Small Form Factor (SFF) Software Defined Radio (SDR) platform is used to implement an experimental cognitive radio for first responders. The system was demonstrated in the 2007 Software Defined Radio technical conference and product exhibition, and is selected for presentation in the demonstrations and experimentation track in the IEEE DySPAN 2008.Different processing tasks are divided between a TI c64x+ DSP and a Xilinx Virtex IV FPGA while an ARM9 core is used to host Greenhills operating system. To tackle the complexity of development, we used progressive simulation based design. Starting from pure simulation of the system, we substitute the models with real modules step by step to build a fully functional modem. To develop the network of the cognitive radio modems, we deploy the available real modems with simulated primary and secondary users in a central simulation. Three real nodes are presented in our demonstration which perform channel sensing and data transmission.