300 research outputs found
On optimal and near-optimal turbo decoding using generalized max operator
Motivated by a recently published robust geometric programming approximation, a generalized approach for approximating efficiently the max* operator is presented. Using this approach, the max* operator is approximated by means of a generic and yet very simple max operator, instead of using additional correction term as previous approximation methods require. Following that, several turbo decoding algorithms are obtained with optimal and near-optimal bit error rate (BER) performance depending on a single parameter, namely the number of piecewise linear (PWL) approximation terms. It turns out that the known max-log-MAP algorithm can be viewed as special case of this new generalized approach. Furthermore, the decoding complexity of the most popular previously published methods is estimated, for the first time, in a unified way by hardware synthesis results, showing the practical implementation advantages of the proposed approximations against these method
Development of a self-tuned drive-train damper for utility-scale variable-speed wind turbines
This thesis describes the development of a procedure that tunes a wind turbine drivetrain
damper (DTD) automatically. This procedure, when integrated into the controller
of any utility-scale variable-speed wind turbine, will allow the turbine to
autonomously and automatically tune its DTD on site. In practice this means that the
effectiveness of the damper becomes independent on the accuracy of the model or the
simulations used by the control engineers in order to tune the damper. This research is
motivated by the fact that drive-train failures are still one of the biggest problems that
stigmatises the wind turbine industry. The development of an automatically tuned
DTD that alleviates the drive-train fatigue loads and thus increases the reliability and
lifetime of the drive-train is thus considered very beneficial for the wind turbine
industry.
The procedure developed begins by running an experimental procedure to collect data
that is then used to automatically system identify a linear model describing the drivetrain.
Based on this model a single band-pass filter acting as a DTD is automatically
tuned. This procedure is run for a number of times, and the resulting DTDs are
compared in order to select the optimal one.
The thesis demonstrates the effectiveness of the developed procedure and presents
alternative procedures devised during research. Finally, insight into future work that
could be performed is indicated in the last chapter of the thesis
BER of MRC for M-QAM with imperfect channel estimation over correlated Nakagami-m fading
In this contribution, we provide an exact BER analysis for M-QAM transmission over arbitrarily correlated Nakagami-m fading channels with maximal-ratio combining (MRC) and imperfect channel estimation at the receiver. Assuming an arbitrary joint fading distribution and a generic pilot-based channel estimation method, we derive an exact BER expression that involves an expectation over (at most) 4 variables, irrespective of the number of receive antennas. The resulting BER expression includes well-known PDFs and the PDF of only the norm of the channel vector. In order to obtain the latter PDF for arbitrarily correlated Nakagami-m fading, several approaches from the literature are discussed. For identically distributed and arbitrarily correlated Nakagami-m channels with integer m, we present several BER performance results, which are obtained from numerical evaluation and confirmed by straightforward computer simulations. The numerical evaluation of the exact BER expression turns out to be much less time-consuming than the computer simulations
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