| Intelligent Reflecting Surface(IRS)is a key technology in the sixth generation(6G)mobile communication system.By adjusting the reflection coefficients of IRS,it can enhance wireless channels and increase the energy of the received signals.Orthogonal Frequency Division Multiplexing(OFDM)is widely used in current communication systems to improve throughput and spectral efficiency.How to use IRS to assist OFDM communication to achieve performance gains in communication rate and system power consumption is a key problem to be solved.However,due to the characteristics of OFDM system and the special structure of IRS,using IRS to enhance OFDM communication system also faces new challenges,mainly including: 1)How to design a low-complexity joint optimization strategy when resource allocation in OFDM system is coupled with the passive beam of IRS? 2)The passive beam of IRS is coupled with all subcarriers,which cannot be designed separately for each subcarrier.It is necessary to design a joint beamforming method? 3)OFDM systems involve frequency-selective channels.How to accurately estimate channel state information(CSI)and optimize transmission.This paper conducts massive research to tackle the above challenges,and has made several innovative achievements,which are summarized as follows:First of all,considering the orthogonal frequency division multiple access(OFDMA)uplink of Cloud Radio Access Network(CRAN),this letter has designed a new scheme for resources allocation,such as sub-band,user power and fronthaul capacity and passive beamforming of IRS.Specifically,a joint optimization problem of system resources and IRS passive beamformer is established to maximize uplink sum-rate.Then the non convex problem is divided into subproblems by Alternative Optimization(AO)algorithm,and a gradient-basedtrategy is proposed for the allocation of sub-bands ? Then,the subproblem is transformed into a solvable convex optimization problem with the help of Continuous Convex Approximation(SCA)technique,do a suboptimal solution of the problem has been obtained.The simulation results show that the proposed algorithm can significantly improve the uplink sum-rate,and has lower complexity than the existing methods.Secondly,for the downlink from the base station to the user,in addition to utilizing IRS to enhance the main OFDM transmission,the symbiotic Io T information transmission is also realized by using the IRS reflection.Specifically,to minimize the transmit power of the base station,the active beamforming of BS and the passive beamforming of the IRS are jointly designed under the limitation of the signal to noise plus interference ratio(SINR)of the user and the Io T signal receiver.Due to the complexity of the optimization problem,the active and passive beamforming are optimized separately,then a suboptimal solution can be obtained by the fractional programming(FP)and semi-definite relaxation(SDR)technique.The simulation results show that slight power is needed to realize the symbiotic transmission,and the proposed beamforming method effectively reduces the transmit power of the base station.Finally,based on the frequency-selective model of IRS,an uplink channel estimation scheme is designed for the IRS-assisted OFDM system,and the downlink transmission is optimized.Specifically,this letter estimates the channel frequency response(CFR)of based on the Least Square(LS)method,and proposes an optimization algorithm based on one-dimensional search to design the reflection pattern of IRS for different pilot symbols,so as to minimize the channel estimation error.Then,based on the estimated channel,a joint optimization algorithm for BS power allocation and IRS beamformer is proposed to minimize the transmission power of the base station.The simulation results verify the accuracy of the channel estimation method,and the optimization algorithm successfully reduces the system power consumption.In addition,the performance gain of frequency-selective model is also revealed,which proves the necessity of introducing it. |
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