Before the fifth generation of mobile communication systems, the transmission and reception links between users in a cellular network were established and routed through the base station. This gradually led to increased traffic on the base station and higher latency in sending and receiving information. However, in the fifth generation, with the use of device-to-device (D2D) communications, users in the cellular network could exchange information directly without the mediation of the base station. Using D2D communication in cellular networks reduces traffic on the base station and decreases signal reception delays. If the D2D communication is of the full-duplex underlay type, although it faces the challenge of self-interference, it enhances spectral efficiency. Due to the nature of full-duplex communication, which involves limited distances between two devices, it reduces the required transmission power and saves energy. Moreover, due to the reduced transmission power, the level of self-interference is significantly lowered. This thesis examines the advantages of using full-duplex underlay D2D communication in cellular networks. For this purpose, a cellular network is considered that includes a base station, cellular users, and D2D users. The locations of these users in the cell follow Poisson point processes. To evaluate network performance, approximations based on stochastic geometry are applied in calculating the coverage probability and the sum of the bit rates for cellular and D2D users. Under certain conditions, where the number of D2D links is sufficiently large, the obtained approximations simplify to a closed-form expression, allowing us to analyze the behavior of the sum of bit rates in relation to various network parameters. It is demonstrated that underlay D2D communication significantly increases the spectral efficiency of the network. Furthermore, it is observed that even a slight reduction in self-interference in full-duplex D2D communications leads to a substantial increase in spectral efficiency compared to half-duplex communications. Finally, the results obtained for coverage probability and sum bit rate are compared with multi-cell networks under standard frequency reuse and fractional frequency reuse scenarios. It is shown that employing multi-cell networks with frequency reuse scenarios increases the coverage probability for users but, conversely, reduces the total bit rate transmitted by users.
