Dieter Schafhuber

MMSE and adaptive prediction of time-varying channels for OFDM systems

We propose decision-directed channel predictors for orthogonal frequency-division multiplexing (OFDM) communications over time-varying channels. Channel prediction is capable of yielding up-to-date channel state information even without regular transmission of pilot symbols. It is thus potentially useful for delay-free equalization, antenna combining, space-time decoding, adaptive modulation, adaptive power control, and adaptive transmit antenna diversity. We derive a minimum mean-square error channel predictor and its efficient discrete Fourier transform implementation. Furthermore, we develop adaptive predictors that do not require any statistical prior knowledge and are able to track nonstationary channel and noise statistics. Simulation results involving an OFDM receiver in which channel prediction is used for delay-free equalization demonstrate the excellent performance of the proposed techniques even for fast time-varying channels.

Pulse-shaping OFDM/BFDM systems for time-varying channels: ISI/ICI analysis, optimal pulse design, and efficient implementation

This paper considers practically relevant aspects and advantages of pulse-shaping orthogonal/biorthogonal frequency division multiplexing (OFDM/BFDM) systems. We analyze the intersymbol/intercarrier interference (ISI/ICI) in such systems when they operate over time-varying channels. Two methods for an ISI/ICI-minimizing pulse design are proposed, and efficient FFT-based modulator and demodulator implementations are presented. Simulations show that for fast time-varying channels, optimized BFDM systems can outperform conventional OFDM systems with respect to ISI/ICI.

Analysis, Optimization, and Implementation of Low-Interference Wireless Multicarrier Systems

This paper considers pulse-shaping multicarrier (MC) systems that transmit over doubly dispersive fading channels. We provide exact and approximate expressions for the intersymbol and intercarrier interference occurring, in such systems. This analysis reveals that the time and frequency concentration of the transmit and receive pulses is of paramount importance for low interference. We prove the (nonobvious) existence of such jointly concentrated pulse pairs by adapting recent mathematical results on Weyl-Heisenberg frames to the MC context. Furthermore, pulse optimization procedures are proposed that aim at low interference and capitalize on the design freedom existing for redundant MC systems. Finally, we present efficient FFT-based modulator and demodulator implementations. Our numerical results demonstrate that for realistic system and channel parameters, optimized pulse-shaping MC systems can outperform conventional cyclic-prefix OFDM systems

Adaptive Wiener filters for time-varying channel estimation in wireless OFDM systems

In wireless OFDM systems, the time-varying channel is often estimated by a Wiener filter-type MMSE estimator based on pilot symbols. Such an estimator, however, requires statistical prior knowledge that is not easily obtained. Here, we propose adaptive Wiener filters for channel estimation that do not require statistical prior knowledge. We also calculate the performance limits of finite-length and infinite-length MMSE estimation. Simulation results demonstrate the good performance of our adaptive estimators.

Kalman tracking of time-varying channels in wireless MIMO-OFDM systems

The performance of coherent MIMO-OFDM systems critically depends on the availability of accurate channel estimates. In wireless communications, the MIMO channel is time-varying and thus has to be tracked by the receiver. In this paper, we develop an extended Kalman filter technique for pilot symbol assisted MIMO-OFDM channel tracking. Our tracking scheme is able to exploit spatial correlations of the channel. It includes on-line estimation of the channels state-space parameters and hence does not require any prior knowledge. Simulations using measured channels demonstrate the excellent performance of our channel tracking scheme.

Adaptive prediction of time-varying channels for coded OFDM systems

We propose adaptive channel predictors for orthogonal frequency division multiplexing (OFDM) communications over time-varying channels. Successful application of the normalized least-mean-square (NLMS) and recursive least-squares (RLS) algorithms is demonstrated. We also consider the use of adaptive channel predictors for delay-free equalization, thereby avoiding the need for regular transmission of pilot symbols. Simulation results demonstrate the good performance of the proposed techniques.

Adaptive identification and tracking of doubly selective fading channels for wireless MIMO-OFDM systems

In wireless MIMO-OFDM systems, accurate channel state information at the receiver is a prerequisite for large diversity and multiplexing gains. Here, we propose a pilot symbol assisted, LMS based, adaptive channel identification/tracking scheme for time-varying, delay-spread MIMO channels. Our scheme features automatic tuning of the LMS adaptation constants and modest computational complexity. No prior knowledge of channel statistics is required. Simulation results demonstrate the good performance of our algorithm for a wide range of SNR, maximum delay, and maximum Doppler.

System capacity of wideband OFDM communications over fading channels without channel knowledge

Assuming PSK modulation, we derive the system capacity of pulse-shaped orthogonal frequency division multiplexing (OFDM) communications over time-frequency selective fading channels in the absence of channel state information at the transmitter and the receiver. We show that capacity tends to zero in the large bandwidth limit and quantify the impact of spread and shape of the scattering function on finite bandwidth capacity

Wireless multicarrier communications via multipulse gabor riesz bases

We introduce multipulse multicarrier (MPMC) modulation, a wireless communication scheme that augments traditional single-pulse multicarrier systems by using multiple pulses at the transmitter and the receiver. The mathematical foundation of MPMC systems is established by the novel concept of multipulse Gabor Riesz bases. We adapt Zak-Fourier domain tools previously developed for multiwindow Gabor frames to analyze and design (bi)orthogonal multipulse Gabor Riesz bases and the corresponding MPMC systems in a computationally efficient manner. Furthermore, explicit expressions for the interference power and the spectral efficiency in MPMC transmissions over time-varying multipath channels are derived. The superiority of MPMC modulation over single-pulse multicarrier systems is finally demonstrated via numerical simulations.

Multipulse multicarrier communications over time-varying fading channels: performance analysis and system optimization

We present an interference and noise analysis for multipulse multicarrier (MPMC) transmission over time-varying fading channels. Based on this analysis, we propose algorithms and guidelines for system optimization (transmit and receive pulse shapes, time-frequency lattice parameters, and power allocation). Numerical simulations illustrate the optimization gain and the superiority of MPMC systems over traditional (single-pulse) multicarrier systems for highly dispersive channels.