Erich Zöchmann

Directional evaluation of receive power, Rician K-factor and RMS delay spread obtained from power measurements of 60 GHz indoor channels

To meet link budgets for millimetre wave wireless communications, antennas with high directivity are essential. This directional view of the channel is evaluated by the important small scale fading parameters receive power, Rician K-factor and the RMS delay spread. These parameters are all derived from frequency swept power measurements. The effect of transmit polarization is very visual in the measurement results. A maximum K-factor of almost 80 was observed. The RMS delay spread is approximately 4 ns at reflective reception.

Comparing antenna selection and hybrid precoding for millimeter wave wireless communications

Due to the small wavelengths at millimetre wave frequencies, antenna arrays with dozens of elements fit easily even in hand-held devices. As RF chains are power hungry and costly, all-digital approaches are impractical and analogue-digital co-designs are likely to come. Common for all approaches is a dimensional reduction of the large MIMO channel. This study compares Antenna Selection (AS) and Hybrid Precoding (HP). In HP systems, channels are estimated employing a set of hybrid codebooks exploiting all antennas. For AS we propose a bistatic MIMO radar method to estimate the channel even with a reduced number of antennas, identical to the number of RF chains.

Low-latency MISO FBMC-OQAM: It works for millimeter waves!

A key enabler for high data rates in future wireless systems will be the usage of millimeter Waves (mmWaves). Furthermore, Filter Bank Multi-Carrier (FBMC) with its good spectral properties has also been considered as a possible future transmission technique. However, many authors claim that multiple antennas and low-latency transmissions, two of the key requirements in 5G, cannot be efficiently supported by FBMC. This is not true in general, as we will show in this paper. We investigate FBMC transmissions over real world channels at 60 GHz and show that Alamoutis space time block code works perfectly fine once we spread (code) symbols in time. Although it is true that spreading increases the transmission time, the overall transmission time is still very low due to the high subcarrier spacing employed in mmWaves. Therefore, coded FBMC in combination with mmWaves enables high spectral efficiency, low-latency and allows the straightforward usage of multiple antennas.

c-LASSO and its dual for sparse signal estimation from array data

We treat the estimation of a sparse set of sources emitting plane waves observed by a sensor array as a complex-valued LASSO (c-LASSO) problem where the usual ź1-norm constraint is replaced by the ź1-norm of a matrix D times the solution vector. When the sparsity order is given, algorithmically selecting a suitable value for the c-LASSO regularization parameter remains a challenging task. The corresponding dual problem is formulated and it is shown that the dual solution is useful for selecting the regularization parameter of the c-LASSO. The solution path of the c-LASSO is analyzed and this motivates an order-recursive algorithm for the selection of the regularization parameter and a faster iterative algorithm that is based on a further approximation. This greatly facilitates computation of the c-LASSO-path as we can predict the changes in the active indices as the regularization parameter is reduced. Using this regularization parameter, the directions of arrival for all sources are estimated. HighlightsThe dual problem to the complex-valued c-LASSO is formulated and analyzed.The dual solution is the output of a weighted matched filter of c-LASSO residuals.The dual solution motivates a selection of the c-LASSO regularization parameter.This gives an order-recursive scheme for selecting the regularization parameter.Approximations lead to the formulation of faster iterative algorithms for its solution.

Exploring the physical layer frontiers of cellular uplink

Communication systems in practice are subject to many technical/technological constraints and restrictions. Multiple input, multiple output (MIMO) processing in current wireless communications, as an example, mostly employs codebook-based pre-coding to save computational complexity at the transmitters and receivers. In such cases, closed form expressions for capacity or bit-error probability are often unattainable; effects of realistic signal processing algorithms on the performance of practical communication systems rather have to be studied in simulation environments. The Vienna LTE-A Uplink Simulator is a 3GPP LTE-A standard compliant MATLAB-based link level simulator that is publicly available under an academic use license, facilitating reproducible evaluations of signal processing algorithms and transceiver designs in wireless communications. This paper reviews research results that have been obtained by means of the Vienna LTE-A Uplink Simulator, highlights the effects of single-carrier frequency-division multiplexing (as the distinguishing feature to LTE-A downlink), extends known link adaptation concepts to uplink transmission, shows the implications of the uplink pilot pattern for gathering channel state information at the receiver and completes with possible future research directions.

Low Complexity Estimation of Frequency Selective Channels for the LTE-A Uplink

3GPP LTE-A uplink channel estimation is based on pilot symbols. For the support of MIMO transmissions, a special reference symbol structure was standardized to perform MIMO channel estimation without excessive overhead. We propose a low complexity channel estimation method in the frequency domain based on correlation that exploits the orthogonal structure of the reference symbols to mitigate inter-layer interference. Further, we introduce a smoothing method to deal with the interference that leads to improved performance at low signal to noise ratio.

Limited feedback in OFDM systems for combating ISI/ICI caused by insufficient cyclic prefix length

In 3GPP LTE communication systems, channel state information is fed back by means of the channel quality indicator, the precoding matrix index and the rank indicator. This limited feedback information is used to adapt the transmission scheme such as to improve the throughput and to reach a certain target block error ratio. Based on the channel impulse response we calculate the interference power caused by insufficient cyclic prefix length. By accounting for this inter symbol and inter carrier interference power in the feedback calculation, we obtain an improved throughput in OFDM systems.

On the Influence of Doubly-Selectivity in Pilot-Aided Channel Estimation for FBMC-OQAM

Superior spectral properties of Filter Bank Multi-Carrier (FBMC) techniques make them an interesting choice for future wireless systems. In order to directly apply well known pilot-symbol aided channel estimation in FBMC, however, the intrinsic imaginary interference has to be canceled. Such cancellation method is usually designed for a doubly-flat channel. In this paper, we investigate the effects of doubly-selectivity, that is, time-selectivity and frequency-selectivity, on the channel estimation Mean Squared Error (MSE) in FBMC. The calculation of the MSE is based on a compact matrix description, allowing us also to find an MSE minimizing channel estimation method that takes the underlying waveform into account.

Resolving the angular profile of 60 GHz wireless channels by delay-Doppler measurements

Previous channel measurement campaigns in millimetre wave bands were mostly focussing on time invariant channels of static environments. Highly directive antennas were swept in azimuth and elevation angle to extract the angular profile of the double-directional radio channel. This angular profile is also visual in the Doppler domain. Therefore, contrary to common Doppler modelling, close scattering objects have to be taken into account. In this paper, a methodology for repeatable estimation of the angular profile through delay-Doppler measurements is proposed. Repeat-able measurements of the static scenario allow for 1) optimization of the probing signals to reduce the geometrical effects and 2) disambiguation of the angular estimates through different measurement positions.

Quantifying the repeatability of wireless channels by quantized channel state information

Repeatability is the prerequisite for scientific evaluation of wireless measurements. However, in real-world scenarios, the channel always slightly changes with time as, for example, trees move in the wind. In this paper, we propose a methodology that uses quantized channel state information and a technique similar to non-substractive SNR-dithering to quantify the repeatablility of wireless channels. Thereby, we introduce a new metric that allows for a comparison of different setups and scenarios in terms of repeatability. In a measurement campaign, we compare (1) a directional link to (2) an outdoor to indoor urban scenario with a fixed receiver and (3) the same scenario with the receiver moving in a circle, thereby experiencing the same high speed channel again and again.