Michal Simko

The Vienna LTE simulators - Enabling reproducibility in wireless communications research

In this article, we introduce MATLAB-based link and system level simulation environments for UMTS Long-Term Evolution (LTE). The source codes of both simulators are available under an academic non-commercial use license, allowing researchers full access to standard-compliant simulation environments. Owing to the open source availability, the simulators enable reproducible research in wireless communications and comparison of novel algorithms. In this study, we explain how link and system level simulations are connected and show how the link level simulator serves as a reference to design the system level simulator. We compare the accuracy of the PHY modeling at system level by means of simulations performed both with bit-accurate link level simulations and PHY-model-based system level simulations. We highlight some of the currently most interesting research questions for LTE, and explain by some research examples how our simulators can be applied.

Pushing the Limits of LTE: A Survey on Research Enhancing the Standard

Cellular networks are currently experiencing a tremendous growth of data traffic. To cope with this demand, a close cooperation between academic researchers and industry/standardization experts is necessary, which hardly exists in practice. In this paper, we try to bridge this gap between researchers and engineers by providing a review of current standard-related research efforts in wireless communication systems. Furthermore, we give an overview about our attempt in facilitating the exchange of information and results between researchers and engineers, via a common simulation platform for 3GPP long term evolution (LTE) and a corresponding webforum for discussion. Often, especially in signal processing, reproducing results of other researcher is a tedious task, because assumptions and parameters are not clearly specified, which hamper the consideration of the state-of-the-art research in the standardization process. Also, practical constraints, impairments imposed by technological restrictions and well-known physical phenomena, e.g., signaling overhead, synchronization issues, channel fading, are often disregarded by researchers, because of simplicity and mathematical tractability. Hence, evaluating the relevance of research results under practical conditions is often difficult. To circumvent these problems, we developed a standard-compliant opensource simulation platform for LTE that enables reproducible research in a well-defined environment. We demonstrate that innovative research under the confined framework of a real-world standard is possible, sometimes even encouraged. With examples of our research work, we investigate on the potential of several important research areas under typical practical conditions, and highlight consistencies as well as differences between theory and practice.

Inter-Carrier Interference Estimation in MIMO OFDM Systems with Arbitrary Pilot Structure

In scenarios with time-varying channels such as intelligent traffic systems or high speed trains, the orthogonality between subcarriers in orthogonal frequency division multiplexing (OFDM) is destroyed leading to inter-carrier interference (ICI). In the literature, ICI equalization algorithms have been proposed; however, they assume perfect channel knowledge at sample level. Unfortunately, existing channel estimation algorithms do not provide accurate channel estimates at high Doppler spreads, prohibiting data transmission with high spectral efficiency. In this paper, we propose an algorithm for ICI estimation that can be applied to OFDM systems with an arbitrary pilot structure. Thus, our algorithm can be applied to any already standardized OFDM system. Our ICI estimator models the channel variation by means of a basis expansion model (BEM). The performance of the estimator and of the subsequent equalization is evaluated in an UMTS long term evolution (LTE) link level simulator. In a Rayleigh fading scenario, the proposed algorithm allows a velocity increase of 150 km/h without throughput degradation. The gain in terms of the post-equalization signal to interference and noise ratio (SINR) is about 3.7 dB at a user speed of 300 km/h.

Doubly dispersive channel estimation with scalable complexity

In this paper, we present an Approximate Linear Minimum Mean Square Error (ALMMSE) fast fading channel estimator for Orthogonal Frequency Division Multiplexing (OFDM). The ALMMSE channel estimator utilizes the knowledge of the structure of the autocorrelation matrix given by the Kronecker product between the time correlation matrix and the frequency correlation matrix. We separate the Linear Minimum Mean Square Error (LMMSE) filtering matrix into two matrices corresponding to individual filtering in frequency and time. The eigenvalues of these two matrices are rank-one approximated by the eigenvalues of the LMMSE filtering matrix. The complexity of the ALMMSE estimator can be scaled by varying the number of the considered number of eigenvalues. Simulation results show that the proposed ALMMSE channel estimator looses only 0.1 dB compared to the LMMSE channel estimator in realistic scenarios.

Optimal pilot symbol power allocation under time-variant channels

Nowadays, most wireless communication systems employ coherent demodulation on the receiver side. Under this circumstance, part of the available transmission resource is reserved and utilized for channel estimation, referred to as pilot symbols. In recent standards, for example long-term evolution (LTE), a certain adjustment is allowed for the power radiated on the pilot symbols. This additional degree of freedom creates space for a further optimization of the system performance. In this article, we consider an orthogonal frequency division multiplexing system and investigate how to distribute the available power between data symbols and pilot symbols under transmissions over time-variant channels so that the overall throughput is maximized. We choose the post-equalization signal-to-interference and noise ratio as the cost function and solve the problem analytically. Simulation results obtained by the Vienna LTE simulator are consistent with the analytical results. With an optimal power distribution between data and pilot symbols, a throughput increase of around 10% can be achieved compared to a system with evenly distributed power between data and pilot symbols.

On performance bounds for MIMO OFDM based wireless communication systems

In this paper, we develop several increasingly tight and restrictive performance bounds for OFDM based MIMO wireless communication systems. Starting with channel capacity as the ultimate upper bound on the throughput, step by step we develop tighter bounds on the practically achievable throughput by incorporating typical design constraints. The presented bounds are applied to a Long Term Evolution system, allowing us to identify dominant sources of performance losses. The investigation reveals flaws of wireless communication systems thereby pointing at directions to efficiently improve performance in system design.

Optimal Pilot Symbol Power Allocation in LTE

The UMTS Long Term Evolution (LTE) allows the pilot symbol power to be adjusted with respect to that of the data symbols. Such power increase at the pilot symbols results in a more accurate channel estimate, but in turn reduces the amount of power available for the data transmission. In this paper, we derive optimal pilot symbol power allocation based on maximization of the post-equalization Signal to Interference and Noise Ratio (SINR) under imperfect channel knowledge. Simulation validates our analytical mode for optimal pilot symbol power allocation.

Adaptive Pilot-Symbol Patterns for MIMO OFDM Systems

Recent standards for cellular transmission systems offer a lot of flexibility, such as the choice of transmission modes, modulation alphabets, coding rates, and precoding matrices. Despite this trend, pilot-symbol patterns in todays standards remain fixed, although such an approach is suboptimal. In this paper, we show how to design optimal pilot-symbol patterns by maximizing an upper bound of a constrained capacity that takes channel estimation errors and Inter Carrier Interference into account. Furthermore, we propose adaptive pilot-symbol patterns that follow changing channel statistics. As a proof of concept, we present throughput simulation results of two competitive systems, a transmission system compliant with the Long Term Evolution (LTE)-standard and an improved system utilizing the proposed adaptive pilot patterns. The transmission system utilizing adaptive pilot patterns outperforms an LTE-standard compliant system in all considered scenarios. The throughput gain for a single input single output system ranges between 3% and 80%. For a 4 × 4 transmission system, the performance gain is significantly higher and can reach up to 850% compared to a conventional LTE system.

Accurate SINR estimation model for system level simulation of LTE networks

This paper presents an SINR prediction model for LTE systems with the aim of improving the accuracy of physical layer models used for higher-than-physical-level simulations. This physical layer SINR abstraction for zero-forcing receivers takes channel estimation errors into account. It allows for the calculation of the post-equalization receive SINR of an LTE MIMO transmission on a per-layer and subcarrier basis. This estimate is further used as input for a link performance model to accurately predict the receiver throughput. The accuracy of the model is validated by simulations for 2×2 and 4×4 MIMO antenna configurations with different levels of spatial multiplexing employing also the precoding matrices defined in the LTE standard. Memory requirements and complexity for the model are quantified as well. The whole simulation environment together with the fully reproducible results of this paper are made available for download from our homepage.

Performance analysis of LTE downlink under Symbol Timing Offset

In this paper, we evaluate the performance of a standardized OFDM system, namely Long Term Evolution (LTE) downlink, with imperfect symbol timing. A closed form expression of the post-equalization Signal to Interference and Noise Ratio (SINR) is derived and compared with results obtained from a standard compliant simulator. Also, we analyzed the channel estimation performance when Symbol Timing Offset (STO) occurs. This work reveals the impact of imperfect synchronization on the link performance. It also allows an accurate and realistic modeling of the physical layer behavior, which can be applied to reproduce results from time-consuming link level simulations.