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Energy-Efficient Resource Allocation for Heterogeneous Cognitive Radio Networks with Femtocells
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3910 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 11, NO. 11, NOVEMBER 2012
Energy-Efficient Resource Allocation for
Heterogeneous Cognitive Radio Networks with
Femtocells
Renchao Xie, F. Richard Yu, Hong Ji, and Yi Li
Abstract—Both cognitive radio and femtocell have been con-
sidered as promising techniques in wireless networks. However,
most of previous works are focused on spectrum sharing and
interference avoidance, and the energy efficiency aspect is largely
ignored. In this paper, we study the energy efficiency aspect
of spectrum sharing and power allocation in heterogeneous
cognitive radio networks with femtocells. To fully exploit the
cognitive capability, we consider a wireless network architec-
ture in which both the macrocell and the femtocell have the
cognitive capability. We formulate the energy-efficient resource
allocation problem in heterogeneous cognitive radio networks
with femtocells as a Stackelberg game. A gradient based iteration
algorithm is proposed to obtain the Stackelberg equilibrium
solution to the energy-efficient resource allocation problem.
Simulation results are presented to demonstrate the Stackelberg
equilibrium is obtained by the proposed iteration algorithm and
energy efficiency can be improved significantly in the proposed
scheme.
Index Terms—Cognitive radio, femtocell, energy efficiency,
Stackelberg game, Stackelberg equilibrium.
I. INTRODUCTION
R
APIDLY rising energy costs and increasingly rigid envi-
ronmental standards have led to an emerging trend of ad-
dressing “energy efficiency” aspect of wireless communication
technologies [1], [2]. In a typical wireless cellular network, the
radio access part accounts for up to more than 70 percent of
the total energy consumption [3]. Therefore, increasing the
energy efficiency of radio networks is very important to meet
the challenges raised by the high demands of traffic and energy
consumption.
Cognitive radio technology, originally proposed to improve
the spectrum efficiency [4], can play an important role in
Manuscript receive d August 10, 2011; revised November 26, 2011 and
April 21, 2012; accepted July 31. 2012. The associate editor coordinating the
re view of this paper and approving it for publication was V. K. N. Lau.
R. Xie is with the Ke y Laboratory of Universal Wireless Communication,
Ministry of Education, Beijing University of Posts and Telecommunications,
Beijing, P.R. China, and also with the Dept. of Systems and Computer Eng.,
Carleton University, Ottaw a , ON, Canada.
F. R. Yu is with the Dept. of Systems and Computer Eng., Carleton
Univ ersity, Ottawa, ON, Canada (e-mail: Richard
yu@carleton.ca).
H. Ji, and Y. Li are with the Ke y Laboratory of Uni versal Wireless
Communication, Ministry of Education, Beijing University of Posts and
Telecommunications, Beijing, P.R. China.
This work was jointly supported by the State Key Program of National
Natural Science of China (Grant No. 60832009), the Natural Science Foun-
dation of Beijing, China (Grant No. 4102044), the National Natural Science
Foundation for Distinguished Young Scholar (Grant No. 61001115), the
National Natural Science Foundation of China (Grant No. 61101113), Huawei
Technologies Canada CO., Ltd., and the Natural Sciences and Engineering
Research Council of Canada.
Digital Object Identifier 10.1109/TWC.2012.092112.111510
improving energy efficiency in radio networks [5]. In cog-
nitive radio networks, secondary users (SUs) can monitor the
surrounding radio environment, dynamically adapt transmis-
sion parameters, and opportunistically utilize the temporarily
unused spectrum resource licensed to primary users (PUs)
[6]. The cognitive abilities have a wid e range of pro perties,
including spectrum sensing and learning-empowered adaptive
transmission, which are beneficial to improve the tradeoff
among energy efficiency, spectrum efficiency, bandwidth, and
deployment efficiency in wireless networks [3], [7], [8].
Some works have been done to consider energy efficiency
in cognitive radio networks. In [9], the authors study the
hierarchy in energy games for cognitive radio networks. The
problem is to maximize the energy-efficiency for each selfish
SU. The authors of [10] study the distributed power control
game to maximize the transmission energy-efficiency for SUs
in cognitive radio networks, where the problem of optimal
power control is formulated as a repeated game. Energy-
efficient power control and receiver design in cognitive radio
networks are studied in [11], where a noncooperative power
control game for maximum energy efficiency for SUs is
proposed under the constraints of fairness and interference
threshold.
On the other hand, femtocell has been considered as a
promising technique, and has been integrated in current and
future radio access networks [12]. The authors of [13] study
the pr oblem of downlink power allocation to maximize the
capacity in a cellular network where a bi-level hierarchy exists.
Then the Stackelberg game model is used to formulate the
optimization problem and Stackelberg equilibrium is solved.
The distributed power allocation strategies for a spectrum-
sharing femtocell n etwork are considered in [14], and the
Stackelberg g ame is form ulated to jointly consider the utility
maximization of the macrocell and the femtocells. Due to its
short transmit-receive distance property, femtocell technique
can also reduce energy consumption, prolong handset battery
life, and increase network coverage [15], [16]. The problem
of energy efficiency for femtocell base station is studied in
[17], where a novel energy saving procedure is designed for
femtocell base stations. In [18], resource sharing and access
control in OFDMA femtocell networks are studied, where
users’ selfish characteristics are considered and an incentive
mechanism is designed for subscribers to share the resource
of femtocell base stations.
Combining cognitive radio with femtocell can further im-
prove the system performance [19]–[22]. The smart cognitive
1536-1276/12$31.00
c
2012 IEEE
XIE et al.: ENERGY-EFFICIENT RESOURCE ALLOCATION FOR HETEROGENEOUS COGNITIVE RADIO NETWORKS WITH FEMTOCELLS 3911
femtocell is considered in [20], where the tradeoff between the
macrocell throughput and the aggregate femtocell throughput
is studied. In [21], the problem of cross-tier interference in
autonomous femtocell networks is studied by using cognitive
radio technology to realize the cognitive radio resource man-
agement. And a strategic game is proposed to implement the
interference mitigation. The interference management for long
term evolution (LTE) networks with femtocells using cognitive
radio technology is investigated in [22]. Then based on a dis-
tributed architecture for LTE networks, the authors recommend
to use two game theoretical mechanisms to mitigate the co-
channel interference.
Although some works have been done for heterogeneous
cognitive radio networks with femtocells, most of previous
works are focused on spectrum sharing and interference avoid-
ance. Consequently, th e energy efficiency aspect in this setting
is largely ignored. In addition, most of previous works assume
that only the femtocell base station (FBS) has the cognitive
capability, without considering the cognitive capability of the
macrocell base station. In this paper, our work is different
from previous works. Some distinct features of this paper are
as follows.
• We focus on the energy efficiency aspect of spectrum
sharing and power allocation in heterogeneous cognitive radio
networks with femtocells. Since both cognitive radio and fem-
tocell are promising technologies to enable energy efficiency
in wireless networks, the interplay between them merits furthe r
research. We use bits/Hz per Joule, which is commonly used in
wireless networks [23], to measure the performance of energy
efficiency.
• To fully exploit the cognitive capability, we consider a
wireless network architecture in which both the macrocell and
the femtocell have the cognitive cap ability, which allows the
macrocell and femtocells to dynamic utilize the spectrum re-
source licensed to primary network and sense the surrounding
channel state in formation. This network architecture is first
proposed in [19], which is of importance in practical cellular
femtocell networks, where the macrocell base stations can
sense the TV band.
• We formulate the energy-efficient resource allocation
problem in heterogeneous cognitive radio networks with fem-
tocells as a Stackelberg game, which has been successfully
used in relay selection and power allocation problem s in co-
operative communication networks [24], among others. Then
a gradient based iteration algorithm is proposed to obtain
the Stackelberg equilibrium solution to the energy-efficient
resource allocation problem.
The rest of this paper is organized as follows. In Section II,
the system model is given, and the Stackelberg game model is
formulated. In Section III, the Stackelberg equilibrium solution
is p resented. Simulation results are presented and discussed in
Section IV. Finally, we conclude this study in Section V.
II. S
YSTEM DESCRIPTION
In this section, the heterogeneous cognitive radio network
with femtocells is presented. Then the problem of spectrum
sharing and resource allocation for energy-efficient communi-
cations is formulated as a three-stage Stackelberg game.
PU
1
w
1
MSU
MSU
I
L
w
PU
PU
PU
PU
PBS
l
1
PBS
PBS
L
FSU
K
1
FSU
FBS
K
1
FBS
PU
MSU
i
l
w
Fig. 1. System model for femtocell-based cognitive radio networks (PBS:
primary base station; PU: primary user; w
l
: spectrum resource from primary
network l; MSU: macrocell secondary user; FBS: femtocell base station; FSU:
femtocell secondary user).
A. System Model
Consider a communication system that consists of primary
networks and a femtocell-based heterogeneous cognitive radio
network, as shown in Fig. 1. The primary n etworks may
operate on the different frequency spectrum.In this case, we
will not consider the interference among the primary networks.
Each primary network can offer a spectrum selling price and
sell part of spectrum resource to the heterogeneous cognitive
radio network with femtocells to earn additional profit. In
the heterogeneous cognitive radio network, there are m ultiple
macro secondary users (MSUs), a cognitive base station (BS),
multiple femtocells, and multiple femtocell secondary users
(FSUs). The cognitive BS and femtocells have the cognitive
capability and can sense the channel state information [25].
Then the cognitive BS allocates the spectrum resource bought
from the primary networks to femtocells or MSUs directly
based on the channel quality condition to maximize its rev-
enue. In each femtocell, there is a femtocell base station (FBS)
to provide service for FSUs, where the FBS is connected to
the cognitive BS over a broadband connection, such as cable
modem or digital subscriber line (DSL). The whole system is
operated in a time-slotted manner, and the primary networks
and the femtocell-based cognitive radio network are assumed
to be perfectly synchronized. To simplify the analysis of the
problem, without loss generality, we assume that there is only
one FSU serviced by the FBS in each time slot.
Under this framework, we assume that there are L primary
networks. Each primary network l is willin g to offer a spec-
trum selling price c
l
and sell its part spectrum resource w
l
of total spectrum W
l
to the heterogeneous cognitive radio
network to maximize its profit. The cognitive BS can buy
the spectrum resource w
l
from primary network l depending
on the spectrum p rice. Then the cognitive BS allocates the
spectrum w
l
to the femtocells or MSUs directly to gain its
revenue. Here, we assume that each femtocell and MSU are
accessed in the form of frequency division multiple access
(including OFDMA), and in each time slot they could only
be allocated one spectrum resource bought from a certain
primary network. Assume there are K
tot
femtocells and I
tot
MSUs directly served by the cognitive BS in th e heteroge-
3912 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 11, NO. 11, NOVEMBER 2012
L
FBS
K
1
w
L
w
1
c
L
c
1l
x
lK
x
lk
K
lK
K
lk
x
1l
K
1
FBS
l
l
w
l
c
FBS
k
Fig. 2. Three-stage Stackelberg game modeling (FBS: femtocell base station;
w
l
: spectrum resource from primary network l; c
l
: spectrum selling price from
primary network l; η
lk
: energy efficiency in femtocell k for spectrum resource
w
l
; x
lk
∈{0, 1} is the spectrum allocation index, where x
lk
=1means
that the spectrum bought from primary network l is allocated to femtocell k,
otherwise x
lk
=0).
neous cognitive radio network. Usually the total number of
femtocells and MSUs requesting to connect to the cognitive
BS is larger than the total number of spectrum resource bought
from the L primary networks, i.e., (K
tot
+ I
tot
) ≥ L.Dueto
the limited spectrum resource bought from primary networks,
we assume that there are at most L out of (K
tot
+ I
tot
)
femtocells and MSUs that can be accessed to the cognitive
BS in each time slot, which can be realized by scheduling
and admission control. Under this assumption, without loss
generality, we assume there are K femtocells and I MSUs
accessed to the cognitive BS, where K + I = L. In each
time slot, when a certain spectrum w
l
is used by a fem tocell,
the aim of the femtocell is to maximize its energy-efficient
communications by allocating its power. Similarly, when the
spectrum is used by MSUs, the cognitive BS performs the
energy-efficient power allocation.
Based o n the discussion above, we can formulate the prob-
lem of resource allocation for energy-efficient communications
in the heterogeneous cognitive radio n etwork as a three-stage
Stackelberg game problem.
B. Problem Formulation
In this subsection, we formulate the energy-efficient re-
source allocation problem as a three-stage Stackelberg game,
as shown in Fig. 2 , which consists of leaders and followers.
Here, we view the up-stage as the leaders that move first,
then down-stage as the followers that move subsequently
by observing the leaders’ strategies. Therefore, in Stage I,
the primary networks are the leaders, and multiple primary
networks offer the spectrum selling price c
l
to the cognitive
BS. In Stage II, the cognitive BS, as the follower in Stage
I, decides to buy the spectrum size w
l
from primary network
l depending on the price c
l
offered by primary network l.
Next, the cognitive BS, as the leader in Stage II, allocates
the spectrum to FBSs or MSUs dir ectly, and performs power
allocation for the MSUs to gain its revenue. The FBSs in stage
III performs power allocation for FSUs. Based on the above
analysis, we know that each stage’s strategy will affect other
stages’ strategies, and the leaders should take the followers’
actions into account when deciding their strategies. Therefore,
to formulate the three-stage Stackelberg game model, we can
use the backward induction method as follows.
When femtocell k obtains spectrum resource w
l
from the
cognitive BS, the FBS aims to maximize energy efficiency in
power allocation, which can be expressed as
η
lk
=
log
2
1+
h
2
lk
p
k
σ
2
p
a
+ p
k
, (1)
where p
a
denotes the additional circuit power consumption
of devices during transmissions [26] (e.g., digital-to-analog
converters, filters, etc), which is independent to the data
transmission power. h
lk
and p
k
are the channel gain and power
allocation on spectrum w
l
for FBS k, respectively. σ
2
is the
additive Gaussian white noise with zero mean and unit varia-
tion. In (1), bits/Hz per Joule is used as the energy efficiency
metric, which is commonly used in wireless networks [23],
[27]. In our work, we assume that the channels are fixed over
the time-period of interest. This simplifying assumption has
been commonly adopted by the game-theoretic studies in the
communication literature [14]. To improve energy efficiency,
less energy should be used to transmit more information data.
Based on (1), the aim of FBS k is to maximize its utility given
spectrum w
l
, which can be expressed as follows.
π
k
=
L
l=1
x
lk
(ς
k
w
l
η
lk
− c
b
w
l
η
lk
)=
L
l=1
(ς
k
− c
b
) x
lk
w
l
η
lk
,
(2)
where w
l
is the spectrum resource bought from primary
network l, ς
k
is the revenue for FBS k,andc
b
is the cost
charged by the cognitive BS due to the allocated spectrum.
x
lk
∈{0, 1} is the spectrum allocation index, where x
lk
=1
means that the spectrum bought from primary network l
is allocated to FBS k;otherwisex
lk
=0. As the energy
efficiency criterion is used, the first term denotes the revenue
gained from the FSU for energy efficient transmission, and
the second term denotes the cost charged by the cognitive
BS for using the spectrum resource. Here, we have ς
k
>c
b
.
Otherwise, the FBSs will not request to access.
When the FBSs finish the energy-efficient power allocation,
there is feedback information to the cognitive BS. For the
cognitive BS, the aim is to buy the size of spectrum from
primary networks and allocate them to the FBSs or MSUs
directly to maximize its revenue. Here, we notice that the
cognitive BS also does the energy-efficient power allocation
for MSUs when the spectrum is allocated to MSUs, which
is similar to (1). If we assume that spectrum demand for
cognitive BS from the prim ary networks is satisfied the linear
demand structure, we can use the following quadratic utility
function [28] for the cognitive BS.
π
b
(w)=
L
l=1
w
l
K
k=1
c
b
x
lk
η
lk
+
I
i=1
ξ
i
x
li
η
li
c
b
x
pk
η
k
−
1
2
L
l=1
w
2
l
+2θ
q=l
w
l
w
q
−
L
l=1
c
l
w
l
,
(3)
where w = {w
1
,w
2
, ..., w
L
} denotes the spectrum bought
from primary networks, η
li
is the energy-efficient transmission
parameter for MSUs, ξ
i
is the cost paid by MSUs to the
cognitive BS, c
l
is the price offered by primary network l,and
θ ∈ [−1, 1] is the spectrum substitutability parameter. θ =1
means that the FBSs or MSUs can switch among the spectrum;
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