Wolfgang Rave

Effects of Phase Noise on OFDM Systems With and Without PLL: Characterization and Compensation

In this paper, we propose an algorithm for suppressing intercarrier interference due to phase noise in coded orthogonal frequency division multiplexing (OFDM) systems. The algorithm approximates the phase-noise waveform by using a Fourier series approximation for the current phase-noise realization. Thereby, it cancels the effects of the phase noise beyond the standard common phase error correction used in contemporary OFDM standards. The algorithm requires that the correlation properties of the intercarrier interference are known. We calculate these properties in terms of the phase-noise spectral correlation matrix for both Wiener and Ornstein-Uhlenbeck phase-noise models, respectively. This modeling corresponds to a free-running oscillator, as well as a phase-locked loop realization of the local oscillator in orthogonal frequency division multiplexing transceivers. For both transceiver configurations, we investigate the performance of the proposed algorithm. It is demonstrated that the new algorithm achieves as much as one order of magnitude better performance in terms of packet/bit error rate when compared to a receiver with only the common phase error suppression.

Dirty RF: a new paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with todays deep sub-micron technology, analog impairments-dirt effects-are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, IQ imbalance, ADC impairments, etc. In the world of dirty RF we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing

Dirty RF: A New Paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with today’s deep sub-micron technology, analog impairments – “dirt effects” – are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, I/Q imbalance, ADC impairments, etc. In the world of “Dirty RF” we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing.

Corners and nucleation in Micromagnetics

The divergence of the stray field in corners and its consequence for micromagnetic calculations was studied numerically in two dimensions for high-anisotropy materials. The results show that no atomistic theory has to be invoked because the singularity is smoothed out already within micromagnetics. The magnetic configurations and its deduced critical quantity, the coercivity, are determined correctly if the configurations are well approximated on the exchange length SQRT(A/Kd). The singularity in the stray field remains only visible in the torque balance where it is compensated by an exchange torque.

Quantitative observation of magnetic domains with the magneto-optical Kerr effect

Abstract A new method for the quantitative determination of the magnetization direction at every point of a sample surface is presented. The method uses our digitally enhanced Kerr microscope and is based on a combination of the longitudinal and the transverse Kerr effect. The information from both effects is digitally combined with pictures taken in different saturated states. The resulting map of magnetization directions is displayed by arrows and additionally with the help of a colour code. Examples taken from complicated stress-governed domain structures on a metallic glass show pictures which are clearly easier to interpret than conventional qualitative Kerr images.

Intercarrier interference due to phase noise in OFDM - estimation and suppression

In this paper we provide an analysis of the intercarrier interference (ICI) due to phase noise in OFDM systems and present an algorithm for its suppression. We examine the general case where phase noise can take any values, thus the ”small” phase noise model is dropped. The statistical properties of the intercarrier interference are analyzed, showing that the ICI is generally a non-gaussian random process which has a large impact on the system performance. Closed form expressions which describe the correlation properties of the constituents of ICI are calculated. An MMSE approach for suppressing ICI in the frequency domain is presented. This approach avoids error propagation to which our previously proposed algorithm was prone. The performance of the suppression algorithm is shown, pointing out the limits for the ICI suppression algorithms in general.

Common phase error due to phase noise in OFDM-estimation and suppression

Orthogonal frequency division multiplexing (OFDM) has already become a very attractive modulation scheme for many applications. Unfortunately OFDM is very sensitive to synchronization errors, one of them being phase noise, which is of great importance in modern WLAN systems which target high data rates and tend to use higher frequency bands because of the spectrum availability. In this paper we propose a linear Kalman filter as a means for tracking phase noise and its suppression. The algorithm is pilot based. The performance of the proposed method is investigated and compared with the performance of other known algorithms.

Relaying in CDMA networks: pathloss reduction and transmit power savings

Relaying has emerged as a field of growing interest for wireless systems. The use of intermediate nodes for relaying information from a source to its destination promises improvements on various levels, ranging from increased connectivity and reduced transmit powers to diversity gains. We examine various propagation models and network parameters and show to which extent the pathloss in cellular wireless systems can be reduced by the use of relay nodes in a two-hop scenario. Having highlighted these potentials, we discuss by means of numerical analysis and system-level simulation under which conditions these savings can be turned into a transmit power reduction for CDMA FDD systems. It becomes evident that the overall performance of the relay system depends on the node density and the relative load.

Quantization of Log-Likelihood Ratios to Maximize Mutual Information

We propose a quantization scheme for log-likelihood ratios which optimizes the trade-off between rate and accuracy in the sense of rate distortion theory: as distortion measure we use mutual information to determine quantization and decision levels maximizing mutual information for a given rate over a Gaussian channel. This approach is slightly superior to the previously proposed idea of applying the Lloyd-Max algorithm to the dasiasoft bitpsila density associated to the L-values. A further data rate reduction can be achieved with entropy coding, because the optimum quantization levels based on mutual information are used with pronounced unequal probabilities.

Combined Kerr-/magnetic force microscopy on NdFeB crystals of different crystallographic orientation

Magnetic force microscopy images of NdFeB crystals of different crystallographic orientation are directly compared with magneto-optical Kerr images. This comparison facilitates the analysis of contrast mechanisms in the MFM images. Different contributions to the contrast arise from the Zeeman interaction energy between (unperturbed) tip and sample on the one hand and the mutual reaction of their magnetizations to the stray fields. These contributions are separated by forming sum and difference images from pictures taken with opposite tip magnetization. This technique yields complementary images of the surface charge pattern of the sample and its local susceptibility.