  This paper considers a retrial queueing model for a base station in cellular
networks where fresh calls and handover calls are available. Fresh calls are
initiated from the cell of the base station. On the other hand, a handover call
has been connecting to a base station and moves to another one. In order to
keep the continuation of the communication, it is desired that an available
channel in the new base station is immediately assigned to the handover call.
To this end, a channel is reserved as the guard channel for handover calls in
base stations. Blocked fresh and handover calls join a virtual orbit and repeat
their attempts in a later time. We assume that a base station can recognize
retrial calls and give them the same priority as that of handover calls. We
model a base station by a multiserver retrial queue with priority customers for
which a level-dependent QBD process is formulated. We obtain Taylor series
expansion for the nonzero elements of the rate matrices of the level-dependent
QBD. Using the expansion results, we obtain an asymptotic upper bound for the
joint stationary distribution of the number of busy channels and that of
customers in the orbit. Furthermore, we derive an efficient numerical algorithm
to calculate the joint stationary distribution.
