Digital Signal Processing Reference
In-Depth Information
Chapter 1
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 con-
texts combined with a potential for cheap high-volume production easily com-
pensates for the high engineering and start-up costs. This unique blend has
made integrated circuits one of the most important developments in the pre-
vious 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 com-
munication
. 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 cir-
cuits 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, digi-
tal 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.
