Vtc2010 Distributed Channel Selection Principles For Ofdma Femtocells With Two Tier Interference
1.
Distributed Channel Selection Principles for
Femtocells with Two-tier Interference
Li-Chun Wang1 , Chiao Lee1 , and Jane-Hwa Huang2
1 National Chiao Tung University, Taiwan
2 National Chi Nan University, Taiwan
Abstract—It goes without saying that the femtocells will be interference. We also compare the random selection scheme,
widely employed in the next generation wireless networks since which randomly selects the sub-channels for transmissions.
the femtocells improve indoor capacity and coverage with low Obviously, using more sub-channels can increase femtocell
power and less cost. However, as the femtocells become pop-
ular, the femtocell users suffer from the complicated two-tier capacity. However, this increases the interference between
interference, including the macrocell-to-femtocell and femtocell- femtocells, and degrades link reliability. Hence, the number of
to-femtocell interference. Therefore, the femtocells pose a difﬁcult sub-channels allowed for a femtocell is the major designing
challenge on managing the interference in a autonomous and dis- parameter in the channel selection scheme. We investigate the
tributed manner. In this paper, we investigate how to distributedly downlink femtocell capacity, with considering path-loss, shad-
select the sub-channels for the OFDMA-based femtocell systems
to reduce interference and to improve indoor capacity under a owing, frequency-selective fading channel. Simulation results
link reliability requirement. We develop the channel-gain oriented demonstrate that the interference from macrocell and femto-
and interference-avoidance oriented distributed channel selection cells signiﬁcantly degrades link reliability. It is shown that
schemes. Simulation results show that the interference from the with appropriately reduce the number of used sub-channels,
macrocell and other femtocells signiﬁcantly degrades femtocell the proposed distributed channel selection schemes can ensure
link reliability and capacity. However, by properly adjusting the
number of used sub-channels, the developed channel selection the link reliability and yield higher capacity than the random
schemes can improve capacity and ensure the link reliability. selection scheme. Simulation results also provide the chan-
nel selection principles for different spectrum allocation and
femtocell density.
I. I NTRODUCTION
With the beneﬁts of low power, low cost, backward com-
In the literature, the capacity and coverage of femtocells
patibility, and single-mode device, the femtocell is a very
are studied in some depth. The work in [1] investigated
promising technique to improve indoor coverage and capacity
the capacity of a CDMA-based femtocell, with considering
for the next-generation mobile system. The femtocell base
the transmit power conﬁguration for femtocells. In [2], the
station (fBS) is a simple and low-priced plug-and-play device,
capacity and coverage for the OFDMA-based femtocell was
which can reuse the licensed spectrum in the indoor envi-
investigated. Both the work in [1] and [2] considered a full-
ronment. Compared to the macrocell base station (mBS), the
loaded case. That is, the femtocell users are busy all the
fBS can use lower power to achieve higher indoor throughput
time and use the whole spectrum to transmit their data. In
because of the shorter communication distance. However, as
addition, the channel selection is not considered. In [3], the
the fBSs are widely deployed, the femtocells will face not only
authors compared channel selection methods in a OFDMA-
the macrocell-to-femtocell interference but also the increasing
based femtocell system. In [3], the whole spectrum is divided
femtocell-to-femtocell interference. Therefore, how to reduce
into three segments, and each femtocell user can choose one
the interference to improve link quality is an essential task in
segment. However, only the femtocell-to-femtocell interfer-
femtocells.
ence is considered in [3]. Different from the works in [1]-[3],
In this paper, we investigate the distributed channel selection
this paper develops the distributed channel selection schemes
principles for the OFDMA-based femtocell networks. We
for the OFDMA-based femtocell networks with considering
consider two spectrum allocation schemes: shared-spectrum
two-tier interference. In addition, the impact of the number of
and exclusive-spectrum allocation schemes. In the ﬁrst one,
the sub-channels allowed to be used by a femtocell user on
the femtocell and macrocell systems share the same spectrum.
link reliability and capacity is investigated.
Therefore, we should considered. In the second one, the
femtocell and macrocell systems are exclusively allocated
with different spectra, and femtocell users suffer from other The rest of this paper is organized as follows. Section II
femtocells’ interference. We develop the distributed channel- introduces the system model, channel models and SINR for the
gain oriented and interference-avoidance oriented channel se- femtocell system. Section III discusses the femtocell capacity
lection schemes for the OFDMA-based femtocell network. maximization problem with the link reliability requirement.
The channel-gain oriented scheme selects the sub-channels In Section IV, different distributed channel selection schemes
with higher link gain to transmit data. The interference- are proposed. The simulation results are shown in Section V.
avoidance oriented scheme chooses sub-channels with lower Concluding remarks are given in Section VI.
2.
TABLE I
M ODULATION C ODING S CHEMES AND EESM PARAMETER (β)
Code Rate Spectrum Minimum EESM
Modulation (Repetition: Efﬁciency SINR factor
default=1) (bit/s/Hz) (β, dB)
QPSK 1/2(4) 0.25 -2.5 dB 2.18
QPSK 1/2(2) 0.5 0.5 dB 2.28
QPSK 1/2 1 3.5 dB 2.46
QPSK 3/4 1.5 6.5 dB 2.56
16-QAM 1/2 2 9 dB 7.45
16-QAM 3/4 3 12.5 dB 8.93
64-QAM 1/2 3 14.5 dB 11.31
64-QAM 2/3 4 16.5 dB 13.8
64-QAM 3/4 4.5 18.5 dB 14.71
GF S are the antenna gains of mBS and fBS. The channel gain
Fig. 1. Femtocell Layout in regular grids between mBS and fBS user is Hj,m , and that between the kth
fBS and fBS user is hk , including the effects of shadowing
j,m
II. S YSTEM M ODELS and fading. Therefore, the CINR of mth sub-carrier on the jth
sub-channel for the user in the ith femtocell is deﬁned as
We consider the OFDMA-based femtocell system with two-
pi GF S hi
tier interference as shown in Fig. 1. We focus on the downlink j,m j,m
L(di )
performance. Each femtocell user experiences interference γj,m = K
. (2)
Pj,m GBS Hj,m pk GF S hk
from other fBSs and mBS. We consider the regular grids of 25 + j,m j,m
+ N0
L(D) L(dk )
femtocells in the macrocell with the coverage radius DM of k=1,k=i
500 m. The house size is 10m-by-10m. Each house has four
rooms, and the fBS is located at the bottom-left corner of the C. Exponential Effective SIR Mapping (EESM)
top-right room with a (0.1 m, 0.1 m) shift from the center of The exponential effective SIR mapping (EESM) method is
the house. The separation distance between fBSs is dsf (m). to map a vector of the per sub-carrier SINR level to a single
AWGN-equivalent SINR [7]. If there are M individual sub-
A. Radio Channel Effects carriers in a sub-channel, the AWGN-equivalent SINR for the
This paper considers the impacts of path-loss, shadowing sub-channel can be expressed as
and frequency selective fading channel as follows. M
1) Path-Loss: The path-loss decays with propagation dis- 1 γj,m
SIN Ref f,j = γef f,j = −β·ln( e− β ). (3)
tance d between the transmitter and the receiver [4], [5]. M m=1
β is an EESM calibration factor to minimize the mean square
LF S (d) = 20 log10 ( 4πd ) , for d ≤ dBP
L(d) = λ
d (1) error between the equivalent SINR by EESM method and
LF S (dBP ) + 35 log10 ( dBP ) , for d > dBP . equivalent SINR from simulation. Table I shows the consid-
The path-loss model is related to wavelength λ of operating ered modulation coding schemes (MCS), the corresponding
frequency. The break-point distance is dBP = 5 m for the SINR threshold, and the EESM parameter β. According to
indoor link and 30 m for the outdoor-to-indoor link. Table I, we can determine the MCS and the spectrum efﬁ-
2) Penetration Loss: The penetration loss is assumed to be ciency SEEESM,j for the used channel with the equivalent
5 dB loss per internal wall for indoor link; and 10 dB per SINR γef f,j .
external wall for outdoor-to-indoor link.
3) Shadowing: Shadowing is modeled by a log-normal D. Link Reliability
X
random variable 10 10 . X is a Gaussian distributed random The link reliability probability is the probability that ef-
variable with zero mean. The standard deviation is 5 dB for fective SINR greater than a predeﬁned SINR threshold γ .
th
the indoor link and 10 dB for the outdoor-to-indoor link. Consider that there are total J sub-channels. Each femtocell
4) Multi-Path Fading: The frequency-selective fading is de- selects ρJ sub-channels for transmission. The sub-channel
scribed by the Stanford University interim-3 (SUI-3) channel usage ratio ρ is deﬁned as the ratio of used sub-channels to
model, which assumes 3 taps with non-uniform delays. the total sub-channels. Let ε be the utility function. If the
j
sub-channel j is selected to transmit data, εj = 1; otherwise,
B. Carrier to Interference-and-Noise Ratio εj = 0. We deﬁne the average link reliability Prel as
We consider the two-tier interference. Let Pj,m and pk
j,m J
be the transmission power of mBS and that of k-th fBS at 1
Prel = εj Pr [γef f,j ≥γth ] (4)
the mth sub-carrier of jth sub-channel. Moreover, GBS and ρJ j=1
3.
where Pr [γef f,j ≥γth ] is the link reliability of jth sub-channel. A. Max-Min Channel-Gain Oriented Selection Scheme
The SINR threshold γth means the minimum SINR require- We explain the procedures in the following.
ment for data transmission. In Table I, we use γth = −2.5 dB (S1) Compare the individual sub-carrier gain in a sub-channel.
as an example. The minimum sub-carrier gain in a sub-channel is
hj = min {hi }, for j = 1, . . . , J
j,m (8)
m=1,2,...,M
III. F EMTOCELL C APACITY M AXIMIZATION
where M is the total number of sub-carriers in a sub-channel,
Capacity and link reliability are both essential factors in and J is the the total number of sub-channels. (S2) Sort hj
the distributed channel selection principle for the OFDMA- as h1 ≥ h2 ≥ ... ≥ hρJ ≥ ... ≥ hJ .
based femtocell systems. From a link reliability perspective, (S3) Select the ﬁrst ρJ sub-channels with higher link gain,
decreasing the number of sub-channels allocated to a fBS can that is, hj > hρJ+1 , for j = 1, ..., ρJ.
decrease the interference effect for the users around to the
fBSs. However, from the link capacity standpoint, increasing
B. Max-Avg Channel-Gain Oriented Selection Scheme
the number of sub-channels allocated to a fBS can provide
higher data rate. This scheme selects the sub-channel according to average
To achieve the tradeoff between capacity and link reliabil- sub-carrier gain, as detailed in the following.
ity, we formulate an optimization problem to determine the (S1) Compute the average sub-carrier gain of each sub-
optimal number of sub-channels allocated to a fBS, aiming channel.
M
to maximize femtocell capacity subject to the link reliability 1
requirement. The femtocell capacity C is deﬁned as the hj = hi , for j = 1, . . . , J.
j,m (9)
M m=1
aggregated throughput of a femtocell, which depends on the
channel selection scheme, the number of used sub-channels, (S2) Follow the steps (S2) and (S3) in Section IV-A.
and the adopted MCS of each sub-channel. Furthermore,
according to the equivalent SINR γef f,j from the EESM C. Min-Max Interference-Avoidance Oriented Selection
calculation and Table I, we can determine the used MCS Scheme
and then the spectrum efﬁciency SEEESM,j . Assume that The procedures are described in the following. We consider
Bj is the bandwidth of a sub-channel. Then, the femtocell the interference from mBS and other fBSs.
J
capacity is equal to C = j=1 εj Bj SEEESM,j . The decision (S1) Compare the interference for each sub-carrier of a sub-
variable in the optimization problem is the channel usage ratio channel. The minimum interference for the sub-carriers in a
ρ. Based on these considerations, the capacity maximization sub-channel is
issue can be formulated as a nonlinear programming problem K
as expressed in the following as Ij = max {Pj,m Hj,m + pk hk }. (10)
j,m j,m
m=1,2,...,M
J k=1,k=i
max C = εj Bj SEEESM,j (5) (S2) Sort Ij as I1 ≤ I2 ≤ ... ≤ IρJ ≤ ... ≤ IJ .
ρ∈[0,1]
j=1 (S3) Select the ﬁrst ρJ sub-channels with lower interference,
that is, Ij < IρJ+1 , for j = 1, ..., ρJ.
subject to:
1
J D. Min-Avg Interference-Avoidance Oriented Selection
ρ= (6) Scheme
J j=1
This scheme selects the sub-channel according to average
Prel ≥ Relth (7) interference for the sub-carriers of a sub-channel.
(S1) Compute the average interference of a sub-channel.
where εj ∈ {0, 1}, and Relth is the link reliability require-
M K
ment. 1
Ij = Pj,m Hj,m + pk hk .
j,m j,m (11)
M m=1
k=1,k=i
IV. D ISTRIBUTED C HANNEL S ELECTION S CHEME (S2) Follow the steps (S2) and (S3) in Section IV-C.
We develop two distributed channel selection principles for
femtocell system: the channel-gain oriented and interference- V. S IMULATION R ESULTS
avoidance oriented schemes. The ﬁrst scheme aims to transmit We investigate the downlink capacity and link reliability
data in the sub-channel with higher link gain. In the contrary, of the OFDMA femtocells by simulations. We consider the
the second one transmits data in the sub-channel with lower in- shared-spectrum allocation and the exclusive-spectrum allo-
terference. Both schemes can operate in a distributed manner. cation schemes for the femtocells and macrocell. We com-
The key parameter is the sub-channel usage ratio ρ as shown pare the channel-gain oriented, interference-avoidance oriented
in (5). We detail the developed channel selection schemes as distributed channel selection schemes and random selection
follows. scheme. We assume the femtocell layout as shown in Fig. 1.
4.
TABLE II
PARAMETERS IN W I MAX AND OFDMA SYSTEM 1
≥γ )
th
0.95
Parameter Value
eff
Carrier frequency 2.5 GHz
Success Probability (CINR
0.9
mBS/fBS Tx power 43,20 dBm
Noise ﬁgure (fBS/MS) 5dB/7dB 0.85
mBS radius, DM 500 m
0.8
Separation distance between fBSs, dsf 20 m/40 m random
interference avoidance oriented with minmax
Antenna gain (mBS/fBS/MS) 8dB/3dB/3dB 0.75 interference avoidance oriented with minavg
channel gain oriented with maxmin
Receiver implementation loss 5 dB channel gain oriented with maxavg
System bandwidth 10 MHz 0.7
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Data sub−channels usage ratio
Sampling frequency 11.2 MHz
FFT size 1024 Fig. 2. Link reliability versus the sub-channel usage ratio in dense
deployment with the exclusive-spectrum scheme
Sub-carrier bandwidth, Bj 10.9375 kHz
Number of null/pilot/data sub-carriers 184,120,720
Number of sub-channels, J 40
1
Sub-carriers of each sub-channel, M 18
Link reliability requirement, Relth 90%
≥γ )
0.95
th
eff
0.9
Success Probability (CINR
0.85
There are 24 femtocells around the considered femtocell, and 0.8
the group of 25 femtocells is uniformly distributed in a macro-
0.75 random
cell with the coverage of 500 m. The separation distances interference avoidance oriented with minmax
interference avoidance oriented with minavg
between fBSs are dsf = 20 m for the dense deployment case, 0.7
channel gain oriented with maxmin
channel gain oriented with maxavg
and 40 m for the sparse deployment case. The link reliability 0.65
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
requirement is Relth = 90%. The nominal system parameters Data sub−channels usage ratio
are listed in Table II. Fig. 3. Link reliability versus the sub-channel usage ratio in dense
deployment with the shared-spectrum allocation scheme
A. Impact of Sub-channel Usage Ratio on Link Reliability B. Impact of Sub-channel Usage Ratio on Capacity
Figure 2 shows the reliability probability against the data
sub-channel usage ratio ρ with the exclusive-spectrum alloca- Figure 4 shows the capacity against the data sub-channel us-
tion, where dsf = 20 m. This ﬁgure shows that the femtocell- age ratio ρ in the exclusive-spectrum scheme, where dsf = 20
to-femtocell interference signiﬁcantly impacts the link relia- m. In this ﬁgure, when fBS uses more data sub-channels with
bility probability. As the sub-channel usage ratio increases, a larger ρ in the random selection scheme, the femtocell user
the link reliability probability decreases due to the increas- can yield higher capacity. Noteworthily, because the macrocell
ing interference from other fBSs. Compare to the random does not interfere with the femtocell, the femtocell can use
selection scheme, the developed distributed channel selection more sub-channels to sent data with a required link reliability.
schemes has better link reliability. In this example, under Besides, the channel-gain oriented scheme can yield higher
the link reliability requirement Prel ≥ 90%, the maximum capacity than other selection schemes since it can select the
allowable channel usage ratio of the random selection scheme sub-channels with higher link gain. In this case, if the link
is 0.5. However, the developed channel selection schemes can reliability requirement Prel ≥ 90% is given, the channel-gain
increase the maximum channel usage ratio to 0.6. oriented scheme with ρ = 0.6 can achieve 42% higher capacity
Figure 3 illustrates the reliability probability for various ρ, than the random selection scheme with ρ = 0.5.
where dsf = 20 m. We consider the shared-spectrum allo- Figure 5 shows the femtocell capacity for various ρ in the
cation scheme and two-tier interference. Compared to Fig. 2, shared-spectrum allocation scheme, where dsf = 20 m. It is
this ﬁgure shows that the interference from mBS remarkably shown that the macrocell-to-femtocell interference degrades
degrades the link reliability. For example, the link reliabil- the femtocell capacity remarkably. Since the macrocell in-
ity probability for the random channel selection scheme at terferes with the femtocell, the femtocell have to use fewer
ρ = 0.5 decreases by 8% as the macrocell and femtocells share sub-channels to ensure link reliability. In this situation, the
the same spectrum. However, the proposed channel selection interference-avoidance oriented scheme is preferred, because
schemes still can improve the link reliability. In the ﬁgure, it can effectively decrease interference and improve capacity.
the sub-channel usage ratio ρ for the random selection scheme In this case, under the link reliability requirement Prel ≥ 90%.
should be less than 0.2 to meet the link reliability requirement the interference-avoidance oriented scheme for ρ = 0.4 can
Prel ≥ 90%. Nevertheless, the proposed channel selection achieve 121% higher capacity than the random selection
schemes can use twice sub-channels for data transmission. scheme for ρ = 0.2.
5.
8
P ≤ 0.9 0.96
rel
≥γ )
7
th
0.94
eff
6
Success Probability (CINR
Capacity (Mbps)
0.92
5
0.9
Prel ≤ 0.9
4
0.88
3 random random
interference avoidance oriented with minmax 0.86 interference avoidance oriented with minmax
interference avoidance oriented with minavg interference avoidance oriented with minavg
2 channel gain oriented with maxmin
channel gain oriented with maxmin
channel gain oriented with maxavg 0.84 channel gain oriented with maxavg
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Data sub−channels usage ratio Data sub−channels usage ratio
Fig. 4. Capacity versus the sub-channel usage ratio in dense deployment Fig. 6. Link reliability versus the sub-channel usage ratio in sparse
with the exclusive-spectrum scheme deployment with the shared-spectrum allocation scheme
7 12
Prel ≤ 0.9
6 10
P ≤ 0.9
rel
Capacity (Mbps)
8
Capacity (Mbps)
5
Prel ≤ 0.9
4 6
3 4
random random
interference avoidance oriented with minmax interference avoidance oriented with minmax
2 interference avoidance oriented with minavg 2 interference avoidance oriented with minavg
Prel ≤ 0.9 channel gain oriented with maxmin
channel gain oriented with maxmin
channel gain oriented with maxavg channel gain oriented with maxavg
1 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Data sub−channels usage ratio Data sub−channels usage ratio
Fig. 5. Capacity versus the sub-channel usage ratio in dense deployment Fig. 7. Capacity versus the sub-channel usage ratio in sparse deployment
with the shared-spectrum allocation scheme with the shared-spectrum allocation scheme
C. Impact of Femtocell Density both the femtocell-to-femtocell and the macrocell-to-femtocell
Figure 6 illustrates the reliability probability against the interference signiﬁcantly degrade the link reliability. We de-
data sub-channel usage ratio ρ in the sparse deployment velop channel-gain oriented and interference-avoidance ori-
case (i.e., dsf = 40 m) with the shared-spectrum allocation. ented channel selection principles to improve capacity and
This ﬁgure shows that the channel-gain oriented selection link reliability. Simulation results show that the developed
scheme can improve the link reliability, compared to the channel selection scheme can achieve at most 121% higher
interference-avoidance oriented selection scheme. In a wireless capacity than the random selection scheme, under the link
system, the link reliability is strongly related to the receive reliability requirement Prel ≥ 90%. It is also shown that the
SINR. To improve the link reliability, we should decrease the interference-avoidance oriented selection scheme is suitable
interference and/or increase the signal power. As the separation for the situation with higher interference to improve link
distance dsf of fBSs increases, the interference from other reliability. In addition, the channel-gain oriented selection
fBSs is reduced. In this situation, increasing signal strength can scheme can be used to enhance capacity for the case with
improve the link reliability more signiﬁcantly than reducing lower interference .
the interference. Because the channel-gain oriented selection
scheme can select the sub-channel with higher link gain to R EFERENCES
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in the OFDMA-based femtocell systems. In the femtocells,
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