Digital Communications Over Analog Channels

It is well known that cost efficiency is one of the driving forces behind on-chip integration of cmos digital circuits. The vast amount of useful application contexts combined with a potential for cheap high-volume production easily compensates for the high engineering and start-up costs. This unique blend has made integrated circuits one of the most important developments in the previous century and one of the driving forces behind today's economy. In the early 1990s, the speed and density of on-chip digital circuit elements achieved a critical mass which has led to the rise of the so-called digital wireless communication. Before this time, the center-of-gravity of wireless communications was almost completely slanted in favor of analog circuits. It is not suggested that wireless communication was limited to the transmission of analog data, though. Transmission of digital information was commonplace, but a major part of the signal processing was housed in the analog domain. Analog circuits were responsible for the signal preprocessing and a clean, refurbished signal was handed over to the digital back-end. The back-end itself, however, did not actively contribute to the actual signal recovery process. At best, digital circuitry was used as the controller for non-time-critical tasks upstream in the physical layer of a transceiver system. This includes, for example, off-line error detection, error correction and the higher-level retransmission protocols. Real-time signal processing such as filtering, automatic frequency correction (afc) and also frame synchronization were taken into account by the analog or mixed-signal circuit blocks.