Modern communications technologies and systems, including those that are wireless, are mostly digital. However, all RF communications ultimately involve transmitting and receiving analog signals; Box 2.1 describes the relationship between digital and analog communication.
Digital signal processing (Box 2.2) is increasingly used to detect the desired signal and reject other “interfering” signals. This shift has been enabled by several trends:
Increasing use of complementary metal oxide semiconductor (CMOS) integrated circuits (Box 2.3) in place of discrete components;
The application of dense, low-cost digital logic (spawned primarily by the computer and data networking revolutions) for signal processing;
New algorithms for signal processing;
Advances in practical implementation of signal processing for antenna arrays; and
Novel RF filter methods.
The shift relies on an important tradeoff: although the RF performance of analog components on a CMOS chip is worse than that of discrete analog components, more sophisticated computation can compensate for these limitations. Moreover, the capabilities of radios built using CMOS can be expected to continue to improve.
The use of digital logic implies greater programmability.3 It is likely that radios with a high degree of flexibility in frequency, bandwidth, and modulation will become available, based on highly parallel architectures programmed with special languages and compilers. These software-defined radios will use software and an underlying architecture that is quite different from conventional desktop and laptop computers, but they will nonetheless have the ability to be programmed to support new applications.
High degrees of flexibility do come at a cost—both financial and in terms of power consumption and heat dissipation. As a result, the wireless transceiver portion (as opposed to the application software that communicates using that transceiver) of low-cost consumer devices is unlikely to become highly programmable, at least in the near future. On the other