Abstract—Link adaptation to dynamically select the data transmission rate at a given time has been recognized as an effective way to improve the goodput performance of the IEEE 802.11 wireless local-area networks (WLANs). Recently, with the introduction of the new high-speed 802.11a physical layer (PHY), it is even more important to have a well-designed link adaptation scheme work with the 802.11a PHY such that its fast transmission rates can be exploited. In this paper, we first present a generic method to analyze the goodput performance of an 802.11a system under the Distributed Coordination Function (DCF), and express the expected effective goodput as a closed-form function of the data payload length, the frame retry count, the wireless channel condition, and the selected data transmission rate. Then, based on the theoretical analysis, we propose a novel MPDU (MAC Protocol Data Unit)based link adaptation scheme for the 802.11a systems. It is a simple table-driven approach, and the basic idea is to pre-establish a best PHY mode table by applying the dynamic programming technique. The best PHY mode table is indexed by the system status triplet that consists of the data payload length, the wireless channel condition, and the frame retry count. At run-time, a wireless station determines the most appropriate PHY mode for the next transmission attempt by a simple table lookup, using the most upto-date system status as the index. Our in-depth simulation shows that the proposed MPDU-based link adaptation scheme outperforms the single-mode schemes and the AutoRate Fallback (ARF) scheme — which is used in Lucent Technologies ’ WaveLAN-II networking devices — significantly in terms of the average goodput, the frame drop rate, and the average number of transmission attempts per data frame delivery.

Power and energy consumption has recently become an important issue and consequently, power-aware techniques are being devised at all levels of system design; from the circuit and device level, to the architectural, compiler, operating system and networking layers. In this survey we concentrate on power-aware design techniques for real-time systems. While the main focus is on hard real-time, soft real-time systems are considered as well. We start with the motivation for focusing on these systems and provide a brief discussion on power and energy objectives. We then follow with a survey of current research on a layer by layer basis. We conclude with illustrative examples and open research challenges. This work provides an overview of poweraware techniques for the real-time system engineer as well as an up-to-date reference list for the researcher.

With the proliferation of wireless local area network (WLAN) technologies, wireless Internet access via public hotspots will become a necessity in the near future. In outdoor areas where the installation of a large number of wired access points is practically or economically infeasible, mobile users located at the edge of the network communicate with the access point at a very low rate and in turn waste network resources. In this work, we promote the use of tetherless relay points (TRPs) to improve the throughput of a WLAN in such environments. We first provide a high level description on how to integrate TRPs in a multi-rate WLAN architecture. We then propose an integer-programming optimization formulation and an iterative approach to compute the best placement of a fixed number of TRPs. Finally, we show in numerical analysis, through a case study based on relay-enabled rate adaptation and IEEE 802.11-like multi-rate physical model with Rayleigh fading, that for a wide range of system parameters, significant performance gain can be achieved when TRPs are strategically installed in the network.

Energy constrained hybrid CSMA/TDMA based Medium Access Control (MAC) scheduling has largely been ignored in litureture regarding Wireless Sensor Networks (WSN). In this paper, we propose a new smart framing scheduling