Meeting the diverse delay requirements of emerging wireless applications is one of the most critical goals for the design of ultra-dense networks. Although the delay of point-to-point communications has been well investigated using classical queueing theory, the delay of multipoint- to-multipoint communications, such as in ultra-dense networks, has not been explored in depth. The main technical difficulty lies in the interacting queues problem, in which the service rate is coupled with the statuses of other queues. In this article, we elaborate on the main challenges in the delay analysis in ultra-dense networks. Several promising approaches, such as introducing the dominant system and the simplified system, to bypass these difficulties are proposed and summarized to provide useful guidance.
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