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Development of an SNMP Managing Application for Adaptation of Coding & Modulation on DVB-S Satellite Modems Is Structured As Follows Development of an SNMP Managing Application for Adaptation of Coding & Modulation on DVB-S Satellite Modems by Dipl.-Ing. Dr.techn. Gerhard VRISK Submitted in a partial fulfilment of the requirements for the degree of Master of Sciences of the Post-graduate University Course Space Sciences (Main Track: Space Communication and Navigation ) Karl-Franzens University of Graz Graz, Austria 2009 Acknowledgments / Danksagung This master thesis has been written at the IKS ( Institute of Communication Networks and Satellite Communications / Institut für Kommunikationsnetze und Satellitenkommunikation ) at Graz University of Technology in the years 2009. First, I would like to thank my thesis advisor, Univ.-Prof. Dipl.-Ing. Dr. Otto Koudelka, Chair of the IKS, who offered me to work on this subject. Furthermore, I would like to thank (em.)Univ.-Prof. DDr. Willibald Riedler, retired Director of the former Institute of Communications and Wave Propagation at the Technical University of Graz and retired Director of the Space Research Institute of the Austrian Academy of Sciences, for his complaisant attendance to act as 2nd reviewer of this master thesis. Last but not least I would like to thank Univ.-Prof. Dr. Helmut Rucker, Research Director at the Space Research Institute of the Austrian Academy of Sciences, especially for his support during the course and generally for organization and managing the 4th MSc University Course Space Sciences . A big kiss goes to my wife Gerlinde for her support und patience during my studies and for the photographs, too. Am Schluss möchte ich mich noch bei meinen Eltern bedanken, für all die Förderung von früh an und für ihre Unterstützung während meinen Studienzeiten. Meinem Vater gewidmet († 2006). Graz, Winter 2008 - 2009 2 Abstract The aim of this master thesis was to develop a software for automated adaption of coding and modulation (called ACM ), implemented for DVB-S ( Digital Video Broadcasting over Satellite ) satellite modems. This software should control transmission parameters and initiate a countermeasure when some fading on the satellite transmission channel appears (by rain, etc.). As satellite modem hardware PARADYN’s DMD20 Universal Satellite Modem was chosen, at which an enhanced set of the DVB-S protocol was yet available, namely DVB- DSNG (Digital Satellite News Gathering ). In the first step as preliminary work the performance characteristics of the different feasible combinations of implemented modulations ( BPSK, QPSK, 8PSK, 16QAM ) and codes ( Viterbi, Trellis ) of the used satellite modem was to determine. For this reason a test setup was built. There the well-known characteristics of BER ( Bit Error Rate ) vs. Eb/N 0 (Ratio between the Energy per Information Bit and Power Spectral Density ) were measured. For this purpose a C++ class was written to communicate with the satellite modem via the IP protocol SNMP (Simple Network Management Protocol ), based on a vendor-specific enterprise MIB (Message Information Base ). The goal of these measuring series was to find the appropriate code and modulation pair for a specified bit error rate. However, a correction of these retrieved Eb/N 0 values, which were estimated originally by the satellite modem, was necessary, depending on utilized modulation. As final step a more-or-less heuristic closed-loop controller strategy was programmed, based on the preliminary measurement data. There the controller distinguishes by means of the currently estimated Eb/N 0 value and a look-up table of pre-defined reference Eb/N 0 values about the used code and modulation pair to keep a particular bit error rate (e.g. 10 -6). As implication the utilized bandwidth is manipulated indirectly to transmit a forced constant data rate. That means in this actual test scenario, when a transmission channel becomes worse there is more bandwidth, respectively a higher symbol rate necessary - to sustain a constant end-to-end stream of bits per seconds. To improve the stability of the controller behavior there were some enhancements implemented, like hysteresis or dampening. Also the trend of collected Eb/N 0 values, accumulated in an observation interval, could be used as basis for follow-up statistical considerations. Accomplished performance improvement could be shown by a number of manual simulations in a simple test setup, whereas several optional parameters of the controller program were varied. On the other hand one can realize that this discrete out-of- band closed-loop implementation has a drawback compared to the high-performance intrinsic implementation of Adaptive Coding & Modulation (ACM) in the next-generation DVB-S2 standard. The reason is the recurrent down-time in the end-to-end transmission each time after changing the modulation or code type caused by new locking between transmitter and receiver. 3 Zusammenfassung Das Ziel der vorliegenden Master Thesis war die Entwicklung einer Software zur automatischen Adaptierung von Kodierung & Modulation eines DVB-S ( Digital Video Broadcasting over Satellite ) Satellitenmodems. Diese Software soll an Hand von Übertragungsparametern die Qualität des Übertragungskanals der Satellitenstrecke überwachen und Gegenmaßnahmen initiieren wenn dort ein Schwund (z.B. hervorgerufen durch Regen) auftritt. Als Hardware wurde das DMD20 Universal Satellite Modem der Fa. PARADYN ausgewählt, welches zusätzlich sogar über ein erweitertes DVB-S Protokoll, nämlich DVB-DSNG (Digital Satellite News Gathering ), verfügte. Als Vorarbeit wurden die Kennlinien des verwendeten Satellitenmodems, in allen implementierten Kombinationsvarianten von Modulierung ( BPSK, QPSK, 8PSK, 16QAM ) und Kodierung ( Viterbi, Trellis ), ermittelt. Dafür wurde eine Testumgebung aufgebaut, in der die bekannten BER ( Bitfehlerrate ) vs. Eb/N 0 (Verhältnis von Energie pro Informationsbit zu spektraler Leistungsdichte ) Kennlinien gemessen wurden. Zum Zweck der Kommunikation mit dem Satellitenmodem wurde eine eigene C++ Klasse geschrieben, basierend auf dem IP Protokoll SNMP ( Simple Network Management Protocol ) und einer herstellerspezifischen MIB ( Message Information Base ). Das Ziel dieser Messreihen war nun die Bestimmung des passenden Kodierung & Modulation Paares jeweils zu einer vorgegebenen Bitfehlerrate. Jedoch war auch eine nachträgliche Korrektur dieser ausgelesenen Eb/N 0 Werte, welche ja vom Satellitenmodem nur geschätzt werden, in Abhängigkeit der Modulation notwendig. Im zweiten Schritt wurde nun eine mehr oder weniger heuristische Strategie eines Reglers, basierend auf den vorher ermittelten Messdaten, programmiert. Der Regler entscheidet an Hand des momentanen Eb/N 0 Wertes aus dem Satellitenmodem und einer vordefinierten Matrix von Referenzwerten über den passenden Kodierung- und Modulationstyp, um eine bestimmte Bitfehlerrate (z.B. 10 -6) zu halten. Die Folge davon ist, dass jeweils die verwendete Bandbreite geändert wird um eine eingeprägte konstante Datenrate übertragen zu können. D.h. in diesem konkreten Testszenario, wenn der Übertragungskanal schlechter wird, wird mehr Bandbreite, bzw. eine höhere Symbolrate benötigt, um einen Datenstrom mit gleich bleibender Rate (d.h. Bits pro Sekunde) Ende-zu-Ende übertragen zu können. Zur Erhöhung der Stabilität des Reglerverhaltens wurden einige Eigenschaftsverbesserungen implementiert, wie z.B. eine Hysteresis oder auch eine Verzögerung der auftretenden Schaltzustände. Auch eine statistische Trendanalyse, basierend auf den ermittelten Werten in einem bestimmten Messintervall, kann herangezogen werden. An Hand einiger manuell durchgeführter einfacher Testläufe, wobei jeweils Parametrisierungen verändert worden sind, konnten die dadurch erreichten Verbesserungen im Verhalten des Reglers gezeigt werden. Jedoch sieht man auch den Nachteil einer externen Regelung im Vergleich zu einer inhärenten Adaptierung von Kodierung & Modulation wie im neuen DVB-S2 Standard beschrieben (ACM). Der Grund sind die auftretenden Unterbrechungen in der Ende-zu-Ende Kommunikation bei jedem Modulierungs- oder Kodierungswechsel, hervorgerufen durch das Neusynchronisieren von Sender und Empfänger. 4 Table of Contents Acknowledgments / Danksagung............................................................................................... 2 Abstract ...................................................................................................................................... 3 Zusammenfassung...................................................................................................................... 4 Table of Contents ....................................................................................................................... 5 References .................................................................................................................................. 6 ETSI Standards....................................................................................................................... 7 RFC Standards........................................................................................................................ 9 List of Abbreviations................................................................................................................ 10 1. Introduction...................................................................................................................... 12 1.1. DVB History ............................................................................................................ 12 1.2. DVB Suite ...............................................................................................................
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