 emerged from military single-carrier, single-band, multiband, and multimode transceivers. Some of these issues will be covered later. and R&D labs and become mainstream technologies. One of these technologies is software-defined radio (SDR). The key point is that SDRs have the ability to go beyond Although much has been discussed in recent years, a simple single-channel, single-mode transceiver technol- good definition of software radio is difficult to generate. ogy with the ability to change modes arbitrarily because This is largely due to the flexibility that SDRs offer, the channel bandwidth, rate, and modulation are all allowing them to take on many different forms that can flexibly determined through software. These character- be changed to suit the need at hand. istics may be changed by direct input, floppy disk, However, SDRs do have characteristics that make over-the-air download, or through the use of careful them unique in comparison to other types of radios. As signal analysis to determine analytically how the the name implies, an SDR is a radio that has the ability to information is coded through a process termed cognitive be transformed through the use of software or redefin- radio [2]. Regardless of the means by which the radio is able logic. Quite often this is done with general purpose reconfigured, a fully implemented SDR will have the digital signal processors (DSPs) or field programmable ability to navigate a wide range of frequencies with pro- gate arrays (FPGAs), as discussed later in the chapter. grammable channel bandwidth and modulation charac- In order to take advantage of such digital processing, teristics. The following list outlines some of the possible traditional analog signals must be converted to and from dynamic characteristics of an SDR: the digital domain. This is accomplished using analog-to- Channel bandwidth digital (ADC) and digital-to-analog converters (DAC). Data rate To take full advantage of digital processing, SDRs keep Modulation type the signal in the digital domain 