______________________________________________________________________ Abstract:In this paper, we analyze airplane oil inventory problem of a base in the Republic of Korea Air Force (ROKAF) which should keep maintaining an appropriate inventory level of oil for the efficient operation of flights. In ROKAF, airplane oil supply is ordered (or delivered) at a constant time period, which means reorder point is not stochastic but deterministic. Since the airplane oil inspection time occupies large proportion of time between a reorder point and the time the oil is available for usage, it is imperative to analyze the inspection time which causes delay in fulfilling an order. In addition, if demands increase dramatically because of a war or an unexpected training, the reorder point could be shortened than in normal situations. In such abnormal circumstances, the shortened reorder point may require more delay for order fulfillment. In this setting, the inspection time endogenously determines the replenishment lead time (or supply lead time) and the pattern of the inventory. Meanwhile, since the procedure of airplane oil inspection is composed of several steps, it may be better that the distributions of inspection times are treated as phase-type distributions. In this context, we use D/PH/1/m queue and counting process of D-BMAP (Discrete-time Batch Markovian Arrival Process) with 2-class to analyze the delay (lead time) for order fulfillment. Using this model, we find a way to derive decision variables such as the number of inventory replenishments and average fill rate.
Cognitive Radio(CR) is the technology that allocates the frequency by using dynamic spectrum access. We proposed a model to calculate the optimal level of the cognitive radiotelegraph, where secondary users opportunistically share the spectrum with primary users through the spectrum sensing. When secondary user with cognitive radio detects the arrival of a primary user in its current channel, the secondary user moves to the idle channel or be placed in the virtual queue. We assume that the primary users have finite buffers and the population of secondary users is finite. Using a two-dimensional Makov model with preemptive priority queueing, we could derive the blocking and waiting probability as well as the optimal level of cognitive radiotelegraph under a various range of parameter circumstances.
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