International Conference on Computer and Communication Engineering (ICCCE 2012), 3-5 July 2012, Kuala Lumpur, Malaysia

978-1-4673-0479-5/12/$31.00 ©2012 IEEE

Advanced Handover Techniques in LTE- Advanced system

Ibraheem Shayea, Mahamod Ismail, Rosdiadee Nordin Dept. of Electrical, Electronic & Systems Engineering

Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia

Shaia2009@yahoo.com, {mahamod, adee }@eng.ukm.my

Abstract— There are always increasing demands of high speed data applications in wireless systems with fast and seamless access to voice and multimedia services and QoS guaranteed such as in Fourth Generation (4G) Long Term Evolution (LTE)-Advanced system. Moreover, as mobility speeds support is expected to reach up to 500 km/h, the handover will occur more frequent, thus the system performance in terms of delay and packet loss will be degraded. Hence, efficient radio resource management including handover techniques, load balancing and interference management are essential. In this paper, we present a comprehensive survey on the advanced handover techniques, requirements and features for LTE-Advanced system. Also, advanced handover techniques are highlighted and discussed such as Fractional Soft Handover (FSHO), Semi Soft Handover (SSHO) and multi-carrier handover (MCHO) that incorporate backward compatibility to the existing system. Meanwhile, FSHO technique based on CA with 5CCs are investigated in term of cell throughput and user’s handover numbers. The result shows that, FSHO with 5 CCs improves LTE-Advanced system in term of cell throughput and numbers of user handover better than Non-CA. Consequently, the existing handover techniques that have been proposed have several advantages, but they are not sufficient to solve hard handover problems. Therefore, a new handover technique is essential required to support fast and seamless handover in LTE-Advanced system. As a result, an advanced handover technique is proposed by combining FSHO, SSHO, and MCHO techniques that can enhance the system performance in term of latency, outage probability and handover reliability especially at cell boundary. Also, the expected output from this hybrid technique can reduce transmission overhead on the network cells by balancing the traffic load in the network cells.

Keywords- LTE- Advanced; Advanced Handover techniques; Fast and Seamless Handover.

I. INTRODUCTION With the strong demand for multimedia services and

broadband wireless applications with higher data rate and wider bandwidth with fast and seamless connectivity everywhere and any time. The IMT-Advanced system has initiated the standardization process for the next-generation mobile communication systems (4G) [1]. Meanwhile, the strong competitive increased between the new wireless technologies pushed Third Generation Partnership Project (3GPP) to developing a new mobile communication standard to keep abreast of recent developments in wireless technology and

achieving the ambitious performance goals of IMT-Advanced mobile systems [1]. For that, 3GPP submitted Long Term Evolution (LTE)-Advanced Release 10 (R10) system to ITU, in order to meet the requirements of the candidate IMT-Advanced system as Fourth Generation (4G) technology.

LTE-Advanced system expected to meet and in many times exceed IMT-Advanced system requirements. The supporting for high mobility speed is one of IMT-Advanced system requirements that should be achieved in 4G. LTE-Advanced system can supports for high mobility speed up to 500 km/h [2]. Moreover, LTE-Advanced is expected to support high Data Rate up to 1Gbps in downlink (DL) and up to 500Mbps in uplink (UL). Also, LTE-Advanced system air interface specifications enhanced link layer handover mechanisms providing short handover interruption time [1].

Accessing multimedia services and broadband wireless application in LTE-Advanced system with high Users Equipment (UE) speed will degrade the reliability and efficiency of the wireless system, especially during handover from the source to the target eNBs. So, the increasing use of wireless technology with very high mobility speed will increase the frequent handover occur. For that, the fast and seamless connectivity with minimum delay during handover from source to target eNBs under different UE speed is one of the greatest goals that are necessary for new wireless systems. LTE-Advanced system is one of the latest mobile communication systems that suffer from lack of fast and seamless connectivity to the UEs.

The handover in LTE-Advanced system is purely based on hard handover. Hard handover is fairly simple with less complexity compared to the soft handover. On the other side, there are some limitations on the hard handover, such as high data loss, disruption time, high outage probability and carrier interferences, thus cause unreliable handover procedure [2], especially for multimedia services and broadband applications. Also, by using hard handover technique, it is difficult to maintain the QoS requirement due to the delay in handover that occurs during eNB migration [3], Meanwhile, providing fast and seamless access to multimedia services and broadband internet application with minimum delay requirements is one of the main goals that should be achieved in LTE-Advanced system, which can be achieved by supporting handover from source to target eNBs [4]. At the moment, there are several

Universiti Kebangsaan Malaysia (UKM).

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handover techniques to allow partially soft hanin LTE-Advanced system, as proposed in [2],[7], but those techniques are not sufficienhandover problems. Therefore, a new handoessentially required.

With the limitation of the existing handoLTE system, it is necessary to determine technique to achieve these goals which includof data, ensure efficiency, reduced outage princreased reliability of handover procedure fogeneration LTE –Advanced system.

In this paper an overview of handover inare provided in section II, followed by advtechniques in LTE/LTE-Advanced systems isection IV an advanced handover features arediscussed, then a hybrid handover technique section V followed by conclusion.

II. HANDOVER In a cellular mobile communication sy

handover can be defined as the process of estaradio link connection from the source to targe(BSs). This handovers could be vertical (intehorizontal (intra technology) handover. A veroften occurring between different wireless techa handover for Mobile Station (MS) from Mobile WiMAX network. On the other sihandover is occurring in the same wireless tecthe handover of UE from cell to other cell system [8].

Handover techniques in wireless communcan be classified into two main handover technfirst technique type is called Hard Handoversecond technique type is called Soft Handovfollowing sections, some basic review techniques will be highlighted and discussed.

A. Hard Handover (HHO) The concept of HHO is to break before m

the old wireless link connection is broken frombefore a new connection is activated to the targcan communicate with one eNB only in eachHHO. That means, after release the connecteNB, the new connection is set up and activatWhereas, after the signal strength from a targthe signal strength from the source eNB thexecution as shown in Fig. 1 [9].

Figure 1. Hard Handover Technique

B. Soft handover (SHO)

(Universiti Kebangsaan Malaysia (UKM))

ndover procedure , [3], [5], [6] and nt to solve hard over technique is

over technique in a new handover de avoiding loss robability and to or the future next

n wireless system vanced handover in section III. In e highlighted and

are proposed in

ystem, the term ablishing a target eted base stations er technology) or rtical handover is hnologies such as LTE network to ide, a horizontal chnology such as such as in LTE

nication network niques types. The r (HHO) and the

ver (SHO). In the about handover

make. That means m the source eNB get eNB. The UE

h time slot during tion from source ed to target eNB. get eNB exceeds he HO start the

e.

The SHO handover techniqmethod. That means a new wireletarget eNB is established while the eNB is maintained. The UE simultadata from several active eNBs [2],there are two main SHO techncommunication system. The first diversity Handover (MDHO) andcalled Fast Base Station Switctechniques will be discussed in the tw

i. Macro Diversity Handover (MDIn MDHO, a list of BSs is main

This set of BSs is called an Active this technique, MS have the abilityBSs in the Active Set as shown in Fand performed data from all the Diall Diversity Set BSs are received from MS. Furthermore, the Neighsignal from MS, but the signal stallow Neighbor BS to be added tosupports fast and seamless handovmore stable and gives better perforseamless handover. On the other sidfor its architecture and during handTherefore, utilizing MDHO will incmore network resources will be common in UMTS systems and also[9], [10].

Figure 2. Macro Diversity

ii. Fast Base Station Switching (FBIn FBSS technique, MS and BS

called a diversity set and communicsimilarly as in MDHO. The MS constations in the Active Set and definBS” based on the received signal sthe only BS of the Diversity Set twith for all UL and DL messages traffic connections as shown in handover supports smoother data target eNBs and less system overhside, FBSS has high data lost latencomparable to MDHO.

iincluding trafficUL and DL communication

Anchor RS2

Neighbor RS4Ac

Active RS3

St

Active BS1

MS

Including traffic

que is make-before-break ess link connection to the old connection with source

aneously receive all services [9]. Under this technique,

niques in wireless mobile technique is called Macro

d the second technique is ching (FBSS).These two wo following sub-sections.

DHO) ntained by the MS and BS. Set or Diversity Set. Under y to communicate with all

Fig. 2. For DL, MS received versity Set BSs. In the UL, and performed information hbor BSs can receive the trength is not sufficient to

o the Diversity Set. MDHO ver. In addition, MDHO is rmance in term of fast and

de, MDHO is more complex dover procedure than HHO. crease system overhead and wasted. This technique is

o applied in WiMAX family

y Handover [11]

BSS) maintain a list of access BS

cates with BS in each frame ntinuously monitors the base nes one BS as an “Anchor strength. The Anchor BS is that MS can communicates including management and Fig. 3 [9]. This type of transition from source to

head than MDHO. In other ncy and outage probability

no traffic

Only signal level measurement

2

tive RS1

Neighbor BS2

tation

No traffic

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Figure 3. Fast Base Station Switching [11]

III. ADVANCED HANDOVER TECHNIQUES IN LTE-ADVANCED SYSTEM

The handover technique in LTE-Advanced systems only supports HHO technique at the moment [2]. In fact, HHO has some advantages compared to MDHO and FBSS. But, HHO has several limitations as mentioned in Section I. Also, providing fast and seamless access is one of the important issues that should be achieved in LTE-Advanced system, especially for the services that required higher data rate and wider bandwidth (i.e. multimedia services and internet application) during handover from source to target eNBs [4].

Due to that, several handover techniques are proposed to solve handover problems in LTE-Advanced system. [2], [3], [5], [6] and [7] are some of the new handover technique that are described and proposed to solve these problems. In this paper some handover techniques are highlighted and discussed, such as Fractional Soft Handover (FSHO), a combination of partial reuse with soft handover techniques, a semi-soft handover, and multicarrier handover techniques. All these techniques are newly introduced in LTE-Advanced system (R10) air interface protocol.

A. Fractional Soft Handover Technique (FSHO) FSHO technique has been proposed in LTE-Advanced

system based on Carrier Aggregation (CA) technique. The main concept of FSHO technique is to partially perform soft handover for VoIP service. In this proposed technique, they classify the service to VoIP and non-VoIP services. During the handover procedure, VoIP services are transmitted from both source and target eNBs, while non-VoIP service are transmitted by source or target eNBs. From the theoretical analysis and simulation results in, they have shown that the proposed FSHO scheme significantly reduces the handover outage probability by saving the radio resource and power consumption, compared with SHO. Furthermore, the proposed FSHO technique maintains the QoS of VoIP service and improves the spectrum efficiency. Finally, the proposed FSHO procedure is backward compatible with LTE handover procedure.

Therefore, FSHO scheme is a competitive choice to enhance the mobility performance in LTE-Advanced system [2].

B. Combined Partial resue and soft handover This scheme proposed an inter-cell interference mitigation

scheme based on a combination of partial reuse and soft handover for an OFDMA DL system. The objective of this scheme is to improve the average cell throughput by considering the data rate fairness among the users, compared to the conventional partial reuse scheme, especially at the cell boundary and during handover occurring from source to the target eNBs in LTE-Advanced system. In addition, utilizing this scheme is resulting in a low soft handover overhead. The concept of this proposed scheme is to select the better signal quality among a partial reuse scheme and a soft handover scheme for the cell edge users [5].

C. Semi Soft Handover technique (SSHO) SSHO technique have been proposed utilizing macro

diversity method, which permit both HHO and SHO advantages for services over multicarrier-based broadband networks to be retained. This hybrid handover method is known as Site Selection Diversity Transmission (SSDT). This technique represents a possible solution for multicarrier systems. The basic concept of SSDT is to selectively transmit each downlink symbol according to channel quality from each BS [3].

In [3], the authors explored a hybrid handover method utilized SSDT for OFDM-based broadband networks, used a zero-padding, referred to as SSHO, which overcomes the drawbacks associated with both HHO and SHO. In addition, a framework for the numerical analysis is presented to measure handover gain and to confirm the superiority of the technique over the forward link of OFDM-based broadband systems. From the system analysis and simulation result published in [3], the SSHO technique provides a lower outage probability than HHO and SHO either for a given number of users or according to the distance from the home BS at high data rates. Thus, SSHO is expected to be widely used in high speed multimedia services over OFDM based broadband networks.

D. Multicarrier Handover Techniques Multicarrier handover is a new technique, which will be in

the LTE-Advanced system. Utilizing multiple carriers handover may support for high-data-rate service and increase cell capacity. In addition, UEs with multicarrier capability can perform fast and seamless handover by keeping connection with the serving eNB and performing handover at the target eNB in a parallel manner [12].

Fig. 4 illustrates the basic concept of multicarrier handover. When MS moves from BS 1 to position 1 the MS received weaker signal strength from BS1, it initiates carrier 1 to trigger carrier 2 to ready for handover process. Meanwhile, carrier 2 can search for the target BS and the list of the neighbor BS is prepared in table based on the received signal strength from BSs [13].

When MS moves to around the location that has equal distances from two BSs, the MS receives the same signal strengths from BS 1 and BS 2, thus triggers carrier 2 to execute

Neighbor BS2

Active RS1

Anchor RS2

Active RS3

Neighbor RS4

Active BS1

/RS

Data are transmitted and received but not processed in BS/RS (MS)

MS

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the handover. While carrier 2 receives the trigger signal from BS1, it starts the execution of handover process. Carrier 2 selects the target BS from its searching process from the table list. Furthermore, as MS is moved from BS1 to BS2, the MS received the signal strength from BS2. After carrier 2 completes the handover process; carrier 1 disconnects its connection with BS1. When MS has reached the hysteresis level, it initiates the handover from BS1 to BS2 by carrier 2 [13].

Figure 4. Multicarrier Handover [13].

IV. ADVANCED HANDOVER FEATURES In [2], [3], [5], [6] and [7] several handover features are

presented and described in LTE-Advanced system to provide seamless handover. In this paper, some advanced handover features are highlighted and discussed in LTE-Advanced system (R10). Fast and seamless handover and legacy supported handover features are newly introduced in LTE-Advanced system (R10). All these advanced handover features are expected to enhance the system performance in term of reduce latency during handover in both the physical and medium access control (MAC) layers, fast with seamless connectivity, low outage probability and good reliability with Legacy supported handover.

A. Fast and Seamless Handover Technique Fast and seamless handover are important features in

wireless systems. Fast handover is a network re-entry procedure with minimum handover latency without any explicit interest in packet loss [14] and interruption time. While, seamless handover are a network re-entry procedure with the capability for UE to contact with the target eNB before initiating a network re-entry control message transaction.

Obviously fast and seamless handover depends on the type of user services. For example, the real-time applications such as video conferencing and streaming media are required high data rate and wider bandwidth. So that, there is decreasing for the connection to these real-time applications (i.e. video conferencing and streaming media) will be probably noted to

the users during handover from the source to the target eNBs. But in other side, browsing a website or transferring a file not required high data rate compare to the real-time applications, so the user does not have noticed anything during handover process. As a result, the important crucial factors for fast and seamless handover are the latency and packet loss. These two factors have to be as small as possible to make the handover fast and seamless [15].

Previous works in [2], [3], [5], [6] and [7] are some of the latest handover techniques that are proposed to provide fast and seamless handover procedure. Fast and seamless connectivity maintained the current session, QoS and Service Level Agreements (SLA) during and after handover without any considerable degradation of the quality of service required by the application, disruption or interruption time for the services [14] [15].

B. Supported for Legacy handover The legacy technologies in LTE-Advanced system are refer

to Global System for Mobile Communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), radio access network (GERAN) and Universal Mobile Telecommunications System (UMTS) families: Wideband Code-Division Multiple Access (WCDMA), high-speed data packet access (HSDPA), and HSDPA+ [12]. LTE-Advanced system (R10) specification is required to be backward compatible with GSM family networks. The latest handover techniques in LTE-Advanced system have to support for Legacy GSM network. Since deployment of LTE-Advanced system might be installed over the existing GSM, EDGE, GERAN, UMTS ,WCDMA,HSDPA, HSDPA+ and LTE network, the LTE-Advanced UEs are designed to support mobility capability across the coexisting deployment of legacy and advanced networks without critical constraints on service continuation.

V. LTE-ADVANCED STANDARD EVALUATION

A. Simulation Scenario In this simulation, 19 eNBs with Omni directional antenna

are considered. Where, each eNB is located at the center of each cell with hexagon shape coverage and radius of 1000 km for each cell. While, 40 users are generated randomly in the source cell (eNB 1) with consider 40 users are kept on during each iteration time, which coverage by source cell (eNB 1). Taking to account each cell serving N users, depend on LTE-Advanced standard capacity. In addition, each mobile has the ability to move to any cell of the 6 eNBs that are located in the first tier around the source cell with random direction selection. The frequency reuse factor (FRF) is considered 1 in order to improve LTE-Advanced standard capacity.

The FSHO based on CA with 5 CCs is implemented in the DL direction from the eNB to the UEs, where each user has ability to contact with the served eNB through all the CCs in the UL and DL based on OFDMA technique. LTE-Advanced standard serve up to 30 users simultaneously in every cell with frequency reused factor 1 based on CA with 5 CCs. Conversely, without CA the serving users will be decreased as well as the number of CCs are decreased. Moreover, all the parameters that are used in our simulation are based on LTE-

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Advanced system environment that have been introduced in [16].

B. Performance Evaluation The Physical frame structure that has been introduced in

[17] is considered in our simulation. Based on that, the cell throughput can be calculated for every sub carriers at the UE side. Where, it is measured in every iteration time during all the simulation time and then taking the average over all the users that are simultaneously active in the cell. The evaluation performance in term of cell throughput is measurement is based on Shannon formula that has been defined in [18]. It can be formulated as the following:

����� � ���� ��� ��� ��������� � ���������� ��� �� ����������� where, BW is the total system bandwidth in Hz, BWeff is the

system bandwidth efficiency, it was introduced in [19], SINR� is the achieved SINR, � is frequency reuse factor, it is assumed to be one (� =1), which means only 1/�th of the spectrum can be used by one cell and SINReff is SINR implementation efficiency and formulated as follow.

����� � ��������� ���������� !�"#$%&'$

()*�+������������������,�

where, SINRk denotes the SINR of the ith subcarrier and the parameter, � which is obtained from link level simulations and is adjusted for each MCS separately. N denotes the number of active OFDM sub carriers [20].

The handover numbers in our simulation is investigated through all the simulation time to see in the effect of CA on LTE-Advanced standard during handover. It can be measured based on the following algorithm using Matlab.

VI. RESULTS AND DISCUSSION In this section, the effect of implementing FSHO based on

CA within 5 CCs in LTE-Advanced system is investigated. These investigations have been evaluated in term of cell throughput and the average numbers of handover for every mobile during all the simulation time from the source to the target eNBs in every iteration times.

Fig. 5 shows the effect of implementing CA technique in LTE-Advanced system, in which provides higher user throughput in everywhere in the cell compared to Non-CA techniques. Consequently, CA achieves around 87.5% gain over Non-CA techniques at any closed or far UE’s positions from the served eNB as shown in Fig. 5.

Figure 5. Throughput Versus Distance for One User only over all the

simulation time.

For that, implementing CA in LTE-Advanced system is resulting in an increase for the cell throughput in everywhere in the cell, which led to decrease user’s handover number at the edge of served eNB. Fig. 6 shows that, the benefit of implementing CA technique in LTE-Advanced system in term of user’s handover numbers. FSHO decrease the number of handover per user around 73.5% over Non-CA technique.

Figure 6. User’s Handover Number over all the simulation time.

VII. PROPOSED HANDOVER TECHNIQUE The existing handover technique support for seamless

handover, but these techniques still have some flaws such as inter-cell interference coordination (ICIC), interference mitigation technologies, latency, unreliability and some data loss during handover. Until now, the available handover

250 300 350 400 450 5000

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Normalized Distance [meter]

Cel

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ough

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0 50 100 150 200 250 300 350 400 4500

0.1

0.2

0.3

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0.6

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Average Number of HO per User

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FSHO-5CCif the No of served MS <= System Capacity

if SINR_Served_eNB(t,j) <= SINR_thrshold Make Handoff from served to the Trget eNBs No_FSHO(j) = No_FSHO (j) + 1;

else Kep commnicat with the served eNB; No_FSHO(j) = No_FSHO (j) + 0;

end else

Make Handoff from served to the Trget eNBs No_FSHO (j) = No_FSHO (j) + 1;

end t refer to the simulation time. j refer to the user’s number.

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techniques still insufficient to solve the hard handover problems. Therefore, a new handover technique is essentially required to improve the handover performance in term of latency, outage probability, interferences, interruption time and reliability during handover, especially at cell boundary.

The effect of FSHO on the system performance in term of outage probability in two different areas, i.e. urban and rural macro cell are investigated in [2]. The performance results of FSHO technique enhanced the system performance in term of outage probability better than hard and soft handover. Meanwhile, Implement SSHO improves system performance in term of outage probability and cell throughput as has been investigated in [3]. From that simulation results, SSHO gives less outage probability compared to HHO and SHO too.

In this paper, a hybrid handover technique is proposed as feature work based on the combination of FSHA and SSHO with Multi-carrier handover techniques. The combination is expected to enhance the system performance in term of latency and outage probability. Also, it will be reduced the transmission overhead on the serving cell as expectation, which results in a balanced network traffic within the system cells.

VIII. CONCLUSION The existing handover technique that is utilized in LTE-

Advanced system is known as HHO. HHO offers reduce architecture and handover procedure complexities. But on the other hand, there are several limitations when performing HHO, such as high latency, handover procedure unreliability, high outage probability and data lost. In this paper, a comprehensive overview of handover including handover techniques and features of the existing handover technique that are used in LTE/LTE-Advanced system are highlighted and discussed. Those handover techniques support seamless handover, but suffer from some flaws such as inter-cell interference coordination (ICIC), interference mitigation technologies, latency, unreliability and some data lost during handover. Moreover, implementing FSHO based on CA has been investigated, which result in improve system performance in term of cell throughput in everywhere in the cell and user’s handover numbers much better than the system that has been implemented with one component carrier only (Non-CA).

A hybrid handover technique is proposed to address the shortcomings of the existing approaches. The hybrid handover scheme is based on the combination of FSHA and SSHO with multi-carrier handover techniques. The combination is expected to enhance the system performance in term of latency, outage probability, interruption time and reliability during handover especially at cell boundary. Also, the combination is expected to reduce the transmission overhead on the serving cell, which balances the traffic load within the system cells in LTE-A context.

ACKNOWLEDGMENT This study is sponsored by Universiti Kebangsaan Malaysia

(UKM) through the university research grant UKM-OUP-2012-182.

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