A relay selection scheme based on geometric optimum in cooperative cognitive systems over mixed channels

This paper proposes a relay selection scheme based on geometric optimum principle to maximize the cognitive link＇ s connectivity with limited interference to primary user in cooperative cognitive systems. A dual-hop cognitive relay system is considered, in which the channel impulse response follows independent non-identical distribution （i. n. d. ） each hop, such as Rician, Rayleigh and Nakagami-m distribution. Then, closed-form expressions in terms of outage probability and average bit error rate （ABER） are obtained using amplify-and-forward （AF） relaying protocol over such mixed fading channels. Furthermore, the best range of the relay is derived. Extensive simulation re- suits are conducted to verify the theoretical analysis, which is useful to the network optimal design.     

生境片断化对植物的遗传影响

生境片断化指大片连续的生境被分割成空间隔离的小生境的现象。通常认为，生境片断化使植物残遗种群由于经历随机遗传漂变和高水平的近交以及基因流的下降，而发生遗传侵蚀，遗传多样性下降，遗传结构改变，变得更加分化。但近期的研究证明，并不是所有的片断化事件都导致植物种群遗传变异丧失，且种群问的基因流甚至有可能增加了。片断化对植物种群的遗传作用还受到诸如世代长短的种间差异、片断化前的丰富度、有性和无性繁殖系统的多样性、种子库以及和传粉媒介及种子传播者的相互作用等的影响。