Capacity and throughput analysis of nanoscale machine communication through transparency windows in the terahertz band
Introduction
Nanotechnology is a fast growing research area aimed at, among many others, the development of extremely small machines capable of performing one of few simple actions. Nanotechnology is nowadays considered an enabling technology for a set of applications in biomedical, environmental, and military fields. Being inherently simple and performing primitive operations only, nanomachines in isolation are not expected to manage advanced tasks. To enable more complex applications such as intra-body drug delivery or cooperative environment sensing, the exchange of information and commands between networking entities and/or external controller is required. The need for coordination and information sharing naturally leads towards the concept of nanonetworks. One promising way to enable networking capabilities is to use wireless communications between nanomachines [1].
There are several techniques proposed for nanomachine communications including the adaptation of biologically-inspired approaches (e.g., molecular, bacterial, neuronal communications) as well as ultra-high-frequency electromagnetic (EM) communications. Within this list, the latter is one of the most promising techniques. The research on EM nanonetworks started few years ago in the context of nanosensor networks [1]. The early studies concentrated on exploring theoretical basis of communications using miniature transceivers and antennas, understanding the set of frequencies nanomachines can use for communication and analyzing channel characteristics. These early investigations have led to the following findings:
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Graphene can be utilized to develop novel nano-transceivers [2], [3] and nano-antennas [4], [5], which can efficiently operate in the Terahertz Band (0.1–10 THz) while satisfying the size constraints of nanomachines.
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The THz Band channel is highly frequency selective and exhibits a unique distance-dependent bandwidth behavior due to the absorption mainly from water vapor molecules [6]. In particular, the THz Band behaves as a single transmission window almost 10 THz wide for distances much below one meter, and as a set of multiple-GHz wide windows for longer distances.
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Ultra-broadband communication schemes based on the transmission of ultra-short pulses are an effective way to exploit the very large bandwidth of the THz Band channel for distances much below one meter [7]. The reception of such pulses can be achieved with a simple energy detector [8].
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Nanomachines will require novel energy-harvesting systems to overcome the limitations of their nano-batteries [9], [10], which will also change the way in which the protocol stack should be designed [11], [12].
For the time being, the focus on EM nanonetworking research has been on increasing the capacity and achievable data rates when utilizing ultra-broadband signals that occupy the entire THz Band. For example, in [6], under realistic energy constraints, it is shown that Terabit-per-second (Tbps) links are possible among nanomachines, but only for distances much below one meter. However, it is relevant to note that, in many prospective applications of EM nanonetworks, the communication range is more important than the data rate. Examples of such applications include transactional type of networks, e.g., environment monitoring networks or command/response type of networks. Thus, it is important to find a way to increase the coverage of a single node.
In this paper, we first propose a way to mitigate molecular absorption by properly choosing the range of frequencies known as transparency windows. Then, we discuss trade-offs and dependencies between the choice of the window, its bandwidth and capacity, and the communication range of a single node. We show that the smart selection of bandwidth with respect to the level of molecular absorption leads to way better performance of our system in terms of communication range and capacity. In particular, our results indicate that for a nominal power of 26.5 nW (equivalent to a 100 fs-long pulse with 0.1 aJ energy) the communication range can be up to 15 m providing satisfactory capacity for transactional applications in the range of 4–11 Mbps. Such a range is only possible when operating in the lower end of the first transparency window 0.1–0.54 THz. Moving up in the spectrum to the next transparency window, the free-space propagation loss dominates the channel characteristics prohibiting long communication distances. Using the whole window bandwidth one can get up to 2 Tbps at 0.01 m.
Motivated by these results, we investigate the achievable throughput of a simple binary digital modulation built on top of a novel width-adaptive pulse-based communication scheme. We also extend the model presented in [6], [7] by incorporating the effect of molecular relaxation. Our results show that, due to the molecular absorption relaxation time, additional pauses between pulse transmissions are required, but reliable communication is possible even for very small signal-to-noise ratio (SNR) values. These results expand the application scope of nanonetworks and illustrate that they are not limited to small coverage areas but can also be used to carry traffic generated by both low-rate transactional and bandwidth-greedy applications at small-to-medium distances. Similar results for macroscale THz Band communication networks have been recently discussed in [13]. As we will discuss in this paper, the distance drastically affects the transparency windows and, thus, our analysis, which is focused on nanoscale machine communication, is novel.
The rest of the paper is organized as follows. In Section 2, we briefly review the THz Band channel characteristics. In Section 3, we identify and discuss the properties of transparency windows in the THz Band and then study trade-offs and dependencies between communication distance, SNR and capacity. The throughput of a width-adaptive pulse-based communication system is analyzed in Section 4 and numerical results are provided in Section 5. Conclusions are drawn in Section 6.
Section snippets
Terahertz Band channel characteristics
The link budget equation for the received power in the THz Band can be represented as where is the power spectral density (p.s.d.) of the transmitted signal, is the p.s.d. of the received signal, is the free-space propagation loss, and is the p.s.d. of the molecular absorption loss. In addition to the two contributions to the path loss, the received signal is also affected by the molecular absorption noise. Note that the thermal noise when using
Communication in transparency windows
In spite of quite complex channel characteristics, there is a way to avoid severe signal distortion by using the transparency windows. In these windows, the transmittance of the medium, in (3), is never smaller than 95% for a certain . This implies that, in these transparency windows both the molecular absorption loss and noise level are as small as feasible. In this section, we will first demonstrate and discuss the propagation characteristics of transparency windows in the THz Band.
Achievable throughput and bit error rate
In this section we analyze the link throughout and the Bit Error Rate (BER) of a simple pulse-based modulation scheme for communication in the transparency windows.
Numerical results
We perform numerical analysis of throughout and associated BER using the simulation study. The major point of the study is the trade-off between correct reception of 1 () and 0 (in the sequence 10, ) caused by different values of power detection threshold and inter-symbol time . Further, we are mainly interested in those regions where the SNR is below than few decibels. In our analysis we are mostly concentrated in qualitative results for low values of SNR.
Conclusion
In this paper, we advocated the use of transparency windows in the THz Band to enable communications between nanomachines. When using these windows the communication range of a single node significantly increases as molecular absorption is close to non-existent. In these windows the free-space propagation loss becomes the dominating factor in channel characteristics. As a result, although there are a number of transparency windows in the THz Band the one allowing us to achieve the larger
Acknowledgments
This work was supported by the FiDiPro program of Academy of Finland “Nanocommunication Networks”, 2012–2016. The authors are also grateful to A. Ozan Bicen (BWN Lab, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA) for helpful discussions and insightful comments in the course of this work.
Pavel Boronin received his M.Sc. degree in Communication Networks and Switching Systems from Saint Petersburg State University of Telecommunications, Russia, in 2012. He is pursuing his studies at the same university and now is a 2nd year Ph.D. student. Since 2014 Pavel has been studying as an exchange Ph.D. student at Tampere University of Technology, Tampere, Finland. His research interests include physics of terahertz radiation, nanonetworks, adaptive routing algorithms for dynamic and
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Pavel Boronin received his M.Sc. degree in Communication Networks and Switching Systems from Saint Petersburg State University of Telecommunications, Russia, in 2012. He is pursuing his studies at the same university and now is a 2nd year Ph.D. student. Since 2014 Pavel has been studying as an exchange Ph.D. student at Tampere University of Technology, Tampere, Finland. His research interests include physics of terahertz radiation, nanonetworks, adaptive routing algorithms for dynamic and static wireless networks.
Vitaly Petrov received his M.Sc. degree in Information Systems Security from SUAI University, St Petersburg, Russia, in 2011. He is now a graduate M.Sc. student at Tampere University of Technology, Tampere, Finland. Since 2014 Vitaly holds a position of Visiting Scholar at School of Electrical and Computer Engineering, Georgia Institute of Technology, GA, USA. From 2011 till 2014 he has been working as a Researcher at the Department of Electronics and Communications Engineering, Tampere University of Technology. Prior to this, from 2008 till 2011 he served as a Junior Researcher with Information Systems Security Department, SUAI, St Petersburg, Russia, as well as an Engineer in Nokia-SUAI joint lab. From May till October 2012 he was a Strategic Intern with Security Research Team at Nokia Research Centre, Helsinki, Finland. Vitaly is a student member of the IEEE and the ACM. His current research interests include Internet-of-Things, Nanonetworks, Cryptology and Network Security.
Dmitri Moltchanov is a Senior Research Scientist in the Department of Electronics and Communications Engineering, Tampere University of Technology, Finland. He received his M.Sc. and Cand.Sc. degrees from Saint Petersburg State University of Telecommunications, Russia, in 2000 and 2002, respectively, and Ph.D. degree from Tampere University of Technology in 2006. His research interests include performance evaluation and optimization issues of wired and wireless IP networks, Internet traffic dynamics, quality of user experience of real-time applications, and traffic localization P2P networks. Dmitri Moltchanov serves as the TPC member in a number of international conferences. He authored more than 50 publications.
Yevgeni Koucheryavy is a Full Professor and Lab Director at the Department of Electronics and Communications Engineering at the Tampere University of Technology (TUT), Finland. He received his Ph.D. degree (2004) from the TUT. Yevgeni is the author of numerous publications in the field of advanced wired and wireless networking and communications. His current research interests include various aspects in heterogeneous wireless communication networks and systems, the Internet of Things and its standardization, and nanocommunications. Yevgeni is an Associate Technical Editor of IEEE Communications Magazine and Editor of IEEE Communications Surveys and Tutorials. Yevgeni is a Senior IEEE member.
Josep Miquel Jornet received the Engineering Degree in Telecommunication and the Master of Science in Information and Communication Technologies from the Universitat Politècnica de Catalunya, Barcelona, Spain, in 2008. He received the Ph.D. degree in Electrical and Computer Engineering from the Georgia Institute of Technology, Atlanta, GA, in 2013, with a fellowship from “la Caixa” (2009–2010) and Fundación Caja Madrid (2011–2012). He is currently an Assistant Professor with the Department of Electrical Engineering at the University at Buffalo, The State University of New York. From September 2007 to December 2008, he was a visiting researcher at the Massachusetts Institute of Technology, Cambridge, under the MIT Sea Grant program. He was the recipient of the Oscar P. Cleaver Award for outstanding graduate students in the School of Electrical and Computer Engineering, at the Georgia Institute of Technology in 2009. He also received the Broadband Wireless Networking Lab Researcher of the Year Award at the Georgia Institute of Technology in 2010. He is a member of the IEEE and the ACM. His current research interests include electromagnetic nanonetworks, graphene-enabled wireless communication, Terahertz Band communication networks and the Internet of Nano-Things.