A model is introduced in this paper to describe the transition boundaries in perpendicular magnetic recording at extremely high density. In contrary to the previous signal generation models, effects of magnetization transition curvature and the track edge fluctuations are adequately included in the new model that is designed to capture the actual transition boundaries, i.e., the variations of magnetization distribution in both down-track and across-track directions. The model is used to predict the readback waveforms based on the detailed information obtained from micromagnetic simulations taking into account the head and media parameters. The model is therefore suitable for magnetic recording at extremely high densities when the impact of the transition curvature and the track edge effect on the recording performance becomes more significant.