Upper panel: square root of the PSD noise TDI-X corresponding to different LISA configurations. The solid curves are the total PSD, while the dashed curves are the contribution from the confusion GWDs. The bundle of curves in the same colour corresponds to T_obs = 1, 2, 4, and 5 yr from top to bottom; Lower panel: sensitivity curves corresponding to different arm lengths and T_obs. The solid curves are the total curve, while the dashed curves are the contribution from the confusion GWDs. The bundle of curves in the same colour corresponds to T_obs = 1, 2, 4, and 5 yr from top to bottom.