4.1 Introduction

Wireless communication devices are composed of three main entities; signaling, physical hardware, and its functionalities. These three main streams, which complement each other, have evolved since the invention of the radio transmission by Guglielmo Marconi. The primitive communications devices had very simple signaling, analog hardware, and limited functionality. In time, each of these entities evolved significantly. Different signaling methods have been invented and used around the world. Furthermore, numerous different wireless communications systems and standards have been developed around the world without any global plan. Recently, the diversity in wireless systems and standards bring some issues on the surface such as interoperability and global seamless connectivity. In parallel, hardware technology evolved significantly, too. Some of the key milestones in this progress are transition from analog hardware to digital hardware and then introduction of sophisticated processors. This is followed by the development of Software Defined Radio (SDR) structures and virtual hardwares that are under development currently. SDR is envisioned initially to be a promising solution for interoperability, global seamless connectivity, multi-standard, and multi-mode issues. Also in parallel, the functionality of wireless devices is increased and they become more and more sophisticated. For instance, the cellular technology was developed to provide voice communications for mobile users initially. However, current cellular phones have multi-functionalities such as internet access, digital camera, Global Positioning System...