Helge Lüders

Polyphase sequence design using a genetic algorithm

The performance of direct sequence code division multiple access (DS-CDMA) systems depends on the autocorrelation and cross-correlation properties of the deployed spreading sequences. Since good autocorrelation characteristics come at the expense of the cross-correlation properties and vice versa, a combination of these two measures needs to be optimized when designing a DS-CDMA system. In this paper, we consider the design of complex-valued spreading sequences with optimized correlation characteristics. In particular, the maximum nontrivial aperiodic correlation values are used to specify the cost or fitness function for the optimization problems. A genetic algorithm is presented for the design of polyphase sequences, namely, Oppermann sequences and modified Walsh-Hadamard sequences. It can be seen from these applications that the genetic algorithm is well suited to efficiently design polyphase sequences especially when the number of parameters for the optimization problem is large.

Performance Evaluation of the Markov Chain Monte Carlo MIMO Detector based on Mutual Information

Recently, MIMO detectors which are based on Markov Chain Monte Carlo (MCMC) simulation techniques have been proposed as alternatives to, e.g., the well-known sphere detector. In this paper, we present a systematic analysis of the performance of MCMC detectors. We study the impact of several parameters such as the list size and the number of independently running chains. As a performance criterion, our analysis is based on the mutual information over an equivalent modulation channel, rather than on coded bit error rates, because this metric is independent of the outer channel code and provides valuable insights over the whole SNR range of interest. Furthermore, we show that combining the MCMC detector with a hard-output sphere detector removes the error floor at high SNR, which is a well-known problem of the MCMC principle.

Improving UMTS LTE Performance by UEP in High Order Modulation

In this contribution we investigate the performance of the UMTS Long Term Evolution (LTE) physical layer using turbo coding and 64QAM with Gray mapping. We show how the mapping of systematic and parity bits to the six different bit positions defining one complex 64QAM symbol influences the convergence of the turbo decoder and thereby the bit error rate (BER) performance as well as number of necessary decoding iterations. Exploiting the unequal error protection (UEP) property of Gray mapped 64QAM results in an SNR performance gain of approximately 2 dB for the non-iterative system and in addition leads to a significant reduction of the necessary decoding iterations when iterative decoding is performed.

A Bit-Mapping Strategy for Joint Iterative Channel Estimation and Turbo-Decoding

In this paper, we investigate turbo-coded transmission over a temporally correlated flat Rayleigh fading channel. Conventionally, channel estimation is performed prior to decoding and is solely based on periodically inserted pilot symbols. We here consider the case that the pilot spacing is not small enough to satisfy the Nyquist criterion for the bandlimited channel process. It is demonstrated that iterative code-aided channel estimation can then significantly improve the bit error rate (BER). Furthermore, we show that the convergence speed of the iterative channel estimation can be significantly accelerated without additional computational power by just carefully choosing the mapping strategy of systematic bits to data symbols.

An improvement of UMTS LTE 64QAM performance

In this contribution we investigate the performance of the universal mobile telecommunications system (UMTS) long-term evolution (LTE) physical layer using turbo coding and 64QAM with Gray mapping. We show how the mapping of systematic and parity bits to the six different bit positions defining one complex 64QAM symbol influences the convergence of the turbo decoder and thereby the bit error rate (BER) performance as well as number of necessary decoding iterations. Exploiting the unequal error protection (UEP) property of Gray mapped 64QAM results in an signal-to-noise ratio (SNR) performance gain of approximately 2 dB for the non-iterative system and leads to goodput gains especially in regions of low SNR. Copyright

On the design of complex-valued spreading sequences using a genetic algorithm

We investigate the design of complex-valued spreading sequences with respect to a combination of different correlation properties. The two classes of complex sequences considered are the Oppermann sequences, which offer a wide range of correlation properties, and the modified Walsh-Hadamard sequences, which have been shown to offer good correlation properties. Since the number of parameters for the optimization problem is large, especially for the modified Walsh-Hadamard sequences, it is difficult if not impossible to use global optimization methods for solving such problems. Thus, we propose to transform the problem with continuous variables to another problem with discrete variables. This problem can then be solved efficiently using a genetic algorithm. These types of algorithms have been successfully applied in various areas, such as neural networks; however, to the authors best knowledge, their use for the design of complex spreading sequences has been rather sparse.

OFDM Turbo DeCodulation with exit optimized bit loading and signal constellations

We propose the combination of Orthogonal Frequency Division Multiplexing (OFDM) and Turbo DeCodulation (TDeC) - a multiple Turbo process consisting of iterative demodulation and iterative source-channel decoding - for transmission of correlated source codec parameters over wireless frequency-selective broadband fading channels. As OFDM systems split up frequency-selective fading channels into orthogonal flat-fading channels, individual modulation signal constellations sets with different bit mapping rules can be assigned to subcarriers taking into account the iterative reception and decoding process. The superior performance of the proposed OFDM-TDeC system is demonstrated by analyzing the achieved residual bit error rate (BER) and parameter signal-to-noise ratio (SNR).