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Title: Multi-User Performance Issues in Wireless Impulse Radio Networks
Authors: Lovelace, William
Advisors: Dr. Keith J. Townsend, Committee Chair
Keywords: impulse radio
rate control
ultra wideband radio
Issue Date: 13-Jan-2005
Degree: PhD
Discipline: Electrical Engineering
Abstract: While Impulse Radio (IR) has shown tremendous potential for high throughput local area networks based on time domain separation techniques, the stringent parametric assumptions required for practical implementation have not been clearly evaluated. Specifically, two of the more common constraints required to meet the projected UWB performance measures are timing tolerances and multi-user interference control. Our work is the first to quantify the effects of timing jitter and tracking on time-hopping UWB multi-user performance. The investigations of these issues show that the performance of binary and 4-ary impulse radio is very sensitive to timing jitter and tracking errors. Simulations find orthogonal pulse position modulation (PPM) out performed binary offset PPM at all jitter levels in thermal and pulse noise. With adequate understanding of the effects of timing jitter an IR receiver can be designed to meet a given performance. However, the control of local user power for a given receiver is not always guaranteed in practical environments or under complete control of the receiver. We propose a simple chip discrimination technique for use with UWB that improves performance for large near/far interference ratios. The technique exploits the unique time domain characteristics that only UWB systems can provide by applying individual chip discrimination prior to the spreading summation. A statistical model is developed that predicts bit error performance for binary offset pulse position modulation (PPM) as a function of near/far density and power for varying discrimination thresholds. Results show substantial improvement using our method for near interferers with near/far power ratios greater than 20 dB. Leveraging the information derived from the chip discrimination approach, as a component to a peer-to-peer MAC layer protocol, we can affect more efficient transmission rate control. The combination of these two techniques greatly improves performance in poor near-far power ratios and out performs fixed parameter links. Adaptation of these methods are applied to a simple ALOHA packet network to illustrate the effectiveness of chip discrimination and rate control to overall network throughput.
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