Erlang Capacity of Multi-Access Systems with Service-based Access ...

Erlang Capacity of Multi-Access Systems with Service-based Access Selection Insoo Koo, Anders Furuskar, Jens Zander, and Kiseon Kim Abstract— In this letter, we provide the lower and upper bounds of Erlang capacity of multi-access systems supporting several different radio access technologies in the multi-service scenario, by considering two extreme operation methods; separate and common operation. In a numerical example with GSM/EDGE-like and WCDMA-like sub-systems, it is shown that the common operation method can provide up to 60% Erlang capacity improvement over the separate operation method when using a near optimum so-called service-based user assignment scheme, with the combined effects of the assignment and the trunking gains. Even in the worst-case, the common operation method still can provide about 15% capacity improvement over the separate operation method, which mainly comes from the trunking gain. Index Terms : Multi-access system, Erlang capacity, operation method.

I. I NTRODUCTION Future mobile networks will consist of several distinct radio access technologies, such as WCDMA or GSM/EDGE, where each radio access technology is denoted as “sub-system.” Such future wireless networks demanding utilizing the cooperative use of a multitude of sub-systems are named multi-access systems. In the first phase of such multi-access systems, the radio resource management of sub-systems may be performed in a separate way to improve the performance of individual systems independently, mainly due to the fact that the terminals do not have multi-mode capabilities. Under such a separate operation method, an access attempt is only accepted by its designated sub-system if possible, and otherwise rejected. Intuitively, improvement of multiple-access systems is expected in a form of common resource management where the transceiver equipment of the mobile stations supports multimode operations such that any terminal can connect to any subsystem[1]. In order to estimate the benefit of multi-access systems, some studies are necessary especially in aspects of quantifying the associated Erlang capacity according to the operation methods. As an example of improving the performance of mulit-access system through the common resource management, for single-service scenario, the “trunking gain” of multiaccess system capacity enabled by the larger resource pool from common resource management has previously been evaluated through simulation in [2], and multi-service allocation is not considered. In multi-service scenarios, it is expected that the capacity of multi-access networks also depends on how users of different services are assigned on to sub-systems. The gain that can be obtained through the employed assignment scheme Manuscript received March 7, 2004. The associated editor coordinating the review of this letter and approving it for publication was Dr. Christos Douligeris. I. Koo and K. Kim are with Department of Information and Communication, Kwang-Ju Institute of Science and Technology, S. Korea. A. Furuskar and J. Zander are with Wireless KTH, Royal Institute of Technology, Sweden This work was supported in part, by IT Professorship program, IITA. Insoo Koo particularly was supported by grant No. R08-2003-000-10122-0 from the Basic Research Program of the KOSEF.

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can be named as “assignment gain”, and further the capacity gain achievable with different user assignment principles has been estimated in [3]. These studies however disregard trunking gains. In this letter, we combine both trunking and assignment gains, and explicitly quantify the Erlang capacity of the multiaccess system in mulit-service scenario. The lower and upper bounds of the Erlang capacity correspond, respectively, to two extreme cases; the separate and common operations. In the case of the common operation, we also consider two kinds of user assignment schemes; service-based user assignment [3] as the best case, and the rule opposite to the service-based assignment as the worst case reference. With the consideration of these extreme cases, the Erlang capacity of multi-access systems would be a useful guideline for operators of multi-access systems. II. E RLANG C APACITY A NALYSIS OF THE M ULTI - ACCESS S YSTEMS Let’s consider a multi-access system consisting of two subsystems supporting voice and data service, and analyze the Erlang capacity of the multi-access system according to two different operation methods; separate and common operations. A. Separate operation of the multi-access systems In the separate operation method of the multi-access system, an access attempt is accepted by its designated sub-system if possible, and otherwise rejected. In order to evaluate the performance of the separate operation in the aspects of traffic analysis, at first we need to identify the admissible region of voice and data service groups in each sub-system. Let Q lv and Qld be the link qualities such as frame error rate that individual voice and data users experience in the sub-system l (l = 1
2), and Q v
min and Qd
min be a set of minimum link quality level of each service. Then, for a certain set of system parameters such as service quality requirements, the admissible region of the sub-system l with respect to the simultaneous number of users satisfying service quality requirements in the sense of statistic, S sub
l can be defined as

Ssub
l =f(n(v
l)
n(d
l))jPr (Qlv (n(v
l)
n(d
l))
Qv
min and Qld(n(v
l)
n(d
l) )
Qd
min)
 %g (1) =f(n(v
l)
n(d
l))j0  fl (n(v
l)
n(d
l) )  1 and n(v
l)
n(d
l) 2 Z+g for l = 1
2 where n(v
l) and n(d
l) are the admitted number of calls of voice and data service groups in the sub-system l respectively,  %

is the system reliability defined as minimum requirement on the probability that the link quality of the current users in the sub-system l is larger than the minimum link quality level, and fl (n(v
l)
n(d
l)) is the normalized capacity equation of the subsystem l. In the case of linear capacity region, for example

fl (n(v
l)
n(d
l)) can be given as fl (n(v
l)
n(d
l)) = alv  n(v
l) + ald  n(d
l) for l=1,2. Such linear bounds on the total number

B. Common operation of the multi-access systems In the common operation of the multi-access systems, the admissible region of the considered multi-access system depends on how multi-services are allocated onto the sub-systems. That is, according to the employed call assign scheme, the admissible region of multi-access system in the common operation can be one of subset of the following set :

of users of each class, which can be supported simultaneously while maintaining adequate QoS requirements, are commonly found in the literature for CDMA systems supporting multiclass services [4,5]. In general, call admission control (CAC) policies can be divided into two categories: number-based CAC (NCAC) and interference-based CAC (ICAC)[6]. In the case of ICAC, a BS determines whether a new call is acceptable or not by monitoring the interference on a call-by-call basis while the NCAC utilizes a pre-determined CAC threshold. In this letter, we adopt a NCAC-type CAC since its simplicity with which we can apply general loss network model to the system being considered for the performance analysis, even though the NCAC generally suffers a slight performance degradation over the ICAC[6]. That is, a call request is blocked and cleared from the system if its acceptance would move the next state out of the admissible region, delimited by Eqn.(1), otherwise it will be accepted. We further assume call arrivals of class j in the sub-system l are initialized as a Poisson process with rate  (j
l) (j = v
d). If a call is accepted then it remains in the cell of its origin for an exponentially distributed holding time with mean 1= (j
l) which is independent of other holding times and of the arrival processes. Then, the offered traffic load of the j -th service group in the sub-system l can be defined as  (j
l) = (j
l) =(j
l) . With these assumptions, it is well known from M/M/m queue analysis that for given traffic loads, the equilibrium probability for an admissible state, N l ( (n(v
l)
n(d
l) )) in the sub-system l, (Nl ) can have a product form on the the truncated state space defined by the call admission strategy such that it is given by

8 (Nl ) =