ENHANCED HANDOVER MECHANISM FOR MULTICAST
AND BROADCAST SERVICES IN IEEE 802.16E SYSTEMS
Min-Gon Kim
Public and Original Technology Research Center, Daegu Gyeongbuk Institute of Science and Technology (DGIST)
Daegu, Korea
Yazan M. Allawi
Department of Information and Communications Engineering, Korea Advanced Institute of Science and Technolog (KAIST)
Daejeon, Korea
Jung-Sook Jang, Jin-Kyu Kang, SangCheol Lee
Public and Original Technology Research Center, Daegu Gyeongbuk Institute of Science and Technology (DGIST)
Daegu, Korea
Minho Kang
Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea
Keywords: IEEE 802.16e, Mobility Management, Handover, Multicast and Broadcast Services.
Abstract: The handover delay time spent in the service connections running at a Mobile Station (MS) in IEEE 802.16e
systems can have a negative impact on real-time applications; especially, a connection for Multicast and
Broadcast Service (MBS) could suffer additional handover delays due to multicast session update (the
process of updating its associated MBS zone), where its MS moves out of its associated MBS zone. Taking
this issue into account, this paper proposes an Enhanced Handover Mechanism (EHM) that can create the
reduction of both the required time to scan neighbor Base Stations (BSs) and the number of the MBS
session update by firstly selecting a neighbor BS guaranteeing a satisfactory level of Received Signal
Strength (RSS) value in the current associated MBS zone as the target BS as many as possible. Simulation
results show that the EHM can create the reduction of the handover delay time of both multicast and unicast
connections while maintaining a satisfactory RSS value of an MS. As a consequence, enhancement of
mobility support for real-time MBS can be achieved while keeping compatibility to IEEE 802.16e systems.
1 INTRODUCTION
One of the major goals of mobile WiMAX (IEEE
802.16e) (1) is to introduce mobility in WiMAX (2),
based upon handover. The motivation of handover
process is to consider the situation that Mobile
Stations (MSs) move out of cell coverage of the
current serving Base Station (BS); specifically, it
aims to keep seamless service to MSs regardless of
their locations. The handover process is initiated
when Received Signal Strength (RSS) of an MS
from the current serving BS is degraded due to high
Bit Error Rate (BER), increasing distance between
the MS and the serving BS, and out of the cell
coverage of the serving BS to support services
(3)(4). Throughout the handover process, the
connection update to the target BS causes an
inevitable delay time spent to sustain the service
connections running at an MS, and thus it can have a
negative impact on Urgent Grant Service (UGS),
real time-Polling Service (rtPS), and Extend Real
Time-Polling Service (ert-PS), which support delay-
sensitive applications (e.g., Video on Demand
(VoD) and Voice over IP (VoIP)) in IEEE 802.16e
12
Kim M., M. Allawi Y., Jang J., Kang J., Lee S. and Kang M. (2010).
ENHANCED HANDOVER MECHANISM FOR MULTICAST AND BROADCAST SERVICES IN IEEE 802.16E SYSTEMS.
In Proceedings of the International Conference on Data Communication Networking and Optical Communication Systems, pages 12-18
DOI: 10.5220/0002933600120018
Copyright
c
 SciTePress

systems. Therefore, it is imperative to put into
consideration the reduction of the initiation phase
time during handover process.
Basically, when the RSS of an MS from the
current serving BS is lower than the handover
threshold value (a predefined value in the Medium
Access Control (MAC) layer), the MS will try to
find a target BS available in its vicinity by scanning
every neighbor BS (cell reselection). Depending on
the number of the scanned neighbor BSs, the
required time to scan neighbor BSs is differently
presented (5). Thus, there have been much effort on
this aspect, the reduction of Layer 2 (L2) handover
preparation delay (5)-(8). In addition, some studies
for fast handover execution have been conducted
(9)-(11). As another layer study, L3 handover
mechanisms have tried to accelerate the L3 handover
for mobile IPv6 (12)(15). However, the
aforementioned studies have just tried to reduce L2
and L3 handover delay without concerning about
additional handover delays from multicast session
update (the process of updating its associated
Multicast and Broadcast Service (MBS) zone in
cases when inter-MBS zone handover is required).
Since MBS zone supports its application based
upon Multicast Connection Identifier (MCID) (11),
the multicast session update is for making the
current serving BS join its associated MBS zone,
where the MS moves out of its current MBS zone.
To the best of our knowledge, there is no study on
the aspect of reducing the inter-MBS zone handover
delay. To reach a comprehensive understanding on
this issue, this paper proposes an Enhanced
Handover Mechanism (EHM), which considers the
reduction of both the required time to scan neighbor
BSs and the number of the MBS session updates by
firstly selecting a neighbor BS guaranteeing a
satisfactory level of RSS value in the current MBS
zone as the target BS as many as possible. The
scanning process to check the RSSs of the MS from
neighbor BSs is continued until finding the BS
included in Class H+ for the reduction of scanning
time (this process adopts the First Satisfaction First
Reservation (FSFR) concept (16)). Simulation
results highlight the fact that the proposed
mechanism can induce the reduction of the handover
delay time regarding both multicast and unicast
connections except the worst case that a full
scanning process and an MBS session update occur
while maintaining a satisfactory level of RSS of an
MS.
2 OVERVIEW OF IEEE 802.16E
HANDOVER PROCESS
2.1 Intra-MBS Zone Handover Process
Fundamentally, the serving BS periodically sends a
neighbor advertisement (MOB-NBR-ADV) message
to its associated MSs at a periodic interval (MOB-
NBR-ADV interval (1)), where the MSs extract
information about neighbor BSs such as the BS ID,
radiation power, frequency assignment, and whether
the BS is co-located with the serving BS
transmitting this message, the scheduling service
supported (e.g., UGS, rtPS, non-real time-Polling
Service (nrtPS), and Best Effort (BE)), mobility and
handover support, their UCD and DCD information.
The MOB-NBR-ADV message information, which
are structured and saved by each MS in a list,
facilitates faster handover process from the serving
BS to the target BS among these BSs.
Once an MS recognizes neighbor BSs and the
current RSS from the serving BS is smaller than the
handover threshold, the MS can start a scanning and
association procedure in order to select a final target
BS. The scanning procedure is conducted through
exchanging scanning request (MOB-SCAN-REQ)
and scanning response (MOB-SCAN-RSP)
messages with the serving BS. Then, the serving BS
will schedule scanning intervals or sleep intervals to
the MS for scanning without terminating the current
connection of the MS to the serving BS.
Specifically, during the scanning process, all DL and
UL transmissions are temporarily paused and the
MS can optionally perform an association with
neighbor BSs by performing an initial ranging
handshaking. The aim of the association procedure
is to enable the MS to acquire and record ranging
parameters and service availability information for
the purpose of selecting a target BS. If the MS
decides to skip the association procedure, it must
perform an initial ranging procedure with the target
BS.
Based on the information from the scanning
process, an initiation of handover process for a target
BS is decided according to some criterions (not
defined in the standard and left open to different
implementations). Therefore, the handover process
can be initiated by any of the MS, the serving BS or
its associated network with either an MS handover
request (MOB-MSHO-REQ) message or a BS
handover request (MOB-BSHO-REQ) message,
including the information of a target BS.
ENHANCED HANDOVER MECHANISM FOR MULTICAST AND BROADCAST SERVICES IN IEEE 802.16E
SYSTEMS
13


Figure 1: Multicast Broadcast Service (MBS) zone handover process for session update.
As the corresponding acknowledgement message of
a handover request, a handover response message
(e.g., MOB-MSHO-RSP or MOB-BSHO-RSP) is
transferred to the MS by the serving BS and then a
handover process is triggered through the MS's
sending a handover indication (MOB-HO-IND)
message to the serving BS. To smooth the progress
of the handover process, the MS synchronizes to
downlink transmissions of the target BS and obtain
DL and UL transmissions parameters. Through the
synchronized link, the MS and the target BS conduct
handover ranging. If whole handover process is
conducted successfully, MS context is terminated;
otherwise, the MS cancels handover at any time
prior to expiration of Resource-Retain-Time interval
after transmitting an MOB-HO-IND message.
Therefore, L2 handover delay (D
HO
L2
) consists of
scanning neighbor BSs (T
SC
), synchronizing the MS
to the target BS (T
SYN
), contention resolution (T
CR
),
ranging (T
RNG
), authentication and key exchange
(T
AUTH
L2
), and registration with the new serving BS
(T
REG
), as expressed by the following equation:
D
L2
HO
=nT
SC
+T
SYN
+T
CR
+T
RNG
+T
L2
AUTH
+T
REG
, (1)
where n is the number of neighbor BSs scanned.
Therefore, the key issue of enhancing the L2
handover performance depends on the reduction of
these factors effectively.
2.2 Inter-MBS Zone Handover Process
Figure 2: WiMAX networks for mobile operators.
Basically, IEEE 802.16e systems consist of three
logical entities (e.g., Fig. 2): (i) MS, (ii) Access
Service Network (ASN), and (iii) Connectivity
Service Network (CSN). The serving BS performs
radio-related functions in ASN, and the CSN
performs IP connectivity services, administrative
functions (e.g., Authentication, Authorization, and
Accounting (AAA)), and admission control for
WiMAX operators. To introduce an MBS in a IEEE
802.16e system, MBS Controller (MBSC) is
included in CSN. An MBS session refers to a single
multicast connection established between the MBSC
and the MSs. The MBSC performs service
provisioning and delivery functions for MBS and
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serves as an entry point for multimedia contents
providers; i.e., when a new connection for multicast
streaming services supporting multimedia contents is
ready, the MBSC initiates the corresponding MBS
connection by performing resource reservation for
forwarding the multicast stream over the WiMAX
networks. When handovers of multicast connections
occur with the MBS session update, they consume
additional handover delay times (11). That is to say,
the handover delay of an MBS connection can suffer
the intra-MBS zone handover delay or inter-MBS
zone handover delay. In case of the intra-MBS zone
handover where an MS with an MBS connection
switches from the serving BS to the target BS in the
MBS zone, only the IEEE 802.16e MAC layer
handover delay is included in the delay (Step. 1
IEEE 802.16e MAC layer handover in Fig. 1). On
the other hand, when it’s imperative to perform
inter-MBS zone handover, where an MS moves out
of the current MBS zone, Steps 2 to 6 in Fig. 1 are
included in the delay. Therefore, the MBS session
update delay (D
MBS
) is given by:
D
MBS
= (T
TB-MBSC
+2T
MBSC-AAA
+P
MBS
AUTH
)
+ (2T
TB-MBSC
+3T
MS-TB
+P
CONN
)…………….. (2)
+ (T
TB-MBSC
+2T
MS-TB
+T
KEY
)
+ (2T
TB-MBSC
+P
MUL
),
where T
TB-MBCC
is the message transmission time
between the target BS and MBSC, T
MBSC-AAA
is the
message transmission time between MBSC and
AAA, P
AUTH
MBS
is the processing time of MBSC
authorization, P
CONN
is the processing time to
establish a multicast connection at an MS, T
MS-TB
is
the message transmission time between an MS and
the target BS, T
KEY
is the processing time to set up a
security key at an MS, and P
MUL
is the processing
time to perform a multicast distribution update at
MBSC. Therefore, in order to enhance the delay
performance of real-time multicast streaming
services, there need to be considerations on methods
for reducing MBS session updates.
3 ENHANCED HANDOVER
MECHANISM (EHM)
To begin with, in order to transmit the information
about MCIDs of neighbor BSs, MOB-NBR-ADV
messages are observed and modified, as follows.
Basically, BSs supporting mobile functionality are
capable of transmitting a MOB-NBR-ADV message
at a periodic interval to identify the network and
define the characteristics of neighbor BS to potential
MS seeking initial network entry or handover. The
message contains the information of neighbor BSs
regarding BSID (24 bits), frequency (8 bits),
scheduling service supported (8 bits), and so on (1).
Specifically, MCIDs supported by neighbor BSs can
be transmitted to MSs by changing scheduling
service supported field (8 bits) in the message to
MCID field (1). For the downlink multicast service,
the same MCID is assigned to all corresponding
MSs on the same channel that participates in this
connection. The MCID field is used for transmitting
the information from 9 to 16 bits of MCID field (8
bits) to MSs. Even though MCID is 16 bits, MSs can
get MCID information because MCID is 0xFEA0-
0XFEFE (1) or 0xFEA0-0XFEFD (17). This implies
that MCIDs can be recognized with the part of 9 to
16 bits.
Based upon the MCID obtained from a MOB-
NBR-ADV message, The MS is capable of deciding
the order for scanning neighbor BSs. As shown in
Fig. 1 and described in Section 2, an initiation of
intra-MBS zone handover delay is smaller than that
of inter-MBS zone handover delay under the same
condition. Therefore, the proposed mechanism, so
called Enhanced Handover Mechanism (EHM),
defines a neighbor BS having the same CIDs as the
MCID supported by the MS as Class + (high class);
otherwise, Class -. If RSS of the MS from the
serving BS is smaller than the handover threshold
value (i.e., handover occurs), the MS firstly scans
the neighbor BSs in Class+ until finding a neighbor
BS having a higher RSS than the (threshold +
hysteresis) value (V
bnd
) for guaranteeing a sufficient
level of RSS of the MS and reducing the ping-pong
effects. This BS is included in Class H+ and then it
will be selected as the target BS. If there is a
neighbor BS in Class H+, the scanning process will
be stopped for adopting the merit of the FSFR
mechanism (16). Otherwise (that is to say, there is
no neighbor BS in Class H+), others in Class + are
included in Class L+. Next, neighbor BSs in Class -
are classified into Class H- or L- depending on their
RSS value. As a result, Table I can be achieved.
Then, with the order of Classes H-, L+, and L-, the
EHM selects a BS having the greatest RSS in each
class as the target BS. By doing so, the number of
multicast session updates for real-time multicast
streaming services can be kept as low as possible
compared to the conventional handover mechanism
with the aforementioned classification of the EHM.
In addition, EHM considers the compatibility to
ENHANCED HANDOVER MECHANISM FOR MULTICAST AND BROADCAST SERVICES IN IEEE 802.16E
SYSTEMS
15

IEEE 802.16e systems, specifically regarding the
MOB-NBR-ADV message format.

4 PERFORMANCE EVALUATION
This section presents performance evaluation of the
EHM compared to the conventional mechanism
(Threshold-based Handover Mechanism (THM)),
which selects a neighbor BS having the highest
possible level of RSS when the RSS value received
from the current serving BS become smaller than the
handover threshold value. Simulation results are
achieved under 1,000,000 seconds and obtained with
the following assumptions: (i) there are 20 MSs,
whose movements follow Random Walk mobility
model (18), and 7BSs, which organize an octagon
shape of a network, (ii) there are two types of
connections in an MS (i.e., multicast and unicast
connections without concerning about application
and QoS type), (iii) all the necessary information
involved in the handover process of the MS can be
exchanged among the different BSs through the
network backbone, and (iv) internal processing time
of the MS is considered as negligible (i.e., the
amount of time spent in constructing and traversing
the short list of BSs is not included in the simulation
results). Besides, the system specification
parameters; the required times for IEEE 802.16e
MAC layer handover, and the required times for
MBS session update are defined as in Table II, III,
and IV, respectively.
Table 2: System Parameters.
Symbol: Description Value Unit
P
Tx
: Transmitter output power 64
W
PL
FS
: Path loss
35.4+35log
10
D
L
dB
F
S
: Shadow fading None
-
G
Tx,A
: Tx antenna gain 10
dBi
G
Rx, A
: Rx antenna gain 0
dBi
N
Tx
: Number of Tx antennas 1
-
N
Rx
: Number of Rx antennas 1
-
S
Rx
: Receiver sensitivity -90
dBm
BW: Channel bandwidth 10
7
Hz
D
S
: OFDM symbol duration 102.9
μs
T
S
: Useful symbol time (87.5%) 91.4 μs
T
G
: Guard time (12.5%) 11.4
μs
DL:UL ratio 28:9
-
P
N0
: Noise power -104
dBm
A
tot
: Total evaluated area
100km*100k
m
-
U: Basic unit of area 100m*100m
-
A
MIN
: Minimum position coordinates in
A
tot
0
-
A
MAX
: Maximum position coordinates
in A
tot
1000
-
SINR
H
: Requested SINR for handover 10
dB
SINR
C
: Required SINR for coverage 8
dB
Table 3: Required times for MBS session update (11).
Symbol: Description
Value Unit
T
a-b
: Message transmission between a and b 5 ms
P
AUTH
MBS
: MBS authorization at AAA 1 ms
P
EST
: Establishing a multicast connection at
an MS
1 ms
T
KEY
: Setting up a security key at an MS 1 ms
P
MUL
: Multicast distribution update at MBSC 1 ms
Table 4: Required times for L2 handover (17) (18).
Symbol: Description Value Unit
T
SC
: Scanning a neighbor BS 5 ms
T
SYN
: Synchronizing the MS to the target BS 10 ms
T
CONT
RES
: Contention resolution 0 to 20 ms
T
RNG
: Ranging 25 ms
T
AUTH
L2
: Authentication and key exchange 25 ms
T
REG
: Registration 10 ms
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
60
80
100
120
140
160
180
200
Handover delay of multicast connection (ms)
Density of BSs in the MBS zone of the network
Conventional
EHM; V
bnd
= 16
EHM; V
bnd
= 13
EHM; V
bnd
= 10
Figure 3: The average handover delay of multicast
connection.
0.10.20.30.40.50.60.70.80.9
0.0
0.2
0.4
0.6
0.8
1.0
Probability of MBS session update
Density of BSs in the MBS zone of the network
Conventional
EHM; V
bnd
= 16
EHM; V
bnd
= 13
EHM; V
bnd
= 10
Figure 4: The probability of MBS session update.
Fig. 3 presents performance of the EHM compared
to the conventional mechanism in terms of handover
delay of multicast connection. Regardless of the
density of BSs in the MBS zone of the network, the
EHM effectively enhance the average handover
delay of multicast connection. This achieved
performance gain is mainly achieved by the reduced
probability of MBS session update, as shown in Fig.
4, due to the defined classes, and partially done by
the reducing the time required to scan neighbor BSs
owing to the FSFR. As another positive effect of the
EHM, Fig. 5 presents the average handover delay of
unicast connection. There is a perceptible
performance enhancement due to the FSFR concept,
although it is not significant compared with the
performance enhancement of multicast connection.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
60
80
100
120
140
160
180
200
Handover delay of unicast connection (ms)
Density of BSs in the MBS zone of the network
Conventional
EHM; V
bnd
= 16
EHM; V
bnd
= 13
EHM; V
bnd
= 10
Figure 5: The average handover delay of unicast
connection.
On the other hand, as a demerit of the EHM owing
to the classification, shown in Fig. 6, a lower level of
RSS values can be induced. The EHM, however, can
keep a satisfactory level of RSS value for supporting
applications. As presented in Figs. 3, 4, 5, and 6, the
level of RSS value is controllable by setting a higher
or lower values of V
bnd
(e.g., a higher value of V
bnd
cause a lower multicast connection handover delay
and a higher RSS value, and vice versa). Thus,
setting the value of V
bnd
can depend on the
requirements of the systems.
5 CONCLUSIONS
This paper proposed an EHM for real-time MBSs in
IEEE 802.16e systems. Differing from the
conventional mechanism, an MS under the EHM
receives Multicast Connection IDentifier (MCID)
information during neighbor advertisement from
neighbor BSs, and thus it can extract the information
of neighbor BSs regarding MBS. Then, the MS
firstly selects the neighbor BS included in the same
MBS zone and having a satisfactory level of RSS
value for guaranteeing a sufficient link quality. The
simulation results substantiate that the EHM can
achieve better performance gain on the aspect of the
multicast and unicast connections due to the
proposed classification and the First Satisfactory
First Reservation (FSFR). Consequently, MBSs, one
of the most important services in IEEE 802.16e
systems, can be well guaranteed with enhanced
mobility management.
ENHANCED HANDOVER MECHANISM FOR MULTICAST AND BROADCAST SERVICES IN IEEE 802.16E
SYSTEMS
17

0.10.20.30.40.50.60.70.80.9
10
12
14
16
18
20
RSS value (dB)
Density of BSs in the MBS zone of the network
Conventional
EHM; V
bnd
= 16
EHM; V
bnd
= 13
EHM; V
bnd
= 10
Figure 6: Performance of the EHM regarding the average
RSS of MSs.
ACKNOWLEDGEMENTS
This work was supported in part by Daegu
Gyeongbuk Institute of Science and Technology
(DGIST) Research Program of the Ministry of
Education, Science and Technology (MEST), the
Development Man-made Disaster Prevention
Technology grant funded by the Korea Government
(NEMA; National Emergency Management Agency)
(No. Nema-09-MD-06), the MKE (the Ministry of
Knowledge Economy) under the ITRC (Information
Technology Research Center) support program
supervised by the IITA (Institute for Information
Technology Advancement) (IITA-2009-(C1090-
0902-0036)), and the IT R&D program of MKE /
KEIT (2009-F-057-01, Large-scale wireless-PON
convergence technology utilizing network coding).
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