Co-Channel Interference Avoidance in Two-Tier L TE...

2013 19th Asia-Pacific Conference on Communications (APCC) , Bali - Indonesia

Co-Channel Interference Avoidance in Two-Tier L TE Femtocell Systems Using Control Information

ISok Chhorn, 2Si-o Seo, 3Mustafa Habibu Mohsini, Department of Computer and Information Science

Korea University, Korea {I chhorn 168, 2sioseo, 3mustafa }@korea.ac.kr

Abstract- Macrocell Base stations, called the Enhanced

NodeB (eNB) are mainly aiming to serve outdoor users. However,

due to insufficient signal strength this solution has difficulty in

serving indoor users. To address this problem, the Femtocell,

called the Home eNB(HeNB), system has been deployed in the

L TE system. Since the HeNBs and the eNB use the same

frequency band and the same frame structure, co-channel

interference between HeNBs and eNB is unavoidable. To solve

this problem, we propose a co-channel interference (CCI) avoidance scheme using the control information in the downlink

subframe. Simulation results show the proposed scheme

outperforms the conventional two-tier L TE femtocell systems in

terms of the outage probability and the system throughput.

Keywords-LTE; femtocells; interference avoidance; control information;co-channel interference

1. INTRODUCTION

A visible increase in data rates can be noticed moving from 3rd generation systems to 4th generation. With such a substantial increase in the data rates, providing good coverage becomes an important issue. It is also estimated that most of these high data rate users use data services from inside buildings. Usually outdoor Macrocell base stations (eNB) are used to provide coverage to indoor users. However, these solutions are insufficient to provide good signal strength to indoor users, because signals experience high path loss in penetrating buildings. Various research, such as wireless repeater and multi hop relay, have been undertaken to resolve this issue. However, it did not provide an original solution owing to these methods depending on local conditions, such as signal statement and eNB's capacity.

Femtocell study has recently become a hot issue. A Femtocell base station (HeNB) is a low power wireless access point that operates in a licensed spectrum to connect standard mobile devices to the mobile operator's network using residential DSL or cable broadband connections [1]. Femtocell benefits might accrue to consumers and operators. For consumers, they will provide improved mobile coverage (better signal strength) in their homes, and high performance mobile data providing faster access to mobile services and multimedia content. For the mobile operator, HeNBs represent a highly cost effective way of increasing both network coverage and capacity in the home

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4Choong-ho Cho Department of Computer and Information Science

Korea University, Korea [email protected]

� - - :interfering signal _---HlIr""---.::.--r1-- :desired signal

� MUE �

Fig. I. interference senarios in overlaying HeNB and eNB

where a large percentage of phone usage takes place [2]. HeNBs allow service providers to extend indoor service coverage, especially where access to the wireless networks would be limited or unavailable. However, as more and more femtocells emerge in the eNB system, unless the spectrum usage is properly optimized, the increased amount of overlaying areas will invoke severe co-channel interferences (CCI) as shown in figure 1 and the overall network capacity might be significantly compromised. The CCI between HeNBs and eNBs has been widely studied [3]-[6]. Since the number and position of the HeNBs are naturally unknown, interference management cannot be easily handled by the operator using traditional centralized network planning and static optimization techniques such as fractional frequency reuse (FFR). Therefore, interference management of HeNB is a key issue to be solved before large scale commercial deployment.

In order to solve problem such as CCI issue, the feasibility of user-deployed HeNBs in the same frequency band as an existing eNB network is investigated to deploy and operate HeNB networks appropriately [7]. To minimize interference between eNB and hundreds of thousands of HeNBs, HeNBs adaptively configures the FFR pattern [8]. An adaptive fractional frequency reuse (AFFR) scheme [9] allocated spectrum bandwidth proportional to the traffic load of each cell in the muti-cell environment. A dynamic frequency reuse schemes were proposed in [10]. However, the use of networks using control information to avoid CCI due to the deployment of the HeNBs within LTE cellular has not been presented. This paper proposes a new scheme that uses the control information to avoid CCI from HeNBs.

2013 19th Asia-Pacific Conference on Communications (APCC), Bali - Indonesia

The remainder of this paper is structured as follows. Section IT explains the system model. Section ITI describes the proposed control information scheme for co-channel interference avoidance between macro and femtocells. Section IV show the performance measurement. Section V shows the simulation results describing the system model for the deployment scenario. The simulation parameters for eNB, HeNB are discussed in this section. Section VI summanzes the results obtained and explains the conclusions achieved.

A. System Model

IT. SYSTEM MODEL

We consider a scenario with seven eNBs in a suburban area where the coverage of each eNB is approximated by a hexagon, as shown in Figure 2 [11]. HeNBs are deployed randomly within the coverage area and reuse the same frequency as the eNB and thus introducing CCI to eNB. Macrocell UEs (MUEs) are located randomly in the coverage area of each cell (I to 50 active MUE per cell, each MUE uses one resource block (RB)) in the resulting downlink (DL) interference. Home eNodeB UEs (HUEs) are located randomly in each house (1 active HUE per HeNB). All of the HUEs are located in the HeNB house; they are located randomly in the active HeNB. MUEs are located in the HeNB blocks.

We consider a single floor indoor scenario where the house size 12 x 12 m. The probability of the HUE being outdoors is 10%. MUEs are allowed in the HeNB house and are randomly located within macro indoor coverage area. The MUE may be located within the 12 x 12 m HeNB house. HeNB houses are not allowed to overlap. The minimum separation from UE to HeNB and from HeNB to eNB is assumed to be 20 and 35cm, respectively. Distribution of HeNB houses is random uniform within eNB coverage area, subject to minimum separation to eNB and non-overlapping constraint.

B. Signal Power Model

The signal power received, P" at MUE and HUE from eNB and HeNB can be expressed as

Pr =1f*IOA-(PLllO)*L, (1)

where PI is the transmit power of eNB and HeNB, PL is path loss, L is the shadowing effect, with log-normal distribution with zero-mean and a standard deviation of cr. The path-loss is modeled according to the micro-urban models TTU-R report [II]. The path loss model includes free space loss, indoor path loss, indoor wall penetration loss, floor penetration loss and the outdoor wall penetration loss. If HUE is inside the same strip, equation (2) will be applied. And if HUE locates in a different strip, equation (3) is adopted.

I'qdH) = 38.46 + 20 loglO JJ + 0.7d2JJ,indoor + q x f"iW, (2) + 18.3 n «n+2)(n+l)-046)

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1:�,9 ,'800

,///600 _ � � � �� _ I _

S G (!)

:6> CiI ISS CiI S i S ®�CiIS e e l � I Q

---- - - - - - eJ t- - - � - iG- - -- -" G Iil I ' G G \

® - - �® Qi- - -IIQ- t -ir; ® - � -CiI- lL Q

____ � ____ p_� _: til �� Q_ <i_

Ci G I II I II : ---® - - ----- � ® ---- .-- -- -

(!) (!) ® Q I G ___ ___ � Q--�----G __ ___ _ e <p " : (i) Ii : - - - - - - - - - - -Gee- - - - - c - ---- -

·10010'-------�----'-----·1000 ·500 500 1000

Fig. 2. Snapshot of simulated Topology

PL(dE) = max(l 5.3+ 37.61oglO D,38.46+ 20IoglO D) + O.7d2D,in door ,(3) L 18 3

((n+2)(n+I)-0.46 L L +qx iW+ . n + ow,! + ow,2

where D denotes the distance between HeNB and HUE . And d21J. indoor refers to the distance inside the strip with the serving HeNB. Lill' is the penetration loss of the indoor walls, which is 5dB. Low, I and Low,2 all represent the penetration loss of outdoor walls correspondingly, which is equal to 20dB. Here q denotes the number of indoor walls, Penetration ratio R is defined as the probability of deploying a working HeNB in an apartment.

Ill. PROPOSED SCHEME

In this section, we propose a new control information scheme (CIS) for CCI avoidance between eNBs and HeNBs. Co-channel HeNBs introduce interference to eNBs when HeNB uses the same frequency with existing eNBs as shown in figure 3(a). In this paper, we consider only the CCI in the DL when deploying HeNB in eNB. In general, downlink control signaling is located in the first 'n' orthogonal frequency division multiple access (OFDMA) symbol of a subframe with the integer n being less than or equal 3. Multiple Physical Downlink Control Channel (PDCCH) intended for different users can be transmitted in the same subframe by multiplexing them in frequency and time.

In CIS, each MUE uses one RB and the three OFDMA symbols in the HeNB have the knowledge of RBs currently the MUEs deploy. The additional PDCCH in HeNB is also assumed to have the knowledge of the RBs currently deployed by eNB. Based on this knowledge it has, the RBs which are not used by MUE are known and the HUE can obtain suitable RBs selected from the set of unused RBs. Correspondingly, the CCI between eNB and HeNB is avoided. Figure 4 shows the procedure of how HeNBs allocate the RB in the proposed scheme.


(a) Resource allocation in in conventional system (current standard

network)

Fig. 4. Resource allocation procesure for HeNB (CIS)

A. SINR Model

(b) Resource allocation in CIS

Fig. 3. Proposed Control Infonnation Schemes

We assume to have additional PDCCR to be mapped onto the resource elements of the existing DL OFDMA control signals in the first slot of each subframe. This additional symbol is mainly intended to enable the CCI avoidance between ReNB and eNB by controlling the ReNB site. Figure 3(b) illustrates the scheduling for eNB and HeNB taking into consideration the proposed PDCCR additional control signal. From figure 3, the eNB can use all the available RBs for transmission time interval and it maps onto three OFDMA symbols for MUE to obtain RB. The HeNB listens in these three OFDMA symbols to see which RBs the eNB is using and use the proposed OFDMA symbol to control the HeNB site.

TV. PERFORMANCE MEASUREMENT

We evaluate the performance measurement in terms of signal to interference plus noise ratio (SINR), system throughput and outage probability.

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The STNR model is defined as the ratio of a signal power to the interference power for the b-th RB in the a-th sub-channel, RBa,b' We assume that X eNBs are placed in a given area and Y HeNBs are deployed in each macrocell's coverage. Also, L

MUEs are serviced by each eNB and F HUEs are serviced by each HeNB. . RBab RBab Under these assumptIOns, let ReNB·. and RHeN·B.. be the ,.m '.j,h power of a received signal for the b-th RB (1 :::; b :::; Z) in the a-th sub-channel (1 :::; a :::; S) from the i-th eNB (1 :::; i :::; X) to the m-th MUE (1 :::; m :::; L) in the i-th eNB coverage and from the j-th HeNB (1 :::; j :::; Y) to the h-th HUE (1 :::; h :::; F) in the j-th ReNB coverage in the i-th eNB coverage, respectively.

The SINR of the MUEi,m for the RBa,b, Y;��m' can be

expressed as (4). No is the white noise power. IRBBas'b and m x,m I=:�; are the power of the interfering signal from the x-th x,Y,m eNB and from the y-th HeNB in the x-th eNB coverage to the MUEi,mfor the RBa,b' wx.m and tVa.b which are binary values are 1 or 0 if eN Bx is in the group of interfering neighbor eNBs for the m-th MUE and the RBa.b is used by the neighbor eNBs or HeNBs, respectively.

The SINR of the HUEi,j,h for the RBa,b, y:�;,.bh' can be ',j, expressed as (5).

Start active new HUE

i = i + 1

RBi available?

HeNB obtains suitable RBs selected from the set of unused RB

RBi allocation

End

i > t

Set t = total number of RBi = 1

NoYes

No

Yes

HeNB listens eNB control Information

HeNB randomly obtains RBs


RRE".b

r� � , �

MU[;,.m = X X y

REllb r HI!i',i,h

N + '" IRE"" . OJ • /If + '" '" IRE"" . o � eNRx,m x,m 't' a,h � � HeNRx,y,m If/a,h x=l,x;t:i x=1 J'=l

B. System Throughput

(5)

After SINR is obtained, user throughput can be calculated in terms of RBs by the Shannon theorem. Using (4), one user can get its received SINR on RBa,h, if RBa,h is allocated to this user. The throughput of MUE can be expressed as follows:

TMUL = Bo · log2 ( 1 + r��;�:m ) . a , (6)

where Bo is the bandwidth of RB and a is defmed as the rate of data transfer resource among total subframe (a=11114, total number of symbol = 14 resource element (RE) per sub frame, PDCCH=3).

The throughput of HUE can be expressed as follows:

THUL = Bo · log2 (1 + r;��h ) . f3 ' I ,J ,h

(7)

where f3 is defined as the rate of data transfer resource among

all subframes (f3 = 1 0114, total number of symbols = 14 (RE), PDCCH=4)

C. Outage Probability

We also analyze the outage probabilities, OeNB,i and 0HeNB,i, for MUEs and HUEs in the i-th eNB coverage and those are calculated by (8).

Nout . Nout o MUE,I 0 . � HUE,i

eNB,i � L ' HeNB,1 � y (8)

where, Nf:rt�,i and NfItb are the numbers of SINR values less than -6dB considering a bit error rate less than 10-6 for MUEs and HUEs, respectively.

V. SIMULATION RESULTS

The simulation was done based on the system model explained in Section II. Tables I and II show the parameters. In this section, we constructed a simulator using MA TLAB to evaluate the performance. The goal of the simulation is to study the performance of the average DL interference from the HeNBs to the MUEs. Key simulation parameters including the propagation models for path loss were performed based on the simulation framework presented in [11].

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TABLE I. ENB SIMULATION ASSUMPTIONS

Parameter Assumptiou Cellular Layout Hexagonal grid

Cell Radius Ikm

Number of sites 7 Cells

Carrier Frequency 2 GHz

Penetration Loss (assumes UEs lOdB are indoors)

Antenna pattern Omni-directional

Total BS Tx power 46dBm (40W)

Traffice model Full buffer

Minimum distance between UE MUE-eNB >=35m

and cell

UE distribution Uniform density with eNB coverage

Threshold SIR of outage -6dB

TABLE IL HENB SIMULATION ASSUMPTIONS

Parameter Assumption Carrier Frequency 2 GHz

Cell Radius 6.77m

Antenna pattern Omni-directional

MiniMax Tx power femtocell O/20dBm

Traffic model Full buffer

Number of symbols for PDCCH 4

Log-normal shodowing standard 4dB

Distribution of HUE within Uniform

HeNB Min separation between HeNB

10 m and HUE

We compare the performance of proposed scheme to the network without HeNB called conventional and a scheme which allocates radio resource randomly selected from entire frequency band to HeNB. We show the system performance of proposed scheme is evaluated in terms of the system throughput and the outage and comparing them with the conventional scheme where radio resources are randomly selected. Two cases are considered: without CIS means MUEs and HUEs are randomly allocate as conventional system (current standard network), and CIS presenting our proposed scheme.

Figure 5 shows the outage probability of MUE versus the number ofMUEs, when 500, 1000, 1500 and 2000 HeNBs are deployed in the eNB coverage. The outage probability increases when the MUEs density is increased according the number of HeNBs. In figure 5, MUE and HUE randomly uses the RBs. This introduces interferences especially in the RB used by both MUE and HUE. The effect seems to be greatly high when employing more HeNBs as this increases the coverage holes, thus preventing other MUE and HUE getting the service.


0.351-;::::::c====='====::c::==='=::;----,-----:---,----�-r-1 ---B--- Co nve nt io nal

-HeNBs/eNB: 500 0.3 .... -----4--HeNBs/eNB: toOO

.......•........•.. �) §,0.25 .D III "[ 0.2 Q)

� 00.15 "" <= <=

� o 0.1

0.05

�HeNBs/eNB: 1500 -----B----HeNBs/eNB: 2000

................. :. O � __ L-__ � __ -L __ -L __ � ____ L-__ L-__ � __ -L __ �

o 5 10 15 20 25 30 35 40 45 50 N umber of MUEs

Fig. 5. The outage probability of MUE without CIS

---B--- Co nve nt io nal

-HeNBs/eNB: 500 0.3 .... -----4--HeNBs/eNB: toDD .............. ;

,§-0.25 :a III "[ 0.2 i!l, � 00.15 "" <=

� 8 0.1

0.05

�HeNBs/eNB: 1500 -----B----HeNBs/eNB: 2000

... . ........... .. ; ....

10 15 20

· . · . · . · . " ............... : ..

25 30 35 N umber of MUEs

40

Fig. 6. The outage probability of MUE with CIS

45 50

Figure 6 shows the outage probability of MUE with CIS. The outage probability is significantly decreased showing that CIS can avoid CCI from HeNB. In additional, the HeNB has knowledge about which RB are used by eNB and based on this knowledge HeNB select and allocate unused RBs from PDCCH, thus avoiding further interference.

Figure 7 shows the DL throughput of MUEs with different HeNBs densities without CIS. eNB throughput was calculated for different numbers of HeNBs; as the numbers of HeNBs increase, the throughput of MUEs decrease due to increasing interference from HeNB when MUE randomly uses RB and HUE uses the same RB.

Figure 8 shows the throughput of MUE with CIS is clearly higher for the deployment of high density HeNBs. In our simulation, we also increased the number of MUEs randomly in the eNB and evaluated the perfonnance improvement of using CIS. This shows that with the increased MUEs, CIS can successfully avoid the CCI introduced from HeNBs.

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20,---,---,---,---,,---,---,---,---,----,---�

18

16

5 10 15 20

. . . _ . . . . . . . . . : ... .

-a- Conventional

-HeNBs/eNB: 500 -----4-- HeNBs/eNB: toOO �HeNBs/eNB: 1500 . -----B----HeNBs/eNB: 2000

25 30 35 40 45 Number of MUEs

Fig. 7. Total MUE throughput without CIS

50

20,---,---,---,---,,---,---,---,---,----,-�

18 - ·

� : : : :. 6 � 12 00 � 10 f-i" 8 ::::i � 6

2

---B--- Co nve nt io nal

-HeNBs/eNB: 500 -----4--HeNBs/eNB: 1000 �HeNBs/eNB: 1500 -----B----HeNBs/eNB: 2000

........ : .............•........•......•......•........ "

...•...... : .......••.......•......

O L-__ L-__ � __ � __ -L __ -L __ -L __ � __ � ____ L-� o 5 10 15 20 25 30 35 40 45

Number of MUEs

Fig. 8. Total MUE throughput with CIS

� :'\ . .. . . ... . ..... .... . 67 · W

-----B----HeNBs/eNB: 500 -HeNBs/eNB: toOO ---B---HeNBs/eNB: 1500

::0 I '0 6 --+-- HeNBs/eNB: 2000 .

..................... : ... : . : ....... : ... . . . ........... ; ..

. ........... . . . . . ......... . ..... . - .. . . . . ....... . . . . ............... - ...... � ......... : ........ ; ..

50

° 0L--- L5---1� 0--�15�� 20L-� 2L5--�30�--3� 5��4O�� 4L 5---c"50· Number of MUEs

Fig. 9. HUE average throughput without CIS


9,---,---,---,---,---,---,---,---,---,---,

� 6 7 W => I 15 6

5

----B-- HeN BsieN B: f:()0 -HeNBsleNB: 1000 ----B-- HeNBsleNB: 1 f:()0 ....... ............ -HeNBsleNB: 2000

.

........... : ...... .. . ................. : .. .

..... '.' ...... . . . . . . .. , . ........ ... . .. .. ...... .... .

10 15 20 25 3J 35 'iO 45 f:() Number 01 MUEs

Fig. 10. HUE average throughput with CIS

Figure 9 shows HUE average throughput without using CIS. The throughputs of HUEs decrease more, when the numbers of

MUE increase, because HUE uses the same RB as MUE. Therefore, it receives interference power from eNB.

Figure 10 shows the HUE average throughput using CIS. The throughput decreases slightly compared to the average HUE throughput without using CIS, because we assumed one symbol for control information in the HeNB of our scheme. In addition, MUE has higher priority to use the RB than does HUE. Figure 10 shows number ofMUE increases, the Average throughput of HUE decreases, for example when MUE increases until 49. The HUEs can only use one RB to avoid the interference introduced to MUEs.

VI. CONCLUSION

We have studied interference avoidance using control information in the two tier L TE femtocell system. The impact of HeNB interference on capacity was investigated. In this paper, we presented a control information scheme (CIS) for HeNBs deployed in the eNB network. We analyzed the STNR, outage, throughput without using the CIS and compared it to the CIS. Simulation results showed the proposed schemes outperform HeNB networks in terms of system throughput and outage probability for MUEs and HUEs. Without using the CIS, when the HeNB densities increase, the MUE outage

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significantly increases. The CIS has shown it can avoid CCI and enhance capacity. Even though we deploy more HeNBs in the eNB, the CIS can avoid interference. However, in the CIS, HUE throughput decreases when the MUE density was increased, because the HUEs use the same RBs as neighboring HUEs, so they get inference from neighboring HeNBs.

ACKNOWLEDGMENT

This work was supported by the Ministry of Knowledge Economy (MKE) of the Republic of Korea. [201185100-10040, Development of proactive energy management technologies for saving the energy cost of residential complex using cooperative sensing devices]

REFERENCES

[I] Femto Forum, http://www.femtoforum.org.

[2] V.Chandrasekhar,.I. Andrews and A. Gatherer, "Femtocell Networks: A survey", in proc. IEEE comm. Mag., Vo1.46, No.9, pp.59-67, Sep. 2008.

[3 ] K. Cho, W. Lee, D. Yoon, K. Hyun, and Yun-Sung Choi, "Resource alloation for orthogonal and co-channel femtocells in a hierarchical cell structure," in proc. IEEE ISCE, pp. 655-656, 25-28 May 2009.

[4] L. T. W. Ho and H. Claussen, "Effects of user-deployed, co channel femtocells on the call drop probability in a residential scenario," in proc. IEEE PIMRC, pp. 1-5, Athens, Greece, Sep 2007.

[5] 1. Guvenc, M. R. Jeong, F. Watanabe, and H. Inamura, "A hybrid frequency assignment for femtocells and coverage area analysis for cochannel operation," in proc. IEEE Commun. Let., pp. 880-882, Dec 2008.

[6] D. L6pez-Perez, G. De La Roche, A. Valcarce, A. Juttner and 1. Zhang, "Interference avoidance and dynamic frequency planning for Wi MAX femtocells networks," in proc. IEEE ICCS, pp. 1579-1584, 19-21 Nov 2008.

[7] H. Claussen, "Performance of macro- and co-channel femtocells in a hierarchical cell structure," in proc. IEEE PIMRC, pp. 1-5, Athens, Greece, Sep 2007.

[8] R. Giuliano, C. Monti, and P. Loreti, "Wi MAX Fractional Freuqency Reuse for Rural Environments," in proc. IEEE Wire!. Commun., pp.60-65, Jun 2008.

[9] S.M.shukry, K.Elsayed, A.Elmoghazy, A. Nassar, "Adaptive Fractional Frequeny Reuse (AFFR) Scheme for Multi-cell IEEE 802.16e Systems," in proc. IEEE HONET, pp. 96-102, Dec 2009.

[10] C. Z. Mostafa, .I. M. Yeong and H . .l.Zygmunt, "Interference Mitigation Using Dynamic Frequency Re-use for Dense Femtocell Network Architectures," in proc. IEEE ICUFN, pp. 256-261, Jun 2010.

[II] 3GPP TR 36.922 R4-092042, "Simulation assumptions and parameters for FOO HeNB RF requirements", May 2009.

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