-generation (1G) cellular was 1.25 MHz bandwidth. Both systems typically utilize based on analog modulation schemes (which did not re- surface acoustic wave (SAW) filter technology to set the quire significant digital processing), it became clear that bandwidth. Since these devices are fixed, it is not pos- due to the limited amount of spectrum and the relative sible (aside from electronically switching filters) to inefficiency of those standards, more efficient means of change the channel bandwidth characteristics. There- spectral usage were required. Therefore, second genera- fore, aside from the modem function, an SDR needs tion (2G) cellular systems such as GSM and IS-95 were additional circuitry that allows other properties of the air developed that took advantage of the emerging DSP interface to be adapted. In practice, the optimal way to technologies. In these early systems, the DSP became do this is to digitize the signal and use digital techniques the modem function and was responsible for taking the for manipulating the channel of interest. This manipu- complex baseband data (I and Q) and determining what lation often occurs in a general purpose DSP or more bitstream was being sent and correcting for errors in- frequently in a digital ASIC with the ability to accom- troduced due to noise, interference, and fading. modate a near continuous range of channel bandwidth, Conceptually, these modem functions were based on data rates, and other physical characteristics in a fully programs running on a DSP and, therefore, could be digital manner. changed simply by changing the program. In fact, over The following figures show the practical evolution time these standards evolved and variations of the stan- path of an Rx SDR architecture. In Figure 2.3-1, a tra- dards were introduced that allowed better efficiency and ditional super-heterodyne receiver is shown with analog higher data-transmission rates. Many of these improve- detection. Figure 2.3-2 adds the DSP function that ments were offered simply by updating