Distance-adaptive modulation is effective at enhancing network capacity as it allows the maximum possible modulation order to be selected for each optical path. However, present single-carrier systems can only select just one modulation order for each optical path and hence the adaptability to transmission characteristics is strictly limited. In contrast, digital subcarrier multiplexing systems can select a combination of modulation orders for multiple subcarriers on each optical path and can flexibly adapt to various transmission characteristics. This paper numerically evaluates the transmission characteristics of digital subcarrier multiplexing systems. The interaction between laser phase noise and chromatic dispersion is well examined by extensive simulations, and two-phase estimation methods are compared. The results show that digital subcarrier multiplexing systems with the appropriate phase estimation method enable longer transmission distances.