GRANET: Graphene-enabled Nanonetworks in the Terahertz Band
In 1959, the Nobel laureate physicist Richard Feynman, in his famous speech entitled "There's Plenty of Room at the Bottom", described for the first time how the manipulation of individual atoms and molecules would give rise to more functional and powerful man-made devices. In his talk, he noted that several scaling issues would arise when reaching the nanoscale, which would require the engineering community to totally rethink the way in which nano-devices are conceived. More than half century later, nanotechnology is providing a new set of tools to the engineering community to design and manufacture devices just a few hundred nanometers in size, which are able to perform only very simple tasks.
For example, one of the early applications of these nano-devices is in the field of nanosensors. A nanosensor is not just a tiny sensor, but a device that makes use of the novel properties of nanomaterials to identify and measure new types of events in the nanoscale. Amongst others, nanosensors can detect and measure physical characteristics of structures just a few nanometers in size, chemical compounds in concentrations as low as one part per billion, or the presence of biological agents such as virus, bacteria or cancerous cells. However, the sensing range of a single nanosensor is limited to its close nano-environment and, thus, many nanosensors are needed to cover significant regions or volumes. Moreover, an external device and the user interaction are necessary to read the actual measurement from the nanosensor.
Similarly to the way in which communication among computers enabled revolutionary applications such as the Internet, we believe that, by means of communication, nano-devices will be able to overcome their limitations and to expand their potential applications. In our vision, nanonetworks will be able to cover larger areas, to reach unprecedented locations in a non-invasive way, and to perform additional in-network processing. Nanonetworks have a vast amount of applications in which classical wireless networks cannot be used. These are classified in four main groups:
|Figure 1 - The Internet of Nano-Things.|
Many nano-device components have already been prototyped and tested. However, there are still several challenges from the device perspective that need to be addressed in order to turn existing nano-devices into autonomous machines. In our vision (Fig. 2), several nano-components such as a nano-processor, a nano- memory and a power nano-system, have to be integrated into a device with a total size a few hundred square micrometers at most. To date, several solutions for these nano-components have been proposed:
|Figure 2 - Nano-Device Hardware Architecture.|
Reducing the size of a metallic antenna down to a few hundreds of nanometers would impose the use of very high resonant frequencies. In particular, a one-micrometer long dipole antenna would resonate at approximately 150 THz. Due to the very limited power of nanosensors, the low mobility of electrons in conventional metals when nanometer scale structures are considered, and the challenges in implementing a nano-transceiver able to operate at this extremely high frequency, the feasibility of communication among nano-devices and nanonetworks would be compromised if this approach was followed.
Alternatively, the use of nanomaterials to fabricate miniaturized antennas can help overcome the aforementioned limitations. A few nano-antenna designs built with carbon-based nanomaterials have been already investigated. For example, CNTs can be used to develop nano-dipole antennas (Fig. 3, right). Using a transmission line model of CNTs it can be shown that, due to the atomic structure of CNT, there exists a kinetic inductance in nano-antennas that dominates over the common magnetic inductance between the nano-antenna and the ground plane. Because of this, the wave propagation speed in a CNT-based nano-antennas can be up to 100 times lower than the propagation speed in the free-space. Resulting from the retarded propagation of EM waves in CNTs, the resonant frequency of nano-antennas can be up to two orders of magnitude below that of a conventional metallic antenna. The possibility to operate at much lower frequencies relaxes the energy and power requirements for the nano-devices.
When it comes to graphene, the propagation of EM waves on infinitely large 2D planes of this nanomaterial has been theoretically analyzed recent works. However, little research has been conducted on EM propagation in GNRs, which is what is mainly needed for the design of a nano-antenna. Amongst others, it has been recently shown that the very large kinetic inductance that has been observed in CNTs can be drastically reduced in GNRs by increasing their width. As a result, both the contact resistance of GNRs and the EM wave propagation speed can be tuned by modifying the dimensions of the GNR. For this, we have recently proposed for the first time a nano-patch antenna based on a GNR (Fig. 3, left) and we have analyzed and compared its performance to that of a nano-dipole antenna based on a CNT. Amongst others, we showed that a 1 micrometer long graphene-based nano-antenna can radiate in the Terahertz Band (0.1-10 THz).
|Figure 3 - GNR-based nano-patch antenna (left) and CNT-based nano-dipole antenna (right).|
Based on our preliminary results, the envisioned nano-devices will be able to communicate in the Terahertz Band (0.1-10 THz). After nano-device design and manufacturing, the Terahertz channel is one of the main aspects that makes the realization of nanonetworks a challenge. The few Terahertz channel models existing to date are aimed at characterizing the communication between devices that are several meters far. In particular, due to the very high attenuation created by molecular absorption (hundreds of dB/m), current efforts both on device development and channel characterization are focused on the communication in the absorption-defined window at 300 GHz. However, thinking of the short transmission range of nano-devices, there is a need to understand and model the entire Terahertz band from 0.1 to 10 THz for distances much below one meter.
In this project, we think of the Terahertz Band (0.1-10 THz) as a single transmission window almost 10 Terahertz wide and develop a new channel model for Terahertz band communications. In particular, we will use radiative transfer theory to analyze the several phenomena affecting the propagation of EM waves in the Terahertz band, by starting from the absorption by molecules found along the path. Stemming from our preliminary results, we will obtain formulations for the:
The expected capabilities of the nano-devices and the peculiarities of the Terahertz Band require the development of novel information modulation and encoding techniques as well as protocols for nanonetworks.
First, existing modulations, which usually consist in encoding the information into one or more parameters of a periodic waveform, are not applicable in nanonetworks because of their relatively complex encoding and decoding processes and their high energy requirements. On the contrary, nano-devices require new modulations able to exploit the huge bandwidth provided by the terahertz channel, while still remaining feasible for their hardware limitations. We will investigate and propose modulation schemes specifically suited for nanonetworks.
Second, MAC protocols used in traditional networks, usually based on carrier sensing (e.g., CSMA and all its variations) need to be revised. One the one hand, information can be transmitted very fast, and, thus, there is “not much to sense”. Within this project, we will propose and design MAC protocols specifically suited for nanonetworks. For example, if the transmitted packets are very short, the probability of collision between nanosensor motes trying to access the channel at the same time will be much lower than in traditional wireless networks. In some scenarios, it is even possible that the probability of collision may be neglected, leading to the integration of MAC protocols in the physical layer. We will investigate novel MAC protocols taking into account the particularities of nanonetworks and the terahertz channel.
Third, the tiny dimensions of nano-devices make them suitable for exploring unreachable locations with unprecedented resolution with respect to traditional devices. However, they also make them vulnerable in front of any kind of external force. For example, nanosensor motes randomly deployed in an open field can be easily taken away by an air flow, or biological nanosensors can be swept away by a body fluid, for example. This may be seen as a shortcoming but can also be exploited in a beneficial way. This random movement of nano-devices will enable novel applications in which mobility of nanosensors can make the difference. Therefore, we will investigate novel routing mechanisms that consider the particular energy requirements and mobility pattern of nano-devices. [Back to top]