05506164 Interference Avoidance and Adaptive Fraction Frequency Reuse in a Hierarchical Cell Structure.pdf

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2010 proceedings.

Interference Avoidance and Adaptive Fraction Frequency Reuse in a Hierarchical Cell Structure AN Ruihong, ZHANG Xin, CAO Gen, ZHENG Ruiming and SANG Lin Wireless Theories and Technologies Lab (W T&T), Beijing University of Posts and T elecommunications School of Information and Communication Engineering, Beijing University of Posts and Telecommunications Beijing, P.R. China, 100876 [email protected]

Abstract —Due to high data rate requirement for indoor radio coverage, femtocells have been proposed as a potential good solution in recent years. Under this configuration, a main design objective is to coordinate the mutual interference between macro and femto cells. In this paper, the flexible assignment of downlink resource in a hierarchical cell structure is investigated. A dynamic frequency assignment technique called adaptive fraction frequency reuse (AFFR) is proposed, by avoidance of severe interference between macro and femto cells. A network model and system level simulations for LTE macrocell/femtocell scenarios are presented. The results show that, AFFR has better average and cell-edge performance compared with co-channel frequency allocation (co-channel FA) and orthogonal frequency allocation (OFA) for both macro and fe mto cells. Keywords- femtocells; adaptive fraction frequency reuse; cochannel interference; interference; frequency allocation

I.

I NTRODUCTION

It is reported that the majority of data services and phone calls will take place in indoor environment. Therefore, indoor coverage will be necessary. One solution to improve indoor coverage is the so-called home base stations [1]. Introduction of femtocells can enable indoor connectivity through existing broadband Internet connections. connections. The T he deployment of femtocells has a few benefits such as improved indoor coverage, reduced bandwidth load in t he macrocell networks, reduced indoor call costs and savings of phone battery [2]. Special attention must be paid to interference mitigation when femtocells are deployed [3], because the mutual interference can not be handled simply by network planning. The deployment ratio and positions of the femtocells are unpredictable, so that it is difficult for operators to make appropriate coordinations. The co-channel interference between femtocells and macrocells has been widely studied [3]-[7]. A conventional mechanism to operate femtocell systems is that femtocell base stations use full frequency bands of macro networks, which called co-channel frequency allocation (co-channel FA) [3]. But this method can increase cross-tier interference. In [4], another way called orthogonal frequency allocation (OFA) is proposed to control co-channel interference. However, it will drive the operators to a reduced spectrum utility, which is extremely expensive and undesirable.

Therefore, co-channel deployment seems appropriate, but technically more challenging. challenging.

to

be

more

Using OFDMA as a multiple-access technique, a good alternative to control co-channel interference between femto and macro cells in LTE system is dynamic assignment of physical resource blocks (PRBs). Femto users and macro users who are interfering with each other will be allocated different PRBs. In this paper, a new dynamic frequency assignment technique is introduced, considering interference and coverage issues, which called adaptive fraction frequency reuse (AFFR). This AFFR scheme makes maximum usage of the resources, and meanwhile reduces the interference brought by femtocells. Ref. [5] and [7] only concern about the effects from femtocells to macrocells, while in this paper, the performance of femtocell users is also discussed. Simulation results show that AFFR has better average and cell-edge performance for both macrocells and femtocells compared with co-channel FA scheme and OFA scheme. The rest of the paper is organized as follows: In section II, a system model of two layer deployment is introduced, and the main design objective is expressed with formula. Section III introduces the existing frequency allocation schemes, such as co-channel FA and OFA, and then presents an AFFR algorithm as dynamic interference avoidance technique for hybrid network scenarios. In section IV, system level simulation results show the increase in capacity and coverage when using AFFR compared with co-channel FA and OFA in this hierarchical cell structure. Finally, in the last section some conclusions are drawn. II. A.

SYSTEM MODEL AND PROBLEM FORMULATION

System Model and Assumptions

The setup consists of a network of seven cells with wraparound, each of which has L L femtocells. The performance of the central base station (BS), called as BS 0, is evaluated. Only one femtocell UE (FUE) per femtocell BS is assumed for simplicity. The number of users per macrocell BS is M , thus, the central cell has M macrocell users (MUEs) and L FUEs. Dense femto cell deployment modeling [8] is used in our model. In each macrocell, a number of femtocell blocks are dropped randomly.

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where P tx and P tx′ denote the transmit power of macrocell BS and femtocell BS on PRB n, respectively. respectively. PLm represents the

′ pathloss between MUE m and its serving BS, while P Lml denotes the pathloss between MUE m and femtocell BS l . N 0 is the additive white Gaussian noise plus inter-cell interference. interference. If PRB n is used by femtocell BS l , x nl = 1 , else x nl = 0 . One FUE per femtocell is assumed, and the received SINR of FUE in femtocell BS l on PRB n can be expressed as Figure 1. A femtocell block

In a dense-urban femtocell modeling, each block represents two stripes of apartments, each of which has 2 by N apartments ( N ( N is 10 in our model). Each apartment is of size 10m × 10m . There is a street between the two stripes, with width of 10m, see Fig. 1. Each femtocell block is of size 10( N + 2)m × 70m . This is to make sure that the femtocell BSs from different femtocell blocks are not too close to each other. Each femtocell block has six floors [8]. The femtocell BS is also randomly placed in each femtocell. All of the FUEs are located in the femtocell apartment, which are dropped randomly in the active femtocells. MUE are dropped randomly throughout the cell. It is possible that some MUEs will be dropped into the femtocell blocks. A number of different deployment configurations have been considered for femtocell BSs [9]. In this paper, closed subscriber group (CSG) mode is investigated. If femtocell BSs are configured with CSG mode, when a MUE approaches a femtocell BS and the signal from the femtocell BS is larger than that from macrocell BS, this MUE still can not hand over to that femtocell BS. For this reason, the interference from femtocell BS may cause “dead zones” in macrocell, as handover can not take place.

SINRl (n) =

• Information exchange is supported between macrocell BSs and femtocell BSs B. Problem Formulation As stated, the main objective is to coordinate the mutual interference between macro and femto cells. To formulate the frequency allocation problem, consider an OFDMA system with N PRBs, and assume one femtocell block per each macrocell for simplicity. Use the definitions in Section A, the received signal to interference and noise ratio (SINR) of MUE m on PRB n can be expressed as

∑ P tx′ l =1

′ ⋅ x nl + N 0 Lml ⋅ P

(2)

k =1,k ≠ l

where P Ll ′ represents the pathloss between FUE in femtocell l

′ denotes the pathloss and its serving femtocell BS, while P Llk between FUE in femtocell l and femtocell BS k . PLl represents the pathloss between FUE in femtocell l and macrocell BS 0. N 0 is the additive white Gaussian noise plus inter-cell interference. If PRB n is used by BS 0, y n = 1 , else y n = 0 . After SINR is calculated, user throughput can be obtained by Shannon theorem and can be calculated calculated in terms of PRBs. Using (1) and (2), one user can get its received SINR on PRB n if PRB n is allocated to this user. The throughput of this user can be expressed as follows: N

C user =

∑ B0 ⋅ log 2 (1 + SINRn ) ⋅ z n ,

(3)

n =1

to this user, z n = 1 , else z n = 0 .

• Exact knowledge of interference source

P tx ⋅ PLm

′ ⋅ x nk + P tx ⋅ PLl ⋅ y n + Ν 0 Llk ⋅ P

,

where B0 is the bandwidth of one PRB. If PRB n is assigned

• Perfect knowledge of users’ channels

L

L

∑ P tx′

The following assumptions are employed for ensuring the analysis:

SINRm (n ) =

P tx′ ⋅ P Ll ′

,

(1)

The throughput of femtocell/macrocell BS will be the sum of its serving UEs. The main idea of the proposed algorithm is to mitigate the mutual interference between macro and femto cells, and then improve the received SINR of both MUEs and FUEs, so that the throughput of both systems can be promoted. III.

EXISTING AND PROPOSED FREQUENCY ALLOCATION SCHEME

This section presents two existing downlink frequency allocation schemes between macrocells and femtocells in the hierarchical cell structure, and then introduces the proposed adaptive fractional frequency reuse (AFFR) scheme. 10MHz bandwidth (BW) for both macrocells macrocells and femtocells femtocells and another 10MHz BW for macrocell only is assumed. Only the performance performance of the first 10MHz BW is investigated, investigated, and femtocells have priority over macrocells to occupy the PRBs


on the first 10MHz BW. 10MHz BW means 50PRBs is available. A. Existing Frequency Allocation Allocation Scheme Fig. 2 shows a co-channel FA scheme. Full frequency bandwidth is allocated to both macrocells macrocells and femtocells. femtocells. Since the co-channel FA scheme shares all frequency band between macro and femto cells, the mutual interferences interferences is very critical. In order to avoid the serious co-channel interference, OFA scheme is proposed [4]. α is defined as the frequency allocation fraction for macrocell BS. That is to say,

α

=

Macro cells bandwidth Total system bandwidth

Figure 2. Co-channel frequency allocation

.

Macrocell BSs use a part of frequency band of the total bandwidth, bandwidth, while femtocell femtocell BSs can only use remaining remaining frequency band. Fig. 3 shows an OFA scheme with α = 0.8 . Although orthogonal FA scheme can avoid the co-channel interferences between macro and femto cells, both macrocell BSs and femtocell BSs can only use part of the full frequency band, which which lead to waste waste of frequency frequency resources. resources. B. Proposed Frequency Allocation Allocation Scheme Co-channel FA scheme will create serious mutual interference while OFA scheme will reduce the proportion of resources used by both macrocells and femtocells. For these reasons, AFFR is proposed. The main idea of the proposed scheme is to suppress the downlink interference from femtocell BSs to both MUEs and FUEs. In order to maintain the performance performance of MUEs, all 50 PRBs can be used at macro Layer. AFFR runs at each time slot and the algorithm is divided into three steps. 1) Share the available spectrum on a non-overlapping basis between neighboring femtocells Using the conception of interference graph [11], when a femtocell BS is powered on, its FUE measures the pathloss from its serving femtocell BS and the surrounding femtocell BSs [12]. Based on these measurements, this femtocell BS determines the set of femtocell BSs that are likely to have an interfering relationship with it by checking whether the pathloss difference difference from this femtocell femtocell BS and its neighboring neighboring TH femtocell BSs to its FUE is above a certain threshold PL threshold PL . The interfering set of femtocell BS l can can be expressed as:

′ − P I l = { femtocell BS k | P Llk Ll ′ > PLTH }, k = 1, 2, ... , L .(4) Then, using a cyclic iterative algorithm, the femtocell BS will be assigned those PRBs (or subbands) that are nonoverlapping with femtocell BSs in its interfering set [10]. That means, on each loop, every femtocell BS chooses one PRB from 50 PRBs randomly except the PRBs this femtocell BS and femtocell BSs in its interfering set have already used. At the end of each loop, the scheduler makes a judgment to decide

Figure 3. Orthogonal frequency allocation ( α = 0.8).

whether new allocation is done during this round. If there is no PRB that has been allocated during this loop, the iterative algorithm is over. After this step, the resources assigned to femtocell BSs that are interfered with each other will be orthogonal. 2) Resource reuse between femtocell BSs In this step, one femtocell BS seeks all the remaining PRBs to find whether there are any other PRBs that can be used based on SINR requirement. requirement. Assume Assume that there is only one FUE associated with a femtocell BS, where the FUE and femtocell BS share the same index for notation simplicity. In order to decide whether PRB n can be used by femtocell BS l , firstly, the SINR of FUE l on PRB n( SINRl (n) ) is calculated supposing PRB n is used by femtocell BS l . If SINRl ( n) is above a certain threshold SINR1TH : SINRl (n) ≥ SINR1TH ,

(5)

then femtocell BS l decides that it can use PRB n, and sends “reuse request” to femtocell BSs in I l . Then the FUE served by femtocell BS k in I l calculates its received SINR on PRB n assuming PRB n is allocated to femtocell BS l . If SINRk (n) is above a certain threshold SINR2TH : SINRk (n) ≥ SINR2TH , HeNBk ∈ I l ,

(6)

the femtocell BS k sends sends “reuse permission” back to femtocell BS l, l, otherwise, femtocell BS k sends “reuse rejection”. If all the received signals are “reuse permission”, femtocell BS l decides that PRB n is assigned to femtocell BS l .


3) Maximum SINR scheduler at macrocell macrocell BSs In this step, one MUE measures its received SINR on all PRBs and reports this information to its main serving BS. First, a macrocell BS selects one MUE randomly, and it means that all MUEs get resources with equal probability. Then once MUE m is granted, its main serving BS selects the PRB on which its SINR is max. n * = arg max ( SINRm (n))

(7)

n =1,... N

TABLE II. Frequency Allocation Scheme

SINR3TH ,

that is

User Throughput(Mbps) Average user throughput throughput

10% user throughput

Co-channel FA

1.31

0.076

OFA

1.15

0.063

AFFR

1.42

0.101

1 0.9

If the received SINR on PRB n* for MUE m is above

MACRO USER THROUGHPUT

0.8

Co-channel FA OFA AFFR

0.7

SINRm (n * ) > SINR3TH ,

(8)

macrocell BS decides that PRB n can be used by MUE m, and can not be allocated to any other MUEs in its serving set. Macrocell BSs continue doing this algorithm until there is no PRB available. This step can avoid MUEs be assigned to the PRBs where the interference brought by femtocell BSs is severe. The received SINR of MUEs can be improved and DL throughput of macro layer can be promoted. IV.

SIMULATION R ESULTS ESULTS

The considered OFDMA system consists of seven base stations with N with N = = 50 PRBs and M = = 10 users. The number of femtocells in each cell is assumed to be 50,100,150 and 200. The detailed parameters are presented in Table I. In the urban deployment, channel model in Table 7 in [10] is used, with minimum distance between UE and femto/macro BS set to be 1m. A. Throughput and SINR of Different Algorithms In this section, throughput and SINR of three different algorithms described in section III are compared. Fig. 4 shows TABLE I.

SIMULATION PARAMETERS

Parameter

Carrier frequency

Value

2000 MHz

Macrocell BS antenna gain

14 dBi

Femtocell BS antenna gain

0 dBi

UE noise figure

9 dB

Maximum femtocell BS TX power

20 dBm

Maximum macrocell BS TX power

46 dBm

α

PLTH SINR1TH / SINR2TH / SINR3TH

0.8 30dB 2dB/2dB/0dB

0.6 F D0.5 C 0.4 0.3 0.2 0.1 0 -40

-30

-20 -10 0 10 20 Femtocell User SINR(dB)

30

40

Figure 4. 4. Femtocell user SINR CDF of three algorithms algorithms

the SINR CDF for the downlink of three schemes with 200 femtocells per macrocell. It is shown that the SINR improvement of FUEs as a result of OFA scheme is only marginal, that is because OFA can not handle the interference caused by other femtocell BSs. However, when AFFR scheme is used, interference from neighboring femtocell BSs can be limited, so AFFR scheme has the best FUE SINR, especially for the cell-edge users. Table II shows the average macro user throughput of three schemes when there are 200 femtocells per macrocell. The average throughput of the MUEs using OFA scheme is the smallest, and the proposed AFFR scheme shows the best performance. performance. The average MUE throughput of AFFR scheme is 1.42Mbps, and shows improved performance by 8.4% than the 1.31Mbps of co-channel FA scheme. The average throughput performance of OFA is worst, which is 1.15Mbps. That is due to the reduced spectrum utility. Fig. 5 shows the simulation results of the throughput for FUEs with 200 femtocells per cell. From Fig. 5, we can see that the throughput of FUEs using OFA scheme is significantly lower than that using co-channel FA and AFFR. That is because when OFA is used, the available bandwidth of each femtocell is only 20% of the total bandwidth. It is also shown that data rate coverage of AFFR is better than that of OFA and co-channel FA. B. Throughput of Different Femtocell Deployment Density Fig. 6 and Fig. 7 show the average cell throughput of femtocells and macrocells vs. different femtocell deployment density. The result of Fig. 6 shows that when femtocell density is very low, such as 50 or 100 femtocells per one macrocell, the


1 0.9 0.8

15 ) s p b M ( 14 l l e c o r c a M13 f o t u p h 12 g u o r h T e 11 g a r e v A 10 50


0.7 0.6 F D0.5 C 0.4 0.3 0.2 0.1 0 2 10

3

4

5

10 10 Femtocell User Throughput(Kbps)

10


75 100 125 150 175 Number Number of Femtocells Femtocells per Macrocell

200

Figure 7. 7. Average cell throughput of macrocell macrocell BSs

Figure 5. Femto user user throughput throughput CDF of three algorithms

30 Co-channel FA OFA AFFR

) s p b 25 M ( t u p h 20 g u o r h 15 T S B l 10 l e c o t m e F 5 0 50

average throughput of macro/femto cells decreases as the increasing of femtocell number. However, for all deployment density, AFFR shows the best performance for macro layers. To sum up, AFFR scheme offers data rate enhancement as well as high spectrum utility in a hierarchical cell structure. R EFERENCES EFERENCES

75 100 125 150 175 Number Number of Femtocells Femtocells per Macrocell Macrocell

200

Figure 6. Average cell throughput of femtocell femtocell BSs

average throughput of femtocells using AFFR is higher than using co-channel FA. OFA scheme has the worst average cell throughput. Fig. 7 shows the average throughput of macrocells vs. different femtocell density. The average throughput of macrocells decreases as the increasing of femtocells number. Similar to Fig. 6, the results of Fig. 7 shows that AFFR has the best macrocell macrocell perform performance, ance, while while OFA is the the worst. worst. V.

CONCLUSION

In this paper, the flexibility of the co-channel deployment of femtocells in an existing macrocell network is investigated. Two existing methods are introduced: orthogonal and cochannel FA schemes. In addition, an adaptive fractional frequency reuse in a hierarchical cell structure is proposed and system level simulation results are analyzed. The simulation results show that AFFR has better coverage and average cell throughput compared with the co-channel FA scheme as well as OFA scheme due to appropriate control of downlink interference from femtocell BSs. Furthermore, the average cell throughput for both macrocells and femtocells in different femtocell deployment density is presented. It is shown that the

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05506164 Interference Avoidance and Adaptive Fraction Frequency Reuse in a Hierarchical Cell Structure.pdf

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