Wireless Multimedia Sensor Networks

Nanotechnologies promise new solutions for several applications in biomedical, industrial and military fields. At nano-scale, a nano-machine can be considered as the most basic functional unit. Nano-machines are tiny components consisting of an arranged set of molecules, which are able to perform very simple tasks. Nanonetworks. i.e., the interconnection of nano-machines are expected to expand the capabilities of single nano-machines by allowing them to cooperate and share information. Traditional communication technologies are not suitable for nanonetworks mainly due to the size and power consumption of transceivers, receivers and other components. The use of molecules, instead of electromagnetic or acoustic waves, to encode and transmit the information represents a new communication paradigm that demands novel solutions such as molecular transceivers, channel models or protocols for nanonetworks. Nanotechnology enables the miniaturization and fabrication of devices in a scale ranging from 1 to 100 nanometers. At this scale, a nano-machine can be considered as the most basic functional unit. Nano-machines are tiny components consisting of an arranged set of molecules which are able to perform very simple computation, sensing and/or actuation tasks. Nano-machines can be further used as building blocks for the development of more complex systems such as nano-robots and computing devices such as nano-processors, nano-memory or nano-clocks.

Communication among nano-machines

Among all of the expected features of future nano-machines, the communication capabilities are also very important. This is the only feature that enables them to work in a synchronous, supervised and cooperative manner to pursuit a common objective. Nano-machines communication can include the two following bidirectional scenarios:

Communication between a nano-machine and a larger system such as electronic micro-devices, and
Communication between two or more nanomachines.

Molecular communication can be used to interconnect multiple nano-machines, resulting in nanonetworks.

Nanonetworks versus Traditional communication networks

Nanonetworks are not a simple extension of traditional communication networks at the nano-scale. They are a complete new communication paradigm, in which most of the communication processes are inspired by biological systems found in nature. Nanonetworks have the following differences with traditional communication networks:

In nanonetworks, the message is encoded using molecules; while in traditional communication networks, the information is encoded in electromagnetic, acoustic or optical signals. Two different and complementary coding techniques can be considered to represent the information in nanonetworks. The first one uses temporal sequences to encode the information, such as the temporal concentration of specific molecules in the medium. According to the level of the concentration, i.e.,the number of molecules per volume, the receptor decodes the received message. For instance, this technique is used by the Central Nervous System to propagate the neural impulses. This technique can be considered similar to those used in traditional networks where time-varying sequences transport the information. The second technique, hereinafter called molecular encoding, uses internal parameters of the molecules toencode the information such as the chemical structure, relative positioning of molecular elements or polarization. In this case, the receiver must be able to detect these specific molecules to decode the information. This technique is similar to the use of encrypted packets in communication networks, in which only the intended receiver is capable to read the information. In nature, molecular encoding is used in pheromonal communication, where only members of the transmitter specie can decode the transmitted message.
The propagation speed of signals used in traditional communication networks, such as electromagnetic or acoustic waves, is much faster than the propagation of molecular messages. In nanonetworks, the information, i.e., molecules, has to be physically transported from the transmitter to the receiver. In addition, molecules can be subject to random diffusion processes and environmental conditions, such as temperature, which can affect the propagation of the molecular messages.
In traditional communication networks, noise is described as an undesired signal overlapped with the signals transporting the information. In nanonetworks, according to the coding techniques, two different types of noise can affect the messages. First, as occurs in traditional communication systems, noise can be overlapped with molecular signals such as concentration level of molecules. This means that another source emits the same molecules used to encode the message, therefore they affect the concentration sensed by the receiver. In nanonetworks, noise can also be understood as an undesired reaction occurring between information molecules and other molecules present in the environment. These reactions can modify the structure of the information molecules and therefore the receiver would not be able to detect the transmitted message.
Text, voice and video are usually transmitted over traditional communication networks. By contrast, in nanonetworks, since the message is a molecule, the transmitted information is more related to phenomena, chemical states and processes .
In nanonetworks, most of the processes are chemically driven resulting in low power consumption. In traditional communication networks the communication processes consume electrical power that is obtained from batteries or from external sources such as electromagnetic induction. We summarize nanonetworks and traditional communication network features in Table 1. Most of the existing communication networks knowledge is not suitable for nanonetworks due to their particular features. Nanonetworks require innovative networking solutions according to the characteristics of the network components and the molecular communication processes.

Nanonetwork components

The first models of nanonetworks are based on those used in Information and Communication Technologies (ICT) for telecommunication networks. In Fig. 1, we show the general concepts of nanonetworks versus existing telecommunication systems. Nanonetworks components are functionally similar to those found in traditional networks. In nanonetworks, we can identify five different components: the transmitter node, the receiver node, the messages, the carrier, and the medium. Each of these components affects the overall communication process, which includes the following steps as Fig. 1b depicts:

1. The transmitter encodes the message onto molecules. 2. The transmitter inserts the message into the medium by releasing the molecules to the environment or attaching them to molecular carriers. 3. The message propagates from the transmitter to the receiver. 4. The receiver detects the message. 5. The receiver decodes the molecular message into useful information such as reaction, data storing, actuation commands, etc.

Short-range communication using molecular Motors

In the framework of nanonetworks, we classify as short-range the communication process that takes place in a range from nm to few mm. Most of the intra-cell communications are based on molecular motors. Molecular motors, e.g., dynein, are proteins or protein complexes that transform chemical energy into mechanical work at a nano-scale. These molecular motors can be found in eukaryotic cells in living organisms.

On the transmitter side, the information molecules are loaded on molecular motors, which transport the information along the microtubules to the receiver. The packets can be encapsulated in vesicles. A vesicle is a fluid or an air-filled cavity that can store or digest cellular products. The objective if this encapsulation of the information is twofold. First, it allows enhancing the compatibility between the information molecule and the molecular motor, enabling the use of diverse types of molecules as information packets. Second, the encapsulation protects the information molecules avoiding them to react with antagonistic receptors present in the medium. The network infrastructure should be deployed prior to the beginning of the communication process. The propagation of molecular motors along a microtubule is unidirectional. The polarity of the microtubule indicates the movement direction of specific molecular motors, e.g., kinesin moves towards the (+) end of the microtubule, and dynein towards the (-) end.

Short-range communication using calcium signaling

Inter-cell communication based on calcium signaling is one of the most well-known molecular communication techniques. It is responsible for many coordinated cellular tasks such as fertilization, contraction or secretion. Calcium signaling is used in two different deployment scenarios. It can be used to exchange information among cells physically located one next to each or among cells deployed separately without any physical contact. Depending on the scenario, the propagation of the chemosignals includes different messengers such as ATP substance, calcium ions (Ca2+) or inositol 1,4,5-triphosphate (IP3) among others.

Similar to natural models, nano-machines should be near each other. Similar to the natural models, the propagation of the information can be performed in two different schemes depending on the deployment of the nanomachines:

Direct access. If nano-machines are physically connected, Ca2+ signals travel from one nano-machine to the next one through the gates, as shown in Fig. 3a. These gates should work similarly to the gap junctions allowing the flux of ions and molecules from one nano-machine to the interior of another nanomachine.
Indirect access. If the nano-machines are not in direct contact, the transmitter nano-machine should be able to release the information molecules to the medium as first messenger in the chemosignal pathway. Information molecules will move through the medium following a diffusion process. Transmitters can encode the information varying the concentration of first messengers as shown in Fig. 3b. Biological systems encode the information on frequency and amplitude of concentration changes, usually referred to as Ca2+ oscillation and Ca2+ spikes.

These two propagation schemes enable the formation of networks supporting multicast or broadcasting transmission mechanisms. The overall communication system could work as follows: transmitter and receiver nano-machines are connected to each other through a signaling network consisting of interconnected nodes, which propagate the information using Ca2+ signals.

Long-range communication using pheromones

Long-range communication is referred to as the communication process in which the distance between transmitter and receiver nano-machines ranges from millimeters up to kilometers. Control and communication of nano-machines in long-range communication can be useful in many applications, such as the military field or environmental monitoring.

The communication is based on the release of molecules that can be detected by a receiver. Once the molecules are released to the medium, they can be affected by several factors, such as antagonist agents, medium flow and temperature, and dispersion. All those factors can be considered as sources of noise, similar to those found in traditional communication channels, and they can compromise the transmission reliability. Since messages consist of molecules, there is a huge quantity of possible combinations that can be used to transmit data. Moreover, messages can be compounded by several different molecules allowing even more combinations to encode the information. The reception of the transmitted molecules is realized by molecular receptors located on the receiver, as depicted in Fig. 4. This phenomenon is based on the ligand-receptor binding process as occurs within calcium signaling triggered by first messengers. A ligand is a molecule that interacts with a protein, by specifically binding to this one. In molecular communication using pheromones, the receptor proteins can be considered as the receiver nano-machine antenna or transducer, which transforms energy contained in the message into a reaction at the receiver.

Related work

I. F. Akyildiz, F. Brunetti, and C. Blazquez, ``NanoNetworking: A New Communication Paradigm," Computer Networks Journal (Elsevier), Vol. 52, pp. 2260-2279, August 2008