Project Report

Wireless communication technology enables the communication of information in applications where wiredlinks are infeasible, such as mobile telephone conversations and battlefield monitoring using wireless sensors. Mobile telephones allow subscribers to download movies and music without a wired connection to the internet, and wireless sensors can be quickly deployed on a battlefield to monitor the movement of military units. Two primary goals of wireless communication research are to improve the energy efficiency and speed of links. Improving energy efficiency extends battery life, allowing devices to operate for long periods of time without the need for recharging. Speed improvements enable the use of high data rate applications, such as real time streaming of high quality video. A report released in 2003 by the Congressional Budget Office regarding the communication speed requirements of U.S. military systems concluded that high data rate applications such as real time video will require a hundred fold increase in speed compared to current networks. For example, if one hundred simultaneous video and data transmissions must be supported in a single location, common requirement within a division or corps headquarters, the speed demand is three orders of magnitude higher than the capability of current military communication systems. One major focus in contemporary wireless research is improving the speed and energy efficiency of networks in which the link between two sources exchanging information is poor, and a relay between the two assists communication. For example, obstacles such as trees, buildings, and rain obstruct wireless signals and cause errors in transmitted information. Assuming the intermediate links between the first source and the relay, and the second source and the relay, are not inhibited by obstacles, the link between between the sources using the relay will have higher performance than the direct link between the sources. A popular technique to improve the throughput of the link between the first source, the relay, and the second source is efficient scheduling. The most common schedule for exchanging information occurs in four steps 1. The first source transmits information to the relay.2. The second source transmits information to the relay.3. The relay transmits information received from the first source to the second source.4. The relay transmits information received from the second source to the first source. This schedule requires four distinct steps to exchange information between the first and second source. These steps are implemented in four distinct time slots, in which only a single device is allowed to transmit at a time. The number of time steps required can be reduced from four to two by appropriate scheduling and wireless receiver design. Advanced techniques exist which allow source one and two to transmit to the relay in the same time slot. The signals transmitted by the devices interfere with one another over the air, and the radio hardware at the relay is designed to process the interfered signal. The relay combines the information received from both sources and broadcasts to both in a single time step, reducing the required number of steps from four to two. The two-step throughput improving technique is very new, having been introduced within the past decade. Practical considerations, such as compensating for hardware imperfections in real wireless receivers, have not been studied extensively. The goal of this work is to analyze and improve the performance of the technique assuming that the hardware in each device which generates reference frequencies for transmitted signals is not perfect. Imperfect reference frequencies result in a loss of energy in the signal received at the relay, increasing the number of errors in the received bits. The effect of imperfect reference frequency was modeled analytically and it was discovered that relay receivers which do not compensate exhibit poor energy performance. A new relay receiver was created which compensates for imperfect frequency without the need for additional hardware at the relay. The energy performance of the new receiver was measured using computer simulation, and it was found that the new receiver is ten to a hundred times more energy efficient than the conventional. Increased efficiency without additional hardware makes the two-step throughput improving technique more viable for real-world conditions such as those found on the battlefield.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Program Officer
Carter Kimsey
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Ferrett Terry R
United States
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