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A Novel Cell Reselection Method in the Scenario of Multi-RAT

Rong Wang, Tao Zhang, Xinlong Luo

School of Information and Communicat ion Engineering

BUPT Beijing, China

[email protected], [email protected],

luoxin [email protected]

Dengkun XiaoResearch Dept of Beijing R&D

Subdivision, WN Huawei Technologies Co., Ltd

Beijing, China [email protected]

Fauzi FannySchool of Electronics and Information Engineering

Beihang UniversityBeijing, China

[email protected]

Abstract—In order to reduce the number of unnecessary UE (User Equipment) cell reselection in Multi-RAT (Multi-Radio Access Technology) that consists of 2G, 3G and LTE for energy saving, a new method on procedure of cell reselection according to UE preference based on priority level is proposed in this paper. Later, this method will impact the measurement action and UE energy saving. The simulation results prove that the proposed scheme shows significant performance improvement on reducing the number of cell reselection and also UE power consumption.

Keywords- Cell reselection, Multi-RAT, UE energy saving

I. INTRODUCTION

Cell reselection is one of the hot issues in Multi-RAT (Multi-Radio Access Technology) 2G, 3G and LTE. And the co-existence of multip le RATs offers mobile operators a powerful means to meet user’s demand. For example, 2G is often used for voice services while 3G and LTE are better for high speed data services. As a matter of fact, different users may have d ifferent applicat ion requirements. However, in current specification and current networks, cell reselection only relys on the signal strength, thus in certain case, it can be considered as disadvantage due to the probability of the UE (User Equipment) selecting LTE is often times greater than selecting 2G or 3G. In this paper the reselection procedure in idle mode will be improved to achieve better performance.

The scope of this paper is to focus on reducing number of cell reselection comparing to the specification 3GPP 36.304 [1]. To satisfy the real networks, the coverage radius scenario is designed such as 2 3G G LTEradius radius radius� �as shown in the Figure 1.

LTE

LTE

GSMGSM

LTE

3G

2G

Figure 1. Network topology for one sector

The prior art of cell reselection in Multi-RAT refers to [1], and it doesn’t mention UE preference during procedure cell reselection. Another former work which is close to the energy saving based on cell selection is introduced in [2], it emphasizes to the triggering condition related to the network traffic. In this paper, the novel method we propose shows the benefits on reducing the cell reselection number and energy saving.

The paper is organized as follows. The classic and proposed cell reselection mechanisms are described in section . Section presents the simulation model including the network topology, UE deployment, parameters for every RAT and so on. In section , the simulat ion result and analysis are given. Finally, conclusions are presented in section .

II. CELL RESELECTION MECHANISM

A. Classic Cell Reselection MechanismAccording to [1], E-UTRAN inter-frequency and inter-

RAT cell reselection rely on whether the thresholds for serving cell and neighbor cell are satisfied. A brief introduction is as follows:

1) Reselection to a cell on higher priority E-UTRAN or inter-RAT frequency than the serving frequency shall be performed if:

2013 International Conference on Cyber-Enabled Distributed Computing and Knowledge Discovery

978-0-7695-5106-7/13 $26.00 © 2013 IEEE

DOI 10.1109/CyberC.2013.82

442

� Neighbor cell has qual X,HighQS >Thresh during

RATTreselection for a cell higher priority EUTRAN or UTRAN RAT/ frequency or,

� Neighbor cell has rxlev X,HighPS >Thresh during

RATTreselection for a cell higher priority GERAN or CDMA2000 RAT/ frequency.

� More than 1 second has elapsed since the UE camped on the current serving cell.

Otherwise, � Neighbor cell has rxlev X,HighPS >Thresh during

RATTreselection for a cell higher priority RAT/ frequency.

� More than 1 second has elapsed since the UE camped on the current serving cell.Where serving cell means the cell on which the UE

is camped, and neighbor cell means the other cells. qualSis the cell reselection quality value, and rxlevS is cell reselection RX level value. X,HighQThresh and

X,HighPThresh separately specifie the qualS threshold and

rxlevS threshold (in dB) used by the UE when reselecting towards a higher priority RAT/ frequency than the current serving frequency. RATTreselection specifies the cell reselection timer value. For each target RAT aspecific value for the cell reselection timer is defined, which is applicable when evaluating reselection towards other RAT.

Figure 2 depicts the workflow of whole procedure. 0.Begin

1.if inter-RAT layers has a priorityhigher than the priority of the

current serving cellY

N

8. reselect the targetcell as serving cell

Y

2.if threshServingLowQis provided

3.if Squal > ThreshX, HighQ(for UTRAN FDD) or

Srxlev > ThreshX, HighP (forUTRAN TDD and GERAN)

Y

7.if More than 1 second haselapsed since the UEcamped on the current

serving cellY

14. not reselect cell

Y

9.if Srxlev >ThreshX, HighP

13.if More than 1second has elapsed

since the UE camped onthe current serving cell

N

N

N

NY

N

4. StartT_reselection Timer

5. Squal ThreshX, HighQ(for UTRAN FDD) or Srxlev

ThreshX, HighP (forUTRAN TDD and GERAN)

6. StopT_reselection Timer

Y

N


11. SrxlevThreshX,

N


Y

Figure 2. workflow for classic cell reselection method

2) Reselection to a cell on lower priority E-UTRAN or inter-RAT frequency than the serving frequency shall be performed if:

� Serving cell has qual Serving,LowQS <Thresh and neighbor cell with lower priority, EUTRAN or UTRAN

RAT/frequency fullfills qual X,LowQS >Thresh during

RATTreselection or,� Serving cell has

qual Serving,LowQS <Thresh and neighbor cell with lower priority, GERAN or CDMA2000 RAT/frequency fullfills

rxlev X,LowPS >Thresh during

RATTreselection .� More than 1 second has elapsed since the UE

camped on the current serving cell.Otherwise, � Serving cell has rxlev Serving,LowPS <Thresh and neighbor

cell with lower priority, RAT/frequency fullfills rxlev X,LowPS >Thresh during RATTreselection .

� More than 1 second has elapsed since the UE camped on the current serving cell.Where Serving,LowQThresh and Serving,LowPThresh

separately specifie the qualS threshold and rxlevS threshold (in dB) used by the UE on the serving cell when reselecting towards a lower priority RAT/frequency.

X,LowQThresh and X,LowPThresh separately specifie the

qualS threshold and rxlevS threshold (in dB) used by the UE when reselecting towards a lower priority RAT/frequency than the current serving frequency.

Figure 3 depicts the workflow of whole procedure.0.Begin

1.if serving cell fulfilsSqual <

ThreshServing, LowQY

N


Y


3.if Squal > ThreshX,HighQ (for UTRAN FDD)or Srxlev > ThreshX,HighP (for UTRAN TDD

and GERAN)

Y

7.if More than 1 secondhas elapsed since theUE camped on thecurrent serving cell

Y


Y

9.if Srxlev >ThreshX, HighPduring a timeinterval

TreselectionRAT


since the UEcamped on the

current serving cellN

N

N

NY

N

4. StartT_reselection

Timer

5. Squal ThreshX,HighQ (for UTRAN FDD)or Srxlev ThreshX,HighP (for UTRAN TDD

and GERAN)


Y

N


Timer

11. SrxlevThreshX,

N


Y

Figure 3. workflow for classic cell reselection method

B. Proposed Cell Reselection Mechanism As for the novel cell reselection mechanism is concerned,

UE’s preference is considered and it is assumed that every UE has one prefer RAT.

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1) Reselection to a cell on higher priority E-UTRAN or inter-RAT frequency than the serving frequency shall be performed if:

� Neighbor cell has qual X,HighQS >Thresh and UE’sprefer RAT is not the RAT type of current serving cell, or it is, but received signal fulfils

rxlev X,newS <Thresh during RATTreselection fors acell higher priority EUTRAN or UTRAN RAT/ frequency or,

� Neighbor cell has rxlev X,HighPS >Thresh during

RATTreselection and if UE’ prefer RAT is not the RAT type of current serving cell, or it is, but received signal quality fulfils qual X,newS <Thresh , for a cell h igher priority GERAN or CDMA2000 RAT/ frequency.

� More than 1 second has elapsed since the UE camped on the current serving cell, but the UE’ prefer cell is not the current serving cell.

Otherwise, � Neighbor cell has rxlev X,HighPS >Thresh during

RATTreselection for a cell h igher priority RAT/ frequency, and UE’ prefer cell is not the current serving cell. or it is, but received signal fulfils

rxlev X,newS <Thresh . � More than 1 second has elapsed since the UE

camped on the current serving cell.Where X,newThresh specifies the qualS threshold or

rxlevS threshold (in dB) used by the UE when reselecting towards a different RAT/frequency and the preference of UE is considered.

Figure 4 depicts the workflow of whole procedure. 0.Begin

1.if inter-RAT layers has apriority higher than the priorityof the current serving cell

Y

N


Y

2.if threshServingLowQ isprovided

3.if Squal > ThreshX, HighQ(for UTRAN FDD) or Srxlev> ThreshX, HighP (for UTRAN

TDD and GERAN)

Y

9.if More than 1 secondhas elapsed since the UEcamped on the current

serving cellY


Y

11.if Srxlev >ThreshX, HighP

17.if More than 1second has elapsedsince the UE campedon the current serving

cell

N

N

N

N Y

N6. Start T_reselection

Timer

7. Squal ThreshX,HighQ (for UTRAN FDD)or Srxlev ThreshX,HighP (for UTRAN TDD

and GERAN)


Y

N


15. SrxlevThreshX,

N


Y

4.if UE s prefer RAT iscurrent serving cell s RAT

5.if UE s receivedsignal fufils Squal <ThreshX,new N

Y

Y

12.if UE s prefer RAT iscurrent serving cell s

RAT


Y

YN N

Figure 4. workflow for novel cell reselection method

2) Reselection to a cell on lower priority E-UTRAN or inter-RAT frequency than the serving frequency shall be performed if:

� Serving cell has qual Serving,LowQS <Thresh and neighbor cell with lower priority, EUTRAN or UTRAN RAT/frequency fullfill qual X,LowQS >Threshon condition that UE’ prefer RAT is not the RAT type of current serving cell, or it is, but received signal fulfils

rxlev X,newS <Thresh during

RATTreselection or,� Serving cell has qual Serving,LowQS <Thresh and neighbor

cell with lower priority, GERAN or CDMA2000 RAT/ frequency fullfill rxlev X,LowPS >Thresh during

RATTreselection , and if UE’s prefer RAT is not the RAT type of current serving cell, or it is, but received signal quality fulfils qual X,newS <Thresh .

� More than 1 second has elapsed since the UE camped on the current serving cell.

Otherwise, � Serving cell has rxlev Serving,LowPS <Thresh and

neighbor cell with lower priority, RAT/frequency fullfill rxlev X,LowPS >Thresh during RATTreselection , and UE’ prefer cell is not the current serving cell. or it is, but received signal fulfils rxlev X,newS <Thresh .

� More than 1 second has elapsed since the UE camped on the current serving cell.Figure 5 depicts the workflow of whole procedure.

0.Begin

1.if serving cell fulfils Squal< ThreshServing, LowQ

Y

N


Y


3.if Squal > ThreshX,HighQ (for UTRAN FDD)or Srxlev > ThreshX,HighP (for UTRAN TDD

and GERAN)

Y



current serving cellY


Y

9.if Srxlev >ThreshX, HighPduring a timeinterval

TreselectionRAT



current serving cell

N

N

N

N Y

N

5. Squal ThreshX,HighQ (for UTRANFDD) or SrxlevThreshX, HighP (forUTRAN TDD and

GERAN)


Y

N

11. SrxlevThreshX,

N


Y

Y6. Start

T_reselectionTimer

4.if UE s preferRAT is current

serving cell s RAT

5.if UE s receivedsignal fufils Squal <ThreshX,new

N

Y

Y


Timer

4.if UE s preferRAT is current

serving cell s RAT


Y

N N

Figure 5. workflow for novel cell reselection method

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III. SIMULATION MODEL

The Multi-RAT network topology and UE deployment are shown in the Figure 6. There are 21 sectors for every RAT, and in the overlapping area of the three RATs totally 420 UEs are randomly deployed. In the figure, the green cells are depitcing GSM, blue ones are representing UMTS cells and the red ones are LTE cells. The black spots are the UEs.

Figure 6. Network topology

The key parameters of the simulation system is set as table below.

TABLE I. SIMULATION PARAMETER

Parameter EUTRAN UTRAN GERANBS

Transmit Power 43dBm 43dBm 43dBm

ISD 500m 650m 1000m

Pathloss Model L(dB) = 128.1+37.6*log(d(km))+20*log(frequency/2)

Penetration 20dB

Antenna pattern, horizontal

(For 3-sector cell sites with fixed

antenna patterns)

� ��

�

��

��

�� m

dB

AA ,12min2

3��

dB3� = 70 degrees, Am = 20 dB

UE Speed 3km/h and 30km/hThe simulation purpose is to find out the UE reselection

number in scenario of Multi-RAT with classic and novel cell reselection method and get the conclusion through the comparison of results. The total simulat ion times are 10s and 3.5min for different scenarios, and UE speed are 3km/h and 30km/h separately.

IV. SIMULATION RESULT AND ANALYSIS

In this section, simulation results of classic and novel cell reselection method is presented, including comparisons of cell reselection number and power consumption.

A. Total UEs’ simulation resultFigrue 7 depicts 420 UEs movement with speed 3km/h

and 30km/h during 10 seconds simulat ion time.

Figure 7. Network topology with UE trace

For all the UEs’ scenario with novel cell reselection method, we set the UE preference uniformly, which means there is an equal probility for all the situtations of 2G, 3G and LTE. And for classic cell reselection method, UEs do not have preferred RAT type.

TABLE II. SIMULATION RESULTS WITH UE SPEED 3KM/H

Parameter with UE Speed 3km/h

Classic cell reselection

Novel cell reselection

Cell reselection number 15 8Effective DRX cycle

detection period 5.76ms 3.072ms

Power consumption 1 0.53Where DRX cycle means the individual time interval

between monitoring paging occasion for a specific UE. Here, taken from [3], measurement of intra-frequency

E-UTRAN cells that is for DRX cycle length[s] = 0.32, then Tdetect, EUTRAN_Intra[s] (number of DRX cycles) = 11.52(36).

We take probability 1/2 fo r wake up state in each DRX cycles so then for effective DRX cycle Tdetect is 11.52/2ms =5.76ms for classic method, and fo r novel method is 8/15*5.76ms = 3.072ms.

Assuming the power consumption of the former method is 1, and we can get the value for later method is 0.53 which means energy is saved, only 53% of the former scenario.

TABLE III. SIMULATION RESULTS WITH UE SPEED 30KM/H






Power consumption 1 0.59Figure 8 depicts the simulation result of cell reselection

number of two algorithms.

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UE speed 3km/h UE speed 30km/h0

20

40

60

80

100

120

140

160

180

200C

ell r

esel

ectio

n nu

mbe

rCell reselection number of total UE

Classic cellreselection

Novel cellreselection

Figure 8. cell reselection number of total UE

Figure 9 depicts the simulat ion result of power consumption of two algorithms.

UE speed 3km/h UE speed 30km/h0

0.2

0.4

0.6

0.8

1

Pow

er c

onsu

mpt

ion

Power consumption comparison of total UE



Figure 9. Power consumption comparison of total UE

As it can be seen from the simulation results , novel method shows benefits on reducing the cell reselection number and energy saving as well. The cell reselection number reduced by 46.67% and 40.74%, and the power consumption is 53% and 59% of classic method.

B. Single UE’s simulation resultFigure 10 describes a UE’s movement in Multi-RAT

network. In the simulation, the UE movement is in constant direction and the total simulation time is 3.5 minutes. UE speed is set to be 30km/h.

Figure 10. Single UE movement with speed 30km/h

For single UE’s scenario, we simulate 3 different situations when UE’s preference is set to be 2G, 3G, and LTE.

TABLE IV. UETYPE IS 2G WITH SPEED 30KM/H






Power consumption 1 0.96

TABLE V. UETYPE IS 3G WITH SPEED 30KM/H






Power consumption 1 0.51

TABLE VI. UETYPE IS LTE WITH SPEED 30KM/H






Power consumption 1 0.44Figure 11 depicts the simulation result of cell reselection

number of two algorithms.

UE preference is 2G UE preference is 3G UE preference is LTE0

5

10

15

20

25

30

35

40

45

50

Cel

l res

elec

tion

num

ber

Cell reselection number of single UE



Figure 11. cell reselection number of single UE

Figure 12 depicts the simulation result of power consumption of two algorithms.

UE preference is 2G UE preference is 3G UE preference is LTE0

0.2

0.4

0.6

0.8

1

Pow

er c

onsu

mpt

ion

Power consumption comparison of single UE



Figure 12. Power consumption comparison of single UE

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As it can be seen from the simulat ion results, no matter which preference the UE has, novel method shows improvements, and when the preference is LTE the simulation performance enhanced the most.

V. CONCLUSION

In this paper, a novel cell reselection method in the scenario of Mult i-RAT has been presented. We provided a detailed description of the classic and novel Multi-RAT cell reselection method, as well as analysis for the performance through cell reselection number and power consumption from system-level simulation.

Simulation results in different situations show that the cell reselection number as well as UE power consumption are significantly reduced when novel method is used. Benefits are obtained and the quality of service increased.

ACKNOWLEDGMENT

This work is supported by cooperation project of Huawei Technologies Co., Ltd and Beijing University of Posts and Telecommunication.

REFERENCES

[1] 3GPP TS 36.304, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode,” section 5.2, available at http://www.3gpp.org.

[2] Yiran Zhu, Yinghui Chen, Wenjing Li, Peng Yu, “A Novel Energy-Saving Cell Selection Mechanism for Cellular Access Networks,”Wireless Communications, Networking and Mobile Computing (WiCOM), IEEE Press, Sept. 2011.

[3] 3GPP R4-081081, “Idle mode performance requirements for 36.133,”May, 2008.

[4] 3GPP TS 25.331, “Radio Resource Control (RRC); Protocol specification Radio Protocol specification,” available at http://www.3gpp.org.

[5] 3GPP TS 25.304 “User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode,” available at http://www.3gpp.org.

[6] Fred Richter, Albrecht J. Fehske, Gerhard P. Fettweis, “Energy Efficiency Aspects of Base Station Deployment Strategies for Cellular Networks,” Vehicular Technology Conference Fall (VTC 2009 –Fall), 2009.9.

[7] Yinan Qi, Muhammad Imran, Rahim Tafazolli, “On the Energy Aware Deployment Strategy in Cellular Systems,” Personal, Indoor and Mobile Radio Communications Workshops (PIMRC Workshops), 2010.9.

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