Sky-polarization-averaged GW signals of IMBBHs modeled with IMRPhenomD (thin lines), overlaid with TianQin (red), LISA (green), and LIGO A+ (blue) sensitivity curves. The merger time is set to 5 years. Dots size scale with total IMBBH mass. Left and right panels correspond to f_b = 0 and f_b = 1, respectively. The black solid line represents a reference IMBBH (m₁ = m₂ = 200,  M_⊙, z = 2, t_c = 5 yr), with red, blue, and black dashed lines marking 1 year, 1 month, and 1 day before merger.