Martin Wrulich

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.

Mutual information based calculation of the Precoding Matrix Indicator for 3GPP UMTS/LTE

This paper presents an efficient method for calculating the Precoding Matrix Indicator (PMI) at the receiver. The PMI is required for MIMO precoding in the downlink of a 3GPP UMTS/LTE system. Our method is based on maximizing the mutual information between the transmitted and received symbols with respect to the precoding matrix applied at the transmitter. The advantage of this method is that it is independent of the symbol alphabet (4/16/64 QAM) and code rate applied, which are signaled by the channel quality indicator (CQI). Although this paper only focuses on the selection of the optimal PMI, such a procedure eventually allows to decouple both problems (CQI and PMI calculation), thereby reducing complexity. The proposed method also provides means to obtain the number of useful MIMO transmission layers, signaled in form of the Rank indicator (RI), by maximizing mutual information also with respect to this value. The performance of the method is evaluated utilizing an LTE downlink physical-layer simulator.

Network-load dependent Partial Frequency Reuse for LTE

Inter-cell Interference (ICI) is a key issue in Orthogonal Frequency-Division Multiple Access (OFDMA) systems. Since OFDMA was proposed to be used in next generation networks several schemes have been investigated for mitigating the ICI. One of the techniques that promises improvement in reducing ICI is Partial Frequency Reuse (PFR). In this paper we investigate a Flexible Bandwidth Allocation (FBA) scheme for PFR depending on the network-load, which allocates bandwidth dynamically in the network. The scheme is based on the assumption that a cell is not loaded homogeneously. We develop a suitable network description to obtain the optimum PFR zone partitioning as a function of the dynamic bandwidth allocation. Thus, our paper presents a general framework for intelligent frequency planning in wireless networks. Compared to simpler PFR schemes, our simulation results show that the cell capacity for reuse-1 can be increased by 2 b/s/Hz when our scheme is used by exploiting the inhomogeneities in the load.

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.

Measurements and channel modeling for short range indoor UHF applications

The recent emergence of short range applications, e.g. RFID (radio frequency identification), requires careful investigation of wave propagation and adequate description of the radio channel. In this contribution, we present indoor channel measurements at 868MHz that expose the transition between the near and far field as well as the effects of fading. Furthermore, we investigate the appliance of a simple statistic channel model, based on the Rician distribution, and show that the underlying assumptions do not hold in general.

Evaluation of HSDPA and LTE: From Testbed Measurements to System Level Performance

This book explains how the performance of modern cellular wireless networks can be evaluated by measurements and simulations With the roll-out of LTE, high data throughput is promised to be available to cellular users. In case you have ever wondered how high this throughput really is, this book is the right read for you: At first, it presents results from experimental research and simulations of the physical layer of HSDPA, WiMAX, and LTE. Next, it explains in detail how measurements on such systems need to be performed in order to achieve reproducible and repeatable results. The book further addresses how wireless links can be evaluated by means of standard-compliant link-level simulation. The major challenge in this context is their complexity when investigating complete wireless cellular networks. Consequently, it is shown how system-level simulators with a higher abstraction level can be designed such that their results still match link-level simulations. Exemplarily, the book finally presents optimizations of wireless systems over several cells. This book: * Explains how the performance of modern cellular wireless networks can be evaluated by measurements and simulations * Discusses the concept of testbeds, highlighting the challenges and expectations when building them * Explains measurement techniques, including the evaluation of the measurement quality by statistical inference techniques * Presents throughput results for HSDPA, WiMAX, and LTE * Demonstrates simulators at both, link- level and system-level * Provides system-level and link-level simulators (for WiMAX and LTE) on an accompanying website (https://www.nt.tuwien.ac.at/downloads/featured-downloads) This book is an insightful guide for researchers and engineers working in the field of mobile radio communication as well as network planning. Advanced students studying related courses will also find the book interesting.

Joint throughput optimized CQI and precoding weight calculation for MIMO HSDPA

In MIMO high speed downlink packet access, the channel adaptation is performed by means of the channel quality indicator (CQI) and the precoding control indicator (PCI). The CQI value is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. The PCI value is associated to a specific precoding vector that is applied to the transmit signal at the basestation. In this work, we derive analytic expressions for the post equalization SINR. These SINR values are then evaluated at the receiver to jointly calculate the CQI and the PCI in order to maximize the data throughput. The SINR expressions are verified not only by simulations but also by outdoor MIMO HSDPA measurements.

Interference aware MMSE equalization for MIMO TxAA

Closed loop transmission diversity will be an important part of the enhancements of todays wireless technologies, e.g. in double-stream transmit antenna array (D-TxAA). Despite the promise of performance gains, closed loop beamforming will introduce an interference scenario in the downlink that cannot easily be handled by the conventional MMSE equalization approach. In this paper, we derive an interference aware MMSE equalizer that uses the information about the beamforming vectors. The proposed equalizer can be calculated by utilizing a virtual channel description incorporating the beamforming weights. To assess the performance, we performed fading simulations with a system level description of the transmission. The derived equalizer shows only a small increase in complexity compared to the conventional approach, but perfectly restores the orthogonality.

Managing the interference structure of MIMO HSDPA: A multi-user interference aware MMSE receiver with moderate complexity

It is known that Wideband Code-Division Multiple Access (W-CDMA) networks are limited by interference more than by any other single effect. Due to the frequency selectivity of the wireless channel, intra-cell interference composed of selfand other-user interference occurs. The intra-cell interference as a result of Transmit Antenna Array (TxAA) HSDPA multi-user transmissions, however, shows a special structure that can be exploited to allow for an efficient interference suppression. The contributions of this article are the following: we (a) propose a suitable system model to derive an intra-cell interference aware Minimum Mean Squared Error (MMSE) equalizer with moderate complexity and investigate its capabilities to suppress the multi-user intra-cell interference, (b) propose solutions to estimate the pre-coding state of the cell both with and without available training, and (c) investigate the resulting throughput performance with physical layer as well as system-level simulations. Our proposed solution can be interpreted as a multi-user extension of the classical MMSE equalizer for HSDPA systems without decoding the undesired users, resulting in only slightly increased complexity.

Intra-cell interference aware equalization for TxAA HSDPA

In TxAA HSDPA downlink a basestation is capable of serving several users simultaneously. The users are distinguished by different spreading sequences and their data chip streams are weighted by individual precoding coefficients. In this contribution we derive an MMSE equalizer for TxAA HSDPA downlink that takes the interference of the other users into account. Compared to the straightforward receiver that neglects the interference of other users, a performance gain of up to 4 dB can be achieved.