Int. J. Communications, Network and System Sciences, 2009, 2, 874-878
doi:10.4236/ijcns.2009.29101 Published Online December 2009 (
Copyright © 2009 SciRes. IJCNS
A Velocity-Adaptive Handover Scheme
for Mobile WiMAX
Caiyong HAO, Hongli LIU, Jie ZHAN
Department of Electronic Information Engineering, Hunan University, Changsha, China
Received March 7, 2009; revised July 27, 2009; accepted September 12, 2009
Mobile WiMAX is a wireless networking system based on the IEEE 802.16e standard. In order to support
mobile, some kinds of handover schemes must be adopted, and the hard handover is defined as mandatory.
Since the data transmission should be paused during the hard handover process, it causes handover delay in
mobile communication. The handover delay makes severe degradation in system performance when imple-
mented in real-time applications such as IPTV and VoIP. The existing draft standard considers only the re-
ceived signal strength when deciding handover initiation. However, the velocity factor also has an important
influence on handover initiation and can not be neglected. To deal with these problems, this article proposes
a velocity-adaptive handover scheme. This scheme adopts dynamic handover threshold according to different
velocity to skip some unnecessary handover stages, reduces handover delay and enhances the network re-
source utilization. The simulation result and performance analysis validate the efficiency of the proposed
Keywords: Mobile WiMAX, Handover, Delay, Velocity-Adaptive
1. Introduction
In order to meet the demand of high data rate in wireless
service for anytime, anywhere, and anyone, the Mobile
WiMAX (Worldwide Interoperability for Microwave
Access) based on the IEEE 802.16e [1] standard is de-
veloped for broadband wireless access as a promising
technology. The IEEE 802.16e is the new, mobile ver-
sion of the old WiMAX specification known as IEEE
802.16-2004 [2], which is a wireless, but fixed, data
transmission scheme for providing broadband connection
for metropolitan areas.
Support for handover (HO) is the most important
amendment in IEEE 802.16e to embrace mobility. The
HO is performed to maintain a continuous data transmis-
sion service for all applications when a Mobile Station
(MS) is moving across cell borders of the BSs (Base Sta-
tion). The IEEE802.16e defines three basic types of HO
[3]: Hard handover (HHO), Macro Diversity Handover
(MDHO) and Fast Base Station Switching (FBSS).
MDHO and FBSS are soft handover. HHO is mandatory
in WiMAX system while the others are optional ones.
HHO adopt break-before-make scheme, the MS stops its
radio link with the serving BS before establishing its
radio link with the target BS. This is a rather simple HO
but causes long HO delay and service disruption for
some applications, especially when the MS is in high
velocity. Thus it is unsuitable for services requiring low
latency. In MDHO or FBSS scheme, a MS is registered
to several BSs at the same time. For MDHO, a MS
communicates with two or more BSs in a given interval,
while for FBSS, a MS communicates with only one BS.
Since both the MDHO and FBSS adopt the make-be-
fore-break scheme, they can improve link quality and
provide better performance for users. In MDHO and
FBSS, the Diversity Set shall be maintained by the MS
and the BS. Since the MS and the BS have to scan and
modify the Diversity Set periodically, these schemes
demand more capacity and multiple channels in terms of
bandwidth efficiency, which give rise to wireless re-
source waste.
Up until now, a few papers have proposed several sche-
mes to deal with the research about HO in IEEE 802.16e.
In [4], fast handover scheme for real-time downlink ser-
vices using fast DL_MAP_IE is suggested, which allow
forward data transmission before the establishment of the
MS registration and authorization. However, it is not
adopted by IEEE 802.16e while some fast handover
schemes such as soft handover are adopted. In [5], it
proposed to associate only one neighboring BS instead of
Copyright © 2009 SciRes. IJCNS
several BSs. But, [5] hasnt given an effective method to
select only one neighboring BS. In [4] and [5], both ne-
glected the HO threshold values influence on HO proc-
In this paper, a velocity-adaptive handover scheme is
proposed to reduce the HO delay and the waste of the
wireless network resource. In WiMAX system, the Re-
ceived Signal Strength Indicator (RSSI) is typically used
as a measure of signal quality. And as soon as the RSSI
form the current serving BS is lower than a threshold and
the RSSI from a potential target BS reaches a threshold,
HO is executed. The most important factor to initiate HO
is the received signal strength and MS mobility. How-
ever, the IEEE802.16e standard considers only the for-
mer. In our scheme, the HO threshold is set variably ac-
cording to the MS's velocity, which can reduce the HO
delay and wireless network resource waste.
The remainder of this paper is organized as follows.
Section 2 describes the HO process in IEEE 802.16e.
Then Section 3 proposes the velocity-adaptive handover
scheme. And the simulation and the performance analy-
sis of the proposed HO scheme are given in Section 4.
Finally the conclusion is provided in Section 5.
2. Handover Process and Analysis
2.1. Handover Process
The HO process defined in the IEEE 802.16e consists of
two phases. In the first phase, network topology acquisi-
tion is carried out before HO initiation. Then the actual
HO process is performed, which includes HO decision,
HO initiation, ranging and network re-entry stage. The
detail explanations of the HO process are given as fol-
2.1.1. Network Topology Acquisition
The BS periodically broadcasts the network topology
information using MOB NBR-ADV messages, which
contains channel information of neighboring BSs such as
the BS ID (Identifier), radiation power, and their UCD
(Uplink Channel Descriptor) and DCD (Downlink
Channel Descriptor) information. Thus, the MS is able to
synchronize with neighboring BSs without listening their
DCD / UCD broadcast messages. Once the MS synchro-
nize with the neighbor BSs, it can start a scanning and
association procedure in order to select a candidate BS
for HO. The scanning procedure is done through ex-
changing MOB SCAN-REQ/RSP messages with the
serving BS. During the scanning process, all downlink
and uplink transmissions are paused and the MS can op-
tionally perform association with the neighbor BSs by
performing initial ranging. To acquire ranging parameter
and service availability information for the purpose of
selecting a potential future HO target BS, the association
Figure 1. HO Process
procedure is performed between the MS and the candi-
date target. Initial ranging process performed during
MS's scanning interval is optional. The MS can reuse this
information for the future HO through the initial ranging
values of associated BS.
2.1.2. Handover Process
HO is executed when a MS migrates from the serving BS
to the target BS. The HO process consists of the follow-
ing stages and is illustrated in Figure 1.
2.2. Handover Analysis
At first, the MS conducts cell reselection with the infor-
mation acquired from network topology acquisition to
evaluate the potential target BSs. If the network topology
has not changed during the process, this stage can be
abbreviated. Such procedure does not terminate the ex-
isting connection to the Serving BS. Then HO Decision
and Initiation is performed, which is the beginning of the
actual HO. The HO Decision consummates with a noti-
fication of MS intent to handover through MOB_MSHO-
REQ or MOB_BSHO-REQ message. The HO is initial-
ized, then the MS synchronize with target BSs downlink
to obtain the downlink and uplink parameters. If the MS
has received a neighbor advertisement earlier, the syn-
chronization procedure can be faster. And if a HO noti-
fication was sent by the serving BS and received by the
target BS via backbone connection, non-contention-based
Copyright © 2009 SciRes. IJCNS
initial ranging can be assigned, which shortens the HO
A good handover scheme should minimize the HO
delay and reduce wireless channel resource waste. How-
ever, some factors degrading the system performance
exist in the HO process. Suppose that the MS moves in a
certain velocity, If the velocity is low, the network to-
pology architecture may maintain the same in a long time,
thus in the cell reselection stage, the MS can use the
same network topology information and skip this stage.
Also, since the received neighbor advertisement (include
BS ID, DCD and UCD) do not change, the MS could
synchronization to target BS downlink by performing
non-contention-based initial ranging. Therefore, the HO
delay is able to shorten. However, for a high velocity, the
channel condition change frequently, which makes the
pre-obtained information become useless. So, during the
actual HO process, the neighboring BSs scanning and
contention-based ranging operation must be performed,
which causes a long HO delay and wireless channel re-
source waste. To deal with these problems the handover
scheme should be adjusted with the velocity.
Once the alternate target BS has been successfully se-
lected, before going into normal operation, network re-
entry process is initiated. It includes MS authorization
and new BS registration. After the success registration
with the target BS, the MS sends MOB_HO_IND mes-
sage and notifies the serving BS that the HO is com-
3. Velocity-Adaptive Handover Scheme
In this section, we propose the velocity-adaptive hand-
over scheme. Firstly, we mainly focus on the HO deci-
sion. The Mobile WiMAX specification [6] defines the
procedures during the HO, but does not include the HO
decision. Typically, the MS makes the HO decision ac-
cording to signal quality, which can be measured by the
RSSI. Figure 2 shows a simple case involving two BSs
and an MS moving away from base station A (serving
BS) toward base station B (target BS).
The Thhandover represents the HO threshold. And the
Thdrop is the point below which the quality of the link
becomes unacceptable, and will lead to excessive packet
loss and the session being dropped. The hysteresis value
(H) is used to eliminate thrashing effect. The idea is
that the MS choose those BSs with high RSSI value
which results in a better link-level communication with
the target BS and a lower bit error rate. Typically, HO
procedures are initiated when the RSSI drops below the
Thhandover (1). Also, HO is executed only if there is another
BS for which the RSSI is at least H higher than the
Thdrop (2). These can be described as fallows:
RSSITh< (1)
Figure 2. Handover decision based on RSSI.
We have known that when the MS moves in a high
velocity, the network channel information will change
frequently, so that some unnecessary stages must be per-
formed, which makes severe degradation in system per-
formance such as HO delay and network resource waste.
Form Equation (1) and (2), it can be known that if the
Thhandover is set higher, the frequency of HO initiated will
be higher, thus the network channel information can be
acquired quickly. It means that the probability of
pre-obtained information used in the HO process (cell
reselection and synchronization to target BS downlink)
will be improved. So it can reduce the HO delay. How-
ever, the frequently performing HO causes a great wire-
less network resource waste. If the Thhandover is set lower,
the HO frequency becomes lower. Since the pre-obtained
information do not update with the changes of channel
condition, the neighboring BSs scanning and conten-
tion-based ranging operation must be performed. There-
fore, the HO delay will be longer, but the wireless chan-
nel resource is consumed less. If the Thhandover maintains
a constant value regardless of the velocity, for a high
Thhandover, the wireless channel resource waste will in-
crease in the low velocity condition, and for a low Th-
handover, the HO delay will increase in the high velocity
To keep the balance of the two facets and cope with
HO delay and wireless channel resource waste problems,
we propose the velocity-adaptive handover scheme. The
scheme changes the HO threshold by dynamically adapt-
ing the Thhandover value based on velocity. It can be ana-
lyzed as follows:
According to the analysis above, H should become
larger with the velocity increase. In the stationary state
(v=0), the network channel topology information keeps
constant, and the HO delay is least, to avoid the unnec-
Copyright © 2009 SciRes. IJCNS
essary network waste, the Thhandover should be setted as
low as possible, so it might be the same value of the Thdrop,
thus the H=0(when v=0). In the highest velocity situa-
tion (v tends to infinity), the network channel topology
information changes so quickly that the improvement of
the Thhandover have nearly no effect on the HO delay, for
some unnecessary stages must be performed. Thus the
H will maintain a nearly constant value when in a very
high velocity. In other situation, the H would be set
larger with the velocity becomes higher. In order to re-
flect this dynamic character, an influence factor r is pro-
posed, which is the index associated with threshold value
and velocity.
where v is the velocity of the MS. It can be acquired
from the received SINR [7]. To make the simulation
simple, the Thdrop is set as a constant value 2dB. Apply-
ing Equations (1), (2) and (3), the initial HO threshold
Thhandover is given by
From the Equation (6), we can acquire that the Thhand-
over can be set different value according to the different
velocity of the MS. When the MS moves in a higher ve-
locity, the Thhandover is set higher, and thus the HO delay
is reduced. When the MS moves in a low velocity, the
Thhandover is set lower. Since the unnecessary HO is re-
duced, the wireless channel resource waste becomes lit-
4. Simulation and Performance Analysis
In order to evaluate the performance of the proposed
handover scheme, a simulation platform is built. The HO
threshold has an influence on HO delay, which depends
on the velocity of the MS. Table 1 shows the main pa-
rameters and the default values used in the simulation.
The simulation was done with MS speeds between 1
and 40 m/s with 1 m/s step. The 40 m/s equals to
144km/h, which is above the 100 km/h limit described in
IEEE 802.16e for a seamless handover. The Oku-
mura-Hata model for small or medium city [8] was used
for evaluating path losses.
When the MS is moving to the border of one BS in a
certain speed, the signal quality of the Serving BS begins
to degrade, if the signal level meets the Equation (5),
initial HO process would be performed, and if the condi-
tion meets the Equation (6), the actual process of HO
would be executed.
In the HO based on constant threshold scheme, we use
the constant threshold value (Drop-threshold 2dB, De-
fault handover-threshold 4dB) when the velocity varied.
Table 1. Simulation parameters.
Simulation Parameters Value
Handover type HHO
Propagation model Okumura-Hata
BS to BS distance 800m
Cell radius 5000m
Number of MS 1
Number of BS 2
Height of MS 2m
Height of BSs 30m
Frequency 5G
Drop-threshold 2dB
Default handover-threshold 4dB
Step of simulation 1 m/s
Speed range 140 m/s
Figure 3. HO delay changes with the velocity.
And, we propose velocity-adaptive HO scheme which
adopts different threshold value according to the differ-
ent velocity (6).
The simulation results of constant threshold HO
scheme and velocity-adaptive HO scheme are shown in
Figure 3.
From Figure 3, we know that when the MS is in a low
velocity (below the 20m/s), there is little influence when
implementing our scheme. However, when the MS mov-
es in a high velocity (above the 20m/s), the HO delay is
obviously reduced in using velocity-adaptive scheme.
In HO based on constant threshold scheme, the HO
delay remained below the 50ms limit (the WiMAX Fo-
rum defines that the Mobile WiMAX supports the HO
delay should be less than 50ms) until the velocity rises
up to 20m/s, apart from a few exceptions that exceeded
the limit only few milliseconds. As the velocity rises, the
HO delay is growing up to 150ms.
In HO based on velocity-adaptive scheme, the HO de-
lay is below the 50ms limit until the velocity is up to
28m/s. After that, the delay increase to 82ms region with
the 40m/s MS velocity. Since the high velocity causes a
rapid changes of the network topology and wireless
Copyright © 2009 SciRes. IJCNS
channel condition and the RSSI is not stable, some steps
such as cell reselection and synchronize to downlink of
the target BS may consume more time, so we can not
always make the HO delay less than 50ms in a high ve-
locity, but in this condition our scheme reduces the HO
delay greatly compared with the HO based on constant
5. Conclusions
In this paper, a velocity-adaptive handover scheme is
presented. According to the existing draft version of
802.16e standard, the HO initiation should be performed
if the RSSI of the serving BS is lower than the threshold.
However, it does not consider the velocitys influence on
the HO process, and the HO threshold is set as a constant.
So the velocity has a bad effect on the HO performance.
To cope with this problem, our scheme is proposed, the
HO threshold is set variably according to the MS mobil-
ity. The simulation results show that HO delay below
50ms limit when the velocity vary from 20m/s to 28m/s.
And HO delay is greatly reduced when the velocity ex-
ceeds 28m/s. Therefore, this velocity-adaptive handover
scheme can provide seamless communication for Mobile
WiMAX in delay-sensitive and high velocity applica-
6. References
[1] IEEE P802.16e/D12, Air interface for fixed and mobile
broadband wireless access systems: amendment for
physical and medium access control layers for combined
fixed and mobile operation in licensed bands, 2005.
[2] IEEE Std 802.16-2004 (Revision of IEEE Std 802.16-
2001), IEEE standard for local and metropolitan area
networksPart16: Air interface for fixed and mobile
broadband wireless access systems, 2004.
[3] WiMAX Forum, Mobile WiMAX-Part I: A technical
overview and performance evaluation, 2006.
[4] S. Choit, G. H. Hwangt, et al., Fast handover scheme for
real-time downlink services in IEEE 802.16e BWA sys-
tems, IEEE Vehicular Technology Conference (VTC
2005), Stockholm, Sweden, Vol. 3, pp. 20282032, May
[5] D. H. Lee and K. Kyamakya, Fast handover algorithm for
IEEE 802.16e broadband wireless access system, IEEE
Wireless Pervasive Computing Conference, pp. 1618
January 2006.
[6] IEEE 802.16e-2005, IEEE Standard for local and metro-
politan area networks-Part16: Air interface for fixed and
mobile broadband wireless access systems-amendent2:
physical and medium access control layers for combined
fixed and mobile operation in licensed band, 2006.
[7] M. D Austin and G. L Stuber, Velocity adaptive handoff
algorithms for microcellular systems, IEEE Transactions,
Vol. 43, pp. 549561, August 1994.
[8] T. Camp, J. Boleng, and V. Davies, A survey of mobility
models for ad hoc network research, Wireless Commu-
nicaitons and Mobile Computing (WCMC), Vol. 2, pp.
483502, May 2002.