4 CHAPTER 1 When radio meets software devices, but rather in base stations and access points, which can take advantage of external power resources. Second, the question where the AD/DA conversion can be performed deter- mines what radio functions can be done in software and hence how reconfigurable a radio can be made. The ultimate goal for software radio is to move the AD/DA conversion as close as possible to the antenna so that all signal processing can be done digitally. However, two technical limitations make it currently infeasible to the AD/DA conversion at the antenna. First, digitization of the RF signal requires the incoming signal to be sampled at least at a rate that is determined by the Nyquist frequency. Additionally, the higher the data rate of the signal, the higher the resolution required to capture the information. Taken together, this means that high-bandwidth, high-frequency RF transmissions require very high sampling rates. The ability to support very high sampling rates, which is especially critical with the use of high-frequency signals in the gigahertz range, limits the range of what can be digitized. To give an example, the typical channels used by an 802.11 WiFi device are 20 MHz wide. To assure that the full 20 MHz is presented to the modem without distortion, it is not unusual for ADC to digitize 40 MHz or so of signal bandwidth. To capture 40 MHz of analog signal bandwidth set by the IF filters without aliasing artifacts, the ADC will probably sample the signal at a rate above