Beam domain propagation characteristics analysis of 6G ultra-massive MIMO channels

In order to fully explore the beam domain channel propagation characteristics, channel measurements and data analyses are conducted for the sixth-generation (6G) ultra-massive multiple-input multiple-output (MIMO) wireless channel. First, the line-of-sight(LoS) and non-LoS(NLoS) channel measurements in an urban scenario are conducted at sub-6 GHz frequency band with ultra-massive uniform linear array (ULA). Second, the ultra-massive ULA is divided into several sub-arrays. Multipath parameters including amplitude, delay, and angle of arrival, are estimated using the high-resolution algorithm. Then, virtual beams are constructed according to the angular resolution of sub-array. Multipaths that belong to different beam angles are grouped and added up. The channel coefficients of different sub-arrays in the same beam direction are compared to show that the non-stationary characteristics in antenna domain are less obvious in beam domain. By analyzing channel coefficients of different beams, obvious sparsity can be observed. The sparsity is more obvious in LoS scenario than that in NLoS scenario. Multipath numbers, powers, and root mean square delay spread (DS) of different virtual beams are also computed and fitted with Gaussian, Gaussian, and uniform distributions respectively. In addition, it is found that the cross-correlation coefficients of different virtual beams in LoS scenario are lower than those in NLoS scenario. The virtual beam resolution has great impact on the cross-correlation coefficients of different virtual beams in NLoS scenario.