TURBO EQUALIZATION - Adaptive Signal Processing: Next Generation Solutions - page 143

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TURBO EQUALIZATION
Philip A. Regalia
Catholic University of America, Washington, DC
3.1 INTRODUCTION
The turbo decoding algorithm [1-7] ranks unquestionably among the major practical
achievements in modern telecommunications, validating the theoretical error correc-
tion capabilities of realizable codes predicted from Shannon's work [8].
The essence of the algorithm is to harness information exchange between lower-
level receiver functions, thus demonstrating the performance advantages achievable
by combining traditionally compartmentalized functions into a bigger picture. In con-
trast to global optimization procedures which can prove unwieldily complex, the
“turbo principle” aims instead to couple basic building blocks boasting computational
efficiency into iterative algorithms that transform information exchange into progres-
sive performance improvements. The standard recipe for turbo receivers, in which
extrinsic information is extracted from one block to usurp the role held for a priori
probabilities within another, can be given countless voicings depending on which
elements are combined; choices include synchronization, equalization, and decoding
in the traditional monouser paradigm, and can be expanded to user separation, key
negotiation, and access control in multiuser settings.
The intent of this chapter is to relate measurable progress in a particular instance of
iterative receiver design, namely turbo equalization which combines equalization and
error correction decoding. This setting is sufficiently rich to illustrate the performance
advantages induced by cooperative information exchange between traditionally separ-
ated receiver functions, yet sufficiently complicated to convince researchers that the