# 陳毅 - 讀書會 - 21/08/25

:::info
- [x] 閱讀 Netmanias-16-EMM Procedure 6. Handover without TAU - Part 1. Overview of LTE Handover
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## Intro

Handover without TAU
- No Tracking Area Update procedure is initiated.
- UE is still connected to the eNB, which cell is in the TAI assigned by the network.

## Overview of LTE Handover
- UE can switch from one base station/cell to another without losing any incoming or outgoing data, and communicate with the network without interruption during such switch.
- This ensures its user is seamlessly served no matter which cell the user is connected to.

The procedure related to handover are as seen in the below:

- UE has an antenna that can search multiple frequency channels over multiple bands.
- when the received signal strength from the UE’s current serving cell is getting weak due to the UE’s travel, shadowing, etc., and the signal from a neighbor cell is getting strong, a handover is initiated.
- the eNB informs the UE in which event the received signal strength should be reported, by sending a configuration message (RRC Connection Reconfiguration message).
- The UE keeps track of the received signal strength of both its serving and neighbor cells.
- Then, eNB will decides whether to initiate a handover or not.

### Measurement
#### 1. Measurement Configuration
由 eNB 提供給 UE，當 RRC Connection 建立時，eNB 會透過 **RRC Connection Reconfiguration** message 告訴 UE 哪些量測資訊需要回報:
- Measurement Object
提供需要由 UE 量測的 E-UTRA cells，包含: Physical Cell ID (PCI), black list cell ID, offset values for each cell, etc.
- Reporting Configuration
哪些 Triggering event 需要 UE 回傳 **Measurement report**。
- Measurement ID
識別 measurement object。
- Quantity Configuration
哪些數值需要由 UE 測量。
- Measurement Gap
指示 UE 要在哪個間隔測量 neighbor cell。

如果:
- neighbor cell 與當前的 serving cell 使用不同頻段
UE 應該先同步到 neighbor cell 的頻率，並在 UL/DL 空閒期間使用測量間隙測量其信號強度。

#### 2. Measurement Report Triggering
- Reporting criteria for E-UTRA report include Events A1, A2, A3, A4 and A5.
- while ones for inter-RAT measurement report include Events B1 and B2
- Event A3 is commonly used in triggering handovers.

#### Event A3

當鄰近小區的訊號強度一定程度的大於當前的服務區，Event A3 就會被觸發，這樣一來，UE 就會回報 measurement results 給 eNB。
eNB 會根據評斷標準決定是否要進行換手。
:::info
**評斷標準**
if A3 is triggered AND the A3 triggering criteria last longer than the time specified as Time-to-Trigger (TTT) period.
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下圖為 symbol 的定義:

### Handover Decision
當 Event A3 被觸發，eNB 會判斷要以哪種方式實施換手流程，換手流程可以根據 EPC Etities 的一些狀態被分為以下幾類:

#### 1. Handover Categorization 1: Whether EPC Entities are Changed or not
根據 UE 連結到的 EPC entities 是否改變，可以將 Handover 區分為幾類:
- Intra-LTE Handover
換手後，UE 連上相同的 MME 與 S-GW。
- Inter-LTE Handover
- Inter-MME Handover
換手後，UE 連上其他 MME 與同樣的 S-GW。
- Inter-S-GW Handover
換手後，UE 連上其他 S-GW 與同樣的 MME。
- Inter-MME/S-GW Handover
換手後，UE 連上不同的 MME 與 S-GW。
- Inter-RAT Handover
換手後，使用了不同的 radio access technology:
- UTRAN to E-UTRAN
- E-UTRAN to UTRAN
#### 2. Handover Categorization 2: Whether EPC Entities are Involved or not
取決於 EPC entity 是否參與換手的決策，LTE Handover 可以是 X2 Handover 或是 S1 Handover。
- X2 Handover
X2 interface 連結了 eNB 與 eNB，如果 Source eNB 與 Target eNB 之間有 X2 Interface 連結，即可使用 X2 Handover。如此以來，就不會有 MME 干涉換手程序。
- S1 Handover
eNB 使用 S1 Interface 連結 EPC (MME for control plane, or S-GW for user packets)
在 X2 Handover 無法觸發的情況下，就會使用 S1 Handover 進行換手。


### Handover Procedure
The handover procedure consists of three phases, preparation, execution and completion phases, as follows:

#### 1. Handover Preparation Phase

不管是透過 X2 或是 S1 發起 Handover procedure，Source eNB 都需要將 UE 的上下文傳送給 Target eNB 以確認 Target eNB 是否足以提供好的服務品質，上下文的內容主要包含了:
- Security context
- QoS context

如果可以，Target eNB 會建立起 transport bearer (DL Packet forwarding bearer) 且分配 C-RNTI 傳送給 Source eNB 供 UE 使用。

#### 2. Handover Execution Phase

在這個階段中:
- Source eNB 會中斷與 UE 之間的 radio link
- Source eNB 將必要的資源 Forward 給 Target eNB
- Target eNB 建立 DRB, DL S1 bearer, C-RNTI, etc.
- 當一切準備就緒，Target eNB 會向 UE 發送 **Handover Command** message

當 Target eNB 所分配的 C-RNTI 傳送給 Source eNB 後，後者會將這段時間的下行封包 Forward 給前者，以確保換手期間 UE 不會遺失訊息。


#### 3. Handover Completion Phase
Once the path is switched, the forwarding bearer used in forwarding DL packets during the handover execution phase is released.

### Handover Interruption Time
在 Handover Preparation Phase，網元會事先分配資源以確保沒有下行封包會在換手期間遺失。
然而，在實際的換手流程上，Handover interruption time 是無可避免的，如果這些中斷時間持續過久，會導致無法支持無間斷服務，使用者便會得到差勁的服務品質。Handover interruption time 包含了:

### Mobility Robustness Optimization
if the signal that UE is receiving from its serving or target cell cannot remain strong enough until the UE completes its radio access to the target cell. Radio Link Failure (RLF) is caused by many different reasons, and can be handover-related or not.

If RLF is caused while a handover is being executed, UE may perform an RRC connection reestablishment procedure, and connect to its serving cell, target cell or another cell.