5306 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 12, DECEMBER 2008
Resource Allocation for Spectrum Underlay in
Cognitive Radio Networks
Long Bao Le, Member, IEEE, and Ekram Hossain, Senior Member, IEEE
Abstract—A resource allocation framework is presented for
spectrum underlay in cognitive wireless networks. We consider
both interference constraints for primary users and quality of
service (QoS) constraints for secondary users. Specifically, inter-
ference from secondary users to primary users is constrained to
be below a tolerable limit. Also, signal to interference plus noise
ratio (SINR) of each secondary user is maintained higher than
a desired lev el for QoS insurance. We propose admission control
algorithms to be used during high network load conditions
which are performed jointly with power control so that QoS
requirements of all admitted secondary users are satisfied while
keeping the interference to primary users below the tolerable
limit. If all secondary users can be supported at minimum
rates, we allow them to increase their transmission rates and
share the spectrum in a fair manner. We formulate the joint
power/rate allocation with proportional and max-min fairness
criteria as optimization problems. We show how to transform
these optimization problems into a convex form so that their
globally optimal solutions can be obtained. Numerical results
show that the proposed admission control algorithms achieve
performance very close to that of the optimal solution. Also,
impacts of different system and QoS parameters on the network
performance are investigated for the admission control, and
rate/power allocation algorithms under different fairness criteria.
Index Terms—Cognitive radio, spectrum sharing, spectrum
underlay, spectrum ov erlay, interference temperature limit, ad-
mission control, rate and power allocation.
I. INT RODUCTION
T
HE emerging high-speed wireless access technologies
and the requirements of different wireless applications
are expected to create huge demand for spectral resources in
the next generation wireless systems. Achieving high spectrum
utilization is, therefore, one of the most critical research
objectives in designing wireless communication systems today.
In fact, it has been reported that cur rent utilizatio n of some
allocated spectrum can be as low as 15 % [1]. Therefore,
there is an increasing interest in developing efficient methods
for spectrum management and sharing which is encouraged by
both industry and FCC [2]. This motivates to exploit spectrum
opportunities in space, time, frequency while protecting users
Manuscript recei ved August 10, 2007; revised October 15, 2007; accepted
October 26, 2007. The associate editor coordinating the re view of this paper
and approving it for publication was M. Guizani. This work was supported
in part by the University of Manitoba Graduate Fellowship (UMGF) and in
part by a grant from the Natural Sciences and Engineering Research Council
(NSERC) of Canada.
L. B. Le is with the Massachusetts Institute of Technology, USA (e-mail:
longble@mit.edu).
E. Hossain is with the Department of Electrical and Computer Engi-
neering, Uni versity of Manitoba, Winnipe g, MB, Canada R3T 5V6 (e-mail:
ekram@ee.umanitoba.ca).
Digital Object Identifier 10.1109/T-WC.2008.070890
of the primary network from excessive interference due to
spectrum access from secondary networks. As described in
[3], users from a secondary network (who are referred to as
secondary users) can access the spectrum owned by the pri-
mary network provider (whose users are referred to as primary
users) using spectrum underlay or spectrum overlay. Spectrum
underlay and overlay techniques are the basis for designing
the emerging cognitive radio networks. Also, in contrast to
resource allocation p roblems in traditional wireless networks
[4]-[6], in cognitive radio networks primary users should be
protected while secondary users access the spectrum.
In the spectrum overlay paradigm, secondary users are only
allowed to access spectrum resources (i.e., channels) owned
by the primary network provider if these channels are not
being used by primary users. Here, spectrum sensing needs to
be performed to avoid possible collision with primary users
[7], [8]. Given that the spectrum opportunities are detected
by a spectrum sensing technique, it is important to coordinate
the spectrum sharing among secondary users in such a way
that different design objectives can be achieved. Centralized
architectures for spectrum sensing and sharing in cognitive
wireless networks were proposed in [9] and [10]. In general,
a m edium access control (MAC) layer protocol with sensing
capability is important to perform fair resource allo cation
among secondary users while avoiding collision with primary
users.
For the spectrum underlay paradigm, it is required that an
interference limit corresponding to an interference temperature
level be maintained at receiving points of the primary n etwork.
A graph-theoretic model for spectrum sharing/access among
secondary users was proposed in [11] where different objective
functions were investigated. In [12], the channel allocation
problem was formulated using game theory. Here, the pro-
posed utility functions capture the interference perceived by
one user on each channel and/or the interference this user
creates to its neighboring users. In these papers, primary
users were not explicitly protected from in terference due to
spectrum access by secondary users. In [13], a heuristic-
based channel and power allocation algorithm was proposed
where interference constraints for primary users were con-
sidered. However, how good the performance of the pro-
posed algorithm compared to the optimal solution was not
known. In [14], a user removal algorithm based on the tree-
pruning algorithm was proposed so that QoS constraints for
secondary users and interference temperature constraints for
primary users are satisfied. The proposed removal algorithm
is, however, computationally extensive.
In this paper, we propose a resource allocation frame-
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2008 IEEE