Architecture and Schemes for Intelligent Mobility Management in Future Mobile Telecommunication Systems Kuo-Hsing Chiang, Nirmala Shenoy Department of Computer Systems Engineering, RMIT University GPO Box 2476v, Melbourne, Victoria, 3000, Australia Email: [email protected], [email protected] Tel: +61 3 9925 5360, Fax: +61 3 9925 5340 Abstract - In this paper, we present a novel terrestrial mobile swap technic is applied to location database management to network architecture that aims at the simplification of further reduce the signalling traffic. In section III C, a mobility management for the terrestrial mobile location updating using macro-cell and the information flows telecommunication networks. The work is in line with the are discussed. A macro/micro cell based intelligent paging UMTS approach. The proposed architecture uses the scheme as applied in a location area is proposed in section III hierarchical cell concept and facilitates strategic placement of D. Section III E describes the location management in pico- database control. A macro-cell based approach for location cell environment and the handover control is briefly discussed updating and a combined macro/micro cell based paging in section III F. Section IV provides discussion and control plays a key role in our proposal. Another significant conclusions. feature of the proposed scheme is that, each micro-cell has the same signalling load, regardless of the location updating II. BASIC ARCHITECTURE traffic. A Location Agent (LAg) that is co-located with Figure 1 illustrates the proposed network architecture. The Signalling Transfer Point (STP) and hierarchically structured proposed network is built on concepts from UMTS and GSM. is also proposed to reduce mobility management signalling The changes proposed by us have been highlighted using traffic to and from the HLR. A swap technic is applied at the Italics. The basic network consists of three sub-networks: location databases, which further reduces the signalling radio access network, core network, and a signalling and traffic. The simplifications achieved through the use of this intelligent network. architecture benefit both location updating and paging • Radio access network provides mainly radio related procedures by optimising the traffic in both cases. functions, the basic local cell site switching (CSS) and transmission functions, and interfaces with core and I. INTRODUCTION signalling networks. A group of pico-cells are The future mobile systems aim at enabling subscribers to connected to one Pico-cell CSS (PiCSS) and a group of communicate with anyone, anytime and anywhere. Mobility micro-cells and one macro-cell are connected to a management is always a key issue for the mobile networks to BSC/CSS. The BSC/CSS coverage area equals to one achieve more efficient location management and handover macro-cell coverage area and also equals to a control. The existing techniques for location management are Location Area. The PiCSS and BSC/CSS are at the location updating (registration) and paging. The first same level of hierarchy and connected to LE/MSC. This generation mobile networks, such as Advanced Mobile Phone type of umbrella architecture has been proposed Service (AMPS), page the Mobile Station (MS) in every cell earlier, but here we have slightly rearranged it and within the network. The second generation mobile networks, utilised this architecture to help strategic placement of such as Global System for Mobile communication (GSM) [1] databases, thereby achieving optimised mobility use Home Location Register (HLR), Visitor Location management. Register (VLR) and Location Area (LA) concepts to achieve • The core network consists of enhanced local and transit more bandwidth efficiency in air interface. In this paper, we switches of B-ISDN and also includes a transmission propose an architecture for an integrated wireless/fixed system to provide switching, call control and network in line with the UMTS approach [2,3] but is very connection control. much superior from the point of view of optimal signalling traffic and shared signalling loads with faster access during • Intelligent and signalling network will provide specific paging. The architecture has been modeled to achieve services and location management. Distributed efficient mobility management by using hierarchical databases will be required in Intelligent Network (IN) distributed database and intelligent paging techniques as to store user related data (user/service profiles) to suggested in the review paper [9]. The basic architecture support access to the network and mobility services. A described in section II, is the significant contribution based on LAg co-located with STP is proposed. The function of which optimal mobility management schemes are proposed. the LAg briefly, is to provide hierarchical pointers to A hierarchical Location Agent concept used for location the VLRs a mobile station is traversing, where the management is explained in section III A. In section III B, the hierarchy is controlled by the mobility of the user. A current mobile network when the LE and MSC are not detailed explanation of this concept follows. integrated.

STP HLR MS initial entry into remote or visited network:

Intelligent and ( Remote or Visited Network ) Signaling Network Signaling Network MS VLRnew STP/LAg HLR STP STP/LAg Location updating (or registration) Location updating (or registration) Location updating (or registration) Core Network LE/MSC/VLR LE/MSC/VLR LE/MSC/VLR Location updating (or registration) ACK Location updating (or registration) ACK

PiCSS BSC/CSS PiBCSS MS moving from VLRold to VLRnew : Radio Access Network ( Remote or Visited Network ) MS VLRold VLRnew STP/LAg HLR

Pico-cells Micro-cell Pico-cells Micro-cell Location updating Macro-cell Location updating Location updating ACK BSC: Base Station Controller HLR: Home Location Register LAg: Location Agent Location updating ACK LE: Local Exchange MSC: Mobile Switching Centre PiCSS: Pico-cell CSS STP: Signaling Transfer Point VLR: Visitor Location Register CSS: Cell Site Switch Figure 1. Proposed Network Architecture Figure 2. Information Flows of the Simplified Location Updating by Lag in Remote or Visited network III. MOBILITY MANAGEMENT In our configuration, the LAgs are distributed at SS7 STPs. A. Location Management Using Hierarchical LAgs at STPs Based on the users’ mobility patterns any one of the LAgs In GSM to locate a mobile user, the HLR is always accessed will be the location agent for that mobile user. For example in first and the locality of connection is not exploited. In the figure 5, LAg111 is at the lowest level for low mobile users, future network, the Local Exchange (LE) may be enhanced to LAg11 at the next level for medium mobile users and LAg 1 include Mobile Basic Call State Model. The LE shall be able at the top level for highly mobile users. MS1 is a subscriber to handle both fixed and mobile calls [4]. Based on this who spends most of his time under the local STP/LAg111 and approach, a local database at the LE can control both fixed his VLR pointer is recorded only at Lag111 while MS3 has and mobile calls that are in the same LE coverage area. To higher mobility and often goes across different STP coverage reduce the signalling traffic when an MS roams to other areas and hence his VLR pointer will be at LAg1. An inherent networks or away from its home LE, a Location Agent (LAg) advantage of this scheme is that the information to be stored concept is used. In our configuration the LAg is distributed at in the LAgs is distributed fairly across the different the SS7 STPs. This means that the SS7 STP will be enhanced hierarchical layers of STP/LAgs. If the LAg acts as a HLR to include extra information. If the MS is roaming to other Agent then for highly mobile users like MS3, having his VLRs under the same LAg coverage area, the LAg will profiles recorded at LAg1 will save some traffic when he record the VLR identifiers. On the other hand, if the MS is in moves from coverage under STP/LAg112 to STP/LAg121. If the remote part of the network or in the other networks, the LAg1 didn’t have his profiles then the user’s information has LAg may acts like HLR Agent and replicates all the necessary to be transmitted via LAg112, LAg11, LAg1, LAg12, and information for the call set-up and mobility management. LAg121 to the new VLR. Our approach has a shorter This will be a distributed HLR in STP level in the network. signalling path to transmit user’s information comparing with Figure 2 illustrates the information flows of the location location forwarding strategy proposed in [10]. updating by LAg in remote or visited network. A STP/LAg The proposed scheme will avoid heavy database processing connects from one to several LE/MSCs. at a particular STP and also reduce the signaling traffic to a remote HLR. The signaling load will be localized and We propose locality of connection to be exploited as distributed more evenly in all STP regions. Analytical studies follows: in Figure 3 the LE is an integrated MSC and SSP of comparing different location management schemes are therefore the call control can be localised at LE/MSC level. being conducted. For example, when a fixed user initiates a call to a mobile user and both users are under the same LE control, the network will first check the local database which is co-located STP/LAg STP/LAg with LE therefore the call can be set up faster and the signalling traffic will be reduced. The other approach shown in Figure 4 is to connect the LEs and MSCs to a common STP LE/MSC LE/MSC LE/MSC LE LE LE that will be the signalling anchor point and therefore saves the MSC MSC signalling traffic to HLR. This approach can be used in the Figure 3. LE and MSC are Integrated Figure 4. LE and MSC are Separated continuously changing [9]. In our proposal, the existing macro-cells are used as LAs and a BSC/CSS control area STP/LAg1 equals to a macro-cell coverage area and also equals to a LA. This is the novelty of our approach as this leads to simplified STP/LAg11 STP/LAg12 approaches to location updating and paging. Figure 7 shows the network planning for the BSC or CSS. In this approach, the macro-cell will broadcast the LA identifier which, could STP/LAg111 STP/LAg112 STP/LAg121 STP/LAg122 be the same as macro-cell identifier to save signalling

VLRold VLRnew overhead. The main advantage of this architecture is to simplify the location updating control through macro-cell. MS1(LAg111) Another important effect is that each micro-cell will have the same signalling load, regardless of the location updating MS2(LAg11) traffic, as it does not have to broadcast the LA identifier. The MS3(LAg1) macro-cell can also act as backup resources during inter Figure 5. Distributed LAg’s for Different Class of User micro-cell handover failures. Because the macro-cell and B. Swap technic for Location Management micro-cells are under the same BSC/CSS control it makes BSC/CSS control of radio resources easier during handover We have further divided the conventional location database and location management. This concept can be extended to (e.g. VLR) into two parts: current and history databases. The the pico-cell and micro-cell environments. The architectural current database records the user/service profiles of the MSs and functional models proposed in this work can be easily that are currently residing in the VLR coverage area and is in scaled to fit the environment. Figure 8 illustrates the the primary storage. The history database is stored in the simplified location updating procedures and information secondary storage. It records the profiles of the MSs that flows. visited the VLR previously in a certain period. The swap technic is used to exchange information between the primary Conventional Location Updating Procedures: and secondary databases. In the conventional mobile network (e.g. GSM, IS-41), the VLRnew STP VLRold HLR location updating algorithm will update the profiles at Location update req. VLRnew and the profiles at VLRold will be deleted. In our Location update resp. (with Auth. parameters or profiles) approach, the profiles at VLRold will be swapped from the Location update req. primary database to secondary database. The profiles are Location update resp. (with or without profiles) deleted only based on the mobility patterns and the timers. By Location cancellation req. via STP11 using the swap technic at VLR, the signalling traffic for Location cancellation resp. via STP11 profiles download will be reduced significantly. The comparison between conventional and our approach is illustrated in Figure 6. There is a profiles download procedure Location Updating Procedures Using Swap technic: every time when the MS roaming from a VLR to another A. MS moves from VLR1 to VLR2: VLR in the conventional approach. In our approach, the profile can be swapped and it is not necessary to be VLR2 STP11 VLR1 HLR downloaded to the VLRs. In conjunction with the hierarchical Location update req. algorithm proposed in the last section, the swap algorithm Location update resp. (with profiles) applied at location databases can further reduce the signalling SWAP at VLR1 traffic for location management. Update location pointer at STP11 C. Location Updating Using Macro-cell Location update resp. (with profiles) In the conventional location updating approach (e.g. GSM), the location updating occurs only in perimeter cells, B. MS returns to VLR1 from regardless of periodical updating. The radio resources VLR2: VLR1 STP11 VLR2 HLR consumed in the perimeter cells are much more than the inner cells. This causes load unbalance for each cell and the Location update req. different design for perimeter and inner cells. Other Location update resp. (without profiles) researchers [11,12] use different dynamic LA management SWAP at VLR2 and updating schemes which need to record the different Update location pointer at STP11 users’ mobility parameters. However, it is generally not easy Location update resp. (without profiles) to have dynamic LA for different MS’s as the MS’s must be SWAP VLR1 able to identify the boundaries of LA’s which are Figure 6. Information Flows of Conventional and Swap Location database xi2R yiR   P(x, y)  {(xi  2R, yi  R),(xi  2R, yi  R),(xi  2R, yi  R),(xi  2R, yi  R)}, Management xi2R yiR 0  x  16,0  y  8 • For example at P2 (16,4), R=2, the first paging area BSC/CSS composes cells {(14,16), (13,15), (15,15), (12,4), (14,4), (16,14), (13,3), (15,3), (14,2)}. • If there is no response from PA1, the rest of cells in the LA LA will be paged. Due to the service reliability reasons, if the MS is still not responding to the first 2 pages, we need to page the whole LA either using macro-cell (if Macro-cell Micro-cells micro-cell paging failed) or micro-cells again. Y Y First PA

Figure 7. Overlaid Cell Planning Model . 8. 8 7 MS BSC/CSSold BSC/CSSnew VLR 7 R 6. (4,6) 6 P3 Monitoring the LA . 5 or macro-cell identifiers 5 R . R P2 4 (8,4) (16,4) . 4 P1 Location updating request 3. (5,3) 3 Location updating request 2. 2 1 1 LA update ...... X X 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 Location updating ACK Location updating ACK * The shadow area is covered a macro-cell and is equal to one BSC/CSS coverage area ** P1, P2, P3 are the last contact cells of the users

Figure 8. Information Flows for Location updating Control Using Figure 9. Planning for Location and Paging Area Macro-cell A description of the macro/micro-cell based paging D. Macro/micro Cell Paging algorithm is given in the flow chart of figure 10. The network In the conventional network, the whole service area is first checks the user’s class whether the user is fast or slow divided into LAs which are a logical group of cells. In our moving. In the case of fast moving user, the macro-cell approach, the LAs are geographically distributed according to paging will be used. If the network realises that user belongs BSC/CSS (macro-cell). We are proposing a combined macro- to slow moving class then the intelligent paging algorithm is cell and micro-cell paging which is depending on the user’s launched. The information flows of an incoming call for fast class (slow/fast). From figure 7 and 9, each macro-cell moving user are shown in figure 11. coverage area is one LA. Each user has his own Paging Area GSM uses a time-triggered periodic location updating to (PA), as we propose a per-user position based PA algorithm trace the MS’s position and status (ON/OFF). In our with more information and intelligent control [6,7] based on approach, when the MS is turning off, the MS will send a user’s speed, last contact cell, etc. [8]. The multiple-step signalling message to inform the network (BSC, VLR, and paging scheme has already been proposed in [13], we plan to HLR) its status so there is no necessity to have periodic use it in our architecture and have proposed a technique location updating to make sure of the MS’s status. This through which we can determine the cells to be paged easily. reduces the signalling traffic considerably and can be easily Hence the BSC/CSS covering one LA will have a number of adapted to the proposed architecture. This signalling message records of paging details for the different users. If a user can be sent by the macro-cell base station. In the case where belongs to the fast moving class then the macro-cell based MS moves outside the network coverage area or moves into a paging scheme may be applied. In general the paging scheme blind spot, there is no signal between MS and network. This is explained below and shown in figure 9. is a critical situation in our approach. During the non- • Each micro-cell’s position is identified with references contactable period, if there is an incoming call to the MS, to X-Y axis. paging will fail and the network will treat the MS as switched off and the following incoming calls will not be paged. Once • R = paging radius and all cells within this radius will be the MS moves into the coverage area and regains the signals, paged. R is dependent on the user’s class (e.g. the MS will send a location updating message to inform pedestrian, car, public transport, etc.). For example in network its status/position immediately. Figure 8, P1 is a user in office, P2 is a pedestrian, and P3 is a user driving a car in the city. • When a user is called, the first paging area = PA1, and is given by P a g i n g R e q u e s t between PiBSC and BSC and the PiBSC communicates directly to the LE. U s e r ' s C l a s s S e l e c t io n F a s t M o v in g Y e s P a g e t h e L A IV. CONCLUSION U s e r ? u s in g M a c r o - c e ll In future mobile network systems, signaling traffic for N o mobility is predicted to be much higher and complicated. In this paper we have approached this problem by suggesting P a g i n g t h e s u r r o u n d i n g c e ll s o f t h e some enhancements to the proposed UMTS architecture, la s t c o n t a c t c e ll w it h in t h e L A resulting in reasonable signaling traffic for mobility a c c o r d in g t o • R " management. The model has been studied with respect to location updating and paging signaling traffic and the corresponding information flows and control points proposed. Y e s M S R e s p o n s e ? S t o p However we further need to study and analyse this model to provide estimation on the actual signaling traffic and perform N o some comparative studies with existing architecture. P a g i n g t h e r e m a i n c e lls in t h e L A REFERENCES [1] M. Mouly, M.-B. Pautet, “The GSM System for Mobile Y e s Communications”, published by the authors, 1992. M S R e s p o n s e ? S t o p [2] T.. Norp, A. J. M. Roovers, “UMTS Integrated with B- ISDN”, IEEE Comm. Mag., Vol. 32, No. 11, Nov. 1994, pp N o 60-65. P a g i n g t h e w h o le L A [3] E. Buitenwerf, G. Colombo, H. Mitts, P. Wright, “UMTS: u s i n g m a c r o - c e ll Fixed Network Issues and Design Options”, IEEE Personal Comm., Vol. 2, No. 1, Feb 1995, pp 30-37. Figure 10. Flow Chart of Paging Control [4] K.-H. Chiang, N. Shenoy, J. Asenstorfer, “Intelligent MS Micro-cell Macro-cell BSC/CSS LE/MSC Handover and Location Updating for a Third Generation BS BS Mobile Network”, IEEE GLOBECOM’98, Vol.4, Nov. 1998,

Incoming call to MS pp 1963-1968. [5] 3GPP, “Technical Report on the Gateway Location Paging MS using macro-cell (for fast moving class user) Paging MS Register”, V1.1.0, 1999-04. Paging response [6] G. L. Lyberopoulos, J. G. Markoulidakis, D. V. Connection setup Polymeros, D. F. Tsirkas, and E. D. Sykas, “Intelligent { Paging Strategies for Third Generation Mobile Telecommunication Systems”, IEEE Trans. on Vehicular OR Paging response Technology, Vol. 44, No. 3, Aug. 1995. pp 543-554. { Connection setup [7] N. E. Kruijt, D. Sparreboom, F. C. Schoute, and Prasad, “Location Management Strategies for Cellular Mobile Networks”, Journal of Electronics and Communication Figure 11. Information Flows of an Incoming Call for Fast Moving User Engineering, Vol. 10, No. 2, 1998, pp 64-72. [8] S. Mishra, O. K. Tonguz, “Most Recent Interaction Area E. Location Management in Pico-cell Environment and Speed-based Intelligent Paging in PCS”, IEEE VTC’97, In figure 1, a functional entity called Pico-cell BSC (PiBSC) Vol .2, May 1997, pp 505-509. which connects to LE is proposed. The functions of the [9] I. F. Akyildiz, J. Mcnair, J. S. M. Ho, H. Uzunalioglu, and PiBSC are similar to BSC. The mobility management in the W. Wang, “Mobility Management pico-cell environment is similar to the micro-cell environment.

F. Handover When the MS moves from a pico-cell to a micro-cell, the forward handover control should be used instead of backward handover control which been used in most of micro-cell and macro-cell cases. This is because there is no connection