As a smart combination of cognitive radio networks and wireless sensor networks,recently introduced cognitive radio sensor network(CRSN) poses new challenges to the design of topology maintenance techniques for dynamic primary-user activities.This paper aims to provide a solution to the energy-efficient spectrum-aware CRSN clustering problem.Specifically,we design the clustered structure,establish a network-wide energy consumption model and determine the optimal number of clusters.We then employ the ideas from constrained clustering and propose both a centralized spectrum-aware clustering algorithm and a distributed spectrum-aware clustering(DSAC) protocol.Through extensive simulations,we demonstrate that DSAC can effectively form clusters under a dynamic spectrum-aware constraint.Moreover,DSAC exhibits preferable scalability and stability with its low complexity and quick convergence under dynamic spectrum variation.
Severe doubly selective channel fading,or the time- and frequency-selective fading,prevailing in highspeed train(HST) communications,has been a critical issue that hinders the improvement of system efficiency and reliability.In this paper,a relay-assisted HST communication system is considered,and a low-complexity but effective scheme is proposed to cooperatively exploit the joint multipath–Doppler diversity of both the direct link and the relay link and thus to mitigate the negative effect of channel fading and improve the system reliability.In particular,using a special precoding structure followed by two-dimensional Fourier transform,the transmit signal is generated which in effect parallelizes the doubly selective fading channel.As a result,in the transform domain,pointwise combining and equalization,which are of linear complexity,become feasible and diversity gain is obtained.The maximal diversity order is then analyzed under the amplify-and-forward relay settings,which reveals that by cooperative relaying the joint multipath–Doppler diversity of both links can be well exploited.Simulation results verified our analysis.
