Flexible mobility management strategy in cellular networks

Transcription

1 Flexible mobility management strategy in cellular networks JAN GAJDORUS Department of informatics and telecommunications (161114) Czech technical university in Prague, Faculty of transportation sciences Konviktska 20, Prague, CZECH REPUBLIC Abstract: - Among other important changes in mobile telecommunications such explosive grow of UMTS network coverage we have seen in last years a huge move from simple mobile terminals to smart devices which we even cannot name just as phones anymore. In location management a procedure how to find user s location in cellular network - there have been published methods minimizing data overhead. Published papers describe distinct approaches to location management but have one thing in common - all the time the mobile terminal is moving across the network cells it is used just one algorithm to minimize network overhead although some other methods may work more efficient if user movement behavior changes. This paper describes some of those methods and later discuss possible combination of distinct algorithms based on the real time evaluation performed by mobile terminal something we can afford to perform inside mobile terminals due to increased computing performance available. Key-Words: - Location Management, Location Update, Paging, Call Delivery 1 Introduction In a wired network, there is a static relationship between a user (terminal) and its location it might be for example the beginning of the called number which determines routing. The terminal number (e.g. IP address) defines its location. In wireless cellular network this prediction is not true at all, therefore some procedure how to find user is needed this is called location management (LM). In mobile cellular wireless network user (mobile terminal MT) may change its location and therefore MT switches between network transmitters / receivers. To handle communication with MT network has to 1) find MT current location by location management methods and 2) keep the communication if MT moves during transmission this is named handover management. This paper is focused mainly on location management in another words process to locate MT location for call or service delivery. As mentioned in abstract, there have been published many distinct methods and approaches to location management in last decade. Although proposed location management (LM) methods vary, one thing they have in common. MT and network core use always one method for location management. As we will show later in this paper there are situations based on current MT movement behavior - when one method is more efficient than another and vice versa. Obviously there have been reasons for this one reason is every algorithm implementation requires some resources in MT and in network core too. But if we focus for a while only to the most used cellular network, UMTS - as reported in [1], global shipment of so-called smart phones grew up just in first quarter 2011 by 83% annually to 101 million units. These devices contain some real time operation systems and among other tasks are capable to evaluate efficiency of different location management methods. This paper is led by the idea of using available computing resources in MT to minimize network overhead for MT localization and to involve MT deeply to process of selecting the most efficient LM strategy in real time. 2 Location management-classification of approaches The grow of mobile users force network operators to increase number of cells in their networks and to reduce of average network's cell size. Therefore there is a strong demand for the efficient location management. ISBN:

2 A cellular network must track the location of each MT in order to route connection to the cell relevant to the MT current location. As typical cellular network has thousands of cells, cells are grouped into so-called Location Areas (LA). LA contains only neighboring cells. Usually MT can move within one LA without updating network about exact location, location update (LA) a process when MT updates network about its location - is performed when MT moves to another LA. If a call is initiated to MT, the network needs to know precisely where MT is. Therefore network asks cells in LA if destined MT is in the cell's range. This process is called Paging. The total cost of locating MT (Total Handover Cost THC) is sum of costs for: - Location Update - Paging update is performed. This condition doesn't depend on any previous MT movement history. From the most common we ll quickly discuss time, distance and movement based LM schemas. Dynamic methods contain personalized profiles and adjust LU frequency for each user. Obviously these methods achieve better results in situations when MT moves predictable by some kind of pattern [4][8]. On a second side, dynamic methods consumes higher computing resources not only within MT but in network as well Static schemas - Time based schema THC = LU HANDOVER + Paging HANDOVER (1) The frequency of LU and paging requests relates with MT movement behavior and time interval between incoming calls as well with network characteristics such as cell size. To minimize paging cost it is possible to increase count of performed LU, but LU can be a very expensive and inefficient operation, especially for MT having long interval between incoming calls. Performed LU generates load on the network core and additionally reduces available bandwidth in the wireless part of the cellular network. Efficient location management schema can significantly reduce the network overhead required in locating mobile devices in a network. 2.1 Location management classification Different proposals exists, we divide them by common characteristics similar as in [2]. Figure 1 classification of LM proposals Figure 2 - Time based strategy In time based schema MT performs location updates periodically, at a constant time interval of. MT needs only an measure interval that has passed since its last LU and the value of may be adjusted for each user depending interval between its destined calls. In [3] it has been found the signaling load within the network may be reduced using this strategy as network knows the MT is turned off if it does not receive a period location update from the MT, thereby not performing unnecessary paging operations. However, using this strategy suggests that users with low movement rates will still perform location updates periodically, thereby generating unnecessary network traffic [4] if they have only moved to surrounding cells or if they have not moved at all. However, in [5] found that this time-based strategy method of location updating performed better than the location area method. Static schemas define some time invariable condition - if this condition is passed, location ISBN:

3 2.1.2 Static schemas - Movement based schema because MT needs to retain information about the starting location in order to calculate the distance D Dynamic schemas The idea behind dynamic proposals comes from the observation a typical user (MT) movement can be predicted from previous movement history. MT movement history is somehow stored in MT profile and is subject of periodical update by recent MT move. Figure 3 movement based strategy This strategy involves an MT performing a location update after a certain number of movements across cell boundaries. The predefined number of movements M is referred to as the movement threshold [6] and the value of M may be adjusted for each user. This schema therefore limits the radius of an MT s location since the last location update to M cells. Hence upon call arrival, the paging operation can be limited to those surrounding cells within radius M. This reduces paging area, thereby conserving network resources Static schemas - Distance based schema Figure 4 distance based strategy In distance-based LU strategy, an MT performs LU when it has moved a distance D from the cell where it performed the last location update. D is a predefined distance, and is referred to as the distance threshold [6]. Using this strategy, location updates may only be performed in the cells at radius D from the last LU cell, thus limiting the paging operation in a similar way to the movement-based strategy discussed above. Distance-based location update strategies are the most difficult to implement [7] from static methods, The way how personal profile is constructed vary from statistical based methods [6][7] to usage of artificial neural networks [8]. As was showed for example in [4][8], dynamic methods are efficient if MT follows an already known pattern but fail in case MT starts to move unexpectedly. 3 Location methods evaluation in real time This section starts with the motivation behind this paper, describe role of mobile terminal in the LM schema selection process followed by description of schema selection algorithm and illustrates total handover cost calculation for static and dynamic schemas. 3.1 Motivations As announced in previous sections, main motivation of this paper is to use available resources in MT for evaluating available schemas real time and to choose the schema which best fits to current MT movement behavior. It has been observed by vast majority of researches [2][3][4][7][8] personal specific / dynamic LM method gives better results than static methods if MT follows its regular patterns. Some of researches focus only on location update cost or only to paging cost, but as noted in (1), the total cost is what really matters and it is wrong to minimize LU cost at the expense of paging cost or vice versa. Another common observation is dynamic LM methods lose their efficiency if MT terminal doesn t follow regular patterns. In certain cases specialized LU / paging schemas result in smaller THC for example in [9] are used geographical data for highly mobile users. Naturally tracking of MT location is user specific problem by its definition. Only MT knows in any ISBN:

4 time its precise location and therefore MT is the most appropriate decision maker. 3.2 MT role in LM schema selection For the schemas comparison we have in MT implemented at least one dynamic method and at least one static method. From dynamic methods we have in MT implemented simple personal profile schema working on the similar principle as in [8] where MT sends LU when visits new cells in network and doesn t send any LU if it is in location previously visited. Paging process then sequentially page only those cells in personal profile with highest probability first. distinguish probabilities for different types of the day (in our example business day and weekend) and hour. From static methods lets calculate total handover cost for movement based (2.1.2), distance based schemas (2.1.1) and schema. MT continuously compares its current location with the location expected by stored profiles and calculates THC. The same calculation is performed for static schemas. Figure 5 illustrates part of cellular network used for testing a regular trip from point 1 to point 6. Road 1 (lower) road represents six stops of public transportation and this trip takes 30 minutes. This regular road and therefore all cells on the road are stored in user s profile. Blue upper road is alternative new (visited for a first time) road. Trip by car from 1 to 6 is about 10 minutes. Distance from start to end is approximately 3.5 km for road 1 and 4.5 km for upper road 2. Figure 5 testing cellular network From (1) total handover cost is the sum of location update cost and paging cost. For all LM methods implemented in MT and network core THC is calculated and method having minimum THC is the most appropriate to current MT movement. Selected schema = min { THC } 1 N (2) Selected schema with lowest handover cost is used until another schema is evaluated as more efficient (lower THC) if MT movement behavior changes. MT then informs network to switch to another LM method together with location update. Because network is not continuously informed about latest MT position, evaluation must be done in MT and MT is responsible for selecting appropriate location management schema. 3.3 Total handover cost calculation Evaluation in this case means calculate handover total cost (1) for all available schemas LM methods must be implemented on both sides in MT and in network core. Simplified algorithm which is continuously performed within MT illustrates figure 6. THC schema #1 MT location +Recent Schema movement... } history Schema THCMIN <> Schema THC CURRENT FALSE THC schema #n TRUE Send LU + switch LM schema Figure 6 continuous THC calculation in MT THC MIN selected Total handover cost calculation for all implemented schemas in the algorithm in figure 6 is obviously the part deserves most attention. Detailed calculations for dynamic schemas are described by researches in related papers e.g. [4][8][9], in next paragraph we compare schemas as described in (3.2). 3.4 MT movement scenarios and THC calculation Let s illustrate THC calculation on regular trip (e.g. from home to office) from point 1 to 6 Figure 5. We are testing these LU strategies as described above - - LSPEED- public transportation avg. speed - average car speed in urban area - update strategy - Stored user profiles strategy [6][7][8] - Geography based [9] Appropriate paging schema are used: ISBN:

5 - For first are paged cells close to the cell where last LU was performed up to max. diameter from last LU allowed by max. speed in network. Cells are paged sequentially. - For the same paging schema is used first are paged cells close to last LU up to diameter D from last LU. - all cells in LA are paged, first the cell where LU was performed last time - For profile based strategy only cells contained in profile are paged. First are paged cells with highest probability where probability is function of time Probability P PROFILE = f(t) - In geography based LM scheme only cells on the tracked road are paged. In our example we have alternative road 2 tracked. For calculation we use average interval between calls 5 minutes.. Following graphs illustrates location update, paging and total handover cost in 30 minutes for two roads from 1 to 6 described above Evaluation well known road Location Update cost Figure 7 Location update handover cost (3) SLOW On the road stored in user profile comes without any surprise based strategy with minimum location update handover. 30 Paging cost 0 10 Figure 8 Paging handover cost SLOW FAST Same situation is for paging handover as MT moves exactly as expected. D strategy produces higher THC when MT moves faster. Total Handover Cost LSPEED PROFILE Figure 9 Total handover cost in 30 minutes trip Finally THC is lowest for profile based strategy if MT moves by pattern and therefore MT uses this strategy during whole experiment Evaluation unknown road Figure 10 LU cost unknown road On figure 10 we can see for first 10 minutes Geo location scheme produces lowest LU cost as we travel on the regular road covered by network similar as in [9]. ISBN:

6 As was demonstrated on regular home-office movement example there is no method producing lowest handover under any MT movement behavior. SLOW Figure 11 Paging cost unknown road Total handover cost We discussed two novel approaches to location management. First is principle of decentralization where the best location management schema at the time is selected. In another words, while mobile terminal is turned on it searches for the most appropriate schema from the group of methods implemented in mobile terminal and the network itself. Second novel approach is to use available resources in mobile terminal and to let mobile terminal itself select the best strategy which suits present users movement behavior the best. LOW SPEED Figure 12 Total handover cost unknown road Travelling on unknown road (from MT perspective) produces lowest THC for Geographic scheme due the road is covered by the network map. As this second route is faster and after 10 minutes we reached final destination, since approximately 15 th minute strategy produces lowest handover and since 30 th minute based scheme has lowest THC. This because since 30 th minute MT again follows regular routine. 4 Conclusion Speaking about location management cost in fact means speaking about certain amount of service traffic needed for maintaining basic network services. Total management cost comprise of two components - location update and paging cost and effort of minimizing one usually lead to cost increase for another. Efficient location management strategy means to minimize the total handover cost for the combination of both location update and paging. In accord with other researches we observe dynamic / personalized location management schemas more efficient - means producing lower service traffic - if mobile users follow some regular pattern learnt from previous mobile movement history. However vast majority of mobile users don't follow their regular routines all the time they are covered by cellular network. References: [1] Canalys research, Worldwide smart phone market report Q [2] T. Kunz, A. Siddiqi, J. Scourias, The Peril of Evaluating Location Management Proposals through Simulations, Wireless Networks 7, , [3] M. M. Zonoozi and P. Dassanayake, User mobility modeling and characterization of mobility patterns,"ieee Journal on Selected Areas in Communications, 15(7): , September [4] A. Sajal, K.Das, LeZi-Update: An Information- Theoretic Approach to Track Mobile Users in PCS Networks Center for Research in Wireless Computing (CReW) [5] I. F. Akyildiz and J. S. M. Ho, Dynamic mobile user location update for wireless PCS networks," Wireless Networks, 1(2): , July 1995 [6] C. Rose and R. Yates, Minimizing the average cost of paging under delay constraints, Wireless Networks, 1(2): , July [7] P. Mutaf and C. Castelluccia: Hash-Based Paging and Location Update Using Bloom Filters [8] A. Quintero, A User Patter Learning Strategy for Managing User's Mobility in UMTS Networks, IEEE transactions on mobile computing, vol. 4, no. 6, November / December 2005 [9] H.Liu,Ch.Liu - A geography based location management scheme for wireless personal communication systems, Vehicular Technology Conference Proceedings, VTC Spring Tokyo IEEE 51st ISBN: