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In this paper, we study the performance of a combined link-and physical-layer approach for the downlink of a dynamic OFDM-FDMA transmission system delivering MPEG-4 video streams. The approach consists of three different mechanisms — queue management, resource allocation, and subcarrier assignment — which can be used separately as well as in combination.(More)
—The simulation models of wireless networks rapidly increase in complexity to accurately model wireless channel characteristics and the properties of advanced transmission technologies. Such detailed models typically lead to a high computational load per simulation event that accumulates to extensive simulation runtimes. Reducing runtimes through(More)
Recently, a lot of research effort has been spent on cross-layer system design. It has been shown that cross-layer mechanisms (i.e., policies) potentially provide significant performance gains for various systems. In this article we review several aspects of cross-layer system optimization regarding wireless OFDM systems. We discuss basic optimization(More)
Within the last decade the OFDM transmission scheme has become part of several standards for wireless systems. Today, OFDM is even a candidate for 4G wireless systems. It is well known that dynamic OFDMA systems potentially increase the spectral efficiency. They exploit diversity effects in time, space, and frequency by assigning system resources(More)
Wireless multi carrier approaches can support the simultaneous transmissions to several wireless terminals , each one getting only some of the available sub-carriers. Judiciously assigning subcarriers to terminals can be used to, e.g., increase the capacity of a wireless cell; however, the resulting assignment problem is difficult. The first subproblem is(More)
In wireless local area networks often a station can potentially associate with more than one access point. Therefore, a relevant question is which access point to select " best " from a list of candidate ones. In IEEE 802.11, the user simply associates to the access point with the strongest received signal strength. However, this may result in a significant(More)
—One of the major challenges in designing cellular Cognitive Radio (CR) networks is the avoidance of Secondary User (SU) interference to so called Primary Users (PUs) operating in the licensed bands. Usually, SU operation has to be interrupted periodically in order to detect PU activity and avoid the respective frequencies. Recently, Dynamic Frequency(More)
Earlier paper have demonstrated that the achievable throughput of OFDM systems can benefit significantly from individual modulation/transmit power selection on a per sub-carrier basis according to the actual gain of individual sub-carriers (so called dynamic OFDM scheme). Usage of such approach requires, however, providing support for additional(More)
Efficiently scheduling and synchronizing parallel event execution constitutes the fundamental challenge in parallel discrete event simulation. Existing synchronization algorithms typically do not analyze event interactions within the simulation model – mainly to minimize runtime overhead and complexity. However, we argue that disregarding event interactions(More)
—Rate adaptation in vehicular networks is known to be more challenging than in WLANs due to the high mobility of stations. Nevertheless, vehicular networks are subject to certain recurring patterns particularly if stations communicate to roadside units. This has lead to the proposal of learning-based rate adaptation schemes which are trained for a certain(More)