@inproceedings {Simula.ND.387,
	title = {A Dynamic Spectrum Access Scheme for Unlicensed Systems Coexisting With Primary OFDMA Systems},
	journal = {The 7th Annual IEEE Consumer Communications and Networking Conference (CCNC)},
	year = {2010},
	publisher = {IEEE},
	type = {Conference},
	abstract = {In this paper, we consider Mobile WiMAX and LTE as the future technologies for primary OFDMA systems and study Dynamic Spectrum Access (DSA) for unlicensed systems coexisting with primary OFDMA systems. We propose an effective DSA scheme that allows an unlicensed system to statistically and opportunistically access the whole spectrum bandwidth during the last consecutive symbols of each primary OFDMA subframe. There is a tradeoff in gaining high normalized throughput by enlarging the access duration in each subframe and keeping low interference caused to the primary system by shorten the access period. Then, we model that tradeoff by deriving the optimization problem that maximizes the normalized throughput of the unlicensed system while guaranteeing the harmful interference constraints set by the primary system. We develop and conduct our simulations in our simulator modified from the powerful WiMAX simulator developed by the WiMAX Forum in Ns-2.},
	isbn = {978-1-4244-5176-0},
	author = {Hai Ngoc {Pham} and P{\r a}l Rolfsen {Gr{\o}nsund} and Paal Einar {Engelstad} and Gr{\o}dalen, Ole},
	editor = {Ott, J{\"o}rg and Balandin, Sergey and Matuszewski, Marcin}
}
@inproceedings {Simula.ND.403,
	title = {Energy Minimization Approach for Optimal Cooperative Spectrum Sensing in Sensor-Aided Cognitive Radio Networks},
	journal = {2010 The 5th Annual ICST Wireless Internet Conference (WICON 2010)},
	year = {2010},
	publisher = {ICST/Create-Net},
	type = {Conference},
	abstract = {In a sensor-aided cognitive radio network, collaborating battery-powered sensors are deployed to aid the network in cooperative spectrum sensing. These sensors consume energy for spectrum sensing and therefore deplete their life-time, thus we study the key issue in minimizing the sensing energy consumed by such group of collaborating sensors. The IEEE P802.22 standard specifies spectrum sensing accuracy by the detection and false alarm probabilities, hence we address the energy minimization problem under this detection accuracy constraint. Firstly, we derive the bounds for the number of sensors to simultaneously guarantee the thresholds for high detection probability and low false alarm probability. With these bounds, we then formulate the optimization problem to find the optimal sensing interval and the optimal number of sensor that minimize the energy consumption. Thirdly, the approximated analytical solutions are derived to solve the optimization accurately and efficiently in polynomial time. Finally, numerical results show that the minimized energy is significantly lower than the energy consumed by a group of randomly selected sensors. The mean absolute error of the approximated optimal sensing interval compared with the exact value is less than 4\% and 8\% under good and bad SNR conditions, respectively. The approximated optimal number of sensors is shown to be very close to the exact number.},
	isbn = {978-963-9799-86-8},
	author = {Hai Ngoc {Pham} and Zhang, Yan and Paal Einar {Engelstad} and Skeie, Tor and Eliassen, Frank},
	editor = {Mahmoud Daneshmand {Chen}}
}
@inproceedings {Simula.ND.377,
	title = {Optimal Cooperative Spectrum Sensing in Cognitive Sensor Networks},
	journal = {The 5th International Wireless Communications and Mobile Computing Conference (IWCMC {\textquoteright}09)},
	year = {2009},
	pages = {1073-1079},
	publisher = {ACM New York, NY, USA},
	type = {Conference},
	abstract = {This paper addresses the problem of optimal cooperative spectrum sensing in a cognitive-enabled sensor network where cognitive sensors can cooperate in the sensing of the spectrum. Such sensor networks are assumed to be power resource constrained. With a given threshold for the accuracy of the spectrum detection, we find the optimal number of cognitive sensors participating in the cooperative spectrum sensing and the optimal sensing interval that minimize the total energy consumption of the cooperative sensing. First, the mathematical lower bound and upper bound for the number of cooperative cognitive sensors are found. Then the optimization problem to minimize the total energy consumed by a group of sensors is presented. Finally, an efficient approximate solution to the optimization problem is proposed. Numerical calculations validate the accuracy and the performance of the proposed scheme. The impact of the noise uncertainty, the choice of the energy detection threshold, and the spectrum bandwidth on the detection accuracy and the minimum total energy consumption is also studied.},
	isbn = {978-1-60558-569-7},
	author = {Hai Ngoc {Pham} and Zhang, Yan and Paal Einar {Engelstad} and Skeie, Tor and Eliassen, Frank}
}
@inproceedings {Simula.ND.375,
	title = {Towards Dynamic Spectrum Access in Primary OFDMA Systems},
	journal = {The 20th Personal, Indoor and Mobile Radio Communications Symposium 2009 (PIMRC{\textquoteright}09)},
	year = {2009},
	pages = {848-852},
	type = {Conference},
	abstract = {OFDMA will be the major transmission technology in the future mobile wireless broadband systems, and is already used in Mobile WiMAX and LTE. In this paper, we use simulations to characterize the spectrum usage of such primary systems. This knowledge is useful for cognitive radio and other technologies for dynamic spectrum access that aim at improving the overall spectrum usage. We calculate the available capacity and analyze the distribution of the spectrum occupancy over time for various network scenarios and traffic models. Based on our analyses, we finally propose directions for improving the dynamic access of spectrum allocated to primary OFDMA systems.},
	author = {P{\r a}l Rolfsen {Gr{\o}nsund} and Hai Ngoc {Pham} and Paal Einar {Engelstad}}
}
@inproceedings {Simula.ND.147,
	title = {QoS-Aware Channel Selection in Cognitive Radio Networks: a Game-Theoretic Approach},
	journal = {IEEE Global Communications Conference (GLOBECOM 2008)},
	year = {2008},
	pages = {1-7},
	publisher = {IEEE},
	type = {Conference},
	abstract = {Abstract-In a cognitive radio wireless network, each node can sense and opportunistically access the under-utilized spectrums in the primary system. Since the unoccupied spectrum is locationdependent and time-dependent, the available spectrums in each node are different.With this spectrum heterogeneity and different Quality-of-Service (QoS) requirement, different node may have different preference in using a particular channel to communicate with its neighboring nodes. In this paper, we formulate this channel selection problem using a cooperative game theoretical approach such that the QoS requirement of each node is guaranteed and the total throughput is maximized. To further improve the performance, a learning negotiation mechanism is introduced. The key motivation is to derive new mixed strategies of the nodes with the reference to the historical profiles of all selected strategies in the past. Simulation results are presented to show the fast convergence of the game, the high efficiency of the learning mechanism, and the effect of mobility.},
	isbn = {978-1-4244-2324-8},
	doi = {10.1109/GLOCOM.2008.ECP.934},
	author = {Hai Ngoc {Pham} and Xiang, Jie and Zhang, Yan and Skeie, Tor},
	editor = {Richard W. {Miller}}
}
