  Recently, molecular communication (MC) has been studied as a micro-scale
communication between cells or molecular robots. In previous works, the MC
channels in unbounded environment was analyzed. However, many of the
experimentally implemented MC channels are surrounded by walls, thus the
boundary condition should be explicitly considered to analyze the dynamics of
MC channels. In this paper, we propose a framework to analyze the frequency
response of one-dimensional MC channels based on a diffusion equation with a
boundary. In particular, we decompose the MC channel into the diffusion system
and the boundary system, and show the relation between the cut-off frequency of
the MC channel and the communication distance based on the transfer function.
We then analyze the frequency response of a specific MC channel and reveal that
the boundary can restrict the communication bandwidth of the MC channel.
