  This paper investigates a spherical transmitter (TX) with a membrane covered
by heterogeneous receptors of varying sizes and arbitrary locations for
molecular communication (MC), where molecules are encapsulated within vesicles
and released from the TX through membrane fusion. Assuming continuous vesicle
generation at the TX and a transparent receiver (RX), we calculate the molecule
release rate, the fraction of absorbed molecules at the TX, and the received
signal at the RX. All obtained analytical expressions are functions of all
receptors locations and sizes, and are validated by particle-based simulations.
Our numerical results indicate that evenly distributed receptors on the TX
membrane can absorb more molecules than randomly distributed receptors or a
single receptor. Furthermore, inspired by the autoreceptor functionality in
synaptic communication, we incorporate a negative feedback mechanism (NFM) at
the TX, such that molecule release stops after a certain period. We then derive
the fraction of molecules that can be reused for the subsequent emissions when
considering both NFM and molecule harvesting. Our numerical results demonstrate
that incorporating NFM can reduce inter-symbol interference (ISI) while
maintaining the same peak received signal as the case without NFM.
Additionally, our results show that TXs incorporating both molecule harvesting
and NFM can achieve a higher energy efficiency and lower error probability than
TXs employing only molecule harvesting or neither functionality.
