I. Introduction
Device-TO-DEVICE (D2D) communication [1] as an emerging communication technology is of great potential to alleviate capacity-shortage problem by offloading traffic from base stations. Besides, cooperative communication [2] (CC) is another new type of wireless communication technology, in which devices can help each other to relay information and consequently improve the spatial diversity. As a result, the network capacity can be enhanced. The integration of D2D and CC can better improve the network capacity. Recently, it is shown in [3] and [4] that cooperative D2D (CD2D) communications can improve the performance of both D2D communications and infrastructure communications at base stations. However, most of current studies on D2D, CC and CD2D communications only consider (i) homogeneous network settings and (ii) associations of devices to a single network. Both considerations (i) and (ii) are neither practical nor applicable to the heterogeneous and multi-accessing features of next generation networks [5], [6].