Communications and Network, 2013, 5, 426-429
http://dx.doi.org/10.4236/cn.2013.53B2078 Published Online September 2013 (http://www.scirp.org/journal/cn)
Copyright © 2013 SciRes. CN
Dropping Rate Simulation for a Handover Scheme Using
Importance Sampling*
Dong Liang, Gan Ding, Wuling Qin, Mugen Peng
Wireless Signal Processing and Network Lab, Key Laboratory of Universal Wireless Communications
(Ministry of Education), Beijing University of Posts and Telecommunications, Beijing, China
Email: liangdong@bupt.edu.cn, dinggan@bupt.edu.cn, qinwulin@bupt.edu.cn, pmg@bupt.edu.cn
Received June 2013
ABSTRACT
The process of changing the channel associated with the current connection while a call is in progress is under consid-
eration. The estimation of dropping rate in handover process of a one dimensional traffic system is discussed. To reduce
the sample size of simulation, dropping calls at base station is considered as rare event and simulated with importance
sampling - one of rare event simulation approaches. The simulation results suggest the sample size can be tremendously
reduced by using importance sam pling.
Keywords: Handover; Importance Sampling; Monte Carlo; Dropping Rate
1. Introduction
Handover is the process of changing the channel (fre-
quency, time slot, spreading code, or combination of them)
associated with the current connection while a call is in
progress [1]. Usually, continuous service is achieved by
supporting handover from one cell to another [2,3]. As
shown in Figure 1, it is often initiated eith er by crossing
a cell boundary or by deterioration in quality of the sig-
nal in the current channel [4].
The handover process starts when the power received
by the mobile station from a neighboring cell’s base sta-
tion (BS) exceeds the power received from the BS of the
current cell by a certain amount, called handover thre-
shold. This is the threshold in the received power, below
which acceptable communication with the BS of the cur-
rent cell is no longer possible [5]. If the power level from
the current BS falls below the receiver threshold prior to
the mobile being assigned to a channel by the target BS,
the call is terminated and the handover attempt fails.
Queuing priority schemes give possibility to reduce the
blocking probability of new calls, where the calls queuing
in handove r queues. Que uing pri ori ty channel assignment
strategy is described in [6]. Analysis of a mobile cellular
system with handover priority and hysteresis control is
given in [7].
Queuing of handover requests is possible, because the
mobile station spends some time in handover area, where
communications with the current BS decrease in depen-
dence of the speed of moving of the mobile station. Each
next request into the handover queue can be served ac-
cording to certain service discipline.
In nowadays broadband wireless networks probabilis-
tic parameters of Quality of Service (QoS) like probabil-
ity of dropped calls because all channels at the BS are
busy is very small, less than10−9. In such cases the Monte
Carlo simulation, which is implemented for probabilities
not less than 10−5 [8] is useless and for estimation of
handover QoS parameters as blocking probabilities is
suggested implementation of rare event simulation.
Rare event simulation helps to speed up the simulation
process, as studied probabilistic parameters of quality of
service have very small probability between 10−8 and 10−12,
and they can’t be reached with standard Monte Carlo.
2. Modeling of Handover Scheme
A simplified handover mechanism is shown in Figure 2,
Figure 1. Handover in cell edge.
*This work was supported in part by the Fundamental
for the Central Universities (Research on efficient algorithm and con-
fidence probability in system level simulatio ns in wireless communica-
tions) (2012RC0113), the Stat e Major Science and Technology Special
Projects (Grant No. 2011ZX03003-002-
Beijing Natural Science Foundation (Grant No. 4131003).