Cryptography Reference
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the complexity of the BCJR-MAP equalizer. As for the MMSE turbo equalizer,
it offers a good compromise between performance and complexity. For many
transmission configurations it leads to performance close to that offered by the
BCJR-MAP turbo equalizer, with reasonable complexity. In addition, unlike
the BCJR-MAP turbo equalizer, the MMSE turbo equalizer can be realized
in adaptive form, thereby jointly performing equalization and tracking of the
channel time variations.
In the second part, we have dealt with the application of the turbo princi-
ple to the domain of multi-user communications in code-division multiple access
systems. We have presented a survey of conventional multi-user detection tech-
niques. In particular, the PIC and SIC methods for cancellation of multi-user
interference have been described. Their particular structures lead to a relatively
simple exploitation of the turbo principle in a multi-user transmission context.
Like for turbo equalization, different detectors can be implemented based on
MMSE filters or matched-filter banks, for example.
In this chapter, we have deliberately limited ourselves to the presentation of
two particular systems exploiting the turbo principle. However, more generally,
any problem of detection or parameter estimation may benefit from the turbo
principle. Thus, the range of solutions dealing with interference caused by a
multi-antenna system at transmission and at reception (MIMO) has been en-
riched by iterative techniques such as the turbo BLAST ( Bell Labs layered space
time ) [11.25]. The challenge involves proposing SISO detectors of reasonable
complexity, without sacrificing data rates and/or the high performance of such
systems.
We can also mention the efforts dedicated to receiver synchronization. In-
deed, the gains in power provided by the turbo principle lead to moving the
systems' operation point towards low signal to noise ratios. Now, conventional
synchronization devices were not initially intended to operate in such dicult
conditions [11.31]. One possible solution is to integrate the synchronization into
the turbo process. A state of the art of turbo methods for timing synchronization
was presented in [11.4]. More generally, when the choice of turbo processing at
reception is performed, it seems interesting, or even necessary, to add a system
to the receiver to iteratively estimate the transmission parameters, like channel
turbo estimation or turbo synchronization.
Among other applications, the uplink of future radio-mobile communications
systems will require higher and higher data rates, with an ever-increasing number
of users. This is the one of the favourite applications of the turbo principle, the
generalization of which will be essential in order to respond to the never-ending
technological challenge posed by the evolution of telecommunications.
Understanding the turbo principle has led to the introduction of novel theo-
retical tools and concepts, like EXIT charts or factor graphs. While the former
enable accurate prediction of the convergence threshold of iterative decoding
schemes, the latter offer a graphical framework for representing complex detec-
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