This paper considers a discrete-time Geo/Geo/1 preemptive priority queue with two
classes of customers. The queue system can be modeled by a quasi-birth-and-death (QBD)
process with infinitely many phases. For the QBD process, we obtain explicit form of the
joint stationary distribution and then we also get the stationary queue length distribution for lower-priority customers.
International Journal of Information and Management Sciences 27 (3), pp.237-251