On The Spectral Efficiency Of Network-assisted Full-duplex System Under Pilot Contamination

In order to improve the coverage of high-frequency networks,the hyper-density collaborative network based on cell-free architecture has attracted widespread attention because of its extraordinary potential for achieving greater capacity communication in 5G and beyond.In view of the fact that the spatial-multiplexing gain of cell-free massive multi-input multi-output(MIMO)can not only improve the spectral efficiency,but also provide flexibility for the choice of duplex mode,this paper focuses on the high-speed development field of cell-free massive MIMO,and studies the spectral efficiency of network-assisted full-duplex(NAFD)cell-free massive MIMO under different channel-estimation methods.This paper investigates the network-assisted full-duplex system with cell-free massive MIMO architecture.The NAFD mode is a novel duplex mode in the space domain,in which the transmitting antenna and the receiving antenna share the same time-frequency resources to communicate with downlink users and uplink users,respectively,which can greatly increase the bidirectional data traffic of a multi-user system.Since the base station(BS)antennas work in half-duplex mode,the self-interference generated in a full-duplex(FD)BS is completely eliminated.Since pilot contamination is an important factor limiting the performance of massive MIMO systems,this paper explores the performance of the NAFD system under pilot contamination caused by different pilot schemes.The academic background of the research is introduced in the introduction in the very beginning,including the development history of mobile communication systems and key technologies of 5G and beyond,outlines MIMO technology,cloud wireless access network and precoding technology,and then describes the contents and current situation of the subject,leading to the focus of this paper.At last,the introduction presents the structure of this paper.In view of the fact that the network full-duplex system supports simultaneous two-way communication at the same frequency,it will inevitably lead to interference channels from uplink users to downlink users and from transmitting antennas to receiving antennas.In order to reduce the impact of the inter-antenna interference channel,the MMSE estimator is utilized to pre-estimate this quasi-static channel and establish its equivalent model based on the estimation error.To further reduce the interference between users,it is proposed that in the channel-estimation stage,the uplink user and the transmitting antenna respectively send the uplink and downlink pilot signals at the same time.Next,based on the constructed equivalent channel model between users and the uplink and downlink equivalent channel models,the analytical expression of the upper bound of the spectral efficiency of the network full-duplex system using MRC/MRT linear technology is derived,and its validity is confirmed by simulation results.In addition,the simulation results also prove from different aspects that under this pilot scheme,the spectral efficiency of the network full-duplex system has a certain degree of improvement relative to the TDD system.Based on the same system model and inter-antenna interference cancellation,a new pilot-multiplexing scheme is proposed: uplink and downlink users share a limited number of pilot sequences and send uplink pilots on the same frequency at the same time.Then,an equivalent channel model between the user and the BS antenna is established with the MMSE linear estimator,and the correlation of the equivalent channel models associated with the same pilot sequence is discussed.Considering the influence of pilot contamination on the channel estimation model,this paper chooses the simpler MRC/MRT linear processing technology.By simulating the effective noise of the received signal as additive Gaussian noise,the spectral efficiency of the network full-duplex system under pilot contamination is explored,and its theoretical upper bound is derived.Finally,the numerical results evaluates the validity of the theoretical upper bound and the superiority of the NAFD system over the TDD system under the number of pilots,antennas or users.An asynchronous channel estimation scheme based on pilot and data multiplexing is proposed in this paper,that is,when uplink users or downlink users send an orthogonal pilot matrix in the reverse link,the transmitting antennas or uplink users send data in the same channel.Assuming that the BS antenna uses the ZF transceiver to process the signal,based on the MMSE estimates of the uplink and downlink channels,the approximate upper bound of the spectral efficiency of the network full-duplex system is studied in theory,which is mainly a function of large-scale fading coefficient and transmission power.Through a series of simulations,the validity of the theoretical upper bound of the spectral efficiency and the performance gap between NAFD and TDD systems is revealed.The results show that the NAFD system can greatly increase the bidirectional traffic.Last but not least,the three pilot schemes involved in this paper are quantitatively compared through numerical simulation.