High-speed Communication Systems Research Articles

High-Speed Railway Communication System Using Linear-Cell-Based Radio-Over-Fiber Network and Its Field Trial in 90-GHz Bands

A linear-cell-based radio-over-fiber (LC-RoF) system is proposed and demonstrated for efficient mobile communication in high-speed trains without hard handover processes. The configuration of the LC-RoF network and possible router architectures are discussed for the field trial test of the proposed system and for advanced systems in the future. Via the LC-RoF network, 90-GHz, millimeter-wave radio access between the ground and a train car is operated in a centralized manner. The resulting throughput of 1.5 Gbit/s is achieved in the field trial test, which is comprised of 250-Mbaud four-subcarrier differential quadrature phase-shift keying with a forward-error correction code implemented in field-programmable gate arrays to the Shinkansen train traveling at 240 km/h without any interruptions in the connections at the change of radio access units.

Ring‐modulator‐based RoF system with local SSB modulation and remote carrier reuse

A full-duplex radio-over-fibre (RoF) system based on an integrated silicon ring modulator is proposed and demonstrated. For the downstream link, a coherent dual-wavelength laser source is coupled to a silicon ring modulator in the central office (CO). Since only one of the optical carriers in the dual-wavelength laser source is aligned to the resonance of the ring modulator, a single sideband (SSB) modulated optical downstream signal is obtained, which is able to combat the power fading introduced by the fibre dispersion. Besides, for the upstream link, the unmodulated optical carrier in the SSB-modulated optical downstream signal is reused by using an optical filter in the remote radio head. After being modulated by the upstream data, the optical upstream signal is transmitted back to the CO. A proof-of-concept experiment is carried out. Error vector magnitudes of 21-GHz downstream and 10-GHz upstream signals are measured, which confirms that the proposed architecture is a promising low-cost solution for future high-speed wireless communication systems.

A 2.5Gbps Transceiver and Channel Architecture for High-Speed Automotive Communication System

A 2.5Gbps Transceiver and Channel Architecture for High-Speed Automotive Communication System

Digital Orthogonal-Filters Enhanced Spatial Modulation for High-Speed Indoor Optical Wireless Communications

Spatial modulation (SM) has been proposed and studied in high-speed indoor optical wireless communication systems to improve the spectral efficiency and system capacity by exploring the spatial domain information of transmitters. However, the correct detection of active transmitter(s) is critical, and it typically requires precise channel state information (CSI) and low correlation between channel gains. To overcome this limit, in this paper an orthogonal-filters enhanced SM scheme is proposed for high-speed indoor optical wireless communication systems, where the use of orthogonal-filters can improve the correct detection of active transmitter(s) significantly. The proposed scheme is experimentally demonstrated, and results show that with 4-pulse amplitude modulation (PAM-4) and two transmitters, 7.5 Gb/s optical wireless data transmission is achieved with 2.5 GS/s symbol rate. The power penalty of proposed scheme on the receiver sensitivity is negligible, and the active transmitter(s) can be correctly detected even under high channel correlations and without CSI. The scalability of proposed orthogonal-filters enhanced SM scheme is also studied, and it is shown that higher dimensions can be realized to achieve higher spectral efficiency, which provides a promising solution in indoor optical wireless communications to achieve ultra-high bit rate.

Recent Developments and Future Challenges in Channel Measurements and Models for 5G and Beyond High-Speed Train Communication Systems

The fast developments of high-speed train (HST) and growing communication demands of users pose new challenges for HST communication systems. To provide reliable communication services, key 5G communication technologies, such as millimeter-wave (mmWave) and massive multiple-input multiple-output (MIMO), and potential architectures, such as coordinated multipoint and mobile relay station, are considered in future railway communications. New technologies and architectures will bring new channel propagation characteristics. Accurate channel models that can capture these channel characteristics play a critical role in the design and test of future railway communication systems. This article reviews the HST channel measurements conducted in different scenarios and frequency bands, and the advanced HST channel models. Then a novel HST channel model incorporating the mmWave and massive MIMO propagation characteristics is presented, and its frequency non-stationarity is analyzed. Finally, some future directions are discussed.

Common-anode LED on a Si substrate for beyond 15 Gbit/s underwater visible light communication

Visible light communication based on light-emitting diodes (LEDs) has become a promising candidate by providing high data rates, low latency, and secure communication for underwater environments. In this paper, a self-designed common-anode GaN-based five-primary-color LED (RGBYC LED) on a Si substrate is proposed and fabricated. The design of a common anode is used to mitigate the saturation effect for a low-frequency component. Additionally, compared with commercially available LEDs that suffer from nonlinearity distortion, applying the designed LED can provide much better and broader linearity according to the measurement results. Therefore, the modulation depth and system performance can be further improved to implement a high-speed underwater visible light communication (UVLC) system. There is no nonlinearity compensation algorithm applied due to the good linearity of the proposed LED; thus, the offline digital signal processing is simplified. We experimentally demonstrate 14.81 Gbit/s 64 quadrature amplitude modulation (QAM)-discrete multitone (DMT) and 15.17 Gbit/s bit-loading-DMT transmissions through a 1.2-m-long underwater channel based on the proposed RGBYC LED with an intrasymbol frequency-domain averaging channel estimation and zero-forcing equalization. As far as we know, this is the highest data rate for an LED-based UVLC system.

Optimized design of single-pump fiber optical parametric amplifier with highly nonlinear fiber segments using a differential evolution algorithm

Fiber optical parametric amplifiers (FOPAs) are regarded as one of the candidate optical amplifiers for future ultralong-distance, large-capacity, and high-speed optical fiber communication systems. FOPAs provide high signal gain and low noise figures compared with other optical amplifiers. However, they suffer from gain fluctuation, which restricts commercial application. Optimization of the dispersion parameters of highly nonlinear fibers (HNLFs) employed as gain media is essential because they have an immediate impact on the flatness of the amplifier. An optimization method for multiple HNLF segments using the differential evolution algorithm is proposed to minimize spectral gain variation. We theoretically yield an FOPA with an average signal gain of 20 dB and gain fluctuation

High responsivity random metal grating couplers for terahertz quantum well photodetectors

Terahertz quantum-well photodetectors (THz QWPs) have advantages in constructing THz high-speed communication and fast imaging systems. In this paper, GaAs/(Al,Ga)As THz QWPs with metal random-grating couples are designed, fabricated, and characterized. Both one-dimensional metal random grating devices and a contrast device with 45°-incident facet are fabricated, and their peak responsivities and photocurrent spectra are characterized with the samples soaked in the liquid helium. Compared with 45°-incident coupling, it is found that normal incidence with the random metal grating is an efficient method to improve the responsivity. The random metal grating THz QWP is ∼400% improved in the peak responsivity.

A Comprehensive Study on the Impact of Various Nano-gratings on MSM-PDs for Enhancement in the Light Absorption

In this paper, we have analyzed metal-semiconductor-metal photodetectors (MSM-PDs) with different nano-grating structures or shapes to improve the light absorption capacity into the device in details for high-speed communication systems and networks. The plasmonic-based MSM-PD structure demonstrates a significant improvement in light absorption capacity for the developed device compared to conventional MSM-PDs i.e., devices that have not employed the nano-gratings. The light absorption capacity of the device is varied with the variation of geometrical shapes and parameters of the nano-gratings, such as the nano-grating height, slit width and so on. These nano-grating structures are assisting in light transmission through the central slit (i.e., subwavelength apertures) efficiently, resulting in the excitation of surface plasmon polaritons (SPPs) as the incident photons interact with the nano-gratings/ nano-corrugations. This improved light transmission in the central slit along with excited SPPs results in resonant light absorption in the device. This means the light trapped inside the central slit is triggered by the SPPs to a higher order magnitude. This causes the light absorption enhancement for the device, i.e., more light is transmitted through the device instead of reflecting back to the surface. The simulation results demonstrated that the light absorption enhancement factor (LAEF) for these devices have improved dramatically due to the nano-gratings. For modeling and simulation of these devices, Opti-FDTD tool is used which is based on finite difference time domain (FDTD) method. The application of these simulated devices is in the range of 800-850-nm. The simulation results are suitable for the design of nano-structured MSM-PDs that can be used in high-speed communication systems and sensor network systems.

Novel Adaptive Distributed Compressed Sensing Algorithm for Estimating Channels in Doubly-Selective Fading OFDM Systems

Doubly-selective (DS) fading channel is often occurred in many orthogonal frequency division multiplexing (OFDM) communication systems, such as high-speed rail communication systems and underwater acoustic (UWA) wireless networks. It is challenging to provide an accurate and fast estimation over the doubly-selective channel, due to the strong Doppler shift. This paper addresses the doubly selective channel estimation problem based on complex exponential basis expansion model (CE-BEM) in OFDM systems from the perspective of distributed compressive sensing (DCS). We propose a novel DCS-based improved sparsity adaptive matching pursuit (DCS-IMSAMP) algorithm. The advantage of the proposed algorithm is that it can exploit the joint channel sparsity information using dynamic threshold, variable step size and tailoring mechanism. Simulation results show that the proposed algorithm achieves 5dB performance gain with faster operation speed, in comparison with traditional DCS-based sparsity adaptive matching pursuit (DCS-SAMP) algorithm.

Nano-optical Switch Based on Photonic Crystal Ring Resonator

In recent trend, the ultra-speed nanophotonics components are highly attractive for the scientific community and next-generation high-speed optical communication systems. In this present work, a novel four-port optical switch is proposed and designed using a two-dimensional photonic crystal platform. The proposed device is composed of waveguides and ring resonator in a square lattice with gallium phosphide rods arranged in an air medium. The plane wave expansion (PWE) method is used to identify the operating wavelength of the proposed device. The functional parameters of the optical switch such as normalized output power, bit rate, response time, extinction ratio, insertion loss, and crosstalk are analyzed using the finite-difference-time-domain method (FDTD). The presented platform is designed with the ultra-fast data rate of 6.52 Tbps, very low crosstalk of − 40.45 dB, and fast response time of 153.33 femtoseconds. Hence, it is tremendously suitable for high speed photonic integrated circuits and switching application.

Joint MAP Detection for OFDM in Presence of Phase Noise from Free Running and Phase Locked Loop Oscillator

This paper addresses a statistically optimal joint maximum a posteriori (MAP) data detection and phase noise (PHN) estimation algorithm for iterative orthogonal frequency division multiplexing (OFDM) receivers, used for high speed and high spectral efficient wireless communication systems. First the cost function for data detection in presence of PHN is formulated with MAP criterion and is optimized further. The optimization is performed in frequency domain with proposed multidimensional circulatory gradient descent algorithm. With reduced computational complexity, the proposed optimization incorporates the statistics of PHN spectral components in prior. The condition of small PHN is relaxed in the proposed algorithm to achieve near optimal performance. Two different models for time varying PHN: produced by free running oscillator and phase locked loop (PLL) oscillator, are presented and implemented for proposed OFDM receiver. Simulation results demonstrate that the proposed optimization achieves “NO PHN” performance for both the PHN models even under multipath channel fading.

Time and reference frequency transfer over 850 nm VCSEL for digital telecommunication network synchronization

Accurate time transfer and synchronization between different network nodes is a key functional requirement in digital communication. Developments in optical fiber-based frequency dissemination techniques have improved optical frequency stability over time to much lower levels. In this work, we experimentally present the reference frequency transfer employing 850 nm vertical cavity surface emitting laser (VCSEL) over 100.3 m OM3 multimode fiber for synchronization of clocks on networked devices such as servers and racks/pod at different data center network nodes. A low-cost power-efficient multimode VCSEL with central wavelength at 844.26 nm is directly modulated with a 2 GHz reference frequency (RF) clock signal, and transferred over 100.3 m of OM3 multimode fiber. The single side band (SSB) phase noise of -104.62 dBc/Hz and −100.70 dBc/Hz is experimentally measured at back-to-back (B2B) and 100.3 m OM3 multimode fiber transmission respectively at a 1 kHz frequency offset. The jitter stability of 0.14 ps and 0.15 ps is experimentally achieved at B2B and 100.3 m fiber transmission, respectively. This work provides an alternative viable approach for the development of time keeping devices in high-speed short-reach optical communication systems.

An Efficient Stabilization Process for Analog Fractionally Spaced Equalizers

An analog fractionally spaced equalizer (FSE) is an efficient solution for high-speed communication system without a clock and data recovery circuit. However, the FSE might become unstable when a sign–sign least mean square tuning algorithm is used, especially when the transmitter pre-distortion is used. Here, an analytical model of the instability is derived, and a low-cost and efficient stabilization process based on dithering is proposed. The analytical instability model is verified in an experimental setup based on a vertical cavity surface emitting laser. Furthermore, it is experimentally demonstrated that in the case of the instability of the FSE, a clear convergence is achieved when applying the proposed stabilization scheme.

Novel Manchester-Based Multilevel Signaling for High-Speed Optical Communication Systems

Abstract In this paper, the performance of a novel multilevel signaling based on Manchester code namely four-level Manchester Coding (4-MC) technique is investigated for next generation high-speed optical fiber communication links. The performance of 4-MC is studied and compared with conventional Manchester modulation and four-level pulse amplitude modulation (4-PAM) formats in terms of receiver sensitivity, spectral efficiency and dispersion tolerance at the bit rate of 40 Gb/s. The bit error rate (BER) calculation model for the proposed multilevel scheme has also been developed. The calculated receiver sensitivity and the chromatic dispersion tolerance at the BER of 10–9 of the proposed scheme are −22 dBm and 67.5 ps/nm, respectively. It is observed that, 4-MC scheme is superior in comparison to 4-PAM by 3.5 dB in terms of receiver sensitivity in back-to-back scenario. Therefore, the proposed scheme can be considered as an alternative to current 4-PAM system.

100 m/500 Mbps underwater optical wireless communication using an NRZ-OOK modulated 520 nm laser diode.

In this paper, we proposed and experimentally demonstrated a long-distance high-speed underwater optical wireless communication (UOWC) system in a laboratory environment by using a low-cost green laser diode (LD) and power-efficient non-return-to-zero on-off keying (NRZ-OOK) modulation. The system successfully achieved a data rate of 500 Mbps through a 100 m tap-water channel by using a pigtailed single-mode fiber 520 nm green LD. The tap water was measured to have an attenuation coefficient comparable to pure seawater. The measured system bit error rate (BER) value of 2.5 × 10-3 was below the forward error correction (FEC) limit of 3.8 × 10-3 with 7% overhead. The distance can be extended if the received optical power is allowed to reduce to the minimum power to meet the data rate requirement. Based on the measured minimum required power and the power decay model in the water channel, the transmission performance was predicted to be 146 m/500 Mbps and 174 m/100 Mbps.

Performance Evaluation of Different Channel Estimation Techniques Using OFDM System on Large and Small Scale Fading Channel Model

The development of internet related applications and their increasing demands have led to advancement in high speed communication systems. For the accomplishment of these increasing demands MIMO channels with multiple transmitting and receiving antennas have been devised. MIMO systems enhance the capacity using spatial multiplexing and interference reduction through spatial dimensioning. Furthermore, MIMO systems mutual with orthogonal frequency division multiplexing (MIMO-OFDM) improves system capacity and mitigation against fading. These systems are the basis of 4G and beyond wireless communication standards. This research work focus on improvement in channel estimation performance using various techniques on standard communication models. Semi blind channel estimation technique is employed for channel estimation, which includes use of training sequence in form of pilots. Block type arrangement is being used, since it’s based on frequency selective channels. LSE and MMSE estimations are the two basic estimation techniques which can be efficiently used to estimate channels in OFDM systems. For MMSE implementation information of noise variance and channel covariance is required. Furthermore it is extremely complex than LSE estimator. DFT and DCT based channel estimation methods are employed to increase the performance and diminish the complexity. Different interpolation methods are been used and analyzed for better performance. Number of subcarriers which also have an impact on estimation performance has also been discussed. Overall channel estimation performance have been examined for both large and small scale fading channel models.

3D non-stationary wideband circular tunnel channel models for high-speed train wireless communication systems

3D non-stationary wideband circular tunnel channel models for high-speed train wireless communication systems

Sub-THz Antenna for High-Speed Wireless Communication Systems

Terahertz (THz) links will play a major role in high data rate communication over a distance of few meters. In order to achieve this task, antenna designs with high gain and wideband characteristics will spearhead these links. In this contribution, we present different antenna designs that offer characteristics better suited to THz communication over short distances. Firstly, a single-element antenna having a dipole and reflector is designed to operate at 300 GHz, which is considered as a sub-terahertz band. That antenna achieves a wide impedance bandwidth of 38.6% from 294 GHz to 410 GHz with a gain of 5.14 dBi. Secondly, two designs based on the same dipole structure but with added directors are introduced to increase the gain while maintaining almost the same bandwidth. The gains achieved are 8.01 dBi and 9.6 dBi, respectively. Finally, an array of 1×4 elements is used to achieve the highest possible gain of 13.6 dBi with good efficiency about 89% and with limited director elements for a planar compact structure to state-of-the-art literature. All the results achieved make the proposed designs viable candidates for high-speed and short-distance wireless communication systems.

Volterra-Assisted Optical Phase Conjugation: A Hybrid Optical-Digital Scheme for Fiber Nonlinearity Compensation

Mitigation of optical fiber nonlinearity is an active research field in the area of optical communications, due to the resulting marked improvement in transmission performance. Following the resurgence of optical coherent detection, digital nonlinearity compensation (NLC) schemes such as digital backpropagation (DBP) and Volterra equalization have received much attention. Alternatively, optical NLC, and specifically optical phase conjugation (OPC), has been proposed to relax the digital signal processing complexity. In this work, a novel hybrid optical-digital NLC scheme combining OPC and a Volterra equalizer is proposed, termed Volterra-Assisted OPC (VAO). It has a twofold advantage: it overcomes the OPC limitation in asymmetric links and substantially enhances the performance of Volterra equalizers. The proposed scheme is shown to outperform both OPC and Volterra equalization alone by up to 4.2 dB in a 1000 km EDFA-amplified fiber link. Moreover, VAO is also demonstrated to be very robust when applied to long-transmission distances, with a 2.5 dB gain over OPC-only systems at 3000 km. VAO combines the advantages of both optical and digital NLC offering a promising trade-off between performance and complexity for future high-speed optical communication systems.