Spectrum-overlay scenarios for wide-band multi-carrier (MC) systems bring new technical challenges along that must be considered during the system design. In such scenarios, the receiver has to perform actions, such as channel estimation and synchronization, in the presence of possibly strong in-band interference. In this paper, we investigate the impact of digitally modulated narrowband interference (NBI) on the iterative algorithm for joint channel estimation and frame synchronization as proposed in [1] for uplink MC-CDMA. Moreover, we modify the algorithm to have better performance in the presence of NBI. The performance of the modified algorithm taking into account the statistics of NBI is verified through computer simulations and compared with the results from [1].