Cryptography Reference
In-Depth Information
Chapter 11
The turbo principle applied to
equalization and detection
The invention of turbo codes at the beginning of the 90s totally revolutionized
the field of error correcting coding. Codes relatively simple to build and decode,
making it possible to approach Shannon's theoretical limit very closely, were at
last available. However, the impact of this discovery was not limited to one sin-
gle coding domain. More generally, it gave birth to a new paradigm for designing
digital transmission systems, today commonly known as the "turbo principle".
To solve certain very complex
a priori
signal processing problems, we can envis-
age dividing these problems into a cascade of elementary processing operations,
simpler to implement. However, today we know that the one-directional succes-
sion of these processing operations leads to a loss of information. To overcome
this sub-optimality, the turbo principle advocates establishing an exchange of
probabilistic information, "in the two directions", between these different pro-
cessing operations. All of the information available is thus taken into account
in solving the global problem and a consensus can be found between all the
elementary processing operations in order to elaborate the final decision.
The application of the turbo principle to a certain number of classical prob-
lems in digital transmission has provided impressive gains in performance in
comparison to traditional systems. Therefore its use rapidly became popular
within the scientific community. This chapter presents the first two systems
having historically benefited from the application of the turbo principle to a
context other than error correction coding. The first system, called turbo equal-
ization, iterates between the equalization function and a decoding function to
improve the processing of the intersymbol interference for data transmission over
multipath channels. The second, commonly called turbo CDMA, exploits the
turbo principle to improve the discrimination between users in the case of a
