Hybrid Overlay/underlay Spectrum Sharing And Performance Analysis For Cognitive Cooperative Relay Networks

In current spectrum allocation mechanism,the static spectrum allocation does not match the dynamic spectrum utilization.As a result,limited spectrum resources are increasingly difficult to meet the growing demand for wireless communication.While cognitive radio has been proposed to improve the utilization efficiency of wireless spectrum greatly,the cooperative transmission is simultaneously introduced to combat channel fading and expand coverage under fading environment,which makes cognitive cooperative networks become a research focus.Based on cooperative spectrum sharing technology,this thesis studies the cognitive two-way relay networks where interference power limit and maximum outage probability constraint are adopted to protect QoS of primary system respectively,and cognitive system is sharing spectrum with primary system by different modes(underlay or hybrid overlay/underlay).Compared to traditional orthogonal forward,non-orthogonal forward,a new relay forwarding method,is investigated in the thesis to improve spectrum efficiency of cognitive relay networks.The main contributions of this thesis are as follows:1.Study of the bidirectional transmission in cognitive relay networks with opportunistic relay selection:Compared to previous one-way relay networks,bidirectional transmission has an incomparable advantage,which could make full use of channel to transmit information and obtain higher spectrum efficiency consequently.Considering the interference from primary transmitter to secondary system,this thesis investigates the bidirectional transmission in cognitive relay networks,and define a new system outage probability for secondary system.Both the exact expression of the outage probability and its high-SNR approximation are derived over Rayleigh fading channels,respectively.We also analyze some results on the outage performance of secondary system by means of changing the network placement.The results show that the full diversity order is achieved only under a fixed interference from primary system.If the interference from the primary transmitter is too strong in practice,then increasing the number of relays can mitigate the performance loss of secondary system.Moreover,a higher outage probability of secondary system is obtained when secondary relays are placed at the symmetrical positions between two sources,which indicates that proper network placement planning should be carefully addressed in practical deployment when performing cognitive two-way relaying with opportunistic relay selection.2.Study of hybrid overlay/underlay spectrum sharing in cognitive relay networks with maximum outage probability constraint:In a practical cognitive relay network,the transmission activity of primary system is bursty due to random service request,and its interference to secondary system is also random.This thesis models the activity behavior as a two states discrete time Markov model with parameters.Compared to previous the peak interference power constraint,maximum outage probability constraint is introduced to protect QoS of primary system,and the exact expression of the outage probability is derived.It is shown that the outage performance of secondary system is affected by the transmit power of primary transmitter and degenerates quickly after a specific threshold,so the interference caused by primary user to secondary user cannot be neglected.The activity behavior of primary system has a great impact on the outage performance of secondary system,especially in less spectrum usage for primary system,which could be compensated by increasing number of secondary relays.Moreover,maximum outage constraint can also cause the outage saturation phenomenon,and we find that the trade-off between two outage performances of primary and secondary system only occurs before a specific threshold,which is closely related with the transmit power limit of secondary system.3.Study of the non-orthogonal decode-and-forward protocol in cognitive relay networks:The high-order diversity gain is obtained by orthogonal forwarding at the expense of spectrum efficiency loss among the researches about cognitive cooperative relay networks.Recently,the non-orthogonal forwarding,a new relay forwarding approach,is never studied in cognitive relay networks.This thesis investigates the non-orthogonal decode-and-forward(NDF)opportunistic relay selection scheme in cognitive cooperative relay networks.The asymptotic expression of the proposed system is derived,and the results reveal that NDF could break through the spectrum efficiency upper limit 1/2 of orthogonal transmission and achieve high diversity gain over 1/2 spectrum efficiency.The outage performance of secondary system is improved with the increasing of relay numbers,but improvement trend keeps slow after a specific number of relays.It is also shown that the non-orthogonal forwarding can achieve better outage performance compared to orthogonal forwarding for which can obtain higher spectrum efficiency.With the increasing of the decoding SNR threshold,the condition to decode successfully is more and more rigorous and the system reduces to the non-cooperative mode at last.