Large Bandwidth-delay Product Research Articles

Pseudospin-polarized slow light waveguides with large delay-bandwidth product

Delay-bandwidth product (DBP) is a key metric in slow light waveguides, requiring a balance between a large group index and broad bandwidth—two parameters that often involve a trade-off. Here, we propose and demonstrate a slow light waveguide with large DBP using a pseudospin-polarized transverse electromagnetic mode. This waveguide features a folded edge configuration that supports a 200% relative bandwidth from quasistatic limit (zero frequency) and an arbitrarily large group index. Owing to the pseudospin-polarized design, the dense folding would not introduce backscattering and the associated group velocity dispersion (GVD). The resulting gapless linear dispersion and pulse transmission behavior in folded edge waveguide are observed in microwave experiments. Our scheme provides a way to overcome the trade-off between group index and working bandwidth in slow light waveguide, which has potential applications in broadband optical buffering, light-matter interaction enhancement, terahertz radiation source and time domain processing.

Large group delay and low loss optical delay line based on chirped waveguide Bragg gratings.

On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23 nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87 ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57 dB/ns and the total insertion loss of the device is 6 dB at the central wavelength near 1550 nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.

Coupled CUBIC Congestion Control for MPTCP in Broadband Networks

Recently, multipath transmission control protocol (MPTCP) was standardized so that data can be transmitted through multiple paths to utilize all available path bandwidths. However, when high-speed long-distance networks are included in MPTCP paths, the traffic transmission performance of MPTCP is severely deteriorated, especially in case the multiple paths’ characteristics are heavily asymmetric. In order to alleviate this problem, we propose a “Coupled CUBIC congestion control” that adopts TCP CUBIC on a large bandwidth-delay product (BDP) path in a linked increase manner for maintaining fairness with an ordinary TCP traversing the same bottleneck path. To verify the performance excellence of the proposed algorithm, we implemented the Coupled CUBIC Congestion Control into Linux kernels by modifying the legacy MPTCP linked-increases algorithm (LIA) congestion control source code. We constructed asymmetric heterogeneous network testbeds mixed with large and small BDP paths and compared the performances of LIA and Coupled CUBIC by experiments. Experimental results show that the proposed Coupled CUBIC utilizes almost over 80% of the bandwidth resource in the high BDP path, while the LIA utilizes only less than 20% of the bandwidth for the same path. It was confirmed that the resource utilization and traffic transmission performance have been greatly improved by using the proposed Coupled CUBIC in high-speed multipath networks, as well as maintaining MPTCP fairness with competing single-path CUBIC or Reno TCP flows.

Photonic Moiré lattice waveguide with a large slow light bandwidth and delay-bandwidth product.

We proposed an effective approach to enlarge the slow light bandwidth and normalized-delay-bandwidth product in an optimized moiré lattice-based photonic crystal waveguide that exhibits intrinsic mid-band characteristics. A flatband corresponding to a nearly constant group index of 34 over a wide bandwidth of 82 nm centered at 1550 nm with near-zero group velocity dispersion was achieved. A large normalized-delay-bandwidth product of 0.5712 with a relative dispersion of 0.114%/µm was obtained, which is a significant improvement if compared with previous results. Our results indicate that the photonic moiré lattice waveguide could advance slow light applications.

Simulation of dynamically tunable broadband and polarization-insensitive electromagnetically induced transparency in Dirac semimetal

The structure proposed in this paper is composed of an outer circular ring resonator (CRR) and four L-shaped resonators (LSRs) in the middle. The destructive interference between bright modes produces a broadband electromagnetically induced transparency (EIT) with a maximum transmission efficiency of 97.6% in both vertical polarization directions. The field distributions and the three-level Λ-type system clarify the physical mechanism of the coupling process between bright modes. The calculated transmission spectrum based on the two-particle model is consistent with the simulated transmission spectrum, which proves the correctness of the results. By introducing hybridization bandgap (HBG) to the structure, the bandwidth of EIT-like effect expands. This structure has a fourfold (C4) rotational symmetry; thus, it is insensitive to the polarization angle. In addition, the transparency window undergoes a redshift as the Fermi energy of Dirac semimetal decreases. Meanwhile, the group delay is also controlled. Due to the wide bandwidth, a large delay bandwidth product (DBP) is obtained. Our work provides a new method for the production of slow-light devices and broadband filters.

Lightweight network-coded ARQ: An approach for Ultra-Reliable Low Latency Communication

Sliding Window RLNC schemes represent a novel, promising and efficient class of algorithms for reliable data transmission over an unreliable link. They combine Random Linear Network Coding (RLNC) with the idea of sliding window from (Automatic Repeat reQuest) ARQ protocols in order to help the basic ARQ mechanism to quickly recover from losses through redundant coded packets. Although very successful, we make the observation that such schemes bear significant limitations stemming from the fact that both the ARQ mechanism and the coding process operate on the same group of packets, called the sliding window. To tackle the problem, we propose the use of two distinct windows; the sliding window, which can be used to optimize the data flow based on the link’s bandwidth-delay product, and the coding window that can be used exclusively in the coding process. We analyze the performance of the proposed strategy and show that it provides an improved coding operation while at the same time reduces the coding complexity. Then, we delineate rapidARQ, a sliding window RLNC protocol that adopts the proposed strategy, and experimentally confirm that it outperforms other state-of-the-art sliding window RLNC schemes. More specifically, it achieves superior throughput-delay performance that better fits in the context of Ultra-Reliable Low-Latency Communication (URLLC) while at the same time it significantly reduces the coding complexity. Moreover, we show that the superior performance of rapidARQ is more prominent in channels with large bandwidth-delay products, a fact that renders its utility to current and future networks more essential.

Fast Beam Training With True-Time-Delay Arrays in Wideband Millimeter-Wave Systems

The best beam steering directions are estimated through beam training, which is one of the most important and challenging tasks in millimeter-wave and sub-terahertz communications. Novel array architectures and signal processing techniques are required to avoid prohibitive beam training overhead associated with large antenna arrays and narrow beams. In this work, we leverage recent developments in true-time-delay (TTD) arrays with large delay-bandwidth products to accelerate beam training using frequency-dependent probing beams. We propose and study two TTD architecture candidates, including analog and hybrid analog-digital arrays, that can facilitate beam training with only one wideband pilot. We also propose a suitable algorithm that requires a single pilot to achieve high-accuracy estimation of angle of arrival. The proposed array architectures are compared in terms of beam training requirements and performance, robustness to practical hardware impairments, and power consumption. The findings suggest that the analog and hybrid TTD arrays achieve a sub-degree beam alignment precision with 66% and 25% lower power consumption than a fully digital array, respectively. Our results yield important design trade-offs among the basic system parameters, power consumption, and accuracy of angle of arrival estimation in fast TTD beam training.

具有宽带宽和高归一化延迟带宽积的光子晶体波导慢光传输

具有宽带宽和高归一化延迟带宽积的光子晶体波导慢光传输

Dispersionless one-way slow wave with large delay bandwidth product at the edge of gyromagnetic photonic crystal

We theoretically, demonstrate a high delay bandwidth product (DBP) and zero group velocity dispersion (GVD) in a two-dimensional one-way slow light waveguide. The waveguide consists of gyromagnetic photonic crystal (GMPC) and a cladding formed by silicon photonic crystal. At the edge of the band, weak interactions (“semi-anticrossing”) between the chiral edge state (CES) mode and the mode localized at the surface of cladding are observed. The group velocity of CES wave can be tuned by adjusting the modal field distribution. As a result, an extraordinarily large value of normalized DBP of 0.63 with a group index of 10.32 and a bandwidth ranging from [Formula: see text] to [Formula: see text] is obtained. This result may contribute to one-way slow light applications in information communication systems.

Proposal of cross‐layer bandwidth assignment with buffer size indication for TCP flow control

AbstractIn this paper, we propose, verify, and evaluate a method combining cross‐layer method and bandwidth allocation to overcome the difficulty of flow control in a network with large bandwidth delay product. When the bandwidth delay product is large, there are problems with the convergence time, efficiency and stability of bandwidth utilization, and fairness among traffics of different round‐trip time. As a result of examining the conventional methods, we propose a novel method that can be realized as a functional addition to the existing transmission control protocol (TCP) flow control with improvement in utilization efficiency, fairness, stability, and convergence time. The method is compared with TCP Reno, FAST TCP, CUBIC, Explicit Congestion Notification, and eXplicit Control Protocol by simulation, and is confirmed as having good characteristics.

Improved slow light performances using photorefractive two-wave mixing.

We experimentally observe an ultralow effective group velocity of 0.9cm/s of light pulses using the two-wave mixing process in an Sn2P2S6 (SPS):Te crystal at a visible wavelength. The time delay can be controlled through the nonlinear photorefractive gain and the input pulse duration. By optimizing the nonlinearity strength, we achieve a maximum fractional delay of 0.79 for a pulse duration of 100ms. Our photorefractive slow light system allows combining low group velocity with large delay-bandwidth product for pulse durations spanning three orders of magnitude.

DVPTCP: A Delay-Driven Virtual Parallel TCP for High-Speed and Lossy Networks

The growing diversification of network architectures and application demands results in poor performance of most existing TCPs, especially in large bandwidth-delay product (BDP) and lossy networks. To tackle this problem, we propose a novel delay-based congestion control algorithm, named DVPTCP, which estimates the network congestion level by using round-trip time (RTT) and adjusts the number of virtual parallel streams dynamically. Related theoretical analyses about throughput and TCP-friendliness are given in detail. According to our extensive simulation tests, our proposal can achieve better bandwidth utilization and TCP-friendliness than the compared schemes in large-BDP and lossy networks.

Performance evaluation of TCP-BIAD in high-speed, long-distance networks

In this paper, the performance of Binary Increase Adaptive Decrease (TCP-BIAD) congestion control algorithm in high-speed long-distance networks is evaluated. As its name implies, this TCP variant is a combination of an enhanced binary increase algorithm during the congestion avoidance phase with the adaptive decrease mechanism of TCP Westwood after a packet loss episode. We also propose a mathematical analysis of the TCP-BIAD paradigm to study the steady-state throughput provided by TCP-BIAD and investigate the intra-protocol friendliness between TCP-BIAD and Additive Increase/Multiplicative Decrease algorithms. Our analysis shows that TCP-BIAD algorithm is exponentially stable, while maintaining an adequately fair and friendly behavior with respect to co-existing TCP-Reno flows. Finally, our results are validated with respect to other TCP variants such as BIC-TCP, CUBIC, HighSpeed TCP, HTCP, Hybla and TCP-Reno by means of computer simulations in networks with large bandwidth-delay products and low sensitivity to RTT values.

A polarization independent electromagnetically induced transparency-like metamaterial with large group delay and delay-bandwidth product

In this study, a classical analogue of electromagnetically induced transparency (EIT) that is completely independent of the polarization direction of the incident waves is numerically and experimentally demonstrated. The unit cell of the employed planar symmetric metamaterial structure consists of one square ring resonator and four split ring resonators (SRRs). Two different designs are implemented in order to achieve a narrow-band and wide-band EIT-like response. In the unit cell design, a square ring resonator is shown to serve as a bright resonator, whereas the SRRs behave as a quasi-dark resonator, for the narrow-band (0.55 GHz full-width at half-maximum bandwidth around 5 GHz) and wide-band (1.35 GHz full-width at half-maximum bandwidth around 5.7 GHz) EIT-like metamaterials. The observed EIT-like transmission phenomenon is theoretically explained by a coupled-oscillator model. Within the transmission window, steep changes of the phase result in high group delays and the delay-bandwidth products reach 0.45 for the wide-band EIT-like metamaterial. Furthermore, it has been demonstrated that the bandwidth and group delay of the EIT-like band can be controlled by changing the incidence angle of electromagnetic waves. These features enable the proposed metamaterials to achieve potential applications in filtering, switching, data storing, and sensing.

High-Capacity and Dispersionless Delay Lines Based on Plasmonic Waveguide Periodically Coupled With Bilaterally Located Ring and Slot Resonators

We propose and study optical delay lines with high capacity and low dispersion based on a plasmonic waveguide periodically coupled with bilaterally located ring and slot resonators. The repetitive unit exhibits plasmon-induced transparency (PIT) with a large delay-bandwidth product due to the interference between the two-mode ring and single-mode slot resonators. By choosing appropriate parameters, the cascaded PIT units can possess a broad transparency window with moderate fluctuations and produce wideband dispersionless slow light. A theoretical model is developed to analyze the proposed structure and the results are validated by finite-difference time-domain simulations. The pulse propagation results indicate that the spectral fluctuations have a negligible effect on the delayed pulse shape. Using multiple cascaded PIT units, we demonstrate that the group index of the waveguide approaches 36, the 3-dB bandwidth is larger than 21 nm, and a storage efficiency of 0.5 bit/unit can be realized with negligible distortion. Due to the high performance, the proposed delay line may find potential applications in future nanoscale integrated photonic systems.

A 2-GHz Bandwidth, 0.25–1.7 ns True-Time-Delay Element Using a Variable-Order All-Pass Filter Architecture in 0.13 $\mu$ m CMOS

An all-pass filter architecture that can be generalized to high orders, and can be realized using active circuits is proposed. Using this, a compact true-time-delay element with a widely tunable delay and a large delay-bandwidth product (DBW) is demonstrated. This is useful for beamforming and equalization in the lower GHz range where the use of $LC$ or transmission line-based solutions to realize large delays is infeasible. Coarse tuning of delay is realized by changing the filter’s order while keeping the bandwidth constant and fine tuning is implemented by changing the filter’s bandwidth utilizing the delay-bandwidth tradeoff. A test chip fabricated in 0.13 $\mu \text{m}$ CMOS process demonstrates a delay tuning range of 250 ps–1.7-ns, over a bandwidth of 2 GHz, while maintaining a magnitude deviation of ±0.7 dB. The filter achieves a DBW of 3.4 and a delay per unit area of 5.8 $\mathrm {ns/mm^{2}}$ . The filter has a worst case noise figure of 23 dB, and −40 dB intermodulation (IM3) distortion for 37 mVppd inputs. The chip occupies an active area of 0.6 mm2, and dissipates 112 mW–364 mW of power between its minimum and maximum delay settings. Computed radiation pattern with four antennas spaced $\mathrm {\lambda _{fmax}}/2$ apart shows ±90° beam steering off broadside.

Service Curve Estimation-Based Characterization and Evaluation of Closed-Loop Flow Control

Closed-loop flow control protocols, such as the prominent implementation transmission control protocol (TCP), are prevalent in the Internet, today. TCP has continuously been improved for greedy traffic sources to achieve high throughput over networks with large bandwidth delay products. Recently, the increasing use for streaming and interactive applications, such as voice and video, has shifted the focus toward its delay performance. Given the need for real-time communication of non-greedy sources via TCP, we present an estimation method for performance evaluation of closed-loop flow control protocols. We characterize an end-to-end connection by a service curve that provides statistical guarantees for arbitrary traffic. The estimation is based on end-to-end measurements at the application level that include all effects induced by the network and by the protocol stacks of the end systems. From our measurements, we identify different causes for delays. We show that significant delays are due to queueing in protocol stacks. Notably, this occurs even if the utilization is moderate. Using our estimation method, we compare the impact of fundamental mechanisms of TCP on delays at the application level: in detail, we analyze parameters relevant for network dimensioning, including buffer provisioning and active queue management, and parameters for server configuration, such as the congestion control algorithm. By applying our method as a benchmark, we find that a good selection can largely improve the delay performance of TCP.

Slow light photonic crystal waveguides with large delay-bandwidth product

We present a slow light photonic crystal waveguide. In our structure design, we displace some air holes perpendicular to the line defect. By tuning the group index, ng, from 11.8 to 40, we attain a wide bandwidth varying from 15.9 to 46.9 nm. The group velocity dispersion is kept below 10−20 m−1 s2, and the normalized delay—bandwidth product of 0.448—is attained at ng=14.8 by this structure. This study uses plane wave expansion and finite-difference time domain methods.

TCP-FIT: An improved TCP algorithm for heterogeneous networks

Wireless networks and large bandwidth delay product (BDP) networks are two types of challenging environments for TCP congestion control. Many congestion control algorithms have been proposed to improve the performance of TCPs in these two environments. Although these improved algorithms can achieve remarkable performance enhancements for either of the two environments, designing a congestion algorithm that performs well over heterogeneous networks that contain both wireless and large BDP links remains a great challenge. In this study, we propose a novel congestion avoidance algorithm called TCP-FIT, which can perform excellently over both wireless and large BDP links while maintaining good fairness with the standard TCP Reno algorithm. To evaluate the proposed algorithm, theoretical analysis is presented for equilibrium, network utilization, stability, TCP friendliness, RTT fairness, and responsiveness. A series of experimental results obtained using network emulators and over the “live” Internet are also presented to demonstrate the significant performance and fairness improvements of TCP-FIT compared with other state-of-the-art algorithms.

Improved Slow Light Capacity In Graphene-based Waveguide.

We have systematically investigated the wideband slow light in two-dimensional material graphene, revealing that graphene exhibits much larger slow light capability than other materials. The slow light performances including material dispersion, bandwidth, dynamic control ability, delay-bandwidth product, propagation loss, and group-velocity dispersion are studied, proving graphene exhibits significant advantages in these performances. A large delay-bandwidth product has been obtained in a simple yet functional grating waveguide with slow down factor c/vg at 163 and slow light bandwidth Δω at 94.4 nm centered at 10.38 μm, which is several orders of magnitude larger than previous results. Physical explanation of the enhanced slow light in graphene is given. Our results indicate graphene is an excellent platform for slow light applications, promoting various future slow light devices based on graphene.