Research On Spectrum Sensing And Access Technology For Cognitive Networks Based On Relay Cooperation

With the rapid development of economy and science,the wireless communication traffic grows fast.However,the frequency spectrum resource is very limited and the distribution of it is uneven.In the context,cognitive radio is proposed and considered to be one of the most effective methods to ease spectrum situation.In cognitive radio networks,unauthorized users(Second Users,SUs)can take up the frequency spectrum of authorized users(Primary Users,PUs)to transmit signals when the PUs don't exist or the interference to PUs below the given threshold.The collaboration technology is widely used in cognitive radio system due to its characteristics such as high efficiency,high reliability.In cognitive radio networks,the collaboration technology mainly divided into two cases.One is SUs act as relays of another SUs,the other is SUs act as relays of PUs.In this paper,we mainly study the second situation in which the channel fading is large between the PUs.The SUs play as the relays of PUs to improve the transmission rates of PUs and increase their own opportunities of access to the spectrum,so improve the performance of the system.At first,this paper established a collaborative cognitive model of the single relay.Second users decide what kind of transmission mode to transmit data according to the results of the sensing in the collaborative cognitive networks.If the PUs is not exist,the SUs can directly use thespectrum;otherwise,SUs have to assist PUs to transmit and SUs take up the rest of time slot after forwarding is complete.The relationship between the sensing time and the system capacity is studied in this paper and the existence of the optimal sensing time slot is proved.And further,three points method is used to obtain the optimal perception time slot and improve system capacity.On the basis of the single relay collaborative research,this article also puts forward a model of multiple relay cognitive cooperation.The spectrum is divided into multiple orthogonal sub-carriers with OFDM technology.Primary users select the channels and the relays to forward the signal firstly,at the same time,PUs allow SUs occupy the rest channels to transmit data.In the proposed model,the main content includes relay selection,channel allocation and power allocation.Simulation results show that there is about 6% increase in the capacity of the motion-aware slot method compared with the traditional fixed-sensing time slot.And also multiple relay selection schemes in this paper can effectively reduce the system cost and improve the cognitive radio system capacity.