space. The locations of BSs (or users/devices) are the points that are generated using spatial point processes. Detailed discussion on different point processes is given in [43-45]. Some of the popular point processes considered in the existing literature to analyze the cellular networks are Poisson point process [32, 34, 46], hard core point process [47-49], and Poisson cluster process [50-52]. For formal and detailed definitions of the point processes, readers can refer to [43, 53]. 5.5 Cooperation Using Device to Device Communication Device to device (D2D) communication has also received great attention in meeting the booming demands and has appeared to be an essential component in the future generation networks [54, 55]. The key difference of the D2D communication with conventional cellular communication is bypassing the BS, i.e., the communication takes place without traversing the core network. Traditionally, all communication must pass through the core network. Therefore, D2D communication is not trans- parent to the cellular network. The methodology where the communication takes place through the core network is suitable only for low data rates service, e.g., voice calls, text messages, etc., where the users are not close to each other to have direct communication. However, the advancements of wireless services, for instance, video sharing, gaming, etc., require high data rates, and devices are most possibly in proximity to one another to establish a direct communication (i.e., D2D communication). Therefore, D2D communication is desirable in such scenarios and can potentially increase the spectral efficiency of the network. D2D communication is also capable of improving throughput, energy efficiency, and delay of the network [56]. The D2D communication can occur in two modes, inband (i.e., using the cellular spectrum) and outband (i.e., on the unlicensed spectrum) [56]. Furthermore, to enhance the performance of cellular users, relaying techniques have been explored using D2D communication in cellular net