Ronald Böhnke

MMSE extension of V-BLAST based on sorted QR decomposition

In rich-scattering environments, layered space-time architectures like the BLAST system may exploit the capacity advantage of multiple antenna systems. We present a novel, computationally efficient algorithm for detecting V-BLAST architectures with respect to the MMSE criterion. It utilizes a sorted QR decomposition of the channel matrix and leads to a simple successive detection structure. The new algorithm needs only a fraction of computational effort compared to the standard V-BLAST algorithm and achieves the same error performance.

Near-maximum-likelihood detection of MIMO systems using MMSE-based lattice reduction

In recent publications the use of lattice-reduction for signal detection in multiple antenna systems has been proposed. In this paper, we adopt these lattice-reduction-aided schemes to the MMSE criterion. We show that an obvious way to do this is infeasible and propose an alternative method based on an extended system model, which in conjunction with simple successive interference cancellation nearly reaches the performance of maximum-likelihood detection. Furthermore, we demonstrate that, a sorted QR decomposition can significantly reduce the computational effort associated with lattice-reduction. Thus, the new algorithm clearly outperforms existing methods with comparable complexity.

Reduced complexity MMSE detection for BLAST architectures

Theoretical and experimental studies have shown that layered space-time architectures like the BLAST system can exploit the capacity advantage of multiple antenna systems in rich-scattering environments. We present a new efficient algorithm for detecting such architectures with respect to the MMSE criterion. This algorithm utilizes a sorted QR decomposition of the channel matrix and leads to a simple successive detection structure. The algorithm needs only a fraction of the computational effort compared to the standard V-BLAST algorithm and achieves the same bit error performance.

MMSE-based lattice-reduction for near-ML detection of MIMO systems

Recently the use of lattice-reduction for signal detection in multiple antenna systems has been proposed. In this paper, we adopt these lattice-reduction aided schemes to the MMSE criterion. We show that an obvious way to do this is suboptimum and propose an alternative method based on an extended system model. In conjunction with simple successive interference cancellation this scheme almost reaches the performance of maximum-likelihood detection. Furthermore, we demonstrate that the application of sorted QR decomposition (SQRD) as a initialization step can significantly reduce the computational effort associated with lattice-reduction. Thus, the new algorithm clearly outperforms existing methods with comparable complexity.

Weighted Sum Rate Maximization for the MIMO-Downlink Using a Projected Conjugate Gradient Algorithm

The maximization of a weighted sum of data rates is an essential point in cross-layer based resource allocation. Several algorithms have been proposed in the literature to solve this problem for the downlink of a multiple antenna system employing dirty paper preceding at the base station. However, they all suffer from a relatively slow convergence if the true number of objective function evaluations is taken into account. In this paper, an improved conjugate gradient method is presented, that takes the power constraint into account in the calculation of the search direction. Its superior convergence properties compared to existing approaches are verified by Monte-Carlo simulations for various scenarios.

SINR analysis for V-BLAST with ordered MMSE-SIC detection

A new way to determine the exact layer-wise SINR distribution for V-BLAST with successive interference cancellation at the receiver is presented. In contrast to previous publications, we do not restrict to zero-forcing, but also consider minimum mean square error interference suppression. It is shown analytically that an optimized detection order has an even larger impact in this case. Numerical examples provide deeper insights into the underlying effects.

Efficient near maximum-likelihood decoding of multistratum space-time codes

Multistratum space-time codes combine the layered transmission strategy of V-BLAST with space-time block coding in order to achieve high data rates and at the same time exploit transmit diversity. Simple detection schemes like successive interference cancellation severely suffer from the strong correlation between the strata, so true maximum-likelihood detection needs to be performed instead, which can be efficiently realized by the sphere decoding algorithm. However, most implementations known from the literature still have relatively high complexity, especially for low signal to noise ratio. Therefore, we propose several modifications that can reduce the computational effort by orders of magnitude while achieving at least near optimum performance.

A Two-Stage QoS Aware Scheduler for Wireless LANs Based on MIMO-OFDMA-SDMA Transmission

In this paper, a QoS-aware cross-layer scheduler for wireless LANs is introduced which can handle parallelised transmissions which are provided by OFDMA or SDMA. The scheduler includes two stages: in the hardware-independent stage, data flows are selected according to their QoS requirements such as throughput and delay. In the hardware-dependent stage, the channel resources are adaptively allocated to the flows according to the user priorities. All flows with a sufficiently high channel capacity are transmitted in parallel by assigning different OFDMA subcarriers or different spatial subchannels to the users. It is shown that the proposed QoS aware scheduler efficiently uses the channel resources in case of TDMA, OFDMA or SDMA and meets the quality-of-service requirements of time-critical flows under varying load and channel conditions.

Diversity vs. adaptivity in multiple antenna systems

An information theoretic analysis of multiple antenna systems shows that the possible gain due to transmitter-sided channel knowledge is usually small for uncorrelated fading. However, it is not obvious how these results translate to real-world transmission strategies. Thus, in this paper the error performance of adaptive bit and power loading with perfect channel state information is compared to that of high-rate multistratum space-time codes relying only on transmit diversity. We demonstrate that the latter may even outperform the adaptive scheme if discrete rate constraints are taken into account.

Multi-user detection in multicarrier-CDMA systems

The upcoming third generation mobile radio system in Europe is based on UMTS (Universal Mobile Telecommunications Standard). In order to supply access to a common transmission channel for several users, UMTS incorporates Code Division Multiple Access (CDMA). Besides a lot of practical advantages, CDMA suffers from multi-user interference limiting spectral efficiency dramatically. However, bandwidth is a very valuable resource and should be used as efficiently as possible. One appropriate mean to increase spectral efficiency of CDMA systems is multi-user detection. This paper gives an overview of different multi-user detection techniques and compares them with the conventional single-user detection including channel coding. Specifically, linear as well as nonlinear multi-user detectors are considered. Efficient realizations of linear detectors are presented and extended to improved nonlinear techniques. It is shown that nonlinear MUD including channel decoding can achieve a spectral efficiency twice as high as that of the well-known GSM standard (Global System for Mobile Communications) employing TDMA and FDMA.ZusammenfassungDie dritte Mobilfunkgeneration in Europa basiert auf dem Standard UMTS (Universal Mobile Telecommunications Standard), der als Vielfachzugriffsverfahren CDMA (Code Division Multiple Access) verwendet. Neben einer Vielzahl praktischer Vorteile besitzt das Codemultiplex den gravierenden Nachteil, dass sich die Teilnehmersignale gegenseitig stören (Mehrnutzerinterferenz), wodurch die spektrale Effizienz solcher Systeme dramatisch reduziert wird. Aufgrund der hohen Preise der Frequenzbänder sollten diese aber so effizient wie möglich genutzt werden. Eine Möglichkeit zur Erhöhung der spektralen Effizienz von CDMA-Systemen stellt dabei die Mehrnutzerdetektion dar. Dieser Artikel gibt einen Überblick über die diversen Ansätze zur Mehrnutzerdetektion und vergleicht sie mit der konventionellen Einnutzerdetektion. Insbesondere werden lineare und nichtlineare Verfahren betrachtet sowie effiziente Realisierungen basierend auf iterativen Strategien diskutiert. Anhand von Simulationsergebnissen wird gezeigt, dass mit der nichtlinearen Mehrnutzerdetektion inklusive Kanaldecodierung eine doppelt so hohe spektrale Effizienz wie in klassischen Zeit- und Frequenzmultiplexsystemen (GSM: Global System for Mobile Communications) erzielt werden kann.