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D5.3 – Report on final THz RF-frontend and antenna and optical RF-frontend for real-time demonstration

TERRANOVA Project of 53

This project has received funding from Horizon 2020, European Union’s

Framework Programme for Research and Innovation, under grant

agreement No. 761794

Deliverable D5.3 Report on final THz RF-frontend and antenna and optical RF-frontend for real-time demonstration Work Package 5 – THz System Technology

TERRANOVA Project

Grant Agreement No. 761794

Call: H2020-ICT-2016-2

Topic: ICT-09-2017 - Networking research beyond 5G

Start date of the project: 1 July 2017

Duration of the project: 30 months

Ref. Ares(2019)4576251 - 15/07/2019



Disclaimer This document contains material, which is the copyright of certain TERRANOVA contractors,

and may not be reproduced or copied without permission. All TERRANOVA consortium

partners have agreed to the full publication of this document. The commercial use of any

information contained in this document may require a license from the proprietor of that

information. The reproduction of this document or of parts of it requires an agreement with

the proprietor of that information. The document must be referenced if used in a publication.

The TERRANOVA consortium consists of the following partners.

No. Name Short Name Country 1

(Coordinator)

University of Piraeus Research Center UPRC Greece

2 Fraunhofer Gesellschaft (FhG-HHI & FhG-IAF) FhG Germany 3 Intracom Telecom ICOM Greece 4 University of Oulu UOULU Finland 5 JCP-Connect JCP-C France 6 Altice Labs ALB Portugal 7 PICAdvanced PIC Portugal

Document Information

Project short name and number TERRANOVA (761794) Work package WP5 Number D5.3 Title Report on final THz RF-Frontend and

Antenna and optical RF-frontend for real-

time demonstration

Version v1.0 Responsible unit FhG Involved units FhG, PIC, ICOM, UPRC Type1 R Dissemination level2 PU Contractual date of delivery 30.06.2019 Last update 15.07.2019

1 Types. R: Document, report (excluding the periodic and final reports); DEM: Demonstrator, pilot, prototype, plan designs; DEC: Websites, patents filing, press & media actions, videos, etc.; OTHER: Software, technical diagram, etc. 2 Dissemination levels. PU: Public, fully open, e.g. web; CO: Confidential, restricted under conditions set out in Model Grant Agreement; CI: Classified, information as referred to in Commission Decision 2001/844/EC.



Document History Version Date Status Authors, Reviewers Description

v0.1 16.05.2019 Draft Thomas Merkle (FhG-IAF) Initial definition of a

document structure

v0.2 31.05.2019 Draft Carlos Castro (FhG-HHI),

Robert Elschner (FhG-HHI)

Refined structure for

Section 4

v0.3 24.06.2019 Draft Ricardo Ferreira (PIC), Ana

Tavares (PIC), Francisco

Rodrigues (PIC), Alexandros-

Apostolos A. Boulogeorgos

(UPRC)

Input for the IM/DD

system in Section 4, input

for Section 3 on spatial

MIMO

v0.4 01.07.2019 Draft Ricardo Ferreira (PIC) Review of Section 4

v0.5 09.07.2019 Draft Thomas Merkle (FhG-IAF),

Robert Elschner (FhG-HHI),

Carlos Castro (FhG-HHI)

Input to Section 3, input to Section 2, merging documents

v0.6 10.07.2019 Draft Thomas Merkle (FhG-IAF) Input to introduction and conclusions. Input to Section 3.

v0.7 11.07.2019 Draft Carlos Castro (FhG-HHI),

Ricardo Ferreira (PIC), José

Machado (ALB), Thomas

Merkle (FhG-IAF)

Revision of structure of Section 4, review of Section 2. Finalization of Section 3, Introduction and Conclusion.

v0.8 12.07.2019 Draft Thomas Merkle (FhG-IAF),

Carlos Castro (FhG-HHI),

Ricardo Ferreira (PIC),

Alexandros-Apostolos A.

Boulogeorgos (UPRC),

Evangelos Papasotiriou

(UPRC), Giorgos Stratidakis

(UPRC), Angeliki Alexiou

(UPRC), Dimitrios Kritharidis

(ICOM), Janne Lehtomäki

(UOULU)

Completion of Executive Summary, review of complete document by all partners, merging all corrections

v1.0 15.07.2019 Final Thomas Merkle (FhG-IAF)

Angeliki Alexiou (UPRC)

Final version for

submission



Acronyms and Abbreviations

Acronym/Abbreviation Description A ADC Analog-to-Digital Converter ATDE Adaptive Time Domain Equalizer AWG Arbitrary Waveform Generator AWGN Additive White Gaussian Noise B B2B Back-to-Back BB Base Band BER Bit Error Rate BF Beamforming C CoRx Coherent Receiver CoTx Coherent Transmitter COTS Commercial Off-The-Shelf / Components Off-The-Shelf CPR Carrier Phase Recovery D DAC Digital to Analog Converter DDS Direct Digital Synthesis DL Down Link DP-IQ Dual Polarization In-phase and Quadrature DRO Dielectric Resonator Oscillator DSP Digital Signal Processing E ECL External Cavity Laser E/O Electrical-Optical ENOB Effective Number of Bits ER Extinction ratio EVM Error Vector Magnitude F FEC Forward Error Correction FPGA Field-Programmable Gate Array H HC High Capacity I I/Q In-phase and Quadrature IEEE Institute of Electrical and Electronics Engineers IF Intermediate Frequency IM/DD Intensity Modulation/Direct Detection ITU International Telecommunication Union ITU-R Radiocommunication sector of the International

Telecommunication Union

L LO Local Oscillator LoS Line of Sight M MAC Medium Access Control



MIMO Multiple Input Multiple Output mmWave Millimeter Wave N NGPON2 Next-Generation Passive Optical Network 2 nLoS Non-Line Of Sight NRZ Non-Return to Zero O OSNR Optical Signal to Noise Ratio P P2MP Point-to-Multi-Point P2P Point-to-Point PAM Pulse Amplitude Modulation PDM Polarization-Division Multiplexing PHY Physical Layer PLL Phased Locked Loop PONs Passive Optical Networks PRBS Pseudo random bit sequence PS Phase shifter Q QAM Quadrature Amplitude Modulation QPSK Quadrature Phase Shift Keying R Rx Receiver S SD-FEC Soft-Decision Forward-Error Correction SISO Single Input Single Output SLS Sector Level Sweep (Phase) SMF Single Mode Fibre SNR Signal to Noise Ratio T TERRANOVA Terabit/s Wireless Connectivity by Terahertz innovative

technologies to deliver Optical Network Quality of Experience in Systems beyond 5G

THz Terahertz Tx Transmitter U UL Uplink ULA Uniform Linear Array UE User Equipment V VCO Voltage Controlled Oscillator VOA Variable optical attenuator W WDM Wavelength Division Multiplexing WP Work Package X XG-PON 10 Gbit/s Passive Optical Network XFP 10Gbit/s Format Pluggable



Contents

1. Introduction ...................................................................................................................... 12

1.1 Scope ....................................................................................................................... 12

1.2 Structure .................................................................................................................. 12

2. System Component Design .............................................................................................. 13

2.1 Final system concept ............................................................................................... 13

2.1.1 THz-only demonstration .................................................................................. 13

2.1.2 Extension to hybrid optical / THz demonstration ........................................... 14

2.1.3 Specification of the 4-Ch BB signal interface ................................................... 14

2.2 Final system component specifications................................................................... 15

2.2.1 DAC/ADC specifications ................................................................................... 15

2.2.2 Baseband amplifier specifications ................................................................... 15

2.2.3 References ....................................................................................................... 16

3. THz Frontend Design ........................................................................................................ 17

3.1 Frontend antenna and integration .......................................................................... 17

3.1.1 Baseband signal mapping and system requirements ...................................... 17

3.1.2 Full-duplex operation ...................................................................................... 19

3.1.3 Basic MIMO considerations for the antenna design ....................................... 19

3.1.4 Fibre-optical THz-wireless antennas ............................................................... 21

3.1.5 Beamforming antennas ................................................................................... 24

3.2 Frontend components for the demonstrators ........................................................ 28

3.2.1 Receiver baseband amplifiers ......................................................................... 28

3.2.2 RTX frontend modules ..................................................................................... 31

3.3 References ............................................................................................................... 34

4. Electro-Optical Integration ............................................................................................... 35

4.1 Optical hardware components ................................................................................ 35

4.1.1 Coherent based optical link ............................................................................. 35

4.1.2 IM/DD optical link ............................................................................................ 42

4.2 Towards system demonstration .............................................................................. 47

4.2.1 Coherent based optical link ............................................................................. 47

4.2.2 IM/DD optical link ............................................................................................ 50

4.3 References ............................................................................................................... 52

5. Conclusions ....................................................................................................................... 53



List of Figures

Figure 1: Block diagram for THz-only demonstration. ............................................................ 13

Figure 2: Block Diagram for hybrid optical - THz demonstration. ........................................... 14

Figure 3: Dual antenna options for the signal mapping between the optical and THz wireless

link. (a) Transmitting and receiving channel separated by frequency, or (b) by polarization.

TERRANOVA targets the Tx/Rx separation by frequency. However, in intermediate steps also

the separation by polarization is required (see also Section 3.1.2). ....................................... 18

Figure 4: Antenna spacing as a function of the transmission distance for different centre

frequency values. .................................................................................................................... 21

Figure 5: CAD design of the high capacity Tx respectively Rx antenna, ready for fabrication.

Approx. size: 200 x 200 x 250 mm³ (LxWxH). The housing includes all analogue frontend

components and connects to the THz optical interface by V-connector cables. .................... 22

Figure 6: Back panel of the preliminary high capacity Tx respectively Rx antenna. The labels

indicate the different interfaces. The final version will include a CFP2-ACO cage. ................ 22

Figure 7: Measurement of the cross-talk between the two Cassegrain antennas. The

calibration reference plane is at the flange of the feed antenna, all measurements are

scalar measurements. ............................................................................................................. 23

Figure 8: Comparison of measured cross-talk at 300 GHz. (a) Measurement to the sky, (b)

measurement with wooden board in 1 m distance in front of the antenna, level sensitive to

the tilt of the board. ................................................................................................................ 24

Figure 9: The 4-channel antenna configurations for beamforming tests (front view). (a) 4

circular horn antennas with 23 dBi gain, element pitch 21.5 mm, (b) 4 rectangular tapered

horns of 16 dBi gain, element pitch 1.25 mm ......................................................................... 25

Figure 10: Block diagram of the basic 4-channel beamforming frontend for digital

beamforming experiments in WP6 ......................................................................................... 25

Figure 11: Back panel of the preliminary beamforming demonstrator 4-channel frontend, top

view shows the I/Q interface. The LO reference signal output can be reconfigured to either

work with an internal or external LO. ..................................................................................... 26

Figure 12: Typical phase noise performance of the reference oscillator at 8.244 GHz. The

carrier power is about 2 dBm. ................................................................................................. 26

Figure 13: Design of experiment for testing the stability of different LO distribution schemes.

(a) DDS based LO generation at 2.061 GHz, (b) DRO based LO generation and distributing at

8.244 GHz, (c) DRO based LO generation at 8.244 GHz and distributing at 100 GHz. ............ 27

Figure 14: CAD of the implementation of the 4-channel frontend. ........................................ 27

Figure 15: Implementation of the variation with a 1:4 LO-splitter at 100 GHz....................... 28

Figure 16: CAD of 2 channel amplifier module used in the WP6 high capacity demonstrator.

The concept includes the same PCB interconnects as used in the RTX module. .................... 29

Figure 17: Fabricated and assembled baseband amplifier test module. (a) Outside view with

connection to the programming interface of the microcontroller, (b) view on chip and RF

printed circuit board. ............................................................................................................... 30

Figure 18: Measured S-parameters of the baseband amplifier test module for the two

channels in use for an internal bias of 4 V. ............................................................................. 30

Figure 19: Dual antenna 2x2 MIMO link for lab bench testing with low gain horn antennas.

Full duplex operation is achieved by two of these links, which are spatially separated. ....... 31



Figure 20: Functional block diagram of the RTX module used in the first phase of WP6. ...... 31

Figure 21: CAD and fabricated RTX module with 23 dBi horn antennas. ................................ 32

Figure 22: Example of the lab bench options with the frontend prototype modules. ........... 32

Figure 23: Test setups for the RF characterization of the Rx and Tx channels. ...................... 32

Figure 24: Measured output power versus IF power and frequency of the Tx channel. ........ 33

Figure 25: Measured conversion gain versus IF frequency at an RF input power of -30 dBm

for the in-phase and quadrature signals of the Rx channel. ................................................... 34

Figure 26: (a) High-level block diagram of a CFP2-ACO. (b) 100G – 200G Tunable CFP2-ACO

from Finisar [4-2] ..................................................................................................................... 35

Figure 27: Block diagram of the CFP2-ACO’s transmitter module. The green box highlights

the actual electro-optical components used in the transmitter’s setup. ............................... 36

Figure 28: Input and output electrical interfaces of the CFP2-ACO. ....................................... 37

Figure 29: Block diagram of the CFP2-ACO’s receiver module. The green box highlights the

actual electro-optical components used in the receiver’s setup. ........................................... 38

Figure 30: Noise-loading setup used to measure the BER vs. OSNR relation of a CFP2-ACO for

32 GBd m-QAM formats. Optical power at the RX port of the CFP2-ACO is kept constant by

means of a VOA, which sets the power to -10 dBm. ............................................................... 39

Figure 31: BER vs OSNR measurements from the real-time optical modem. Experimental data

for m-QAM formats has been compared to the theoretical expectation of said modulation

formats. ................................................................................................................................... 40

Figure 32: Constellation diagrams for back-to-back (a) 32 GBd 4-QAM, (b) 32 Gbd 8-QAM,

and (c) 32 GBd 16-QAM as measured at the optical modem. ................................................ 41

Figure 33: Target THz system using a MIMO configuration combined with an IM/DD optical

transmission system for the central office-to-radio direction. ............................................... 42

Figure 34: Experimental setup used to transmit one of the two components (I or Q) of the

THz signal. ................................................................................................................................ 43

Figure 35: Laboratory infrastructure: (a) real-time oscilloscope; (b) AWG; (c) MZM;

(d) photodiode; (e) RF driver for MZM; (f) RF driver after photodiode;

(g) and (h) power supply for RF driver; (i) polarization controller for the laser. ..................... 44

Figure 36: Experimental results with the BER vs. received power

for QPSK (NRZ) and 16QAM (4-PAM). ..................................................................................... 45

Figure 37: Spectrum and Eye Diagram samples at -2 dBm before (a) and after (c) signal

equalization for QPSK, and before (b) and after (d) signal equalization for 16QAM for B2B. 45

Figure 38: Experimental setup used to transmit the in-phase and quadrature component

of the THz signal. ..................................................................................................................... 46

Figure 39: Sample of the eye diagram of the in-phase (a, d) and

quadrature (b, e) components and constellation (c, f) at 3 dBm

respectively for QPSK (a, b, c) and 16QAM (d, e, f) after signal equalization. ........................ 47

Figure 40: Block diagram of the fibre extension use case based on THz technologies. The

CFP2-ACO and the Optic-THz interface ensure the transformation of the optical signals into

electrical waveforms, which are fed into the THz elements for data wireless transmission. . 48

Figure 41: Front panel of the preliminary 100G/200G coherent optic-THz interface as

illustrated in Figure 40. The labelling indicates the different parts of this interface, which are

further described in the text below. ....................................................................................... 48

Figure 42: Chassis for the optic-THz interface with a CFP2-ACO. ........................................... 49

Figure 43: Amplitude voltages used in the experimental validation for the I or Q component,

considering 0 dBm as optical received power. ........................................................................ 51



List of Tables

Table 1: Specification of the 4-channel baseband signal interface. ........................................ 14

Table 2: Specification of the 4-channel DAC/ADCs. ................................................................ 15

Table 3: Specification of the baseband amplifiers. ................................................................. 16

Table 4: Key specifications of the Cassegrain antenna. .......................................................... 21

Table 5: Initial specifications of the baseband amplifiers. ...................................................... 29

Table 6: Transmitter specifications for a class 2 CFP2-ACO client. ......................................... 37

Table 7: Receiver specifications for a class 2 CFP2-ACO client. .............................................. 39

Table 8: System specifications ................................................................................................. 50

Table 9: Equipment requirements........................................................................................... 50

Table 10: System specifications ............................................................................................... 52



Executive Summary

The deliverable “D5.3 - Report on final THz RF-frontend and antenna and optical RF-frontend

for real-time demonstration” completes the research and development on the “THz System

Technologies” as part of Work Package (WP) 5. The most important goal of the work package

was to develop the component technologies required in WP6 (“THz Demonstrator

Implementation and Validation”) to a level “ready for implementation”. The practical

implementation within the two system demonstrators will be actually part of WP6. This will

also include the optimization and re-design of some of the components where necessary. One

of the implications of the work in WP6 will be the development of co-design capabilities by

validating the prototype components in the system, by system modelling and by re-design and

optimization. All THz components, which are required in WP6 to start the system

demonstrations, were designed and fabricated respectively are in fabrication and will return

for assembly and testing within the next 4-8 weeks. The design of the components also

provided a good insight into the real-world implementation problems of hybrid fibre optical –

THz wireless links. With the designed set of components it will be also possible to

experimentally investigate the stability of THz beamforming systems and validate beam

forming algorithms as for example proposed in D5.2.

The main high-level objectives of this deliverable are:

Consider intermediate test setups for testing sub-systems of the final demonstrators

Finalize the system architectures to be demonstrated and the interface specifications

Design all hardware components that are required for the system implementation,

and

Experimentally validate critical sub-systems and key hardware components.



1. INTRODUCTION

One of the major objectives of work package (WP) 5 of TERRANOVA was to develop the “THz

System Technologies” required for the implementation of the planned demonstrators in WP6.

The work on the components in WP5 touched several important interdisciplinary topics, e.g.

spectral efficient THz transceivers above 200 GHz, end-to-end optimized hybrid optical – THz

wireless links, signal conversion between optical and electrical carriers, coherency and

synchronization, and THz antennas for applications that require high gain antennas or

beamforming antennas. With the completion of WP5 many of the practical implications of the

TERRANOVA candidate architectures were understood in more detail. This gives not only

important insight on the co-design and optimization of an E2E analogue hybrid optical-THz

wireless communication system, but also on the main cost drivers of commercial

implementations of such concepts. The stability of THz beamforming communication systems

is a topic hardly covered. The stability of THz communication systems is not only a function of

the bandwidth of the modem, but depends also on the phase noise mask and required thermal

and mechanical stability of the components. The designed components and experiments

behind in WP5 provide a platform to assess and resolve some of these critical questions with

impact on system architecture decisions.

1.1 Scope

The deliverable “D5.3 - Report on final THz RF-frontend and antenna and optical RF-frontend

for real-time demonstration” completes the research and development on the “THz System

Technologies” as part of Work Package (WP) 5. This deliverable focuses on the design of the

required hardware components in WP6. A critical milestone has been achieved with finalizing

all THz system component designs. The fabrication of most of the components has been

completed or will be completed within the next 4-8 weeks, which also means that the

experimental work in WP6 can start with minor delays.

1.2 Structure

The organization of this document is as follows:

Section 2 (System Component Design) presents the final system concepts to be tested in

WP6 and the interface specifications to which the components need to comply with. All

subsequent chapters cover the required hardware components for the implementation.

Section 3 (Frontend Design) summarizes the designed THz RF frontend and antenna

components and their integration. It covers as well the mapping of the two polarizations

of the optical fibre to the THz frontend and some of the differences that need to be

considered in intermediate lab bench tests. The antenna for the high capacity

demonstrator was evaluated for the implementation of a full-duplex link in that context.

The design, fabrication and test of the analogue THz frontend modules together with the

baseband receiver amplifiers are finally summarized.

Section 4 (Electro-Optical Integration) addresses the hardware implementation of the

required interfaces between the fibre optical and THz wireless link. The section focuses

first on the optical components as part of the optical link, which are characterized stand-

alone for the IM/DD link and the coherent link (back-to-back), meaning without the THz

wireless link connected. Furthermore, the modem of the coherent optical link is tested

with the THz wireless link, with no optical link connected.



2. SYSTEM COMPONENT DESIGN

For the successive implementation of the demonstrators for WP6, the THz frontend acts either

as a host or client for the optical link. For simplicity, in Section 2.1.1, the optical fibre is omitted

and the THz frontend directly connects to the baseband unit. In this case the THz frontend is

the client. In the case of the final hybrid fibre optical – THz wireless link of Section 2.1.2, the

THz frontend acts as a host. A management data input/output (MDIO) interface is used to

send and receive control signals, information regarding the status of the elements, and data

signals between the ACO host and the ACO client. Furthermore, the MDIO interface can be

also used to implement adaptive schemes based on the status of the transmission system.

Following this, the MDIO interface is used in the final demonstration of an optical - THz system

not only to transport the data signals from the baseband unit to the ACO client, but also to

optimally adjust the operating point of the different stages in the optical transponder. In this

regard, the MDIO interface is in charge of controlling the driving voltages of the amplitude

and phase optical modulators, the activation and configuration of optical attenuators,

activation and adjustment of the Tx laser, flag readout, among others.

2.1 Final system concept

In this section, we will describe the final system concepts for the demonstrations in

TERRANOVA. Most importantly, the baseband interfaces between the different building

blocks will be identified and specified to fulfil the requirements from the DAC/ADC

components of the real-time baseband unit.

2.1.1 THz-only demonstration

The first demonstration step is a THz-only demonstration, consisting of a real-time baseband

unit (BBU) and a THz link as shown in Figure 1. The real-time BBU comprises a 4-channel DAC,

a 4-channel ADC and a 2x2 MIMO MAC + PHY DSP. The THz link comprises 2x I/Q THz Tx

frontends, 4 horn antennas and 2x I/Q THz Rx frontends, as well as a set of 4 external baseband

amplifiers at the receive side in order to adapt the signal level to the ADC.

The interface between these two building blocks is a bi-directional 4-channel baseband signal

interface, required to support 4 analogue electrical baseband signals (2 x I/Q) that form the

2x2 MIMO I/Q signal at the THz carrier.

Figure 1: Block diagram for THz-only demonstration.

2x Tx

THz front-end

2x Rx

THz front-end

2x2 MIMOMAC + PHY

DSP

High-Speed 4-Ch ADC

High-Speed 4-Ch DAC

GbE

Control

Interface

BER

EVM

CSI

Computer

2x horn antennas

2x horn antennas

4x BB AMP

BBU Side THz Side

4-Ch BB Signal

Interface



2.1.2 Extension to hybrid optical / THz demonstration

For the extension to the hybrid optical - THz demonstration, an optical link is inserted between

the real-time BBU and the THz link, as shown in Figure 2. The optical link consists of two E/O

interfacing boards and the optical fibre connection. In order to keep the setup and the

components modular, the hybrid optical - THz demonstration will keep the same 4-channel

baseband signal interface as described before, but now having two of them.

Figure 2: Block Diagram for hybrid optical - THz demonstration.

2.1.3 Specification of the 4-Ch BB signal interface

One important implication of the definition of the 4-channel baseband signal interface in

Figure 1 and Figure 2 is that the interface needs to be symmetric, i.e. the input and output

directions need to support the same specifications, which are listed in Table 1.

Table 1: Specification of the 4-channel baseband signal interface.

Type Symmetric w.r.t. to input and output

Signal type Analogue electrical baseband signals

Number of channels Up to 4

Connector type Coaxial SMA (differential preferred)

Coupling AC

Termination 50 Ohm single-ended 100 Ohm differential

Signal level Minimum: 125mVpp single-ended 250mVpp differential Maximum: 350 mVpp single-ended 700 mVpp differential

Bandwidth >16 GHz

Linearity Support of multilevel analogue signals

2x Tx

THz front-end

2x Rx

THz front-end

2x2 MIMOMAC+PHY

DSP

High-Speed 4-Ch ADC

High-Speed 4-Ch DAC

GbE

Control

Interface

BER

EVM

CSI

Computer

2x horn antennas

2x horn antennas

BBU Side

4x BB AMP

CF

P2

-AC

O

Inte

rfa

ce

b

oa

rd

Inte

rfa

ce

b

oa

rd

Optical

connection

(SSMF)

CF

P2

-AC

O

E/O Side

4-Ch BB Signal

Interface

E/O Side THz Side

4-Ch BB Signal

Interface



2.2 Final system component specifications

To support the envisioned demonstrations, the DAC/ADC as well as the baseband amplifiers

will be specified in the following.

2.2.1 DAC/ADC specifications

In order to support symbol rates up to 32 GBd and modulation order up to 16-QAM, the

required DAC/ADC specifications are listed in Table 2.

Table 2: Specification of the 4-channel DAC/ADCs.

DAC/ADC bandwidth >16 GHz

DAC/ADC nominal resolution 8 bit

DAC/ADC ENOB [2-1] > 4 bit

Connector type SMA differential

DAC output signal level Typical: 125mVpp single-ended 250mVpp differential Maximum: 225mVpp single-ended 450mVpp differential

ADC input signal level Typical: 125mVpp single-ended 250mVpp differential Maximum: 350mVpp single-ended 700mVpp differential

If we were to increase the data rate of the system even further (i.e. upgrade the system a 64-

QAM format at a symbol rate of 32 GBd), we would need to revise the specifications of the 4-

channel DAC/ADCs accordingly. Compared to the information contained in Table 2, this

DAC/ADCs should exhibit an ENOB (Effective Number of Bits) value of at least 5.5 bit [2-1] and

input/output signal levels around 15-20% higher than the values for 32 GBd 16-QAM.

However, as the system’s symbol rate remains the same, the required bandwidth would stay

constant at > 16GHz.

2.2.2 Baseband amplifier specifications

With an estimated THz frontend output signal level of around 25 mVpp, the baseband

amplifiers need to comply with the specifications in Table 3.



Table 3: Specification of the baseband amplifiers.

Gain 20dB

Output power > 10 dBm

S11/S22 < -10 dB

Bandwidth > 20 GHz (3dB)

NF < 10 dB

Coupling AC

Connector type SMA differential

Automatic gain control Depends on expected link variability

2.2.3 References

[2-1] T. Pfau, X. Liu, S. Chandrasekhar, “Optimization of 16-ary Quadrature Amplitude

Modulation Constellations for Phase Noise Impaired Channels”, European

Conference on Optical Communications (ECOC), paper: Tu.3.A, Geneva, 2011.

[2-2] G. Khanna, el at, “Single-carrier 400G 64QAM and 128QAM DWDM field trial

transmission over metro legacy links”, IEEE Photonics Technology Letters, 29(2),

pp. 189-192.



3. THZ FRONTEND DESIGN

For the planned WP6 demonstrators, the THz wireless transmitter and receivers, the antenna

and the components that generate the local oscillator need to be integrated in a single

portable housing, which will be connected to the electro-optical components of the optical

fibre link. In the course of WP6, different components will be replaced by components of

higher integration or performance. The implementation strategy and timeline for the

demonstrators can be found in D6.4. This section presents the designed analogue THz

frontends and antennas, which are in fabrication or already available. The electro-optical

interface and integration of the hybrid THz wireless – fibre optical link are the subject of

Section 4.

3.1 Frontend antenna and integration

TERRANOVA requires for the experimental demonstrators different antenna solutions and

interfaces between the front-end and the antenna, which introduces an increased complexity

due to the needed variety of component designs. The HC (high capacity) demonstrator

requires very high gain antennas (>50 dBi), which should be capable of supporting two

polarizations. Cassegrain reflector antennas were chosen over lens antennas because they can

be implemented in a more planar form factor and offer more flexibility in the feed design3. In

addition, D2.2 and D5.1 discussed the use of two frequencies to separate the transmitting and

receiving channel (also further referred to as up and down link). The BF (beamforming)

demonstrator uses an existing 4 element fan-beam antenna array composed of individual

waveguide horns, spaced nearly half a wavelength apart. For the exploration of 4x4 MIMO

configurations, the horn array can be substituted by individual horn antennas. The following

sections discuss the impact of the baseband signal mapping on the complete frontend and

antenna solution and present the designed antenna components and frontend integration for

the demonstrators in WP6.

3.1.1 Baseband signal mapping and system requirements

The antenna solution and its integration with the frontend need to comply with the optical

coherent 2x2 MIMO transmission scheme of the fibre as discussed in multiple occasions of

the project (D2.2, D5.1, D5.2). IM/DD is an attractive alternative solution to optically transmit

the THz-wireless IQ-baseband components to a remote signal processing unit. The different

polarizations will be mapped to different optical wavelengths. This solution can be further

down-scaled to a SISO case.

WP6 has the goal to show the feasibility of the end-to-end (E2E) fibre-optical THz wireless

hybrid link by experimentally demonstrating different sub-systems but also a complete hybrid

optical-THz wireless link. The variety of options introduces a huge complexity to the

component design. The work was focusing on the 2x2 polarization MIMO case though it leads

to more complex frontends, mostly due to the high interface complexity. Two Rx and two Tx

with IQ baseband pairs in differential GSSG configuration must be routed in the analogue

domain (see also Section 4). Exploiting differential signals has many advantages. However, it

requires a tight integration with the optical modem and complicates the I/O periphery of the

3 Frensnel lenses can be also built with low profile. The feed design of the lens leads to a large volume since the focal length is in the order of the diameter. In comparison, the Cassegrain antenna with its subreflector reduces the volume when comparing very high gain configurations (>45 dBi).



chip. For this reason, the first generation of the components for the demonstrators will use

single-ended signalling, while a frontend with differential signalling based on the

specifications of this deliverable will be fabricated to replace this version in the very last phase

of the project.

Figure 3 gives an overview of the options using two separate antennas for the transmitting

and receiving channel. Eventually a single antenna solution will be desired, especially to

implement spatial MIMO schemes, as a means to further increase the link capacity.

Figure 3: Dual antenna options for the signal mapping between the optical and THz wireless

link. (a) Transmitting and receiving channel separated by frequency, or (b) by polarization.

TERRANOVA targets the Tx/Rx separation by frequency. However, in intermediate steps also

the separation by polarization is required (see also Section 3.1.2).

The two polarizations of the optical fibre, which carry each an IQ signal, can be mapped to

two frequencies or to two orthogonal polarizations of the THz link. The remaining degree of

freedom, either two polarizations or two frequencies, are used for the separation of the

down-link and up-link.

In both schemes, a combined optical-THz wireless 2x2 MIMO channel is created, which the

optical modem needs to compensate for (interference cancelation). The two signal mapping

possibilities as discussed in Figure 3 require different critical passive components for the

implementation (feed horns, orthomode-transducers, polarizers, diplexers and filters), which

in turn influence the frontend chip architecture, layout and packaging.

From an electrical point of view, the passive components at the antenna introduce critical

extra-losses, especially with tight frequency plans. The implementation with rectangular and

circular waveguide technology achieves the lowest losses possible. In contrast, planar

transmission line technologies typically lead to considerably higher losses (e.g. microstrip,

coplanar waveguide, centred striplines, etc.) unless the filter orders are reduced. Low cost

fabrication of rectangular waveguides, however, requires 2.5D micromachining technologies,

which can be composed of thin sheets. This technique can also be applied to the fabrication

of circular waveguides but they are limited to the broadside (perpendicular) direction. The

cost increases with the number of sheets and micromachining approaches require a large

market volume to achieve low-cost. In TERRANOVA, all waveguide components are fabricated

by CNC wire cutting suitable for prototyping, since the needed R&D quantities are too small

for micromachining.



3.1.2 Full-duplex operation

State-of-the-art commercial point-to-point links require full duplex (FDX) operation (i.e. the

simultaneous transmission in both directions) in order to fulfil the low latency requirements

of mobile networks beyond 5G, and in order to compete with fibre optical solutions. This is

even more important for the concept of TERRANOVA, since the available standard coherent

optical modems do not operate in time division duplex (TDD). The transmit and receive

channel can be separated in space, polarization or frequency to increase the isolation between

the channels. Most critical is the crosstalk of the transmitter to the receiver at the same node

(near end cross-talk) that saturates the receiver or distorts the incoming received signal from

the remote radio station (far end node). It is an engineering challenge to implement full duplex

solutions at THz frequencies. Although TERRANOVA focuses on specific aspects of hybrid fibre-

optical THz wireless links, full duplex is a prerequisite to properly test the wireless links in real-

time over long distances, since single channel loop-back operation [3-1] is complicated for this

scenario.

TERRANOVA’s choice is to separate the transmitting and receiving channel by frequency (see

Figure 3a). The polarization is exploited for mapping the two optical polarizations to comply

with the optical modem. This is the most intuitive approach and has less stringent

requirements on the orthogonality of the two modes since crosstalk can be cancelled in the

digital domain by the optical modem. A single antenna reflector solution is preferred to reduce

cost, form factor, and to be able to exploit spatial MIMO architectures. The architectures of

Figure 3 can be transferred to single antenna solutions by dual-feed reflectors. However, the

designs are complex and challenging in detail. Since the focus is placed on the proof of concept

of the overall hybrid fibre optical - THz-wireless link concept, the single antenna solution was

not investigated.

The polarization MIMO architecture can be implemented best by a dual-polarized single feed

antenna instead of using waveguide orthomode transducers (OMT). It also maintains

compatibility to the reflector antenna, which means that the reflector does not need to be re-

designed. The separation of the frequencies is achieved by two parallel links, which requires

only bandpass filters and omits additional diplexers in the antenna feed network. In contrast,

a spatial MIMO architecture requires diplexers for each antenna to separate the frequencies.

3.1.3 Basic MIMO considerations for the antenna design

Although the Tx/Rx separation by frequency is preferred (or in other words mapping the two

polarizations of the fibre to two polarizations of the antenna), intermediate experimental

steps for testing and debugging in the lab lead to additional architectures, as seen in [3-1].

Here, the two polarizations of the fibre are mapped to two spatially separated antennas. In

the ideal case these two links can be considered to be independent (dual simplex, DSX) but in

more realistic cases they have to be considered as line-of-sight (LoS) MIMO links.

Spatial multiplexing in the LoS MIMO system can be obtained by focusing the Rx arrays on the

Tx arrays. Once the arrays steer the beam along the desired direction, they can be considered

antenna elements (AEs) of a virtual MIMO system. Let us assume a 2 × 2 system with 2

antenna elements at both the TX and the RX and that the distance between the Tx and Rx is

R. The path difference between the 2 Rx AEs can be obtained as



𝛥𝐿 = √𝐷2 + 𝑅2 − 𝑅 ≈𝐷2

2𝑅

where D stands for the AE spacing. Moreover, the corresponding phase difference can be

evaluated as

𝜑 =2𝜋

𝜆𝛥𝐿 ≈

𝜋

𝜆 𝐷2

𝑅

where λ is the wavelength. By assuming a uniform linear array (ULA) antenna, the Rx array

response to the first and second Tx AEs can be respectively expressed as

𝒂1 = [1 exp(𝑗𝜑)]

and

𝒂2 = [exp(𝑗𝜑) 1]

Hence, the correlation can be obtained as

𝜌 =|𝒂1

𝐻 𝒂2|

||𝒂1||||𝒂2||

which, after some algebraic manipulations can be obtained as

𝜌 =sin(2𝜑)

2 sin (𝜑)

In order to nullify the correlation, we set

sin(2𝜑) = 0

and, since 𝜑 ≠ 0, the above expression lead to the following condition

2𝜑 = 𝜋

or equivalently

𝐷 = √𝑅 𝜆

2

Next, we graphically illustrate the antenna spacing selection as a function of the transmission

distance and the wavelength (Figure 4). From this figure, we observe that, for a given

frequency as the transmission distance increases, the antenna spacing should be increased.

Moreover, for a fixed transmission distance, as the frequency increases the antenna spacing

should be decreased. In other words, in order to select the appropriate antenna spacing, we

need to take into account the transmission frequency and distance. For this reason, a rather

flexible modular lab setup is used to explore the options in WP6 first experimentally before

starting higher system integration. The LoS MIMO architecture is difficult to be implemented

in a single module with fixed integrated antenna for lab experiments. This problem is

addressed by the modular concept of Section 3.2, which will be replaced by a functionally

higher integrated solution.



Figure 4: Antenna spacing as a function of the transmission distance for different centre

frequency values.

3.1.4 Fibre-optical THz-wireless antennas

The high capacity demonstrator considers links up to 1 km; therefore, the specifications of the

high gain antenna were derived for this particular scenario. The nominal key figures are

summarized in Table 4.

Table 4: Key specifications of the Cassegrain antenna.

Parameter Value

Gain 55 dBi

Frequency bandwidth 250 – 290 GHz

Electrically relevant diameter 220 mm

Half power beam width 0.4 deg

Half power spot size @ 1 km 6.4 m Note: The antenna will be operated up to 310 GHz in the first series of experiments.

In the first step of WP6, a THz downlink and an optical up-link will be explored, first directly

connected to the optical modem, next implemented as a fibre extender, and at last both the

up- and down-link should be supported by the THz wireless link. WP5 explores possible

hardware solutions and provides component designs for implementing this plan in WP6.

The Cassegrain reflector and the frontend components will be integrated together for the first

series of tests. The corresponding CAD design is shown in Figure 5.

0 100 200 300 400 500 600 700 800 900 1000

R (m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

D (

m)

f=275 GHz

f=300 GHz

f=325 GHz

f=350 GHz

f=375 GHz

f=400 GHz



Figure 5: CAD design of the high capacity Tx respectively Rx antenna, ready for fabrication.

Approx. size: 200 x 200 x 250 mm³ (LxWxH). The housing includes all analogue frontend

components and connects to the THz optical interface by V-connector cables.

The Cassegrain antenna is fed by a circular corrugated horn with rectangular waveguide

flange. For the dual-polarized feed implementation, a second version with circular waveguide

flange of diameter 0.99 mm is used. A planar transmission line to circular waveguide dual

transition accomplishes the broadband excitation of two orthogonal modes in this case.

Figure 6: Back panel of the preliminary high capacity Tx respectively Rx antenna. The labels

indicate the different interfaces. The final version will include a CFP2-ACO cage.

The back panel of the antenna and frontend is shown in Figure 6, where

1) and 2) IQ-baseband signals, single-ended for the THz Tx and Rx, respectively. The THz

Rx baseband signals are amplified by 20 dB to comply with the specifications of the



CFP2 ACO optical Tx interface specifications as described in Chapter 4. The revision of

the 4-channel baseband amplifier is used. The CFP2 ACO Rx baseband signal need to

be attenuated to comply with the optimum operating conditions of the THz Tx

(~ 100 mVpp). Coaxial attenuator pads are used.

3) Local oscillator (LO) reference output. One LO reference signal will be available per

reflector antenna when polarization mapping is employed, respectively two when

frequency mapping is employed.

4) DC connector, +-6V for all RF modules. The dielectric resonator oscillator (DRO) is

biased separately (+5V) to avoid the risk of interference due to common power supply

rails. The optimization and miniaturization are engineering tasks and omitted at this

proof-of-concept stage.

5) Control interface which allows to turn off the Rx and Tx independently for test and

debugging purposes or for minimization of crosstalk, which is important for channel

sounding and the measurement of atmospheric losses, for example.

The back panel can be exchanged to fit to new frontend and antenna interface architectures.

The project plans a redesign and miniaturization of the housing once the first series of outdoor

tests have been completed to prove the concept. At this stage, the individual components will

be integrated at chip and system level, e.g. the baseband amplifiers and the frontend chip, or

the THz interface and the antenna housing.

The HC demonstrator plans to implement the full duplex operation by spatially separating the

transmit and receive channel at the same frequency. The near end cross-talk is a very critical

value for this approach, which was determined by the experimental setup of Figure 7.

Figure 7: Measurement of the crosstalk between the two Cassegrain antennas. The

calibration reference plane is at the flange of the feed antenna, all measurements are

scalar measurements.



The measured power at the output of the frequency multiplier-by-36 (X36) was measured

with a power meter and with the spectrum analyser. This provides the offset for an absolute

power measurement with the spectrum analyser to the first order. The transmit power at the

flange of the feed horn of the Tx antenna was -4 dBm, which is slightly higher than the transmit

power that will be used in the transmission experiments. The measured received signal of the

Rx antenna is compared in Figure 8 for a measurement to the sky and with a wooden board

(to avoid saturation of the receiver) at 1 m distance in front of the two antenna. The indoor

scenario was measured to the ceiling and showed a similar reflection, subject to the exact

levelling of the antenna. This experiment shows that the isolation with no obstacles in front

of the antenna is higher than 90 dB. In earlier deliverables, it was proposed to separate the

transmitting and receiving channel in frequency. This result indicates that the transmitting

and receiving channels can be also spatially separated at the same frequency with the

Cassegrain setup. This simplifies the frontend hardware and allows the operation in the

252-292 GHz band in agreement with the expected ITU-R regulations at the WRC-2019

conference.

Figure 8: Comparison of measured crosstalk at 300 GHz. (a) Measurement to the sky, (b)

measurement with wooden board in 1 m distance in front of the antenna, level sensitive to

the tilt of the board.

3.1.5 Beamforming antennas

Although the presented 4-channel BF solution presented in this section does not support

duplex operation, the solution is still very suitable for experimentally exploring the major

limits of [coherent] beamforming at THz frequencies in WP6. On the one hand, a

reconfigurable platform was needed, and on the other hand, a certain degree of mobility

should be preserved. The design of the final antenna configurations is presented in Figure 9.

The frontend can either operate with 4 circular horn antennas of 23 dBi gain (spaced 21.5 mm

apart), or with 4 rectangular tapered horns of 16 dBi gain (spaced 1.25 mm apart). The latter

version will be predominately used in WP6, yet the first version may offer additional options,

e.g. operating in a beam switching mode.



(a) (b)

Figure 9: The 4-channel antenna configurations for beamforming tests (front view).

(a) 4 circular horn antennas with 23 dBi gain, element pitch 21.5 mm, (b) 4 rectangular

tapered horns of 16 dBi gain, element pitch 1.25 mm

Both depicted solutions are self-contained analogue 4-channel frontends that can be directly

connected to the baseband IF interface boards of the baseband 4-channel real-time modem.

Figure 10 provides an overview of the frontend components and functions. The design of the

complete experiment leads to variations that will be explained in more detail with the help of

Figure 13 below. For all implemented variations, the LO reference signal at 8.244 GHz is

derived from an internal DRO, but it can be also provided by an external signal generator.

Figure 10 shows the simplified block diagram of the transmitter array. The same block diagram

applies to the receiver array.

Figure 10: Block diagram of the basic 4-channel beamforming frontend for digital

beamforming experiments in WP6

The whole frontend requires two DC supply voltages of +6V and +5V. The first one is used for

the DRO only, while the latter one provides the DC bias for all other active components. The

total current is 400 mA/channel, which amounts to approximately 2A for the total frontend

including the supply for other components like micro-controllers, etc. The LO reference signal

is also available for phase synchronization by the stolen carrier approach, which serves as a

reference case to compare with the autonomous digital carrier recovery of the modem. The

corresponding back-panel with all required I/Os is shown in Figure 11.



Figure 11: Back panel of the preliminary beamforming demonstrator 4-channel frontend, top

view shows the I/Q interface. The LO reference signal output can be reconfigured to either

work with an internal or external LO.

One overall goal of WP5 and WP6 is to investigate and derive the required phase noise mask

of the LO signal for broadband and narrowband modems, and the project should provide an

experimental platform for testing the impact of the phase noise experimentally. For this

reason, the LO reference signal can be optionally filtered by a 10 MHz bandpass filter at

8.244 GHz. This limits the phase noise mask of the DRO (Figure 12). The DRO phase noise

resembles the phase noise of the DDS-(Direct Digital Synthesis)-based solution (reference

case) between 2 kHz and the cut-off frequency of the bandpass filter (10 MHz). The drift due

to thermal and / or mechanical variations has a major effect on the phase noise close to the

carrier. The lower corner frequency determines the complexity of the bias supply, thermal

management, packaging and system integration.

Figure 12: Typical phase noise performance of the reference oscillator at 8.244 GHz. The

carrier power is about 2 dBm.



The stability of THz beamforming frontends, especially for MIMO or hybrid beamformers, will

be studied more specifically with three distinct experiments of which Figure 13 provides a

simplified overview. The DDS-based LO generation at 2.061 GHz was the initial solution at the

start of the project. This solution uses four DDSs which are synchronized by a master-slave

configuration. The second approach generates the reference signal at 8 GHz before

distributing it to the frequency multipliers. The third option splits the reference signal at

100 GHz and distributes the copies to the individual Rx/Tx channels.

Figure 13: Design of experiment for testing the stability of different LO distribution schemes.

(a) DDS based LO generation at 2.061 GHz, (b) DRO based LO generation and distributing at

8.244 GHz, (c) DRO based LO generation at 8.244 GHz and distributing at 100 GHz.

The designed frontend covers all three cases. A depiction of the CAD implementation of the

frontend can be observed in Figure 14. The fabrication of all components has been completed

and the assembly of the system is currently in progress. Due to missing third party

components to complete the frontend assembly, the experiments will be carried out with a

delay in September 2019, which is considered as a part of WP6.

Figure 14: CAD of the implementation of the 4-channel frontend.



The splitting of the LO at 100 GHz requires a 1:4 waveguide splitter that must exactly fit both

the frontend and the antenna. A rat-race combiner, which is depicted in Figure 15b, was

designed for that purpose. The frontend can be reconfigured to accommodate the combiner

as shown in Figure 15a.

(a) (b)

Figure 15: Implementation of the variation with a 1:4 LO-splitter at 100 GHz.

3.2 Frontend components for the demonstrators

This section summarizes the designed and fabricated analogue frontend components that will

be used to support the development and demonstration of the high capacity system

demonstrator for lab bench tests with low gain antennas and for outdoor experiments with

the high gain Cassegrain antennas. The integration of the individual functions at module and

chip level for miniaturization will be carried out in WP6 after testing the system concept. An

integrated transmit and receive module was designed and fabricated that operates between

270 and 320 GHz, which contains also the option to add the baseband amplifier chips, and

finally integrate them on the Tx/Rx chip, as described in the deliverable D5.1. The initial

baseband amplifier module of D5.1 was re-designed for that purpose. Both modules, the RTX

module and the baseband amplifier module will be described in more detail in the following

sections. In total four sets of modules have been fabricated and are currently being assembled

and tested.

3.2.1 Receiver baseband amplifiers

In order to comply with the specified baseband signal levels at the THz receiver side, as

described in more detail in Section 4, broadband baseband amplifiers were designed at chip

level together with modules for testing. The typical received baseband signal levels are

between 20 mVpp and 50 mVpp (which means between -30 and -22 dBm single-ended at 50

Ohm). This lead to the initial baseband amplifier specifications of Table 5, which will be further

refined and expanded after the first few system tests. The expected output signal levels will

be between 200 mVpp and 500 mVpp (-10 and -2 dBm), or the power back-off will be > 6 dB.



Table 5: Initial specifications of the baseband amplifiers.

Parameter Value

Typ. gain 20 dB

Frequency bandwidth (3-dB) > 25 GHz

Typ. noise figure NF 4 dB

P-1dB output referred compression point > 4 dBm

Input / output match |S11|, |S22| < -10 dB

The CAD design of the final module version, which will be used for the first high capacity

demonstrator tests, is shown in Figure 16. The baseband amplifier module can be directly

connected to the RTX module to avoid additional cables.

Figure 16: CAD of 2 channel amplifier module used in the WP6 high capacity demonstrator.

The concept includes the same PCB interconnects as used in the RTX module.

Figure 16 also indicates that the module uses identical interconnect boards as in the RTX

modules. Upon successful first system tests, the baseband amplifier can be integrated into the

current RTX module generation with little modifications. The baseband amplifier chip was

initially designed for testing a 2-channel single chip Tx. However, the floor plan of the new Tx

chip will route the IQ signals of the two polarizations to two opposing sides of the chip (north-

south locations), which is the same in the RTX module for the coaxial connectors. Only two of

the four channels of the baseband amplifier chip are used at the moment for that reason on

each side. The dual-polarized 2x2 MIMO architecture will be supported by two baseband

amplifier modules, compatible to the RTX module architecture in the series of the first tests.

One of the fabricated modules is shown Figure 17. The bias settings of the module can be

programmed similar to all other modules from the outside (Figure 17a), and the currents at

the chip I/O pads can be monitored and logged, which allows for an accurate comparison

between the on-chip measured performance and the performance of the chips after

packaging in the module. A close-up view of the RF printed circuit boards (PCBs) and the

connection to the chip is depicted in Figure 17b. The layout is consistent with the layout of

the PCBs in the RTX module, although one side will connect directly to the Rx chip. Upon

successful system tests, the board and amplifier chip will be integrated into the RTX module.



(a) (b)

Figure 17: Fabricated and assembled baseband amplifier test module. (a) Outside view with

connection to the programming interface of the microcontroller, (b) view on chip and RF

printed circuit board.

The measured S-parameters are shown in Figure 18. The amplifier chip’s 3-dB bandwidth (on-

wafer) is beyond 40 GHz although the modules still exhibits a noticeable slope of 0.15 dB / GHz

or -3.0 dB gain at 20 GHz due to the insertion loss of the two PCBs. This slope refers to two

PCBs and when integrating the chip in the module the 3-dB bandwidth will double for that

reason.

Figure 18: Measured S-parameters of the baseband amplifier test module for the two

channels in use for an internal bias of 4 V.



3.2.2 RTX frontend modules

For the implementation of the high capacity demonstrator intermediate setups in the lab are

planned before going over to outdoor experiments. The dual antenna 2x2 MIMO link will be

emulated by two separate low gain feed horns (23 dBi). The polarizations of the fibre are

mapped to the frontend as depicted in Figure 19. The transmitting and receiving channel (or

up-link and down-link) are realized by two spatially separated links, which allows the full-

duplex operation using the same carrier frequency.

Figure 19: Dual antenna 2x2 MIMO link for lab bench testing with low gain horn antennas.

Full duplex operation is achieved by two of these links, which are spatially separated.

The block diagram of the developed frontend RTX module is shown in Figure 20. The module

was designed with the requirement that either two Rx or two Tx channels can be realized in a

single module, but also an Rx channel together with a Tx channel. The polarization can be

rotated by E/H twists, which are implemented by a quarter-wavelength shim with bow-tie

aperture. The standard gain horns depicted in Figure 20 have a rectangular waveguide flange

and support only one polarization for that reason. Both channels use the same LO reference

signal. The splitting of the reference signal occurs at 100 GHz after the first frequency

multiplier stage (X12). The individual channels can be disabled by an external logic signal.

Figure 20: Functional block diagram of the RTX module used in the first phase of WP6.

The fabricated module is shown in Figure 21. The size of the module is approximately 60 x 60

x 23 mm³ (LxWxH). An integrated micro-controller sets the bias conditions of the Rx, Tx and

X12 chip. The module operates with a single bias supply of 6V. In total four more modules

were fabricated of the first prototypes presented in this deliverable. Those modules are

currently being assembled and tested. With the four modules different duplex scenarios (FDD,

TDD, PDD) can be tested at the lab bench, especially with different antenna spacing, as

indicated in Figure 22. Although the two channels of each module operate with the same LO,

the depicted 2x2 MIMO duplex configuration allows to also separate the uplink and downlink

by different carrier frequencies (FDD). The PDD is realized by adding the waveguide twists as



depicted in Figure 20, while the TDD option is realized by using either the two enable logic

signals or directly at the IF.

Figure 21: CAD and fabricated RTX module with 23 dBi horn antennas.

Figure 22: Example of the lab bench options with the frontend prototype modules.

The first modules were configured as Rx/Tx modules while the additionally fabricated modules

were configured as two-channel Tx respectively two-channel Rx modules. The RF continuous

wave test setup and typical RF measurement results are summarized in Figure 23 to Figure 25.

Figure 23: Test setups for the RF characterization of the Rx and Tx channels.



The Tx channel was measured as a function of the IF input frequency for an IF input power

level of -8 dBm, which still corresponds to a linear operating range. The measured slope of the

output power as a function of the IF input frequency decreases towards higher input power

levels; however, this is practically less relevant due to the required power back-off for higher

order modulation schemes. The saturated output power deviates from the on-chip measured

power levels by approximately 2 dB. A chip variation with higher output power is being

currently assembled. For the high capacity demonstrator in WP6, a transmit power of -8 dBm

is targeted.

Figure 24: Measured output power versus IF power and frequency of the Tx channel.

The Rx channel was measured as a function of the RF input frequency at a fixed LO frequency

(300 GHz). The x-axis of the graphs in Figure 25 show the offset frequency of the received RF

signal from the LO (IF frequency), for the upper and lower sideband of the receiver. The input

power level of the RF signal was -30 dBm to simplify the absolute reference calibration with a

power meter. In typical practical scenarios the input power is 10 – 15 dB lower. In both cases,

the Tx and Rx measurements, the LO input power at 8.333 GHz was -4 dBm.



Figure 25: Measured conversion gain versus IF frequency at an RF input power of -30 dBm

for the in-phase and quadrature signals of the Rx channel.

3.3 References

[3-1] C. Castro, R. Elschner, T. Merkle, and C. Schubert, “100 Gbit/s terahertz-wireless real-

time transmission using a broadband digital-coherent modem,” IEEE 5G World Forum,

30.09. – 02.10.2019, Dresden.



4. ELECTRO-OPTICAL INTEGRATION

One of the key enabling technologies of TERRANOVA is embedding the THz wireless link into

the optical fibre networks as an enabler for a hybrid optical-THz system, and, as previously

reported, both coherent and direct detection designs will be deployed for this approach. This

chapter intends to report the required components for the demonstrator, including block

diagrams with integrated designed boards or devices, interface specifications and component

measurements and characterizations. For the sake of simplicity, the demo will focus on a

unidirectional network link and seeks to keep the same real-time digital signal

processing (DSP) unit for both approaches. While the coherent approach seeks to employ

commercial CFP2-ACO transceivers, the IM/DD based solution plans to employ simplified

modulator and photodiode modules looking for higher flexibility.

4.1 Optical hardware components

In order to evolve to the final demonstrator, the different optical link scenarios should be well

established and the performance of the different modules and interfaces should be

characterized. This section introduces considerations and results towards the final system

demonstrator and its hardware characterization both based on the coherent and

IM/DD transceivers.

4.1.1 Coherent based optical link

In a hybrid optical-THz coherent system, we envision to realize the opto-electrical (O/E)

interface via the use of modules based on CFP2-ACO technology. In this section, we focus on

the design, specifications, and operational parameters of CFP2 analogue coherent optical

modules according to the implementation agreement by the Optical Internetworking

Forum (OIF) [4-1].

(a) (b)

Figure 26: (a) High-level block diagram of a CFP2-ACO. (b) 100G – 200G Tunable CFP2-ACO

from Finisar [4-2]

In broad terms, a CFP2-ACO module is a pluggable device, whose main function is to perform

O/E and E/O conversions for spatially multiplexed channels (i.e. polarization-division

multiplexing) in optical communication systems. Nevertheless, this functionality is

appropriate for our vision regarding hybrid optical-THz links, since instead of feeding the



electrical signals obtained from the CFP2-ACO into a DSP stage (at the interfacing point) we

would redirect them into the THz elements to prepare them for wireless transmission. At the

THz Rx, we would just need to perform a similar action in reverse order to continue

transmitting the data signals over a second fibre link. Figure 26 illustrates a modified high-

level block diagram of the CFP2-ACO-based O/E interface.

Transmitter

The schematic diagram of the CFP2-ACO’s optical transmitter module as shown in OIF’s

implementation agreement is shown in Figure 27, where the main functional building blocks

of the transmitter are highlighted. Along the system there are multiple monitoring stations

that communicate through an MDIO interface to ensure the correct operation of the different

opto-electrical elements.

Regarding the functionality of the transmitter module, a laser generates an optical signal onto

which data is modulated via an arrangement of Mach-Zehnder modulators (MZM). These

optical modulators are driven by 4 electrical data signals: in-phase (I) and quadrature (Q)

components for 2 spatially multiplexed channels. After the MZM, signals that correspond to

the X and Y polarization axes are multiplexed using a polarization beam combiner and the

optical signal is then amplified before being launched into the fibre that is connected at the

Tx port of the CFP2-ACO.

Figure 27: Block diagram of the CFP2-ACO’s transmitter module. The green box highlights

the actual electro-optical components used in the transmitter’s setup.



The CFP2-ACO clients must comply with certain electrical and optical specifications to

guarantee the correct operation of the devices. Based on this, the implementation agreement

defines operating ranges for the Tx, which have been summarized in Table 6.

Table 6: Transmitter specifications for a class 2 CFP2-ACO client.

Parameter Tx Interface

Differential voltage (host) 200 – 450 mVpp

Electro-optical group delay variation

0 – 30 ps

Low corner cutoff frequency 1000 kHz

IQ Timing skew -5 - 5 ps

XY Timing skew -8 – 8 ps

Skew variation 2 ps

PN (Differential) intrapair timing skew

1 ps

Tunable Tx laser wavelength range

1530-1565 nm

Wavelength tuning time < 1 min

Optical signal Tx power -2 – 1 dBm

Optical signal Tx power stability

< 1 dB

Symbol rate 32 GBd

Modulation formats 4/8/16-QAM

Pulse shaping non-return to zero

In addition to the previous specifications, the CFP2-ACO pluggables also have DC blocking

capacitors on the RF signals lanes, as shown in Figure 28.

Figure 28: Input and output electrical interfaces of the CFP2-ACO.



Receiver

The CFP2-ACO’s optical coherent Rx module is depicted in Figure 29. Similarly, to the Tx

module’s schematic, the main functional building block has been highlighted. In regards to its

functionality, the CFP2-ACO’s Rx port receives a dual-polarized optical signal, whose X and Y

components are then separated by a polarization beam splitter, each one is then fed into a

90° hybrid (90° mixer in Figure 29). Depending on the pluggable’s design, the local oscillator

(LO) source is either a separate laser or a laser shared both by Tx and Rx. The 90°hybrid splitter

in combination with photodiodes and differential amplifiers allows the separation of the I and

Q signal components of their corresponding polarizations, which results again in the 4

electrical data signals: XI, XQ, YI, and YQ.

Figure 29: Block diagram of the CFP2-ACO’s receiver module. The green box highlights the

actual electro-optical components used in the receiver’s setup.

The CFP2-ACO clients must comply with certain electrical and optical specifications to

guarantee the correct operation of the devices. Following this, the implementation agreement

defines operating ranges for the transmitter, which have been summarized in Table 7.



Table 7: Receiver specifications for a class 2 CFP2-ACO client.

Parameter Rx Interface

Differential voltage (host) 300 – 700 mVpp

Low corner cutoff frequency 1000 kHz

IQ Timing skew -3 - 3 ps

XY Timing skew -8 – 8 ps

XY Skew variation -3- 3 ps

PN (Differential) intrapair timing skew

1 ps

Tunable local oscillator laser wavelength range

1530-1565 nm

Wavelength tuning time < 1 min

Optical signal Rx power -18 – 0 dBm

Electrical power up of receiver

< 2 min

Reference curves from a CFP2-ACO-based real-time coherent system

In order to have a point of reference for future implementations of hybrid optical-THz

transmission systems, we have investigated the bit error rate (BER) performance of a standard

commercially available CFP2-ACO module in combination with an optical modem capable of

performing DSP actions in real-time. The basic noise-loading setup is shown in Figure 30. In

this configuration, the optical modem generates 32 GBd electrical signals using m-QAM

formats of order 2, 3, and 4, which result in net data rates corresponding to 100 Gbit/s,

150 Gbit/s, and 200 Gbit/s, respectively. The CFP2-ACO converts these electrical signals into

the optical domain and transmits them over the fibre in a polarization-multiplexed manner at

a launch power of 0 dBm. After a very short fibre link (~1 m), additive white Gaussian

noise (AWGN) is added to the signal via a 3 dB optical coupler. Moreover, a variable optical

attenuator (VOA) has been placed directly at the output of the CFP2-ACO’s transmitter port

to control the optical signal-to-noise ratio (OSNR) of the resulting combined signal.

Figure 30: Noise-loading setup used to measure the BER vs. OSNR relation of a CFP2-ACO for

32 GBd m-QAM formats. Optical power at the RX port of the CFP2-ACO is kept constant by

means of a VOA, which sets the power to -10 dBm.

Real-time optical modem

CFP2-ACO

RxTx

AWGN

VOA3 dB

couplerVOA -10 dBm

OSNR



Then, a second VOA has been placed before reaching the optical modem to control the power

at the Rx port of the CFP2-ACO in order to keep the Rx optical power constant regardless of

the amount of noise added to the system in the noise-loading stage. Following this, the optical

power at the CFP2-ACO’s receiver port is -10 dBm. Subsequently, the CFP2-ACO converts the

signals into the electrical domain and DSP is performed by the optical modem in real-time.

Since the optical modem is a commercial product, it is not possible to provide a

comprehensive overview of the DSP algorithms used; however, it is safe to assume that

standard techniques [4-3] are employed. Furthermore, the modem estimates the OSNR of the

received optical signal and outputs the BER of the complete system.

Figure 31: BER vs OSNR measurements from the real-time optical modem. Experimental data

for m-QAM formats has been compared to the theoretical expectation of said modulation

formats.

Figure 31 depicts the OSNR vs. BER experimental results of the three supported QAM formats

and compares them to the theoretical curve of said modulation format.

In addition, Figure 32 shows the constellation diagrams of 4/8/16-QAM in back-to-back (B2B)

configuration (fibre between TX and Rx port of the CFP2-ACO < 1m).

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0 5 10 15 20 25 30 35 40 45

BE

R

OSNR [dB]

Theory 4-QAM

32 GBd 4-QAM

Theory 8-QAM

32 GBd 8-QAM

Theory 16-QAM

32 GBd 16-QAM



a)

b)

c)

Figure 32: Constellation diagrams for back-to-back (a) 32 GBd 4-QAM, (b) 32 Gbd 8-QAM,

and (c) 32 GBd 16-QAM as measured at the optical modem.



4.1.2 IM/DD optical link

Since the fibre reach may not be a critical issue for the application scenario of the previously

discussed THz system, where the fibre length is expected to be in the order of magnitude of

those distances observed in optical access links (tens of kilometres, at most), a solution based

on intensity modulation and direct detection (IM/DD) transceivers can be also used. This

proposal comprises an architecture that uses two optical wavelengths to separately transmit

the in-phase and quadrature signal components of the THz signals, simplifying the optical

system using only amplitude modulation. Modulation schemes of higher order such as 16QAM

or 64QAM are transparent for this technology, and at the receiver side only signal equalization

may be required to compensate bandwidth limitations for high symbol rate signals.

Figure 33 shows the proposed system based on a multiple-input multiple output (MIMO)

solution for the THz transmission, wherein the optical system is used as an intermediate

medium between the central office and the radio station. For the optical transmission, and

compared to the coherent solution, only the Ix and Qx signal components are propagated, and

two optical modulators are used to map both signals into two different wavelengths. If QAM

signals are employed, the signals in the optical domain may be shaped using pulse amplitude

modulation (PAM). At the receiver side of the optical system, both signals are separated using

a wavelength multiplexer (WM) and then direct detected using two photodiodes. To enable

the connection with the MIMO radio system, both received Ix and Qx signal components

should be separated using a 3-dB electrical splitter. In addition, electrical delay lines must be

employed to synchronize all signals. At the receiver antenna, the electrical signals may be

directly connected to ADCs and compensated by the BBU’s real-time DSP. Note that the MIMO

configuration is only a use case example, i.e. the SISO configuration may also be easily

assumed.

SSMF

Optical Setup – IM/DD system

PIN

PIN

WM

DSP

High-speed 4-Ch DACs

2x2 MIMO link

Real-time DSP THz front-end / link

Ix

Qx

SMA

SMA

XIy

Qy

X

High-speed 4-Ch ADCs

SMA

SMA 2 x Tx

3 dB splitter

Ix

Qx

Iy

Qy

32 Gbaud

32 Gbaud

2 x Rx

λ1

λ2

3 dB splitter

Ix

Qx

Iy

Qy

100 GHz

λ2λ1

MZM

MZM

Figure 33: Target THz system using a MIMO configuration combined with an IM/DD optical

transmission system for the central office-to-radio direction.

The main advantage of this solution is that the signal may be directly sent to the THz

transmitter, if the signal is not strongly affected by fibre distortions such as chromatic



dispersion, since this construction does not feature DSP between the optical domain and the

wireless system.

Characterization of the IM/DD system using one wavelength

The experimental setup depicted in Figure 33 is used to characterize the performance of the

optical IM/DD system used to transmit the in-phase or the quadrature component of the THz

quadrature amplitude modulation (QAM) signal. For the transmitter side, an MZM

(TERRANOVA infrastructure) is driven by a 64 Gsa/s arbitrary waveform generator (AWG)

(Agilent M8195A) with 18 GHz analogue bandwidth. The electrical signal is a 32 Gbaud signal

based on a pseudo random bit sequence (PRBS) of 213-1 using raised cosine shaping with a

roll-off factor of 0.1. Both QPSK and 16QAM modulation formats are employed to

demonstrate the feasibility of the system to emulate a network of 50 Gb/s and 100 Gb/s. Note

that the setup depicted in Figure 34 only illustrates the transmission of a single signal

component (in-phase component). Therefore, in the optical domain the signal corresponds to

a NRZ and 4-PAM for QPSK and 16QAM, respectively. At the AWG output, an RF driver

(TERRANOVA infrastructure) is used to feed the MZM with 5 dB gain. Moreover, a distributed

feedback laser (DFB) with around 1 MHz-linewidth at 1546.16 nm is used as an optical source.

The transmitted power is set at approximately 0 dBm at the fibre input and then, the signal is

transmitted over 10 km of a standard single mode fibre (SSMF) (α=0.21 dB/km,

D=16.5 ps/nm/km, γ=80 µm²) and at the receiver a VOA set the received power. After being

detected by the photodiode (TERRANOVA infrastructure), the electrical signal is amplified by

5 dB using an RF driver (TERRANOVA infrastructure) and sampled by a real-time oscilloscope

at 50 Gsa/s with an analogue bandwidth close to 16 GHz. The signal is then processed offline

with DSP: resampling is performed to maintain two samples per symbol, and a 13-tap-long

adaptive equalizer is then used to compensate the signal according to the constant modulus

algorithm (CMA). Figure 35 shows some photos of the experimental setup and the devices we

used.

SSMF

MZM PIN

λ1

50 Gsa/s Real-time

Scope

64 Gsa/s AWG

Offline DSP

Offline DSP

PRBSRaised Cosine

ShapingResample

Adaptive Equalizer

BER

System to be valited

Figure 34: Experimental setup used to transmit one of the two components (I or Q) of the

THz signal.



Figure 35: Laboratory infrastructure: (a) real-time oscilloscope; (b) AWG; (c) MZM;

(d) photodiode; (e) RF driver for MZM; (f) RF driver after photodiode;

(g) and (h) power supply for RF driver; (i) polarization controller for the laser.

Figure 36 depicted the received sensitivity for both QPSK (NRZ) and 16QAM (4-PAM) scenarios

for B2B, 6 and 10 km of SSMF. The considered BER limit of 3.8×10-3 corresponds to the

7% hard-decision forward error correction (HD-FEC). The reported received sensitivity for B2B

was -15.5 and -10.5 dBm respectively for NRZ and 4-PAM, and the difference between them

is caused due to the lower tolerance for SNR of the 4-PAM modulation format. The impact of

the fibre chromatic dispersion is slightly higher for the 4-PAM, but lower than 2 dB for

10 km fibre.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)



Figure 36: Experimental results with the BER vs. received power

for QPSK (NRZ) and 16QAM (4-PAM).

Figure 37 shows an example of the eye diagram and spectrum at -2 dBm in B2B before (a) and

after (c) signal equalization in the DSP for QPSK, and before (b) and after (d) for 16-QAM,

showing the impact of the bandwidth limitations mainly due to the 18 GHz AWG and 16 GHz

real-time oscilloscope.

Figure 37: Spectrum and Eye Diagram samples at -2 dBm before (a) and after (c) signal

equalization for QPSK, and before (b) and after (d) signal equalization for 16QAM for B2B.

Characterization of the IM/DD system using two wavelengths

The experimental setup depicted in Figure 38 is used to validate the propagation of both I and

Q components of the THz signal. Compared to the previous experimental setup, the optical



system is basically replicated to transmit two different signals using two wavelengths. After

the two MZM, both signals are 3 dB coupled and then propagated in the same optical fibre.

The wavelengths considered for this experiment are 1546.16 and 1550.16 nm (4 nm

wavelength separation) by using DFB lasers with ~1 MHz-linewidth, and at the Rx a WM

separates both signals for two different PIN receivers. The signals are then mixed in digital

domain in order to form a complex THz signal (e.g. 16QAM) and recovered using the offline

DSP.

SSMF

MZM PIN

λ1

64 Gsa/s AWG

Offline DSP

PRBSRaised Cosine

Shaping ResampleAdaptive Equalizer

BER

MZM PIN

WM

λ2

50 Gsa/s Real-time

Scope

Offline DSP

System to be valited

Figure 38: Experimental setup used to transmit the in-phase and quadrature component

of the THz signal.

Due to lab limitations, only two RF drivers were available for this validation (used at the

transmitter), and therefore after PIN detector the two drivers were not used and are

represented in Figure 38 with a dashed line. Due to this limitation, results of receiver

sensitivity were not obtained, since the optical received power at the PIN detector must be

higher (>0 dBm) in order to detect a suitable electrical voltage (>30 mVpp) for the real-time

oscilloscope.

The important considerations to be taken into account with this validation are the output

power and the delay between the two transmitted signals, i.e. the I and Q components. The

output power between the two signals should be approximately the same to obtain a

balanced constellation, and this can be accomplished by adjusting the electrical or optical

power at the transmitter or receiver side. The delay between the two signals must be

compensated to maintain both signals synchronous using for instance an electrical delay line

as represented in blue in the schematic of Figure 38. If the wavelength separation between

the two optical channels is reduced, the delay is also reduced. This is due to the chromatic

dispersion of the optical fibre, wherein different wavelengths in the fibre propagate at

different speeds.

Figure 39 shows an example of a constellation and eye diagram for QPSK (a, b, c) and

16QAM (d, e, f) after 10 km SMMF transmission, where the received power was measured to

be 3 dBm. The performance of the signal may be improved by using the RF drivers at the

receiver. The power difference between the received in-phase and quadrature signals was

compensated by adjusting the optical power of each channel at the transmitter. For B2B, no

delay was observed between the two signals. However, for 10 km of fibre a delay of around

0.7 ns was observed, corresponding to approximately 22 symbols at 32 GBd signals.



Figure 39: Sample of the eye diagram of the in-phase (a, d) and

quadrature (b, e) components and constellation (c, f) at 3 dBm

respectively for QPSK (a, b, c) and 16QAM (d, e, f) after signal equalization.

For the reported IM/DD system calibration only the following considerations should be taken

into account:

The MZM is calibrated by defining the DC bias voltage and adjusting the amplitude of

the electrical signal through the RF driver.

The laser power at the MZM input must be optimized for the X-polarization using for

instance polarization controllers as shown in in Figure 38.

The delay between the in-phase and quadrature signals must be removed using an

electrical delay line.

The electrical power difference between the in-phase and quadrature signals at the

output of the system must be reduced either by adjusting the electrical or the optical

power of the different components.

4.2 Towards system demonstration

This section describes the hardware specifications required for the demonstrator of both

solutions presented in Section 4.1.

4.2.1 Coherent based optical link

In order for hybrid optical-THz systems to be built, it is necessary to produce O/E interfaces

that are capable of seamlessly allowing the transmitted signals to cross over from the optical

to the electrical domain and vice versa. For this reason, we have placed our focus on CFP2-ACO

modules, since they are able to perform O/E and E/O conversion without performing DSP

tasks. For example, if we consider the fibre extension use case (Figure 40), both ends of the

system feature CFP2-ACO clients on optical modems, where Tx and Rx DSP is performed. The



transmission line, however, is meant to be considered as a single channel, i.e. the THz system

must be transparent. In this regard, the optic-THz interface should only perform a domain

conversion.

Figure 40: Block diagram of the fibre extension use case based on THz technologies. The

CFP2-ACO and the Optic-THz interface ensure the transformation of the optical signals into

electrical waveforms, which are fed into the THz elements for data wireless transmission.

Furthermore, Figure 40 also illustrates an interface between the CFP2-ACO and the THz

elements. A picture of this interface is shown in Figure 41.

Figure 41: Front panel of the preliminary 100G/200G coherent optic-THz interface as

illustrated in Figure 40. The labelling indicates the different parts of this interface, which are

further described in the text below.

The following is a description of the connectors, LEDs, and ports on the preliminary interface’s

front panel:

1) CFP2-ACO socket: socket where the CFP2-ACO should be plugged in.

Op

tic-THzI/F

THz Tx

THz Rx

CFP2-ACO

Op

tic-THzI/F

THz Tx

THz Rx

CFP2-ACOFiber connections Fiber connections

Spatially-multiplexed wirelessTHz link



2) Status LEDs: provide information regarding the current status of the modem, in the

case an internal FPGA is used. This feature is not enabled at the moment.

3) Control USB port: USB port to control and configure the CFP2-ACO.

4) FPGA USB port: USB port to connect to an internal FPGA in the coherent optic-THz

interface. This feature is not enabled in the current version.

5) Tx Analogue IN: CFP2-ACO’s transmitter RF input ports (4 differential pairs, which

correspond to in-phase and quadrature for each channel in a polarization-multiplexed

optical transmission; it could also be operated with single-ended signals at the cost of

a degradation in terms of performance). These RF signals are the output of the THz

receiver (THz Rx) after down-conversion into the baseband.

6) Rx Analogue OUT: CFP2-ACO’s receiver RF output ports (4 differential pairs, which

correspond to in-phase and quadrature for each channel in a polarization-multiplexed

optical transmission; single-ended signals are available, but unused ports should be

terminated with a 50 Ohm load). These baseband RF signals become the input of the

THz transmitter (THz Tx); then, they will be up-converted into the THz band and

radiated into free-space. NOTE: The optical to electrical channel and polarity (p/n)

mappings of both Tx and Rx ports depends on the CFP2-ACO module plugged into the

optic-THz interface.

7) Pull handle: handle to turn off and remove the interface.

8) Status LED: shows the current operational status of the interface (blue: interface

is off).

9) Status LED: shows the current operational status of the interface (green: interface

is on).

As a means to provide the optic-THz interface with some degree of autonomy and flexibility,

a chassis to hold the interface has been designed. The design of the complete CFP2-ACO +

optic-THz interface is presented in Figure 42 as a stand-alone device. The design of the

interface front panel differs slightly from the one in Figure 41, but the changes are purely

aesthetical and do not modify its performance or operation in any way.

Figure 42: Chassis for the optic-THz interface with a CFP2-ACO.



The introduction of these interfaces in the communication system is not expected to modify

the signals in any major way; these interfaces are meant to purely allow the conversion from

the optical to electrical domain and vice versa. Following this, the DSP at the receiver will just

observe the sum of all the impairments and distortions along the transmission line (optical

and wireless channels) and try to correct them all using a particular algorithm scheme. It is

therefore of great importance to understand the limitations of standard DSP techniques in

optical communication systems with regards to the joint compensation of optical and wireless

channel distortions [4-4].

Finally, based on the characteristics of the coherent communications system (optical and THz

wireless) and of the opto-electrical interface, the specifications of the demonstrator have

been summarized in Table 8.

Table 8: System specifications

Symbol rate 32 Gbaud

Polarization multiplexing Yes (spatially multiplexed channels)

Modulation formats QPSK

Bit rate 100 Gb/s

Laser power ≤ 16 dBm

Optical power per channel (Tx) ≥ 0 dBm

Wavelengths 1550 nm

Wavelength Multiplexer None

Maximum received optical power -5 dBm

Receiver sensitivity > -15 dBm (QPSK)

Optical link distance To be determined

Type of transmission Unidirectional

4.2.2 IM/DD optical link

The optical infrastructure for the IM/DD demonstrator was selected to support 32 Gbaud

signals, and for this purpose the equipment summarized in Table 9 is required. Most of these

elements have already been characterized (Section 4.1).

Table 9: Equipment requirements

Units Device Analogue bandwidth

RF Connector

4 RF Driver 40G 32 GHz K Connector Female, AC Coupled

2 Intensity Modulator 40G (MZM) 30 GHz V Connector

2 Photodiode 40G 41 GHz K Connector Female, DC Coupled



The RF driver is required to drive the optical modulator and to boost the electrical power after

direct detection at the receiver. The RF driver is a wide band amplifier designed for 40 GHz

applications with a maximum gain of 30 dB, which is manually adjustable, providing a

single-ended high voltage signal with an analogue bandwidth of 32 GHz.

The intensity modulator is designed with 30 GHz analogue bandwidth. It is of high linearity,

with a low driving voltage Mach-Zehnder Interferometer (MZI) design, supporting a maximum

RF input power of 27 dBm. Its extinction ratio is > 30 dB, operating from 1535 nm to 1610 nm,

and with an insertion loss less than 5 dB. The photodiode is a wide band PIN detector

supporting > 40 GHz signals, with an operating wavelength from 850 nm to 1650 nm and a

maximum optical input level of 10 dBm. Both devices are compatible with the RF driver

previously reported. On the other hand, in order to calibrate the modulator, only an

adjustment of the electrical signal’s amplitude via the RF driver is required. Furthermore, the

RF driver provides also a DC output that can be used to set the DC bias of the modulator.

The interface between the electrical and the optical system is based on SMA k-type female

connectors, where two connectors are required at the transmitter and receiver for the

in-phase and quadrature signal components, respectively. At the Tx side, since the MZM is

powered by an AC coupled electrical driver with adjustable gain (up to 30 dB) as represented

in Figure 34 and Figure 38, the input electrical signal may have a wide range voltage variation.

Figure 43 shows an example of the amplitude voltages used in the experimental investigation

previously reported in Section 4.1. As illustrated in the schematic, electrical attenuators can

be also employed to enhance the system’s operational voltage. At the Rx side, the electrical

signal (after the PIN detector) is DC coupled and its amplitude depends on the optical received

power. This signal is typically in the order of a few millivolts, and must be therefore amplified.

The example of Figure 43 corresponds to the 0 dBm optical power at the PIN receiver.

For -6 dBm for instance, the voltage at the PIN output decreases for 0 to 10 mV, and if a driver

gain of approximately 5 dB is employed, the output varies between -140 mV and 140 mV.

Table 10 reports a summary of the specifications for the demonstrator using the IM/DD optical

link interface.

SSMF

MZM PIN

-50 to 50 mV -1.5 to 1.5 V 0 to 30 mV -500 to 500 mV

AWG

-150 to 150 mV

Scope

~5 dB gain~5 dB gain

Figure 43: Amplitude voltages used in the experimental validation for the I or Q component,

considering 0 dBm as optical received power.



Table 10: System specifications

Symbol rate 32 Gbaud

Polarization multiplexing No (medium capacity transmission)

Modulation formats QPSK / 16QAM

Bit rate 50 / 100 Gb/s

Laser power ≤ 20 dBm

Optical power per channel (Tx) ≥ 0 dBm

Wavelengths 1532.68 and 1533.47 nm

Wavelength Multiplexer 40 GHz bandwidth at 1532.68 and 1533.47 nm

Maximum received optical power 10 dBm

Receiver sensitivity > -16 dBm (QPSK) | > -10 dBm (16QAM)

Optical link distance 10 km

Type of transmission Unidirectional

4.3 References

[4-1] Optical Internetworking Forum (OIF), “Implementation Agreement for CFP2-Analogue

Coherent Optics Module”, 2016. [Online] Available: https://www.oiforum.com/wp-

content/uploads/2019/01/OIF-CFP2-ACO-01.0.pdf. [Accessed: 10- Jul- 2019].

[4-2] Finisar, “200G/100G Tunable C-Band CFP2-ACO Analog Coherent Optical Transceiver”.

[Online] Available: https://www.finisar.com/optical-transceivers/ftlc3322x3nl.

[Accessed: 10- Jul- 2019].

[4-3] R. Elschner, F. Frey, C. Schmidt-Langhorst, J. K. Fischer, C. Schubert, “Data-aided DSP

for Flexible Transceivers”, Advanced Photonics Congress, paper: SW1C.1 (invited), San

Diego, 2014.

[4-4] C. Castro, R. Elschner, C. Schubert, “Analysis of Joint Impairment Mitigation in a Hybrid

Optic-THz Transmission System”, IEEE 20th International Workshop on Signal

Processing Advances in Wireless Communications (SPAWC), Cannes, 2019.



5. CONCLUSIONS

The final deliverable of WP5, D5.3, has presented the designed hardware components that

will be used in WP6 to construct the PHY layer for TERRANOVA’s final demonstrators, i.e. the

high capacity demonstrator and the beamforming demonstrator. In addition, the system

requirements and final system architectures for the high capacity demonstrator were

described in order to summarize all relevant information for the system implementation of

the hybrid fibre optical – THz wireless link with the proposed coherent dual-polarized 2x2

MIMO approach. An attractive alternative approach to coherent optical links is the IM/DD

optical link, an idea which was introduced and investigated in previous reports (D5.1 and

D5.2). For both optical link options, the required hardware components were prepared and

tested independently. The tests of the complete optical wireless system are performed in the

framework of WP6.

Section 2 provided an introduction to the final system concept. In addition, this chapter

identified the required specifications for the baseband components (BBU, opto-electrical

interface, and baseband amplifiers) that will be used in the realization of the TERRANOVA

demonstrators. One of the goals of WP6 is to test those specifications experimentally and

derive requirements and models for refined THz and optical frontend components, which will

eventually allow the co-design and optimization of the complete fibre optical – THz wireless

system.

The wireless THz frontend components necessary for the work in WP6 were presented in

Section 3. The report focuses on the frontend antenna and the integration with the frontend

components. It was highlighted that the intermediate steps towards the high-capacity

demonstrator require additional components or component variations, which increases the

practical complexity of the project. For this reason, a modular approach was designed, which

can address the varying needs in the different phases of WP6. The dual-polarized 2x2 MIMO

architecture will be also supported by a new chip generation. These new THz devices, which

have been designed based on the specifications stated in this deliverable, will replace some

of the existing frontend components and will be used in the demonstrators at the end of the

project. The beamforming demonstrator plans to address different LO generation

architectures. One of the goals is the investigation and experimental derivation of a phase

noise mask for future THz systems at 300 GHz. This phase noise mask is not only a function of

the digital carrier recovery but also the mechanical and thermal stability of the sub-

components. All components necessary to support the outlined experiment have been

designed and presented in this deliverable.

Section 4 provided a detailed insight into the E/O integration, i.e. the interface that allows the

seamless interconnection of the optical link to the THz wireless components. Since the system

performance of the THz elements has been explored in previous reports, the first part of this

chapter focuses exclusively on the optical elements used to realize the O/E interface, while

providing experimental results that demonstrate the expected performance of the optical

components. In this regard, both coherent detection and IM/DD optical systems have been

investigated. Furthermore, based on these experimental results, the transmitter and receiver

specifications for the hardware components have been determined and summarized.

Subsequently, in order to describe the TERRANOVA demonstrators from a system perspective,

the setup diagrams that illustrate possible constructions for coherent and IM/DD transmission

systems were presented alongside their corresponding O/E interfaces.

deliverable d5.3 report on final thz rf-frontend and ... · d5.3 – report on final thz...

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