The modem: the most underrated player in communication

Ceilidh Hoffmann

Projet Planete


Historically the design of efficient modems has been overshadowed by the need to construct good channel codes and decoding strategies that attain information rates close to Shannon capacity. In fact, the role of modulation and demodulation is transparent in the derivation as well as in the proof of Shannon s famous channel capacity equation. However, modern-day ommunications services would not be possible without modems. What are the intricate details and entailments in the design of efficient modems? What is the efficiency of modems currently on the market or drawing boards? What is the significance of modem design in reaching Shannon s ultimate limit in communications? These are some of the many intriguing questions I wish to address in my talk.

The history of modulation is much older than its younger sibling coding even though the latter has enjoyed most of the limelight. Modems were first used in analog communication whereas coding was a birth child of Shannon and its disciples the coding theorists for digital communication. We are quite familiar with encoding and decoding techniques whose names are attributed to their inventors, for example Hamming, Huffman, BCH, Reed-Solomon, Viterbi, Fano, BCJR, etc. Interestingly, most communications researchers and engineers are unaware of the names of early and latter day pioneers who devised efficient modulation and demodulation methods.

I will briefly describe some past and present modem techniques used in the communications industry over various (wireless, copper, fiber) media. Then I will also discuss several benchmarks used to evaluate the performance of modems. As case studies, I will highlight some poor choices and design flaws of modem selection in past and current wireless standards.

During the talk, it will become apparent that modem designs are thrusted forward by researchers and practitioners with technical background and expertise in areas that are quite different from those in the coding domain. To this day, the coding world revolves around the discrete additive Gaussian channel model. The prerequisites for mastering coding theory are abstract algebra and introductory probability --- it is a discipline most suited for mathematicians. In contrast, the design of efficient modems requires a good understanding of signals and systems. Since the modem designers must deal with real numbers and waveforms (compared to coding theorists who work in the Galois field), a hands-on experience with laboratory testing equipment is a plus. With the advancement in digital signal processing techniques, the theory and practice of modem design has also undergone a paradigm shift from analog to digital domains. An important by-product of this shift is the software-defined radio architecture. I will discuss some subtle issues that arise when the modem is embedded in DSP hardware.

If time permits, I will cover the topic of convergence between theory and practice. The most revealing form of research-to-product is in intellectual properties (i.e. patents). Many advanced modem techniques never appeared in IEEE-type technical journals; the only source of information of these techniques is the patent archives. As a case example, I will discuss one of Einstein s handful patents: a pump for a better refrigerator. It is fascinating to know that Einstein, being a theoretical physicist, was also an inventor of gadgets that have practical value! As a side note, the connection between his pump, our Mother Earth with her depleted ozone layer and the nuclear war will be described.

[Ceilidh Hoffmann]
[Projet Planete]