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CHAPTER 2

LTE MOBILITY

2.1 Mobility Management

In the wireless communication, the UE change its location regularly and the network have to keep tracking of UE present location. This mobility of UE may have different patterns such as pedestrians, vehicles and outer space motion etc. The mobility also brings advantages to the end users. By this the end user would have voice or real time video connections in high speed vehicles. In contrast to it, Mobility has also brought more complexities to the network system which results in to complex network management. Hence the main task of 3GPP LTE was to build a network system which promises a seamless mobility of UE with simple network management.

The mobility procedures can be classified into two modes namely idle mode and connected mode. The network controls transition of UE between the idle and RRC (Radio resource control) mode according to its activity and mobility.

2.1.1 LTE Mobility in Idle Mode (Cell Selection & Reselection)

Idle mode UE, as the name suggests that UE is not actively communicate with the network and have not been assigned the network resources. For example, when the UE is turn on for the first time, it starts the initial cell selection procedure. It receives the broadcast channels in the E-UTRAN cell of a Public Land Mobile Network (PLMN).The UE start to check the network information received from the broadcast channels and select a suitable cell with good radio quality. The main purpose of this procedure is to ensure that the UE could connect to the network resources as soon as possible. This procedure is known as Initial Cell Selection. Here it should be mentioned that the UE may have some stored information about the available or carrier frequencies and cells in the neighborhood. This information may get from the system or has been acquired in the past by UE.

After this initial cell selection, the UE have to register itself to PLMN of the selected cell. During a while, the UE will continue to find a better cell as compared to the present one for reselection of cell.

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Here the UE will use the cell reselection criterion. We may differentiate the cell reselection into two types namely as Intra-frequency cell reselection and Inter-frequency / RAT (Remote Access Technology) cell reselection.

Figure 2-1: Idle Mode Overview [1]

2.1.1.1 Intra-Frequency Reselection

Intra-frequency cell reselection is following the cell ranking criterion. The network defines some neighbor cells to the UE in order to measure its radio quality for reselection procedure. Cell ranking is used by UE to find a good radio quality cell for camping. This ranking is based on the Rs criterion for the serving cell and Rn for the neighboring cells. i.e

Rs = Qmeas,s + Qhyst

Rn = Qmeas,n + Qoffset

Whereas Qmeas is the Reference Signal Received Power (RSRP) measurement quantity, Qhyst denotes the power domain hysteresis to avoid the UE ping-pong and Qoffset is an offset value to control different frequency or cell specific characteristics. Hence the UE can reselect the best ranked neighbor cell if it has good radio quality then the serving cell for a longer time than Treselction(gives the cell reselection timer value).

Similarly, the network may also not allow the UE to reselect some cell known as blacklisted cells. Some limits have been defined [1] in order to minimize the number of measurements. So if ‘SServingCell’ (Specify a threshold for intra frequency in dB) is high enough, then UE are not allowed to do any intra-frequency, inter-frequency or inter-system measurements. The range for intra-frequency measurements is that if

SServingCell ≤ SintraSearch.

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Figure 2-2: Idle mode intra-frequency cell reselection algorithm [1]

2.1.1.2 Inter-Frequency/ RAT Reselections

In E-UTRAN LTE, Absolute priority based reselection method is introduced for the inter-frequency/ RAT reselection. Here the UE will prioritize the different frequencies or RATs in the form of layers. Each layer is being given priority and UE use those criteria for reselection procedure. Hence if a UE wants to reselect another layer with good services, so it will have check the threshold level denoted by Threshx, high.

Similarly, for the inter-frequency measurements, the following relation must come true which is SServingCell ≤ SnonintraSearch.

2.1.2 Tracking Area Optimization

Tracking Area is the area/cell tracked by an eNodeB. During the cell selection/ reselection procedure, there is a chance that UE may change its tracking area. This tracking area is being controlled by the MME. The larger the tracking area, the lesser the signaling load and vice versa. Similarly, the smaller tracking area has minimum paging signaling. This tracking area in UTRAN is known as a Routing area.

Figure 2-3: Tracking area concept [1]

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2.2 LTE Mobility in RRC-Connected Mode (Handovers)

In RRC-connected mode, the UE have an active communication with E-UTRAN in order to transmit and receive data. Both the network and the UE is trying to maintain an active radio connection between each other. In addition to it, the UE is also measuring the radio quality of the neighbor cells.

Moreover, when the UE found a cell of good quality as compared to the current one, the eNodeB allows the UE to handover to that cell. This LTE handovering from the eNodeB to other cells, may from same LTE carrier (Intra-frequency handover), to LTE cells on other carriers (Inter-frequency handover) as well as cells of different RAT.

2.2.1. Intra-LTE Handovers

In cellular communication, the handover means the process of transferring an ongoing call or data session from one channel to another channel while connected to the core network. E-UTRAN has the following principles for the handovers procedure:

E-UTRAN has the authority to make handovering of the UE. Hence the network has to decide the time and cell for UE handovering.

E-UTRAN assigns the parameters to UE for measurement and UE send a measurements report to network. After then the network decides the handover according to the UE measurements.

After completing the handovering, the S1 connection between the target eNodeB and the core network is also updated. This process is known as Late path switch. While the core network have no control on the handovering of UE.

2.2.1.1 Handover Procedure

The intra-frequency handover procedure can be shown in figure 2-4. As mentioned in the figure, there is user plane connection between UE and eNodeB as well as to the System Architecture Evolution Gateway (SAE GW). The eNodeB have the S1

connection with MME. When the UE know that the target cell have achieved the measurement reporting threshold level, it sends a report to the source eNodeB. The source eNodeB establishes the signaling connection and GPRS Tunneling Protocol (GTP) to the target cell.

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Figure 2-4: Intra-frequency handover procedure [1]

When the target eNodeB find the availability of resources, the source eNodeB will command the UE to handover to target cell. The UE have the capability to make direct connection with target eNodeB. As this whole procedure has been done on the E-UTRAN level so the core network have no information regarding handovering. Finally, the target eNodeB updated S1 connection with core network and the process is named as late path switching.

2.2.1.2 X2 Based Handover

The X2 based handover scenario will be explain in handover procedure section. The X2

is the interface between the two eNodeB’s to communicate with each other. The X2-AP is the protocol used for the communication over this interface. The X2 based handovering is performed between the two eNodeBs which are directly connected to each other via X2 interface. Here the source eNodeB and target eNodeB is served by the same MME and SGW.

2.2.1.3 S1 Based Handover

The S1 interface provide the connectivity between the eNodeB and Evolved packet core (EPC) which is further divided into two interfaces. One interface is used by the control plane while other one is for user plane.S1 based handover scenario take place when the source and target eNodeB’s are served by different MME’s. It means that there will be no direct X2 connectivity between the two eNodeB’s.

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Here the source eNodeB will be serving by the source MME and source SGW and the target eNodeB will be served by the target MME and target SGW. While the source and target MME’s can communicate over S10 interface. Hence all the handovers decision will take place on the S1 interface, so it’s known as S1 based handover.

2.2.2 Handover Signaling

Here we will define the signaling messages which have been transferred between the UE, E-UTRAN and core network elements (MME & GW). We can divide the procedure into three parts namely as Handover preparation, Handover execution and Handover completion.

2.2.2.1 Handover Preparation

The source eNodeB instruct UE via MEARSUREMENT CONTROL message to check the radio quality of neighboring eNodeBs for handovering. This message contains the measurement reporting threshold. If UE finds a target cell which may fulfills the required threshold level, so it report to the source eNodeB via MEASUREMENT REPORT. The source eNodeB makes the handovering decision on the UE report. So the source eNodeB send a request to target eNodeB for handovering of UE via HANDOVER REQUEST. The target eNodeB will accept the handover request by sending the HANDOVER REQUEST ACKNOWLEDGE to the eNodeB.

Figure 2-5: Handover preparation [1]

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2.2.2.2 Handover Execution

In handover execution, the source eNodeB instructs the UE via HANDOVER COMMAND to make handover to target eNodeB. Similarly, the source eNodeB start sending the downlink packets to the target eNodeB. The UE do make the synchronization with the target eNodeB via a RACH (Random Access Channel) procedure. In reply the target eNodeB gives the uplink allocation and timing advance information to UE. Finally, the UE sends HANDOVER CONFIRM to the target eNodeB and ready to receive data from the target eNodeB.

Figure 2-6: Handover execution [1]

2.2.2.3 Handover Completion

Here the target eNodeB informs the MME about handovering of the UE by sending the PATH SWITCH message. In the same way, the MME inform the Serving Gateway (SGW) about the new location of UE via USER PLANE UPDATE REQUEST message. Hence the SGW switches the downlink path to the target eNodeB and also inform the MME via USER PLANE UPDATE RESPONSE message. Similarly, the MME confirms the PATH SWITCH message with the PATH SWITCH ACK message. Finally, the target eNodeB request the source eNodeB to release the UE related resources by sending RELEASE RESOURCE message.

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Figure 2-7: Handover Completion [1]

2.2.3 Inter-LTE Handovers

Inter-LTE handover have almost the same procedure as compared to the intra-LTE handovering. But the main difference is that, upon inter-LTE handover the whole AS-configuration have to be signaled while in Intra-LTE, there is no need of wide range of signaling [2].

The procedure for Inter-LTE handover can be illustrated in figure 2-8. In the Inter-LTE handover, the UE can send a MEASUREMENT REPORT message to source eNodeB for handovering. Similarly, the source eNodeB may also request the target RAN (Radio Access Network) for handovering by sending the HANDOVER PREPARATION REQUEST. In this message, the source eNodeB also provides the information to target RAN about the UE capabilities. Hence, the target RAN responds with HANDOVER COMMAND to the source eNodeB.

During this while, the source eNodeB instruct the UE for handovering via MOBILITY FROM EUTRA COMMAND message. On receiving this message, the UE start to make connection with the target node.

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Figure 2-7: Inter-LTE Handover [2]