3302 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 8, NO. 6, JUNE 2009
Secondary
Cognitive Radio K
Secondary
Cognitive
Radio 2
Secondary
Cognitive
Radio 1
Secondary
User Base
Station
Primary User Base
Station
Primary User
Transmitter
Fig. 1. The primary-secondary user communications system model.
although we limit ourselves to one primary transmitter for the
simplicity of exposition, the p roposed scheme can be extended
to include more than one primary user. Since the key ingre-
dient in the proposed concept of dynamic spectrum leasing is
the interaction between the primary system and the secondary
system, a single primary user is enough to demonstrate the key
aspects of dynamic spectrum leasing while avoiding extrane-
ous complications. However, in a realistic network, of course,
there will be multiple primary transmitters and their own
internal interactions will add an important dimension to th e
problem of dynamic spectrum leasing. There is one primary
receiver and one common secondary receiver in the system
(again,generalization to more than one is possible). The cross
correlation coefficients between the signalling waveforms of
the k-th secondary user and that of a primary user is denoted
by ρ
kp
, between a primary user and the k-th secondary user
is by ρ
pk
and between the k-th and the j-th secondary
users is by ρ
jk
for all k, j ∈{1, ···,K}. For simplicity,
throughout we will assume that ρ
kp
= ρ
pk
= ρ
sp
,forall
k ∈{1, ···,K}. The channel gain between the k-th secondary
user and the common secondary receiver is h
sk
, between the
k-th secondary user and the primary receiver is h
pk
, between
the primary user and the primary receiver is h
p0
, and between
the primary user and the common secondary receiver is h
s0
.
In the proposed formulation, the p rimary user can adapt its
interference cap, denoted by Q
0
, which is the maximum total
interference the primary user is willing to tolerate from all
secondary transmissions. However, the primary user should
always first strive to achieve its target SINR to ensure its
required QoS. This is an important constraint in the concept of
dynamic spectrum leasing since it is expected that the primary
system should first focus on its communication needs and
spectrum leasing is only an option to improve the spectrum
utilization. Note that, the QoS requirement in conjunction with
the c hosen interf e rence cap will directly determine the primary
user’s transmit power level. By adjusting the interference cap,
the p rimary user can indirectly control the total transmit power
the secondary users impose on the channel at any given time.
All secondary users adapt their transmission powers to achieve
a certain transmission quality. However, their transmission
powers must be carefully controlled in order to ensure low
interference to the primary user (within the a llowed interfer-
ence cap) as well as to other secondary users. We use P
0
and
p
k
to represent transmission powers of the primary user and
the k-th secondary user, respectively.
In the above cognitive dynamic spectrum leasing network,
the primary and secondary users interact with each other by
adjusting their actions in response to those of the others:
the primary u ser by adjusting its interfere nce cap (which, in
turn, determines its transmit power) and the secondary users
by controlling their transmit power levels. In essence, both
primary as well as secondary users act as rational decision
makers, thereby making game theory a n atural framework to
analyze and predict the behavior of this system. Formally, we
model our proposed scheme as the following non-cooperative
game:
1) Players: K = {0, 1, 2, ..., K},where0-th user is taken
to be the prima ry user and k =1, 2, ..., K represents the
k-th secondary user.
2) Action space: P = Q×P
1
×P
2
... ×P
K
,whereQ =
[0,
¯
Q
0
] represents the p rimary user’s action set and P
k
=
[0,
¯
P
k
],fork =1, 2, ..., K, represents the k-th secondary
user’s action set.
¯
Q
0
and
¯
P
k
represent, respectively, the
maximum allowed interference cap of the primary user
and the maximum allowed transmission power of the
k-th secondary user. The action vector of all users is
denoted by p =[Q
0
,p
1
, ..., p
K
],wherep
k
∈P
k
and
Q
0
∈Q. The action vector excluding the action of the
k-th user, for k =0, 1, 2, ..., K, is customarily denoted
by p
−k
.
3) Utility function: We use u
k
(p
k
, p
−k
) , ∀k =1, 2, ..., K
to represent the k-th secondary user’s utility function
and u
0
(Q
0
, p
−0
) to represent the primary user’s utility
function.
Throughout this paper, we assume that the primary receiver
is based on a matched-filter detector since we are limiting our-
selves to a primary system with only a single user. However,
it is possible to modify the proposed scheme for situations
in which the primary receiver can b e an ad vanced multiuser
detector, as will be required when one considers a primary
system with multiple transmitters. A ssuming a matched- filter
based primary receiver, the primary user’s target SINR is
defined as:
¯γ
0
=
h
2
p0
P
0
Q
0
+ σ
2
, (1)
where P
0
and Q
0
represent the primary user’s transmission
power and its chosen interference cap, respectively, and σ
2
is
the variance o f the additive noise at the primary receiver. Note
that, since Q
0
is the maximum interference from secondary
users the primary user is willing to tolerate at any given
time, ¯γ
0
in (1) represents the worst-case transmission quality
the primary user can expect with its chosen Q
0
. Since this
worst-case SINR needs to guarantee a required QoS constraint,
the prima ry user’s tr ansmit power is thus directly determined