Research

RESEARCH INTERESTS

• Molecular Communication
• Cognitive Radio Networks
• Green Communications
• Detection & Estimation
• Ad Hoc Networks
• Mathematical Modeling

Molecular Communication via Diffusion (MCvD) Basics

Molecular communication is a new communication paradigm that uses molecules for information transmission. Similar to traditional communication systems, several factors constitute limits over the performance of this communication system. Molecular Communication via Diffusion (MCvD) is an effective and energy efficient method for transmitting information across nanonetworks. In MCvD, the information is transmitted between the transmitter and receiver through the propagation of certain molecules via diffusion. In MCvD, the transmitters stay in a fluid medium and they emit modulated molecules. The information is transmitted via the propagation (diffusion) of emitted molecules through the environment. Diffusion causes the molecules to propagate and spread throughout the environment. Finally, at the receiver side, molecules react with the receptors over the receiver node surface and are removed from the environment. In nature, most of the receptor types remove the information-carrying molecules from the environment once they arrive at the receiver. This means molecules contribute to the signal once in a short duration. If the receptors do not remove molecules from the environment, they have other mechanisms to guarantee that each molecule contributes to the signal only once (e.g., acetylcholinesterase breaks down the molecules in neuromuscular junctions).

In this communication system, information is sent using a sequence of symbols which are spread over sequential time slots with one symbol in each slot. The symbol sent by the transmitter is called the “intended symbol”, and the demodulated symbol at the receiver is called the “received symbol”. An MCvD system has five main processes: encoding, emission, propagation, absorption, and decoding. Generally, the information is modulated on any physical property or the release timing of the messenger molecules with certain method, which physically travels to the receiver side.

Propagation Process

The molecules, called messenger/information molecules, function both as a information and a carrier. These molecules can be of many types of chemical compounds such as DNA fragments, proteins, peptides or specifically formed molecules. The messenger molecules are the information particles in molecular scale. In this scale, the movement of particles inside a fluid is modeled by Brownian motion or diffusion process. The motion is governed by the combined forces applied to the messenger molecule by the molecules of the liquid due to thermal energy.

In n-dimensional space, the total displacement, r in one time step can be found by considering displacement at each dimension. Movement at each dimension in one time step is modeled independently and follows a Gaussian distribution, N(0, 2DΔt), where D and Δt are the diffusion coefficient and the step time.

Modulation Process

The task of the particle emission process is to modulate the particle concentration at the transmitter according to the input symbol, modulation type, and the waveform of the signal. In one symbol duration, the number of released molecules may be spike like at the start of the slot or may be spread over the slot. Releasing all the molecules for a symbol may be better compared to spreading it over the symbol slot in terms of symbol demodulation. The capabilities of the transmitter node, however, may necessitate the spreading it over the symbol duration. If the transmitter node has less space for storing the synthesized messenger molecules it would necessitate sending the synthesized messenger molecules before the storage areas are full. Hence, depending on the capabilities of the transmitter node, sending one peak at the start of the symbol duration may be possible or not.

Symbols can be modulated over various “messenger molecule emission properties” at the sender, e.g., concentration, frequency, phase, molecule type, to form a signal. Some of the modulation techniques utilizes carrier waves, however, some do not. Using carrier waves makes the handling of Inter Symbol Interference (ISI) more complex.

• Amplitude-based Modulations

The concentration of the sent/received messenger molecules is used as the amplitude of the signal in Concentration Shift Keying (CSK), hence it represents data as variations in the amplitude of a carrier wave or pulse. CSK is analogous to Amplitude Shift Keying (ASK) in classical communication. Representing b bits requires 2^bdifferent amplitudes. If the carrier wave is single point pulse and we use presence and absence of carrier wave to indicate binary “1” and “0”, respectively, it is called Molecular Concentration On-off Keying (MC-OOK) which is a special type of Binary CSK (BCSK). Amplitude-based modulation symbols may also be modulated on a square wave. If there are four amplitude levels for two bit symbols then it is called Quadrature CSK (QCSK).

• Molecule Type-based Modulations

Molecule type is used to represent information in Molecular Shift Keying (MoSK). Representing b bits requires 2^b different molecules with similar properties. The transmitter releases one of these molecules based on the current intended symbol. The receiver decodes the intended symbol based on the type and the concentration of the molecule received during a time slot.

• Frequency-based Modulations

Representing the data through discrete frequency changes of a carrier wave of number of messenger molecules is called Molecular Frequency Shift Keying (MFSK). In this modulation scheme, number of molecules sent is not constant during the symbol duration and changes according to cosine wave. Since we cannot send negative number of molecules we have to shift the amplitude to up.

Challenges

Most crucial challenge in this domain is to characterize and model the propagation of information-carrying molecules. In the scope of wireless communications, many models and characterizations are developed for electromagnetic waves. To predict and understand the channel response, it is necessary to develop the propagation and arrival modeling in nanonetworking domain. The diffusion dynamics and modeling are mature research areas, however when the receiver is an absorber then the problem becomes much more complex. In general, molecular communication systems and receptors are designed to guarantee that each molecule contributes to the signal only once. Therefore, first passage processes are studied to form the basis of the channel characteristics.

After physical channel problems are studied and practiced, more predictable and robust modulation techniques can be proposed and implemented. Some of the modulation techniques are introduced in this chapter, however there are unrealistic assumptions including perfect modulation, demodulation, and synchronization. Nanonetworking-enabled nodes are assumed to be capable of synthesizing the required molecules and to be able to store as much as required. There may be constraints on the transmitter receiver pairs for more realistic scenarios. These assumptions should be relaxed in the future in order to make these modulation schemes more feasible.

Another issue in molecular communication domain is the interference. Sources of interference may differ, however due to channel characteristics inter symbol interference is at a crucial point. Therefore, ISI mitigation techniques compatible with nanonetworking-enabled nodes should be designed and implemented. Arrival of diffusing particles spans a huge duration, however on the other hand we are trying to shorten the symbol duration. Therefore, ISI becomes an important problem to be solved. Considering the simplicity and the expected outcomes of the nanomachines, one can deduce that these nonomachines should cooperate and communicate. Their collaborative actions are expected, therefore multi user interference should also be considered.

Hence the main challenges can be listed as:

• channel and molecule arrival modeling
• efficient modulation schemes
• inter-symbol-interference (ISI) due to heavy tail nature of the molecule arrivals
• co-channel-interference (CCI)
• testbed implementations