The Research Based On The Difference Of Channels For Dynamic Spectrum Access

Dynamic spectrum access technology is one of the key technologies of cognitive radio system. The ordinary dynamic spectrum access technology mainly includes the spectrum sensing technology and the spectrum access technology. The second user and the primary user may compete with each other when getting access to the channel through spectrum sensing technology. For the ordinary spectrum access technology, the switch on the unoccupied channel for the second user is randomly based on the CSMA protocol, so the random access strategy have a negative affect on the performance of the whole cognitive radio system. Therefore, in this thesis, the traditional dynamic spectrum access is overrode by combining the prediction technology based on the HMM with the quality awareness access strategy.First of all, the current research on the dynamic spectrum access technology is introduced in this thesis, and spectrum dynamic spectrum access technology based on prediction is mainly introduced.Secondly, directing at the low probability to find out the spectrum hole and high probability between the second user and the primary user in the traditional dynamic spectrum access technology, the channel state prediction scheme based on the HMM model is proposed in this thesis. The next state of the primary user is decided by comparing the joint probability of the current sequence and all the next possible state in the forward algorithm. After the channel state determined, the new sequence is trained and the parameters of the predictor is updated by the HMM predictor and the next state is predicted by the new HMM predictor again. So HMM model can quickly update the model parameters, and get a more accurate result of the prediction. Finally, the performance of the proposed scheme and the traditional scheme are analyzed. The conclusion rate between the second user and the primary user is reduced in the proposed scheme, r the accuracy of the prediction result is raised and the performance of the cognitive radio system is improved.Finally, in view of inadequate access strategy in spectrum access, the quality awareness access scheme is proposed in this thesis. The new parameters, quality factor, is proposed in this thesis, the channel gain and noise power density channel of each channel are estimated through the HMM model and verified the effectiveness of the quality factor from the perspective of simulation. Besides, the estimation of the channel environment is theoretically deduced and then the feasibility of the quality factor is proved. Finally, this thesis constructed the simulation environment which was made up with three channels with different quality and a second user. After finished the simulation in the multi-channel environment, this thesis verified that the proposed dynamic spectrum access scheme improve the performance of the cognitive radio system from the perspective of channel switching rate, access rate for the second user.