  In this paper, we propose a symbol interval optimization algorithm in
molecular communication with drift. Proper symbol intervals are important in
practical communication systems since information needs to be sent as fast as
possible with low error rates. There is a trade-off, however, between symbol
intervals and inter-symbol interference (ISI) from Brownian motion. Thus, we
find proper symbol interval values considering the ISI inside two kinds of
blood vessels, and also suggest no ISI system for strong drift models. Finally,
an isomer-based molecule shift keying (IMoSK) is applied to calculate
achievable data transmission rates (achievable rates, hereafter). Normalized
achievable rates are also obtained and compared in one-symbol ISI and no ISI
systems.
