# Cooperative Crossing Cache Placement in Cache-Enabled Device to Device-Aided Cellular Networks
## 1 Introduction
In [23], the average probability of successful file transmission was maximized to determine the success of file transmission in a wireless cache-enabled system of small cells. The authors in [24] described a probabilistic cache placement scheme that takes into account the cache-aided throughput; the scheme measures the density of successfully served requests through the local cache of the requesting user or through nearby devices via D2D transmission. However, these works ignored quality of service (QoS) requirements for communication (i.e., delay), which can change the optimal cache placement in BSs and users. Nevertheless, significant work has been done on D2D cache-enabled networks to minimize delay. In [25], the author investigated the network performance of D2D communication with a delay constraint, by modeling delay as the total time for transmission, retransmission and network access. The authors in Reference [26] proposed a D2D underlaid cellular network that minimized the average transmission delay in both cellular and D2D modes. In [27], a cooperative D2D caching scheme for a 5G wireless network was used to minimize the delay of content retrieval, in which cached content would be received by the requesting users from the caches of both users and the BS. Further, the authors in [27] enhanced the system performance by allowing multiple content delivery methods. However, a delay assignment scheme is a separate problem, which is out of the scope of the present paper. To the best of our knowledge, [28] is the only work to propose the successful transmission probability in a D2D cache-enabled network while guaranteeing successful content transmission within a certain time. Whereas, we focus our attention on analyzing the system performance in the cooperative crossing cache placement scheme while guaranteeing the successful content transmission, with considering the impact of transmission device availability, to facilitate maximization of the total offloading probability. More specifically, the main contributions of this paper are summarized as follows:  
1. We consider a cache-enabled D2D-aided cellular network, in which content delivery traffic can be offloaded via the local cache; if it is not present in the local cache or the user has no cache ability, it is offloaded via a D2D link; otherwise, via a cellular link, in consideration of the impact of transmission device availability on the effectiveness of the cache placement scheme. We model the device availability, which indicates whether a transmission device can handle the requested content within a required sending time while achieving optimal probabilistic caching. 2. We formulate the cooperative crossing cache placement problem, aiming to maximize the offloading probability for the network. In contrast to the cache placement scheme in [21], the cooperation content offloading is derived while guaranteeing the successful content transmission by D2D transmitters or BSs. 3. We analyze the optimization of the proposed scheme by exploiting the structure of difference of convex (DC) functions and an easily implemented algorithm with low complexity is employed through DC programming. 4. We conduct simulations that show that the proposed scheme outperforms Most Popular Content (MPC) scheme [29] and Coop. BS/D2D caching scheme [30] in terms of the offloading probability. The remainder of this paper is organized as follows. In Section 2, we describe the system model. In Section 3, we present the offloading probability analysis. We optimize the cooperative crossing cache placement problem in Section 4. In Section 5, simulation results are discussed and the conclusions of the study are presented in Section 6.