POWER CONTROL IN COGNITIVE RADIO SYSTEMS BASED ON SPECTRUM SENSING SIDE INFORMATION Karama Hamdi, Wei Zhang, and Khaled Ben Letaief The Hong Kong University of Science and Technology Communications, ICC '07. IEEE International Conference

Outline 2  Introduction  System Model  Power Control Based on Spectrum Sensing Side Information  Numerical Result  Conclusion  Comments

Introduction 3  Cellular network bands are overloaded in most parts of the world but amateur radio or paging frequencies are not.  Some important abilities should be provided by the cognitive radio [5]  spectrum sensing, dynamic frequency selection, transmit power control  Challenging problem  The interference which occurs when a cognitive radio accesses a licensed band but fails to notice the presence of the licensed user. [5] S. Haykin, “Cognitive radio: brain-empowered wireless communications,” IEEE J. Sel. Areas in Commun., vol. 23, pp. 201–220, Feb

Introduction (cont’d) 4  In [8], A power control rule was presented to allow cognitive radios to adjust their transmit powers in order to guarantee a QoS to the primary system  Base on the locations of the licensed user  In this paper, we present a power control approach in cognitive radio systems base on spectrum sensing side information  To mitigate the interference to the primary user due to the presence of cognitive radios

Introduction (cont’d) 5  This approach consists of two steps  the shortest distance between a licensed receiver and a cognitive radio is derived from the spectrum sensing side information  the transmit power of the cognitive radio is determined based on this shortest distance to guarantee a quality of service for the licensed user

System Model 6 Primary transmitterPrimary receiver Decodable region Protection region Cognitive radio PTx’s transmit power: Q p CR’s transmit power: Q c Δ (dB): the signal attenuation of the distance Rd μ (dB): the margin of protection

System Model (cont’d) 7  In the secondary system,  A cognitive radio is considered to work in the same frequency band as the primary system  “Listening before accessing”  Let d (m) denote the distance between the primary transmitter and the cognitive radio  Computing d will be a challenging problem  QoS of the primary receiver  Limit the transmit power Q c of the cognitive radio

System Model (cont’d) 8  The worst case scenario  The primary receiver is located on the crossing point between the boundary of the protection region and the line from the primary transmitter to the cognitive radio  can guarantee a good QoS for the primary receiver in operation at any location inside the protection region  The transmit power of the cognitive radio  inflicts tolerable interference on the primary receiver  depends on the SNR loss ( μ + ψ ) in dB  The SNR loss due to the distance d is give by η = ψ + Δ  The transmit power control problem is essentially converted to  The problem of evaluating the SNR loss η due to d for a given μ and Δ

Power Control Based on Spectrum Sensing Side Information 9  Firstly propose an idea of determining the distance d between the primary transmitter and the cognitive radio from spectrum sensing  The transmit power of the cognitive radio can be controlled based on the distance d in order to guarantee a QoS to the primary receiver

Power Control Based on Spectrum Sensing Side Information (cont’d) 10  Spectrum Sensing Side Information T: the observation time, x(t): the received signal at the cognitive radio s(t): the transmitted signal from the primary transmitter n(t): the zero-mean additive white Gauassian noise (AWGN) with variance σ 2 h: the Rayleigh fading channel coefficient  The instantaneous SNR is defined as

Power Control Based on Spectrum Sensing Side Information (cont’d) 11  The energy of the received signal, denoted by Y, is collected in a fixed bandwidth W  and a time slot duration T and then compared with a pre-designed threshold λ  If Y > λ then the cognitive radio assumes that the primary system is in operation, i.e., H 1. Otherwise, it assumes H 0.

Power Control Based on Spectrum Sensing Side Information (cont’d) 12  The average probability of false alarm, detection and missing of energy detection over Rayleigh fading channels can be given by, respectively [11] False alarm Detection Missing Detection :denotes the average SNR at the cognitive radio E γ [.]: the expectation over the random variable γ which is Rayleigh distributed. Γ: the gamma function [11] A. Ghasemi and E. S. Sousa, “Collaborative spectrum sensing for opportunistic access in fading environments,” (DySPAN’05),

Power Control Based on Spectrum Sensing Side Information (cont’d) This figure show that when the average SNR increases the probability of missing becomes smaller 2. For a specified average SNR, a larger P f will result in the decrease of Pm because of the decrease of the threshold used in energy detection

Power Control Based on Spectrum Sensing Side Information (cont’d) 14  The path loss due to the distance d can be given by  We can obtain a relationship between P m and d (or η ) for the given Q p / σ 2 and α. It is obvious that the distance d (or η ) can decided by P m

Power Control Based on Spectrum Sensing Side Information (cont’d) 15 For a fixed distance d, A higher transmit SNR can get a better sensing performance, i.e. a lower P m, because the received SNR is enhanced. Numerical results demonstrate that when the cognitive radio is far from the primary transmitter, a high probability of missing is obtained.

Power Control Based on Spectrum Sensing Side Information (cont’d) 16  The probability of missing P m can be calculated as follows Let, Y i denotes the energy collected by the cognitive radio in the time slot i. Once P m is determined, d (or η ) can be obtained from the previous formula

Power Control Based on Spectrum Sensing Side Information (cont’d) 17  Transmit Power Control for Cognitive Radio  SINR decodability SNR where Q p ‘ and Q c ‘ denote the received signal power from the primary transmitter and the cognitive radio, respectively α and μ are constants It can be seen from the above formula, that the value of the allowable Q c depends on the SNR loss ψ

Power Control Based on Spectrum Sensing Side Information (cont’d) 18  In this work, we consider the worst case that the primary receiver is located at the closet point to the cognitive radio. (fig 1.)  where Q c max denote the maximum value of Q c in dB d has been derived from the spectrum sensing side information previously

Power Control Based on Spectrum Sensing Side Information (cont’d) 19  Proposed Power Control Algorithm:  Step1: Calculate the P m (probability of missing)  Step2: Derive d or η  Step3: Calculate Q c max

Numerical Results 20 This shows the proportional relationship between Pm and the SNR loss due to the distance d.

Numerical Results (cont’d) 21 It demonstrates that the allowable transmit power of the cognitive radio can be increased when a heavy SNR loss occurs between the cognitive radio and the primary receiver

Conclusion 22  Develop a power control approach which intelligently adjusts the transmit power of the cognitive radio  while maintaining a QoS for the primary user  The transmit power is controlled by the spectrum sensing side information  the probability of missing which actually includes the implicit location information of the primary user.

Comments 23  Only use the sensing information to make the power control decision.  Evaluating the distance as an important parameter to make the decision  Consider the worst case, rather than find out a general solution.  Performance consideration