Secondly, the dissertation exhibits various categories of existing techniques on spectrum sensing in CR, including matched filter detection, energy detection, cyclostationary feature detection in local detection; AND-rule, OR-rule, K-rule in multiple nodes sensing .However, local detection is compromised when a user experiences shadowing or fading discount wedding gowns effects.So thirdly, cooperative spectrum sensing is employed to detect the presence of primary user quickly and accurately. We adopt the cooperative schemes with orthogonal transmission in a TDMA system which has been recently proposed by G. Ganesan and Y.G. Li. A cooperative scheme ,in which cognitive users U1 and U 2operative in the same band to cooperate by using the user U 2to act as a relay for custom made prom dresses U1 under the AF protocol while the cognitive user U1 receives less power from the primary user. But in this scheme impacts of some important parameters are neglected. We first propose a novel cheap shoes blog 2010 model to depict cooperative sensing performance accurately, the feasibility of our model is well explained and supported by extensive simulations. Then we analyze the sensing performance using our model, and exhibit some new results and their interpretations. We find that, for a user, the improvement in sensing wedding dresses 2010 performance is fairly more sensitive to the ratio of path loss exponent between the channel from primary user to cognitive user and the channel from cognitive user to cognitive user (blog wedding dresses 2010). Also , the decrease of sensing performance due to the increasing of the angleφis impacted byα2 /α1,andφdenotes the angle between the straight line from PUto U1 and the straight line from PUto U 2.Moreover, gain is also pretty sensitive toα2 /α1when the transmit powers is varied, and cooperative gain of U1 can be neglected if the transmission power of cognitive user is much smaller than that of PU.It is hard to achieve cooperation gain for U1 if cognitive user’s transmissions attenuate faster with distance than primary transmissions do.
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