Research Of Robust Power Allocation Algorithm Based On OFDM Cognitive Radio Network

Wireless communications technology has become a key element in the development of the modern society.In our daily life,wireless devices such as garage door openers,TV remote controllers,cellular phones,personal digital assistants(PDAs),and satellite TV receivers,etc.are all based on the wireless communications.With the use of lot of wireless devices and the increase of the wireless applications,the scarcity of spectrum resources has been the essential problem in the near future since this resource is limited.To solve this issue,cognitive radio technology is considered one of the important and useful solution and is widely concerned.Cognitive radio aims to improve the spectrum efficiency,meanwhile emphasizing the cooperative spectrum sharing between different kinds of users.A basic cognitive radio network consists of authorized user(primary user or PU)and unauthorized user(secondary user,or cognitive user or SU).In order to achieve cooperation or spectrum sharing with primary users,cognitive radio technique permits secondary users to access spectrum holes belonging to primary users through spectrum detection,sensing and other methods by adjusting system parameters,as long as the normal communication of PUs is not affected.At present,the research on cognitive radio is mainly focusing on spectrum sensing and resource allocation or power control.SUs can detect spectrum holes and obtain more opportunities to access the authorized spectrum,then reasonably allocate transmit power with some interference constraints to ensure basic communication quality of PUs.Precisely speaking,the interference from the transmitters of SUs to the receivers of PUs cannot exceed the interference threshold or called interference temperature.According to the above principles,two resource allocation schemes are proposed from different perspectives in this thesis.The main contributions of this thesis are summarized as follows.(1)From the perspective of energy efficiency,considering the issue of energy consumption(to minimize total transmit power),a distributed cognitive radio network with multi-user is proposed to reduce complexity of information exchange.With practical channel state information,a robust resource allocation scheme is presented based on nominal optimization problem(non-robust problem)while all channel estimation errors are taken into account.The objective is to minimize the total transmit power under signal to interference noise ratio(SINR)constraint,interference threshold constraint and transmit power budget constraint.And the channel state information errors are assumed to be bounded with ellipsoid and interval sets.The original optimization problem is converted into a convex one through the Worst-case approach.The simulation results prove that the suboptimal solution can achieve a satisfactory performance for both SUs and PUs at the same time.(2)In order to comprehensively consider parameter errors of a cognitive radio system,this thesis combines spectrum sensing error or imperfect spectrum sensing with robust power allocation problem.The proposed robust power allocation scheme is divided into two parts.In the first part,SU detects and estimates which spectrums are free and for the decision on whether to access or not.However,the detection results may be wrong,due to false alarm probability and false detection probability.And the interference constraint is composed with co-channel interference and side-band interference together;in the second part,in considering the channel parameter uncertainties and using the Worst-case approach and the Cauchy-Schwarz inequality,we make the optimization problem be in the worst case while using the Lagrange dual algorithm to get the optimal power allocation and obtain maximum transmit data rate.It is concluded that the imperfect spectrum sensing can still better evaluate detection result and avoid the unnecessary interference to PU.In addition,since the proposed robust power allocation algorithm considers the channel uncertainties in advance,it can effectively combat parameter perturbations and ensure basic communication of SUs.