Abstract—In Long Term Evolution (LTE) networks, Mobility

Robustness Optimization (MRO) and Mobility Load Balancing (MLB) are two important functions to auto-optimize the network performances. There is a close correlation between them, as they both choose adjusting handover parameters as optimization actions. The conflict may occur between the two functions when they adjust the same handover parameter in opposite directions. This can not improve the performances but waste network resources. In this paper, we present a novel scheme to solve the problem. In order to prevent the occurrence of the conflict, we set an allowed range for MLB in which the handover problems can be prevented. Analyses and simulation results demonstrate that the proposed scheme can effectively solve the conflict problem and improve the performances of both functions.

I. INTRODUCTION In recent years, with the rapid evolution of wireless

communication networks, handover parameters have become more complex and larger, manual setting of these parameters is a time-consuming task and parameter updating after the initial deployment is also considered too costly. To reduce the operational complexity in wireless communication networks, Self-Organizing Network (SON) is seen as the promising area to simplify the management by auto-optimization [1]. There- fore, SON is introduced to Long Term Evolution (LTE) and currently discussed in 3rd Generation Partnership Project (3GPP) standardization [1] [2].

Mobility Robustness Optimization (MRO) and Mobility Load Balancing (MLB) are two important functions in SON. MRO aims to minimize handover problems [2] [3] [4], while MLB aims to cope with the unequal traffic load between cells [2] [5]. Although the two functions operate independently with each other, there is a close correlation between them, as they both choose adjusting handover parameters as optimization actions. It’s possible that the conflict occurs when they adjust the same handover parameter in opposite directions, e.g., handover parameter is adjusted by MLB to balance the load between cells and then changed back by MRO to minimize the handover problems [6]. The conflict may lead to closed loop between the two functions, which will decrease the network performances seriously.

To the best of our knowledge, the conflict problem has only been discussed in 3GPP standardization. In order to solve the problem, it is proposed that MLB should get the high priority and MRO should be suspended for a while during the operation of MLB [6]. However, the proposal sacrifices the performance of MRO largely. This will bring about handover

problems which can decrease user’s QoS. Furthermore, the proposal is in contradiction to the specification, as the specification declares that the load balancing shall not affect the user’s QoS negatively beyond what a user would experience at normal mobility without load-balancing [2].

In this paper, we present a novel scheme to solve the conflict problem. We notice that the reason of the conflict is the inappropriate operation of MLB. If MLB adjusts handover parameters in an incorrect way, handover problems will occur and the conflict problem will follow on. In the proposed scheme, we set an allowed range for MLB to make sure that MLB operates correctly. Because of the restriction on MLB by the allowed range, handover problems are prevented and the conflict is avoided. The rest of this paper is organized as follows: Section II describes the operational principles of MRO and MLB, and explains the reason of the conflict. Section III presents the proposed scheme to solve the problem. Section IV evaluates the performances by system level simulation and Section V concludes the paper.

II. DESCRIPTION OF THE CONFLICT In this section, at first we introduce the handover triggering

procedure which is closely related with MRO and MLB, then we describe the operational principles of the two functions. At last, we explain the reason of the conflict.

A. Handover triggering procedure in LTE The handover triggering procedure in LTE is illustrated in

Fig.1. The x-axis represents the position of User Equipment (UE) and the y-axis represents the signal strength received by UE from E-UTRAN NodeB (eNB) in its cell. While a UE moves from cell1 to cell2, it measures the signal strengths of the two cells and checks whether the signal strengths satisfy the entering condition (1) for event A3 of cell1[7]. If the entering condition is satisfied and lasts for duration of Time To Trigger (TTT), UE will send a measurement report to eNB1. After that, a handover procedure from eNB1 to eNB2 will be initiated on UE.

)( ,21112 CIOHMM −+> (1) Where: - M2: measurement result of cell2; - M1: measurement result of cell1; - H1: hysteresis parameter of cell1for event A3; - CIO1,2: cell specific offset set by cell1 for cell2.

Conflict Avoidance between Mobility Robustness Optimization and Mobility Load Balancing

Zhiqiang Liu, Peilin Hong, Kaiping Xue, Min Peng The Information Network Lab of EEIS Department,

University of Science and Technology of China (USTC), Hefei, China, 230027

liuzq@mail.ustc.edu.cn

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.

B. Operational principle of MRO Incorrect handover parameter settings will bring about

handover problems, such as Radio Link Failure (RLF) and ping-pong handover. RLF affects user’s experience seriously since it causes the detachment of UE from the serving eNB. The immediate cause of RLF is that the signal strength of UE’s serving cell is too low and the interference is too high [8]. Inappropriate handover triggering will lead to the occurrence of RLF. If handover is triggered too early, as the signal strength of the target cell is too low, RLF will occur shortly after the handover procedure. If handover is triggered too late, as the signal strength of the source cell is already too low, RLF will occur before the handover is initiated or during the handover procedure [2]. Ping-pong handover does not affect use’s QoS, but it wastes the network resources. As illustrated in Fig.2, H2 is the hysteresis parameter of cell2 and CIO2,1 is the cell specific offset set by cell2 for cell1. The incorrect handover parameter settings cause the overlapped area, in which the entering conditions for event A3 of the two cells are both satisfied. When a UE goes through that area, ping-pong handovers will occur between the two cells [7].

MRO can detect the handover problems in the cell through information gathering on UEs [2]. Its objective is to minimize RLFs and ping-pong handovers by auto-optimizing the handover parameters (H, CIO, TTT). If RLF by too early handover is detected, MRO will postpone the handover triggering procedure; on the contrary, if RLF by too late handover is detected, MRO will make the handover triggering earlier. Ping-pong handover can also be solved by MRO, the solution is to adjust handover parameters to satisfy (2). After that, the overlapped area as shown in Fig.2 is eliminated and ping-pong handovers are avoided.

0CIOHCIOH 122211 >−+− )()( ,, (2)

C. Operational principle of MLB MLB aims to cope with the unequal traffic load between cells. It monitors the load periodically and adjusts handover parameter CIO when the traffic load is unbalanced [2]. Here is an example of MLB. We assume that cell1 has heavy load and its neighbour cell2 has light load, the MLB function of cell1 starts up and chooses cell2 to balance. In the operation procedure, eNB1 increases CIO1,2 to make the handover triggering earlier, therefore UEs moving from cell1 to cell2 will handover out of cell1 earlier and then the load in cell1

will be cut down. Meanwhile, eNB1 informs eNB2 to adjust CIO2,1 lower, therefore the handovers from cell2 to cell1 will be postponed and the increase of the additional load in cell1 will be slowed down.

D. Reason of the conflict between MRO and MLB In the original scheme, although MRO and MLB operate

independently, they are closely related to each other. The two functions both optimize network performances through adjusting the handover parameters, therefore conflict may occur when they adjust the same parameter in opposite directions. Here is an example as shown in Fig.3. For the purpose of load balancing in cell1, MLB increases CIO1,2 to move UEs out of cell1 earlier. Unfortunately, the value of entering condition for event A3 of cell1 is changed too low, and RLF by too early handover occurs after the operation of MLB. MRO detects the RLF problem and adjusts CIO1,2 lower to postpone the handover triggering procedure. However, after a while, MLB changes it back once more. The conflict occurs and leads to closed loop, another kind of “ping-pong”. Similarly, the inappropriate operation of MLB may cause ping-pong handover or RLF by too late handover, which can also lead to the occurrence of the conflict.

The conflict problem between MRO and MLB is urgent to be solved as it decreases the performances of both functions. On one hand, handover problems are brought during the conflict. This will affect user’s experiences and waste network resources. On the other hand, because of the “ping-pong” effect between the two functions, the operation of MLB loses its efficiency. This may lead to the long duration of heavy load conditions, which will bring about more blocking calls and dropping calls.

Fig.1. The entering condition for event A3 is satisfied at point A; UE sends a measurement report to trigger handover implement at point B; the handover procedure accomplishes at point C.

Fig.2. In the overlapped area, ping-pong handovers will occur between cell1 and cell2 when a UE goes through.

Fig.3. Conflict due to inappropriate operation of MLB

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.

III. THE PROPOSED SCHEME In this section, we present a simple but effective scheme to

solve the conflict. The scheme we describe is based on the restriction on the operation of MLB. In addition, a novel MLB procedure is designed to bring the proposed scheme into effect. We then describe the principle of the proposed scheme and the designed MLB procedure.

A. Principle of the proposed scheme As described in section II, if MLB operates inappropriately, handover problems will occur and the conflict problem will follow on. Therefore the key to solving the conflict is to prevent MLB from adjusting handover parameters in an incorrect way. In other words, the conflict will not occur as long as the operation of MLB causes no handover problems.

The proposed scheme is based on the restriction on the operation of MLB. In order to avoid the conflict, we set an allowed range for MLB to prevent handover problems during its operation. The proposed scheme requires MRO to record the minimum value and maximum value of entering condition for event A3 than cannot cause RLFs while MRO deals with RLF problems. Besides, in order to get the allowed range, an inquiry procedure for neighbor’s handover parameters is also required.

As illustrated in Fig.4 (a), (H1 - CIO1,2,early) is the minimum value and (H1 - CIO1,2,late) is the maximum value of entering condition for event A3 that cannot cause RLFs in cell1. While UEs move from cell1 to cell2, RLFs by too early or too late handover will occur if (H1 - CIO1,2) is set outside the shaded area. Similarly, as shown in Fig.4 (b), (H2 - CIO2,1,early) is the minimum value and (H2 - CIO2,1,late) is the maximum value that cannot cause RLFs in cell2. UEs moving from cell2 to cell1 will handover successfully if (H2 - CIO2,1) is set inside the shaded area of cell2. We assume that the MLB function of cell1 intends to balance load to cell2, eNB1 increases CIO1,2 and informs eNB2 to decrease CIO2,1. As shown in Fig.4 (a)-(b), in order to prevent the RLF problems, (H1 - CIO1,2) and (H2 - CIO2,1) should be changed inside the shaded areas during the operation of MLB. Besides, the entering conditions for event A3 of the two cells should satisfy (2) to prohibit the occurrence of ping-pong handovers. To assume that CIO1,2 and CIO2,1 are adjusted to CIO1,2' and CIO2,1' respectively after the MLB procedure, the restriction on the handover parameters during the operation of MLB can be summarized as (3).

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(3)

The purpose of the first inequality in (3) is to prevent RLFs by too early handover while UEs move from cell1 to cell2; the second inequality is to prevent RLFs by too late handover while UEs move from cell2 to cell1; the last one is to avoid ping-pong handovers between the two cells. Furthermore, we can get the allowed range of CIO1,2' and CIO2,1' presented as (4) which is transformed from (3). The allowed range is calculated by eNB1 after the inquiry for cell2’s handover parameters (H2, CIO2,1, CIO2,1,late). During the operation of MLB, the adjustment on CIO1,2' and CIO2,1' is restricted in the allowed range to prohibit the occurrence of the conflict.

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B. MLB procedure in the proposed scheme In order to bring the proposed scheme into effect, a novel MLB procedure is designed as shown in Fig.5. The procedure can be described as follows: 1) eNB1 inquires about the handover parameters of cell2

( H2, CIO2,1, CIO2,1,late); 2) eNB2 responses to the inquiry; 3) eNB1 calculates the allowed range from the handover

parameters of the two cells(H1, CIO1,2, CIO1,2,early, H2, CIO2,1, CIO2,1,late), and works out the optimum values of CIO1,2' and CIO2,1';

4) eNB1 requests eNB2 to adjust CIO2,1; 5) eNB2 accepts the request and adjusts CIO2,1 to CIO2,1'; 6) eNB1 adjusts CIO1,2 to CIO1,2'.

In the procedure, the operation of MLB is restricted in the allowed range, therefore the handover problems are prevented and the conflict is avoided.

(a). If (H1 - CIO1,2) is set inside the shaded area of cell1, RLFs by too early handover and too late handover will be prevented while UEs move from cell1 to cell2

(b). If (H2 - CIO2,1) is set inside the shaded area of cell2, RLFs by too early handover and too late handover will be prevented while UEs move from cell2 to cell1

Fig.4. The entering conditions for event A3should be set inside the shaded areas during the operation of MLB

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IV. PERFORMANCE EVALUATION In this section, we evaluate the proposed scheme by

dynamic system level simulation including MRO, MLB and handover functions. In the simulation topology, there are 61 cells and wrap-around technique is used to avoid the edge effect. The simulation is driven in the Manhattan scenario [9]. UEs move in the street represented by the bold lines as shown in Fig.6. UEs’ probability of turning in the crossings is 0.5. The average call length of UEs is 20s, and after the idle state of 20s’ duration, UEs will connect to the network once more. Call admission control adopts Queueing Priority (QP) scheme, i.e. calls are accepted whenever there are free channels [10]. Other simulation parameters can be found in table I. RLF rate and ping-pong rate are used to evaluate the performance of MRO. Meanwhile, call blocking rate and call dropping rate are used to evaluate the performance of MLB.

A. Operations of MRO and MLB In the simulation, MRO and MLB both choose adjusting CIO as optimization actions. The method to detect RLFs and ping-pong handovers refers to [8] and [9]. After detecting 3 ping-pong handovers within 50s, or 2 RLFs by too early / too late handover within 200s in its cell, MRO begins operating. The traffic load in each cell is measured once 5s. MLB starts up when the resource usage of its cell reaches 90%, and it only choose the neighbor cell whose resource usage is lower than 80% to balance the load. If a cell accepts one neighbor’s request of MLB, it will reject other neighbors’ requests.

B. Simulation results This section presents a comparison between the original scheme and the proposed scheme. In the original scheme, MLB operates without restriction; in the proposed scheme, the operation of MLB is restricted in the allowed range to avoid the conflict problem.

Fig.7 shows the RLF rate and Fig.8 shows the ping-pong rate varying the number of UEs in the simulation topology. It is obvious to view that the proportions of handover problems in the proposed scheme are lower than that in the original scheme. Especially in 3kmph case, the ping-pong rate is much reduced in the proposed scheme, which is kept below 1%. However, the rate in the original scheme is above 15%. This is due to the fact that in the proposed scheme, with the restriction on MLB, the conflict is avoided and the handover problems

are prevented during the operation of MLB; in the original scheme, MLB operates without restriction, conflict occurs frequently and unnecessary handover problems are brought. Besides, it can be noted that with the growth of the number of UEs in the simulation topology, the proportions of handover problems rise in both schemes. The reason is that larger number of UEs causes more heavy load conditions and more operations related to load balancing in cells, therefore the probability of the conflict increases and more handover problems are brought.

As illustrated in Fig.9 and Fig.10, call blocking rate and call dropping rate have higher percentages in the original scheme than that in the proposed scheme, especially in the large number of UEs conditions. There are two reasons for this. One reason is that in the original scheme, inappropriate operation of MLB brings a lot of ping-pong handovers which waste the network resources of the adjacent cells seriously. For another reason, due to the “ping-pong” effect between MRO and MLB, the operation of MLB loses its efficiency. Therefore, the heavy load conditions last longer and more blocking calls

Inquire the handover parameters

Response(H2, CIO2,1, CIO2,1,late)

Calculate the allowed range and work out the optimum new settings

Request eNB2 to adjust (CIO2,1')

Adjust CIO1,2

eNB1 eNB2

Accept the request

Adjust CIO2,1

Fig.5. MLB procedure in the proposed scheme

Fig.6. Simulation topology

TABLE I

SIMULATION PARAMETERS

Parameter Assumption Cell layout Hexagonal grid, 61cell sites

Inter-site distance 1732m

Carrier frequency 2GHz

Bandwidth 5MHz

eNB power 43dBm

Distance-dependent path loss 128.1 + 37.6log10(R), R in km

Shadowing standard deviation 10dB

Correlation distance of Shadowing 10m

Time to Triger (TTT) 100ms

Hysteresis parameter (H) 3dB

Initial cell specific offset (CIO) configuration 0dB

Number of UEs 700~1300

Traffic on UE 144kbps, real-time

UE speeds 3kmph, 30kmph

Simulation time 2000s

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and dropping calls are brought. Besides, it can be deduced that more operations of MLB are needed in the original scheme. At last, in order to prove our analyses, we present the number of operations of MLB in both schemes as shown in Fig.11. It is obvious to view that with the growth of number of UEs in the simulation topology, more operations of MLB are needed. Besides, it can be noted that there are more operations of MLB in the original scheme than that in proposed the scheme. These phenomena coincide with our analyses above.

In sum, in contrast with the original scheme, the proposed scheme can achieve better performances of both functions.

V. CONCLUSION In this paper, we have proposed a novel scheme to solve the

conflict problem between MRO and MLB. We noticed that reason of the conflict is the inappropriate operation of MLB, therefore we set an allowed range for MLB to prevent the handover problems. In order to bring the proposed scheme into effect, we have designed a novel MLB procedure. Simulation has been done to evaluate the performances. The simulation results have demonstrated that the conflict can be avoided effectively in the proposed scheme.

VI. ACKNOWLEDGEMENT This work is supported by the National Natural Science

Foundation of China under Grant No. 60903216, 60772033, National High-Tech Key program of China under Grant No. 2009AA012002 and the National S&T Major Project of China under Grant No. 2008ZX03003-005.

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Long Term Evolution”, Nomor Research GmbH, Munich, Germany, May 2008.

[2] 3GPP standardization, “Self-configuring and self-optimizing network use cases and solutions (Release 9)”, TR 36.902 v1.2.0, May 2009. http://www.3gpp.org/

[3] André Schröder, Henrik Lundqvist, and Giorgio Nunzi, “Distributed Self-Optimization of Handover for the Long Term Evolution”, Proceedings of the 3rd International Workshop on Self-Organizing Systems, Vienna, Austria, 2008, pp.281-286.

[4] M. I. Tiwana, B. Sayrac, and Z. Altman, “Statistical Learning for Automated RRM: Application to eUTRAN Mobility”, IEEE ICC 2009, Dresden, Germany, 2009, pp.1 - 5.

[5] Ridha Nasri, Zwi Altman, “Handover Adaptation for Dynamic Load Balancing in 3GPP Long Term Evolution Systems”, Conf. on Advanced in Mobile Computing Multimedia (MoMM2007), Jakarta, Indonesia, 2007, pp.145-154.

[6] CMCC, “Dependencies among SON use cases and CCO priority”, San Francisco, US, 3GPP TSG RAN WG3 Meeting #64, R3-091032, May 2009. http://www.3gpp.org/

[7] Alcatel-Lucent, “Exchange of handover parameters directly between eNBs”, San Francisco, US, 3GPP TSG RAN WG3 Meeting #64, R3-091294, May 2009. http://www.3gpp.org/

[8] NTT DOCOMO, INC., “Evaluation model for Rel-8 mobility performance”, Seoul, Korea, 3GPP TSG RAN WG1 Meeting #56bis, R1-091578, March 2009. http://www.3gpp.org/

[9] Nokia, Nokia-Siemens Networks, “LTE Rel-8 Handover Performance in Manhattan Environment”, Seoul, Korea, 3GPP TSG RAN WG1 Meeting #56bis, R1-091559, March 2009. http://www.3gpp.org/

[10] C. Chang, C. J. Chang, and K. R. Lo, “Analysis of a hierarchical cellular system with reneging and dropping for waiting new calls and handoff calls,” IEEE Trans. Veh. Technol., vol.48, no.4, 1999, pp.1080–1091.

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Fig.7. RLF rate

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Fig.8. Ping-pong rate

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Fig.9. Call blocking rate

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Fig.10. Call dropping rate

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Fig.11. Number of operations of MLB

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