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 in the 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 then combined. 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 Terms—Heterogeneous 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 user assignment 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 RAT selection 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
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 its performance 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 WIRELESS NETWORKS
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 the heterogeneous 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.
Internet Internet
Internet RAT-2
RAT-1
RAT-3
JRRM
Multimode terminal
Splitting of a downlink session into two packet streams
Media server
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 of RATs 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 to accommodate 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, Bj and T0j are 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
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 of radio 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, bi. 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. The different 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 λniand λhirespectively.
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/ȝi.
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:
Ω=(mi,j,k,ni,j,k:i=1,,I, j=1,,J,k=1,,K) (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,k
denotes the number of ongoing handoff class-i calls (or sub streams of handoff class-i calls) allocated k bbu in RAT j. Let S denote the state space of all admissible states of the group of co-located cells as it evolves over time. An admissible state s 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≤k≤K)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:¦ ¦ ¦ ¦
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= = = =
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Joint call admission decisions are taken in the arrival epoch.
Every time a new or handoff class-i call 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 the arrival of a handoff class-i call 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 state s does not evolve.
2) Admit the class-i call into only one RATs (no session splitting) in which case the state s evolves.
3) Admit the class-i call into a set of RATs (with session splitting) in which case the state s evolves.
Thus, the call admission action space A can be expressed as follows:
} ,..., , , { ,
: ) , , , , , ( {
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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 aih) ∈ 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 aih) ∈ A1 means 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 aih) ∈ A2 means 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 aih
) ∈ Aj means 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:
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n (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. ai
n (or ai h) = (1&2&3) means split the call session into three substreams and accept the new (or handoff) class-i call 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
k j
newi,,
ρ and ρhani,j,k denote the load generated by new class-i calls and handoff class-i calls, respectively, in RAT-j. Let
n
μi
/
1 and 1/μihdenote the channel holding time of new class-i call and handoff class-i call respectively, and let λni,j,k and
h k j i, ,
λ 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,
i j k
n i n
k j i
newij , , , ,
, = ∀
μ
ρ λ , (4)
i j k
h i h
k j i
han ij , , , ,
, = ∀
μ
ρ λ (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 (s∈∈∈S∈ ).
From the detailed balance equation, P(s) is obtained as:
S n s
m s G
P
I
i ijk
n han k
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m J new
j K
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, ρ
ρ (6)
where G is a normalization constant given by:
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ρ (7)
A. New Call Blocking Probability
A new class-i call 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 Sbi ⊂S denote the set of states in which a new class-i call is blocked in the group of collocated cells. It follows that the new call blocking probability (NCBP),
bi
P , for a class-i call in the group of co-located cells is given by:
¦
∈
=
bi i
S s
b P s
P ( ) (8) B. Handoff Call Dropping Probability
A handoff class-i call 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 S S
di ⊂ denote the set of states in which a handoff class-i call is dropped in the group of co-located cells. Thus the handoff class-i call dropping probability (HCDP) for a class-i call,
di
P
, in the group of co- located cells is given by:
¦
∈
=
di i
S s
d P s
P ( ) (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 call blocking/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 { , ih∈
n i a
a .The system parameters used are
as follows: T0,1 = 0.5B1, T0,2 = 0.5B2, T0,3 = 0.5B3, b1 = 6 bbu, μ1=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
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 to admit 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
Call blocking/ dropping probability Pb w ithout session splitting
Pb w ith session splitting Pd w ithout session splitting Pd 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 the proposed 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 with that 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 Call blocking/ dropping probability Pb without session splitting
Pb with session splitting Pd without session splitting Pd with session splitting