Christian Mehlführer

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.

Calculation of the spatial preprocessing and link adaption feedback for 3GPP UMTS/LTE

This paper presents an efficient method for calculating the Precoding Matrix Indicator (PMI), Rank Indicator (RI) and Channel Quality Indicator (CQI) at a Long Term Evolution (LTE) User Equipment (UE). The indicators are required for spatial preprocessing and link adaption in the downlink of a 3GPP UMTS/LTE system. To reduce the computational burden for the UE, our method decomposes the problem into two separate steps, one of jointly evaluating the PMI and RI based on a mutual information metric and one of choosing the CQI value to achieve a given target Block Error Ratio (BLER) constraint. The performance of the method is evaluated utilizing an LTE downlink physical layer simulator. The influence of estimated channel knowledge on the feedback choice is investigated for Least Squares (LS) and Linear Minimum Mean Squared Error (LMMSE) channel estimators.

Low complexity approximate maximum throughput scheduling for LTE

In this paper we address the challenge of multiuser scheduling in the downlink of 3GPP UMTS/LTE. Long Term Evolution (LTE) imposes the constraint of using the same code rate, modulation order and transmit power for all resources a User Equipment (UE) is scheduled onto. This, in addition to the lack of channel knowledge, prohibits theoretical concepts such as capacity maximization to be applied for resource allocation. Based on the Channel Quality Indicator (CQI) feedback we derive a linearized model for multiuser scheduling. In contrast to other proposals we use Mutual Information Effective SNR Mapping (MIESM) to calculate an average CQI value for all UE resources. This enables a rate increase while still guaranteeing an imposed Block Error Ratio (BLER) constraint. The proposed framework can also be applied to implement other scheduling strategies. This is demonstrated by comparing different standard schedulers in terms of achieved throughput and fairness.

Vienna MIMO testbed

While the field of MIMO transmission has been explored over the past decade mainly theoretically, relatively few results exist on how these transmissions perform over realistic, imperfect channels. The reason for this is that measurement equipment is expensive, difficult to obtain, and often inflexible when a multitude of transmission parameters are of interest. This paper presents a flexible testbed developed to examine MIMO algorithms and channel models described in literature by transmitting data at through real, physical channels, supporting simultaneously four transmit and four receive antennas. Operation is performed directly from Matlab allowing for a cornucopia of real-world experiments with minimum effort. Examples measuring bit error rates on space-time block codes are provided in the paper.

Experimental Evaluation of Adaptive Modulation and Coding in MIMO WiMAX with Limited Feedback

We evaluate the throughput performance of an OFDM WiMAX (IEEE 802.16-2004, Section 8.3) transmission system with adaptive modulation and coding (AMC) by outdoor measurements. The standard compliant AMC utilizes a 3-bit feedback for SISO and Alamouti coded MIMO transmissions. By applying a 6-bit feedback and spatial multiplexing with individual AMC on the two transmit antennas, the data throughput can be increased significantly for large SNR values. Our measurements show that at small SNR values, a single antenna transmission often outperforms an Alamouti transmission. We found that this effect is caused by the asymmetric behavior of the wireless channel and by poor channel knowledge in the two-transmit-antenna case. Our performance evaluation is based on a measurement campaign employing the Vienna MIMO testbed. The measurement scenarios include typical outdoor-to-indoor NLOS, outdoor-to-outdoor NLOS, as well as outdoor-to-indoor LOS connections. We found that in all these scenarios, the measured throughput is far from its achievable maximum; the loss is mainly caused by a too simple convolutional coding.

Carrier frequency synchronization in the downlink of 3GPP LTE

In this paper, we investigate carrier frequency synchronization in the downlink of 3GPP Long Term Evolution (LTE). A complete carrier frequency offset estimation and compensation scheme based on standardized synchronization signals and reference symbols is presented. The estimation performance in terms of mean square error is derived analytically and compared to simulation results. The impact of estimation error on the system performance is shown in terms of uncoded bit error ratio and physical layer coded throughput. Compared to perfect synchronization, the presented maximum likelihood estimator shows hardly any performance loss, even when the most sophisticated MIMO schemes of LTE are employed.

Throughput Maximizing Multiuser Scheduling with Adjustable Fairness

We address the problem of downlink multiuser scheduling in practical wireless networks under a desired fairness constraint. Wireless networks such as LTE, WiMAX and WiFi provide partial channel knowledge at the base station/access point by means of quantized user equipment feedback. Specifically in 3GPPs LTE, the Channel Quality Indicator (CQI) feedback provides time-frequency selective information on achievable rates. This knowledge enables the scheduler to achieve multiuser diversity gains by assigning resources to users with favourable channel conditions. However, only focusing on the possible diversity gains leads to unfair treatment of the individual users. To overcome this situation we propose a method for multiuser scheduling that operates on the boundary of the achievable multiuser rate region while guaranteeing a desired long term average fairness. Our method is based on a sum utility maximization of the alpha-fair utility functions. To obtain a given fairness, quantified with Jains fairness index, it is necessary to find an appropriate α, which we obtain from the observed CQI probability mass function (pmf).

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.

An accurate and low complex channel estimator for OFDM WiMAX

In this paper, we present an approximate LMMSE (ALMMSE) channel estimator for OFDM WiMAX. The ALMMSE estimator utilizes the inherent correlation between the channels of neighboring subcarriers to improve the least squares estimate by a spectral smoothing filter. We show that the covariance matrix of the channel, required for the calculation of the spectral smoothing filter, can be estimated from the LS channel estimate of the same receive frame. The complexity of the proposed ALMMSE estimator is considerably lower than the complexity of the exact LMMSE estimator while the performance loss is only about 0.3 dB in SNR. Our evaluation of the channel estimator performance is based on outdoor measurements in realistic WiMAX scenarios.