Feedback-based LTE-Downlink Measurements .Feedback-based LTE-Downlink Measurements ausgef¼hrtzumZweckederErlangungdes

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  • institute oftelecommunications

    Diplomarbeit

    Feedback-basedLTE-Downlink Measurements

    ausgefhrt zum Zwecke der Erlangung desakademischen Grades eines Diplom-Ingenieurs

    unter der Leitung vonUniv. Prof. Dipl.-Ing. Dr.techn. Markus Rupp

    Dipl.-Ing. Martin LerchInstitute of Telecommunications

    eingereicht an der Technischen Universitt WienFakultt fr Elektrotechnik und Informationstechnik

    Martin Klaus Mller, B.Eng.

    Matr. Nr.: 1026943Hauslabgasse 14/23, 1050 Wien, sterreich

    Wien, im November 2013

  • I hereby certify that the work reported in this thesis is my own,and the work done by other authors is appropriately cited.

    Martin Klaus Mller11.11.2013

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  • Abstract

    The predictions for cellular traffic in the next few years show an exponential growthfor mobile data. New transmission techniques and the application of efficient algo-rithms are required in order to make the best use of the scarce resources in mobilecommunications to be able to satisfy this growing demand.Following the evolution of mobile cellular systems, the last releases of UMTS1 werealready capable of applying MIMO2-techniques to significantly increase the possi-ble throughput per cell. This trend went on with the introduction of LTE3 and itssuccessor LTE-A4. Even more antennas are supported and the system gets moreflexible by providing a large set of predefined modulation and coding schemes.Thus each link can be used with close to optimal transmission-parameters.The Vienna MIMO Testbed was designed to measure the performance of variouswireless transmission standards. Up to now, this was achieved without using afeedback link. However, very flexible LTE and LTE-A transmission-modes cannotbe evaluated this way. To overcome these restrictions, a feedback mechanism hasbeen integrated in the testbed. This allows to measure advanced features andtransmission modes of LTE and LTE-A.How this feedback is incorporated for the measurement of such advanced modes isdescribed in this thesis. Topics under investigation are the impact of retransmis-sions on throughput and block error ratio, frequency selective scheduling and theperformance of advanced transmission modes, such as CLSM5 and MU-MIMO6

    techniques. The adaptation of the testbed-software for measuring the aforemen-tioned aspects is detailed in this work. For every transmission mode, measurementresults are provided.

    1Universal Mobile Telecommunications System2Multiple Input Multiple Output3Long Term Evolution4LTE-Advanced5Closed Loop Spatial Multiplexing6Multi User MIMO

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  • Kurzfassung

    Fr das Daten-Aufkommen in zellulren Mobilfunknetzen wird ein exponentiellesWachstum vorhergesagt. Neue bertragungstechniken und der Einsatz von ef-fizienten Algorithmen sind notwendig, um den grten Nutzen aus den sprlichvorhandenen Ressourcen zu ziehen, um so den wachsenden Bedarf zu befriedigen.Vielversprechend ist der Einsatz von MIMO1-Techniken. Sie ermglichen es, denDatendurchsatz in einer Zelle deutlich zu erhhen. In dem MobilfunkstandardLTE2 und dessen Nachfolger LTE-A3 wurde der Einsatz von MIMO ausgeweitet.Noch mehr Antennen knnen eingesetzt werden und das System wird flexiblerdurch die Bereitstellung einer groen Anzahl von vordefinierten Modulations- undCodierungs-Arten. So kann jeder bertragungspfad annhernd optimal genutztwerden.Das Vienna MIMO Testbed wurde entworfen, um die Leistung von unterschiedlichenMobilfunk-Standards messtechnisch zu ermitteln. Bis jetzt war dazu keine Rck-fhrung von Informationen vorgesehen. Sehr flexible bertragungs-Modi von LTEund LTE-A knnen so aber nicht untersucht werden. Um diese Einschrnkungenaufzuheben, wurde eine solche Rckfhrung von Informationen in den Messauf-bau integriert. Dadurch wird die Messung von fortschrittlichen Mechanismen undbertragungs-Modi von LTE und LTE-A ermglicht.Wie diese zurckgefhrten Informationen in Verbindung mit solchen fortschrit-tlichen Modi in die Messung eingebunden werden, wird in dieser Arbeit beschrieben.Untersuchte Themen sind der Einfluss von Sendewiederholungen auf den Daten-durchsatz, Frequenz-selektive Ressourcen-Verteilung und die Leistungsfhigkeitvon fortschrittlichen bertragungs-Modi wie CLSM4 und MU-MIMO5-Techniken.Wie die Software des Messaufbaus fr die Messung der soeben genannten Aspekteangepasst werden muss, wird in dieser Arbeit erlutert. Fr jeden bertragungs-Modus werden Messergebnisse in der Arbeit vorgestellt.

    1Multiple Input Multiple Output2Long Term Evolution3LTE-Advanced4Closed Loop Spatial Multiplexing5Multi User MIMO

    vii

  • Acknowledgements

    My first words of gratitude extend to my supervisor, Prof. Markus Rupp. It washe who made this work possible and was always there to provide support and ad-vice when necessary.

    I would like to thank my second supervisor, Martin Lerch, who helped me un-derstand my work in depth by discussing even the smallest detail.

    My thanks also go to my colleagues at the Institute of Telecommunications,especially Stefan Schwarz for the invaluable support with simulation-issues.

    My special thanks go to my good friend and colleague Michael Meidlinger, onwhose work this thesis is based upon. Thank you for never being short of an an-swer, no matter what the problem is!

    Finally, and most importantly, I want to thank my friends and my family,especially my parents, for being a continuous source of support and for beingthere, when I need them most.

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  • Contents

    1 Introduction 1

    2 Theoretical Background 32.1 LTE Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    2.1.1 OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.3 Channel Coding and HARQ . . . . . . . . . . . . . . . . . . 82.1.4 Adaptive Modulation and Coding . . . . . . . . . . . . . . . 11

    2.2 LTE MAC-Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.1 Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.2 Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.3 LTE Transmission-Modes . . . . . . . . . . . . . . . . . . . 15

    3 Measurement Setup and Methodology 193.1 Vienna MIMO Testbed . . . . . . . . . . . . . . . . . . . . . . . . . 19

    3.1.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.1.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    3.2 Simulation Environment . . . . . . . . . . . . . . . . . . . . . . . . 213.2.1 LTE-Simulator . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.2 Simulator - Testbed Interface . . . . . . . . . . . . . . . . . 233.2.3 Measurement Emulation . . . . . . . . . . . . . . . . . . . . 24

    3.3 Measurement Methodology . . . . . . . . . . . . . . . . . . . . . . . 243.3.1 Measurement/Simulation Process . . . . . . . . . . . . . . . 263.3.2 Data Generation . . . . . . . . . . . . . . . . . . . . . . . . 283.3.3 Offline Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 29

    3.4 Implementation of LTE-Modes . . . . . . . . . . . . . . . . . . . . . 303.4.1 Implementation Differences . . . . . . . . . . . . . . . . . . 303.4.2 Scheduler Implementation . . . . . . . . . . . . . . . . . . . 31

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  • 4 Measurement- and Simulation-Results 374.1 Comparison of Different Scheduling Granularities . . . . . . . . . . 37

    4.1.1 LTE Transmission-Mode 4, CLSM . . . . . . . . . . . . . . . 374.1.2 LTE-A Transmission-Mode 9, CLSM . . . . . . . . . . . . . 38

    4.2 HARQ-Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.3 ZF MU-MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    5 Conclusion and Outlook 43

    Bibliography 45

    A Acronyms 48

  • Chapter 1

    Introduction

    Since the first commercial deployment of Long Term Evolution (LTE), the suc-cessor of Universal Mobile Telecommunications System (UMTS), already severalyears have passed. But the evolution of mobile communication has not stoppedthen and first LTE Advanced (LTE-A) systems are about to be rolled out. Thisnew standard is equipped with several new features, further enhancing the perfor-mance in order to keep pace with the always growing demand for more datarate.Since there is almost no practical experience with LTE-A, the search for new algo-rithms suited for the new environment and capable of exploiting the new structureis quite prevalent in research and is mostly pursued by employing numerical sim-ulations for evaluation-purposes.Simulation results are a good way to check the feasibility of new algorithms, butmeasurements under realistic circumstances are necessary to evaluate the perfor-mance in a real mobile communication system. Only there, the effect of all aspectsthat have not been modelled or have been abstracted for the simulations can beidentified. The Vienna MIMO Testbed as described in [6], [12] can be used forthe measurements of various mobile communication standards, among them theLTE-downlink. An example for such measurements can be found in [16]. There,the first part of the measurement-procedure is to transmit signals for all possi-ble sets of transmission parameters for every measurement point. The optimalset is determined afterwards, by evaluating all received files and choosing thosethat yield the highest throughput for every transmission-constellation. While theadvantage of this approach is, that no real-time feedback is necessary, there aretwo factors that restrict the measurement to certain transmission modes. For atransmission mode with a discrete set of transmission-parameters, the number ofcombinations that have to be transmitted is limited, but the computational effortcan get out of hand. For the current hardware used for the offline evaluation, not

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