TERAPOD is a Horizon 2020 project funded by the European Commission (Grant number: 761579). The project aims to investigate and demonstrate the feasibility of ultra-high bandwidth wireless access networks operating in the terahertz (THz) band. The proposed TERAPOD THz communication system will be developed, driven by “beyond 5G” (B5G) usage scenario requirements, will be demonstrated in a “first adopter” operational setting (a data centre) and will significantly progress innovations across the whole communications protocol stack.
TERAPOD pursues the ambitious vision of the short-range Tbps wireless connectivity paradigm, by exploiting 3 of the most promising emerging THz device technologies, namely resonant tunneling diodes, uni-traveling carrier photodiodes, and Schottky barrier diodes to enable the development and integration of the building blocks required for ultra-broadband communications in the Terahertz spectrum. TERAPOD employs a holistic approach where multiple technologies are explored simultaneously. An indoor Datacenter scenario is targeted to demonstrate the feasibility of ultra-high bandwidth short range Terahertz communication. A packaged device is intended to demonstrate the feasibility covering channel and device characterization, antenna design with appropriate Physical, MAC and Network layer communication protocols.
The projects aim to implement and demonstrate technological advancements for Datacenter environment in the following domains,
- Reliable, high efficiency and high-power THz RTD sources
- Low barrier diodes for operation as THz mixer
- Power combination of multiple THz sources
- Novel measurement and characterization techniques for THz devices
- Channel measurements and characterization for Datacenter environment
- Novel substrate integrated THz antennas
- PHY and MAC layer THz communications protocols targeting various use cases
- Standardization and Regulation (IEEE, ITU, WRC)
The aim of TERAPOD is to investigate and demonstrate the feasibility of ultra-high bandwidth wireless access networks operating in the Terahertz band. The project will focus on end to end demonstration of the THz wireless link within a Data Centre as a Proof of Concept deployment, while also investigating other use cases applicable to beyond 5G. The project seeks to bring THz communication a leap closer to industry uptake through leveraging recent advances in THz components, a thorough measurement and characterization study of components and devices, coupled with specification and validation of higher layer communication protocol specification. Mainly, there are four key TERAPOD objectives which underpin the project work plan:
- Advance the Technology Readiness Level (TRL) of THz communication devices and systems from the laboratory towards industrial and SME uptake, within the context of B5G usage scenario requirements.
- Demonstrate the feasibility of THz communication systems in B5G scenarios through a fully integrated “first adopter” data centre demonstrator.
- Address the non-technical barriers to adoption of THz communication in the areas of regulation and standardisation.
- Promote scientific research and innovation of THz communications systems in Europe
1. Terapod mixers and transceivers
The communications links at 300 GHz envisaged in TERAPOD require a new generation of transceivers and mixers. Fig. 1 shows a 300 GHz transceiver developed at ACST which features a 300 GHz transmitter and a 300 GHz receiver which is intended for frequency-modulated continuous-wave (FMCW) radar applications and imaging. This technology will be developed in the scope of the TERAPOD project
Figure 1: A 300 GHz transceiver developed at ACST. Figure 2: Schottky-based subharmonic 300 GHz mixer from ACST.
Fig 2 shows a sub-harmonic mixer with local oscillator frequency from 135-160 GHz and RF input frequency from 270-320 GHz. It is able to down-convert a signal in the 300 GHz frequency range to an intermediate frequency (IF). The IF signal is matched with 50 Ω and can be tuned from DC-18 GHz. This device is used for heterodyne reception, typically for telecommunication systems, radars, imaging etc. The Noise Figure (NF) represents the degradation (in dB) of the signal-to-noise ratio (SNR) of the RF signal after the receiver. The Noise Temperature is another method of quantifying this effect using a different notation.
Figure 3: The noise performance for the mixer.
2. Terahertz 25 Gbps Wireless Bridge
The main aim of TERAPOD is to enable the use of THz wireless technologies in data centres. This breakthrough would improve network performance in general and in particular, would greatly enhance the capacity for switching and reconfigurability. A key part of the first year of the project has been the development of a bench-top laboratory demonstrator. It is important to recognise that the core of the network in a data centre is based on optical links. It was therefore decided that the key user-based scenario would be a wireless bridge which could connect photonic network elements.
At the laboratory of one of the TERAPOD partners (UCL), the first bench-top demonstration of such a THz wireless bridge integrated on an optical network link has been performed. The demonstration was done using TERAPOD devices for both the transmitter and receiver. The bridge achieved a record throughput of 25 Gbps for a single change and 75 Gbps using three optical channels transmitted simultaneously across the wireless link.
Figure 4: The 25 Gbps wireless bridge at the UCL labs.
This early demonstration shows the viability of the TERAPOD system plan for integration with a data centre network and was presented at OFC 2018: H. Shams, T. Li, C. C. Renaud, A. J. Seeds, R. Penty, M. Fice and I. White Digital Radio over Fiber Distribution using Millimetre Wave Bridging OFC 2018, paper Th2A.69; https://doi.org/10.1364/OFC.2018.Th2A.69
The project work continues to develop this lab set-up into a robust packaged system solution suitable for full demonstrations in situ in a real data centre at TERPOD partner Dell-EMC in Cork, Ireland.
3. Channel measurement at DELL EMC and TSSG Data Centres
The measurement campaigns were carried out to measure the characteristics of Terahertz channel.
Figure 5: Measurement setup Figure 6: Power Delay Profile
Two measurement campaigns were carried out in Cork and Waterford within two data centers, measurements were collected for the top of the rack channel and inter-rack links, taking into account the presence of reflectors with different electromagnetic properties. Recently the channel signature was extracted; it consists of power delay profile, the attenuation profile and AOD/AOA mapping.
4. Communication protocols:
TSSG will be responsible for communication protocols model and simulation for TERAPOD project, the aim is to boost THz wireless throughput to 100Gbps and to push the communication delay to less than 1ms using the 300 GHz transmission window. During the first year of the project, models were proposed for the physical layer, data link layer and infrastructure and basic simulation was performed. Models consist of physical layer block diagrams, DLL modules such as framing, error control and buffering and possible topologies to be considered.
Networking functionalities will be also investigated, it includes nodes discovery, nodes relaying, path diversity and routing and link management. Figure 4.1 shows the evolution of buffer memory for different DLL considerations, Figure 4.2 the evolution of P2P throughput as a function of BER, BER statistics should be gathered from the demonstrated.
Figure 7: Evaluation of number of frames in buffer. Figure 8: Achieved throughput for different scenarios.
5. The Device Characterization
THz device characterisation TERAPOD will produce a range of THz transmitters and receivers based on RTD, UTC-PD and SBD technologies. These devices need to be carefully characterised in order to quantitatively evaluate and compare their performance. The challenging task of obtaining accurate measurements in the THz domain is being addressed by the TERAPOD partner NPL (Teddington, UK). It has established a test apparatus based on a lamellar interferometer for measuring the broadband frequency spectrum of devices, and a set-up for determining the beam profile of THz emitters.
Heterodyne detection has long been established as the most accurate, high-resolution, traceably calibrated technique for measuring signal frequencies and spectral profiles. Heterodyne measurements use a local oscillator (LO) of known frequency, amplitude and phase which is mixed with the source signal to produce a signal at the difference frequency that is proportional to the amplitude of the source. This lower frequency output signal can be more easily detected and analyzed using low-frequency circuits.
Unfortunately, laboratory signal analysers are not suitable for broad spectral profile THz sources since the bandwidth is limited by the input waveguide. Instead a free-space broadband interferometer may be employed: the interferogram produced by the device is a Fourier transform of the source spectrum, which can be recovered by applying an FFT (fast Fourier transform) algorithm to the acquired data. In order to fully characterize a THz emitter, its spectrum must be measured using both a narrow-band heterodyne signal analyzer and a broadband free-space optical interferometer, which is the focus of NPL work.
Lamellar interferometer: A type of interferometer that is particularly suitable for broadband spectroscopy at THz frequencies is a Michelson interferometer with a lamellar mirror, where a split mirror acts as both a beam-splitter and a moveable mirror. The lamellar mirror consists of two parts, each comprising several lamellae or “fingers”, with one part being fixed and the other moveable (Fig. 4). Although the frequency resolution is low (~1 GHz, limited by the mirror scan length) this design avoids using a separate beamsplitting element, is polarization insensitive and can be ultra-broadband.
Figure 9: Schematic showing principle of operation (left) and photograph (right) of the lamellar split mirror at NPL.
Emitter beam profile measurement: Characterization of emitter (transmitter) beam profiles and detector (receiver) acceptance cones has long since been accepted as an essential tool in designing microwave and mmwave communication systems. There are extensively developed and well understood techniques for antenna characterisation, and specialised facilities are available to perform the required measurements. However, none of these as yet exists for THz devices. Electronic THz emitters produce relatively low powers (commonly <100 µW), have short wavelengths (<1 mm), and there is a lack of compact, high sensitivity detectors. These factors combine to make spatial characterisation of THz beams severely challenging.
A THz beam profiler for laboratory use has been established at NPL. The apparatus is based on a commercial pyroelectric detector and features an aperture which is raster scanned across the emitter. An example of the output is shown in Fig 10.
Figure 10: Test measurement of Gaussian THz emitter using the NPL THz beam profiler.
Dr. Alan Davy (Waterford Institute of Technology)