Posts Tagged ‘Shannon limit’

Gallager’s LDPC error-correcting codes

September 28, 2007

Error-correcting codes are a technology that most people don’t think much about, if they even know they exist, but these codes work quietly in the background to enable such things as cell phones and other wireless technology, cable and satellite TV, and also the internet, including DSL, fiber-optic communications, and good old-fashioned dial-up modems. The modern communications revolution could not have begun without these codes.

So what’s the idea behind these codes? There’s a lot to say, and many textbooks have been written on the subject, so I’ll only give the briefest of introductions. [Some excellent textbooks I recommend include MacKay's textbook which I've already reviewed, McEliece's "Theory of Information and Coding", and Lin and Costello's "Error Control Coding." See also this post for two forthcoming books available online.] EDIT: I’ve added some more information about LDPC decoders, with pointers to available software, in this post.

The basic idea is that we want to transmit some bits which represent some piece of text or picture or something. Unfortunately, when we transmit those bits, they need to travel through some channel (say a wire or through the air) and when they are received, the receiver only gets a noisy version of each bit. For example, each bit might be flipped independently from a 0 to a 1 or vice versa with some small probability (this is called the binary symmetric channel; many other channel models exist too).

To combat this noise, we send extra bits; so instead of sending say the 1000 bits that represent our message, we might send 2000, where the extra 1000 “check” bits have some known relationship to the original 1000. Both the transmitter and receiver agree on that relationship ahead of time; that is the “code.” Of course, all 2000 bits are now subject to the noise, so some of those extra bits could be flipped. Nevertheless, if the noise is small enough, the receiver can try to “decode” the original 1000 bits by finding the configuration of the 2000 bits which obeys all the constraints and is most probable.

In 1948, Claude Shannon proved a theorem that essentially said that if the noise in the channel was small enough, and if the number of extra bits that you were willing to send per original bit was large enough, that one could design very long codes, that if optimally decoded, would always remove all the noise and recover the transmitted message.

(By the way, it is this amazing property that codes can remove 100% of the noise that means that we can watch crystal-clear high-definition TV coming in over the airwaves, something I very much appreciate when I watch a football game these days. When advertisers talk about “digital this” or “digital that,” they really mean “error-corrected digital”.)

As an example of Shannon’s theorem, if one was willing to use one extra bit for every original bit, and the percentage of flipped bits in your binary symmetric channel was less than the Shannon limit of about 11%, his theorem tells you that codes exist that will reliably remove all the noise. However, Shannon’s proof was non-constructive; he didn’t tell us what these wonderful codes were. Shannon also proved a theorem that if the noise was higher than the “Shannon limit,” no codes exist that can reliably correct the noise.

So error-correcting coding theory deals with the problems of designing codes, and efficient encoders and decoders for those codes, that work as close to the Shannon limit as possible. Many theorists invented many interesting and useful codes and encoders and decoders over the years, but until the 1990′s, it still seemed a distant dream to most coding theorists that we would be able to find practical codes that performed near the Shannon limit.

What is very strange is that the best codes and decoders that were discovered in the 1990′s were actually a rediscovery of codes and decoders invented by Robert Gallager in the early 1960′s, for his Ph.D. thesis. Gallager’s thesis introduced “low density parity check” (LDPC) codes, and their decoding algorithm, the famous “belief propagation” decoding algorithm. His thesis also introduced many other important ideas in coding theory, including “density evolution,” simplified “bit-flipping decoders,” and analysis methods for optimal LDPC decoders. It is a truly remarkable work, that every aspiring coding theorist should read. Fortunately, it’s available online.


How is it possible that this work was forgotten? Well, in 1968, Robert Gallager wrote a magnificent textbook on information theory, called “Information Theory and Reliable Communication,” where he explained most of the information and coding theory known at the time, but neglected to talk about LDPC codes! I’m not sure why. Possibly he thought that he’d already covered the material in the 1963 monograph that was made from his thesis, or perhaps he didn’t think LDPC codes were practical at the time. In fact, the decoder was too complex for 1960′s technology, but Moore’s Law took care of that, although only now are LDPC codes widely replacing previously-developed codes in communications systems.

So the moral of the story is: if you write a ground-breaking Ph.D. thesis about a remarkable technology that is 30 years ahead of its time, please don’t forget to mention it in your classic textbook a few years later. Not a moral too many of us have to worry about, but still…


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