The document provides an overview of 4G and 5G mobile network architectures as well as an introduction to device-to-device (D2D) network technology. It describes the key features and components of 4G networks including the evolved packet core and how 5G networks aim to achieve much higher data rates and connectivity for many more devices. The document also outlines the benefits of D2D communication, how it can operate in both licensed and unlicensed spectrum, and some potential applications including multi-user cooperative communication and vehicle-to-vehicle networks.