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Terahertz Technologies to Deliver Optical Network Quality of Experience in Wireless Systems Beyond 5G
Alexandros-Apostolos A. Boulogeorgos, Angeliki Alexiou, Thomas Merkle, Colja Schubert, Robert Elschner, Alexandros Katsiotis, Panagiotis Stavrianos, Dimitrios Kritharidis, Panteleimon-Konstantinos Chartsias, Joonas Kokkoniemi, Markku Juntti, Janne Lehtomaki, Antonio Teixeira, Francisco Rodrigues
TL;DR
The article identifies critical technology gaps for Tbps wireless connectivity and presents a concept for blending wired-optical and wireless-THz links. It reports approximately 300 Gbps over 1 km for a current photonic radio link and outlines the enablers needed for further development.
Problem
Critical technology gaps must be identified and addressed to support Tbps wireless connectivity beyond current 100 Gbit/s capabilities.
Method
The article develops a blended wired-optical and wireless-THz link concept and examines its requirements, baseband interfaces, transceiver frontends, DSP algorithms, and PHY/MAC protocols.
Results
Approximately 300 Gbps over a wireless distance of 1 km is identified as the maximum data rate supported by the current photonic radio link.
Takeaways & Limitations
Channel modeling, novel PHY and MAC protocols, transceiver RF frontends, and baseband DSP algorithms are identified as fundamental enablers for the THz system.
Takeaways & Limitations
The shared link budget between optical and wireless links is identified as a first challenge.
Abstract
from arXiv · showhide
This article discusses the basic system architecture for terahertz (THz) wireless links with bandwidths of more than 50 GHz into optical networks. New design principles and breakthrough technologies are required in order to demonstrate Tbps data-rates at near zero-latency using the proposed system concept. Specifically, we present the concept of designing the baseband signal processing for both the optical and wireless link and using an end-to-end (E2E) error correction approach for the combined link. We provide two possible electro-optical baseband interface architectures, namely transparent optical-link and digital-link architectures, which are currently under investigation. THz wireless link requirements are given as well as the main principles and research directions for the development of a new generation of transceiver frontends, which will be capable of operating at ultra-high spectral efficiency by employing higher-order modulation schemes. Moreover, we discuss the need for developing a novel THz network information theory framework, which will take into account the channel characteristics and the nature of interference in the THz band. Finally, we highlight the role of pencil-beamforming (PBF), which is required in order to overcome the propagation losses, as well as the physical layer and medium access control challenges.
I. INTRODUCTION
Beyond-5G THz networks are motivated by growing demand for high-rate, QoE-oriented wireless connectivity and limited 5G spectrum capacity. The paper proposes co-designing optical and wireless links while developing new RF, interface, modeling, waveform, coding, beamforming, and MAC technologies.
- Growing wireless-device use and bandwidth-intensive internet services increase demand for high-data-rate transmission.
- Limited 5G spectrum capacity and inefficient handling of large QoS- and QoE-oriented data motivate THz communications.
- THz networks require breakthrough concepts, including joint optical-wireless baseband DSP and broadband, spectrally efficient RF frontends above 275 GHz.
- The proposed vision targets Tbps wireless connectivity, exceeding the current 100 Gbit/s target by a factor of 10 through increased spectrum efficiency.
- The central design shift is from joint optimization toward co-design across optical and wireless links, backhaul and access, channels, waveforms, signals, coding, beam patterns, and MAC schemes.
- Progress beyond 5G depends on identifying technology gaps and developing, optimizing, and demonstrating appropriate enablers.
II. BEYOND 5G SCENARIOS, APPLICATIONS AND REQUIREMENTS
Beyond-5G scenarios demand Tbps-class wireless links with constraints on cost, range, reliability, and latency. The paper outlines rural backhaul, local access, and cyber-physical applications together with research and validation requirements.
- Networks beyond 5G must address performance limitations in bandwidth, transmission and processing delay, cost, and energy consumption.
- Research should validate little-explored technologies through early-stage de-risking, theoretical models, experimental evaluation, and system validation.
- THz wireless access for cyber physical systems: Cyber-physical access scenarios require Tbps-class connectivity with almost zero latency of approximately 1 ms and fast response constraints.
III. THZ WIRELESS-OPTICAL RADIO SYSTEMS
The paper proposes a photonic radio that aligns THz wireless links with coherent optical-transponder technology, using shared modem and DSP progress to connect the two domains. This approach targets data-rate compatibility between optical and THz links.
- System objective: The proposed system seeks data-rate compatibility between fiber-optical and THz wireless links to extend optical-network QoS and QoE.The concept is motivated by matching the rates of the optical and wireless portions of the combined system.
- Technology basis: Optical transponder chipsets are expected to support symbol rates up to 64 Gbd and modulation up to 64-QAM.Higher-order modulation up to 256-QAM or 1024-QAM at 64 Gbd has also been demonstrated experimentally in testbeds.
- Photonic radio concept: The photonic radio aligns THz wireless systems with coherent fiber-optic transponder technologies rather than conventional RF-over-Fiber.The approach is introduced to distinguish it from RoF and exploit optical coherent-transceiver developments.
- Photonic radio concept: Coherent optical-transceiver modems and associated DSP provide a basis for implementing THz wireless links.The paper highlights technological progress in optical modems and digital signal processing.
A. System Overview
The photonic-radio architecture maps baseband signals between a digital modem, optical transponder, and THz RF frontend. It uses coherent optical transmission and maps IQ components to the wireless frontend.
- System architecture: The generic photonic-radio concept maps baseband data signals between the RF frontend and optical transponder for backhaul and access scenarios.The same system concept can be directly applied to wireless access.
- System architecture: A digital baseband unit connects to a pluggable coherent optical transceiver through an optical link at the electro-optical interface.The architecture uses coherent optical-fiber transmission between the digital baseband and optical frontend.
- Signal mapping: Each optical polarization carries a QAM signal, while two baseband IQ components are mapped to the RF frontend.The optical and wireless interfaces therefore share quadrature-based signal representations.
- Signal mapping: Unlike RF-over-Fiber, photonic radios do not modulate the RF carrier onto the optical carrier.This avoids requiring ultra-broadband optical modulators and photodiodes, while using native fiber-optical baseband infrastructure.
B. Baseband Interface Architectures
The paper compares transparent and digital optical-link architectures for photonic radios. They trade off joint DSP and compactness against standardized interfaces, distance, capacity, and local processing requirements.
- Transparent optical-link architecture: The transparent optical-link architecture converts between optical and wireless signals without additional DSP elements visible to higher protocol layers.It may use commercially available analog coherent optical pluggable modules.
- Transparent optical-link architecture: Transparent links require the digital modem to address impairments from both the optical and wireless links through PHY-level DSP co-design.Analog mapping between optical and wireless signals remains a challenge, particularly for MIMO systems.
- Architecture challenges: A shared optical-wireless link budget may limit transmission distance or capacity, while modem-to-antenna separation can increase latency and restrict tracking speed.These constraints jointly affect the deployment choices for transparent architectures.
- Transparent optical-link architecture: The transparent architecture potentially integrates the optical and wireless frontend into one analog module with improved compactness and power consumption.This benefit is contrasted with the additional processing and link-budget constraints.
- Digital optical-link architecture: The digital optical-link architecture uses Ethernet-compliant pluggable modules but restricts optical-link distance and capacity to standardized specifications.A dedicated PHY near the THz antennas increases wireless-system complexity and power consumption.
- Architecture selection: The two architectures can also be combined, so proof-of-concept demonstrators are needed to identify the most suitable design for each application scenario.The digital architecture limits optical-wireless DSP co-design mainly to MAC and radio-resource-management functions.
C. THz Wireless Link Budget
The link-budget analysis estimates THz wireless data-rate limits under representative 300 GHz assumptions and highlights the importance of antenna gain, modulation order, coding, and atmospheric-loss modeling. Current capabilities support hundreds of gigabits per second, with higher projected limits as optical transponders advance.
- Propagation and implementation: Atmospheric attenuation and free-space propagation losses must be modeled accurately, while coding and linearization influence the power backoff needed for longer-range links.For short optical links of a few kilometers, the THz wireless link budget sets an upper-level power of 10 dBm.
- Link-budget assumptions: The analysis assumes a 10 dB receiver noise figure, at least 64 GHz bandwidth, 0 dBm linear transmit power, and an FEC threshold of BER = 2 × 10^-2.These values are close to those derived from recent 300 GHz experimental demonstrators.
- Link-budget assumptions: A 55 dBi antenna gain is assumed at both link ends, corresponding to a 225 mm aperture diameter and 80% aperture efficiency.The assumed directional antennas address the severe propagation-loss conditions of THz links.
- Transceiver constraints: Current III-V receiver performance limits modulation order to 128 because of the tradeoff between mixer linearity and receiver noise figure.Optical transponder chipsets are expected to support up to 64-QAM.
- Estimated data-rate bounds: 800 Gbps per link using two antenna polarizations is projected with coherent optical systems supporting 128-QAM.The paper presents these values as feasible within the next few years under predicted chipset progress.
- System implication: Data-rate compatibility between the optical and THz portions is presented as a plausible vision, without including the additional potential of full LOS MIMO architectures.The stated vision therefore remains bounded by the evaluated link-budget assumptions and architecture scope.
IV. DESIGN PRINCIPLES AND TECHNOLOGY ENABLERS
THz wireless-network design requires co-designing signals, codes, protocols, channel models, and pencil-beam antenna arrays. The section frames channel characteristics and frontend beamforming architectures as key enabling technologies.
- THz network design should jointly develop signals, codes, protocols, channel models, and pencil-beam antenna arrays.
- Channel characteristics below 1 THz and frontend architectures above 275 GHz are identified as important technology areas.
- Possible THz beamforming architectures are included among the enabling technologies under investigation.
A. THz Channel
The THz channel combines distance-, frequency-, and humidity-dependent losses, making channel modeling central to link and network design. FSPL dominates below 370 GHz over increasing distances, while molecular absorption dominates above 370 GHz and varies with atmospheric conditions.
- Frequency-dependent path-loss pre-equalization becomes relevant over several hundred meters, especially with spectrally efficient modulation.
- Combined optical-wireless channel models support the proposed end-to-end error-correction approach.
- THz propagation loss includes free-space path loss, antenna-aperture effects, and molecular absorption, whose impact increases with distance.
- Molecular absorption depends strongly on atmospheric water vapor, so regional propagation properties matter for standardization.
- Below 370 GHz, FSPL dominates total loss as distance increases; above 370 GHz, molecular absorption dominates.
- Below 1 THz, transmission windows with minimum molecular absorption can support long-distance links.
- Doubling water-vapor mixing ratios roughly doubles molecular-absorption loss on a dB scale.
- Heavy rain and dense fog add approximately 10 dB loss, while normal conditions can reduce total path loss by 10−20 dB.
B. THz Transceiver Frontend
THz transceiver frontends must combine high bandwidth, power, beamsteering, and integration capabilities with baseband compensation for higher-order modulation. The section highlights advances in semiconductor devices and the remaining packaging and circuit challenges.
- Broadband THz transceiver design faces both technological and circuit-implementation challenges.
- III-V HEMT and HBT technologies have dominated frontends because of their power and low-noise capabilities.
- CMOS high-frequency transistor solutions still lack bandwidth, while multichannel sub-band architectures can counter this limitation at greater frontend complexity.
- 300 GHz devices with bandwidths above 50 GHz have been realized for THz communications.
- Higher-order modulation requires baseband DSP to mitigate IQ imbalance, amplifier nonlinearity, and local-oscillator phase noise.
- Feedforward carrier-phase recovery can improve robustness at higher symbol rates, while thermal phase-noise floors limit high-symbol-rate operation.
- The baseband interface must support up to 64 Gbd, at least 64-QAM, optical-fiber mapping, and pre-equalization.
- Integrated optical-transceiver packaging creates technological challenges but may reduce system costs and enable commercial form factors.
C. Pencil Beamforming and Resource Management
Pencil beamforming is central to overcoming THz propagation loss, but narrow beams create stringent antenna, synchronization, channel-estimation, MAC, and resource-management requirements. Small arrays can reach high data rates, although larger arrays increase integration and power burdens.
- Reducing active antenna elements is an important enabler because array pitch constrains transceiver architecture and integration.
- A four-element array can support up to 200 Gbps with a 15° maximum antenna opening angle at a 10 m link distance.
- Increasing the array to 8 or 16 elements can raise data rate or improve the trade-off between data rate and opening angle.
- Beamsteering for limited arrays and scanning angles can use digital-baseband time shifts, LO-path phase shifts, or combinations of both.
- Distance-aware modulation and coding should adapt transmission bandwidth by using the entire window, its center, or its sides for different link distances.
- PBF requires accurate channel-state information, but channel estimation becomes infeasible as antenna counts increase.
- Cooperative beamforming with independent local oscillators requires tight synchronization and coordination among neighboring nodes.
- Narrow PBF beams require accurate pointing and automatic beam steering, while low-complexity synchronization mechanisms remain necessary.
V. CONCLUSIONS
The article presents a co-design concept for wired-optical and wireless-THz links and identifies the technologies and research directions needed to advance THz systems. Because development remains at an early stage, the final architecture and features cannot yet be specified precisely.
- The paper presents a concept for blending wired-optical and wireless-THz links, including their requirements and possible applications.
- The THz system and baseband interface remain at an early development stage, making the final architecture and features difficult to specify precisely.
- The identified enablers include channel modeling, novel PHY and MAC protocols, transceiver RF frontend design, and baseband DSP algorithms.
- Baseband DSP algorithms are intended to mitigate hardware constraints, while novel pencil-beamforming schemes are also identified as enablers.
- The article discusses a co-design principle and important research directions for developing THz systems.