Research On Cooperative Communication Technology In Cognitive Radio Networks

Along with the development of wireless communication technology,the kind and number of communication equipments increase rapidly,and the categories of wireless communication business tend to be diversified.Communication networks nowadays require a stronger data processing ability and a larger transmission capacity,and as a result,the demand for spectrum resources also increases dramatically.Considering the scarcity of wireless spectrum resources and the fact that most spectrum resources have been exploited,improving the efficiency of spectrum utilization becomes an increasingly urgent task.Due to the burst nature of communication services,in most cases,the spectrum will not be occupied by the licensed users for a long time.Therefore,most of the spectrum resources are under-utilized.In this context,an opportunistic spectrum sharing technology,named cognitive Radio(CR)technology,appears.The CR technology allows the unauthorized cognitive users to sense the surrounding spectrum environment periodically and get access to the spectrum holes without affecting the quality of service(QoS)of PUs' communication.Since it greatly alleviates the contradiction between the growing shortage of spectrum resources and the increasing wireless business,it has attracted considerable attention from both academia and industry.Considering the case that the transmitter and the receiver are separated in a long distance,or the case that there are many obstructions between the transmitter and the receiver,due to the strong channel fading,it is impossible to establish a reliable direct link.In cognitive radio networks,how to deal with the channel fading and ensure the signal transmission are worthy of attention.Cooperative communication refers to the communication where users not only transmit their own packets,but also forward the other users' packets,which avoid the packet transmission over channels with poor qualities.As a result,the outage probability can be decreased and the overall system performance can be enhanced.In a heterogeneous scenario where the authorized network coexists with the cognitive radio network,on one hand,the cognitive user can get access to the spectrum holes to transmit their own packets,which satisfies its transmission demand and enhances the spectrum efficiency.On the other hand,as a reward of utilizing the licensed spectrum,the cognitive user will help forward the authorized user's packets,which ensures its transmission stability and expands the coverage.This kind of cooperative cognitive radio network brings a double-win relationship between the two users.Therefore,it has a broad research prospect.There have been several cooperation schemes proposed by works at home and abroad,and the corresponding performances are analyzed.Most of these works are based on the assumption that the instantaneous channel state information(CSI)or buffer state information(BSI)can be acquired.However,in practice,considering the fast variation of channel states and the overhead due to information feedback,the CSI and BSI are usually become outdated when they are acquired,which makes it hard to optimize the cooperation schemes.Although it is barely possible to obtain precise instantaneous CSI or BSI,we can get the channel characteristics by long-term statistics.Therefore,in this paper,we first propose a cooperation scheme,which is based on long-term channel statistics,for traditional wireless communication networks.Under the proposed scheme,the scheduler will allocate transmission opportunities for different links' transmission with different probabilities,based on channels statistics and users' QoS requirements.We then extend the scheme to cognitive radio networks,where the cognitive user will decide when to get access to the licensed spectrum and whether to transmit its own packets or help forward the authorized user's packets,according to the sensing results.In this case,transmission chances will not be wasted as long as there are packets waiting to be transmitted in the system.For both scenarios,we construct a queuing model to analyze the packet arrival and the packet service process,based on which we further derive some important performance parameters,including the average buffer length,the stable throughput region,and the average packet transmission delay,for the analyzed systems.We mainly analyze the impacts of environmental parameters on the system performance,and put an emphasis on the throughput and delay tradeoffs among users.