  In conventional molecular communication (MC) systems, the signaling molecules
used for information transmission are stored, released, and then replenished by
a transmitter (TX). However, the replenishment of signaling molecules at the TX
is challenging in practice. Furthermore, in most envisioned MC applications,
e.g., in the medical field, it is not desirable to insert the TX into the MC
system, as this might impair natural biological processes. In this paper, we
propose the concept of media modulation based MC where the TX is placed outside
the channel and utilizes signaling molecules already existing inside the
system. We consider signaling molecules that can be in different states which
can be switched by external stimuli. Hence, in media modulation based MC, for
information transmission, the TX stimulates the signaling molecules to encode
information into their state. In particular, we elaborate media modulation for
the group of photochromic molecules, which undergo light-induced reversible
transformations, and study the usage of these molecules for information
transmission in a three-dimensional duct system. We develop a statistical model
for the received signal which depends on the distribution of the signaling
molecules in the system, the reliability of the state control mechanism, and
the randomness of molecule propagation. Furthermore, we analyze the performance
of media modulation based MC in terms of the bit error rate (BER). We show that
media modulation enables reliable information transmission, which renders a TX
inside the channel unnecessary.
