CHAPTER 2.4a The software defined radio as a platform for cognitive radio only general purpose processors (GPPs), but also digital signal processors (DSPs), field-programmable gate arrays (FPGAs), and custom computing machines (CCMs). Furthermore, SDR must support the RF and intermediate frequency (IF) hardware that is necessary to interface the computing hardware with radio signals. This support is largely a tuning structure coupled with a standardized interface. Finally, SDR is not a generic information tech- nology (IT) solution in the way that database management is. SDR deals explicitly with the radio domain. This means that context is important. This context is most readily visible in the application programming interface (API), but is also apparent in the strict timing requirements in- herent to radio systems, and the development and debugging complexities associated with radio design. This chapter is organized as follows: Section 2.4a.2 in- troduces the basic radio hardware architecture and the processing engines that will support the cognitive func- tion. Section 2.4a.3 discusses the software architecture of an SDR. Section 2.4a.4 discusses SDR software design and development. At present, many SDRs utilize a Software Communications Architecture (SCA) as a middleware to establish a common framework for waveforms, and the SCA is covered in some detail in this section. Section section is to explore hardware for SDR from a radio standpoint. Figure 2.4a-1(a) shows a basic radio receiver. As one example based on the basic radio receiver archi- tecture, Figure 2.4a-1(b) shows a design choice made possible by digital signal processing techniques, in which the sampling process for digital signal processing can be placed in any of several locations and still provide equivalent performance. 2.4a.2.1 The block diagram The generic architecture tour presented here traces from the antenna through the radio and up the protocol stack to the application. RF externals Many radios may achieve satisfactory performance with an antenna consisting of a passive conductor of resonant length, or an array of conductors that yield a beam pat- tern. Such antennas range from the simple quarter- wavelength vertical to the multi-element Yagi and its wide bandwidth cousin, the log periodic antenna. An- tennas used over a wide frequency range will require an