Expectation Propagation as Turbo Equalizer in ISI Channels Articles uri icon

authors

  • SANTOS, IRENE
  • Murillo Fuentes, Juan Jose
  • BOLOIX TORTOSA, RAFAEL
  • ARIAS DE REYNA, EVA
  • MARTINEZ OLMOS, PABLO

publication date

  • January 2017

start page

  • 360

end page

  • 370

issue

  • 1

volume

  • 65

International Standard Serial Number (ISSN)

  • 0090-6778

Electronic International Standard Serial Number (EISSN)

  • 1558-0857

abstract

  • In probabilistic equalization of channels with intersymbol interference, the BCJR algorithm and its approximations become intractable for high-order modulations, even for moderate channel dispersions. In this paper, we introduce a novel soft equalizer to approximate the symbol a posteriori probabilities (APP), where the expectation propagation (EP) algorithm is used to provide an accurate estimation. This new soft equalizer is presented as a block solution, denoted as block-EP (BEP), where the structure of the matrices involved is exploited to reduce the complexity order to O LN2 , i. e., linear in the length of the channel, L, and quadratic in the frame length, N. The solution is presented in complex-valued formulation within a turbo equalization scheme. This algorithm can be cast as a linear minimum-mean-squared-error (LMMSE) turbo equalization with double feedback architecture, where constellations being discrete is a restriction exploited by the EP that provides a first refinement of the APP. In the experiments included, the BEP exhibits a robust performance, regardless of the channel response, with gains in the range 1.5-5 dB compared with the LMMSE equalization.

keywords

  • terms-expectation propagation (ep); bcjr; complexvalued; turbo equalization; isi; sum-product algorithm; decision-feedback; factor graphs; intersymbol interference; sequence estimation; bcjr algorithm; linear codes; mimo