教育部補助「行動寬頻尖端技術跨校教學聯盟計畫-行動寬頻網路與應用-小細胞基站聯盟中心計畫」 Small Cell創新應用與服務專題 課程單元：LTE換手(Handover)

概述 水平(horizontal)換手 使用相同的無線存取 技術的基站之間切換 垂直(vertical)換手 使用不同的無線存取技術的基站之間切換，譬如從戶外的4G換到室內的WiFi網路 硬式(hard)換手 (“break-before-make”) 與新的基站建立新的無線連接之前，先斷開目前服務的基站之無線連線 軟式(soft)換手 (“make-before-break”) 與目前服務的基站之連線未斷開之前，即與新的基站建立新的連接

簡介 換手(handover亦稱為handoff)是移動管理機制的一部分，用於維持UE從基地台無線訊號涵蓋區域移動到另一區時，仍能保持網路暢通，不至於通訊中斷 LTE換手屬網路控制(network-controlled)的機制? 於TS 36.300第10節詳述

簡介 UE保持RRC-connected狀態，換手過程包括： 無線電鏈路從eNB 1釋放並在eNB2重新建立 控制面(control plane)切換到eNB 2(或後端的MME) 用戶面(user plane)切換到eNB 2和(或後端的S-GW)

換手程序：分三大階段 Handover Preparation Handover Execution Handover Completion Search new link Network discovery Network selection Handover negotiation Setup new link Layer 2 connectivity IP connectivity* Transfer connection Handover signaling Context transfer Packet reception

LTE換手程序 當UE與eNB建立RRC連接時，eNB透過RRC Connection Reconfiguration訊息告知UE在哪個情況下應回報接收到的信號強度。UE監看目前服務它的基站與相鄰基站的收訊強度，透過Measurement Report訊息週期地或依指定事件之觸發向eNB報告收訊強度。eNB收到後，檢視所回報的資訊和基站的負載狀態來決定是否啟動換手程序，選擇新的目標(target)基站，再透過節點之間交換訊息完成網路連結重新設定的程序。

LTE換手程序 Measurement Report Triggering (量測報告的觸發時機) The UE measures the signal strength of its serving cell and neighbor cells. Then it reports the results to eNB, periodically or when a measurement event is triggered as one of the reporting criteria set by the measurement configuration is satisfied. Reporting criteria for E-UTRA (Evolved Universal Terrestrial Radio Access) report include Events A1, A2, A3, A4 and A5 while ones for inter-RAT (Radio Access Technology) measurement report include Events B1 and B2. Event A3 is commonly used in triggering handovers.

LTE換手的觸發事件有若干種，到底有哪些？ 除了A3，尚有哪些？有何不同

訊號量測報告 (1/3) UE量測與服務eNB之間的無線信號狀況(L1測量)，再向服務的eNB發送L3測量報告 L1 (Layer-1)測量 UE測量參考信號接收功率的差異(difference of reference signal receiving power) 此差異值類似信號與干擾之噪聲比(SINR, signal to interference noise ratio) Reference Symbol Received Power (RSRP)為UE量測downlink reference signal的平均訊號強度

訊號量測報告 (2/3) UE量測與服務eNB之間的無線信號狀況(L1測量)，再向服務的eNB發送L3測量報告 L1 (Layer-1)測量 L1測量每10ms進行一次，週期TL1 第n次的L1測量 s: serving eNB t: target eNB

訊號量測報告 (3/3) L3測量 UE的RRC層(L3)對L1量測數據進行取樣(sampling)過濾 UE在L3測量週期(TL3)結束時，向服務eNB發送測量報告 L3測量的計算方式：NL3是TL3期間L1測量的次數 (a: filter coefficient)

A3事件觸發換手 由eNB決定是否進行換手 A3事件： (H 為門檻值，稱為遲滯邊界，hysteresis margin) 另設置Time-to-trigger (TTT)計時器 若所有L3測量在TTT計時內均滿足A3事件，則啟動換手 (H 為門檻值，稱為遲滯邊界，hysteresis margin)

資料來源 https://goo.gl/FoFRcy http://lteuniversity.com/get_trained/expert_opinion1/b/donhanley/archive/2013/11/20/stuck-in-the-middle-with-you-part-2.aspx 資料來源 https://goo.gl/FoFRcy

Measurement Event A3 進入條件: Mn + Ofn + Ocn - Hys > Ms + Ofs + Ocs + Off 離開條件: Mn + Ofn + Ocn + Hys < Ms + Ofs + Ocs + Off (Reference Signal Received Power) When the signal strength of a neighbor cell (sum of signal strength and offset, MNbr = Mn + Ofn + Ocn) becomes greater than that of the UE’s serving cell (sum of signal strength and offset, MSer = Ms + Ofs + Ocs), AND the difference is greater than the value of A3 offset (Off), Event A3 is triggered, and the UE reports the measurement results to the eNB. Hysteresis (Hys) indicates the value of a handover margin between the serving cell and the target cell. The eNB decides to trigger a handover if A3 is triggered AND the A3 triggering criteria last longer than the time specified as Time-to-Trigger (TTT) period.

決定換手(Handover Decision) 當偵測到UE回報A3事件時，eNB決定要對哪個目標基站執行何種型態的換手。換手的類型可分成二大類： (1) EPC元件是否更動； (2) EPC元件是否涉入 “EPC元件是否更動”又細分為 Intra-LTE換手：服務UE的MME與S-GW不因換手而改變 Inter-LTE換手：服務UE的MME或S-GW因換手而更動 Inter-MME換手：服務UE的MME更動，但S-GW未變 Inter-S-GW換手：服務UE的S-GW更動，但MME未變 Inter-MME/S-GW換手：服務UE的MME與S-GW均更動 Inter-RAT換手：使用不同無線接取技術的基站之間切換 3G基站換到4G 4G基站換到3G

決定換手(Handover Decision) “EPC元件是否涉入”又細分為 X2換手：新舊eNB彼此交換訊息以進行程序，不需要MME的介入 S1換手：若X2換手不可行，則採S1換手。來源eNB透過MME與目標eNB溝通，以實現換手程序

X2換手與S1換手範例 ECM: Evolved Packet System Connection Management DRB: Data Radio Bearer ECM: Evolved Packet System Connection Management. A Non-Access Stratum (NAS) state which reflects the connectivity (either ‘Idle’ or ‘Connected’) of a UE with the Evolved Packet Core (EPC). DRB: Data Radio Bearer. A radio bearer which carriers user data as opposed to control plane signalling.

LTE換手程序：使用X2介面與否

補充：穿隧協議(Tunneling 1/3) Header Payload Original IP Packet Header https://technet.microsoft.com/en-us/library/cc739294(v=ws.10).aspx Header Payload Outer Encapsulated IP Packet Endpoint Tunnel (隧道) IP-in-IP tunneling例

補充：穿隧協議(Tunneling 2/3) 另例：群播(multicast, 1對多通訊模式)經常利用穿隧的機制，讓資料封包仍能行經未支援群播功能的網路加以傳送 Source 隧道端點 https://technet.microsoft.com/en-us/library/cc739294(v=ws.10).aspx Header Payload Outer

補充：穿隧協議(Tunneling 3/3) http://sugerent.tistory.com/125

換手程序：Handover Preparation For S1 handover, MME gets involved through S1 signaling Source eNB sends the user’s UE context (security context, QoS context) to the target eNB to check if the target eNB is capable of providing satisfying service quality Target eNB establishes a transport bearer for DL packet forwarding and then allocates a C-RNTI (Cell Radio Network Temporary Identifier) that the UE needs to use when accessing the eNB, and forwards the same to the source eNB

換手程序：Handover Execution Once the resources needed for packet forwarding between two eNBs are allocated, and the new resources for the UE are allocated at the target eNB (a DRB, DL S1 bearer, C-RNTI) during the preparation phase, the two eNBs are ready for a handover. Then, the source eNB orders the UE to perform a handover by sending a Handover Command message.

換手程序：Handover Completion Once the UE completes its radio access to the target eNB, the UE’s bearer path (DL S1 bearer) is connected to the target eNB, instead of the source eNB. Once the path is switched, the forwarding bearer used in forwarding DL packets during the handover execution phase is released.

換手中斷時間 (Handover Interruption Time) 換手程序進行期間，網路連接重新配置，UE端、基站(或後端的網路節點)皆須費時完成設定，在此期間內，UE無法收送資料封包

使用X2介面進行訊息交換的Intra-LTE換手 To handover a UE from a source eNodeB (S-eNB) to a target eNodeB (T-eNB) using the X2 interface when the MME and SGW are unchanged http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用X2介面進行訊息交換的Intra-LTE換手 在沒有EPC參與的情況下執行X2換手程序，即在S-eNB和T-eNB之間直接交換準備訊息。在S-eNB的資源釋放由T-eNB觸發完成 http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用X2介面進行訊息交換的Intra-LTE換手：細部步驟 (1/3) UE is in connected state and a data call is up. Data packets are transferred to/from the UE to/from the network in both directions (DL as well as UL). The network sends the MEASUREMENT CONTROL REQ message to the UE to set the parameters to measure and set thresholds for those parameters. Its purpose is to instruct the UE to send a measurement report to the network as soon as it detects the thresholds. The UE sends the MEASUREMENT REPORT to the S-eNB after it meets the measurement report criteria communicated previously. The S-eNB makes the decision to hand off the UE to a T-eNB using the handover algorithm; each network operator could have its own handover algorithm. The S-eNB issues the RESOURCE STATUS REQUEST message to determine the load on T-eNB (this is optional). Based on the received RESOURCE STATUS RESPONSE, the S-eNB can make the decision to proceed further in continuing the handover procedure using the X2 interface. The S-eNB issues a HANDOVER REQUEST message to the T-eNB passing necessary information to prepare the handover at the target side (e.g., UE Context which includes the Security Context and RB Context (including E-RAB to RB Mapping) and the Target cell info). http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用X2介面進行訊息交換的Intra-LTE換手：細部步驟 (2/3) The T-eNB checks for resource availability and, if available, reserves the resources and sends back the HANDOVER REQUEST ACKNOWLEDGE message including a transparent container to be sent to the UE as an RRC message to perform the handover. The container includes a new C-RNTI, T-eNB security algorithm identifiers for the selected security algorithms, and may include a dedicated RACH preamble and possibly some other parameters (i.e., access parameters, SIBs, etc.). The S-eNB generates the RRC message to perform the handover, i.e, RRCCONNECTION RECONFIGURATION message including the mobilityControlInformation. The S-eNB performs the necessary integrity protection and ciphering of the message and sends it to the UE. The S-eNB sends the eNB STATUS TRANSFER message to the T-eNB to convey the PDCP and HFN status of the E-RABs. The S-eNB starts forwarding the downlink data packets to the T-eNB for all the data bearers (which are being established in the T-eNB during the HANDOVER REQ message processing). In the meantime, the UE tries to access the T-eNB cell using the non-contention-based Random Access Procedure. If it succeeds in accessing the target cell, it sends the RRC CONNECTION RECONFIGURATION COMPLETE to the T-eNB. http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用X2介面進行訊息交換的Intra-LTE換手：細部步驟 (3/3) The T-eNB sends a PATH SWITCH REQUEST message to the MME to inform it that the UE has changed cells, including the TAI+ECGI of the target. The MME determines that the SGW can continue to serve the UE. The MME sends a MODIFY BEARER REQUEST (eNodeB address and TEIDs for downlink user plane for the accepted EPS bearers) message to the SGW. If the PDN GW requested the UE’s location info, the MME also includes the User Location Information IE in this message. The SGW sends the downlink packets to the target eNB using the newly received addresses and TEIDs (path switched in the downlink data path to T-eNB) and the MODIFY BEARER RESPONSE to the MME. The SGW sends one or more “end marker” packets on the old path to the S-eNB and then can release any user plane / TNL resources toward the S-eNB. The MME responds to the T-eNB with a PATH SWITCH REQ ACK message to notify the completion of the handover. The T-eNB now requests the S-eNB to release the resources using the X2 UE CONTEXT RELEASE message. With this, the handover procedure is complete. http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用X2介面進行訊息交換的 換手流程 Intra- LTE http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/ EMM: Evolved Packet System Connection Management. A Non-Access Stratum (NAS) state which reflects whether a UE is registered with the Mobile Management Entity (MME) – either ‘registered’ or ‘deregistered’. ECM: Evolved Packet System Connection Management. A Non-Access Stratum (NAS) state which reflects the connectivity (either ‘Idle’ or ‘Connected’) of a UE with the Evolved Packet Core (EPC).

使用X2介面進行訊息交換的 換手流程 Intra- LTE

使用S1介面進行訊息交換的Intra-LTE換手 http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

使用S1介面進行訊息交換的Intra-LTE換手 http://www.3glteinfo.com/intra-lte-handover-using-x2-interface/

在S1換手前、後的UE狀態和連線訊息 將分為三部分呈現流程

在S1換手前、後的UE狀態和連線訊息 Before S1 handover EMM: Evolved Packet System Connection Management. A Non-Access Stratum (NAS) state which reflects whether a UE is registered with the Mobile Management Entity (MME) – either ‘registered’ or ‘deregistered’. ECM: Evolved Packet System Connection Management. A Non-Access Stratum (NAS) state which reflects the connectivity (either ‘Idle’ or ‘Connected’) of a UE with the Evolved Packet Core (EPC). Before S1 handover The UE stays in EMM-Registered and ECM/RRC-Connected and keeps all the resources allocated by E-UTRAN and EPC.

在S1換手前、後的UE狀態和連線訊息 S1 Handover Preparation The source eNB (eNB A in the figure) chooses a target eNB (eNB B in the figure) to handover to, based on the neighbor cell list information and the information on the signal strength of the neighbor cells included in the Measurement Report message. Next, it decides to perform a S1 handover instead, and prepares to perform one through MME. Both eNBs communicate with the MME through S1AP signaling. 

在S1換手前、後的UE狀態和連線訊息

參考資料 Netmanias Technical Documents, http://bit.ly/2rBzAIf 2015 (v12.5.0) 3GPP TS 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 2012 (v11.1.0) 3GPP TR 36.839, Evolved Universal Terrestrial Radio Access (E-UTRA); Mobility Enhancements in Heterogeneous Networks