The book explores the transformative journey of mobile communications, tracing the evolution from fifth-generation (5G) cellular networks to the promising paradigm of Device-to-Device (D2D) communication. It delves into the technological advancements, challenges, and opportunities arising from this transition, offering insights into spectral efficiency enhancement, resource allocation, standardization efforts, and cutting-edge techniques shaping the future of mobile connectivity. Through comprehensive analysis and exploration, the book aims to provide a deep understanding of the dynamic landscape of mobile communications and its impact on efficiency, connectivity, and innovation in the modern era.