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Handover in the 3GPP Long Term Evolution

(LTE) Systems

Jihai Han, Bingyang Wu National Mobile Communication Research Laboratory, Southeast University, Nanjing, 210096, China

Email: [email protected], [email protected]

Abstract --One of the main goals of E-UTRAN is to provide

seamless access to voice and multimedia services with strict

delay requirements, which is achieved by supporting

handover from source cell to target cell. Proper handover

algorithms can make the system increased capacity, better

coverage, higher throughput, reduced latency requirements.

In this paper we give an overview of the LTE intra-access

handover procedure, the simulation approach, and several

latest handover algorithms which can enhance performance

of the LTE systems are introduced. Finally the results show

that the handover in the LTE system is complicated and

imperfect, proper handover algorithms in the LTE system

should be researched as quickly as possible.

Keywords- eNodeB; Handover; LTE; Outage probability;

Semi-soft Handover;

I. INTRODUCTION

3G WCDMA system are being deployed all over the

world, while the technologies are being enhanced with

improvements continuously that can guarantee increased

user bit rates and provide better capacity and coverage

performance. In the LTE and LTE-Advanced systems,

Mobility enhancement is an important aspect. LTE and

LTE-Advanced systems should support mobility for

various mobile speeds up to 350km/h (or even up to

500km/h) [1]. With the moving speed even higher, the

handover will be more frequent. Therefore, the handover

performance becomes more crucial, especially for real

time service.

The E-UTRAN architecture is comprised of eNodeBs,

Mobility Management Entity (MME), and System

Architecture Evolution (SAE) Gateways (Figure.1). The

eNodeBs are connected to the MME/S-GW by the S1

interface whereas X2 interface is interconnecting between

the eNodeBs. The latter exists between neighboring nodes

that needs to communicate with each other. The X2

interface is used also on U-plane for temporary user

downlink data forwarding during the inter-eNodeB

handover [2].

S 1

S 1

S 1

S 1

X 2X

2

Figure.1.E-UTRAN architecture

The rest of this paper is organized as follows. Section

II gives the overview of the Long Term Evolution (LTE)

systems and the E-UTRAN architecture. The specific

handover procedures are introduced in Section III.

Section IV gives the simulation approach of the handover

procedure, which is most widely used in the systems. In

Section V, several novel handover algorithms that can

improve the performance of the systems are analyzed.

Finally we give a conclusion and discuss the future work

in Section VI.

II. OVERVIEW OF HANDOVER

There are two main handover technologies in wireless

communication systems, hard handover and soft handover.

Hard handover is a break-before-make method. It means

that a new wireless link connection with the target

eNodeB should be set up after the release of the

connection with the source eNodeB. Soft handover is a

make-before-break method. It means that a new wireless

link connection is established with the target eNodeB

while the connection with source eNodeB is maintained.

The UE simultaneously receive all services data from

several active eNodeBs [3].

A handover procedure can typically be divided into

four parts: the measurements control, the measurements

report, the handover decision, the handover execution [4].

Measurements control and measurements reports can be

978-1-4244-9003-5/10/$26.00 ©2010 IEEE

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considered as handover measurements. In the LTE system,

handover measurements are achieved by the signaling

interaction between the measurements control and the

measurements reports. Handover measurements are made

Figure.2. Intra-MME/S-GW handover

in downlink and processed in the user-equipment (UE).

Processing is done to filter out the effect of fast-fading.

These processed measurement results are reported back

to the base-station (eNodeB) in a periodic or event based

manner. Hence a handover is initiated based on the

processed handover measurements and if certain criteria

are met then the target cell becomes the serving cell

performing the network procedures with the assistance of

the UE.

Handover technology have many decision criterions,

the main criterions are as follows: Reference Signal

Received Power (RSRP); Reference Signal Received

Quality (RSRQ); Received Signal Strength Indicator

(RSSI); Signal Noise Ratio (SNR); Carrier interference

ratio (CIR); Signal interference plus noise ratio(SINR).

Received Signal Strength Indicator is the most widely

used criterion in the systems. Handover algorithms that

presented in [5] [6] are both based on received signal

strength (RSS) measurements.

III. HANDOVER PROCEDURE OF LTE SYSTEM

In LTE systems, active mode mobility managements

are distributed, the eNodeBs are making the handover

decisions without involving MME/S-GW. The necessary

handover information is exchanged between eNodeBs

via the X2 interface. MME/S-GW is notified with a

handover complete information after a new connection is

established between UE and the target eNodeB. After the

reception of the information, the MME/S-GW switch the

path. So, there is a time (Detach Time) when the UE is

not connected to the systems. The solution method of the

problems is the temporary forwarding of user data from

the source cell to the target cell. But the forwarding of

the user data can make more delay to the systems and

finally impact the performance of the systems. We give

some novel handover algorithm that can decrease the

delay of the systems which will be involved in the

Section V.

The Figure.2.gives a more detailed description of the

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intra-MME/S-GW handover procedure we can summary

the main steps of the handover procedure as followes:

0. The UE context in the source eNodeB contains

information regarding roaming restrictions that

where provided either at connection construction

or at the last Timing Advance (TA) update.

1. The source eNodeB configures user equipment

(UE) measurement procedures according to the

area restriction information.

2. The handover is triggered by the UE that sends a

Measurement Report to the source eNodeB.

3. The source eNodeB makes Handover Decision

based on the Measurement Report and the Radio

Resource Management (RRM).

4. The source eNodeB sends a Handover Request

to the target eNodeB which contains all the

relevant handover informations.

5. The Admission Control may be performed by

the target eNodeB dependent on the received

information from the source eNodeB to increase

the likelihood of a successful handover, if the

resources can be granted by target eNodeB. If

the resources can not be granted, the target

eNodeB rejects the admission.

6.

The target eNodeB saves the context, preparesL1/L2 for handover and respond to the source

eNodeB with a Handover Request Ack that

provides information for the establishment of

the new radio link.

7. The target eNodeB generates the RRC message

to perform the handover, i.e RRC Connection

Reconfigure Message including the Mobility

Control Information, to be sent by the source

eNB towards the UE. The UE receives the RRC

Connection Reconfigure Message with required

parameters and is commanded by the source

eNodeB to perform the HO. The UE does not

need to delay handover execution for delivering

the HARQ/ARQ responses to source eNodeB.

8. The source eNodeB sends the Sn Status Transfer

Message to the target eNodeB to convey uplink

PDCP SN receiver status and downlink PDCP

SN transmitter status of the E-RABs for which

PDCP status preservation applies (i.e. for RLCAM). The source eNodeB may omit sending this

message if none of the E-RABs of the UE shall

be treated with PDCP status preservation.

9. After receiving RRC Connection Reconfigure

Message which includes the Mobility Control

Information, UE performs Synchronisation to

target eNodeB and accesses the target cell via

RACH, following a contention-free procedure if

a dedicated RACH preamble was indicated in

the Mobility Control Information, or following a

contention-based procedure if no dedicated

preamble was indicated.

10. The target eNodeB responds with UL allocation

and timing advance.

11. When the UE has successfully accessed the

target cell, the UE sends the RRC Connection

Reconfiguration Complete Message to confirm

the handover, along with an uplink Buffer Status

Report to the target eNodeB to indicate that the

handover procedure is completed for the UE.

The target eNodeB verifies the C-RNTI sent in

the RRC Connection Reconfiguration Complete

Message. The target eNodeB can begin sending

data to the UE,now.

12. The target eNodeB sends a Path Switch Request

Message to MME to inform that the UE has

changed cell.

13. The MME sends an User Plane Updata Request

Message to the S-GW.

14. The S-GW switches the downlink data path to

the target side. The S-GW sends one or more

"end marker" packets on the old path to the

source eNodeB and then could release any

U-plane resources towards the source eNodeB.

15. S-GW sends an User Plane Updata Response

Message to MME.

16.

The MME confirms the Path Switch Message

with the Path Switch Ack Message.

17. After sending UE Context Release, the target

eNodeB informs success message of handover

to the source eNodeB and triggers the release of

resources by the source eNodeB. The target

eNodeB sends the message after the Path Switch

Ack Message is received from the MME.

18. Upon reception of UE Context Release message,

the source eNodeB releases radio and C-plane

related resources associated to the UE context.

The service quality experienced by the end user

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during handover is affected by: the Detach Time during

which the UE is not connected to the system; the delay of

the forwarded packets; the delay difference between the

direct path and the forwarded path (after path switching

there can be packets in the system that are forwarded and

in the same time packets are arriving in the new path to

the target eNodeB) and the amount of discarded UL

packets. The delay difference between the direct path and

the forwarded path can cause out of order delivery of

downlink packets, duplicate TCP segments and TCP

timeouts. The duplicate packets might arrive in uplink as

well due to retransmission of the discarded PDUs.

IV. HANDOVER EVALUATION MECHANISMS

The Simulation is the most commonly used handover

evaluation mechanism. Several simulation models fit forthe evaluation of different types of handover algorithms

and different deployment scenarios have been proposed

and used in literatures. The simulation approach contains

many features of the cellular system and the cellular

environment in the evaluation framework. The approach

provides a commonly comparison method of different

handover algorithms, and also provides the performance

of the cellular systems [4]. Despite being cost-effective,

measurements made at the eNodeBs for the handover

performance evaluation are not very useful, and they

cannot characterize small-area performance. The field

measurements are useful, but they are time-consuming

and expensive. Software simulation provides fast, easy,

and cost-effective evaluation. The analytical approach

gives insight into handover behavior quickly, while

simulations are required for complex scenarios. Hence,

the combination of the analytical approaches and the

simulation approaches can be very powerful. Simulation

models usually consist of one or more of followingcomponents: the cell model, the propagation model, the

traffic model, and the mobility model [7].

The cell model-Cell planning strategies in microcells

and macrocells are different. The cells can be considered

as hexagons while considering handover between two

eNodeBs in the neighborhood of two, three, or more cells.

A macrocellular system is sometimes simulated as the

49-cell toroidal systems or the 19-cell toroidal systems [3]

that with the uniformly distributed traffic. Reference [8]discusses the microcell cell planning in the Manhattan

environment. The city is modeled as a chessboard with

squares representing blocks and streets located between

the blocks.

The Propagation Model - The design of spectrally

efficient wireless communication systems require a

detailed understanding of radio propagation environment.

The characteristics of the radio channel vary greatly with

the operating frequency, and the mode of propagation,

e.g., line-of-sight (LoS) radio links, diffraction scattering

reflection, and satellite links. In this paper the emphasis

is on land mobile radio channels that are typical of

terrestrial cellular mobile radio systems, although many

of the concepts will apply to other types of channels as

well [9]. Different propagation models exist for outdoor

and indoor propagation and for different types of

environment (e.g., urban and rural) [10]. Macrocells and

microcells have different propagation characteristics.

Reference [11] presents signal attenuation measurements

for microcells and shows the conventional propagation

models (e. g. Hata and Okumura models) are not valid

for the microcell environment. The main features of the

models discussed have been experimentally validated in

the literature. For example, Reference [12] suggests path

loss, the slow fading, and the fast fading models for the

microcellular systems based on actual measurements.

Reference [13] descries the computer models of Rayleigh,Rician, log-mormal, and the land mobile satellite fading

channels based on processing of a white Gaussian

random process. The propagation model usually consists

of a path loss model, a slow fading model, and a fast

fading model.

The path loss model: in the macrocells, the path loss

models is used for several aspects of cell planning such

as the eNodeB placement, the cell sizing, and frequency

reuse[14]. The path loss models of Hata and Okumuracan be used for macrocells. But microcells have different

models for LOS and NLOS propagation.

The slow fading or the large-scale fading model:

According to [12], the distribution of the slow fading

component is close to a log-normal distribution for a

majority of LoS and NLoS streets with different standard

deviations. The distribution is a truncated log-normally

distributed variation. Reference[15] show an exponential

autocorrelation model for shadow fading in mobile radiochannels. The results show that the model fit is good for

moderate and large cells; the predictions are less accurate

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for microcells due to multipath.

The fast fading or small-scale fading model: Fast

fading is usually modeled as a Rician distribution where

parameter K (Rice factor) varies with distance. When

K=0, the variation is Rayleigh fading. Fast fading can

usually be neglected since it gets averaged out due to a

short correlation distance relative to that of the shadow

fading.

The Traffic Model-Traffic can be assumed to be

uniform distribution for macrocells. But in the microcells,

road structures should to be considered, and traffic can be

allowed only along the streets. The new call arrival

process is modeled as an independent Poisson process

with a certain mean arrival rate. The new call durations

are independent exponential random variables with acertain mean. In some simulation scenarios, the statistics

of dwell time can be useful [16]. Dwell time is defined as

the average time spent by an UE in a cell with handover.

The Mobility Model-The UEs which have different

velocities follow a truncated Gaussian distribution [7].

V. LATEST HANDOVER ALGORITHMS FOR LTE

AND LTE-ADVANCED SYSTEMS

As we known, handover can be classified with hard

handover and soft handover. Soft handover can only used

in the CDMA systems, so we should use hard handover

technology in the LTE and LTE-Advanced systems. But

the hard handover has its defects: the outage probability

is high and the handover procedure may be unreliable.

Reference [17] show that combined partial reuse and soft

handover can improve the cell edge combined in

OFDMA system.

In paper, we give some latest handover algorithms

with a site selection diversity technique termed semi-soft

handover for the multicarrier systems. Reference [18]

presents a handover technique, referred to as semi-soft

handover utilizing macro diversity, which permit both

hard and soft handover advantages for services over

multicarrier-based broadband networks to be retained.

Reference [3] proposes a fractional soft handover scheme

based on the carrier aggregation. The main idea is to

partially perform soft handover for VoIP, but non-VoIP

service is only transmitted from source eNodeB or targeteNodeB. A velocity-based bicasting handover scheme is

to efficiently utilize backhaul network resource in Fourth

Generation (4G) mobile systems, which could lead to an

aggressive consumption of resources at the backhaul

network [19]. When the scheme is widely adopted for the

real time services and the demand for these services

increase, the amount of the backhaul network resources

consumed due to bicasting will increase tremendously.

So we proposed a velocity-based bicasting scheme which

reduces the bicasting time and improve the backhaul

network resource utilization. Our scheme uses a latest

concept of bicasting threshold determined on the basis of

specific mobile speed groups. Handover prediction has

been considered an effective technology for improvement

of the LTE systems handover performance. Although not

a few techniques have been proposed to achieve this goal

including handover preparation based on cross-layer

optimization and mobility prediction, the fact is that their

gains are not often as high as their cost. To overcome

such weaknesses, reference [20] proposes the simple

handover prediction technique which is based on a novel

user mobility model to approximate simulation the laws

of the user mobility actions. We develop a user mobility

database to assist the mobility prediction based on the

user mobility history records.

VI. CONCLUSION

Handover is an integral component of the cellular

communications. The efficient handover algorithms can

enhance the system capacity and the service quality cost

effectively. And different system deployment scenarios

present different constrains on handover procedure. In

the LTE and LTE-Advanced systems, the hard handover

can be used. But the hard handover has its shortcoming,

for example, the high handover outage probability, large

delay. So in the paper, under the analysis of the handover

procedure, we give the novel handover algorithms whichcalled the semi-soft handover. The simulations in [3] [18]

show that the performance of the semi-soft handover is

better than the hard handover in the systems. They also

show that the scheme not only attains lowest handover

outage probability, but also improves the reliability of

VoIP service. Handover technology represents one of the

radio resource management tasks carried by the cellular

systems. Some other resource management functions,

which include the admission control, channel assignment,

and power control are also important for radio resource

management. So when we research the performance of

the handover, the admission control, channel assignment,

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and the control also should be considered.

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