Page 21 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 3 – Internet of Bio-Nano Things for health applications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 3

ties of the MC channel, transmitter and receiver architec‑ that can be detected by the receiver via ield‑effect.
tures as follows: The strength of ionic screening depends
exponentially on the distance of bound information
• Intersymbol Interference (ISI): Due to the slow na‑
molecules from the surface of the receiver’s
ture of molecular diffusion in MC channel, severe
transducer channel. Numerous solutions exist in
ISI occurs in both forward and backward direction,
the biosensing literature that partially overcome
which is the main factor limiting the communication this widely‑observed problem. For example, using
rate. The effect of ISI is less pronounced in low‑ small‑size receptors, e.g., aptamers, can allow the
based MC channels; however, the slow reaction ki‑ bound information molecules to approach the
netics at the receiver surface might compound the receiver surface, increasing their effective charge
ISI, as revealed in [72]. Therefore, MC techniques [76, 77]. Alternatively, high‑frequency AC biasing at
should account for ISI, either removing it or redu- the receiver, exploiting the oscillating dipole
cing its effects. moments of the bound information molecules, can be
employed to overcome the ionic strength in exchange
• Nonlinearity and Time‑variance: The nonlinearity
for increased complexity on the receiver side [78].
of the MC system results from the nonlinear trans‑
• Low Communication Rate: Slow diffusion and re‑
mission and propagation dynamics, and the reaction‑
action kinetics of molecules might result in very low‑
based receiver mechanisms. On the receiver side,
in particular, the saturation of the receiver could communication rates, as shown in some of the recent
practical MC demonstrations. These physical limi‑
have substantial effect on the detection performance.
tations call for new modulation and detection tech‑
Therefore, the developed modulation and detection
techniques should account for nonlinearity. Time‑ niques that simultaneously exploit multiple proper‑
variance can result from the luctuations in the low ties of molecules, e.g., concentration and type, to
conditions, as well as from the time‑varying molecu‑ boost the communication rate for MC systems.
lar interference level in the channel.
Modulation techniques in MC fundamentally differ from
• Molecular Interference: The existence of other that in conventional EM communications, as the mo-
molecules in the MC channel can originate from an ir‑ dulated entities, i.e., molecules, are discrete in
relevant biological process, or another MC system co‑ nature, and the developed techniques should be robust
existing in the same channel. The interference against highly time‑varying characteristics of the MC
manifests itself on the receiver side, as the channel, as well as inherently slow nature of the
selectivity of receptors against information propagation mechanisms [8]. Exploiting the observable
molecules is far from ideal in practice, and thus, characteristics of molecules, researchers have proposed
many different types of molecules having inite to encode information into the concentration, type, or
inity with the receptors, could also bind the release time of the molecules [13, 79]. The simplest
same receptors, resulting in considerable modulation method proposed for MC is on‑off keying
interference at the received signal. To overcome this (OOK) modulation, where a binary symbol is
problem, new detection methods exploiting the represented by releasing a number of molecules or
frequency‑domain characteristics of the receiver not releasing any [80]. Similarly, using a single type
reaction and transducing processes can be de‑ of molecule, concentration shift keying (CSK), that is
veloped to increase the selectivity [57]. Moreover, analogous to amplitude shift keying (ASK) in traditional
the receptor cross‑talk resulting from multiple types wireless channels, is introduced in order to increase the
of molecules can be exploited to develop new modu‑ number of transmitted symbols by encoding
lation techniques to boost the communication rate. information into molecular concentration levels [81].
Molecular information can also be encoded into the type
• Noise: In addition to particle counting noise and
of molecules, i.e., molecule shift keying (MoSK) [79], or
ligand‑receptor binding noise, which are well inves‑
into both the type and the concentration of molecules to
tigated in the MC literature, the physical architecture
boost the data rate [82]. Additionally, the release order
of the receiver can lead to new noise sources. For
of different types of molecules [83], and the release time
example, in nanomaterial‑based designs, thermal
of single type of molecules [84] can be modulated to en‑
noise and electronic noise, e.g., 1/f noise, of the
code information in MC. Finally, in [85], authors propose
receiver can be expected to severely undermine the
the isomer‑based ratio shift keying (IRSK), where the in‑
reliability of communication.
formation is encoded into the ratio of different types of
• Ionic Screening: One of the main problems parti- isomers in a molecule, i.e., molecule ratio‑keying.
cularly observed at FET biosensor‑based receivers To overcome the noisy and ISI‑susceptible nature of MC
is the ionic screening in physiologically relevant channels, several channel coding techniques which are
luids, which decrease the SNR tremendously. The adopted from EM communications, e.g., block and con‑
ions in the channel luid can cause the screening of volution codes, or developed ically for MC, such
electrical charges of information molecules, as the ISI‑free coding scheme employing distinguishable
resulting in reduced effective charge per molecule molecule types, have been studied. Detection is by far

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