, reduce energy consumption, and expand the access number of the cell users, this paper constructs a system model for multiple pairs of D2D users to communicate through one relay, and describes the session process and data transmission process while network coding (NC) is introduced to promote the performance. For the purpose of further improving system performance, this paper proposes a relay selection method that combines the relay location, communication content, residual energy and system capacity. The proposed method not only has a low complexity (easy for implementing), but also can greatly increase the achievable system capacity after network coding assisting communication and relay selection, which is proved through numerical results.

设备对设备（D2D）通信有效地减轻了基站的负担，解决了通信资源短缺的问题。借助中继辅助的通信技术，D2D可以扩大覆盖范围并提高可靠性。但是可能没有足够的中继用户来匹配每个D2D用户。另一方面，为了节省用户资源，减少能耗，扩大小区用户的接入数量，本文构建了多对D2D用户通过一个中继进行通信的系统模型，并描述了会话过程。同时引入了网络编码（NC）以提高性能的数据传输过程。为了进一步提高系统性能，本文提出了一种中继选择方法，该方法将中继位置，通信内容，剩余能量和系统容量。数值结果证明，该方法不仅复杂度低（易于实现），而且可以大大提高网络编码后的通信和中继选择的可实现系统容量。