Cost and Reliability Estimation of Radio Access Network Structures for ...

Cost and Reliability Estimation of Radio Access Network Structures for the 4G Systems A. Krendzel, Y. Koucheryavy, J. Harju e-mail: {krendzel, yk, harju}@cs.tut.fi Tampere University of Technology, Institute of Communication Engineering, P.O. Box 553, FIN-33101, Tampere, Finland. http://www.cs.tut.fi/tlt/

Currently, there is an evolution in wireless communications from GSM Phase 2+ cellular systems towards the Fourth Generation (4G) wireless multimedia systems. The 4G systems should offer significantly higher bit rates than 2 Mb/s, also should have higher capacity and lower bit cost then those of the Third Generation (3G) cellular systems and be able to support all types of new and future multimedia services. These requirements will make the 4G Radio Access Network (RAN) different from current RANs [1]. It is expected that in the 4G systems a load on both the Radio Network Controller (RNC) signal processing equipment and the entrance links between the RNC and Base Stations (BSs) will be heavier [2]. It may raise valuable funds for the RAN construction. In [1,2,3] a new and innovative RAN structure for the 4G systems from points of view of a load and routing capabilities has been proposed and analytical proved. This structure is called a cluster-type configuration or, in other words, it known as a ring structure. In this paper, the ring structure of the 4G RAN physical links with respect to its reliability and cost is considered. The objective of the paper is to make the quantitative estimation of reliability and cost parameters of the ring structure, to compare it with those of other structures and to give recommendations on applying different structures of a physical links configuration for the 4G RAN. We start the paper with problem estimating of the reliability parameters for structures consisting of one, two, three and more rings, and a radial or a tree structure. As a quantitative parameter of the structure reliability, the average number of knocked out BSs is calculated. Such BSs cannot serve mobile terminals because of damages in the RAN physical links connecting BSs to each other. The BS is knocked out if there are damages in the ring on both side of this BS. It is assumed that BSs are connected to each other by optical fibers, because optical fiber links are preferred as the dominant links to construct the 4G RAN [4]. Problem It is given: N is a number of rings in the RAN, N ∈ {1,2,…}; L is a number of damages in the RAN, L ∈ {0,1,2…}; M is a number of BSs in a ring, M ∈ {1,2,…}. It is necessary: to find an average number of knocked out BSs depending on the number of rings in the RAN, the BSs number in the ring, the damages number in the RAN - Xˆ ( N , M , L) ∈{1,2,…,z}. We present the method based on the probability theory principles to solve the problem. The final expression for calculations of the average number of BSs that are not able to serve mobile terminals as a result of damages in a 4G RAN ring structure is the follows ( M + 2)( − ln p0 ) 2 p0 ( M + 2)( 2 N + 1)( − ln p0 ) 3 p 0 Xˆ ( N , M , L ) = + 3( N + 1) 6( N + 1)( N + 2 )

(1)

The final formula for calculations of the average number of knocked out BSs of a radial structure (N=0) is given by ( − ln p0 ) 3 p0 Xˆ ( M , L) = ( − ln p0 ) p 0 + ( − ln p0 ) 2 p 0 + 2

(2)

The probability of zero damage (p0 ) corresponds to the availability in term of the reliability theory [5]. Relationships of the average number of knocked out BSs from the rings number N and the BSs number in each ring M when very low availability (p0 =0.3) is shown in Fig. 1. It is seen that with increasing the ring number the RAN reliability is considerably raised up.

8 Normalised length of links, r

Average number of knocked out BSs

2

1.5

1

0.5

0

0

5 10 Number of BSs

6

4

2

15

0

5

10

15

20

Number of BSs

one ring structure two rings structure three rings structure four rings structure five rings structure radial structure

radial structure one ring structure two rings structure three rings structure

Fig. 1. Relationships of the average number of knocked out BSs from the rings number and the BSs number in each ring when p 0 = 0.3

Fig. 2. Relationships of the normalised length of links from the ring number and BSs number in the RAN

In the second part of our paper we calculate cost parameters of ring and radial structures and compare them with each other. The normalised length of links connecting BSs to each other as a quantitative parameter of cost is considered. Problem It is given: M is a number of BSs in the ring, M ∈ {1,2,…}; N is a number of rings in the RAN, N ∈ {1,2,…}; coordinates of BSs. It is necessary: to find the normalised length of links connecting BSs to each other depending on the number of BSs and rings in the RAN - L ( M , N ) . The expression for calculation of the cost parameter of ring structures has the following view L (M , N ) =

2 r( M + N ) , M

(3)

where r is radius of the internal circle of cells. For calculation of the cost parameter of a radial structure of the RAN the following expression is obtained   L (M , N = 0 ) = 2 r, M = 1,6  12 r + 3, 46 r( M − 6 ) , M = 7,12  L( M , N = 0) = M   L( M , N = 0) = r (4 M − 15 ) , M = 12 ,18  M

(4)

Relationships of the normalised length of links from the BSs number and the rings number in the RAN are presented in Fig. 2. It is seen that if the number of BSs in the RAN is from 1 to 7 then the cost parameter of the radial structure is more beneficial than the ring structures one. If the number of BSs is 7 and more then the ring structures are preferable to radial from the viewpoint of cost. In the third part of the paper we compare different structures with each other taking into consideration both cost and reliability and give a number of recommendations how to choose a RAN physical links structure depending on a BSs number in the 4G RAN. REFERENCES [1] Y.Yamao, H. Suda, N.Umeda, N. Nakajima, “Radio access network design concept for the fourth generation mobile communication system,” Proc. VTC2000-Spring, vol. 3, pp. 2285-2289, 2000. [2] T. Otsu, Y. Okajima, N. Umeda, Y. Yamao, “Network architecture for mobile communications systems beyond IMT2000,” IEEE Personal Commun. Mag., vol. 8, no. 5, pp. 31-37, October 2001. [3] T. Otsu, N. Umeda, Y. Yamao, “System architecture for mobile communications systems beyond IMT-2000,” GLOBECOM'01, vol. 1, pp. 538 –542, 2001. [4] Y. Aburakawa, T. Otsu, Y. Ymao, “Fiber and free-space hybrid optical networking for new generation mobile radio access network,” the 5th international symposium on wireless personal multimedia communications, vol. 2, pp. 586 –590, 2002. [5] Telephone network and ISDN, Quality of service, network management and traffic engineering, CCITT, Blue Book, vol. II, Fascicle II.3, pp. 317-365, Geneva, 1989.