with an external PA. The LNA will 2 GHz, or yttrium-iron-garnet (YIG) circulators at fre- normally have a tunable filter with it. Therefore, it is quencies above 2 GHz. necessary to be able to provide digital interfaces to the The typical software-defined RFFE begins notionally external RF components to provide control of tuning with receiving a signal and filtering the signal to reflect frequency, transmit/receive mode, VSWR and transmit the range of frequency covered by the intended signals. power-level sensing, and receive gain control. For a spread spectrum wideband code division multiple In all of these cases, a general-purpose SDR architec- access (WCDMA) signal, this could be up to 6 MHz of ture must anticipate the possibility that it may be called bandwidth. In order to assure that the full 6 MHz is CHAPTER 2.4a The software defined radio as a platform for cognitive radio presented to the modem without distortion, it is not variety of waveforms; they may ultimately be expected unusual for the ADC to digitize 12 MHz or so of signal to demodulate. Today we would summarize that a typical bandwidth. In order to capture 12 MHz of analog signal SDR should be able to provide at least 266 MIPS of GPP, bandwidth set by the IF filters without aliasing artifacts, 32 Mbytes of random access memory (RAM), 100 MIPS the ADC will probably sample the signal at rates above of DSP, and 500 equivalent gates of FPGA-configurable 24 million samples per second (Msps). After sampling logic. More performance and resources are required for the signal, the digital circuits will shift the frequency of sophisticated waveforms or complex networking appli- the RF carrier to be centered as closely as possible to cations. Typically, the GPP will be called upon to perform 0 Hz direct current (DC) SO that the signal can again be the protocol stack and networking functions, the DSP filtered digitally to match the exact signal bandwidth. will perform the physical layer modulation and de- Usually this filtering will be done with a cascade of 