Sebastian Caban

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.

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.

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.

Measurement-Based Performance Evaluation of MIMO HSDPA

In this paper, we report on the results of physical-layer multiple-input-multiple-output High-Speed Downlink Packet Access (HSDPA) throughput measurements. These measurements were carried out in two different environments: 1) an alpine valley and 2) a city. In addition to the standard compliant single- and two-transmit-antenna HSDPA schemes, we defined and measured a four-transmit-antenna HSDPA scheme to explore future enhancements of the standard. To analyze the implementation loss, we introduce the so-called achievable mutual information and compare it with the actually measured data throughput. We find that the measured data throughput is far from the optimal, leaving room for future receiver optimizations. Furthermore, we find that the achievable mutual information is far from the channel capacity. Thus, optimizations in the standard could further improve HSDPA performance.

Testbedding MIMO HSDPA and WiMAX

Abstract—Modern wireless communication systems employMIMO and feedback—two properties that make it especiallydifficult to measure the performance of such systems withreasonable effort in actual outdoor scenarios.In this paper, we will present a time, cost, and manpower effi-cient measurement approach to evaluate the throughput achievedby such systems. Summer/winter, large distance outdoor-to-indoor/outdoor, urban/alpine measurements have been carriedout to successfully test this approach. Exemplarily, we report onthe throughput gains (over TX power and base-station-antenna-rotation) of standard compliant 2x2 MIMO HSDPA and IEEE802.16-2004 WiMAX when, for example, employing improvedchannel coding methods. I. M OTIVATION Modern wireless communication systems rely on complexalgorithms to squeeze out the last bit of performance froma radio link. They do so by exploiting the characteristics of achannel in an ingenious way. In the end—although sometimesa good benchmark—it is not the BER or the theoreticallyachievable capacity that counts, but the throughput actuallyachieved by the system under investigation.Determining the throughput performance of ingenious trans-mission schemes, on the other hand, is a different story. Todo so, usually, the communication system is simulated in e.g.M

Measuring the physical layer performance of wireless communication systems: Part 33 in a series of tutorials on instrumentation and measurement

Measuring the physical layer performance of wireless radio communication systems is one important step in clearly understanding their behavior in real-world, that is, non-simulation, environments. Unfortunately, measurements in wireless communications are extremely expensive and time-consuming. In fact, they require researchers to deal with nonartificial, realizable, real-world problems. They also require these researchers to have experience in computer engineering, telecommunication engineering, electrical engineering and often even mechanical engineering. Finally, they require teamwork to set up complete systems instead of dealing with single, isolated, numerical environments.

Directional evaluation of receive power, Rician K-factor and RMS delay spread obtained from power measurements of 60 GHz indoor channels

To meet link budgets for millimetre wave wireless communications, antennas with high directivity are essential. This directional view of the channel is evaluated by the important small scale fading parameters receive power, Rician K-factor and the RMS delay spread. These parameters are all derived from frequency swept power measurements. The effect of transmit polarization is very visual in the measurement results. A maximum K-factor of almost 80 was observed. The RMS delay spread is approximately 4 ns at reflective reception.