Comparison of the characteristic strain of our sample with $\sqrt{f_{\mathrm{g}\mathrm{w}}{S}_{\mathrm{n}}(f_{\mathrm{g}\mathrm{w}})}$ for T_obs = 4 yr of observations, where S_n(f_gw) represents the noise power spectral densities for TianQin (dotted line; Huang et al. 2020), LISA (dashed line; Robson et al. 2019), Taiji (densely dashdot-ted line; Liu et al. 2023), DECIGO (dot-dashed line), and BBO (solid line) (Yagi & Seto 2011; Sun & Li 2023; Sun et al. 2024). The stars and hexagons denote the PSR systems and WD systems, respectively.