166 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 1, JANUARY 2013 An Investigation on LTE Mobility Management
Ren-Huang Liou, Yi-Bing Lin, Fellow, IEEE, and Shang-Chih Tsai
Abstract—Mobility management in Long Term Evolution (LTE) is different from that in the third generation mobile telecom networks. In LTE, the Mobility Management Entity (MME) is responsible for the mobility management function. The MME is connected to a large number of evolved Node Bs (cells) that are grouped into the Tracking Areas (TAs). The TAs are further grouped into TA Lists (TALs). When a User Equipment (UE) moves out of the current TAL, it reports its new location to the MME. If the LTE network attempts to connect to the UE, the MME asks the cells in the TAL to page the UE. In LTE paging, the MME may sequentially page a cell, the TA of the cell, and/or the TAL of the cell. This paper investigates the performance of LTE paging, and provides the guidelines for the best paging sequence of cells.
Index Terms—Location update, long term evolution (LTE), mobility management, paging Ç
1INTRODUCTION
N a mobile telecom network, the locations of the User 4 identity broadcast from eNB 7. Since TA 4 is included in IEquipments (UEs) are tracked so that incoming calls can TAL 1, the UE still resides in the same location. When the be delivered to the UEs. Typical mobility management UE moves to Cell 9 (Fig. 1(2)), the received TA 5 identity procedures include location update and paging. When a (broadcast from eNB 9) is not found in TAL 1, which means UE moves from one location to another location, the UE that the UE has moved out of the current location. In this reports its new location to the network through the location case, the UE executes the location update procedure to update procedure. When an incoming call to the UE inform the MME that it has left TAL 1. The MME then arrives, the network identifies the location of the UE via assigns a new TAL to the UE. In Fig. 1, the new TAL is TAL the paging procedure. 2 ¼fTA4; TA5; TA6g. Note that the TAL is assigned on a In Long Term Evolution (LTE), the Mobility Management per-user basis (i.e., TALs for different UEs may have Entity (MME; Fig. 1a) is responsible for the mobility different sizes and shapes), and the newly assigned TAL management function [1], [2], which is connected to a group may be overlapped with the previously assigned TAL (e.g., of evolved Node Bs (eNBs; the LTE term for base stations; see TAL 2 is overlapped with TAL 1 in Fig. 1). We consider the Fig. 1b). The radio coverage of an eNB (or a sector of the central policy [1] that assigns a new TAL whose central TA eNB) is called a cell (see the dashed squares; Fig. 1c). Every includes the cell where the UE currently resides. In Fig. 1(2), cell has a unique cell identity. The cells are grouped into the the UE resides in TA 5, and TAL 2 ¼fTA4; TA5; TA6g is Tracking Areas (TAs; e.g., TA 1 contains Cell 1 and Cell 2 in centered at TA 5. In the central policy, the TALs may be Fig. 1d). Every TA has a unique TA identity (TAI). The TAs overlapped. For example, TA 4 is included in both TAL 1 are further grouped into TA Lists (TALs) [1]. In Fig. 1, TAL 1 and TAL 2. consists of TA 2, TA 3, and TA 4 (Fig. 1e). When an incoming call to the UE arrives, it may incur A UE stores the TAL that includes the TA where the UE large paging traffic if all cells in the TAL page the UE resides. In Fig. 1(1), the UE is covered by Cell 5, and the TAL simultaneously. To resolve this issue, we implement three it stores is TAL 1 ¼fTA2; TA3; TA4g. If the LTE network paging schemes in LTE. In this paper, an “interacted cell” attempts to connect to the UE, it asks the cells in the TAL refers to a cell where the UE is paged, makes calls, or (e.g., Cell 3-Cell 8) to page the UE. Every eNB periodically performs location update. In other words, the interacted broadcasts its TAI. The UE listens to the broadcast TAI and cell is the cell through which the UE had the interaction checks if the received TAI is in its TAL. If so, it means that with the network. the UE does not move out of the current location. In Fig. 1, when the UE moves from Cell 5 to Cell 7, it receives the TA . Scheme CT (Cell-TAL). When an incoming call arrives, the MME first asks the last interacted cell to page the UE. If fails, all cells in the TAL are asked to . R.-H. Liou and S.-C. Tsai are with the Department of Computer Science, National Chiao Tung University, No. 1001, University Road, Hsinchu page the UE. City, Taiwan 300, R.O.C. E-mail: {rhliou, tsaisc}@cs.nctu.edu.tw. . Scheme TT (TA-TAL). When an incoming call . Y.-B. Lin is with the Department of Computer Science, National Chiao arrives, the TA of the last interacted cell is asked to Tung University, No. 1001, University Road, Hsinchu City, Taiwan 300, R.O.C., the Institute of Information Science and the Research Center for page the UE. If fails, all cells in the TAL are asked to Information Technology Innovation, Academia Sinica, Nankang, Taipei, page the UE. Taiwan, R.O.C., and King Saud University. E-mail: [email protected]. . Scheme CTT (Cell-TA-TAL). When an incoming call Manuscript received 8 Aug. 2011; revised 2 Oct. 2011; accepted 11 Nov. 2011; arrives, the MME first asks the last interacted cell to published online 28 Nov. 2011. page the UE. If fails, the TA of the last interacted cell is For information on obtaining reprints of this article, please send e-mail to: asked to page the UE. If fails again, all cells in the TAL [email protected], and reference IEEECS Log Number TMC-2011-08-0447. Digital Object Identifier no. 10.1109/TMC.2011.255. are asked to page the UE.
1536-1233/13/$31.00 ß 2013 IEEE Published by the IEEE CS, CASS, ComSoc, IES, & SPS LIOU ET AL.: AN INVESTIGATION ON LTE MOBILITY MANAGEMENT 167
Fig. 1. LTE mobility management architecture.
Basically, the central policy and the three paging schemes configuration does exist in real environment [9]. We will we described for LTE mobility management partially extend the 1D model to a 2D model through simulation. implement the movement-based and the distance-based Fig. 2 illustrates the TAL configuration where a rectan- location updates [3], [4], [5] with the Shortest-Distance-First gular represents a cell. In this configuration, a TAL contains (SDF) paging [4], [6]. Although these schemes have been NT TAs and each TA covers NC cells. In a TAL, the TAs are intensively studied in the literature, they have not been sequentially labeled from 1 to NT ,andthecellsare exercised in any commercial mobile telecom network sequentially labeled from 1 to NCNT . To simplify our because their implementations are not feasible. Specifically, discussion on the central policy mentioned in Section 1, we in the distance-based location update, the UE is required to assume that NT is an odd number. Following the central have the cell topology information (i.e., the distance relation- policy, the TAL is overlapped with each of its adjacent TALs ship between cells) [3], [7], [8], which can not be practically by NCbcNT =2 cells. Therefore, when the UE leaves the implemented in a real network. In the SDF paging, it is current TAL from Cell NC NT , the entrance cell of the newly difficult to dynamically define the neighboring cells (when assigned TAL is Cell NC bcNT =2 þ 1. Similarly, if the UE the radio coverage changes, the “adjacent cells” may also leaves the current TAL from Cell 1, the entrance cell of the change). Through the predefined TA configuration, LTE can newly assigned TAL is Cell NC ðbcNT =2 þ 1Þ. partially implement the distance-based scheme with the SDF In most commercial 3G mobile telecom networks, the base paging for commercial operation. In this paper, we show that stations are grouped into Location Areas (LAs) [10]. When the LTE mobility management outperforms third-generation (3G) UE moves from old LA to new LA, a location update is mobility management by capturing the advantages of the performed. When an incoming call arrives, all cells in the LA distance-based scheme with the SDF paging. We propose an of the UE will page the UE. The 3G mobility management analytic model to evaluate the performance of the TAL- scheme is a special case of the TAL-based location update based location update with the above three paging schemes. with the TT paging where the size of an LA is the size of a This paper is organized as follows: Section 2 introduces TAL, and NT ¼ 1. the TAL-based location update. Section 3 proposes an analytic model for modeling the TAL-based scheme. 3ANALYTIC MODELING Section 4 investigates the performance of the TAL-based scheme by numerical examples, and the conclusions are This section models the TAL-based location update and the given in Section 5. paging schemes. We first describe the input parameters and output measures. Fig. 3 illustrates the timing diagram for the cell crossings 2 TAL-BASED LOCATION UPDATE and the incoming call arrivals. In this figure, we assume This section describes the TAL-based location update by that the intercall arrival interval tc ¼ 2 1 is an exponen- considering one-dimensional (1D) random walk model for the tial random variable with the mean 1= c. Let tm;i ¼ tiþ1 ti UE movement. This configuration significantly simplifies be the cell residence time between the ði 1Þth cell crossing the description and provides better demonstration. Also, 1D and the ith cell crossing. Assume that tm;i is independent 168 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 1, JANUARY 2013
Fig. 2. The TAL configuration. and identically distributed random variable with the mean where 1= m, the variance V , and the Laplace transform fmðsÞ. N N N T 1 N T 1 When the UE makes cell crossing, the UE moves to the 1 p C ðb 2 cþ Þ 1 p C b 2 cþ 1 p 1 p right-hand side neighboring cell with routing probability p, A ¼ and B ¼ : ð4Þ 1 1 p NC NT þ1 1 1 p NC NT þ1 and moves to the left-hand side neighboring cell with p p probability 1 p. We consider the following three output C C C measures: Now we derive p;CT , p;TT , and p;CTT . Fig. 4 illustrates the state-transition diagram for the TAL-based location . Cu: the expected number of location updates during tc update. In this figure, state j represents that the UE resides . C p;x: the expected number of cells that page the UE in Cell j of the TAL, where 1 j NCNT . For 2 j when an incoming call arrives, where x 2fCT; NC NT 1, the UE moves from state j to state j þ 1 with TT;CTTg probability p, and moves from state j to state j 1 with . C : the expected number of polling cycles [6] before d;x probability 1 p. the UE is found, where x 2fCT;TT;CTTg. Note The state transition from state 1 to state N ðbN =2cþ1Þ that the maximum number of polling cycles for the C T CT, the TT, and the CTT schemes are 2, 2, and 3, represents that the UE leaves the current TAL from Cell 1 respectively. and enters Cell NC ðbNT =2cþ1Þ of the newly assigned TAL. It is clear that the smaller the above output measures, the Similarly, the transition from state NCNT to state better the performance of the location update and paging NC bNT =2cþ1 represents that the UE leaves the current schemes. TAL from Cell NCNT and enters Cell NC bNT =2cþ1 of the We first derive Cu. Let E½M be the expected number of newly assigned TAL. Let i be the steady-state probability cell crossings before the UE leaves the current TAL (i.e., the that the UE resides in Cell i. From Fig. 4, we obtain the expected number of cell crossings between two consecutive following balance equations: location updates). Then, Cu can be computed as E t 8 ½ c m > ð1 pÞ ; for i ¼ 1 Cu ¼ ¼ : ð1Þ > iþ1 E½M E½tm;i cE½M > NT NT > p þð1 pÞ ; for 2 i N ;N > i 1 iþ1 C 2 C 2 In [11], we have derived E½M with the following final > > formats. If p ¼ 0:5, > NT > þ 2 i NC þ 1 1 > 2 > NT NT > E½M ¼NC þ 1 NC þ 1 : ð2Þ <> NT 2 2 and NC þ 1 þ 1 i 2 i ¼ > If p 6¼ 0:5, > NC NT 1 > "#> p þð1 pÞ > i 1 iþ1 N N 1 B N NT 1 > NT A C T þ C 2 > þ p NC NT ; for i ¼ NC þ 1 E½M ¼ > 2 1 þ A B 2p 1 > p i 1 þð1 pÞ iþ1 "#> > NT NT > þð1 pÞ ; for i ¼ N þ 1 1 B NCNT þ 1 A NC þ 1 > 1 C þ 2 ; :> 2 1 þ A B 2p 1 p i 1; for i ¼ NC NT : ð3Þ ð5Þ LIOU ET AL.: AN INVESTIGATION ON LTE MOBILITY MANAGEMENT 169
Fig. 5. State-transition diagram for 1D random walk model with two absorbing states.
and 8 2 39 N < N T þN 2 = 1 p2 p C 2 C ¼ 1 þ 4 15 NC NT 1 p : 2p 1 1 p ; ("# "# Fig. 3. Timing diagram for the cell crossings and the incoming call p NC 1 p 1 ð1 pÞ2 arrivals. 1 þ þ 1 p 2p 1 p pð1 2pÞ 2 39 By rearranging (5), if p ¼ 0:5, i can be rewritten as N 1 N T 1 = 4 1 p C 2 5 8 1 1: > N p ; > T > i 1; for 1 i Nc þ 1 > 2 > ð9Þ > NT NT P > N þ 1 ; for N þ 1 C 1 c Probability can be solved by using C T ¼ 1 and < 2 2 i i¼1 i N (6)-(9). i ¼ T ð6Þ ðkÞ > Nc þ 1 q i > 2 Let i;j be the probability that the UE moves from Cell > > to Cell j after k cell crossings without any location update > NT > NCNT i þ 1 1; for Nc þ 1 2 k k :> these k cell crossings). Let q ð Þ and q ð Þ be the N N : i;0 i;NC NT þ1 C T probabilities that the UE initially stays in Cell i and moves out of the current TAL from Cell 1 and Cell NC NT at If p 6¼ 0:5 and NT ¼ 1, i is rewritten as ðkÞ ðkÞ the kth cell crossing, respectively. Probabilities qi;j , q i;0 , k and q ð Þ are derived as follows. To compute the i ¼ 1; for 1 i NC: ð7Þ i;NC NT þ1 probability that the UE leaves the current TAL at the kth ðkÞ ðkÞ If p 6¼ 0:5 and NT 6¼ 1, i is rewritten as cell crossing (i.e., q and q ), Fig. 5 modifies Fig. 4 by ()"# i;0 i;NC NT þ1 8 adding two absorbing states 0 and N N þ 1 and removing > 2 i 2 C T > 1 p p N N =2 1 > 1 þ 1 1; the transitions from state 1 to state Cðb T cþ Þ and > 1 p 2p 1 1 p > from NCNT to NCbNT =2cþ1. Let qi;j be the one-step > N > T transition probability from state i to state j. Fig. 5 illustrates > for 2 i NC þ 1 > () "# 2 the state-transition diagram, where the transition probabil- > i 2 > 1 p2 p ity matrix Q ¼ðq Þ of the random walk is > 1 þ 1 i;j > 1 0 1 > 1 p 2p 1 1 p > 2 3 1000 000 < N i N T 1 B C p C 2 p B1 p 0 p 0 000C i ¼ 4 15 ; B C > NC NT B 01 p 0 p 000C > 1 p 2p 1 B ...... C > Q B ...... C > ¼B ...... C > N N B C > for N T þ 1 C C @ A > ()"#"# 2 2 0000 1 p 0 p > 2 NC NT i 1 > ð1 pÞ 1 p NC NT 0000 001 > ðNC NT þ2Þ ðNC NT þ2Þ: > 1 þ 1 ; > 1 2p p p > ð10Þ > N :> T k for NC þ 1 i NC NT 1; ðkÞ ð Þ ðkÞ 2 Let Q ¼ðqi;j Þ be the transition matrix. From (10),Q Q is computed by the matrix multiplication QðkÞ ¼ k Q. ð8Þ i¼1 qðkÞ q ðkÞ q ðkÞ Based on i;j , i;0 and i;NC NT þ1 are computed as
Fig. 4. State-transition diagram for TAL-based location update. 170 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 1, JANUARY 2013 q ; for k 1 ðkÞ i;0 ¼ paper to investigate the impact of variance for cell residence q i;0 ¼ ðkÞ ðk 1Þ ð11Þ qi;0 qi;0 ; for k>1 times. From (15), (14) is rewritten as 8 and 1 > 2 ( <> 1 m 1 1 V m ; for k ¼ 0 c V m cþ1 qi;N NT þ1; for k ¼ 1 ðkÞ C ðkÞ¼ 1 2 k 1 q ¼ ðkÞ ðk 1Þ ð12Þ > 2 2 i;NC NT þ1 : m V m 1 V m qi;N N þ1 qi;N N þ1; for k>1: 1 ; for k>0: C T C T c 1V m cþ1 V m cþ1 Given that the UE resides in Cell i when the previous ð16Þ incoming call arrives (i.e., Cell i is an “interacted cell”; see 1 ðk;nÞ in Fig. 3), let i be the probability that after k cell Let C be the probability that the UE resides in the last crossings and n location updates, the UE moves back to the interacted cell when an incoming call arrives. From (6)-(9), last interacted cell. If n ¼ 0, then the last interacted cell is (13) and (16), C is expressed as Cell i.Ifn 1, the last interacted cell is the cell where the UE ðk;nÞ X1 NXC NT Xk performs the nth location update. From (11) and (12), is i ¼ ðkÞ ðk;nÞ: ð17Þ computed as C i i k¼0 i¼1 n¼0 8 > 1; for k ¼ 0 and n ¼ 0 > ðkÞ In the right-hand side of (17), the UE makes k cell crossings > qi;i ; for k>0 and n ¼ 0 > during tc with probability ðkÞ (0 k 1) and resides in <> Xk ðÞk j;n 1 ðjÞ Cell i with probability i (1 i NCNT ) when the previous ðk;nÞ N q i;N N þ1 ¼ N T þ1 C T ð13Þ i > j¼1 C 2 incoming call arrives. Therefore, C is the summation of the > ðk;nÞ > k j;n 1 j product ðkÞ over all possible ðk; i; nÞ pairs (i.e., 0 > þ ðÞ q ð Þ ; for k n 1 i i > NT i;0 : NC 2 þ1 n k 1and 1 i NC NT ). 0; for k
1 2 1 V m Cp;TT ¼ T NC þ 1 T NC þ NC NT ; ð21Þ fmðsÞ¼ : ð15Þ V ms þ 1 and We consider the Gamma distribution because it has been 8 shown that the distribution of any positive random variable < Cp;CT ; for NC ¼ 1 can be approximated by a mixture of Gamma distributions Cp;CTT ¼ C þð T CÞð1 þ NCÞ ð22Þ [12]. The Gamma distribution was used to model UE : þð1 T Þð1 þ NC þ NCNT Þ; for NC 6¼ 1: movement in many studies [5], [6], [9] and is used in this LIOU ET AL.: AN INVESTIGATION ON LTE MOBILITY MANAGEMENT 171
2 2 Fig. 6. Performance results for 1D (p ¼ 0:5 and NC NT ¼ 45) and 2D configurations (p ¼ 0:25 and NC NT ¼ 45 ), where m= c ¼ 5 and V ¼ 1= m.
Similar to the derivations for Cp;x, Cd;x is computed as CTT schemes, the last interacted cell may not be the center cell of the TA. Cd;CT ¼ C þ 2ð1 C Þ¼2 C; ð23Þ . The previous analytic models [5], [6] assume that the UE moves from a cell to one of the neighboring cells Cd;TT ¼ T þ 2ð1 T Þ¼2 T ; ð24Þ with the same probability. In our model, the UE can move to each of its neighboring cells with different and probabilities. 8 < Cd;CT ; for NC ¼ 1 Our analytic model is used to validate the discrete event simulation model (reviewers, please see the supplementary Cd;CTT ¼ : C þ 2ð T CÞþ3ð1 T Þ; ð25Þ ¼ 3 C T ; for NC 6¼ 1: document, which can be found on the Computer Society Digital Library at http://doi.ieeecomputersociety.org/ The major differences between our analytic model and the 10.1109/TMC.2011.255). Simulation experiments show that previous models for the movement-based and the distance- the discrepancies between the analytic (i.e., (1), (20)-(25)) and based schemes [5], [6] are described as follows: simulation results are within 1 percent. After the simulation . In the movement-based and the distance-based of 1D cell configuration has been validated by the analytic schemes, a location update or an incoming call resets model, the validated simulation flowchart is extended to the center cell of the residing area [6] to be the last accommodate the 2D mesh cell configuration (i.e., Manhat- interacted cell. On the other hand, the TAL-based tan-street layout). location update resets the central TA of the TAL to be the TA of the last interacted cell (i.e., the last 4NUMERICAL EXAMPLES interacted cell may not be the center cell of the TAL). Therefore, our analytic model is more compli- This section investigates the performance of the TAL- cated than those for the movement-based and the based location update and paging schemes. We first point distance-based location updates. out that the performance results for the 1D configuration . In the SDF paging, the last interacted cell is the center are consistent with those for the 2D configuration. For x 2 cell of the subareas (i.e., the TA) [6]. In the TT and the fCT;TT;CTTg, Fig. 6a plots Cp;x for the 1D configuration, 172 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 1, JANUARY 2013
2 Fig. 8. Effects of m= c and p on Cp;x (NC ¼ 9, NT ¼ 5 and V ¼ 1= m). Fig. 7. Effects of p, m= c, and NC on Cu (NC NT ¼ 45). boundary cell of the new TAL. Now consider p ¼ 0:5.If and Fig. 6b plots Cp;x for the 2D configuration. Figs. 6c and NC ¼ 1, then it is unlikely that the UE will move out of the 6d plot Cu and Cd;x for both the 1D and the 2D TAL in first few cell crossings. On the other hand, if configurations, respectively. In the 1D configuration, we NC ¼ 45, then the UE will move out of the TAL at the first consider the TAL size NC NT ¼ 45 and p ¼ 0:5. In the 2D cell crossing with probability 0.5. In other words, for a fixed mesh configuration, we consider a 45 45 cell structure, NC NT value, if p is small, E½M decreases as NC increases. and the UE moves to one of the four neighboring cells For example, for NCNT ¼ 45 and p ¼ 0:5, (2) indicates that with the same routing probabilities 0.25. This figure shows E½M ¼529 for NC ¼ 1 and E½M ¼45 for NC ¼ 45. Since that the trends of Cp;x, Cu, and Cd;x are similar for both the Cu is inversely proportional to E½M (see (1)), for a fixed 1D and the 2D models. Therefore, our observations on the NC NT value, Cu increases as NC increases for a small p (the 1D model are also valid for the 2D model. For other input solid curves in Fig. 7). parameter setups, same results are observed, and the On the other hand, for a fixed N N value, if p is large, details are omitted. C T E½M increases as N increases. In the extreme case, when The remainder of this section shows the results for the C p ¼ 1, (3) is rewritten as E½M ¼N NT .IfN N ¼ 45,we 1D configuration because several nontrivial observations C 2 C T E½M ¼23 N ¼ 1 E½M ¼45 N ¼ 45 can be easily explained through the analytic model of this have for C and for C . p C N simple configuration. Therefore, for a large , u decreases as C increases (the dashed curves in Fig. 7). 4.1 Analysis of the Cu Performance As mentioned in Section 2, the 3G location update is a In the 1D configuration, it suffices to consider 0:5 p 1 special case of the TAL-based scheme where NT ¼ 1 (i.e., in our study. In practice, a small p (e.g., p ¼ 0:5; solid lines NC ¼ 45; see the 4 symbol in Fig. 7). When p ¼ 0:5, the in Fig. 7) represents the movement of a pedestrian or a TAL-based scheme with NT 6¼ 1 can reduce about 90 percent vehicle in local roads, which exhibits locality. A large p of the 3G location update cost. When p ¼ 0:85, the TAL- (e.g., p ¼ 0:85; dashed lines in Fig. 7) represents the based scheme with NT 6¼ 1 incurs extra 4-36 percent cost movement of a vehicle in highways. When p increases, over the 3G location update. the UE tends to move to one direction. Therefore, Cu 4.2 Analysis of the Cp;x Performance increases as p increases. In Fig. 7, the values of the dashed = p V N curves (p ¼ 0:85) are higher than those of the solid curves This section investigates the effects of m c, , , and C C (p ¼ 0:5). on p;x. When = increases (i.e., more cell crossings during m c . Effects of m= c: For all x 2fCT;TT;CTTg, Fig. 8 t C c), more location updates are expected (i.e., u increases indicates that Cp;x increase as m= c increases. When = as m c increases). In Fig. 7, the values of the curves m= c increases, it is more likely that the UE is far ( m= c ¼ 50)arehigherthanthoseofthe curves away from the last interacted cell when an incoming = 5 ( m c ¼ ). call arrives, and thus higher Cp;x are expected. When The effect of NC on Cu can be explained as follows. When m= c is small, the low-mobility UE is more likely to NC ¼ 1, the entrance cell of the UE is the center cell of the be found in the last interacted cell, and there is no new TAL. On the other hand, when NC ¼ 45 (i.e., NT ¼ 1 or need to page the TA. In this case, we observe that the TAL only has one TA), the entrance cell of the UE is the Cp;CTT Fig. 9. Effects of V and p on Cp;x (NC ¼ 9 and NT ¼ 5). m= c is large, the UE is unlikely to reside in the last Due to Fact 1, when V is small and m= c > 1, the interacted cell, and paging the last interacted cell UE is unlikely to be found in the last interacted cell, 2 incurs extra paging cost. Therefore, Cp;CTT is slightly and Cp;CT >Cp;TT (in Fig. 9a, for V 1= m, the larger than Cp;TT . The effect of m= c on Cp;CT is more values of the curves are higher than those of significant than that on Cp;CTT , and than that on Cp;TT . the curves). On the other hand, Cp;CT . Effects of V : For a fixed m value, we have the The effect of V on Cp;CT is more significant than that following facts about the variance V of the cell on Cp;CTT , and than that on Cp;TT . In summary, when V is small, Cp;CTT Fig. 10. Effects of NC and p on Cp;x (NC NT ¼ 45). indicates that when V is large and m= c < 1, Cp;TT lowest Cp;x value can be found in the CTT scheme increases as NC increases. In this case, the UE is when 5 NC 15 (or 3 NT 9). Note that when more likely to be found in the last interacted cell NC ¼ 1, Cp;CT ¼ Cp;TT ¼ Cp;CTT because a TA only (Fact 2), and it is a waste to page the TA (same result contains one cell and paging the TA is the same as is observed for large V with m= c > 1). This extra paging the cell (see (23)). On the other hand, when cost becomes significant as the TA size is large (i.e., NC ¼ 45 (i.e., NT ¼ 1), a TAL only contains one TA, Cp;TT increases as NC increases). As we previously and Cp;CT ¼ Cp;CTT . stated, the CTT scheme takes advantages of the CT As mentioned in Section 2, the 3G paging is a special case and the TT schemes to yield the best performance of the TT scheme where NT ¼ 1 (i.e., NC ¼ 45; see the 4 for all NC values. This figure also indicates that the symbol in Fig. 10). Fig. 10 indicates that the TAL-based Fig. 11. Effects of m= c, p, and V on Cd;x ( NC ¼ 9 and NT ¼ 5). LIOU ET AL.: AN INVESTIGATION ON LTE MOBILITY MANAGEMENT 175 paging schemes can reduce 21-97 percent of the 3G paging ACKNOWLEDGMENTS cost. For all input parameter setups under our study, the This work was supported in part by NSC 100-2221-E-009- TAL-based paging schemes always outperform the 3G 070, Chunghwa Telecom, IBM, Arcadyan Technology paging in terms of the C performance. p;x Corporation, ICL/ITRI, Nokia Siemens Networks, and the 4.3 Analysis of the Cd;x Performance MoE ATU plan. Fig. 11 plots Cd;x against m= c, p, and V . The effects of m= c, p, and V on Cd;x are similar to those on Cp;x described in REFERENCES Section 4.2, and the details are omitted. It is clear that [1] 3GPP, “General Packet Radio Service (GPRS) Enhancements for Cd;TT Cd;CT Cd;CTT . As stated in Section 2, the 3G paging Evolved Universal Terrestrial Radio Access Network (E-UTRAN) simultaneously asks all cells in the LA to page the UE, and Access,” Technical Specification 3G TS 23.401, version 10.0.0 (2010-06), 2010. thus the polling cycle is 1. The TAL-based paging schemes [2] 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and incur extra 0.01-1.08 polling cycles over the 3G paging for Evolved Universal Terrestrial Radio Access Network (E-UTRAN),” high mobility (Fig. 11a), and incur extra 0.01-0.21 polling Technical Specification 3G TS 36.300, version 10.1.0 (2010-09), 2010. [3] cycle over the 3G paging for low mobility (Fig. 11b). A. Bar-Noy, I. Kessler, and M. Sidi, “Mobile Users: to Update or Not to Update?” Wireless Networks, vol. 1, no. 2, pp. 175-185, 1995. [4] J.S.M. Ho and I.F. Akyildiz, “Mobile User Location Update and Paging Under Delay Constraints,” Wireless Networks, vol. 1, no. 4, 5CONCLUSIONS pp. 413-425, 1995. This paper investigated the performance of LTE mobility [5] R. Chen, S. Yuan, and J. Zhu, “A Dynamic Location Management Method of Personal Communication System,” Proc. E-Tech, pp. 1- management. We considered the central policy for location 9, July 2004. update and three paging schemes based on the concept of [6] I.F. Akyildiz, J.S.M. Ho, and Y.-B. Lin, “Movement-Based Location TA and TAL. Our study indicates the following results: Update and Selective Paging for PCS Networks,” IEEE/ACM Trans. Networking, vol. 4, no. 4, pp. 629-638, Aug. 1996. . [7] V.W.-S. Wong and V.C.M. Leung, “Location Management for The central-based LTE location update outperforms Next-Generation Personal Communications Networks,” IEEE the 3G location update by 90 percent for p ¼ 0:5 Network, vol. 14, no. 5, pp. 18-24, Sept./Oct. 2000. (i.e., when the user movement exhibits locality). On [8] I.F. Akyildiz, J. McNair, J.S.M. Ho, H. Uzunalioglu, and W. Wang, the other hand, the LTE scheme incurs extra 4-36 “Mobility Management in Next-Generation Wireless Systems,” Proc. IEEE, vol. 87, no. 8, pp. 1347-1384, Aug. 1999. percent cost over the 3G location update for p ¼ 0:85 [9] Y.-B. Lin and Y.-C. Lin, “WiMAX Location Update for Vehicle (i.e., when the user tends to move to one direction). Applications,” Mobile Networks and Applications, vol. 15, no. 1, We note that when p 0:75, the TAL-based scheme pp. 148-159, Feb. 2010. always outperforms the 3G location update. [10] Y.-B. Lin and A.-C. Pang, Wireless and Mobile All-IP Networks. John Wiley & Sons., 2005. . The TAL-based paging schemes outperform the 3G [11] S.-R. Yang, Y.-C. Lin, and Y.-B. Lin, “Performance of Mobile paging by 21-97 percent in terms of the number of Telecommunications Network with Overlapping Location Area paged cells. Among the TAL-based paging schemes, Configuration,” IEEE Trans. Vehicular Technology, vol. 57, no. 2, pp. 1285-1292, Mar. 2008. the TT scheme outperforms the CT scheme when the [12] F.P. Kelly, Reversibility and Stochastic Networks. Wiley, 1979. variance V of the cell residence time is small (i.e., the [13] Y.-B. Lin, “Per-User Checkpointing for Mobility Database Failure user movement pattern is regular) and the UE moves Restoration,” IEEE Trans. Mobile Computing, vol. 4, no. 2, pp. 189- 194, Mar./Apr. 2005. frequently (i.e., m= c is large). The CT scheme [14] S.-R. Yang, S.-Y. Yan, and H.-N. Hung, “Modeling UMTS Power outperforms the TT scheme when V is large or m= c Saving with Bursty Packet Data Traffic,” IEEE Trans. Mobile is small. The CTT scheme takes advantages of both Computing, vol. 6, no. 12, pp. 1398-1409, Dec. 2007. the CT and the TT schemes, which has the best [15] D.-W. Huang, P. Lin, and C.-H. Gan, “Design and Performance Study for a Mobility Management Mechanism (WMM) Using performance in most input parameter setups under Location Cache for Wireless Mesh Networks,” IEEE Trans. Mobile our study. Computing, vol. 7, no. 5, pp. 546-556, May 2008. . For the number of polling cycles, the TAL-based [16] N. Nasser, A. Hasswa, and H. Hassanein, “Handoffs in Fourth paging schemes incur extra 0.01-1.08 polling cycles Generation Heterogeneous Networks,” IEEE Comm. Magazine, vol. 44, no. 10, pp. 96-103, Oct. 2006. over the 3G paging. Among the TAL-based paging schemes, the TT scheme outperforms the CT and the Ren-Huang Liou received the BS and MS CTT schemes. degrees in computer science from National Chiao Tung University (NCTU), Hsinchu, Tai- In summary, if network signaling costs for location update wan, R.O.C., in 2007 and 2009, respectively. He and paging are major concern, then the CTT scheme should is currently working toward the PhD degree at be selected. If the number of polling cycles is major concern, NCTU. His current research interests include Voice over Internet Protocol (VoIP), mobile then the existing 3G mobility management should be computing, and performance modeling. selected for the users with high mobility and regular movement patterns (i.e., with small V ). For low mobility users, the extra overheads incurred by the LTE paging can be ignored. Finally, the central-based LTE mobility manage- ment creates several interesting research issues. In the future, we will investigate the MME failure restoration based on dynamic TAL assignment [13], power saving [14], mobility management for mesh-mode LTE [15], handoff in hetero- geneous network [16], and so on. 176 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 1, JANUARY 2013 Yi-Bing Lin is a vice president and life chair Shang-Chih Tsai received the BSCS degree professor in the College of Computer Science, from National Chiao Tung University (NCTU), National Chiao Tung University (NCTU), and a Hsinchu, Taiwan, R.O.C., in 2008. He is currently visiting professor of King Saud University. He is working toward the MSCS degree at NCTU. His also with the Institute of Information Science and current research interests include design and the Research Center for Information Technology analysis of a personal communications services Innovation, Academia Sinica, Nankang, Taipei, network and performance modeling. Taiwan, R.O.C. He is the author of the books Wireless and Mobile Network Architecture (Wi- ley, 2001), Wireless and Mobile All-IP Networks (John Wiley, 2005), and Charging for Mobile All-IP Telecommunications (Wiley, 2008). He has received numerous research awards including the 2005 NSC distinguished researcher award and the 2006 Academic . For more information on this or any other computing topic, Award of the Ministry of Education. He is a fellow of the IEEE, ACM, please visit our Digital Library at www.computer.org/publications/dlib. AAAS, and IET.