  The simulation of diffusion-based molecular communication systems with
absorbing receivers often requires a high computational complexity to produce
accurate results. In this work, a new a priori Monte Carlo (APMC) algorithm is
proposed to precisely simulate the molecules absorbed at a spherical receiver
when the simulation time step length is relatively large. This algorithm
addresses the limitations of the current refined Monte Carlo (RMC) algorithm,
since the RMC algorithm provides accurate simulation only for a relatively
small time step length. The APMC algorithm is demonstrated to achieve a higher
simulation efficiency than the existing algorithms by finding that the APMC
algorithm, for a relatively large time step length, absorbs the fraction of
molecules expected by analysis, while other algorithms do not.
