Christoph Windpassinger

Precoding in multiantenna and multiuser communications

In this paper, Tomlinson-Harashima Precoding for multiple-input/multiple-output systems including multiple-antenna and multiuser systems is studied. It is shown that nonlinear preequalization offers significant advantages over linear preequalization which increases average transmit power. Moreover, it outperforms decision-feedback equalization at the receiver side which is applicable if joint processing at the receiver side is possible, and which suffers from error propagation. A number of aspects of practical importance are studied. Loading, i.e., the optimum distribution of transmit power and rate is discussed in detail. It is shown that the capacity of the underlying MIMO channel can be utilized asymptotically by means of nonlinear precoding.

Low-complexity near-maximum-likelihood detection and precoding for MIMO systems using lattice reduction

We consider the lattice-reduction-aided detection scheme for 2/spl times/2 channels recently proposed by H. Yao and G.W. Wornell (see Proc. IEEE Globecom, 2002). By using an equivalent real-valued substitute MIMO channel model, their lattice reduction algorithm can be replaced by the well-known LLL (Lenstra-Lenstra-Lovasz) algorithm (Lenstra, A.K. et al., 1982), which enables the application of the algorithm to MIMO systems with arbitrary numbers of dimensions. We show how lattice reduction can also be favourably applied in systems that use precoding and give simulation results that underline the usefulness of this approach.

Lattice-reduction-aided broadcast precoding

A precoding scheme for multiuser broadcast communications is described, which fills the gap between the low-complexity Tomlinson-Harashima precoding and the sphere decoder-based system of Peel et al. Simulation results show that, replacing the closest-point search with the Babai approximation, the full diversity order supported by the channel is available to each user, as in the system of Peel et al., and unlike Tomlinson-Harashima precoding, which suffers some diversity penalty. The complexity of the scheme is similar to that of Tomlinson-Harashima precoding.

MIMO precoding for decentralized receivers

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

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.

Multiple-symbol differential sphere decoding

In multiple-symbol differential detection (MSDD) for power-efficient transmission over Rayleigh fading channels without channel state information, blocks of N received symbols are jointly processed to decide on N-1 data symbols. The search space for the maximum-likelihood (ML) estimate is therefore (complex) (N-1)-dimensional, and maximum-likelihood MSDD (ML-MSDD) quickly becomes computationally intractable as N grows. Mackenthuns low-complexity MSDD algorithm finds the ML estimate only for Rayleigh fading channels that are time-invariant over an N symbol period. For the general time-varying fading case, however, low-complexity ML-MSDD is an unsolved problem. In this letter, we solve this problem by applying sphere decoding (SD) to ML-MSDD for time-varying Rayleigh fading channels. The resulting technique is referred to as multiple-symbol differential sphere decoding (MSDSD).

Precoding and loading for BLAST-like systems

In this contribution we study transmission over MIMO channels, where channel state information available at the transmitter enables the use of precoding as weak as an optimization of rate and power distribution over the parallel subchannels in the transmission system (loading). It is shown that spatial loading provides substantial gains and a revised version of the V-BLAST algorithm is given to calculate the matrix filters required for Tomlinson-Harashima precoding, or equivalently decision-feedback equalization.

Improved MIMO precoding for decentralized receivers resembling concepts from lattice reduction

The paper presents an improved version of precoding, i.e., nonlinear pre-equalization, for situations with one central transmitter (e.g., base station) and a number of distributed receivers (e.g., mobile terminals). The new method is based on Tomlinson-Harashima precoding, applied to multiple-input-multiple-output (MIMO) channels, and inspired by the concept of lattice-reduction-aided detection in MIMO communication systems. We show that, by using the proposed technique, a considerable degree of diversity can be gained over linear pre-equalization, as well as conventional precoding.

Signal processing in decision-feedback equalization of intersymbol-interference and multiple-input/multiple-output channels: a unified view

Digital transmission over dispersive channels which suffer from intersymbol interference (ISI) and/or over multiple-input/ multiple-output (MIMO) channels which exhibit multi-user interference is reviewed. In particular, the digital signal processing required at the receiver for performing decision-feedback equalization is studied. The aims of processing the received signal are discussed and compared for ISI channels, MIMO channels, and MIMO ISI channels. The calculation of the optimum filters for these applications is presented in a unified way, and it is shown that each aspect in one field has its immediate correspondence in the other application.

Modulo-lattice reduction in precoding schemes

This paper discusses the use of modulo arithmetics in the precoding scheme for N-dimensional data symbols. The transmission scheme employing modulo reduction mod-/spl Lambda/ precoding is an attractive strategy when transmitting over AWGN channels, which suffer from additive noise, and an additional interference term, which is already known at the transmitter side. The capacity of channels and the use of optimum lattice quantizers for minimum error entropy approach applying numerical optimization are also studied.