Clemens Novak

Quantization for soft-output demodulators in bit-interleaved coded modulation systems

We study quantization of log-likelihood ratios (LLR) in bit-interleaved coded modulation (BICM) systems in terms of an equivalent discrete channel. We propose to design the quantizer such that the quantizer outputs become equiprobable. We investigate semi-analytically and numerically the ergodic and outage capacity over single- and multiple-antenna channels for different quantizers. Finally, we show bit error rate simulations for BICM systems with LLR quantization using a rate 1/2 low-density parity-check code.

MIMO-IDMA: Uplink Multiuser MIMO Communications using Interleave-Division Multiple Access and Low-Complexity Iterative Receivers

Interleave-division multiple access (IDMA) has recently been proposed as an alternative to CDMA. IDMA employs user-specific interleavers combined with low-rate channel coding for user separation. It can outperform coded CDMA when iterative receivers are used, and it allows the design of multiuser detectors with moderate complexity. In this paper, we extend IDMA to MIMO multiuser systems employing spatial multiplexing. We also develop an iterative receiver for MIMO-IDMA that incorporates an efficient soft multiuser detector whose complexity is linear in the number of users. Both flat-fading and frequency-selective MIMO channels are considered. The performance of the proposed MIMO-IDMA system is assessed through simulation results.

The effect of unreliable LLR storage on the performance of MIMO-BICM

Future semiconductor technologies will suffer from reliability issues, which requires a paradigm shift from the assumption of 100% reliable computations to fault-tolerant signal-processing systems. In this paper, we illustrate how the impact of unreliable circuits can be taken into account in the performance analysis of wireless communication systems. To this end, we consider the example of a multiple-input multiple-output (MIMO) system using bit-interleaved coded modulation in which a soft-output detector delivers quantized log-likelihood ratios (LLRs) to the channel decoder through an interleaver memory suffering from bit-flips due to reliability issues of the silicon implementation.

Log-likelihood ratio clipping in MIMO-BICM systems: Information geometric analysis and impact on system capacity

The clipping of log-likelihood ratios (LLRs) in soft demodulators for multiple-input multiple-output (MIMO) systems with bit-interleaved coded modulation (BICM) was recently observed to allow for enormous complexity savings. In this paper we first provide an information-geometric interpretation of LLR clipping as information projection onto a log-convex manifold. Then we study the system capacity of MIMO-BICM systems that use LLR clipping. Our results show that strong LLR clipping is possible without significant capacity loss. We finally propose an LLR transformation scheme which is necessary for approaching capacity in case of strong clipping. The usefulness of this LLR transformation is illustrated by numerical simulations for MIMO-BICM systems employing low-density parity check (LDPC) codes.

Synthesis and Preliminary Biological Evaluation of 2′-Substituted 2-(3′-Carboxybicyclo[1.1.1]pentyl)glycine Derivatives as Group I Selective Metabotropic Glutamate Receptor Ligands

The first series of 2′‐substituted 2‐(3′‐carboxybicyclo[1.1.1]pentyl)glycine derivatives, (2R)‐ and (2S)‐(2′,2′‐dichloro‐3′‐carboxybicyclo[1.1.1]pentyl)glycine (10) and (11), and 2‐(2′‐chloro‐3′‐carboxybicyclo[1.1.1]pentyl)glycine (12) were synthesized and evaluated as mGluR ligands. Compounds 11 and 12 were shown to be competitive group I mGluR antagonists. These results are also discussed in light of docking studies with both the active (closed) and inactive (open) conformations of mGluR1.

IDMA for the Multiuser MIMO-OFDM Uplink: A Factor Graph Framework for Joint Data Detection and Channel Estimation

Interleave-division multiple access (IDMA) has recently been proposed as a promising alternative to code-division multiple access (CDMA). In this paper, we consider the use of IDMA within a multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) multiuser system employing higher-order modulation and transmitting over frequency-selective MIMO channels. Based on a factor graph/message passing framework and the sum-product algorithm, we devise an iterative receiver that jointly performs pilot-aided channel estimation, multiuser detection, and channel decoding. The use of Gaussian message approximations results in a receiver complexity that scales only linearly with the number of users. A further complexity reduction is obtained by a novel selective message updating scheme. We also present a simulation-based comparison of the maximum rate achievable with our MIMO-OFDM-IDMA receiver with the information-theoretic capacity of the multiple-access channel. Finally, we provide simulation results illustrating the bit error rate (BER) performance of our receiver. It is observed that the proposed turbo-like integration of channel estimation in the iterative multiuser detection and channel decoding scheme yields a dramatic BER reduction, and that the proposed selective message updating scheme results in a significant reduction of complexity.

Factor graph based design of an OFDM-IDMA receiver performing joint data detection, channel estimation, and channel length selection

We present a factor graph based design of a receiver for pilot-assisted OFDM-IDMA systems transmitting over frequency-selective channels. The receiver performs joint iterative multiuser data detection and channel estimation with a complexity that is linear in the number of users, and it includes estimation of the channel length. Simulation results demonstrate large performance gains compared to OFDM-IDMA receivers using separate MMSE channel estimation.

Low-complexity MIMO-BICM receivers with imperfect channel state information: Capacity-based performance comparison

This paper provides a performance comparison of soft MMSE and max-log demodulators for multiple-input multiple-output (MIMO) bit-interleaved coded modulation (BICM) systems with imperfect channel state information. We use the capacity of the equivalent modulation channel as a code-independent performance metric. Our results show that the conventional approach of mismatched demodulation is noticeably inferior to optimal demodulators that are derived by conditioning on the channel estimate right from the beginning. Numerical simulations further demonstrate the importance of appropriate pilot power allocation.

Blind Monte Carlo detection-estimation method for optical coherence tomography

We consider the parametric analysis of frequency-domain optical coherence tomography (OCT) signals. A Monte Carlo (Gibbs sampler) detection-estimation method for determining the depths and reflection coefficients of tissue interfaces (reflective sites in the tissue) is proposed. Our method is blind since it estimates the instrumentation-dependent “fringe” function along with the tissue parameters. Sparsity of the detected interfaces is enforced by an impulse detector and a modified Bernoulli-Gaussian prior with a minimum distance constraint. Numerical results using synthetic and real signals demonstrate the excellent performance and fast convergence of our method.

Low-Complexity Factor Graph Receivers for Spectrally Efficient MIMO-IDMA

Interleave-division multiple access (IDMA) has recently been introduced as an attractive alternative to CDMA. IDMA employs user-specific interleavers combined with low-rate channel coding for user separation. In this paper, we consider a MIMO-IDMA system with increased spectral efficiency due to the use of higher-order symbol constellations. Based on a factor graph framework and the sum-product algorithm, we develop an iterative turbo multiuser receiver. Gaussian approximations for certain messages propagated through the factor graph lead to a complexity that scales only linearly with the number of users. To further reduce complexity, we introduce a selective message update scheme. Numerical simulations demonstrate the performance of the proposed receiver algorithms.