# Cooperative Crossing Cache Placement in Cache-Enabled Device to Device-Aided Cellular Networks
## 1 Introduction  
The proliferation of smartphones in the last decade has led to a sharp increase in data traffic, challenging the capacity of network infrastructure and mobile devices. To cope with this amount of traffic, Device-to-Device (D2D) communications become a key 5G feature for enhancing the performance of cellular network and supporting a variety of use-cases such as network offloading, public safety, Internet of Things (IoT) and Vehicle-to-Everything (V2X) communication [1]. In [2], the relationship between offloading gain of the system and energy cost of each helper user in cache-enabled D2D communications was presented and a user centric protocol to control the energy cost for a helper user to transmit a file was introduced. The best trade-off between energy consumption and the quality of the multimedia stream was provided in Reference [3] where theD2D communications make use of the unlicensed spectrum (e.g., WiFi Direct). In [4], public safety network and D2D communications were investigated to provide the optimal communication route for network. In addition, the authors showed a new step to the provisioning phase for network survivability against network failure and can be executed in an incremental fashion. Ref. [5] proposed a novel policy for device caching that combines the emerging technologies of D2D and mmWave communication to enhance the offloading and content retrieval delay performance of the cellular network. In [6], the caching and scheduling policies were optimized to maximize the probability of successful offloading and an optimal scheduling factor for a random scheduling policy to control network interference was obtained. In [7], the traffic offloading was maximized by an interference aware reactive caching mechanism. A problem of configuring a set of transmitters to provide service coverage to a set of receivers where the signal-to-interference ratio (SIR) representing service coverage condition was proposed in [8; 9]. Caching has been emphasized as an efficient technique for offloading data traffic by storing content at the network edge (e.g., in base stations (BSs) or devices), handling wireless access requests directly, rather than fetching content from the core network [10]. The quest for a suitable caching strategy, cooperative content offloading prevents redundancies in user demands and reduces duplicate content transmissions [11; 12]. Upon receiving a request for content, a device obtains the requested content from the local storage if the content is locally cached; otherwise, it obtains the content from devices through device-to-device (D2D) communication. If there is no device nearby, a BS handles the requested content. By caching content based on popularity in BSs with high storage capacities, the traffic load in the core network is reduced considerably [13]. It is worth noting that devices with the desired cached content are not always available for content retrieval, owing to user uncertainty (e.g., storage capacity constraints, resource constraints and limitations to battery consumption) and this can reduce the service quality. Several recent studies have focused on various cache placement scenarios in wireless D2D caching networks, namely cache placement through D2D communication [14; 15; 16; 17], cache placement through cellular communication (i.e., caching in BSs) [18; 19; 20] and cache placement in heterogeneous cache-enabled networks [21; 22]. An optimal caching placement scheme was developed to maximize the offloading probability in [21], in which offloading originates from local caching, D2D sharing and helper transmission. The throughput of the cache-enabled network was characterized in terms of the average ergodic rates and outage probability in a three-tier network [22]. The average probability of successful content delivery was derived in [20], where it was necessary to quantify the success of content delivery during caching placement.