Josep Colom Ikuno

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.

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.

A Novel Link Error Prediction Model for OFDM Systems with HARQ

This paper presents a link error prediction model capable of accurately predicting the block error ratio in OFDM systems with hybrid ARQ. The modeling of both the channel coding and the combining of the retransmissions solves many of the issues with present models. Our model is based on accumulated conditional mutual information, mutual information effective SNR averaging, and simulated AWGN performance curves. As a consequence, our model is fading-insensitive and takes into account the non-ideality of rate matching. The model can be applied to any OFDM system that utilizes rate matching as a means to achieve adaptive modulation and coding and HARQ. We demonstrate the application of our model in LTE and simulate its accuracy. The whole simulation environment together with the results of this paper are made available for download at our homepage.

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.

Capacity density optimization by fractional frequency partitioning

This paper presents a fractional frequency reuse optimization scheme based on capacity density. It assigns a given user to the frequency subband with maximum achievable capacity density (bit/s/m2). We formulate the optimization problem and solve it by simulation. Unlike in previous work, the problem is solved assuming interference limitation in all subbands. A sectorized layout with three-dimensional antenna radiation patterns is utilized, which includes the effect of antenna downtilting on the signal-to-interference-plus-noise ratio distribution. The simulation results show that the proposed scheme outperforms conventional Reuse-1- and Reuse-3 schemes in terms of average- and cell-edge performance.