1788 IEEE COMMUNICATIONS LETTERS, VOL. 16, NO. 11, NOVEMBER 2012 Seamless Handover for LTE Macro-Femto Networks Based on Reactive Data Bicasting Tao Guo, Atta ul Quddus, and Rahim Tafazolli Abstract—Seamless mobility support is a key technical require- ment to motivate the market acceptance of the femtocells. The current 3GPP handover procedure may cause large downlink service interruption time when users move from a macrocell to a femtocell or vice versa due to the data forwarding operation. In this letter, a practical scheme is proposed to enable seamless handover by reactively bicasting the data to both the source cell and the target cell after the handover is actually initiated. Nu- merical results show that the proposed scheme can signiﬁcantly reduce the downlink service interruption time while still avoiding the packet loss with only limited extra resource requirements compared to the standard 3GPP scheme. Index Terms—Seamless handover, data bicasting, femtocells, LTE. I. I NTRODUCTION F EMTOCELL has been recognized as a promising solution to tackle the explosive growth of mobile broadband usage, especially resulting from prevalent multimedia applications, in cellular networks [1]. It is usually deployed by the users at their own premises and connected to a mobile operator’s network via residential broadband access such as Digital Subscriber Line (DSL). Thus, it provides a cost-effective way for mobile network operators to ofﬂoad cellular network trafﬁc. From the user side, better Quality of Service (QoS) are expected by deploying femtocells compared to other competitors such as Wi-Fi by exploiting advanced air interface and more tight coupling with the mobile cellular networks. Extensive effort has been made to integrate femtocells into the 3GPP Long Term Evolution (LTE) standard [2]. When a user moves from a macrocell to a femtocell or vice versa, seamless mobility should be supported by the employed Handover (HO) procedure such that the HO is not perceptible to the users. Two main Key Performance Indicators (KPIs) are Service Interruption Time (SIT) and packet loss. In current 3GPP standard, the same hard HO procedure as for inter-macro mobility is used for macro-femto mobility. A data forwarding procedure is used to forward the Downlink (DL) packets from the source to the target during HO to enable lossless packet delivery. Remarkable DL SIT up to hundreds of milliseconds may be perceived by the users in HO due to the data forwarding latency along the delay-prone residential backhaul of the femtocells, which easily exceeds the delay jitter target of a typical Voice overIP (VoIP) ﬂow (30 ms) [3] Manuscript received July 15, 2012. The associate editor coordinating the review of this letter and approving it for publication was I. Guvenc. The authors are with the Centre for Communication Systems Research, University of Surrey, Guildford, GU2 7XH, United Kingdom (e-mail: {t.guo, a.quddus, r.tafazolli}@surrey.ac.uk). This work has been performed in the framework of the ICT project ICT- 4-248523 BeFEMTO, which is partly funded by the European Union. Digital Object Identiﬁer 10.1109/LCOMM.2012.091712.121562 A proactive data multicasting scheme have been proposed in [4] to support fast macro-femto handover. A dynamic set of candidate target cells is maintained for a User Equipment (UE) based on the radio signal measurement. A cell will be added to this set if the signal strength received from this cell is above certain threshold. The data will be multicasted to all the cells in the candidate set as long as this set is not empty. This scheme signiﬁcantly reduces the DL SIT by making the data available at the potential target cells before the HO. However, proactively multicasting the data to all the cells in the candidate set may consume numerous backhaul and buffer resources because: (1) the size of the candidate set may be large due to the dense deployment of the femtocells; (2) the actual handover process may occur later or even not occur at all. [5] proposes to determine the data multicasting threshold based on the UEs’speed to avoid too early trigger for low-speed UEs and too late trigger for high-speed UEs. However, UE speed estimation is a non-trivial operation. In addition, given the short transmission range of a femto base station, the accuracy requirement for determining the proper multicasting threshold is challenging. Finally, given that the hard handover is the only HO procedure supported by LTE so far [2], the maintenance of the candidate set may require considerable modiﬁcations to the standard. In this letter, we propose a reactive data bicasting scheme to reduce the DL SIT during HO by making relatively mod- erate modiﬁcations to the 3GPP procedure. Compared to the proactive multicasting schemes, the proposed scheme triggers the data bicasting after the HO is actually initiated by the source cell, and thus, can signiﬁcantly relax the backhaul and buffer resource requirements. An optional drop-head buffering mechanism only requiring a very small buffer size can be used at the core network to avoid the packet loss during HO. II. 3GPP HO PROCEDURE A network-controlled UE-assisted hard HO procedure is speciﬁed in 3GPP for the HO between Evolved Node Bs (eNBs) and Home Evolved Node Bs (HeNBs) as shown in Fig. 10.1.2.1.1-1 [2]. If the HO criterion is satisﬁed, the source eNB/HeNB will send a HO Required message to the Mobility Management Entitie (MME) in the Evolved Packet Core (EPC). The MME will authenticate it and send a HO Request to the target HeNB/eNB. If the target decides to admit this request based on resource availability, it will send a HO Request Ack to the MME and the MME will send a HO Command to the source. Upon receiving the HO Command, the source will send a Radio Resource Control (RRC) Conn. Reconf. to the UE to perform the HO. Meanwhile, the source will send a Sequence Number (SN) Status Transfer message 1089-7798/12$31.00 c  2012 IEEE