Ping-pong Reduction using Sub cell Movement Detection Zoltán Fehér 1 , András Veres 2 , Zalán Heszberger 1 1 HSNLab, Dept. of Telecommunications and Media Informatics Budapest University of Technology and Economics Budapest, Hungary {zoltan.feher, heszberger}@tmit.bme.hu 2 Ericsson Research Ericsson Hungary LTD. Budapest, Hungary andras.veres@ericsson.com Abstract—In this paper we demonstrate an effective way to classify repeated handovers (ping-pongs) in mobile broadband networks. The paper also presents a method that can significantly reduce unwanted ping-pongs in the network. The method combines a sub cell movement detection method and ping-pong detection to decide when it is most effective to apply handover threshold tuning (pinning) without increasing the risk of late or failed handovers. The algorithm was evaluated based on live network measurements. Keywords-mobile broadband; ping-pong; movement detection; network management; self-optimization I. INTRODUCTION In mobile systems handovers are controlled by evaluating radio measurements performed by the terminal. If the measurement from a new cell becomes better than the old one (plus some thresholds are evaluated), a handover may be initiated. Since measurements fluctuate due to natural reasons, certain thresholds and smoothing are applied. The setting of these thresholds and smoothing parameters are not obvious and may be a subject to optimization. Ping-pong handover is a potentially undesirable phenomenon, in which the terminal performs frequent handovers between the same pair of cells back and forth, in a short time period. Reduction of undesired ping-pong handovers is an important task of mobile network management. Moreover, in mobile broadband systems like HSPA, or LTE, ping-pongs may have more undesirable impact than for traditional voice services. Mobile broadband users spend more time in active state typically. It is more likely that a ping-pong situation is maintained for prolonged time than during typically much shorter voice conversations. Packet switched mobile systems are also optimized for data transmission primarily; the extra capacity required to serve large number of ping-pong handovers comes with a non-negligible cost. The amount of ping-pong type handovers may account to approx. 40-60% of all handovers based on measurements in numerous networks. Another negative aspect of ping-pongs is their potentially adverse affect on mobile broadband services. When the user equipment (UE) switches between serving cells transmission is halted for short time, buffer transfer may happen. Delay or throughput sensitive applications e.g., real-time gaming, real- time interactive applications or high-speed file transfer may be harmed due to frequent handovers [1]. The shortcoming of existing ping-pong reduction solutions is that all ping-pongs are treated the same i.e., they are considered bad and should be eliminated. In reality ping-pongs are not equal. The solution to the problem could be optimized better when knowing the ping-pong situation deeper. Most existing solutions [2][3][4] are common in that on detecting a ping-pong handover, they become more and more conservative i.e., they increase some thresholds. As a result, the pinning decision increases the probability of late and failed handovers causing service interruption. This paper classifies ping-pongs into several categories and applies different actions to different situations. For this, the paper involves a new type of information, which is the terminal mobility on the sub-cell level. There are solutions [5][6][7] which use different types of location info to assist in the reduction of unnecessary handovers but the resolution of these solutions is not precise enough to be sufficiently effective. Also mobility models are used to predict if the UE has a moving or stationary state but these solutions can have significant error ratio [5][8] and operate on a macro scale rather than on a sub- cell level. The paper is organized as follows. In Section II ping-pong detection and sub-cell level movement detection method is introduced. Section III presents the different classes of ping- pong cases. Finally, in Section IV and V the hand-over elimination logic and its results are presented. II. PING-PONGS VS. MOBILITY Due to movement the radio conditions change, and when thresholds change, causing handovers. In this case handovers are necessary to avoid failed handovers. Such handovers may be done repeatedly between the same base stations. If handovers between two cells happens back and forth in an appropriate time user is in a ping-pong sequence. Ping-pong can happen naturally, when the terminal is moving, passing buildings, trees and other obstacles. In this case the affect of ping-pong is much less of a concern than if the terminal is doing ping-pongs while being completely stationary. If the terminal is stationary, there is no real need for such handovers. As a consequence of the stationary state, radio signal quality 978-1-4673-0990-5/12/$31.00 ©2012 IEEE