Intra - LTE (Long Term Evolution) HAND OVER
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PRESENTED BY
SRAVANTHI.A

[attachment=13525]
Handover
Definition: The process of changing the current radio channel to a new radio channel
Why Handover: To avoid the termination of call when mobile unit moves from one Base station area to the other base station area
Different ways a handover
Network Evaluated: the network makes the handover decision
Mobile Evaluated: the UE makes the handover decision and informs the network about it. In this instance, the final decision will be made by the network based upon on the Radio Resource Management.
Hard handover: existing connection must be broken before the new one is established. One example of hard handover is when frequencies are changed
Soft handover: is a make-before-break method. It means that a new connection is established with target eNodeB while the old connection with source eNodeB is maintained
Performance targets of LTE
Spectral efficiency two to four times more than with HSPA(High Speed Packet Access) Release 6
Peak rates exceed 100 Mbps in downlink and 50 Mbps in uplink
Packet switched optimized
High level of mobility and security
Round trip time <10 ms
Optimized terminal power efficiency
High speed vehicular mobility: 120 to 350 km/h
Handover interruption times
Handover type Interruption time(ms)
Intra-frequency 27.5
Inter-frequency 40-60
LTE network architecture
eNodeB logical connections
1. E-UTRAN
NodeB
(eNodeB)
2. MME:
3. Gateway:
Serving GW (S‑GW)
PDN GW (P‑GW).
MME(Mobility Management Entity) connections to other logical nodes and main functions
System architecture for 3GPP Access networks
Types of LTE handovers
Intrafrequency intra-LTE handover : A handover inside
E-UTRAN
Interfrequency intra-LTE handover: A handover to E-
UTRAN from other RAT(Radio Access Technology) or from
E-UTRAN to another RAT
Inter-RAT towards LTE
Inter-RAT towards UTRAN(Universal Terrestrial Radio
Access Network) handover
Inter-RAT towards GERAN(GSM/EDGE radio
access network) handover
Inter-RAT towards cdma2000® system handover
Reason for Hard handover in LTE
One of the main goals of LTE is to provide
- fast - seamless handover
The eNodeBs form a self-organizing network through their interconnection with the X2 interface which leads to low latency in LTE. Also, there will be no soft handover—the benefits that were created by the soft handover now aim at the self-organizing network of eNodeBs.
‘Hard’ handovers, meaning that there is a short interruption in service when the handover is performed.
‘Seamless’ handover is the attempt to provide a given QoS also during the process of Handover
E-UTRAN(LTE) handover principles
The handovers are network controlled. E-UTRAN eNodeB decides when to make the handover and what the target cell is.
The handovers are based on the UE measurements.
The UE measurements and measurement reporting is controlled
by parameters given by E-UTRAN.
The handovers in E-UTRAN are targeted to be lossless by using packet forwarding between the source and the target eNodeB.
The core network S1 connection is updated only when the radio
handover has been completed. This approach is called Late path
switch. The core network has no control on the handovers.
Intra-frequency handover procedure
User plane switching in handover
Intra-LTE Handover Signalling
Handover preparation
Handover execution
Handover completion
Handover Preparation
Handover Decision
- - For intra-frequency handovers, the UE needs to be connected to the cell with the lowest path loss
- For inter-frequency and inter-RAT handovers, the eNodeB can also take the load and service information into account.
Admission control
- the uplink interference can be minimized by allowing the UE to connect to the cell with lowest path loss. If there are simply no resources in the target cell, the network may need to release the connection to avoid excessive interference.
Handover Execution
The source eNodeB sends the status information to the target eNodeB indicating the packets that were acknowledged by the UE.
The UE makes the final synchronization to target eNodeB and accesses the cell via a RACH (Random Access Channel) procedure.
Handover Completion
X2 based Handovers
X2-based handover without Serving GW relocation:
when the MME and the Serving GW is are unchanged.
The presence of IP connectivity between the Serving GW and the source eNodeB, as
well as between the Serving GW and the target eNodeB is assumed.
X2-based handover with Serving GW relocation:
when the MME is unchanged but the Serving GW is to be relocated.
The presence of IP connectivity between the source Serving GW and the source
eNodeB, between the source Serving GW and the target eNodeB, and between the
target Serving GW and target eNodeB is assumed.
Handover types based on radio page link failure
Backward handover
Radio Link Failure (RLF) handover
Both of these handover procedures require the source eNB to prepare a target cell for handover concurrently with the handover decision (i.e., the UE’s context must be available and resources must be reserved at the target cell when the UE accesses the target cell), otherwise, the UE transitions to idle-state where it attempts to complete the handover procedure by transitioning back to connected-state via a procedure called Non-Access Stratum (NAS) recovery.
Backward Handover
RLF(Radio Link Failure) Handover
LTE NAS Recovery Procedure
The source eNB does not prepare the target cell for handover.
With NAS recovery, the UE does not remain in connected-state. Instead, upon re-establishment failure, the UE transitions from connected-state to idle-state and attempts to establish a new connection.
The transition to idle state incurs additional delay. No data forwarding, so all of the data buffered in the source eNB is lost.
This will consume valuable backhaul and core network resources by triggering TCP retransmissions which will negatively impact TCP throughput performance.
Problem
A short interruption in service during a long FTP session (e.g. large file download) may be tolerable, while an interruption in a VoIP call or a streaming video session or short FTP session (e.g. image download) or a latency sensitive gaming application may not.
While the LTE handover procedures defined provide mobility support, they may not be suitable for all scenarios and could result in unsatisfactory user experience even when compared to legacy 2G and 3G systems.
Solution: LTE Forward Handover
Forward handover offers advantages
Successful even if the radio conditions are not good
Shorter interruption in service due to the ability to set a more aggressive RLF timer value. This also reduces the number of drive tests needed when deploying base stations and optimizing the network.
New nodes can be added on an ad-hoc basis in hot-spots without the need for extensive drive tests to recompute optimal RLF timers
Performance Comparison
An image download during handover
The download time improved by ~50% for an average user in a LTE 10 MHz system.
Higher average user throughput during handover and hence a much better user experience. (13.3 Mbps vs. 7.6 Mbps average throughput)
CONCLUSION
Different ways of Handover, LTE Network architecture, types of handovers, Intra LTE signalling X2-intra LTE handover, backward handover, RLF handover, NAS Recovery, forward handover, performance comparison.
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