till provide interface. Finally, SDR is not a generic information tech- equivalent performance. 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 2.4a.2.1 The block diagram visible in the application programming interface (API), but is also apparent in the strict timing requirements in- The generic architecture tour presented here traces from herent to radio systems, and the development and the antenna through the radio and up the protocol stack debugging complexities associated with radio design. to the application. This chapter is organized as follows: Section 2.4a.2 in- troduces the basic radio hardware architecture and the RF externals processing engines that will support the cognitive func- Many radios may achieve satisfactory performance with tion. Section 2.4a.3 discusses the software architecture of an antenna consisting of a passive conductor of resonant an SDR. Section 2.4a.4 discusses SDR software design and length, or an array of conductors that yield a beam pat- development. At present, many SDRs utilize a Software tern. Such antennas range from the simple quarter- Communications Architecture (SCA) as a middleware wavelength vertical to the multi-element Yagi and its to establish a common framework for waveforms, and wide bandwidth cousin, the log periodic antenna. An- the SCA is covered in some detail in this section. Section tennas used over a wide frequency range will require an 2.4a.5 discusses applications as well as the cognitive antenna tuner to optimize the voltage standing wave functionality and languages that support cognitive soft- radio (VSWR) and corresponding radiation efficiency. ware as an application. Section 2.4a.6 discusses the de- Each time the transceiver changes frequency, the antenna velopment process for SDR software components. Section tuner will need to be informed of the new frequency. It 2.4a.7 then discusses cognitive waveform development. will either 