A Handover Algorithm for LTE System based on the Target Cell Pre- Bearer in High-speed Railway Environment

Transcription

1 A Handover Algorithm for LTE System based on the Target Cell Pre- Bearer in High-speed Railway Environment Broadband Communication Networks Lab Beijing University of Posts and Telecommunications, Beijing , China {Barryluan, Wumuqing. Abstract This paper evaluates the handover performance of an optimized A3 event handover algorithm of LTE system, which is based on the target cell pre-bearer procedure in the high-speed railway (HSR) scenario. The paper introduces this handover algorithm and the handover signaling procedure based on the target cell pre-bearer procedure. These have been designed and implemented in a dynamic system level simulator and have been studied to select different handover parameter sets for HOM (Handover hysteresis margin) and TTT (Time- to-trigger) in the different velocity interval of the UE (User Equipment) for the high-speed railway simulation scenario. The different HOM and TTT parameters have been simulated in a dynamic system level simulation platform and have been investigated the influence to radio link failure (RLF) via the simulation. The results suggest that this kind of handover algorithm has better handover performance in the velocity internal 250km/h and 350km/h. 1. Introduction Keywords: LTE, Handover, High-Speed Railway, TTT, HOM, RLF Universal Terrestrial Radio Access Network Long-Term Evolution (UTRAN LTE) is known as Evolved UTRAN (E-UTRAN). It is a system currently under development within the 3rd Generation Partnership Project (3GPP) [1]. 3GPP has adopted Orthogonal Frequency Division Multiple Access (OFDMA) as radio access technology (RAT) for LTE system. The main objective of Long Term Evolution are: (1) the provision of peak cell data rates up to 100Mbps in downlink (DL) and up to 50Mbps in uplink (UL) under various mobility and network deployment scenarios; (2) user throughput improved by factor of 2~3 in uplink and downlink compared to High Speed Packet Access (HSPA); (3) user-plane latency below 5ms for small IP packet in an unloaded network; (4) improved spectrum efficiency by factor of 2~3 in uplink and downlink respectively compared to HSPA [2][3][4]. With the rapid development of in China high-speed railway, there is a requirement for mobility support with high performance up to the speed of 250km/h, even up to 350km/h. LTE system is consisted of three elements: evolved-nodeb (enodeb), Mobile Management Entity (MME), and Serving Gateway (S-GW)/Packet Data Network Gateway (P-GW). The enodeb performs all radio interface related functions such as packet scheduling and handover mechanism. MME manages mobility, user equipment (UE) identity, and security parameters. S-GW and P-GW are two nodes that terminate the interface towards E-UTRAN and Packet Data Network, respectively [5]. This paper is focus on the high-speed railway scenario, which deploys the enodeb consisting of Base Band Unit (BBU) and Radio Remote Unit (RRU) along railway line as demonstrated in Figure 1. At present, there are two kinds of handover technologies in wireless communication systems: hard handover and soft handover. Hard handover is a break-before-connect (BBC) mode. Handover in LTE system is hard handover according to 3GPP. Although hard handover reduces the complexity of the LTE network architecture, hard handover type may result in data being lost and higher handover failure ratio. Therefore, a handover mechanism to avoid loss of data and handover failure ratio is necessary for LTE system. To provide high data rate services in high velocity environment within LTE system, an efficient, self-adaptive and robust handover algorithm is required. Hence handover technique becomes an important research area since the proposition of LTE by 3GPP in 2004 [6][7]. Generally, handover procedure within LTE system is divided into the following steps: the measurements control, the measurements report, the handover decision and the handover execution [8]. Handover measurements include measurements control and measurements reports, which is fundamental and crucial for International Journal of Advancements in Computing Technology(IJACT) Volume4, Number 22,December 2012 doi: /ijact.vol4.issue

2 handover performance. Handover measurements can be made in the downlink between the source cell (also shown the serving cell) and the target cell (also shown the neighboring cell) and processed in the user equipment (UE). Based on the measurement of either reference signal received power (RSRP) or reference signal received quality (RSRQ) or both, a measurement report is done in order to make a reselection or handover decision procedure. Then by radio resource control (RRC) signaling in the uplink, the report is delivered to the souring cell. If certain handover measurement criterion is met, the reasonable handover is estimated. At last, handover is executed by handing in UE control power to the target cell from the serving cell [9]. Several papers have been published to optimize the HO performance for LTE recently. A generalized model has been proposed to analyze the impact of propagation environment and velocity on the handover performance of the UE for LTE systems. The proposed algorithm reduces the call dropping rate during handover at the cell boundary of urban cells [10]. Two kinds of handover algorithms have been introduced in the LTE system [11][12]. And the simulation results show that these both algorithms have better handover performance. Figure 1. enodeb deployment along railway line. The paper is organized as follows: In Section 2, LTE handover mechanism is introduced including handover measurement model and RLF. The Pre-bearer handover algorithm for LTE system will be introduced detailedly in high-speed railway scenario and pre-bearer handover signaling procedure will be shown in Section 3. Then in Section 4 simulation results are analyzed and finally conclusions are drawn in Section LTE Handover Mechanism 2.1 Measurement Model The enodeb controlled handover is based on the measurement taken by the UE and execution taken by the source enodeb. The measurement model is shown in Figure 2, which has been used as working assumption in 3GPP for handover. At point A measurements (samples) is internal to the physical layer. It means the UE measures RSRP by adding reference symbols power within the sub-frame over the complete measurement bandwidth. The UE measures RSRP, which includes pathloss, antenna gain, log-normal shadowing and fast fading, averaged over all the reference symbols within the measurement bandwidth. Then Layer 1 filtering can average the measured samples at point A and is sampled at point B after every measurement period. Layer 3 filtering performed on the measurements provided at point B. The behavior of the Layer 3 filtering is standardized and the configuration of the layer 3 filtering is provided by RRC signaling. Filtering reporting period at point C equals one measurement period at point B. A measurement is gotten at point C after processing in the layer 3 filtering. The reporting rate at point C is identical to it at point B. This measurement is used as input for one or more evaluation of reporting criteria. Evaluation of reporting criteria checks whether actual measurement report is necessary at point D. Measurement report information (message) is sent on the radio interface at point D [13] [14]. 632

3 Figure 2. Handover measurements filtering and reporting For formula (1), RSS is the measurements after Layer 1 filtering, also shown as RSRP. RSS is the measurements after Layer 3 filtering. T m is the measurement period of Layer 1 and T u is known as Layer 3 filtering period. is named as the forgetting factor and the value is between 0 and 1, so T u is an integer multiple of T m. The relative influence on RSS of the recent measurement and older measurements is controlled by the forgetting factor. With the increase of, RSS will be closer to the recent measurements of RSS ; whereas RSS will be closer to the older measurements of RSS. In Figure 3, it is shown the relation between Tm and T u. RSS nt (1 ) RSS n 1 T RSS nt, n 1, 2 (1) m m m T T (2) m u Figure 3. The Filtering Period of Layer 1 and Layer Radio Link Failure Figure 4. Radio link failure procedure In Figure 4, it shows the RLF procedure as a whole. When the radio link quality deteriorated serious enough in the first phase, a Radio Link Failure is triggered to lead interruption in communication service and a cell re-selection, then handover procedure will be established. In principles, there are several factors to trigger RLF procedure: (1) HO delay is extensive enough; (2) there is high interference in the handover area; (3) when the UE is at the edge of coverage. The UE always monitor the DL link quality via the measurement of cell-specific reference signal. The radio link quality has two the quality thresholds. One is the threshold Q, which is the threshold of the triggered RLF out 633

4 procedure; the other is the threshold Q, which is the threshold of the cancelling RLF procedure. The in threshold Q out represents the level at which the DL radio link cannot be reliably received and correspond to10% Block Error Rate (BLER) on the Physical Downlink Control Channel (PDCCH). The threshold Q shall correspond to 2% Block Error Rate [15] [16]. in 3. The Pre-Bearer Handover Algorithm In Figure.5, the flowchart of the pre-bearer handover algorithm is based on GPS information. At first, the UE perform the measurement procedure. After the UE receives the data, the UE processes the data and gets the velocity of the UE. When S (the velocity of the UE) is equal or less than Sthreshold, the UE triggers normal HO procedure, otherwise the UE triggers the pre-bearer HO procedure based on GPS. Then the UE received the longitude and latitude of the UE periodically. The distance L is calculated between the position of the UE and pre-bearer triggering reference point. While D is equal or less than Dthreshold, the UE sends the measurement report to the source enodeb. Then the pre-bearer procedure is triggered. When the UE enters into the handover area, handover algorithm adopts the optimized A3 event handover algorithm based on the velocity of the UE [11]. In table 1, it shows the deployment of the enodeb and Serving Gateway (S-GW). The list of enodeb includes the following attributes: evolved Cell Global Identifier (E-CGI), Pre-bearer reference point, enodeb Number and S-GW Number. In this paper, pre-bearer reference point is predefined based on the deployment of the enodeb before the UE enters into the handover area. As soon as the movement direction of the UE (The UE is just a train) for high-speed railway is decided, the list of the whole enodeb is also confirmed. E-CGI is ordered according to the deployment of the enodeb. There is one handover reference point near the handover area in the mobility direction of the train. S-GW Number is shown to communicate with the related enodeb Number. The pre-bear handover has a mechanism, which eliminates Ping-Pong handover, that the UE is not able to trigger handover back from the target enodeb to the source enodeb after it finishes handover from the source enodeb to the target enodeb. Because the target enodeb has been confirmed, the source enodeb can know which enodeb the UE will trigger the handover to. So the pre-bearer handover algorithm won t be happen to Ping-Pong handover. At the same time, it will add the risk of RLF. Table 1. enodeb/s-gw Deployment List E-CGI Pre-bearer Triggering enodeb Number S-GW Number reference point E-CGI(0) (A0,B0) enodeb(0) S-GW(N-1) E-CGI(1) (A1,B1) enodeb(1) S-GW(N) E-CGI(2) (A2,B2) enodeb(2) S-GW(N) E-CGI(n) (An,Bn) enodeb(n) S-GW(N) E-CGI(n+1) (An+1,Bn+1) enodeb(n+1) S-GW(N+1) In Figure.6, it shows the main steps of the pre-bearer handover procedure. It is as follows: Firstly, the pre-bearer procedure is triggered by the UE that sends a measurement report to the source enodeb including the longitude and latitude information. It means while the UE reaches to prebearer reference point, the UE sends the measurement report to the source enodeb according to the location information. The source enodeb makes pre-bearer procedure decision based on the measurement report and Radio Resource Management (RRM) information. A pre-bearer request message is sent from the source enodeb to the target enodeb. This message includes all the relevant information, which the target enodeb shall bear for handover preparation. The target enodeb saves the context of the UE, prepares L1/L2 for handover preparation and responds to the source enodeb with a pre-bearer request ack message which provides information for the establishment of the new radio link (admission control). At this time, the pre-bear procedure is finished. The target enodeb has prepared the relevant resource for the handover. 634

5 Secondly, the train begins to enter the handover area and the UE receives the reference signals from the source enodeb and the target enodeb. When A3 event evaluation criteria is met, the handover measurement report will be sent to the source enodeb. When it estimates the handover measurement report to meet the handover requirement, the source enodeb transfers all the necessary information to the UE in the HO command. Thirdly, from this time the source enodeb stops sending and receiving over the air. It begins to forward DL data to the target enodeb. The UE detaches from the source enodeb, synchronizing to the target enodeb. During this time (Detach time) there is no radio connectivity to the system. The UE is disconnected from the source enodeb and it is not connected to the target enodeb. At last, the UE sends Handover Confirm message to the target enodeb about the success message of handover. Up to this time the target enodeb buffers DL data received from the source enodeb. After receiving this message, it starts transmitting the buffered data to the UE. The target enodeb sends Handover Complete message to initiate data path switching. The UE location information is updated to the target enodeb at MME/S-GW after receiving the Handover Complete message and it performs the path switching after which packets are directly sent to the target enodeb. Then the MME/S-GW confirms the path switching by sending a Handover Complete Ack message. After receiving this message, the target enodeb sends a Releases Resource message to the source enodeb that can flush its forwarded DL data buffer that was stored in for case of fallback. In this paper, RLF is the most important key performance indicators (KPI) during the handover procedure of high-speed LTE network because RLF represents most of handover failure ratio. The system level simulation platform studies the variety of RSRPs of the source enodeb and the target enodeb during the handover area on the condition of the different forgetting factor. It also studies the relation of RLF and TTT on the condition of the different HOM based on the different velocity of the train. In table 2, it shows the simulation parameters of the system level simulation platform. Figure 5. The Flowchart of the Pre-bearer Handover algorithm 635

6 UE Source enodeb Target enodeb MME S-GW PDN GW Downlink and uplink data 0. X2-based handover decision Pre-bearer procedure 1.Measurement Control UL Allocation 2.Pre-bearer Measurement Reports 3.Pre-bearer decision 4.Pre-bearer Request Legend L3 signalling L1/L2 signalling User Data DL Allocation 6.Pre-bearer Request Ack 5. Admission Control 7.HO Measurement Reports Handover preparation 9. Handover Command 8.Handover decision Detach from S-eNodeB and synchronize T-eNodeB Handover execution Deliver buffered and in transit Packets to T-eNodeB 10.SN status transfer Data Forwarding 11.Synchronisation and RACH Access 12.UL allocation + TA for UE 13.Handover Confirm Downlink data Uplink data 14.Path Switch Request 15.Modify Bearer Request 16a.Modify Bearer Request 16b.Modify Bearer Response Handover completion Downlink data 17.Switch DL path End marker End marker 20.UE Context Release 19.Path Switch Request Ack 18.Modify Bearer Response 21.Release Resource 22.Tracking Area Update procedure Figure 6. Handover signalling procedure based on pre-bearer procedure 636

7 Table 2. Handover simulation parameters Parameters Values UE number 1 enodeb 2 Carrier Frequency Bandwidth Antenna Gain Antenna Max Transport Distance Antenna Direction Parameter Distance to railway line Handover area Length Handover execution time RRU Transmitting Power Path Loss HOM TTT Channel scenario 2.6GHz 10MHz More than 18dBi 1600m Horizontal 120/150deg Vertical 65deg 50m 300m 5s 47dBm Cost 231 Hata model 1.0dB, 2.0dB, 3.0dB 0~120km/h: 0, 120, 240, 360, 480ms 120~250km/h:0, 90, 180, 270, 360ms 250~350km/h: 0, 60, 120, 180, 240ms Mountain 4. Simulation Results 4.1 Layer 3 Filtering Simulation Results In Figure 7, it shows the L3 filtering RSRPs which the UE receives from source enodeb and the target enodeb when the forgetting factor is 1/2, 1/4, 1/6, and 1/8 in the mountain scenario. When is 1/2, the variety of the RSRP is very acute because this result represents the instantaneous variety of the shadow fading. The purpose of L3 filtering is to eliminate the effect of the shadow fading and fast fading so that the handover procedure can be triggered at the right time. The variety of the RSRP becomes lower with the reduction of. But RSRP is close to the older the measurements. By the tradeoff between the fluctuation of the measurements and the timeliness of the measurement, we adopt 1/6 in this paper. 637

8 4.2 Handover Simulation Results Tm=10ms, a=1/2, Sourcing enodeb L3 Filtering RSRP Target enodeb L3 Filtering RSRP Tm=10ms, a=1/4 Sourcing enodeb L3 Filtering RSRP Target enodeb L3 Filtering RSRP RSRP (dbm) RSRP (dbm) Distance (m) Distance (m) (a) (b) Tm=10ms, a=1/6, Sourcing enodeb L3 Filtering RSRP Target enodeb L3 Filtering RSRP Tm=10ms, a=1/8 Sourcing enodeb L3 Filtering RSRP Target enodeb L3 Filtering RSRP RSRP (dbm) RSRP (dbm) Distance (m) Distance (m) (c) Figure 7. L3 Filtering RSRP of the Source enodeb and Target enodeb based on different (d) 20 VoIP=50, V=350Km/h, HOM=1.0dB 5 VoIP=50, V=350Km/h, HOM=2.0dB 18 RLF RLF Probability(%) Probability(%) TTT (ms) TTT (ms) (a) Figure 8. RLF simulation result for V=350km/h (b) 638

9 40 35 VoIP=50, V=250Km/h, HOM=1.0dB RLF VoIP=50, V=250Km/h, HOM=2.0dB RLF Probability(%) Probability(%) TTT (ms) TTT (ms) (a) Figure 9. RLF simulation result for V=250km/h Figure 8 and Figure 9 show the simulation results when the velocity of the UE is 350km/h and 250km/h. The purpose of the simulation is to analyze the relation between RLF and TTT when HOM adopts the different values to find the right handover parameter sets to satisfy the requirement. For the velocity of the UE is 350km/h, the RLF ratio is decreased with the growth of the TTT and HOM. When HOM and TTT is 2dB, 120, 180ms and 240ms, RLF ratio is lower than 0.5%, even is zero. These handover parameter sets can ensure to the QoS requirement of the radio communication for LTE systems. For the velocity of the UE is 250km/h, RLF ratio is higher than it in 350km/h. It is because RLF is easier to reach to RLF procedure triggering criteria with the reduction of the velocity in 500ms interval. When HOM and TTT is 2dB, 270ms and 360ms, RLF ratio is lower than0.5%. These handover parameters sets meet the QoS requirement of LTE system. 5. Conclusion In this article we have proposed a pre-bearer handover algorithm for LTE system which takes advantage of already existing A3 event handover algorithm. The algorithm combines the pre-bearer procedure for high-speed railway and the optimized A3 event handover algorithm based on the velocity of the UE. According to the handover simulation result, it is clearly shown that the pre-bearer handover algorithm has gotten lower RLF ratio via the adopting the different HOM and TTT parameter sets in different velocity interval. 6. Acknowledgment The research is supported by The National Science and Technology Major Projects (No.2011ZX ) and Beijing Natural Science Foundation (No ). 7. References [1] 3GPP TR V7.3.0 ( ), Requirements for Evolved UTRA [2] 3GPP TR V7.0.0 ( ), Physical layer aspects for Evolved [3] Mohmmad Anas, Francesco D. Calabrese, Performance Evaluation of Received Signal Strength Based Hard Handover for UTRAN LTE Vehicular Technology Conference, VTC2007- Spring. IEEE 65th [4] Cheng-Chung Lin, K. Sandrasegaran, Optimization of Handover Algorithms in 3GPP Long Term Evolution System, Modeling, Simulation and Applied Optimization (ICMSAO), th International Conference on Publication Year: 2011, Page(s): 1 5. [5] S. Sesia, M. Baker, and I. Toufik., LTE-The UMTS Long Term Evolution. John Wiley and Sons Ltd, [6] H. G. Myung, "Technical Overview of 3GPP LTE" May 18, (b) 639

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