Showing papers by "Johannes B. Huber published in 2002"

MIMO precoding for decentralized receivers

Abstract: This paper presents a modified version of Tomlinson-Harashima type precoding applicable to a setting with decentralized receivers, e.g. in a DS-CDMA downlink scenario. Precoding, i.e., nonlinear pre-equalization, is an interesting strategy for significantly simplifying signal processing at the receiver side (mobile terminals) at only moderate additional complexity at the base station.

Tomlinson-Harashima precoding in space-time transmission for low-rate backward channel

Abstract: In this paper, Tomlinson-Harashima precoding, a nonlinear pre-equalization technique, is proposed for transmission over multiple-input/multiple-output channels. Instead of equalizing intersymbol interference (temporal equalization) here spatial equalization, i.e., equalization of multi-user interference is performed. If only a low-rate backward channel is available for communicating channel state information back from the receiver to the transmitter, a compromise precoder setting, calculated from (medium-term) average channel knowledge in combination with linear residual equalization at the receiver side is proposed. Compared to an optimal adjustment of the precoder, i.e., perfect channel state information at the transmitter, only small losses have to be accepted.

On prefilter computation for reduced-state equalization

Abstract: In advanced time-division multiple-access (TDMA) mobile communications systems, reduced-state equalization algorithms have to be employed because high-level modulation is used in order to improve spectral efficiency. Reduced-state equalizers yield only high performance, if the overall discrete-time system to be equalized is minimum-phase. Therefore, in general, a discrete-time prefilter has to be inserted in front of equalization. For prefilter computation, several approaches are investigated in this paper. For the finite impulse response (FIR) prefilter case, which seems to be more relevant for practical applications than the in finite impulse response case, we discuss a method based on minimum mean-squared error decision-feedback equalization and a novel approach based on linear prediction (LP). The LP method seems to be very robust and requires an only moderate amount of computational complexity. Here, the prefilter consists of the cascade of a channel-matched filter and a prediction-error filter, which may be viewed as a finite-length approximation to the noise whitening part of the ideal prefilter transfer function. A key observation of the paper is that the proposed cascaded structure enables a very efficient prefilter computation because a prediction-error filter can be calculated via the Levinson-Durbin algorithm. Simulation results are given, which demonstrate that the performance of reduced-state equalization with proper FIR prefiltering is close to that of equalization combined with ideal all-pass prefiltering. Furthermore, it is shown that high performance can be obtained for TDMA mobile communications systems, if the LP scheme is employed for prefiltering.

A novel iterative multiuser detector for complex modulation schemes

Abstract: A novel multiuser detector for direct sequence code division multiple access is proposed. The receiver performs iterated soft decision interference cancellation (ISDIC) based on multiuser interference suppression filters designed for minimization of the mean-square error. Assuming a complex modulation format, we show that the multiuser interference becomes rotationally variant in the course of the iterations. Regarding this rotational variance in the design of the multiuser interference suppression filter, the presented iterative multiuser detector achieves significant performance gains compared with conventional ISDIC employing a standard minimum mean-squared error filter which is optimum only for rotationally invariant multiuser interference.

Design of \Multiple{Turbo{Codes" with Transfer Characteristics of Component Codes

Abstract: | For the construction of low rate turbo{codes, the concept of multiple parallel concatenatedcodes, i.e. the \multiple{turbo{code" has been suc-cessfully employed e.g. in [13, 16]. Analysis of thisstructure has been done via simulations [16] or theanalysis of equivalent coding schemes [13].In this paper we extend the method of EXIT charts[5], that has been applied to multiple{turbo{codesconsisting of identical component codes via serial{to{parallel conversion [13], to the analysis of asym-metric structures which employ difierent componentcodes. With this method, convergence analysis of thewhole class of multiple{turbo{codes and their designbecomes tractable. Keywords : Turbo{Codes, Convergence Analysis.I. Introduction Recently new constructions of turbo{codes [13, 16] have beenfound that outperform the original turbo{code [3] in both, thewaterfall and error{°oor regions. Additionally, these codeshave a signiflcantly lower decoding complexity.One of the ways to flnd such improved coding schemes,is the technique of code design via EXIT charts [5], whichhas been applied very successfully for serial concatenations[6] and classical turbo{codes [7]. In [13] it has been usedto analyze symmetric multiple{turbo{codes, which consist ofidentical constituent codes. This structure has been convertedto an equivalent serial concatenation of an outer repetitioncode and an inner rate{1 scrambler, that fltted within theframework of EXIT charts.In this paper we will focus on multiple{turbo{codes, asdescribed in Section II. We extend the method of conver-gence analysis by means of EXIT charts to the general caseof asymmetric multiple{turbo{codes, where we are no longerrestricted to use identical constituent codes. The properties ofthe component codes are determined by measuring their trans-fer characteristics in Section III. In Section IV, an abstractmodel for the information processing within the iterative de-coder of a multiple{turbo{code is established. This model en-ables us to determine the convergence properties of any codingscheme without simulation of the decoding procedure or theentire digital communication scheme. An algorithm for thisconvergence analysis is given in Section V. Examples for suc-cessful code designs of asymmetric multiple{turbo{codes aregiven in Section VI. Finally, in Section VII some conclusionsare given.

Soft-output-decoding: Some aspects from information theory

Abstract: Recent literature presents methods for the analysis of concatenated coding schemes by solely characterizing the behavior of the component codes [4], [12], [16], [9], [7]. Component codes are analyzed either analytically using unique properties of special component codes, e.g., single-parity-check code or accumulator, or via simulations. The goals of this paper are to find fundamental insights into concatenated codes by analyzing the input-output relation of their components from an information-theoretic point of view. We derive the Information Processing Characteristic (IPC), which completely characterizes the behavior of a coding scheme comprising encoder, code and decoder for the general class of linear codes. For time invariant convolutional codes it is studied how the IPC can be obtained in practice. (Less)

Iterative soft decision interference cancellation receivers for DS-CDMA downlink employing 4QAM and 16QAM

Abstract: Three promising receiver algorithms performing iterative soft decision interference cancellation (ISDIC) for a synchronous DS-CDMA system employing random spreading sequences and BPSK or 4QAM transmission, respectively, over an AWGN channel were proposed in Muller et al (1998) and Lampe et al (2001) In this paper, we extend these ISDIC receivers for a DS-CDMA downlink transmission over multipath channels when employing 4QAM and 16QAM or general square QAM constellations, respectively Additionally, modified variants including the channel decoder into the iteration loop are investigated It can be shown that the proposed schemes clearly surpass the standard RAKE receiver and approach the respective single user bounds within 05 to 2 dB in a UMTS scenario

Channel model for watermarks subject to desynchronization attacks

Abstract: One of the most important practical problems of blind Digital Watermarking is the resistance against desynchronization attacks, one of which is the Stirmark random bending attack in the case of image watermarking. Recently, new blind digital watermarking schemes have been proposed which do not suffer from host-signal interference. One of these quantization based watermarking scheme is the Scalar Costa Scheme (SCS). We present an attack channel for SCS which tries to model typical artefacts of local desynchronization. Within the given channel model, the maximum achievable watermark rate for imperfectly synchronized watermark detection is computed. We show that imperfect synchronization leads to inter-sample-interference by other signal samples, independent from the considered watermark technology. We observe that the characteristics of the host signal play a major role in the performance of imperfectly synchronized watermark detection. Applying these results, we propose a resynchronization method based on a securely embedded pilot signal. The watermark receiver exploits the embedded pilot watermark signal to estimate the transformation of the sampling grid. This estimate is used to invert the desynchronization attack before applying standard SCS watermark detection. Experimental results for the achieved bit error rate of SCS watermark detection confirm the usefulness of the proposed resynchronization algorithm.

Decision-feedback equalization for CDMA downlink

Abstract: A well-known receiver strategy for a linearly modulated signal transmitted over a frequency-selective channel is channel equalization. Recently it was proposed to employ a minimum mean-squared error (MMSE) channel equalizer for the downlink of CDMA. In this paper, we introduce a new receiver concept using MMSE channel equalization as a first stage and MMSE decision-feedback equalization (DFE) utilizing soft feedback from the decoding unit as a second stage. Both schemes are compared for the downlink of CDMA. It turns out, that after channel decoding we gain about 1 dB compared to conventional MMSE channel equalization.

Coded continuous phase modulation with low-complexity noncoherent reception

Abstract: Coded continuous phase modulation based on a feedback-free modulator with noncoherent detection is discussed. Low-complexity receiver processing is achieved by using only two or three linear filters for demodulation and applying noncoherent sequence estimation with reduced-state Viterbi decoding and simple branch metric calculation. Overall, the proposed noncoherent receiver provides significant advantages over previously presented approaches.

Signal shaping for peak-power and dynamics reduction in transmission schemes employing precoding

Abstract: Precoding, i.e., nonlinear pre-equalization, at the transmitter side has been proved to be a very efficient strategy for channel equalization in single-carrier digital transmission schemes. It enables the application of coded modulation in a seamless fashion. A drawback of precoding is that the signal at the input of the decision device exhibits a huge dynamic range. Based on dynamics shaping, a combined precoding/shaping technique introduced in the paper by Fischer et al. (1995), a new shaping strategy is developed in this paper. This technique enables a flexible tradeoff among: (1) reduction of the average transmit power; (2) avoidance of peaks in the transmit signal in order to facilitate line driver implementation; and (3) restriction of the maximum amplitude at the receiver side to a prescribed value. Over a wide range, all three demands can be met simultaneously. As the scheme is fully compatible with Tomlinson-Harashima (1971, 1972) precoding, it can replace the precoder even in existing and standardized schemes. Simulation results for a typical digital subscriber line scenario show the achievable gains.

Extrinsic and intrinsic information in systematic coding

Abstract: The analytical separation of extrinsic and intrinsic parts of the decoder output connects new analysis methods e.g., the EXIT charts, to traditional performance measures (e.g., bit error ratio, first error event probability) and their soft-output equivalents.

Grundlagen der Wahrscheinlichkeitsrechnung für iterative Decodierverfahren

Abstract: In dieser Ubersichtsarbeit werden die Grundgleichungen der iterativen Decodierung mittels elementarer Verfahren der Wahrscheinlichkeitsrechnung abgeleitet und zusammengestellt. Dabei zeigt sich, dass durch direkte Reprasentation einer Symbolzuverlassigkeit mittels der a-posteriori-Wahrscheinlichkeit diese Beziehungen einfach darzustellen sind, und die Prinzipien der iterativen Decodierung dadurch leicht fasslich formuliert werden konnen. Die Vorteile anderer gebrauchlicher Methoden zur Reprasentation von Symbolzuverlassigkeiten und deren Wechselbeziehungen werden diskutiert.

Data-Aided Symbol Timing Estimation for Linear Modulation

Abstract: Summary Timing phase estimation (TPE) plays the key role in feedforward (FF) symbol timing.For reasons of performance often data-aided (DA) methods are preferred. But frequently, they turn out to be critical with respect to theimplementation and the spectrum efficiency (due to the required overhead).This paper presents three DA TPE methods for quadrature pulse amplitude modulation (PAM). In spite of their very low complexity, these methods closelyapproach the theoretical limit for timing estimation with respect to the estimation variance, even at low SNR. This enables power efficient transmission.Further, they employ a CAZAC (constant amplitude, zero auto-correlation)sequence as training-sequence (TS), or a sequence with similar correlationproperties. Since such sequences are suited for almost all DA receiver tasks, a high spectrum efficiency can be obtained by the use of a single TS. A generalization of the proposed methods for DA TPE withrespect to the choice of the TS is also shown. The presented methods can be applied to noncoherent receivers. They are suited for high data-rate applications, since they can work with an oversampling factor of 2.

Sendevorrichtung und Empfangsvorrichtung Transmitting device and receiving device

Abstract: Bei einem Sende/Empfangs-Konzept wird ein Redundanz-hinzufugender Codierer mit einer Coderate kleiner oder gleich 0,5 verwendet, um zwei Datenstrome fur zwei unterschiedliche Sender zu erhalten, die an raumlich unterschiedlichen Positionen angeordnet sind. In a transmission / reception concept, a redundancy-adding Direction encoder with a code rate is less than or equal to 0.5 is used to obtain two data streams to two different stations, which are located at spatially different positions. Beide Sender senden in demselben Frequenzband. Both stations broadcast in the same frequency band. Im Empfanger wird das Empfangssignal von einem ersten Abtaster synchron zum ersten Sender abgetastet, und von einem zweiten Abtaster synchron zum zweiten Sender abgetastet, um ein erstes und zweites Empfangssignal zu erhalten, welche einem Trellis-Decodierer zugefuhrt werden, um eine decodierte erste und zweite Empfangs-Untergruppe von Codeeinheiten zu erhalten, welche wiederum einer Berechnungseinrichtung zugefuhrt werden, um die Interferenzsignale zu berechnen, die dann zur Interferenzreduktion mit den entsprechenden Empfangssignalen kombiniert werden. In the receiver the received signal by a first scanner is scanned synchronously to the first transmitter, and scanned by a second scanner in synchronism with the second transmitter, to obtain a first and a second reception signal which are fed to a trellis decoder to provide a decoded first and second receiving to obtain subgroup of code units, which in turn are fed to a calculating means to calculate the interference signals, which are then combined into interference reduction with the corresponding received signals. Das iterative Konzept ermoglicht eine Interferenzreduktion fur Empfangssignale, die von zwei raumlich getrennten, jedoch beide im gleichen Frequenzband sendenden Sendern bestimmt werden. The iterative approach allows for interference reduction for receiving signals, which are determined by two spatially separated but both transmitting in the same frequency band transmitters. Das Empfangerkonzept zeigt eine schnelle Konvergenz und ermoglicht somit, das die beiden Sender im selben Frequenzband senden, was in einer Reduzierung der benotigten Bandbreite um die Halfte im Vergleich zu einem entsprechenden Senderkonzept resultiert, bei dem der erste und der zweite Sender bei unterschiedlichen Frequenzen senden. The receiver concept shows a fast convergence and therefore allows that the two transmitters transmit in the same frequency band, resulting in a reduction in the required bandwidth by half as compared to a corresponding transmitter concept at the sending of the first and second transmitters at different frequencies.

Construction of low-rate power-efficient coding schemes and their application to CDMA

Abstract: We propose a novel design method, which is an extension to the EXIT charts introduced by ten Brink (see IEE Electronics Letters, vol.35, no.10, p. 806-808, 1999), for low-rate power-efficient codes. An exemplary design of a rate -1/4 code shows more than 1 dB more coding gain than the original turbo-code of rate -1/2. Using this code in a CDMA scheme while reducing the spreading by a factor of 2 helps improving the overall performance of the multi-user scheme significantly.