Terahertz Band Communications: Applications, Research Challenges, and Standardization Activities

Transcription

1 Terahertz Band Communications: Applications, Research Challenges, and Standardization Activities Tampere University of Technology, Tampere, Finland Vitaly Petrov:

2 Motivation for THz communications (1) Trends in Wireless Networks 1 Tbps 1 Gbps 1 Mbps 1 Kbps Ethernet IEEE First Alphanumeric Pager Wide Area Paging GSM (2G) IEEE AE IEEE Z IEEE U UMTS (3G) IEEE BA LTE-A (4.5G) LTE (4G) Cellular LAN q Wireless Terabit-per-second (Tbps) links will become a reality within the next 5 years* * IEEE d Task Group, 2014

3 Path loss and capacity trade offs q Spatial loss E.g. free-space loss for omnidirectional antennas q Shannon Capacity Limit Link-level performance in case of best modulation and coding scheme L P ( f, d) =! # " 4π fd c 0 $ & % 2 C = Blog 2 (1+ SNR)

4 Practical benefits of higher frequencies q Smaller antenna size λ/2 and λ/4 for 10 MHz = 15 (7.5) m λ/2 and λ/4 for 1 GHz = 15 (7.5) cm λ/2 and λ/4 for 1 THz = 150 (75) mcm q MIMO (!) Massive MIMO è Higher capacity Adaptive MIMO è Interference cancellation q Devices miniaturization Micro and Nano Scale networks

5 What is the THz band? Definitions and advantages THz (IEEE: 0.3 3THz) Advantages of the THz band: q Very large amount of bandwidth available (~10THz) o Enabling Tbit/s links with 0.1bit/sec/Hz -> sounds feasible q Miniaturized antennas (λ~1mm for 300GHz) o Enabling technology for interactions of micro-scale objects (buzzword: Nanonetworks ) q Still penetrate visually non-transparent objects o Can work in environments, where Visible Light can hardly, such as box, pocket, device with a plastic cover... [!] Enabler technologies are coming...

6 Macro generators of the THz radiation q Equipment is available from late 1990s* 1. Lasers: Quantum cascade lasers (QCL) Far infrared lasers (FIR) 2. Free electron based: Schottky diodes Travelling Wave Tubes (TWT), etc. Size and power requirements Poor performance at room temperature *D. Grischkowsky et al., "Far-Infrared Time-Domain Spectroscopy with TeraHz Beams of Dielectrics and Semiconductors, The Journal of the Optical Society of America B, October 1990

7 Graphene and Carbon Nano Tubes (CNTs) q One atom thick carbon material q Produced by Andre Geim, K. Novoselov in 2004 Nobel prize 2010 q Major electrical property: Extremely high electrical conductivity q Derivatives: Carbon Nanotubes (CNT) Graphene Nanoribbons (GNR) Feasibility of micro- and nano-scale antennas

8 Proposal 1. Resonant-tunneling diode + voltage oscillator q Significantly decrease the size of THz signal generators and detectors By Rohm, Japan, 2011* Frequency: 300 GHz Estimated price: 1.3 USD Achieved rate: 1.5 Gbps Estimated rate: 30 Gbps Size: 1.5 x 3 cm * Rohm Semiconductor Press-release, November 2011

9 Proposal 2. Optical rectification for continuous-wave terahertz emission q Significantly decrease the size of THz antennas By Astar, Singapore and Imperial College, London in 2012* Size of few hundreds nanometers Operational at room temperature* Size: 255 х 341 nm *H. Tanoto et al., Greatly enhanced continuous-wave terahertz emission by nanoelectrodes in a photoconductive photomixer, Nature Photonics, January 2012

10 Proposal 3. SPP waves and plasmonic antennas q Enhance the performance of THz signal generators and detectors 1. Increase efficiency 2. Decrease losses In theory, operational at room temperature* [!] Many decisive applications envisioned * J. M. Jornet and I. F. Akyildiz, "Graphene-based Plasmonic Nano-antenna for Terahertz Band Communication in Nanonetworks," IEEE Journal on Selected Areas in Communications (JSAC), December 2013

11 Envisioned application (1) Backhaul for mmwaves cell q Backhaul rate should be higher than of the fronthaul o GHz o Static link o Alignment during the installation o Low interference with mmwaves spectrum

12 Envisioned Application (2) Terahertz Information Shower Main features: q Data rates: up to 100Gbit/s* q Communication rage: 0.1-5m One of the potential deployment strategies for THz access points Areas to be deployed in: q Gates with high traffic Metro, highway entrance q Dense environments Shopping mall, airport *IEEE d Application Requirements Document, IEEE /0304r16, May 2015

13 Envisioned application (3) Security-sensitive communication q Health monitoring, E-payments, etc. q Similar benefits as for military: o Fast signal degradation with distance o Substantial bandwidth for almost any handshakes Beneficial to study the suitability of: o PHY layer security o ID-based crypto systems

14 Envisioned application (4) Ubiquitous connectivity with micro-world q Micro-scale communications between everything Ø THz and VLC are almost the only solutions, operational at both micro- and macro-scales *I. F. Akyildiz, J. M. Jornet and C. Han, "Terahertz Band: Next Frontier for Wireless Communications," Physical Communication (Elsevier) Journal, September 2014

15 Envisioned application (5) On-board communications q May solve complexity and scalability issues q Homogeneous system structure q Capacity of THz channel is sufficient for on-board and intra-chip communications *Q. J. Gu, "THz interconnect: the last centimeter communication," in IEEE Comm. Mag. April 2015

16 Envisioned application (6) Terahertz mobile access Illustration from Akyildiz et al. Terahertz band: Next frontier for wireless communications, 2014 System-level performance analysis is required Link level characteristics: q Extensive bandwidth: THz q Theoretical capacity: Tbits/s q Effective communication range: <50m Truly 5G (Beyond 5G) technology [!] Is THz comm a silver bullet? - No

17 Major challenges with THz mobile access A Eff = λ 2 4π q Design of THz electronics: so-called THz gap q Molecular absorption at THz q Inherently small antenna size o o Issues with heat dissipation Issues with communication range (high path loss) [!] Leading to the fundamental limits

18 Limit 1: Antenna heat dissipation Approach We balance the consumed and the dissipated power and apply the Stefan Boltzmann law: T a antenna temperature, T r room temperature, η antenna efficiency, h air air heat transfer coefficient, σ Stefan Boltzmann const. Antenna size: λ/3/2

19 Limit 1: Antenna heat dissipation Results Temperature < 50 C: q 0dBm till 300GHz q -10dBm till 1THz q -20dBm till 3THz Higher in either frequency or power? Other radiation principles Larger number of elements [!] Massive antenna arrays are needed

20 Limit 2: Path loss at THz frequencies Approach We compare two cases: 1) Directional + Omni (MxM + 1) 2) Directional + Directional (MxM + MxM) Let us write a path loss equation (S target SNR): For the free-space path loss, range can be expressed as

21 Limit 2: Path loss at THz frequencies Results Parameters: q P Tx = 0dBm q Target SNR = 5dB q 10GHz bandwidth Effective communication range: Dir + Omni: <2m Dir. + Dir.: <50m [!] Both Tx and Rx antennas have to be directional to get reasonable range

22 Limit 3: Efficiency of distributed MAC Approach For the sake of example, we assume legacy IEEE distributed coordination function (DCF) and OFDM. Then, spectral efficiency is limited to: Finally, if N FFT is the number of OFDM symbols and SIFS consists of 4 symbols, maximal spectral efficiency is

23 Limit 3: Efficiency of distributed MAC Results Parameters: q aka. IEEE signaling assumed q SIFS = 4 OFDM frames q N FFT = 512 Effective range (for 10 users): 10ms TXOP: 50m [5G]: 1ms TXOP: 15m 0.1ms TXOP: 5m [!] Latency-bounded applications have to either work over short links or apply centralized MAC

24 Conclusions: practical limitations of THz mobile access Outcomes: 1. Massive antenna arrays MUST be used 2. Antennas MUST be directional AT BOTH Tx and Rx 3. Intelligent MAC MUST be used for delay-critical apps Envisioned colonization of the THz spectrum: GHz by IEEE d Task Group o High antenna gains (20dBi+) o [!] Sketch is to be ready by mid Around 1THz and 1-1.5THz by leading academic units o Graphene/CNT/plasmonic nano-antennas/etc. o Extreme antenna gains (50dBi+) 3. Micro-scale communications with individual (~omnidirectional) antennas at 1THz+ by academia

25 Summary: Open R&D challenges Ø THz band is, most probably, a next frontier for wireless communications, immediately after mmwaves q Major advantage: o Potentially Tbit/s wireless links few meters long q Major issues: o Hardware / electronics o Propagation: Absorption and small antenna area q Major unsolved communication challenges: q PHY: Reliable P2P interaction over the THz band LoS blockage, massive scattering, high pathloss q Link: Channel access with dynamic beam steering q Network: Nodes discovery and addressing [!] Huge room for further R&D