A study of impact of inband signalling and realistic channel knowledge for an example dynamic OFDM-FDMA system

Technische Universität Berlin, Berlín, Berlin, Germany
European Transactions on Telecommunications (Impact Factor: 1.35). 01/2005; 16(1):37-49. DOI: 10.1002/ett.1030
Source: DBLP

ABSTRACT Dynamically assigning sub-carriers of orthogonal frequency division multiplexing (OFDM) systems to multiple different terminals in a cell has been shown to be beneficial in terms of different transmission metrics. The success of such a scheme, however depends on the ability of the access point to inform terminals of their newest sub-carrier assignments as well as on the accuracy of the channel state information used to generate new assignments. It is not clear whether the overhead required to implement these two functions consumes all of the potential performance increase possible by dynamically assigning sub-carriers. In this paper, a specific MAC structure is selected enabling the operation of a dynamic OFDM system, incorporating a signalling scheme for dynamically assigned sub-carriers. Based on this structure, we study the overhead required for a dynamic sub-carrier scheme; a static variant that serves as a comparison case. We investigate the performance difference of these two schemes for various scenarios where at first signalling and then realistic channel knowledge is added to the system model. Average throughput and goodput per terminal serve as figures of merit; the number of terminals in the cell, the transmission power per sub-carrier, the delay spread and the movement speed of the terminals are varied. We find that a realistic overhead model decreases the performance of both static and dynamic schemes such that the overall ratio favours in all cases except for higher speeds the dynamic rather than the static scheme especially in realistic system environments. Copyright © 2005 AEIT.

Download full-text


Available from: James Gross, Jun 17, 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Multiuser dynamic orthogonal frequency division multiple access (OFDMA) can achieve high downlink capacities in future wireless networks by optimizing the subcarrier allocation for each user. When it comes to the integration into current wireless local area network (WLAN) standards, dynamic OFDMA raises several implementation issues which are neglected in theoretical papers. Putting this emerging approach into practice requires to treat these issues accordingly and to demonstrate the feasibility of the system design. In this paper, we propose a dynamic OFDMA integration for the physical layer of the widespread IEEE 802.11a standard. To test our implementation and demonstrate its practical relevance we use a pragmatic approach: We prototype multiuser dynamic OFDMA on a real-time software-defined radio testbed for WLANs. We discuss details of our implementation and provide measurements showing that it does not introduce significant overhead into the IEEE 802.11a system at high subcarrier allocation quality. We particularly focus on the problems of our integration as well as the concepts and limitations of the used testbed.
    Testbeds and Research Infrastructure for the Development of Networks and Communities, 2007. TridentCom 2007. 3rd International Conference on; 06/2007
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nutzer drahtloser Netze müssen häufig Verbindungsabbrüche und niedrige Datenraten in Kauf nehmen. Um dieses Problem effizient zu lösen, ist kooperatives Weiterleiten (sog. cooperative relaying) vielversprechend. Theoretisch verspricht dieser Ansatz hohe Diversitätsgewinne, wenn unkorrelierter Kanalschwund (sog. fading), volles Kanalwissen, perfekte Konnektivität und ideale Protokolle vorausgesetzt werden. Jedoch wurden bisher weder diese idealistischen Annahmen systematisch verifiziert, noch die hohen Gewinne experimentell belegt. Es ist das Ziel dieser Dissertation, diese Lücke zwischen Theorie und Praxis zu schließen. Dies erfolgt in drei Schritten. Zunächst wird analysiert, inwiefern praktische Annahmen die Leistung idealer Kooperationsverfahren verringern. Daraus folgen neue theoretische Werkzeuge mit denen das leistungsfähigste Kooperationsprotokoll auch unter realistischen Einschränkungen ausgewählt werden kann. Um unter diesen Einschränkungen hohe Gewinne zu erreichen, werden im zweiten Schritt neue Resourcenzuteilungsverfahren vorgestellt. Abschließend wird ein Prototyp für kooperative lokale Netze (sog. WLANs) implementiert und in aufwändigen Feldversuchen untersucht. Diese Messungen demonstrieren nicht nur die hohe Leistung der vorgestellten Verfahren sondern belegen erstmals, dass kooperative Systeme die theoretisch vorhergesagten Gewinne auch erreichen. Dies zeigt, dass kooperatives Weiterleiten bereits heute effizient, praktikabel und reif für die Standardisierung ist. Mobile users often experience communication outage and low data rate. To efficiently and economically cope with this problem, cooperative relaying is promising. In theory, this new approach shows tremendous diversity gains with uncorrelated fading channels, full channel knowledge, perfect network connectivity, and ideal protocol operation. However, so far these idealistic assumptions are not validated and cooperation's high gains are not experimentally proven. Bridging this gap between theory and practice is targeted in this thesis. We do so in three steps. First, we analyze how realistic assumptions decrease the performance of ideal cooperation. This provides new theoretical tools to choose the beneficial relaying protocol with limited channel knowledge and limited network connectivity. Second, we develop practical resource allocation techniques to maintain cooperative gains even under realistic assumptions. Third, we implement a prototype for cooperative Wireless Local Area Networks (WLANs). Our extensive field measurements (e.g., using an actual train to move the cooperating terminals) not only demonstrate the feasibility and high performance of our solutions. Moreover, our measurements provide the missing experimental proof that, even under realistic constraints, cooperation achieves the performance order promised by theory. Even with today's technology and in real mobile scenarios, letting nodes cooperate is feasible, efficient, and ready for standardization.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The paper studies a distributed OFDMA system in which the transmitting nodes (source and relay) have access to channel information and to some interference-related information. Distributed subchannel allocation is modelled as a noncooperative game where the players are the source nodes. Channel-aware source nodes opportunistically select their subchannels while a channel-aware relay node assigns the input subchannels at relay to appropriate output subchannels in a way that maximizes the sum of utilities in terms of signal-to-noise ratio. Numerical examples illustrate the performance of distributed subchannel assignment; opportunistic channel allocation implies a trade-off between the average capacity and fairness.
    Proceedings of the 2nd International Conference on Performance Evaluation Methodolgies and Tools, VALUETOOLS 2007, Nantes, France, October 22-27, 2007; 01/2007