  This paper investigates the channel impulse response (CIR), i.e., the
molecule hitting rate, of a molecular communication (MC) system employing an
absorbing receiver (RX) covered by multiple non overlapping receptors. In this
system, receptors are heterogeneous, i.e., they may have different sizes and
arbitrary locations. Furthermore, we consider two types of transmitter (TX),
namely a point TX and a membrane fusion (MF)-based spherical TX. We assume the
point TX or the center of the MF-based TX has a fixed distance to the center of
the RX. Given this fixed distance, the TX can be at different locations and the
CIR of the RX depends on the exact location of the TX. By averaging over all
possible TX locations, we analyze the expected molecule hitting rate at the RX
as a function of the sizes and locations of the receptors, where we assume
molecule degradation may occur during the propagation of the signaling
molecules. Notably, our analysis is valid for different numbers, a wide range
of sizes, and arbitrary locations of the receptors, and its accuracy is
confirmed via particle-based simulations. Exploiting our numerical results, we
show that the expected number of absorbed molecules at the RX increases with
the number of receptors, when the total area on the RX surface covered by
receptors is fixed. Based on the derived analytical expressions, we compare
different geometric receptor distributions by examining the expected number of
absorbed molecules at the RX. We show that evenly distributed receptors result
in a larger number of absorbed molecules than other distributions. We further
compare three models that combine different types of TXs and RXs.
