  The arrival of molecules in molecular communication via diffusion (MCvD) is a
counting process, exhibiting by its nature binomial distribution. Even if the
binomial process describes well the arrival of molecules, when considering
consecutively sent symbols, the process struggles to work with the binomial
cumulative distribution function (CDF). Therefore, in the literature, Poisson
and Gaussian approximations of the binomial distribution are used. In this
paper, we analyze these two approximations of the binomial model of the arrival
process in MCvD with drift. Considering the distance, drift velocity, and the
number of emitted molecules, we investigate the regions in which either Poisson
or Gaussian model is better in terms of root mean squared error (RMSE) of the
CDFs; we confirm the boundaries of the region via numerical simulations.
Moreover, we derive the error probabilities for continuous communication and
analyze which model approximates it more accurately.
