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Abstract A heterogeneous wireless network supporting

multihoming gives multi-mode terminals the flexibility to be

simultaneously connected to more than one radio access

technologies (RATs). Existing joint call admission control

(JCAC) algorithms designed for heterogeneous wireless networks

block or drop an incoming call when none of the available

individual RATs in the heterogeneous network has enough

bandwidth to support the incoming call. Consequently, high

bandwidth-demanding calls can easily be blocked or dropped inthe network, especially during the peak hours. In order to reduce

this problem of call blocking/dropping, this paper proposes a

JCAC algorithm that selects multiple RATs for an incoming call

when none of the available individual RATs has enough bbu to

accommodate the incoming call. Selection of multiple RATs for

an incoming call entails that the packet stream of the incoming

call will be split among the selected RATs. The aim of the

proposed JCAC algorithm is to admit an incoming call (that

cannot be admitted into any of the available single RATs because

of high load in the RATs) into two or more RATs. The residual

bandwidths in the selected RATs are combined to support the

incoming call, and the packet stream of the call is split among the

selected RATs, thereby reducing call blocking/dropping

probability. At the receiver, the split packet streams are thencombined. An analytical model is developed for the proposed

JCAC algorithm, and its performance is evaluated in terms of call

blocking/dropping probability. Simulation results show that the

JCAC algorithm reduces call blocking/dropping probability in

heterogeneous wireless networks supporting multihoming.

Index TermsHeterogeneous wireless network, multihoming,

Joint radio resource management, joint call admission control,

radio access technology, Markov chain, mobile terminal.

I. INTRODUCTION

It is envisaged that next generation wireless networks

(NGWN) will be heterogeneous, combining existing and new

radio access technologies to provide high bandwidth access

anytime, anywhere for multimedia services [1-3].

The motivation for heterogeneous wireless networks arises

from the fact that no single radio access technology (RAT) can

provide ubiquitous coverage and continuous high QoS levels

across multiple smart spaces, e.g. home, office, public smart

Manuscript received July 11, 2010. This work is supported in part by

Telkom, Nokia Siemens Networks, TeleSciences and National Research

Foundation, South Africa, under the Broadband Center of Excellence

program.

spaces, etc [4]. This motivation has lead to the deployment of

multiple RATs in the same geographical areas. Consequently,

the coexistence of different RATs has necessitated joint radio

resource management (JRRM) for enhanced QoS provisioning

and efficient radio resource utilization.

A heterogeneous wireless network supporting multi-homing

gives multimode terminals the flexibility to be simultaneously

connected to more than one RAT. Such simultaneous

connections entails that that packet stream of a session from a

multimode terminal will be split among multiple RATs in the

heterogeneous wireless network.

A number of joint call admission control (JCAC) algorithms

have been proposed for heterogeneous wireless networks, and

a review of these JCAC algorithms appear in [5]. However,

these JCAC algorithms block or drop an incoming call when

none of the available individual RATs in the heterogeneous

network has enough bandwidth to support the incoming call.

Consequently, high bandwidth-demanding calls can easily be

blocked or dropped in the network, especially during the peak

hours.

In [6], Furuskar et al proposed service-based userassignment algorithms for heterogeneous wireless networks.

The performance of the proposed algorithms was evaluated for

a two-RAT heterogeneous wireless network comprising GSM

and WCDMA. The proposed algorithm selects just one RAT

for each call. Session splitting and multiple-RAT selection

were not considered in the study.

In [7], Falowo et al proposed a Joint Call Admission

Control Algorithm for Fair Radio Resource Allocation in

Heterogeneous Wireless Networks Supporting Heterogeneous

Mobile Terminals. However, session splitting and multiple-

RAT selection were not considered in the study.

Xavier et al [8] presented a Markovian approach to RATselection in heterogeneous wireless networks. They developed

an analytical model for RAT selection algorithms in a

heterogeneous wireless network comprising GSM/EGDE and

UMTS. The proposed algorithm selects just one RAT for each

call. Session splitting and multiple RAT selection were not

considered in the study.

This paper proposes a JCAC scheme that reduces call

blocking/dropping probability by selecting multiple RATs for

an incoming call when none of the available single RATs has

enough bandwidth to accommodate the incoming call.

Joint Call Admission Control Algorithm for Reducing

Call Blocking/dropping Probability in Heterogeneous

Wireless Networks Supporting Multihoming

Olabisi E. Falowo

Department of Electrical Engineering, University of Cape Town, South Africa

IEEE International Workshop on Management of Emerging Networks and Services

978-1-4244-8865-0/10/$26.00 2010 IEEE 611

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In wireless networks, dropping an ongoing call is more

annoying to users than blocking a new call. Therefore, handoff

calls are usually prioritized over new calls. The proposed

JCAC algorithm prioritizes handoff calls over new calls by

using different rejection thresholds for new and handoff calls.

The contributions of this paper are twofold. First, a JCAC

algorithm for reducing call blocking/ dropping probability in

heterogeneous wireless networks is proposed. Second, an

analytical model is developed for the proposed scheme, and itsperformance is evaluated in terms of new call blocking

probability and handoff call dropping probability.

To the best our knowledge, this is the first work using

multiple RAT selection and session splitting for reducing call

blocking/dropping probability in heterogeneous wireless

networks.

The rest of this paper is organized as follows. In section II,

the proposed JCAC algorithm is described. In section III, the

system model is presented. A Markov model is developed for

the JCAC scheme in section IV. In section V, the performance

of the JCAC scheme is investigated through simulations.

II. PROPOSED JOINT CALL ADMISSION CONTROL FOR

HETEROGENEOUS WIRELESSNETWORKS

The proposed JCAC scheme uses session splitting and

multiple RAT selection to reduce call blocking/ dropping

probability in heterogeneous wireless networks. For example,

when a new call arrives in a heterogeneous wireless network

and no single RAT in the heterogeneous network has enough

basic bandwidth units (bbu) to accommodate the incoming

call, the existing JCAC schemes will block the call. However,

with session splitting between two or more RATs, it may be

possible to admit the incoming call by combining the residual

basic bandwidth units in two or more RATs. Consequently,

the overall call blocking/ dropping probability in theheterogeneous wireless network will be reduced.

Fig. 1 illustrates multiple RAT selection and session splitting

between the selected RATs. As shown in Fig 1, none of the

individual RATs in the heterogeneous wireless network has

enough bbu to admit the incoming call because the RATs are

almost fully loaded. However, a combination of the residual

bbu in RAT 1 and RAT3 will be sufficient to accommodate the

incoming call. Therefore, the proposed JCAC algorithm

selects RAT 1 and RAT 3 for the call. The session is split

between the two selected RATs.

InternetInternetInternetRAT-2

RAT-1

RAT-3

JRRM

Multimodeterminal

Splitting of a downlink

session into twopacket streams

Mediaserver

RAT-J

Fig. 1. Splitting of a session between two RATs in a J-RAT heterogeneous

wireless network.

When a new call (session) arrives, the proposed JCAC

algorithm, which resides in the JRRM module, decides

whether the call can be admitted into the network or not, as

well as whether the call should be split among multiple RATs

or not (note that not all classes of calls can be split), and what

RAT(s) will most suitable to admit the incoming call. The

JCAC scheme makes the above decisions based on the class of

calls, bandwidth requirement of the call, and current load in

each of the available RATs.

The joint call admission scheme will then selects for the

incoming call, a set of n RATs (0 n J) from the available

RATs in the heterogeneous network. J is the total number ofRATs in the heterogeneous wireless networks and n is the

number of RATs selected. n = 0 implies that the incoming call

cannot be admitted into the heterogeneous network. Therefore,

the call is blocked or dropped. n =1 implies that the incoming

call can be admitted into just one of the available RATs.

Hence there is no need for session splitting. n > 1 implies that

the incoming call will admitted into more than one RAT.

Therefore the session will be split among n RATs.

The proposed JCAC algorithm tries to admit an incoming

call into a single RAT (i.e. without session splitting) if any of

the available RATs that can support the call has enough bbu to

accommodate the incoming new (or handoff) class-i call.

If none of the available single RATs has enough bbu toaccommodate the incoming call, two RATs that have the

highest residual bandwidth that can support the service class of

call will be selected for the call (with session splitting). If no

combination of two RATs has enough bbu to accommodate the

call, three RATs that can support the service class of the call

will be selected for the call, and so on. If no combination of

RATs has enough bbu to support the incoming call, the call

will be rejected.

In order to maintain lower handoff dropping probability

over new call blocking probability, different threshold, Bjand

T0jare used for rejecting new and handoff calls, respectively,

in RAT-j.

III. SYSTEM MODEL AND ASSUMPTIONS

A heterogeneous wireless network that supports

multihoming and consists of J number of RATs with co-

located cells is considered in this paper. Cellular networks

such as GSM, GPRS, UMTS, EV-DO, LTE, etc, can have the

same and fully overlapped coverage, which is technically

feasible, and may also save installation cost [9, 10]. Fig. 2 and

Fig. 3 illustrate a two-RAT heterogeneous cellular network.

Fig. 2, adapted from [11], is a typical heterogeneous cellular

network comprising 3G-WCDMA and LTE OFDMA. Fig 3

shows the co-located cells of the two-RAT heterogeneous

wireless networks.

Fig. 2. Two-RAT heterogeneous cellular network with co-located cells.

LTE

OFDMA

3G

WCDMA

Multi-Mode

Terminal

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Fig. 3. Co-located cells of a two-RAT heterogeneous cellular network.

Radio resources are jointly managed in the heterogeneous

network and each cell in RAT j (j =1,,J) has a total of Bj

basic bandwidth units (bbu). The physical meaning of a unit ofradio resources (such as time slots, code sequence, etc) is

dependent on the specific technological implementation of the

radio interface. However, no matter which multiple access

technology (FDMA, TDMA, CDMA, or OFDMA) is used,

system capacity can be represented in terms of effective or

equivalent bandwidth. Therefore, in this paper, bandwidth

required by a call is denoted by bbu, which is similar to the

approach used for wireless networks in [12].

The approach used in this paper is to decompose a

heterogeneous cellular network into groups of co-located cells.

As shown in Fig. 3, cell 1a and cell 2a form a group of co-located

cells. Similarly, cell 1b and cell 2b form another group of co-located

cells, and so on.

A newly arriving call will be admitted into one or multiple

cells in the group of co-located cells where the call is located.

For example, in the two-RAT heterogeneous wireless network

shown in Fig. 3, an incoming call from a multimode terminal

(MT) can be admitted into either of the two RATs (cell 1b or

cell 2b) in the group of collocated cells. Alternative, the call

can be admitted into both RATs (cell 1b and cell 2b), with

session splitting. Otherwise the call is blocked.

The correlation between the groups of co-located cells

results from handoff connections between the cells of

corresponding groups. Under this formulation, each group of co-

located cells can be modeled and analyzed individually. Therefore,

this paper focuses on a single group of co-located cells.

The heterogeneous network supports I classes of calls. Each

class-i call requires a discrete bandwidth value, b i. Each class

is characterized by bandwidth requirements, arrival

distribution, and channel holding time. Some classes of calls

(e.g. video streaming) may support session splitting whereas

some other classes of call (e.g. voice) may not support or

require session splitting. Generally, high-bandwidth

demanding calls may require session splitting to reduce call

blocking/dropping probability in heterogeneous wireless

network. For example, a layered-coded video consists of base

layer and enhance layers. Thus, the different layers of a layer-

coded video session can be split among multiple RATs. Thedifferent layers are then combined at the receiver.

Following the general assumption in cellular networks, new

and handoff class-i calls arrive in the group of co-located cells

according to Poisson process with rate ni and

h

i respectively.Note that the arrival rates of a split Poisson process are also

Poisson [13].

The channel holding time for class-i calls is exponentially

distributed with mean 1/i.

IV. MARKOV MODEL

The JCAC policy described in section III can be modeled as

a multi-dimensional Markov chain. The state space of the

group of co-located cells can be represented by a (2*I*J*K)-

dimensional vector given as:

),,1,,,1,,,1:,( ,,,, KkJjIinm kjikji ==== (1)

The non-negative integer mi,j,k denotes the number of

ongoing new class-i calls (or sub-streams of class-i calls)allocated k bbu in RAT j, and the non-negative integer ni,j,kdenotes the number of ongoing handoff class-i calls (or sub

streams of handoff class-i calls) allocated k bbu in RAT j. Let

Sdenote the state space of all admissible states of the group of

co-located cells as it evolves over time. An admissible states

is a combination of the numbers of users in each class that can

be supported simultaneously in the group of co-located cells

while maintaining adequate QoS and meeting resource

constraints. k )1( Kk is an integer and it is the number of

bbu allocated to call or substream of a call in a particular

RAT. K is the maximum number of bbu that can be allocated

to any class-i call (i.e. without session splitting).

The state S of all admissible states in the group of co-located cells is given as:

= = ==

= =

+

====

I

i

I

i

K

k

jkji

K

k

kji

I

i

K

k

jkji

kjikji

jBknkm

jTkm

KkJjIinmS

1 1 1

,,

1

,,

1 1

,01,,

,,,,

}..

.

:),1,,,1,,,1:,({=

(2)

Joint call admission decisions are taken in the arrival epoch.

Every time a new or handoff class-icall arrives in the group of

co-located cells, the JCAC algorithm decides whether or not to

admit the call, and in which set of RAT(s) to admit it. Note

that a call admission decision is made only at the arrival of a

call, and no call admission decision is made in the group of co-

located cells when a call departs. When the system is in state s,

an accept/reject decision must be made for each type of

possible arrival, i.e., an arrival of a new class-i call, or thearrival of a handoff class-icall in the group of co-located cells. The

following are the possible JCAC decisions in the arrival epoch.

1) Reject the class-i call (new or handoff) in the group of

collocated cells, in which case the statesdoes not evolve.

2) Admit the class-i call into only one RATs (no session

splitting) in which case the statesevolves.

3) Admit the class-i call into a set of RATs (with session

splitting) in which case the statesevolves.Thus, the call admission action space A can be expressed as

follows:

},...,,,{,

:),,,,,({

210

11

Jhi

ni

hI

hnI

n

AAAAaa

aaaaaA

== (3)

1)()},&)1(&...&2&1{(

)()},&)1((),...,&2(),...,3&2(),&1(),...,2&1{(

1)(},,...,2,1{

1)(,},0{

222

111

0000

===

==

===

===

JJ

JJ

J

J

J

CAnJJA

CAnJJJJA

CAnJA

CAnAinelementsofnumberA

RAT 1RAT 1

RAT 2RAT 2

A group ofA group of

coco--locatedlocated

cellscells

1a1a 1b1b

1c1c

2a2a2b2b

2c2c

MTMT

RAT 1RAT 1

RAT 2RAT 2

A group ofA group of

coco--locatedlocated

cellscells

1a1a 1b1b

1c1c

2a2a2b2b

2c2c

MTMT

613

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where ain denotes the action taken on arrival of a new class-i

call within the group of co-located cells, and aih denotes the

action taken on arrival of a handoff class-i call from an

adjacent group of co-located cells. ain (or ai

h) A0 means no

RAT is selected for an incoming class-i new (handoff) call,

therefore, the new (or handoff) class-i call is rejected in the

heterogeneous wireless network. ain(or ai

h) A1means one

RAT is selected for the call, therefore, there is no session

splitting and the new (or handoff) class-i call is accepted into

the selected single RAT. ain(or ai

h) A2means two RATs are

selected for the call, therefore, there is session splitting and the

new (or handoff) class-i call is split into two substreams and

admitted into the selected two RATs. ain(or ai

h) Ajmeans j

RATs are selected for the incoming call. Thus, there is session

splitting and the new (or handoff) class-i call is split into j

substreams and admitted into the selected j RATs.

For example, in the J-RAT heterogeneous wireless network

shown in Fig. 1, if J=3. It follows that:

)}3&2&1{()},3&2(),3&1(),2&1{(},3,2,1{},0{ 3210 ==== AAAA

)}3&2&1(),3&2(),3&1(),2&1(,3,2,1,0{, h

i

n

i

aa

where ain (or ai

h)=0 means reject the new (or handoff) class-i

call. ain(or ai

h) = 1 means accept the new (or handoff) class-i

call into RAT-1. ain (or ai

h) = (1&2) means split the call

session into two substreams and accept the new (or handoff)

class-i call subsreams into RAT-1 and RAT-2. ain (or ai

h) =

(1&2&3) means split the call session into three substreams and

accept the new (or handoff) class-icall subsreams into RAT-1,

RAT-2, and RAT-3.

Based on its Markovian property, the JCAC algorithm can

be model as a (2*I*J*K)-dimensional Markov chain. Let

kjinew

,, and

kjihan

,, denote the load generated by new class-i

calls and handoff class-i calls, respectively, in RAT-j. Letni/1 and

hi/1 denote the channel holding time of new class-i

call and handoff class-i call respectively, and let n kji ,, and

hkji ,, denote the arrival rates of new class-i call (or sub-stream

of new class-i call) and handoff class-i call (or sub-stream of

handoff class-i call) allocated k bbu in RAT j , respectively,

then,

kjini

nkji

newji

,,,,

,=

, (4)

kjih

i

hkji

hanji

,,,,

,=

(5)

From the steady state solution of the Markov model,

performance measures of interest can be determined by

summing up appropriate state probabilities. Let P(s)denotes

the steady state probability that system is in state s (sS).

From the detailed balance equation,P(s)is obtained as:

SsnmG

sP

I

i kji

n

han

kji

m

newJ

j

K

k

kji

kji

kji

kji = = = =1 ,,,,1 1

!

)(

!

)(1)(

,,

,,

,,

,, (6)

where Gis a normalization constant given by:

= = =

=Ss

I

i kji

nhan

kji

mnew

J

j

K

k nm

G

kji

kji

kji

kji

1 ,,,,1 1!

)(

!

)( ,,,,

,,

,,

(7)

A. New Call Blocking Probability

A new class-icall is blocked in the group of co-located cells if

the selected RAT(s) do not have enough bbu to accommodate

the new call. Let SSbi denote the set of states in which a

new class-icall is blocked in the group of collocated cells. Itfollows that the new call blocking probability (NCBP),

ibP , for

a class-icall in the group of co-located cells is given by:

=

ib

i

Ss

b sPP )( (8)

B. Handoff Call Dropping Probability

A handoff class-icall is dropped in the group of co-located

cells if the selected RAT(s) do not have enough bbu to

accommodate the handoff call. Let SSid denote the set of

states in which a handoff class-icall is dropped in the group of

co-located cells. Thus the handoff class-i call droppingprobability (HCDP) for a class-icall,

idP , in the group of co-

located cells is given by:

=

id

i

Ss

d sPP )( (9)

V. SIMULATION RESULTS

In this section, the performance of the proposed JCAC

scheme is evaluated via simulation, using a three-RAT

heterogeneous cellular network. Only one class of calls namely

video streaming is considered in this paper because of high

computational overhead of evaluating the callblocking/dropping probability. In the example, an incoming

new or handoff call can be admitted into a single RAT or split

into two equal substreams and then admitted into any two of

the available three RATs that are least loaded (i.e.

))3&2(),3&1(),2&1(,3,2,1,0{, hini aa .The system parameters used are

as follows: T0,1 = 0.5B1, T0,2 = 0.5B2, T0,3 = 0.5B3, b1 = 6 bbu,

1=0.5, k{3, 6}. In this illustration, if a single RAT is

selected for a call, k=6, if two RATs are selected for a call

(with session splitting), k=3 in each of the two RATs.

The performance of the proposed JCAC scheme is

compared with the performance of a JCAC scheme that does

not allow multiple RAT selection/session splitting.Fig. 4 shows the effect of varying the new call arrival rate with

NCBP (Pb) and HCDP (Pd) for the two JCAC schemes when

B1 = 20, B2 = 20, B3 = 20. As showed in Fig. 4, for the two

JCAC scheme, NCBP (Pb) increases with increase in call

arrival rate. However, the Pb of the proposed JCAC scheme is

always less that the corresponding Pb of the JCAC scheme that

does not incorporate multiple RAT selection/ session splitting.

Similarly, the HCDP (Pd) for the two JCAC schemes

increases with call arrival rate. However, the Pd of the

proposed JCAC scheme is always less that the corresponding

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Pd of the JCAC scheme that does not incorporate multiple

RAT selection and session splitting.

Moreover it can been seen that Pd is always less than the

corresponding Pb because handoff calls are prioritized over

new calls by using different call rejection thresholds for new

and handoff calls as earlier mentioned in Section I .

The proposed JCAC scheme reduces Pb and Pd of incoming

calls by combining the residua bbu of two RATs to admit calls

when none of the available single RATs has enough bbu toadmit the call. In the scenario considered in this paper, when

two RATs are selected for a call the bbu required by the call is

shared equally among the selected RATs.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Call arrival rate

Callblocking/droppingprobability Pb w ithout session splitting

Pb w ith session splittingPd w ithout session splittingPd w ith session splitting

Fig. 4. Call blocking/ dropping probability against call arrival rate: B1 = 20,

B2 = 20, B3 = 20.

Fig. 5 shows the effect of varying the new call arrival rate with

NCBP (Pb) and HCDP (Pd) for the two JCAC schemes when

B1 = 10, B2 = 20, B3 = 30 As showed in Fig. 5, the Pb and Pd

for the two JCAC schemes follow a similar trend as that of Fig.

4. In Figure 5, it can be seen that the Pb and Pd for theproposed scheme are less than the corresponding Pb and Pd of

the JCAC scheme that does not support multiple RAT

selection and session splitting. Moreover it can be seen that

Pd is always less than the corresponding Pb.

Fig. 5. Call blocking/ dropping probability against call arrival rate: B1 = 10,

B2 = 20, B3 = 30.

VI. CONCLUSION

In this paper, a JCAC scheme that uses multiple RAT

selection and session splitting to reduce call blocking/

dropping probability in heterogeneous wireless networks

supporting multihoming has been proposed. An analytical

model has been developed for the proposed JCAC scheme

using two performance metrics namely new call blocking

probability and handoff call dropping probability. Performance

of the proposed JCAC scheme is evaluated and compared withthat of a JCAC scheme that does not support multiple RAT

selection and session splitting. Simulation results show that the

proposed JCAC scheme reduces call blocking/ dropping

probability in the heterogeneous wireless network.

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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Call arrival rate

Callblocking/droppingprobability

Pb without session s plittingPb with session s plittingPd without session s plittingPd with session s plitting

615