Research On Performance Analysis And Power Allocation In The Underlay CR-NOMA System

With the continuous development of wireless communication technology in the 5G,limited spectrum resources can hardly meet the massive demand for future equipment.Therefore,how to ensure the quality of communication while expanding the access scale of users has been extensively studied.The CR-NOMA system which combines Non-Orthogonal Multiple Access(NOMA)technology with Cognitive Radio(CR)technology is considered to be a solution that can meet the needs of mobile communication in the 5G era for large-scale access,high reliability and throughput.Some research on CR-NOMA system has made some achievements,but there are still some shortcomings.For example,some researches only consider flat fading channels in multi-carrier system,ignoring intercarrier interference,and ignore the analysis of symbol error rate when evaluating system performance which leads to the performance cognitive bias.The work based on relay selection and power allocation in multi-relay cooperative systems is not comprehensive enough,and it does not involve the research on outage probability of secondary user(SU)nodes,system throughput,and optimal power allocation at transmitters and relays.Therefore,in order to gain more insights into system performance and improve the throughput and interrupt performance SU system,this paper comprehensively considers the above factors,improves corresponding power allocation schemes for different systems and derives closed form expressions of key performance to obtain better insights into system performance and optimize the performance of system.The specific research contents are summarized as follows:Firstly,the outage performance and symbol error rate performance of multicarrier underlay CR-NOMA system are studied.The multicarrier-based SU in the system use all the subcarriers of the primary user(PU)in the underlay mode,and use the Successive Interference Cancellation(SIC)at the receiver to control the symbol error rates(SER).In order to maximize the SU throughput,the power allocation scheme is improved.The power allocated to SU subcarriers is optimized while fixing the PU transmission power and ensuring the minimum PU throughput.When analyzing the system performance,considering the L-tap multipath Rayleigh fading channel,the closed form expression of SU outage probability is derived.At the same time,the average SER expression and its approximate closed form expression of PU and SU on each subcarrier are derived.The accuracy of the derived expression is verified by extensive Monte Carlo simulation.The power allocation scheme is implemented in a system based on orthogonal frequency division multiplexing to evaluate the SER performance of the system under different channel estimation methods by extensive simulation.The derivation of closed form expression provides more insights into parameter selection when evaluating system performance.The simulation results show that the minimum throughput of PU in the system is guaranteed,the SU system throughput is improved by multicarrier system and SIC,the outage probability is reduced by the increase of signal-to-noise ratio and the number of subcarriers,noise cancellation and SER performance is improved by adjusting noise cancellation and SU maximum transmitting power.Secondly,in order to reduce the outage probability of system and improve the throughput of SU system,the cooperative underlay CR-NOMA system under the partial relay selection(PRS)scheme is studied.The system uses K half duplex Decode and Forward(DF)relays to assist SU transmitter to transmit information to SU receiver.In order to analyze the outage performance,the outage probability expression of SU under imperfect SIC is derived.The interference threshold constraint of the PU receiver and the maximum transmit power constraint of the secondary user are considered.In order to provide more insights on outage performance and simplify the complexity of the derived expression,the approximate outage probability expression of SU in the high transmission power region is derived.On this basis,in order to maximize the throughput of the secondary user system,the optimal power allocation factors of the transmitter of secondary user,the selected relay and the combination of the two are determined respectively.Simulation results show that compared with random relay selection and fixed relay selection,the outage probability of SU is significantly reduced and the throughput is significantly improved under the PRS scheme considered.In addition,compared with the equal and random power allocation scheme,the proposed optimal power allocation factor can significantly reduce the outage probability of SU and improve the system throughput.