Why Digital???

Comparing the block diagrams for analog and digital communication, respectively, we see that the digital communication system involves far more processing. However, this is not an obstacle for modern transceiver design, due to the exponential increase in the computational power of low-cost silicon integrated circuits. Digital communication has the following key advantages.

Optimality 

For a point-to-point link, it is optimal to separately optimize source coding and channel coding, as long we do not mind the delay and processing incurred in doing so. Due to this source-channel separation principle, we can leverage the best available source codes and the best available channel codes in designing a digital communication system, independently of each other. Efficient source encoders must be highly specialized. For example, state of the art speech encoders, video compression algorithms, or text compression algorithms are very different from each other, and are each the result of significant effort over many years by a large community of researchers. However, once source encoding is performed, the coded modulation scheme used over the communication link can be engineered to transmit the information bits reliably, regardless of what kind of source they correspond to, with the bit rate limited only by the channel and transceiver characteristics. Thus, the design of a digital communication link is source-independent and channel-optimized. In contrast, the waveform transmitted in an analog communication system depends on the message signal, which is beyond the control of the link designer, hence we do not have the freedom to optimize link performance over all possible communication schemes. This is not just a theoretical observation: in practice, huge performance gains are obtained from switching from analog to digital communication.

Scalability 

While a single digital communication link between source encoder and decoder, under the source-channel separation principle, there is nothing preventing us from inserting additional links, putting the source encoder and decoder at the end points. This is because digital communication allows ideal regeneration of the information bits, hence every time we add a link, we can focus on communicating reliably over that particular link. (Of course, information bits do not always get through reliably, hence we typically add error recovery mechanisms such as retransmission, at the level of an individual link or “end-to-end” over a sequence of links between the information source and sink.) Another consequence of the source-channel separation principle is that, since information bits are transported without interpretation, the same link can be used to carry multiple kinds of messages. A particularly useful approach is to chop the information bits up into discrete chunks, or packets, which can then be processed independently on each link. These properties of digital communication are critical for enabling massively scalable, general purpose, communication networks such as the Internet.

Such networks can have large numbers of digital communication links, possibly with different characteristics, independently engineered to provide “bit pipes” that can support data rates. Messages of various kinds, after source encoding, are reduced to packets, and these packets are switched along different paths along the network, depending on the identities of the source and destination nodes, and the loads on different links in the network. None of this would be possible with analog communication: link performance in an analog communication system depends on message properties, and successive links incur noise accumulation, which limits the number of links which can be cascaded.

The preceding makes it clear that source-channel separation, and the associated bit pipe abstraction, is crucial in the formation and growth of modern communication networks. However, there are some important caveats that are worth noting. Joint source-channel design can provide better performance in some settings, especially when there are constraints on delay or complexity, or if multiple users are being supported simultaneously on a given communication medium. In practice, this means that “local” violations of the separation principle (e.g., over a wireless last hop in a communication network) may be a useful design trick. Similarly, the bit pipe abstraction used by network designers is too simplistic for the design of wireless networks at the edge of the

Internet 

physical properties of the wireless channel such as interference, multi path propagation and mobility must be taken into account in network engineering.

Why analog design remains important??

While we are interested in transporting bits in digital communication, the physical link over which these bits are sent is analog. Thus, analog and mixed signal (digital/analog) design play a crucial role in modern digital communication systems. Analog design of digital-to-analog converters, mixers, amplifiers and antennas is required to translate bits to physical wave-forms to be emitted by the transmitter. At the receiver, analog design of antennas, amplifiers, mixers and analog-to-digital converters is required to translate the physical received wave-forms to digital (discrete valued, discrete time) signals that are amenable to the digital signal processing that is at the core of modern transceivers. Analog circuit design for communications is therefore a thriving field in its own right, which this textbook makes no attempt to cover. However, the material in Chapter 3 on analog communication techniques is intended to introduce digital communication system designers to some of the high-level issues addressed by analog circuit designers. The goal is to establish enough of a common language to facilitate interaction between system and circuit designers. While much of digital communication system design can be carried out by abstracting out the intervening analog design (as done in Chapters 4 through 8), closer interaction between system and circuit designers becomes increasingly important as we push the limits of communication systems, as briefly indicated in the epilogue.

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