Impact of the Transmission Scheme on the Performance in Wireless LANs Andreas Könsgen, Andreas Timm-Giel, Carmelita Görg 1 , and Ronald Böhnke 2 1 Communication Networks 2 Communications Engineering Center for Computing Technologies (TZI) University of Bremen Otto-Hahn-Allee 1, 28359 Bremen, Germany Abstract. In wireless LANs, different multi-user access methods such as TDMA, OFDMA and SDMA are available which can be used with or without channel knowledge at the transmitter and a single antenna (MISO) or multiple antennas (MIMO) at the receiver. A cross-layer scheduler is considered which can be configured with these different PHY methods as well as with knowledge about application requirements and channel conditions at the MAC layer. The scheduler computes priorities on the MAC layer that are handed over to the physical layer in order to keep quality-of-service constraints such as throughput and delay. In this paper, it is demonstrated that controlling the priorities by a QoS aware resource allocation method allows to meet the requirements by the appli- cations under various channel conditions. MISO-SDMA has a relatively small performance penalty in comparison to MIMO-SDMA which gives the best result. For MIMO-TDMA and -OFDMA, channel knowledge at the PHY layer does not result in essential performance enhancement. Keywords: Wireless LAN, cross-layer, MIMO. 1 Introduction Wireless LANs have to meet increasing requirements nowadays and in the fu- ture: high data rates for each user, high spectral efficiency in the sense of a high total capacity and meeting several types of QoS requirements for different applications. Up to now, most protocol stacks are designed according to the OSI model which defines seven layers from the physical layer up to the application layer, with an increasing degree of abstraction from the physical hardware. In legacy protocol stacks, these different protocol layers have been optimised indepen- dently of each other. This separation is in particular problematic for the design of the two lowest layers, which are the MAC and the PHY layer, because there are close mutual dependencies between these two layers. The QoS requirements have already to be considered by selecting the physical transmission method. Moreover, the actual channel conditions and the effects of these conditions for F. Granelli et al. (Eds.): MOBILIGHT 2009, LNICST 13, pp. 303–314, 2009. c  ICST Institute for Computer Sciences, Social-Informatics and Telecommunication Engineering 2009