19.6 Problems 583 presented, which focused on the exploration of cognitive radio systems from the varying perspectives of spectrum sensing, the mobile unit, and the base station element. Each design faces unique challenges and in this chapter we present the motivations, architectures, challenges, and learned experiences from each testbed. The first platform is developed using the Berkeley emulation engine, reconfig- urable 2.4 GHz radio modems, and fiber link interface for connection between BEE2 and radios. The software architecture consists of Simulink-based design flow and the BEE2-specific operating system that provide an integrated environment for implementation and simple data acquisition during experiments. The testbed is particularly suited for the development of physical and network layer functional- ities and their experimental characterization in realistic wireless scenarios, with advanced capabilities, such as real-time high-speed signal processing and protocol implementation, support for multiple network interactions, and multiple antenna operation. This testbed is used for the design, implementation, and experimental studies of a spectrum sensing functionality. Motorola describes a fundamentally flexible, low-power transceiver RFIC and how it enables a small form-factor, experimental CR platform. The platform can receive and transmit signals of many wireless protocols at carrier frequencies from 10 MHz to 4 GHz with channel bandwidths from 8 kHz to 20 MHz. Selected