Thanks for figuring that out. The reason for this is the function “sym_solve” in channel_quantizers.LTE_MIMO_Grassmann_pred_fs. The object channel_quantizers.LTE_MIMO_Grassmann_pred_fs is utililzed for predictive Grassmannian quantization, which is a method I have proposed during my PhD time.
You can deactivate this by setting LTE_params.feedback.codebook = ‘PERFECT’; in LTE_load_advanced_parameters.m. This would imply that perfect channel state information at the transmitter is considered in case of zero-forcing/block-diagonalization based multi-user transmission.