  In a diffusion-based molecular communication system, molecules are employed
to convey information. When propagation and reception processes are considered
in a framework of first passage processes, we need to focus on absorbing
receivers. For this kind of molecular communication system, the characteristics
of the channel is also affected by the shape of the transmitter. In the
literature, most studies focus on systems with a point transmitter due to
circular symmetry. In this letter, we address propagation and reception pattern
for chemical signals emitted from a spherical transmitter. We also investigate
the directivity gain achieved by the reflecting spherical transmitter. We
quantify the power gain by measuring the received power at different angles on
a circular region. Moreover, we define three metrics, i.e., the half-power
pattern-width, the directivity gain, and the peak time of the signal, for
analyzing the received signal pattern.
