Research On Beam Domain Channel Theory For Ultramassive MIMO

Ultramassive multiple-input multiple-output(MIMO)technology is a promising solution for 6G communication systems with increasing data transmission rates,channel capacity,and spectral efficiency.However,ultramassive MIMO systems have shown special characteristics such as spherical wavefront(SWF),millimeter wave(mm Wave),and time-evolving characteristics that pose challenges to the implementation of communication systems.The recently proposed beam domain channel theory can effectively reflect the above-mentioned characteristics by converting the spatial antenna domain to beam domain through unitary matrix transforming.Determining the possible beam positions in advance,beams with more concentrated power and higher accuracy can improve the system reliability and accuracy,significantly reducing the system complexity,and covering various practical communication scenarios.The main work and innovations of this paper are summarized as follows with respect to the characteristics demonstrated by the beam domain channel of super massive MIMO.(1)Due to various obstacles in the rich scattering environment,the ultramassive MIMO in the beam domain at mm Wave band exhibits complex features that have not been considered in previous studies.In this paper,a novel beam domain channel model(BDCM)with mixedbouncing clusters for ultramassive MIMO scenarios at mm Wave bands is proposed.In this proposed BDCM,the line-of-sight(Lo S)ray,single bouncing ray,and double-bouncing ray are simultaneously considered.Moreover,the non-stationarity of the propagation channel is modeled by determining visibility regions(VRs)of the single-bouncing and double-bouncing clusters,respectively.Based on this proposed model,the spatial-temporal correlation function(ST-CF)is obtained and simulated results show that the single-bouncing clustersmake a significant contribution on the ST-CF.Finally,impact of the single-bouncing clusters on the other statistical property,such as channel capacity,is also presented.Results suggest that ignoring the non-stationarity and power contribution of the single-bouncing rays may lead to inaccurate evaluation of the channel capacity.The proposed model may be used in the 6G ultramassive MIMO communication systems at mm Wave bands.(2)For the near-field effects caused by the large array size under the ultramassive MIMO systems,a beam domain channel model based on spherical wavefront is proposed in this paper.The model considers the Lo S and NLo S rays,supports dual mobility and the movement of the scatterer,and simplifies the computational complexity by using a two-dimensional parabolic approximation to calculate the spherical wavefront distance between the transceiver and the clusters.Based on this model,the ST-CF is derived to analyze the effect of introducing spherical wavefront on spatial-temporal correlation.The simulation results show that,with guaranteed system accuracy by beam spread verification,the spherical wavefront has a significant impact on channel correlation.