Ultra-broadband Communications Research Articles

E-band telecom-compatible 40 dB gain high-power bismuth-doped fiber amplifier with record power conversion efficiency

Multi-band transmission is one of the key practical solutions to cope with the continuously growing demand on the capacity of optical communication networks without changing the huge existing fiber base. However, ultra-broadband communication requires the development of novel power efficient optical amplifiers operating beyond C- and L-bands, and this is a major research and technical challenge comparable to the introduction of the seminal erbium-doped fiber amplifiers that dramatically changed the optical communication sector. There are several types of optical fibers operating beyond C- and L-bands that can be used for the development of such amplifiers, specifically the fibers doped with neodymium, praseodymium, thulium, and bismuth. However, among these, Bi-doped fibers are of special interest as the most promising amplification medium because, unlike the others, different Bi-associated active centers allow amplification in an enormous band of overall width of 700 nm (1100–1800 nm). Such spectral coverage can be obtained by using different host materials, such as aluminosilicate, phosphosilicate, silica, and germanosilicate glasses. Here, we report a novel Bi-doped fiber amplifier with record characteristics for E-band amplification, including the highest power conversion efficiency among telecom-compatible E-band amplifiers reported to date. This bismuth-doped fiber amplifier (BDFA) features a maximum gain of 39.8 dB and a minimal noise figure of 4.6 dB enabled by 173 m Bi-doped fiber length. The maximum achieved power conversion efficiency of 38% is higher than that of L-band Er-doped fiber amplifiers. This performance demonstrates the high potential of BDFA for becoming the amplifier of choice in modern multi-band optical communication networks.

40-GHz Band Photodiode-Integrated Phased Array Antenna Module for Analog-Radio over Fiber toward Beyond 5G

In 5th generation (5G) and Beyond 5G mobile communication systems, it is expected that numerous antennas will be densely deployed to realize ultra-broadband communication and uniform coverage. However, as the number of antennas increases, total power consumption of all antennas will also increase, which leads to a negative impact on the environment and operating costs of telecommunication operators. Thus, it is necessary to simplify an antenna structure to suppress the power consumption of each antenna. On the other hand, as a way to realize ultra-broadband communication, millimeter waves will be utilized because they can transmit signals with a broader bandwidth than lower frequencies. However, since millimeter waves have a large propagation loss, a propagation distance is shorter than that of low frequencies. Therefore, in order to extend the propagation distance, it is necessary to increase an equivalent isotropic radiated power by beamforming with phased array antenna. In this paper, a phased antenna array module in combined with analog radio over fiber (A-RoF) technology for 40-GHz millimeter wave is developed and evaluated for the first time. An 8×8 phased array antenna for 40-GHz millimeter wave with integrated photodiodes and RF chains has been developed, and end-to-end transmission experiment including 20 km A-RoF transmission and 3-m over-the-air transmission from the developed phased array antenna has been conducted. The results showed that the 40-GHz RF signal after the end-to-end transmission satisfied the criteria of 3GPP signal quality requirements within ±50 degrees of main beam direction.

Hierarchical Bandwidth Modulations for Ultra-Broadband Communications in the Terahertz Band

Terahertz (THz)-band (0.1–10 THz) communication will be key in enabling high speed wireless links due to the wide available bandwidths. At THz frequencies, the path-loss is governed by high spreading loss due to small antenna apertures and by molecular absorption loss due to water vapor. The latter also determines the available transmission bandwidth, which shrinks with distance. Modulations that consider the high propagation loss and the distance-dependent bandwidth are needed to fully exploit the THz channel’s bandwidth. Using a hierarchical constellation to simultaneously service users at symbol rates, Hierarchical Bandwidth Modulation (HBM) leverages molecular absorption to increase aggregate data rates in a broadcast system while offering flexibility to receivers experiencing high path loss. This paper introduces HBM and evaluates its performance. The symbol error rate performance for a 4/M-QAM HBM system is derived and verified using simulations. These results are used to define the design constraints for an HBM system: the HBM functional region and transition region. The functional region is verified using an experimental testbed for ultrabroadband communications. The results show that with proper design HBM successfully achieves its goal to exploit the distance-dependent characteristics of the THz channel, to spatially multiplex users, and to increase the system capacity.

A Review on Massive MIMO Antennas for 5G Communication Systems on Challenges and Limitations

High data rate transfers, high-definition streaming, high-speed internet, and the expanding of the infrastructure such as the ultra-broadband communication systems in wireless communication have become a demand to be considered in improving quality of service and increase the capacity supporting gigabytes bitrate. Massive Multiple-Input Multiple-Output (MIMO) systems technology is evolving from MIMO systems and becoming a high demand for fifth-generation (5G) communication systems and keep expanding further. In the near future, massive MIMO systems could be the main wireless systems of communications technology and can be considered as a key technology to the system in daily lives. The arrangement of the huge number of antenna elements at the base station (BS) for uplink and downlink to support the MIMO systems in increasing its capacity is called a Massive MIMO system, which refers to the vast provisioning of antenna elements at base stations over the number of the single antenna of user equipment. Massive MIMO depends on spatial multiplexing and diversity gain in serving users with simple processing signal of uplink and downlink at the BS. There are challenges in massive MIMO system even though it contains numerous number of antennas, such as channel estimation need to be accurate, precoding at the BS, and signal detection which is related to the first two items. On the other hand, in supporting wideband cellular communication systems and enabling low latency communications and multi-gigabit data rates, the Millimeter-wave (mmWave) technology has been utilized. Also, it is widely influenced the potential of the fifth-generation (5G) New Radio (NR) standard. This study was specifically review and compare on a few designs and methodologies on massive MIMO antenna communication systems. There are three limitations of those antennas were identified to be used for future improvement and to be proposed in designing the massive MIMO antenna systems. A few suggestions to improve the weaknesses and to overcome the challenges have been proposed for future considerations.

5G/B5G mmWave Cellular Networks with MEC Prefetching Based on User Context Information.

To deal with recent increasing mobile traffic, ultra-broadband communication with millimeter-wave (mmWave) has been regarded as a key technology for 5G cellular networks. In a previous study, a mmWave heterogeneous network was composed of several mmWave small cells overlaid on the coverage of a macro cell. However, as seen from the optical fiber penetration rate worldwide, it is difficult to say that backhaul with Gbps order is available everywhere. In the case of using mmWave access under a limited backhaul capacity, it becomes a bottleneck at the backhaul; thus, mmWave access cannot fully demonstrate its potential. On the other hand, the concept of multi-access edge computing (MEC) has been proposed to decrease the response latency compared to cloud computing by deploying storage and computation resources to the user side of mobile networks. This paper introduces MEC into mmWave heterogeneous networks and proposes a content prefetching algorithm to resolve such backhaul issues. Context information, such as the destination, mobility, and traffic tendency, is shared through the macro cell to the prefetch application and data that the users request. Prefetched data is stored in the MEC and then transmitted via mmWave without a backhaul bottleneck. The effectiveness is verified through computer simulations where we implement realistic user mobility as well as traffic and backhauling models. The results show that the proposed framework achieved 95% system capacity even under the constraint of a 1 Gbps backhaul link.

A versatile experimental testbed for ultrabroadband communication networks above 100 GHz

Communication at terahertz (THz) frequencies is envisioned as a near-future landmark in wireless networking and a key component of the sixth generation (6G) wireless systems and beyond. In the last decade, major progress has happened in terms of device technology development as well as THz-wave propagation and channel modeling. In order to advance THz communication and networking research, there is tremendous necessity in developing programmable software-defined hardware and architectures that operate at THz frequencies and are able to process signals with tens to hundreds of GHz of bandwidth, thus making most out of moving to THz band. This paper presents a versatile testbed for conducting wireless experimental research above 100 GHz. The platform consists of multiple sets of analog front-ends at three different frequencies between 100 GHz and 1 THz as well as three different digital signal processing back-ends, able to manipulate more than 10 GHz of bandwidth in real-time. Implementation details and early experimental results to demonstrate the platform capabilities are presented.

Intelligent Radio for Next Generation Wireless Communications: An Overview

This article discusses various issues on intelligent radio (IR) from a perspective of intelligent UE and intelligent BS. Unlike a traditional UE, an intelligent UE can automatically access channel resources, cooperate with other UEs, and manage interferences. To implement such an intelligent UE, innovative techniques should be used, such as smart resource sensing and grant-free transmission. The core of IR lies in the development of thinking hardware, which can collect/process vital information of the environment and manage its resources optimally to support ultra-broadband communications. In particular, an integrated radar-communication unit is one of the most critical environment sensors in an intelligent UE, which helps it to acquire accurate location and mobility information of its neighbors for cooperation and interference avoidance. The traditional physical layer designs (such as beamforming and MIMO) should be revolutionized for the development of IR. ML and other AI algorithms play an important role in both BS and UEs. The communication protocols should be re-designed as well. The IR can offer a very high spectrum efficiency and power efficiency and is one of the most important research topics for future-generation wireless communications.

Compressed Detection for Pulse‐Based Communications in the Terahertz Band

Terahertz (THz) band (0.1‐10 THz)‐based electromagnetic communications are envisioned as a key technology to enable future high‐data‐rate short‐range ultrabroadband communications. However, one of the fundamental bottlenecks is the efficiency of the analog‐to‐digital converters (ADCs) considering the formidable challenge of sampling the signal at the Nyquist rate eventually increasing transceiver design complexity. The Compressed sensing (CS) framework enables the successful reconstruction of sparse signals from a small set of projections onto a random vector which would lead to sub‐Nyquist rate sampling. In this paper, THz band channel estimation based on the theory of CS is developed. The proposed approach exploits the fact that transmitting an ultrashort pulse through a multipath THz channel leads to a received THz signal that can be approximated by a linear combination of a few atoms from a predefined dictionary, yielding thus a sparse representation of the received signal. The fundamental idea is in the design of the dictionary of atoms that closely matches the transmitted pulse leading thus to a higher probability of CS reconstruction. The Orthogonal Matching Pursuit (OMP) algorithm is used to identify the strongest atoms in the projected signal. This reconstructed signal is subsequently used as a reference template in a correlator‐based detector. The bit error rate (BER) performance of the proposed detector is analyzed and compared with the conventional CS‐based channel estimation and reconstruction approach. Extensive simulations show that, for different design parameters, our proposed detector outperforms the traditional CS‐based correlator receiver for the same sampling rate leading thus to a much‐reduced use of analog‐to‐digital resources. Moreover, the proposed detector has been shown to reduce the hardware complexity of the receiver by significantly reducing the number of parallel mixer‐integration branches.

Narrow plasmon resonances enabled by quasi-freestanding bilayer epitaxial graphene

Exploiting the underdeveloped terahertz range (~1012–1013 Hz) of the electromagnetic spectrum could advance many scientific fields (e.g. medical imaging for the identification of tumors and other biological tissues, non-destructive evaluation of hidden objects or ultra-broadband communication). Despite the benefits of operating in this regime, generation, detection and manipulation have proven difficult, as few materials have functional interactions with THz radiation. In contrast, graphene supports resonances in the THz regime through structural confinement of surface plasmons, which can lead to enhanced absorption. In prior work, the achievable plasmon resonances in such structures have been limited by multiple electron scattering mechanisms (i.e. large carrier scattering rates) which greatly broaden the resonance (>100 cm−1; 3 THz). We report the narrowest room temperature Drude response to-date, 30 cm−1 (0.87 THz), obtained using quasi-free standing bilayer epitaxial graphene (QFS BLG) synthesized on (0 0 0 1)6H–SiC. This narrow response is due to a 4-fold increase in carrier mobility and improved thickness and electronic uniformity of QFS BLG. Moreover, QFS BLG samples patterned into microribbons targeting 1.8–5.7 THz plasmon resonances also exhibit low scattering rates (37–53 cm−1). Due to the improved THz properties of QFS BLG, the effects of e-beam processing on carrier scattering rates was determined and we found that fabrication conditions can be tuned to minimize the impact on optoelectronic properties. In addition, electrostatic gating of patterned QFS BLG shows narrow band THz amplitude modulation. Taken together, these properties of QFS BLG should facilitate future development of THz optoelectronic devices for monochromatic applications.

Lithographically Defined Plasmonic Graphene Antennas for Terahertz-Band Communication

Graphene-based plasmonic antennas could enable ultrabroadband communication among nanodevices in the Terahertz band (0.1–10 THz). Despite significant progress in graphene synthesis, the fabrication, manipulation, and placement of precisely formed graphene-based nanostructures still pose many practical challenges. In this letter, by comparing analytical solutions with numerical simulations, we show that lithographically defined graphene antennas, circumscribed by a focused ion beam (FIB), should behave as though they were electromagnetically isolated provided the width of the surrounding cut is comparable to the plasmonic wavelength. This result opens the door to practical and more advanced antenna designs.

TeraNets: ultra-broadband communication networks in the terahertz band

Terahertz Band communication is envisioned as a key technology to satisfy the data rate requirements of future wireless communication networks, and enable new applications both in classical networking domains as well as in novel nanoscale communication paradigms. Major recent advancements in THz Band technologies are helping to finally close the so called THz Gap and bring practical THz Band communication networks one step closer. This paper surveys the state of the art in THz Band device technologies, and highlights the challenges and potential solutions from the communication and networking perspective as well as in terms of experimental testbeds. Ultimately, a roadmap is defined for the development of THz Band systems as the new frontier in wireless communications.

Ultra broadband indoor channel measurements and calibrated ray tracing propagation modeling at THz frequencies

Ultra broadband communication systems operated at THz frequencies will require the thorough knowledge of the propagation channel. Therefore, an extensive measurement campaign of 50 GHz wide indoor radio channels is presented for the frequencies between 275 and 325 GHz. Individual ray paths are resolved spatially according to angle of arrival and departure. A MIMO channel is recorded in a 2×2 configuration. An advanced frequency domain ray tracing approach is used to deterministically simulate the THz indoor propagation channel. The ray tracing results are validated with the measurement data. Moreover, the measurements are utilized for the calibration of the ray tracing algorithm. Resulting ray tracing accuracies are discussed.

Short-Range Ultra-Broadband Terahertz Communications: Concepts and Perspectives

We propose the concept of ultra-broadband terahertz communication, based on directed non-line-of-sight (NLOS) transmissions. Potential applications of such a system supporting multi-gigabit data rates are given, and put into the context of currently emerging WLANs/WPANs. The technology and propagation constraints serve as boundary conditions for the determination of the required antenna gain to support ultra-broadband communication. Resulting high-gain antenna requirements will necessitate highly directed transmissions. We propose the use of omni-directional dielectric mirrors to support directed NLOS paths. Their performance is investigated with ray-tracing simulations of a terahertz propagation channel in a dynamic office environment, which is calibrated with measured building-material and mirror parameters. We demonstrate that a directed NLOS path scheme will make a terahertz communication system robust to shadowing. Furthermore, we show that dielectric mirrors covering only parts of the walls will significantly enhance the signal coverage in a typical indoor scenario.