  This paper investigates the estimation of different parameters, e.g.,
propagation distance and flow velocity, by utilizing two fully-absorbing
receivers (RXs) in a one-dimensional (1D) environment. The time-varying number
of absorbed molecules at each RX and the number of absorbed molecules in a time
interval as time approaches infinity are derived. Noisy molecules in this
environment, that are released by sources in addition to the transmitter, are
also considered. A novel estimation method, namely difference estimation (DE),
is proposed to eliminate the effect of noise by using the difference of
received signals at the two RXs. For DE, the Cramer-Rao lower bound (CRLB) on
the variance of estimation is derived. Independent maximum likelihood
estimation is also considered at each RX as a benchmark to show the performance
advantage of DE. Aided by particle-based simulation, the derived analytical
results are verified. Furthermore, numerical results show that DE attains the
CRLB and is less sensitive to the change of noise than independent estimation
at each RX.
