Meik Dörpinghaus

Improving MIMO phase noise estimation by exploiting spatial correlations

Phase locked loops (PLL) for RF carrier synthesis often employ oscillators that insert a considerable amount of time varying phase noise into the received signal. That noise must then be removed in the digital baseband receiver. This phase noise is an indivisible superposition of noise components from receiver and transmitter. Regarding systems with multiple transmit and receive antennas (MIMO) and if multiple PLL for carrier synthesis are used each of the superposed phase noise processes per transmit and receive antenna pair can be measured at the receiver. This paper provides a new scheme for high SNR scenarios that exploits spatial correlation between these overlaying phase noise processes at the receiver in order to improve estimation and compensation of the phase noise. Therefore the Wiener filter approach is applied.

Joint Reduction of Peak-to-Average Power Ratio and Out-of-Band Power in OFDM Systems

The high peak-to-average power ratio is a major drawback of OFDM systems. Many PAPR reduction techniques have been proposed in the literature, among them a method that uses a subset of tones that do not carry any data, but are modulated such that the PAPR of the resulting time domain signal is minimized. Another problem of OFDM systems is the high out-of-band power caused by the sidelobes of the modulated tones. The OBP can be reduced by modulating reserved tones at the edges of the occupied spectrum so that the sidelobes of the data carriers are reduced. In this paper, we propose to consider both optimization problems jointly. This way, the amount of PAPR and OBP reduction can be significantly enhanced in comparison to a system that performs two separate optimization steps. Furthermore, the joint reduction algorithm offers more flexibility, because the relative weighting of the two optimization criteria can easily be changed, resulting in a smooth trade-off curve.

Characterization of Non-Stationary Channels Using Mismatched Wiener Filtering

A common simplification in the statistical treatment of linear time-varying (LTV) wireless channels is the approximation of the channel as a stationary random process inside certain time-frequency regions. We develop a methodology for the determination of local quasi-stationarity (LQS) regions, i.e., local regions in which a channel can be treated as stationary. Contrary to previous results relying on, to some extent, heuristic measures and thresholds, we consider a finite-length Wiener filter as realistic channel estimator and relate the size of LQS regions in time to the degradation of the mean square error (MSE) of the estimate due to outdated and thus mismatched channel statistics. We show that for certain power spectral densities (PSDs) of the channel a simplified but approximate evaluation of the matched MSE based on the assumption of an infinite filtering length yields a lower bound on the actual matched MSE. Moreover, for such PSDs, the actual MSE degradation is upper-bounded and the size of the actual LQS regions is lower-bounded by the approximate evaluation. Using channel measurements, we compare the evolution of the LQS regions based on the actual and the approximate MSE; they show strong similarities.

On the Gain of Joint Processing of Pilot and Data Symbols in Stationary Rayleigh Fading Channels

In many typical mobile communication receivers, the channel is estimated based on pilot symbols to allow for a coherent detection and decoding in a separate processing step. Currently, much work is spent on receivers which break up this separation, e.g., by enhancing channel estimation based on reliability information on the data symbols. In this paper, we evaluate the possible gain of a joint processing of data and pilot symbols in comparison to the case of a separate processing in the context of stationary Rayleigh flat-fading channels. Therefore, we discuss the nature of the possible gain of a joint processing of pilot and data symbols. We show that the additional information that can be gained by a joint processing is captured in the temporal correlation of the channel estimation error of the solely pilot-based channel estimation, which is not retrieved by the channel decoder in case of separate processing. In addition, we derive a new lower bound on the achievable rate for joint processing of pilot and data symbols. Finally, the results are extended to multiple-input multiple-output channels.

Decision Making in the Arrow of Time

We show that the steady-state entropy production rate of a stochastic process is inversely proportional to the minimal time needed to decide on the direction of the arrow of time. Here we apply Walds sequential probability ratio test to optimally decide on the direction of times arrow in stationary Markov processes. Furthermore, the steady-state entropy production rate can be estimated using mean first-passage times of suitable physical variables. We derive a first-passage time fluctuation theorem which implies that the decision time distributions for correct and wrong decisions are equal. Our results are illustrated by numerical simulations of two simple examples of nonequilibrium processes.

On the achievable rate of stationary Rayleigh flat-fading channels with Gaussian input distribution

For a Gaussian input distribution, we investigate the achievable rate of a stationary Rayleigh flat-fading channel under the assumption of unknown channel state information at transmitter and receiver side. The law of the channel is presumed to be known to the receiver. In addition, we assume the power spectral density of the fading process to be compactly supported. The contribution of the present paper is the derivation of an upper bound on the achievable rate for the special case of a rectangular power spectral density depending on the SNR and the spread of the power spectral density. For comparison, we also give a lower bound on the achievable rate which is already known from and holds for an arbitrary power spectral density.

Enhanced Predictive Up/Down Power Control for CDMA Systems

In this paper we derive an enhanced power control algorithm, fitting into the up/down control scheme, as it is considered in the frequency division duplex (FDD) mode of the current 3GPP standard. Analysis of the classical up/down power control scheme unveils, that with increasing velocities the power control performance degrades, as the fixed step size power control is not able to track the channel fading properly. For the uplink we derive a nonlinear control algorithm generating the up/down power control commands accounting for the future of the channel fading process. Simulations show that this algorithm in combination with perfect future channel state information can partially mitigate the drawbacks of a fixed step-size up/down power control. A prerequisite for predictive power control is the acquisition of the future channel state information. In this paper we deduce a robust and adaptive structure for the prediction of the channel fading process in the context of a power controlled code division multiple access (CDMA) system based on least mean square (LMS) adaptation. Link level simulations show a signal to noise and interference ratio (SINR) gain in terms of the block error rate, enabling a decrease of the target SINR and thus leading to an enhanced spectral efficiency.

Energy-Efficient Transceivers for Ultra-Highspeed Computer Board-to-Board Communication

Enabling the vast computational and throughput requirements of future high performance computer systems and data centers requires innovative approaches. In this paper, we will focus on the communication between computer boards. One alternative to the bottleneck presented by copper wire based cable-bound communication is the deployment of wireless links between nodes consisting of processors and memory on different boards in a system. In this paper, we present an interdisciplinary approach that targets an integrated wireless transceiver for short-range ultra-high speed computer board-to-board communication. Based on our achieved results and current developments, we will also estimate energy consumption of such a transceiver.

On the Achievable Rate of Stationary Rayleigh Flat-Fading Channels With Gaussian Inputs

In this work, a discrete-time stationary Rayleigh flat-fading channel with unknown channel state information at transmitter and receiver side is studied. The law of the channel is presumed to be known to the receiver. For independent identically distributed (i.i.d.) zero-mean proper Gaussian input distributions, the achievable rate is investigated. The main contribution of this paper is the derivation of two new upper bounds on the achievable rate with Gaussian input symbols. One of these bounds is based on the one-step channel prediction error variance but is not restricted to peak power constrained input symbols like known bounds. Moreover, it is shown that Gaussian inputs yield the same pre-log as the peak power constrained capacity. The derived bounds are compared with a known lower bound on the capacity given by Deng and Haimovich and with bounds on the peak power constrained capacity given by Sethuraman et al.. Finally, the achievable rate with i.i.d. Gaussian input symbols is compared to the achievable rate using a coherent detection in combination with a solely pilot-based channel estimation.

Achievable rate with receivers using iterative channel estimation in stationary fading channels

We study the achievable rate with receivers based on synchronized detection and iterative code-aided channel estimation for stationary Rayleigh flat-fading channels. The main idea behind this type of receivers is that — additionally to the pilot symbols which are used for the initial channel estimation and coherent detection/decoding — the channel estimation is enhanced by iteratively feeding back reliability information on the data symbols acquired by the channel decoder. For a specific type of such a receiver, we derive an upper bound on the achievable rate. Based on an approximation of the upper bound, which is not a closed-form expression, we are able to upper-bound the possible performance gain when using this specific receiver based on code-aided channel estimation in comparison to receivers using synchronized detection with a solely pilot based channel estimation. Furthermore, we compare this approximate upper bound with a lower bound on the achievable rate with joint processing of data and pilot symbols given in [1]. In addition, we show which part of the mutual information between the transmitter and the receiver cannot be exploited by the given receiver structure using synchronized detection in combination with iterative code-aided channel estimation.