Stefan Pratschner

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.

A mutual coupling model for massive MIMO applied to the 3GPP 3D channel model

Massive Multiple-Input Multiple-Output (MIMO) has become one of the key technologies for future mobile communication systems. Although there is a variety of potential benefits, unresolved implementation issues of massive MIMO are manifold. In this work, we consider the issue of mutual coupling in large antenna arrays. A known matrix model that describes array coupling effects is considered. This coupling model is augmented by a matching network in order to provide a universal coupling model that is applicable to any channel model. Impact of array coupling is then shown by applying the coupling matrix on the 3rd Generation Partnership Project 3D channel model. We show that the matching network leads to decorrelation and significantly reduces capacity losses due to mutual coupling in the context of massive MIMO.

Single-user and multi-user MIMO channel estimation for LTE-Advanced uplink

In LTE-Advanced (LTE-A) demodulation reference symbols are employed for pilot aided channel estimation to perform coherent detection. Since these reference symbols are allocated on the same time-frequency positions for all users in Multi-User MIMO (MU-MIMO) operation or for all spatial layers in Single-User MIMO (SU-MIMO), they are designed to be code-domain orthogonal in order to be separable at the receiver. This orthogonality is obtained by cyclically shifting a reference signal base sequence, where the specific cyclic shift values are signaled to each user. In this work we show formal equivalence of SU-MIMO and MU-MIMO for LTE-A uplink in the context of channel estimation. We propose a standard compliant mapping that assigns cyclic shifts to users such that SU-MIMO estimation methods are applicable also for MU-MIMO transmissions. Further we show the trade-off between the number of active users and the channels frequency selectivity in MU-MIMO operation due to the residual channel estimation error.

Efficient multi-user MIMO transmissions in the LTE-A uplink

Single-User Multiple-Input Multiple-Output (MIMO) transmissions are possible since Rel. 10 of 3GPP LTE-A. The Closed Loop Spatial Multiplexing (CLSM) transmission mode enables up to a four fold increase in spectral efficiency compared to Rel. 8, yet it requires complex User Equipment (UE) hardware supporting more than one transmit antenna. The closed loop spatial multiplexing transmission mode enables up to a four fold increase in spectral efficiency compared to Rel. 8, yet it requires complex UE hardware supporting more than one transmit antenna. As it is likely that the Base Station (BS) has more receive antennas than the UE transmit antennas, employing Multi-User (MU) MIMO transmissions in the uplink can lead to significant improvement in cell throughput. We formulate MU MIMO transmissions with linear receivers as a compact matrix system model. Based on our model we present a post-equalization SINR equation, allowing the calculation of a quantized feedback parameter, the Channel Quality Indicator (CQI) value. Within this framework we reveal possible MU gains through known scheduling algorithms although the frequency selective channel is approximated by its arithmetic mean.

CARBON FIBER REINFORCED POLYMER WITH ISOTROPIC 60 GHZ REFLECTIVITY

Carbon fiber reinforced polymer (CFRP) is measured as reflector material for millimeter waves at 60 GHz. Reflectivity is measured to characterize material anisotropy in a mono-static setup. Disc shaped material samples are rotated in steps of one degree. Four commonly employed CFRP are investigated: unidirectional fibers, plain-weave, twill-weave and fiber shreds. Results show that the unidirectional CFRP and twill-weave CFRP are anisotropic, while the remaining materials are isotropic within measurement accuracy.

Versatile mobile communications simulation: the Vienna 5G Link Level Simulator

Research and development of mobile communications systems require a detailed analysis and evaluation of novel technologies to further enhance spectral efficiency, connectivity and reliability. Due to the exponentially increasing demand of mobile broadband data rates and challenging requirements for latency and reliability, mobile communications specifications become increasingly complex. For this reason, analytic analysis as well as measurement-based investigations of link-level methods soon encounter feasibility limitations. Therefore, computer-aided numeric simulation is an important tool for investigation of wireless communications standards and is indispensable for analysis and development of future technologies. In this contribution, we introduce the Vienna 5G Link Level Simulator, a Matlab-based link-level simulation tool that facilitates research and development in mobile communications. Our simulator enables standard compliant setups according to 4G LTE, 5G NR and even beyond, making it a very flexible simulation tool. Offered under an academic use license free of charge to fellow researchers, it considerably enhances reproducibility in wireless communications research.

Dependable wireless connectivity: insights and methods for 5G and beyond

Dependability is a measure of availability and reliability of systems/services. In the context of communication systems, dependability is governed by the coverage probability of the network under prescribed service requirements, by the latency of data transmissions as well as by the transmission error probability. Achieving dependable connectivity can be very challenging, especially within wireless mobile communications, where the transmission channel is often prone to severe fading and strong interference. Current generations of cellular mobile communication systems (4G and below) can mainly provide best effort services and are not well equipped to achieve a sufficiently high level of dependability as required by many novel applications, such as, road-safety relevant information exchange in vehicular communications, as well as, wireless remote operation of robots and drones. Standardization bodies have already recognized the market potential of such use-cases for mobile communications, and correspondingly efforts are ongoing to enhance the fifth generation of cellular systems (5G) towards ultra-reliable low-latency transmission. In this paper, we provide insights gained by our research work within the Christian Doppler Laboratory for Dependable Wireless Connectivity for the Society in Motion with respect to factors influencing the dependability of 5G mobile cellular systems, and we present our achievements over the past two years for enhancing the robustness, reliability and efficiency of dependable wireless communications.ZusammenfassungDie Zuverlässigkeit (Engl. dependability) eines Systems oder eines Dienstes ist ein Maß für dessen Verfügbarkeit und Ausfallsicherheit (Engl. reliability). In Mobilfunksystemen wird Zuverlässigkeit hauptsächlich durch die Netzabdeckung für vorgegebene Servicanforderungen bestimmt sowie durch die Fehlerwahrscheinlichkeit und die Latenzzeit der Datenübertragung. Zuverlässigkeit drahtloser Verbindungen zu gewährleisten, stellt eine große Herausforderung dar, da grundlegende physikalische Eigenschaften des drahtlosen Übertragungsmediums, wie Kanalschwunderscheinungen und Interferenzen, inhärente Beeinträchtigungen verursachen. Aktuelle Generationen von Mobilfunksystemen (4G und seine Vorgänger) bieten vorwiegend sogenannte “best effort” Dienste an und sind nicht ursprünglich dafür geschaffen, Anwendungen mit strikten Zuverlässigkeitsanforderungen hinreichend sicher zu unterstützen. Neuartige Anwendung von drahtlosen Übertragungssystemen, wie zum Beispiel der Austausch verkehrssicherheitsrelevanter Informationen in der Fahrzeugkommunikation oder die drahtlose Steuerung von Robotern und Drohnen, verlangen jedoch nach einem sehr hohen Grad an Zuverlässigkeit. Das Marktpotential solcher Anwendungen wurde bereits von Standardisierungsgremien erkannt, und entsprechende Weiterentwicklungen von Mobilfunksystemen der fünften Generation (5G) in Richtung sehr geringer Latenzzeiten und hoher Ausfallsicherheit sind derzeit in vollem Gange. In unserer Forschungsarbeit im Christian Doppler Labor für Zuverlässige Drahtlose Konnektivität für eine Gesellschaft in Bewegung erforschen und entwickeln wir Methoden, um die Zuverlässigkeit von Mobilfunksystemen zu verbessern. Mit dem vorliegenden Artikel geben wir einen Überblick über unsere Erkenntnisse und die daraus resultierenden Weiterentwicklungen von Mobilfunktechnologien.

Angle-dependent reflectivity of twill-weave carbon fibre reinforced polymer for millimetre waves

Twill-weave carbon fibre reinforced polymer is measured as reflector material for electromagnetic waves at 60 GHz. The reflectivity at millimetre wavelength shows a predominant direction, which coincides with the diagonal pattern of the twills top layer. In the remaining directions the investigated 2/2 twill-weave composites reflectivity is almost isotropic.