Martin Lerch

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.

LTE Downlink Performance in High Speed Trains

Long Term Evolution (LTE) is expected to substitute the Global System for Mobile Communications (GSM) as the radio access technology for railway communications. Recently, especial attention has been devoted to high-speed trains since this particular environment poses challenging problems in terms of performance simulation and measurement. In order to severely decrease the cost and complexity of high-speed measurement campaigns, we have proposed a technique to induce effects caused by highly-time varying channels on Orthogonal Frequency-Division Multiplexing (OFDM) signals while conducting measurements at low speeds. In this work, we evaluate this technique by comparing the results of LTE measurements at different velocities as well as by simulations. Additionally, we use this technique to show the performance of LTE for high-speed train scenarios up to 600 km/h. To accomplish this, we use both a controlled high-speed measurement setup as well as a channel model developed according to the guidelines of the ITU Radiocommunication Sector (ITU-R) for the evaluation of radio interface technologies for IMT- Advanced systems.

Experimental Validation of the OFDM Bit Error Probability for a Moving Receive Antenna

For an orthogonal frequency-division multiplexing system with pilot-symbol aided channel estimation, we compare the measured bit error ratio to the theoretical bit error probability. In order to measure mobile physical systems, we utilize the Vienna Wireless Testbed which has been augmented by a rotation wheel unit. The analytical solution assumes Rayleigh fading, additive Gaussian noise, and an arbitrary linear interpolation method to estimate the unknown channel taps. Our measurements confirm our assumptions and demonstrate convincingly that our theoretical expressions accurately model the true physical behavior, even for speeds of up to 100\,km/h.

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.

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.

Experimental assessment of 5G-candidate modulation schemes at extreme speeds

The radio access technology for railway communications is expected to migrate from GSM for Railways (GSM-R) to fourth generation (4G). Recently, considerable attention has been devoted to high-speed trains since this particular environment poses challenging problems in terms of performance simulation and measurement. In order to considerably decrease the cost and complexity of high-speed measurement campaigns, we have proposed a technique to induce effects caused by highly-time varying channels on multicarrier signals by conducting measurements at low speeds. This technique has been proved to be accurate for the cases of WiMAX and Long Term Evolution (LTE) standardized waveforms. In this work, we evaluate experimentally the performance of this technique by employing modulation schemes proposed for fifth generation (5G) systems. More specifically, we compare the performance obtained by transmitting Filter Bank Multicarrier (FBMC) schemes with two different prototype filters at different velocities in a controlled measurement environment.

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.

Experimental comparison of fast-fading channel interpolation methods for the LTE uplink

Due to the pilot structure, channel interpolation in the LTE uplink is a challenging problem in fast fading scenarios. In this paper different channel interpolation methods that include channel estimates from the previous and from the subsequent subframe are compared to methods using only one or two channel estimates from the actual subframe. The presented results are obtained by measurements performed on a wireless testbed that allows for reproducible measurements at velocities of up to 400 km/h.

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.

Methods to Perform High Velocity LTE Experiments at Low Velocities

LTE is designed to support user velocities of up to 500 km/h where experiments are expensive, time-consuming and dangerous. Fortunately, such experiments can be emulated at lower velocities by time-stretching the transmit signals. This method preserves the spatial properties of the mobile radio channel but performs a spectral compression. In this paper, we propose a new set of methods that preserve the spectral properties by inserting additional subcarriers. Next, we compare the method of time-stretching to the new set of methods by applying them to LTE downlink signals. We evaluate the properties and the main drawbacks of the proposed methods by simulations using standard-compliant LTE signals. Finally, we highlight practical aspects to be considered when implementing the proposed high speed emulation techniques.