| Research Abstract |
Recently, genetic algorithms attract one's attention as an optimization technique from the view point not only of statistics but also various fields of engineering. In this research, we examine the applicability of genetic algorithm to design methods of spread-spectrum slotted ALOHA communication systems under given various conditions such as the packet dropping probability and the average packet transmission delay. In spread-spectrum slotted ALOHA communication systems, there exist a number of parameters to be optimally designed, for example, the packet retransmission algorithm, the number of retransmission trials (retransmission cutoff) to be allowed, coding rate for forward error correction, and so forth. Particularly, the combinatorial effect of an exponential backoff scheme and retransmission cutoff on the stability of frequency-hopping slotted ALOHA systems with finite population was investigated in terms of the catastrophe theory. In the systems, the packet retransmission probab
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ilities are geometrically distributed with respect to the number of experienced unsuccessful transmissions and a packet will be discarded after a certain number of unsuccessful transmissions. Expressions which should be satisfied at equilibrium points are first derived. Then, the cusp point and the bifurcation sets are numerically evaluated. Finally, we visualize how the exponential backoff scheme and retransmission cutoff effect on the bistable region. Numerical results show that the exponential backoff scheme can mitigate bistable behavior of the system with finite population. However, it is also revealed that there is asymptotically no effect of the exponential backoff scheme on the stability of the system with infinite population. Furthermore, we have found that genetic algorithms can be applicable in designing slotted ALOHA communication systems. Further studies include ; 1) optimization of slotted ALOHA communication systems in conjunction with spread-spectrum techniques, 2) optimization under more realistic conditions such as the 95% packet transmission delay, and 3) derivation of more precise expressions for the packet dropping probability. Less
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