receiver elements example. Referring to the single-carrier block diagram in Figure In this case, both the digital preprocessing and the 2.3-7, while keeping in mind that this applies to the general purpose DSP are reconfigured, respectively, by multi-carrier example as well, a fully developed SDR will changing the digital channel filter from GSM to CDMA have all signal elements that are programmable. and by loading the new processing code into the DSP. The antenna is no exception, and unfortunately it is Since GSM channels are 200 kHz wide, this example one of the weakest elements in an SDR [1]. Since most could easily be reconfigured as a 102-channel GSM antenna structures have a bandwidth that is a small receiver. percentage of its center frequency, multiband operation Analog Control Nyquist Filter 1 Filter Antenna Output Output Output Tuning Control Loop Output Figure 2.3-5 Multi-carrier CDMA example. Software defined radio CHAPTER 2.3 Figure 2.3-6 Multi-mode spectrum with IS-95 and narrowband carriers. can become difficult. In the many applications where tuned near television and audio broadcast services where single bands of operation are used, this is not a problem. transmit power levels can exceed 100 kW. This can be However, for systems that must operate across several especially problematic for multi-carrier receivers where orders of frequencies such as the SpeakEasy discussed many orders of signal magnitude must be dealt with. If all earlier, the antenna must be tuned by some means to of the signals are of interest, then it will not be possible to track the operating frequency to maintain operating ef- filter the stronger signals, and the resulting receiver must ficiency. While it is true that just about any antenna can have a relatively large signal dynamic range [8]. be impedance matched to the active electronics, there is Most receivers require a low-noise amplifier or LNA. usually a sacrifice in the link gain that potentially results An SDR should ideally incorporate an LNA that is ca- in an antenna