Optimal Power Control in Interference-Limited Cognitive Radio Networks

Optimal Power Control in Interference-Limited Cognitive Radio Networks Invited Paper Shimin Gong, Ping Wang, Dusit Niyato Centre for Multimedia and Network Technology (CeMNet), Nanyang Technological University, Singapore Abstract—Power control for multiple cognitive radios is investigated for broadcast service under the coverage of primary base station. Multiple secondary broadcasting base stations are assumed to operate in the same frequency band as that of primary system. However, the performance of the primary users can be degraded due to the interference from secondary base stations. Therefore, to cope with this problem, a new interference metric is considered to account for the impact on the broadcast service of the primary system. This new interference metric focuses on the percentage of interfered primary receivers. Furthermore, we consider the broadcast service of secondary receivers to be optimized for the utility function. To maximize the total utility of coexisting secondary receivers, an iterative method is proposed for the optimal power allocation. Simulation results show that the proposed algorithm converges to the optimal power level while guaranteeing interference level to the primary system.

Keywords – Cognitive radio, broadcast service, optimization. I. I NTRODUCTION With the rapid growth of wireless applications, there is a dramatic increase in the demand for spectrum frequency bands. However, since the radio spectrum has been allocated to license holders for exclusive use, the spectrum becomes a scarce resource. The shortage of spectrum is an obstruction to the development and deployment of new wireless applications. On the other hand, field measurement reports [1] showed that the usage of spectrum is extremely inefficient. To balance the pressure of increasing demand and under-utilization of licensed spectrum, U.S. Federal Communications Commission (FCC) admitted Cognitive Radio (CR) technology [2] as a promising solution to tackle this embarrassment. CR technology enables unlicensed users (i.e., secondary users) to opportunistically access the spectrum bands allocated to the licensed users (i.e., primary users), while maintaining an acceptable interference. Generally, there are several metrics to define and measure the interference to primary users. Overlapping time [3] measures the collision duration between the transmissions of primary and secondary users. This metric is practically important when the duration of interruption is critical to the quality of service (QoS) of the primary user, e.g., the voice communication of primary users that lasts much longer than packet transmission of secondary users. When the average packet lengths of both primary and secondary users are in the same scale, collision probability [4],

defined as the number of collided packets over total number of packets in the perspective of primary user, is considered as the interference metric. Another metric is the interference power, which determines the noise plus interference floor to the primary user. Then, the interference constraint violation probability [5] can be derived to guarantee QoS requirements of primary user. Given any of these interference measures, the secondary users are required to intelligently adjust their operating bands, transmitting power or data rates, and any other performance related parameters. In this paper, we address the power control problem of multiple secondary users under the same operating spectrum channel of primary users. Power control algorithms [6] [7] in traditional wireless networks are no longer valid in cognitive radio networks. There are extensive works [5] [8] [9] on finding new methods that adapt to highly dynamic spectrum environment. In [8], a method was proposed to maximize the data rate of secondary users without bringing extra outage probability for primary user. In [5], the primary user has some tolerance, and the power control aims to maximize data rate of secondary users subject to interference limit. [10] and [11] proposed a dynamic programming method to optimally allocate transmit power in each time slot. In [12], the relationship between transmit power of secondary user and the detection probability of spectrum opportunity was investigated based on Poisson model of the primary network. All the above works considered uni-cast service for primary users, i.e., the interference from secondary users to a particular primary user. Different from the work in literature, in this paper, we consider broadcast service for primary users (e.g., TV service). In this case, the interference metric defined for a particular primary user may not be applicable. Therefore, we introduce a new interference metric which requires that the total number of interrupted (i.e., interfered) primary users is less than a certain percentage of the total primary users. That is, the primary receiver1 interruption ratio should be restricted to a small threshold according to QoS requirements of primary receivers. For example, there are 100 primary receivers spatially distributed in the coverage of the primary base station, and several secondary base stations are operated in the same band and same area. The broadcast service from 1 In

this paper, we use “node”, “receiver”, and “user” interchangeably.

the primary base station can tolerant some percentage of failure, e.g., 5%. That means at most 5 primary receivers can be interrupted by secondary base stations. The rest of the paper is organized as follows. Section II describes the system model and problem definition. Section III presents the iterative algorithm for power control. Simulation results are given and explained in Section IV. Finally, Section V concludes this work with some remarks on possible future directions.

We consider one primary system with many primary receivers (i.e., nodes) spatially distributed in the coverage of primary base station. There are multiple secondary base stations operating in the same band as that of primary base station (Fig. 1). Both primary and secondary base stations provide broadcast service to their receivers. The secondary base stations aim to maximize their total utility while limiting the interference to the primary receivers. We assume that the secondary base stations are able to exchange control information among each other. A. Utility of Secondary Systems Since the broadcasting secondary base stations operate in the same band, we should also take the interference among secondary systems into the definition of secondary utility. As secondary receivers are located at different locations, for simplicity, the effect of interference is just considered at each secondary base station. Thus, the utility of a secondary system is defined as, Ã ! X pm P U (p) = ln 1 + , (1) no + i6=m,i∈S gi,m pi m∈S

where S denotes the set of all secondary base stations with |S|=M , gi,m represents the channel gain from secondary base station i to m. pm is the transmit power of secondary base station m, and p = [p1 , p2 , . . . , pi , . . . , pM ] is a vector of the transmit power of all secondary base stations. The aggregate utility is the summation of utility of all secondary base stations, while the utility of each secondary base station increases with its transmit power and decreases with the interference. Next, we consider the constraint due to performance requirement of primary receivers in this broadcast service. B. Primary Receiver Interruption Constraint Interference is brought to primary base station if the QoS requirement, i.e., minimum signal to noise plus interference ratio (SINR) γ0 , cannot be guaranteed. Let L = {1, 2, . . . , `, . . . , N } denotes a set of primary receivers. The SINR at any primary receiver is defined as follows: P h` pt n0 + m∈S α`m pm

=

h` pt n0 +α` pT

, ∀` ∈ L,

Primary Receiver

Secondary BS

Fig. 1: Primary and secondary networks.

II. S YSTEM D ESCRIPTION

γ` =

Primary BS

(2)

where h` is the channel gain from primary base station with transmit power pt to primary receiver `, and

α` =[α`1 , · · · , α`m , · · · , α`M ], α`m denotes the channel gain from the secondary base station m to primary receiver `. Let η denote the maximum threshold of primary receiver interruption ratio. That is, up to η|L| of the primary receivers may have the SINR lower than γ0 . The total number of interrupted primary receivers is obtained from X X NI = 1{γ` E` } , (3) `∈L

`∈L

E` = hγ`0pt

where − n0 denotes the acceptable interference level from secondary base stations. 1{γ`