Research On Spectrum Handoff Mechanism In Cognitive Radio Networks

The rapid development of wireless communication technology promotes the increase of wireless devices and emerging wireless businesses continually,which makes the allocation of available spectrum resources become saturated.The shortage of radio spectrum resources becomes a bottleneck,which restricts technologies to develop further.However,studies show that the existing spectrum allocation policy makes the average spectrum utilization very low.Cognitive radio technology can greatly improve the spectrum efficiency and ease the contradiction between the scarcity of spectrum resources and the growing spectrum demand,by allowing the secondary users to share the authorized bands.As one of the key cognitive radio technologies,spectrum handoff is able to guarantee the continuity and reliability of the secondary users in the communication process.Based on the hidden Markov theory,queuing theory and dynamic programming theory,spectrum handoff model and the associated primary arrival and service rate parameter estimation problem,target channel sequence selection problems are deeply studied,and the corresponding solutions are proposed.Firstly,for the reason that the primary user arrival rate and service rate are hard to get in time,a parameter estimation algorithm is proposed based on the hidden Markov model in this paper,providing decision basis for the spectrum handoff of secondary users.On the basis of the characteristics of cognitive radio networks,the preemptive queuing theory model is used to describe the priority relationship between the primary user and the secondary user,and the corresponding queues are established,respectively.On this basis,the state transition matrix of the primary user and confusion matrix are derived,so that the state transition process of the primary user queue and the secondary user detection process are described as the hidden Markov random process.The result sequence of the secondary user spectrum detection is regarded as a training sequence,which can be used to calculate the hidden Markov model parameters through the forward-backward algorithm,and thus the estimation of the arrival rate and the service rate are indirectly realized.Compared with traditional statistical methods,the proposed algorithm is real-time,accurately estimates the arrival and service rate of the primary user,and reflects the status of the traffic intensity,so that a decision support is provided for the secondary user to perform the optimal spectrum handoff scheme.In addition,the proposed algorithm can flexibly adjust the length of an observable sequence based on the estimation accuracy and complexity requirement.Secondly,spectrum handoff problems are further studied in a resume mechanisms-based and a repeat mechanisms-based preemptive priority queuing model,respectively.In the resume mechanism-based preemptive queuing model,the queuing method with hybrid queuing and parallel service is taken,the spectrum handoff behaviors of the secondary user are analyzed on this basis,and thus the extended data delivery time is derived.According to the principle that minimizes the extended data delivery time,an adaptive spectrum handoff scheme is proposed based on the busy probability.The proposed spectrum handoff model reduces the extended data delivery time and improves the performance of spectrum handoff of secondry users,compared with the existing one.In order to extend the spectrum handoff model study further,the repeat mechanisms-based preemptive priority queuing model is proposed.Based on the analysis of the spectrum handoff behaviors of the secondary user,the extended data delivery time and system sojourn time are derived.Based on the analysis of effects of different service time distributions on the extended data delivery time of secondary user,a repeat mechanisms-based adaptive spectrum handoff scheme is proposed.In addition,an admissible region is studied,where the secondary user can access channels under a given delay limitation condition,providing a reference for the secondary user to work out access guidelines.Finally,for energy efficiency issues in the spectrum handoff process of the secondary user,a target channel sequence algorithm is proposed based the energy efficiency in this paper.For the description of the spectrum handoff behaviors of secondary users,this paper takes a full consideration of multiple channels,multiple spectrum handoffs,competition between secondary users and primary channel busy probability.Based on the analysis of the spectrum handoff behaviors of the secondary user,transmitting energy consumption,sensing energy consumption,idling energy consumption,witching energy consumption and unit bandwidth transmission rate are analyzed during the entire communication process.Thus,an optimization problem for maximizing the energy efficiency is established.Then,the original optimization problem is transformed into a new optimization problem by applying the fractional programming.The new optimization problem is viewed as a multi-stage decision-making process,and dynamic programming equations are formulated.Combining the dynamic programming with the bisection algorithm,the numerical solution of the optimization problem is obtained.The proposed algorithm can achieve higher energy efficiency,compared with the existing one based on minimum cumulative delay.