Content uploaded by Tao Jiang
Author content
All content in this area was uploaded by Tao Jiang on Feb 01, 2015
Content may be subject to copyright.
Throughput Improvement for OFDMA Femtocell
Networks Through Spectrum Allocation and Access
Control Strategy
Mingjie Feng, Da Chen, Zhiqiang Wang, Tao Jiang and Daiming Qu
Wuhan National Laboratory for Optoelectronics
Department of Electronics and Information Engineering
Huazhong University of Science and Technology
Wuhan, 430074, China
Email: tao.jiang@ieee.org
Abstract—In this paper, we propose a novel scheme to improve
the throughput of femtocell networks. The proposed scheme
consists of two steps: spectrum allocation for femtocells and
dealing with nearby macro-users with access control strategy.
Since the positions of femtocells are relatively stable, we firstly
deal with inter-femtocell interference and formulate it as an
integer programming problem. Then we analyze the effects of
different access strategies on system throughput for each sub-
channel and propose a novel access control strategy. Finally,
the superiority of the proposed scheme is verified by simulation
results.
Index Terms — femtocell, spectrum allocation, through-
put, access control.
I. INTRODUCTION
Femtocell is regarded as an effective solution for the indoor
coverage problem and provide high data rates in future wire-
less networks [1]. However, the spectrum utilized by femtocell
is provided by wireless operator, resulting in interference with
existing cellular networks and interference among different
femtocells. To deal with the interference problem, previous
research focused on power control [2], [3] and spectrum
partitioning [4], [5]. In [6], the author proposed a fractional
frequency reuse (FFR) scheme that mitigate the interference,
but the interference reduction is at the expense of decreased
spectrum utilization. An adaptive fractional frequency reuse
scheme had been proposed in [7], and the frequency reuse
factor is determined according to specific interference level.
In [8], [9], cognitive radio was applied to address cross-
tier interference, where the femtocell opportunistically utilize
the available channel by recognizing interference signature.
However we still face the problem of miss detection and
false alarm, and the optimal channel selection strategy is
not well studied yet. An alternative way to handle cross-tier
interference is considering access control [10]. For the closed
access scheme, only registered users are served by femtocell,
while all the users are permitted to access to femtocell in
open access scheme. Study to analyze capacity under different
access schemes in the uplink has been conducted in [11].
In [12], the author defined the femtocell coverage area, and
calculated the average throughput in each area with different
access strategies in the downlink.
In this paper, we firstly deal with inter-femtocell interference
by formulating the femtocell spectrum allocation as an integer
programming problem. In our assumption, closely deployed
femtocells can not use the same channel, and we aim to
maximize the resource block of all femtocells. After that,
for both uplink and downlink spectrum of macrocell, we
analyze the throughput of each interfering nearby macro-user
and femto-user, then schedule the access scheme in order
to maximize the throughput of the whole cellular system.
By allocating proper spectrum resource for each femtocell
and providing the optimal access strategy for each interfering
macro-user, our scheme offer better throughput performance
compared with several conventional schemes.
The remainder of this paper is organized as follows. The
system model considered in this paper is described in Section
II. The proposed femtocell spectrum allocation is discussed in
Section III. Then, the access strategy for each user is analyzed
in Section IV. The performance of the proposed scheme is
verified in Section V. Finally, conclusions are given in Section
VI.
c
R
th
D
Inner region
Outer region
Femtocell Base station
Femtocell Area
Macrocell Base station
Fig. 1. System model.
978-1-4577-1719-2/12/$26.00 ©2012 IEEE 387
II. SYSTEM MODEL
The system model of this paper is shown in Fig. 1. We
consider an OFDMA cellular system, and the radius of macro-
cell and femtocell is R
c
and R
f
, respectively. The channel
of a macrocell with bandwidth of B
c
is divided into N sub-
channels, and the bandwidth of each sub-channel B
u
is equal
to the bandwidth required by a macro-user. For simplicity,
we assume that the N sub-channels are sufficient for macro-
users, thus each macro-user correspond to one sub-channel
in our analysis. We assume both femtocells and macro-users
are randomly distributed in the macrocell area, each macro-
user is allocated with random and different sub-channel. The
transmission power of macro-user P
u
has 5 levels according
to the distance with macrocell base station, and the macrocell
base station use fixed transmission power of P
c
. We consider
a Rayleigh fading channel with unit average power, and
experience path loss and wall penetration loss. The wall
penetration loss and the path loss exponent is denoted as L
and α, respectively. The noise power is σ
2
.
The femtocell spectrum allocation method has several cate-
gories in previous study. One category is that femtocell located
in a macrocell area utilize the spectrum of this macrocell, thus,
interference management is required to coordinate between
two tiers. For the other category, femtocell utilize the spectrum
of other macrocell [13], [14], but interference problem still
exist in cell-edge area. In [12], the macrocell area is divided
into inner region and outer region with boundary D
th
, which
indicate that femtocells located in the two regions have differ-
ent features. In this paper, we allocate the inner region with the
spectrum of another macrocell, while allocate the outer region
with the spectrum of the corresponding macrocell. For the
inner region, due to longer distance with other macrocell and
the low transmission power femtocell base station, interference
between femtocell and other macrocell is neglectable in our
study. For the outer region, the distance between femtocell
base station and macrocell base station is relatively further,
therefore we can mainly focus on dealing with the interfer-
ence between cell-edge macro-user and interference between
femtocell base station and macrocell base station is mitigated.
III. FEMTOCELL SPECTRUM ALLOCATION
In this section, we consider maximize the total spectrum
bandwidth of all femtocells while mitigate inter-femtocell
interference and guarantee a basic resource allocation for
each femtocell. Define the interference list as a M × M
binary matrix with element F
j
i
, where F
j
i
= 1 indicates that
femtocell i and j will have inter-femtocell interference when
they utilize the same sub-channel, and F
j
i
= 0 indicates that
femtocell i and j will not bring interference to each other even
they utilize the same sub-channel. Therefore, as shown in Fig.
2, the interference list can be obtained by
F
j
i
=
1, D
j
i
< R
f
+ R
i
0, else
. (1)
where D
j
i
is the distance between two femtocell base stations,
and R
i
is the interfering radius of femtocell. We only consider
f
R
Femtocell area
Femtocell area
i
R
f
R
i
R
j
i
D
Fig. 2. Inter-femtocell interference.
path loss in this section to obtain R
i
, and R
i
can be obtained
using the path loss model between two femtocells.
Denote W as the spectrum resource of all femtocells, δ
m
n
is an indicator and is defined as
δ
n
m
=
1, sub − channel n is assigned to femtocell m
0, else
(2)
In order to ensure the fairness among femtocells, and avoid
unnecessary resource allocation, the spectrum resource allocat-
ed to each femtocell should have an upper bound and denoted
as A
m
. Therefore, the spectrum allocation of femtocell is
formulated as
W =
N
P
n=1
M
P
m=1
δ
n
m
max
δ
W
s.t.
δ
n
i
δ
n
j
F
j
i
= 0
N
P
n=1
δ
n
m
≤ A
m
(3)
However, obtaining the optimal solution of this problem by
exhaustive searching brings huge complexity since the number
of all possible combinations is exponentially increasing with
the number of femtocells multiplexing the number of sub-
channels (M × N). Therefore, we define the independent
femtocells set, and consider a resource block of 10 sub-
channels for analysis. In an independent femtocell set, a
femtocell only brings interference to femtocells in the same
set, while does not bring interference to femtocells in other
independent sets. With this approach, we can obtain the
optimal solution of spectrum allocation in each independent
set with lower complexity, hence the optimal solution is the
combination of the solutions of these independent sets. By
388
considering a resource block of 10 sub-channels, we assign
the N/10 resource blocks for femtocells and the computational
complexity is obviously reduced. In our model, femtocells in
different regions utilize different sets of sub-channels, thus we
should conduct spectrum allocation for femtocells located in
inner region and out region separately.
Note that the value of A
m
also affects the final result,
and it is related with the density of femtocells. Obviously,
femtocell that is interfered by more other femtocells will be
allocated with less spectrum resource to maximize the total
spectrum resource while avoiding inter-femtocell interference.
Therefore, when A
m
is large, the fairness among femtocells
will not be satisfactory, while when A
m
is small, the number
of total spectrum resource will significantly reduced. In this
paper, we set A
m
to be a constant that consider both fairness
and the total benefits, future work may search the optimal
value of A
m
with respect to femtocell density.
IV. THROUGHPUT ANALYSIS BASED ACCESS CONTROL
STRATEGY
Since femtocells located in the inner region utilize the spec-
trum of another macrocell and thus the cross-tier interference
is neglectable, this section focus on addressing the cross-tier
interference in outer region. We stand on the point of wireless
operator, and aim to maximize the throughput of the cellular
system. Previous research [10], [12] had shown the fact that
there is a tradeoff in the throughput performance between
femtocell and macrocell, and femto-users tend to adopt closed
access to maintain high data rates while macro-users prefer
open access to improve their QoS especially in cell-edge area.
In this paper, we analyze the throughput relationship between
interfering macro-users and femto-users in detail, then propose
a novel access control strategy and the procedure to realize it.
A. Throughput analysis on each sub-channel
In our scheme, the interference management is mainly
accomplished by femtocell base station. Since frequent fre-
quency switching and handover will deplete the battery power
of a macro-user equipment (MUE), we assume that macro-user
will not conduct channel switching for both access strategies.
For analytical tractability, we neglect the difference between
uplink and downlink of femtocell users and use the downlink
for our study.
For sub-channels used by the uplink of the macrocell,
if a sub-channel is occupied by a macro-user and nearby
femtocells at the same time, femtocells are interfered by this
macro-user, and have the choice of open or closed access.
Closed access means that femtocell continue occupying the
sub-channel while tolerate the interference, while open access
means the sub-channel is used to serve macro-user and not
available for femtocell. Therefore, the sum throughput on this
sub-channel for both access strategies can be represented as
Closed access:
T
u
=B
u
log
2
1 +
P
f
R
−α
g
0
P
u
LR
−α
g
0
+ σ
2
+ B
u
log
2
1 +
P
u
D
−α
g
0
P
cochannelFBS
P
f
LD
−α
g
0
+ σ
2
(4)
Open access:
T
u
= B
u
log
2
1 +
P
u
LR
−α
g
0
P
cochannelFBS
P
f
LD
−α
g
0
+ σ
2
(5)
Where R and D denote the distance between user and
femtocell base station, macrocell base station, respectively. g
0
is the exponentially distributed channel power with unit mean.
For sub-channels used by the downlink of the macrocell,
macro-user is interfered by co-channel femtocells, and inter-
fering femtocells also have the choice of open or closed access.
In closed access, femtocells should reduce transmission power
on the sub-channel to guarantee the QoS of macro-user, while
in open access, the sub-channel is used to serve macro-user and
not available for femtocell. Therefore, the sum throughput on
this sub-channel for both access strategies can be represented
as
Closed access:
T
u
=B
u
log
2
1 +
P
f0
R
−α
g
0
P
c
LD
−α
g
0
+ σ
2
+ B
u
log
2
1 +
P
c
D
−α
g
0
P
cochannelFBS
P
f0
LR
−α
g
0
+ σ
2
(6)
Open access:
T
u
= B
u
log
2
1 +
P
f
LR
−α
g
0
P
cochannelFBS
P
f0
LR
−α
g
0
+ σ
2
(7)
Where P
f0
is the adjusted transmission power of femtocell
base station.
B. Access control strategy
In our model, we assume the initial system state is closed
access, and the access results will be updated after a period of
time. Both femtocell base station (FBS) and MUE can identify
the received signal, and macrocell base station (MBS) schedule
the whole system through control channels.
1) uplink: For the uplink sub-channels of macrocell, when
MUE and femtocells are utilizing the same sub-channel, the
access control strategy has the following steps:
• If half of the spectrum of a FBS is used for MUE
transmission, this FBS is unavailable for open access.
• On each sub-channel, FBS detects the interference sig-
nal of nearby MUE, then obtain the throughput loss
if adopting open access. The throughput of macro-user
389
if adopting open access can be obtained based on the
received signal strength of MUE. The calculation results
and the availability information are sent to MBS for
scheduling.
• The throughput of MUE when adopting closed access
is easily obtained at the MBS, thus the throughput gain
from closed access to open access for MUE is also
obtained. Then, MBS compare the maximal throughput
gain of MUE if access to an available FBS with the sum
throughput loss of all interfering FBSs. If the throughput
gain of the MUE is greater than the throughput loss of
all FBSs, the MUE is scheduled to access to the FBS. If
the throughput gain of MUE is less than the throughput
loss of all FBSs, the MUE continue served by MBS in
the uplink.
• If the MUE is determined to access to a FBS, reducing
the transmission power of MUE may further improve the
system throughput since lower power will generate less
interference to other FBSs while the reduced power is
sufficient for open access transmission. Thus, the MBS
replace the transmission power of MUE to lower levels
and repeat the calculation. Hence, the optimal transmis-
sion power of MUE when access to a FBS can be found.
• MBS sends to access result and transmission power
information to MUE.
2) downlink: For the downlink sub-channels of macrocell,
when MUE and femtocells are utilizing the same sub-channel,
the access control strategy has the following steps:
• If half of the spectrum of a FBS is used for MUE
transmission, this FBS is unavailable for open access.
• MUE detects and identifies the interference signals of
nearby FBSs. Then MUE reports the received signal
strength of MBS and each interfering FBSs to MBS.
• MBS informs each of the interfering FBS, and each FBS
reduce the transmission power on this sub-channel to P
f0
.
• Each interfering FBS calculate the throughput loss on this
sub-channel if adopting open access and send the results
to MBS. The calculation results and the availability
information are sent to MBS for scheduling.
• MBS calculates the throughput of MUE with both ac-
cess schemes based on power information reported from
MUE. Then, MBS compare the maximal throughput gain
of MUE when served by an available FBS with the sum
throughput loss of all interfering FBSs. If the throughput
gain of the MUE is greater than the throughput loss of
all FBSs, the MUE is scheduled to access to the FBS.
Otherwise, the MUE will continue served by MBS in the
downlink.
• If the access scheme is open access, MBS will inform
the corresponding FBS to serve the macro-user.
V. SIMULATION RESULTS
The performance of the proposed scheme is verified by
computer simulation with system parameters shown in Table
I. Interference power threshold P
i
is set to determine whether
TABLE I
SIMULATION PARAMETERS
P
c
43dBm
P
f
20dBm
P
f 0
15dBm
P
u
10dBm,15dBm,20dBm,25dBm,30dBm
R
c
500m
R
f
30m
D
th
250m
B
c
20MHz
B
u
200KHz
α 3.5
L 0.8
σ
2
-105dBm
A
m
70
P
i
-60dBm
two femtocells can use the same sub-channel in Section
III, and whether to handle access control in Section IV. To
evaluate the spectrum allocation of the proposed scheme, the
proposed scheme is compared with conventional schemes.
For the validation of the access control strategy, we compare
the proposed scheme with closed access scheme and open
access scheme. We assume that in open access scheme, any
interfering macro-users will be served by femtocells.
Fig. 3 depicts the total throughput of femto-users and
macro-users with different spectrum allocation schemes. In
the universal frequency reuse scheme, all femtocells located in
the macrocell utilize the spectrum of this macrocell, and each
femtocell utilize the whole spectrum of this macrocell. For the
improved universal frequency reuse scheme, femtocells locat-
ed in the inner region utilize the spectrum of another macrocell
, while femtocells located in the outer region utilize the spec-
trum of this macrocell, and each femtocell utilize the whole
spectrum of the corresponding macrocell. We can observe that
when the number of femtocell is small, universal frequency
reuse scheme and improved universal frequency scheme can
obtain higher throughput than the proposed scheme, but when
the number of femtocell gets larger, the proposed scheme offer
better performance. This is because there is enough space for
spatial reuse of spectrum when there are few femtocells, and
each femtocell can use the whole spectrum with slight inter-
ference to each other. As the more femtocells are deployed,
the spectrum resource upper bound A
m
is necessary to reduce
interference. We can also infer that the optimal value of A
m
is related to the number of femtocells. As the number of
femtocells increases, A
m
should decrease, which means in
dense deployed situation, the constraint of resource allocation
for each femtocell should be tight to reduce interference and
improve the whole system performance. The proposed scheme
could offer better performance since the maximal number of
sub-channels are allocated to each femtocell while avoiding
inter-femtocell interference.
Fig. 4 shows the total throughput performance compared
390
0 20 40 60 80 100
0
1
2
3
4
5
6
7
8
9
Total System Throughput (Gbps)
Number of Femtocell
Improved Universal Reuse
Universal Reuse
Proposed
Fig. 3. Total system throughput for different spectrum allocation schemes.
with fully open access and closed access scheme. In order to
better demonstrate the effectiveness of the proposed scheme,
we make the comparison for the sum throughput of interfering
users, and use the spectrum allocation strategy of universal
frequency reuse. As expected, the proposed access control
strategy could achieve higher throughput than fully open and
closed scheme while guarantee the benefits of both macro-
users and femto-users. With user selection and power adjust-
ment in the proposed strategy, each macro-user is given with
the best access strategy to maximize the total throughput.
0 20 40 60 80 100
0
5
10
15
20
25
30
35
Number of Femtocell
Sum Throughput of Interfering Users (Mbps)
Proposed
Closed Access
Open Access
Fig. 4. Comparison of the throughput of interfering users with different
access strategies.
VI. CONCLUSION
In this paper, we considered the design of a spectrum
allocation and access control strategy for femtocell networks.
The spectrum allocation is based on femtocell location, and
aim to maximize the resource for femtocells while mitigate
inter-femtocell interference. We also considered finding the
optimal access strategy for macro-users to improve the total
system throughput while mitigate the cross-layer interference.
The simulation results demonstrate that the proposed scheme
provide throughput improvement compared with conventional
schemes.
ACKNOWLEDGMENT
This work was supported by the Project-sponsored by SRF
for ROCS, SEM, the National & Major Project with Grant
2012ZX03003004, the National Science Foundation of China
with Grant 61172052 and Grand 60872008, the Program for
New Century Excellent Talents in University of China under
Grant NCET-08-0217, the Research Fund for the Doctoral
Program of Higher Education of the Ministry of Education
of China under Grant 200804871142, the Science Found for
Distinguished Young Scholars of Hubei in China with Grant
2010CDA083, and Major Program of Ministry of Science and
Technology of China (No. 2010ZX03003-002-03).
REFERENCES
[1] V. Chandrasekhar, J. G. Andrews, and A. Gatherer, “Femtocell networks:
a survey,” IEEE Commun. Mag., vol. 46, no. 9, pp. 59-67, Sep. 2008.
[2] H.-S. Jo, C. Mun, J. Moon, and J.-G. Yook, “Interference mitigation
using uplink power control for two-tier femtocell networks,” IEEE Trans.
Wireless Commun., vol. 8, no. 10, pp. 4906-4910, Oct. 2009.
[3] H.-S. Jo, C. Mun, J. Moon, and J.-G. Yook, “Self-optimized coverage
coordination in femtocell networks,” IEEE Trans. Wireless Commun., vol.
9, no. 10, pp. 2977-2982, Oct. 2010.
[4] V. Chandrasekhar, J. G. Andrews, “Spectrum allocation in tiered cellular
networks,” IEEE Trans. Commun., vol. 57, no. 10, pp. 3059- 3068, Oct.
2009.
[5] I. Guvenc, M.-R. Jeong, F. Watanabe, and H. Inamura, “A hybrid frequen-
cy assignment for femtocells and coverage area analysis for cochannel
operation,” IEEE Comm. Lett., vol. 12, no. 12, pp. 880-882, Dec. 2008.
[6] R. Giuliano, C. Monti, and P. Loreti, “WiMAX fractional freuqency reuse
for rural environments,” IEEE Wireless Commun., vol. 15, no. 3, pp. 60-
65, June. 2008.
[7] Heui-Chang Lee, Dong-Chan Oh, and Yong-Hwan Lee, “Mitigation of
Inter-Femtocell Interference with Adaptive Fractional Frequency Reuse,”
in Proc. IEEE International Conference on Communications, pp. 1-5,
May. 2010.
[8] Chiao Lee, Jane-Hwa Huang, Li-Chun Wang. “Distributed Channel
Selection Principles for Femtocells with Two-Tier Interference,” in Proc.
IEEE Vehicular Technology Conference, pp. 1-5, May. 2010.
[9] Yang-Yang Li, Macuha. M., Sousa. E.S., Sato. T.. Nanri. M., “Cognitive
Interference Management in 3G femtocells,” in Proc. IEEE Personal,
Indoor and Mobile Radio Communications, pp. 1119-1122, Sep. 2009.
[10] Guillaume de la Roche, Alvaro Valcarce, David Lpez-Prez, and Jie
Zhang, “Access Control Mechanisms for Femtocells,” IEEE Commun.
Mag., vol. 48, no. 1, pp. 33-39, Jan. 2010.
[11] P. Xia, V. Chandrasekhar, and J. G. Andrews, “Open vs. closed access
femtocells in the uplink,” IEEE Trans. Wireless Commun., vol. 9, no. 12,
pp. 3798-3809, Dec. 2010.
[12] Han-Shin Jo, Ping Xia, and Jeffrey G. Andrews, “Downlink Femtocell
Networks: Open or Closed?,”in Proc. IEEE International Conference on
Communications, pp. 1-5, June. 2011.
[13] Ju Yong Lee, Sueng Jae Bae, Young Min Kwon, Min Young Chung,
“Interference Analysis for Femtocell Deployment in OFDMA Systems
Based on Fractional Frequency Reuse,” IEEE Comm. Lett., vol. 15, no.
4, pp. 425 - 427, Mar. 2011.
[14] Haddad. Y, Porrat. D, “A Two-Tier Frequency Reuse Scheme,” in Proc.
IEEE Personal, Indoor and Mobile Radio Communications, pp. 203 -
207, Sep. 2010.
391
