A Simultaneous Network Search Scheme for Fast Roaming and

Handover of the Simultaneous Voice and LTE Mobile Devices

Minsuk Ko, Myungchul Kim and Sungwon Kang

Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea

Keywords: Fast Roaming and Handover, Simultaneous Voice, LTE.

Abstract: Most of the countries in the world are currently providing 2G or 3G mobile communication services with

limited coverage of LTE services. Therefore when an LTE service user moves into or around such

countries, the LTE terminal needs to roam or handover to 2G or 3G network for service quality. For LTE

terminals that can be connected to 3GPP Legacy (such as GSM, UMTS, and HSPA) and CDMA networks,

it normally takes more than 2 minutes to select one of several candidate networks for roaming and 20

seconds to 2 minutes to handover to other networks before getting out of the LTE service area, which is

considered very long by many LTE users. This paper proposes a scheme for fast roaming and handover that

simultaneously searches for multiple networks with a Dual RF LTE terminal for both voice and data

communication. In our experiment with the simultaneous network search implemented in a commercial

LTE terminal with dual RF, the network selection time for roaming was reduced by 15% to 40% and the

delay time for handover in LTE network was reduced by 90%.

1 INTRODUCTION

Generations of mobile communication technology

include the 1st Generation (1G) to the 4th

Generation (4G) in accordance with data

transmission speeds which have been classified by

International Telecommunication Union (ITU).

Only voice communication was available in

analog 1G. Data transmission became possible in the

2nd Generation (2G), which was the first digital

mobile communication technology. 2G includes

Global System for Mobile communications (GSM)

in Europe and Code Division Multiple Access

(CDMA) in the U.S.A. region. However, their data

transmission speed was 14.4 to 64 Kbps so that only

SMS and email could be possible.

3G has opened the real mobile communication

era. The 3rd Generation Partnership Project (3GPP)

in the European region commercialized Wideband

Code Division Multiple Access (WCDMA), which

includes Universal Mobile Telecommunications

System (UMTS) and High Speed Packet Access

(HSPA). On the other hand, the 3rd Generation

Partnership Project2 (3GPP2) in the U.S.A. region

commercialized the Code Division Multiple Access

(CDMA2000), which includes 1xRTT and

Evolution-Data Only (EV-DO).

While 3GPP was commercialized as the 4G

technology with the Long Term Evolution (LTE) in

Korea, U.S.A., etc., 3GPP2 gave up developing a 4G

technology. Although another 4G technology,

Mobile WIMAX, was developed, it is used only in a

small number of countries. Therefore, LTE is a

predominant 4G technology. However, currently just

a few countries including Korea, U.S.A., etc., have

started providing LTE and the rest of the countries

are still providing 2G or 3G only. Therefore, when

the LTE users move into the countries with no LTE

networks, voice and data services must be provided

through 2G or 3G. Also, even in the countries with

LTE network, its coverage may not be as wide as

that of 3G, in which case LTE terminals need to

handover to 3G network when LTE network is not

available.

Since mobile communications are evolutionary

technologies, a mobile device for a new wireless

network should support the existing ones. Also, the

limited coverage of a new wireless network calls for

fast roaming and handover between different

wireless networks as they are essential for service

quality, which operators and terminal manufacturers

all aspire to provide (Tu, 2013).

One approach for fast roaming and handover is

to directly exchange necessary information between

Ko M., Kim M. and Kang S..

A Simultaneous Network Search Scheme for Fast Roaming and Handover of the Simultaneous Voice and LTE Mobile Devices.

DOI: 10.5220/0005037600360043

In Proceedings of the 11th International Conference on Wireless Information Networks and Systems (WINSYS-2014), pages 36-43

ISBN: 978-989-758-047-5

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

two networks. Another is to make a mobile device

search for available network fast, which can save

cost and time compared to the former approach. In

particular, since LTE terminals with a Dual RF

supporting existing wireless networks can be used

for simultaneous network search, the network

selection time for roaming and handover can be

significantly reduced.

1.1 CS Domain and PS Domain

A network comprises of the access network and the

core network. The access network is used for users

to access to the core network and the core network

provides various services including telephony to the

users who are connected through the access network.

The core network in turn consists of the Circuit

Switched (CS) domain, which is for communications

such as voice and SMS, and the Packet Switched

(PS) domain, which is for data communication.

In the CS domain, one exclusive physical route

between a caller and a callee is established until a

telephone call connection is terminated. The route is

maintained during communication so that the service

is stable and delay is short. On the other hand, in the

PS domain, a physical route is established only when

data is transmitted in the form of packets and then

the route is disconnected until next packets are

created to send. Network resources are used only

when data are transmitted so that the resources are

managed efficiently but delay may be longer than in

the CS domain.

1.2 Simultaneous Voice and LTE

Since the LTE technology supports only the PS

domain, various technologies were proposed for

voice communication, for example, Voice over LTE

(VoLTE), Circuit Switched Fallback (CSFB), and

Simultaneous Voice and LTE (SVLTE).

For VoLTE, IP packet is used for voice

communication in LTE network. However, the LTE

service area is still not sufficiently wide and there

are interoperability issues among VoLTE services

by different providers.

CSFB is a technology used by the LTE service

provider who was a 3GPP 3G service provider. It

provides 3G WCDMA or GSM for voice

communication by disconnecting LTE if

communication is needed during data

communication. Since LTE is disconnected during

voice communication, both voice and data

communications are performed in 3G network

(Tanaka, 2009). Paper (Tu, 2013) studies the inter-

play of voice and data in operational LTE networks

and assesses how the popular CSFB-based voice

service affects the IP-based data sessions in 4G LTE

networks, and vice versa. Their findings reveal that

the interference between them is mutual.

SVLTE is a technology used by the LTE service

provider who was a 3GPP2 3G service provider.

Unlike CSFB, SVLTE can simultaneously transmit

data through LTE when voice communication uses

CDMA network (Qunhui, 2011). Terminal for this

service uses a dual RF: one RF to acquire and

the other to acquire and register to the LTE network

for data communication. With a dual RF, data packet

can be transmitted in the LTE network while voice

communication is using the 1xRTT network and the

terminal can separately register to LTE and CDMA

networks by using different band classes. LTE

terminal using a dual RF has two modems, which

are connected to separate RF transceivers and

antennas for communication to networks: one

modem for data communication using LTE and

3GPP Legacy and the other for voice

communication using CDMA. Each modem can

communicate with the other using message passing.

1.3 Network Registration Procedure

The general procedure for mobile communication

terminal to register to a network has three steps as

follows: In Step 1, a mobile terminal acquires time

and frequency synchronization of the base station

using information such as frequency list, mobile

country code, and mobile network code in the SIM

card. In Step 2, the mobile terminal receives network

information that is periodically broadcasted from

network after successfully performing Step 1. The

network information, which is broadcasted by the

base station, includes network type, mobile country

code, and basic information for network connection.

In Step 3, the mobile terminal performs a

registration procedure to the selected network after

successful execution of Step 2. In this procedure,

Steps 1 and 2 are performed by the terminal itself

without assistance of the base station as the network

can recognize the terminal only in Step 3. In general,

the most time consuming of these steps is Step 1 as

it acquires time and frequency synchronization.

2 CONVENTIONAL NETWORK

REGISTRATION

This section introduces the conventional network

registration procedure and analyzes the conventional

roaming and handover.

ASimultaneousNetworkSearchSchemeforFastRoamingandHandoveroftheSimultaneousVoiceandLTEMobile

Devices

2.1 Conventional Network Registration

Procedure

In the conventional network registration procedure

depicted in Figure 1, the terminal first tries to

acquire time and frequency synchronization. If the

terminal fails to acquire time and frequency

synchronization, the network is considered to be

unavailable. If the terminal acquires time and

frequency, it tries to obtain network information that

is broadcasted by the network periodically. If the

terminal fails to obtain network information, the

network is considered to be unavailable. If the

terminal obtains network information, it tries to

the network is considered to be unavailable. If the

terminal registers to the network, the network

registration has been successfully completed.

In general, service providers based on LTE and

CDMA network select a network registration

method for roaming and handover of SVLTE

terminal. Therefore, network registration is

performed in the order of LTE, HSPA, 1xRTT,

UMTS, and GSM. A mobile terminal searches for an

available network in this order so it searches for

LTE, HSPA, and 1xRTT networks sequentially prior

to searching for UMTS and GSM networks.

Figure 2 shows the conventional network

selection procedure. The terminal sets network

priority for search. The priority depends on the

policies of service providers. Network search for

registration is performed according to the priority

order. The conventional network registration

procedure is shown in Figure 1. If network

registration fails, it tries again for a next network

according to the priority order. If the terminal

registers to a network, the network selection has

been successfully completed.

When the priority is in the order of LTE, EVDO,

and UMTS and only UMTS network is available, the

terminal tries search in that order and then finally

registers to UMTS network.

Handover from LTE network to CDMA network

is important when the LTE coverage is not wider

than the CDMA coverage. Since the terminal

performs the procedure for handover to another

network before getting out of the LTE service area,

seamless data service can be provided to users even

though the data transfer rate becomes somewhat

lower.

LTE network may inform available CDMA

network information to a terminal for efficient

handover. CDMA network information, which LTE

network provides, includes frequency information

Figure 1: Conventional network registration procedure.

Start

Set network priority to

Network registration

procedure (Fig.1)

Registration

completed?

Success

Yes

Try again for next

network in priority order

Figure 2: Conventional network selection procedure.

and pseudo noise code (PN Code) of its neighboring

base stations. A terminal measures the signal

strengths of the neighboring CDMA base stations

periodically even though it is registered to LTE

network. Therefore, a terminal performs handover

from LTE to CDMA network based on the measured

WINSYS2014-InternationalConferenceonWirelessInformationNetworksandSystems

Figure 3: Conventional network registration procedure.

signal strength before getting out of the LTE service

area.

However, LTE network may not inform

available CDMA network information to a terminal.

In this case, the terminal selects CDMA network

after getting out of the LTE service area.

Because the service area of CDMA network is

wider than that of an LTE network, the terminal

does not perform handover from CDMA to LTE

network. However, since LTE has a higher priority

than CDMA, handover occurs when the terminal

searches for an LTE network periodically while it is

registered to CDMA network.

2.2 Time Analysis of the Conventional

Roaming and Handover

In this analysis, the time taken for network

registration for roaming was measured using an LTE

terminal with Exynos application chipset (Tanaka,

2009) operating in LTE, CDMA (EVDO, 1xRTT),

and 3GPP Legacy while signals of LTE and CDMA

networks are not available and only HSPA network

is available.

Figure 3 shows experimental results of network

registration for conventional roaming. Network

Terminal

LTE

network

3GPP Legacy

network

CDMA

network

Try to acquire synchronization of LTE network

1. LTE network search/registration

Try to acquire synchronization of CDMA network

Success of synchronization acquisition of LTE network

Network priority order:

1. LTE

2. CDMA (EVDO/1xRTT)

3. 3GPP Legacy (HSPA/UMTS/GSM)

LTE network information acquisition

Try LTE registration

Success of LTE registration

Success of synchronization acquisition of CDMA network

CDMA Network information acquisition

2. Out of LTE coverage

CDMA Network Registration procedure

Success of CDMA Network Registration

Figure 4: Conventional procedure for handover from LTE

to a CDMA network.

priority for search is in the order of LTE, CDMA

(EVDO/1xRTT), and 3GPP Legacy. The terminal

tries to acquire time and frequency synchronization

from the LTE network with the highest priority first.

Also it is confirmed that there is no available LTE

network by failure of its acquisition, which took 1 to

2 seconds. The terminal tries to acquire time and

frequency synchronization from CDMA network

based on the network priority order. However, it is

confirmed that there is no available CDMA network

by failure of acquisition of time and frequency of

CDMA network which took 140 seconds. Finally,

the terminal acquires time and frequency

synchronization of UMTS network with the lowest

priority. Then it receives network information which

UMTS network broadcasts periodically and then

completed registration which took 25 seconds.

Handover time from LTE to CDMA was

measured in the area that both LTE and CDMA

networks are available. LTE network doesn't provide

information of CDMA network to a terminal. The

signal strength of LTE was gradually reduced while

it is registered to LTE network with a higher

priority. We measured the time from when LTE

signal was reduced to the extent that normal service

ASimultaneousNetworkSearchSchemeforFastRoamingandHandoveroftheSimultaneousVoiceandLTEMobile

Devices

is not possible to when LTE terminal with Exynos

application chipset completes handover to CDMA

network.

Figure 4 shows the conventional procedure for

handover from LTE to CDMA network. The

terminal completes registration to the LTE network

with the highest priority. The strength of LTE signal

was gradually reduced for the experiment. Since the

terminal doesn't receive CDMA network information

from LTE network, it doesn't search for a CDMA

network while it is registered to LTE network. When

the strength of LTE signal is reduced to the extent

that normal service is impossible, the terminal

begins to search for a CDMA network. It took 30

seconds to 2 minutes to acquire time and frequency

synchronization of CDMA network, receive network

information, and complete registration.

The terminal performs search based on the

frequency list stored in the SIM card. The search

time varies according to the location in the

frequency list. That is, if the frequency, which is the

same as network frequency, is ahead in the

frequency list, it takes approximately 30 seconds

and, if it is behind, it takes up to 2 minutes.

The terminal searches for networks in the order

of LTE, EVDO, 1xRTT, UMTS, and GSM as

described in Section 2. In the roaming area where

neither LTE nor CDMA network is available, the

terminal can acquire UMTS or GSM network, in

which roaming is possible only after completion of

search for LTE, EVDO, and 1xRTT network. The

search time depends on the frequency of search and,

in general, several minutes are required.

Usually the terminal performs handover to

another network before getting out of the LTE

service area. However, if the signal strength from

the LTE network is reduced abruptly or LTE

network doesn't inform available neighboring

network information, the terminal is disconnected

from the LTE network, in which case it should

perform an initial network selection to acquire

CDMA.

3 SIMULTANEOUS NETWORK

SEARCH SCHEME

We propose a simultaneous network search that can

replace the conventional sequential one. The

proposed search scheme reduces network search

time for roaming and handover by a simultaneous

search for multiple networks.

Figure 5: Proposed network registration procedure.

3.1 Scheme to Reduce Time for

Network Registration in Roaming

and Handover

For SVLTE terminal, two RFs are used at the same

time for both voice and data. Therefore, if two

networks can be searched simultaneously by using

both RFs, the search time can be reduced. In

particular, since networks with a lower priority for

roaming and handover can be searched while high

priority networks are searched, the search will be

very efficient.

As with the network registration procedure in

Section 1.3, Steps 1 and 2 for acquisition of time and

frequency synchronization and reception of network

information are performed by the terminal itself

without assistance of the base station. Therefore, it is

possible that LTE terminal performs Steps 1 and 2 to

both of LTE and CDMA networks simultaneously.

WINSYS2014-InternationalConferenceonWirelessInformationNetworksandSystems

Figure 6: Proposed network selection procedure.

Figure 5 shows the proposed network

registration procedure. The terminal tries to acquire

time and frequency synchronization of network. If

the terminal fails to acquire it, the network is

considered to be unavailable. If the terminal

succeeds, it tries to receive network information that

is periodically broadcasted by the network. If the

terminal fails to receive network information, the

network is considered to be unavailable. If the

terminal receives network information, it checks

whether the selected network has the highest

priority. If the selected network doesn't have the

highest priority, the terminal just waits for network

registration. If the selected network has the highest

priority, network registration is performed.

Figure 6 describes the proposed network

selection procedure using the UML activity diagram

(Object Management Group, 2011) to explicitly

show parallelism. The terminal first sets network

priority for search. The priority depends on the

policies of service providers. The order used in this

paper is LTE, CDMA, and 3GPP Legacy. The

terminal tries to register to LTE network with the

highest priority and to CDMA network

simultaneously. If the terminal registers to LTE

network, the network selection reaches the “LTE

network registration successful” state and

terminates. If the terminal fails to register to LTE

network, it tries to register to 3GPP Legacy network.

This procedure is performed while CDMA network

registration is performed as in Figure 5. So if the

terminal that has currently registered is the highest

priority one – LTE network, then it completes the

registration process. Otherwise it goes to the "Wait

for registration" state. Then if and only if when LTE

registration is not successful, it checks whether

CDMA network information has been received. If

the terminal has received CDMA network

information, it registers to CDMA network and

network selection terminates.

If the terminal hasn't received CDMA network

information, it checks if network information from

3GPP Legacy network has been obtained. If it has,

the terminal registers to 3GPP Legacy network and

network selection terminates. Otherwise, the initial

selection of LTE is resumed.

LTE network may not inform available CDMA

network information to a terminal. In this case, the

terminal selects a CDMA network after getting out

of the LTE service area. In this case, it takes the

terminal the same amount of time as the initial

network selection to obtain available CDMA

network. While a terminal is registered to LTE

network, it measures the signal strength of the

network periodically in order to reduce the network

selection time. If LTE signal is lower than a fixed

level, it begins to search for a CDMA network

regardless of whether the available network

information from the LTE network is received.

When a CDMA network is found, time and

frequency synchronization and network information

acquisition are performed except for its network

registration. If the signal strength of LTE network is

not high enough for normal service, it begins

registration to the CDMA network that it already

selected.

3.2 Roaming and Handover Time

Analysis for the Proposed Scheme

The time required for roaming was measured using

an LTE terminal that uses the Exynos application

chipset (Tanaka, 2009) and supports LTE, CDMA

(EVDO, 1xRTT), and 3GPP Legacy while signals of

LTE and CDMA networks are not available and

only HSPA network is available.

Figure 7 shows experimental results with the

proposed roaming scheme. Network priority for

search is in the order of LTE, CDMA

(EVDO/1xRTT), and 3GPP Legacy. The terminal

tries to acquire time and frequency synchronization

ASimultaneousNetworkSearchSchemeforFastRoamingandHandoveroftheSimultaneousVoiceandLTEMobile

Devices

Terminal

LTE

network

3GPP Legacy

network

CDMA

network

Try to acquire synchronization of LTE network

1. Time and frequency synchronization / 2. Network information acquisition

Try to acquire synchronization of CDMA network

Failure of synchronization acquisition of LTE network

Try to acquire synchronization of 3GPP Legacy network

Failure of synchronization acquisition of CDMA network

Success of synchronization acquisition of 3GPP Legacy network

wait for search result for

CDMA network

Try 3GPP Legacy registration

Success of 3GPP Legacy Registration

3. Network registration procedure

Network priority order:

1. LTE

2. CDMA (EVDO/1xRTT)

3. 3GPP Legacy (HSPA/UMTS/GSM)

3GPP Legacy network information acquisition

Figure 7: Roaming scenario for experiments with the

proposed scheme.

from LTE network with the highest priority first.

The terminal tries to acquire time and frequency

synchronization from CDMA network with the next

priority at the same time because two RF modems

for LTE and CDMA can work simultaneously. It is

confirmed that there is no available LTE network

because it has failed to acquire time and frequency

synchronization of LTE while it is acquiring time

and frequency synchronization of CDMA. Then the

terminal tries to acquire time and frequency

synchronization of UMTS. Since signal of a UMTS

network is available, the terminal acquires time and

frequency synchronization of 3GPP Legacy and then

receives 3GPP Legacy network information. Though

the terminal acquires time and frequency

synchronization of 3GPP Legacy and receives

network information, it should wait for search result

for CDMA network without registering to 3GPP

Legacy network because it is trying to acquire time

and frequency synchronization of CDMA. The

terminal performs registration to 3GPP Legacy

network for roaming after confirming that there is no

CDMA network by failure of acquisition of time and

frequency synchronization of CDMA.

As described above, the time taken to acquire

time and frequency synchronization of CDMA was

longer than those of LTE (about 2 seconds) or 3GPP

(140 seconds at maximum), which is much shorter

than that of CDMA, and the time taken for

acquisition of time and frequency synchronization of

3GPP Legacy was 25 seconds, which is longer than

that of LTE but shorter than that of CDMA.

Compared with the conventional method, which

took 167 seconds to select a network for roaming,

our proposed method took about 140 seconds to

search for LTE, 3GPP Legacy, and CDMA networks

simultaneously.

The time for handover to CDMA network for

LTE terminal with the Exynos application chipset

based on the proposed method was measured in the

same experimental environment described in Section

2. Figure 8 shows experimental results using a

commercial SVLTE terminal with the proposed

handover procedure.

The terminal completes registration to the LTE

network with the highest priority. The strength of

LTE signal was gradually reduced in the experiment.

When the strength of LTE signal is reduced to a

fixed level, the terminal begins to search for CDMA

networks. In this paper, we set the LTE signal

strength level to -100 dBm to begin searching for

CDMA network. The terminal confirms availability

of CDMA network and receives network

information but just maintains time and frequency

synchronization without registration. If the terminal

considers that signal strength of LTE network is not

high enough for normal service, it begins to register

to the CDMA network that it already selected. The

time for registration to CDMA network is less than 1

second, which is shorter than that for initial

registration to CDMA network. Therefore, the

terminal can perform fast handover to CDMA

network for data communication when it moved out

of the LTE service area.

In the conventional method, the same experiment

took 20 seconds to 2 minutes. In the proposed

scheme, the time was reduced by up to 90% with the

exact reduction rate depending on the CDMA

frequency. Such a high performance improvement is

made possible by using parallelism and also

performing Steps 1 and 2 in advance in network

registration procedure.

Since two modems search for a network to

select, it consumes more battery than a sequential

search. But the battery is used just for the initial

network selection. So battery consumption is not

significant. Also the scheme to reduce the time for

handover from LTE to CDMA based on the early

WINSYS2014-InternationalConferenceonWirelessInformationNetworksandSystems

search of CDMA network may consume more

battery where LTE signal is weak but service is

available. In this paper, the threshold to begin an

early search was set to 100dBm. Battery

consumption can be reduced by decreasing the

number of early searches. More detail

experimentation and analysis will be performed

further.

4 CONCLUSIONS

The 4G era has begun with the LTE technology.

High-speed LTE will provide users with various

convenient services such as video conference and

streaming.

This paper proposed a scheme for reducing

network registration time for roaming and handover

time from LTE to CDMA network and demonstrated

that network registration time for roaming was

reduced by 15% through reduction of time for time

and frequency synchronization and network

information acquisition of UMTS and GSM. The

time depends on the frequency of searching and can

be reduced by as much as 45%. Handover time from

LTE to CDMA can be reduced by over 90% through

reduction of CDMA network search where LTE

network doesn't provide CDMA network

information.

The proposed scheme makes use of the existing

Dual RF in a commercial LTE terminal, and works

without change of network configuration and with

only change in LTE terminal. As a result, it gives

benefits such as fast roaming and handover to users

and network operators.

ACKNOWLEDGEMENTS

This work was supported by the National Research

Foundation of Korea Grant funded by the Korean

Government (NRF-2012R1A2A2A01008244).

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ASimultaneousNetworkSearchSchemeforFastRoamingandHandoveroftheSimultaneousVoiceandLTEMobile

Devices