Faculty of Engineering, School of Electrical and Information Engineering, MSc Dissertation / For the effective implementation of a wireless local area network (WLAN) within a building, a complete understanding of indoor signal propagation is required. This paper compares three empirical propagation prediction models with regards to efficiency and accuracy. To achieve this, a software prediction tool was developed using C++ which allows an end user to quickly draw a building floor plan using user specified drawing materials. It also has the ability to calculate the required empirical parameters from entered measurements but this was found to produce results similar to that when theoretical empirical parameters were used. The accuracy of the prediction tool was gauged by comparing its outputs, using the different empirical models, to measurements. In doing so it was determined that two of the models produced functional levels of accuracy in which 93% and 82% of the simulated results were within 15 and 10 dB of the measured results respectively for the most accurate of the models used. All three empirical models were found to have computational times low enough, less than 5 minutes for an average building, as to allow for interactive WLAN design.

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No description available.

In this paper in order to improve the performance of the IEEE 802.11e WLAN (Wireless Local Area Network), we propose a simple MAC layer access control mechanism SCAS (Self-Conscious Congestion Avoidance Scheme). During the contention period, if the traffic load is high, we adaptively temporary stop some STAs (stations) contending for the channel to reduce the collision rate according to the network conditions. SCAS is on the MAC layer. Hence, it can not only be applied to the IEEE 802.11e medium access method (EDCA [2]) but also be applied to AEDCF [12]. Besides, each STA can operate SCAS by itself not through AP (Access Point) or higher layer. Finally we evaluate the performance of SCAS on EDCA and AEDCF through simulations. Results show that SCAS can not only reduce collision rate of EDCA and AEDCF efficiently but also improve mean delay time and system throughput especially in high traffic load conditions.

Abstract Voice over IP (VoIP) is one of the fastest growing applications on the Internet. Concurrently, 802.11 Wireless LANs (WLANs) have become ubiquitous in residential, enterprise, campus and public networks. Currently the majority of traffic on WLANs is data traffic but as more people use wireless networks as their primary access medium, a greater portion of traffic will be real-time traffic such as VoIP traffic. Unfortunately 802.11 networks are designed to handle delay-insensitive, bursty traffic and perform poorly for VoIP streams. Experimental and analytical results have shown that a single 802.11b access point operating at the maximum 11 Mbps rate can support only 5 to 10 VoIP connections simultaneously. Intuitively, an 11 Mbps link should support approximately 85 bi-directional 64Kbps (G.711) streams. The reason for this under-utilization lies primarily in the Distributed Coordination Function (DCF) used by 802.11 MAC layer. The problem can be addressed by using the optional Point Coordination Function (PCF). However PCF is not widely implemented in commodity hardware nor likely to be. There is a similar problem with the proposed 802.11e standard for quality of service. To solve these problems we propose Virtual PCF, a legacy-client compatible solution to increase the number of simultaneous VoIP calls. We implement Virtual PCF, a scheme which employs a variety of techniques to improve both uplink and downlink VoIP QoS. This alleviates delays and packet loss due to DCF contention and doubles the number of supported VoIP sessions.

No description available.

No description available.

Abstract Voice over IP (VoIP) is one of the fastest growing applications on the Internet. Concurrently, 802.11 Wireless LANs (WLANs) have become ubiquitous in residential, enterprise, campus and public networks. Currently the majority of traffic on WLANs is data traffic but as more people use wireless networks as their primary access medium, a greater portion of traffic will be real-time traffic such as VoIP traffic. Unfortunately 802.11 networks are designed to handle delay-insensitive, bursty traffic and perform poorly for VoIP streams. Experimental and analytical results have shown that a single 802.11b access point operating at the maximum 11 Mbps rate can support only 5 to 10 VoIP connections simultaneously. Intuitively, an 11 Mbps link should support approximately 85 bi-directional 64Kbps (G.711) streams. The reason for this under-utilization lies primarily in the Distributed Coordination Function (DCF) used by 802.11 MAC layer. The problem can be addressed by using the optional Point Coordination Function (PCF). However PCF is not widely implemented in commodity hardware nor likely to be. There is a similar problem with the proposed 802.11e standard for quality of service. To solve these problems we propose Virtual PCF, a legacy-client compatible solution to increase the number of simultaneous VoIP calls. We implement Virtual PCF, a scheme which employs a variety of techniques to improve both uplink and downlink VoIP QoS. This alleviates delays and packet loss due to DCF contention and doubles the number of supported VoIP sessions.

In this thesis, the relation between the fractal structure and the performance of the antennna is discussed. From the simulation and theoretical analysis , we can conclude that the 10dB impedance bandwidth of the monopole decreases when the iteration level of the fractal structure increases . Then we use the fractal structure to design the LTCC antenna. We design and manufacture an LTCC antenna for WLAN IEEE 802.11a. The simulation and measured results are analyzed and discussed. Finally, we provide a new method which can reduce the cross polariztion level. And this method is used to reduce the cross polarization of the monopole using the second iteration level of the Minkowski curve. It is shown that the method does work from the measurement. This method can reduce the cross polarization of the fractal antenna and hence find more application in communication systems.

Thesis (M.S. in Electrical Engineering) Naval Postgraduate School, Dec. 2001. / Thesis advisor: John McEachen. "December 2001." Includes bibliographical references (p. 95). Also available online.