# Social-Aware-Based Resource Allocation for NOMA-Enhanced D2D Communications
## 2 System Model  
System model in this paper is shown in Figure 1. \(M\) cellular users, \(N\) D2D content requesters and \(K\) D2D content providers are randomly distributed in the cell which centered in the base station (BS). Cellular users \(\{C_{1},C_{2},\ldots,C_{M}\}\) communicate with the BS through dedicated channels \(\{SC_{1},SC_{2},\ldots,SC_{M}\}\). Each cellular user is assigned with a dedicated and orthogonal channel. Hence, it can be assumed that cellular user \(C_{m}\) is assigned with the channel \(SC_{m}\) without loss of generality. \(\{DR_{1},DR_{2},\ldots,DR_{N}\}\) is the set of D2D content requesters (receivers) and \(\{DT_{1},DT_{2},\ldots,DT_{K}\}\) is the set of D2D content providers (transmitters). There are \(F\) files that can be requested in the system. D2D content requesters ask for each file with a probability of \(p\). The request file set can be represented as \(\left\{req\right\}_{n}=\{e_{s}\}_{s\in F}\). \(e_{s}=1\) indicates \(File_{s}\) is requested, otherwise \(e_{s}=0\). Assume that all files are cached at the base station. D2D content providers cache each file with a probability \(q\). The cache file set can be represented as \(\left\{cache\right\}_{k}=\{e_{s}\}_{s\in F}\). \(e_{s}=1\) indicates \(File_{s}\) is cached, otherwise \(e_{s}=0\). A D2D content requester can obtain contents in two ways: (1) from D2D content providers around him through D2D communication. Through establishing D2D communication links with D2D content providers by reusing cellular user's channel, he can get content files from users in proximity. (2) From the BS through traditional cellular communication. The D2D content requester would choose the first way preferentially in order to reduce the cellular traffic pressure. By selecting appropriate D2D content providers to form a D2D cluster, D2D content requester can receive several desired contents from multiple D2D content providers in the cluster simultaneously through NOMA. D2D content requester correctly demodulates the received content from different D2D content providers with serial interference cancellation (SIC) technology and NOMA transmission protocol. If none of the D2D content provider in proximity has cached any file the requester wants, D2D content requester should choose the second way to obtain contents. After D2D clusters have been formed, each D2D cluster reuses the uplink cellular channel for D2D communication. We assume that each cellular channel can be reused by multiple D2D clusters while each D2D cluster can only reuse one cellular channel. D2D communication can be established through coordinating interference between cellular users and D2D users. This paper does not consider cellular users' power optimization problem, and the transmit power can be set to a fixed value. Due to the fact that mobile devices are dominated by users, it is more reasonable to pair D2D users by considering social relationship between them. According to the system model in physical domain in Figure 1, a corresponding system model in social domain can be establi  
Figure 1: System model (physical domain). In practical network environment, channel conditions along with system interference can be described with physical domain. In this paper, D2D users in D2D clusters communicate by reusing the uplink cellular channels. Therefore, D2D content providers may cause interference to the BS and D2D content requesters may suffer interference from cellular users. We can represent the system model in physical domain as a graph \(G\big{(}V_{p},E_{p}\big{)}\), where \(V_{p}\) indicates users and \(E_{p}\) denotes the data transmission quality for channel. \(G\big{(}V_{p},E_{p}\big{)}\) indicates if user's communication requirement can be met in physical domain. Users' social attributes can be represented as social domain \(G(V_{s},E_{s})\), where \(V_{s}\) indicates the users corresponding to users in physical domain and \(E_{s}\) denotes the social relationship between D2D users. Social relationship can be defined as \(S_{k,n}\), \(S_{k,n}\in[0,1]\).