 the modem operates in place of the analog detector and can also software. While consumers seldom experienced these function as the baseband modem. This allows the exact benefits for a number of economic reasons, the in- demodulation functions to change as needed while the frastructure side did benefit from these upgrades and is other channel characteristics are fixed. Figure 2.3-3 in- benefiting from many of these software updates in the cludes a wideband ADC and digital preprocessing before migration from 2G to 2.5G and ultimately to 3G, most the modem that allows the physical channel character- notably in the evolution of the CDMA2000 and UMTS istics to be adapted as necessary. Figure 2.3-3 also shows standards [15, 16]. what a full SDR might look like. An architecture such as While the evolution of the modems used for GSM and this may find use in diverse areas such as multi-mode CDMA is an aspect of SDR, other factors such as in- systems capable of simultaneously processing several compatibility of these two standards drives the second standards at once or simply as a manufacturing option to aspect of SDR. While CDMA is primarily a North simplify production and inventory issues. American and Asian standard and GSM is a standard in Europe and the rest of world, in reality both of these standards are overlayed in many countries. Ideally, ser- Applications and need for SDR vice providers would like to purchase one piece of equipment that would work with both standards. Un- Interoperability fortunately, these and other standards are incompatible in terms of bandwidth, modulation format, and data rate. The military is not the only agency in need of in- Traditional radios, even those with DSP modems, teroperability. As numerous agencies, both domestic and operate with fixed bandwidths and therefore prevent international, have responded to various natural and crossfunctionality. A typical GSM system works with manmade disasters around the world, communications CHAPTER 2.3 Software defined radio Analog Control Loop C