Research On The Performance Of Adaptive Modulation In Underlay Cognitive Network

Along with the development of the society, the wireless communication technology come out new style day by day, from its birth to well known to be widely used by people, it faced with such a problem that the limited spectrum resource and the explosive growth of the demand-that is to say spectrum is short of supply. This kind of phenomenon always hindering the study of the wireless communications from the birth of the area if wireless communications to the new technology of 3G and 4G. We can described the phenomenon graphically as the contradiction between the allocation of static spectrum and the demand of wireless network, its the basic and the difficult contradiction of the construction of the wireless network. Only do we change the current resource allocation scheme or use a better resource allocation strategy, can we make the spectrum resource be effectively used to alleviate the lacked problem caused by unreasonable allocation, and also promote the wireless communication developed more vigorous. The cognitive radio can sense the spectrum environment and adjust the transmit parameters according to the sensed facts and realizing the effective utilization of the spectrum. In addition, the cognitive network technology can sense the communication environment based on the artificial intelligence,analyze the environment and adjusting the working parameters of the system on the basis of specific decision and learning methods to promote the more effective utilization of the spectrum, finally, through the constantly study of the environment to meet the demanding of the intelligence.Moreover, cooperative relay technology has became a hot topic due to the technology can improve the channel capacity effectively. With the coming of the 21 st century and the popularization and development of the network, spectrum get dried up, and because of the limited spectrum resource and can't be reborn, people's focus returned to a new integration of exiting resource to improve the high utilization. All kinds of new technologies have mushroomed at this time, and the traditional relay technologies cooperate with the cognitive network and has created a new field in communication network that is cognitive relay network. But as the new technology developed, the new problem also came, the main matters in the cognitive relay networks are: the interference of the primary users, the additional power limitation, the lower spectrum efficiency and the predetermined interference and so on, in this situation, the traditional relay theory here are looked so weak,so that there must be a new theory and technology must be put forward. In view of the above problems, the paper mainly discussed under the multi-primary users and multi-relays and with the underlay method realize the sharing of the spectrum, we have a research in the performance of the relay network with the adaptive modulation.The work of this paper are as follow:(1)Considering the serious shadow of the secondary transmitter and the receiver, we ignore the transmission of the direct link, and only take the relay transmission into consideration.At that point, there are multiple primary users and multiple relays available, the SU use the spectrum of the PU with Underlay method and opportunistic AF relay method, we discussed the outage probability and the bit error rate and we give the simulation to verify our analysis. The results show that: the relay and PU adjacent to the predetermined threshold do a certain influence on the performance and the growth of the relay have a positive influence on outage probability and BER.(2)In the situation of there exists a direct link between the transmitter and the receiver, it is also different in the performance because of the different receive methods in the secondary receiver. At this point, we discussed the outage probability and the BER when the secondary receiver using the maximal ratio combining(MRC) and the selection combing(SC). Finally, the simulation verified out theory analyze. The results showed that under the direct transmission, the method of the adoption of MRC has a preferred performance to the adoption of SC which depended by the principle the method work. Moreover the number of the relay and the threshold do a positive effect on the performance.(3)On the basis of the above two situations, we had a comparison of the performance about without direct link and the direct with MRC and with SC methods, and from the simulation results we found the performance of direct link transmission is better than without the direct link. And under the direct transmission, the MRC method is better.