Multiplexing Transmission Research Articles

High-security optical transmission system with multi-dimensional multiplexing based on quaternion chaotic encryption.

In this paper, we propose a multi-dimensional multiplexing scheme for space division multiplexing optical transmission systems based on quaternion chaotic encryption. A constellation compression shaping mapping method is designed to replace the traditional 2n mapping scheme, which leads to flexible encoding modulation. In order to achieve orthogonality between data symbols and effectively suppress crosstalk, the spectrally superposed transmission of three-dimensional (3D) constellation data is carried out by code division multiplexing and 4D carrier-less amplitude phase joint modulation techniques based on orthogonal basis. The Chen's chaotic model is utilized to generate the rotation angle of the constellation points, which enables data encryption without changing the transmitted power, enhancing system's security. The feasibility and superiority of the proposed scheme are successfully verified by constructing an experimental platform for a seven-core fiber transmission system.

Highly secure 3D-OFDM transmission scheme based on two-level noise masking key-accompanying transmission over seven-core fiber.

In this Letter, we propose a highly secure three-dimensional orthogonal frequency division multiplexing (3D-OFDM) transmission scheme based on two-level noise masking key-accompanying transmission. The original signal is encrypted with a spherical constellation to ensure the system's security with a 4D Lorenz-like model. The key realizes two-level noise masking by introducing additional noise bits at the bit level and hiding in a noise-like spherical shell at the constellation level. Moreover, the proposed method of placing the key in the encrypted signal can simultaneously transmit the encrypted signal and the key. A 101.06-Gb/s 3D-OFDM encrypted signal with the proposed scheme over a 2-km 7-core fiber experiment was successfully implemented. Experimental results show that the security performance of the system can be guaranteed under the conditions of partial key leakage and key misplacement at the illegal receiver. At the same time, the key masking degree (KMD) of the proposed two-level noise masking can reach 3267, which effectively guarantees the safe transmission of the key.

On the BER Performance of Constant Envelope OFDM in Frequency Selective MIMO Channels With ML Detection

In this contribution, we investigate the issue of constant envelope orthogonal frequency-division multiplexing (CE-OFDM) transmission in multipath fading channels, by quantifying its bit-error rate (BER) performance with the aid of maximum likelihood (ML) detection in multiple-input multipleoutput (MIMO) configurations. Through theoretical derivation, we demonstrate the advantage of CE-OFDM over its traditional counterparts as original OFDM and discrete Fourier transformspread-OFDM (DFT-s-OFDM) in terms of BER performance, which is ultimately supported by the simulation results.

OCDM-SDM Optical Communication System Utilizing Four-Dimensional Constellation Mapping

In this paper, an orthogonal chirp division multiplexing (OCDM) Space division multiplexing (SDM) optical communication utilizing four-dimensional constellation mapping is proposed. A new 16-point four-dimensional constellation is designed, and its constellation figure of merit (CFM) can reach 1.17. A direct detection orthogonal chirp division multiplexing (DD-OCDM) transmission scheme based on four-dimensional constellation mapping is designed, and 32.8 Gb/s OCDM signal transmission is performed across 2 km seven core fiber through experiments, which verifies the performance of the proposed constellation. Under the condition of multi-core multiplexing and the bit error rate (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , the receiver sensitivity of the OCDM signal, which is based on the proposed four-dimensional constellation mapping based on associated constellation mapping (16ACM-4D), has a gain of 0.59 dB, 3.26 dB, and 5.73 dB compared with the OCDM signals based on QPSK, 16QAM, and 16PSK, respectively. The results show that the proposed OCDM short-reach optical communication scheme based on four-dimensional constellation mapping has good bit error performance and anti-interference ability, and has broad application prospects in the future short-reach optical communication.

Multi-Timescale Channel Customization for Transmission Design in RIS-Assisted MIMO Systems

The performance of transmission schemes is heavily influenced by the wireless channel, which is typically considered an uncontrollable factor. However, the introduction of reconfigurable intelligent surfaces (RISs) to wireless communications enables the customization of a preferred channel for adopted transmissions by reshaping electromagnetic waves. In this study, we propose multi-timescale channel customization for RIS-assisted multiple-input multiple-output systems to facilitate transmission design. Specifically, we customize a high-rank channel for spatial multiplexing (SM) transmission and a highly correlated rank-1 channel for beamforming (BF) transmission by designing the phase shifters of the RIS with statistical channel state information in the angle-coherent time to improve spectral efficiency (SE). We derive closed-form expressions for the approximation and upper bound of the ergodic SE and compare them to investigate the relative SE performance of SM and BF transmissions. In terms of reliability enhancement, we customize a fast-changing channel in the symbol timescale to achieve more diversity gain for SM and BF transmissions. Extensive numerical results demonstrate that flexible customization of channel characteristics for a specific transmission scheme can achieve a tradeoff between SE and bit error ratio performance.

Toward Zero-Energy Devices: Waveform Design for Low-Power Receivers

The use of a low-power wake-up receiver (WUR) can significantly reduce the device’s energy consumption. To incorporate such WURs into existing communication systems, the waveform design needs to facilitate simple detection yet be compatible with the orthogonal frequency division multiplexing (OFDM) transmission. In this letter, we propose a novel phase design for the wake-up signal (WUS) so that the time-domain distortion of the WUS can be minimized. A majorization-minimization (MM) method is proposed to address the waveform design problem. Simulations show that the proposed scheme can achieve a near-optimal detection performance under different system parameters.

3.61 Pbit/s S, C, and L-Band Transmission With 19-Core Single-Mode Fiber

We experimentally demonstrate a 3.61 Pbit/s generalized mutual information (GMI) estimated throughput by space division multiplexing (SDM) and wavelength division multiplexing (WDM) combined transmission over 19-core single-mode fiber. Covering 17 THz bandwidth with a 25-GHz grid in S, C, and L band, 680 wavelength channels are used to deliver 24.5 GBaud probabilistically constellation-shaped (PCS) 256 quadrature amplitude modulation (QAM) and PCS-64QAM signals. The measured data rate shows a great potential for S/C/L-band transmission to dramatically enhance the achievable data rate in multi-core fibers.

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Fast Beam Search Based on Recorded Combinations for Distributed Massive MIMO Systems Using Sub-THz Wave Bands

Massive multiple-input multiple-output (mMIMO) systems using sub-Terahertz (sub-THz) wave bands are viewed as a promising way to increase greatly the capacity of future mobile communications because they can provide a wider bandwidth than conventional wireless communications. Distributed mMIMO systems can improve the spatial multiplexing transmission performance by reducing spatial correlations among the MIMO channels in line-of-sight environments. Unfortunately, beam search is more complex than in conventional wireless communications, because each access point (AP) requires a very narrow beamwidth to compensate for the significant propagation attenuation in sub-THz waves and beam search must involve all distributed APs. To address this problem, this paper proposes a fast beam search method to limit the beam-sweep directions of beam sweep based on the beam combinations selected for the distributed APs in the past. Simulations show that the proposed method maintains the transmission capacity while greatly reducing the number of search beams compared to conventional all-AP exhaustive beam search. The results show based on 16 APs that while matching the transmission capacity attained by all-AP exhaustive or two-stage beam search methods, the proposed method reduces the number of search beams by APs (excluding the initial AP executed exhaustive beam search) by 90.8% at the 50th percentile and 62.5% at the 95th percentile compared to two-stage beam search for 50,000 sets of user equipment used for recording beam combinations before evaluating the performance.

Simulation of Coherent Electromagnetic Waves in Wavelength Division Multiplexing (WDM) Transmission

This study analyzes the application of Wavelength Division Multiplexing (WDM) in fiber optic networks which aims to find the wavelength, WDM optical spectrum and modes, as well as the CPR estimated phase and modes. In this study WDM allows the simultaneous transmission of different data streams through a single optical fiber, using different wavelengths. This research was conducted using the python OptiCommPy module. This module is used to perform modeling of complex optical fiber transmission systems by considering the various parameters and disturbances involved in optical transmission. The results obtained from this study are that WDM networks can use full or limited wavelength conversion, depending on the wavelength conversion capability of each network node. Whereas multifiber networks use fiber pools between network nodes, and multifiber WDM networks can be implemented without or with full wavelength conversion. This research can be a guide for designing coherent electromagnetic waves in WDM transmissions using the OptiCommPy python module.

Single-wavelength polarization- and mode-division multiplexing free-space optical communication at 689 Gbit/s in strong turbulent channels.

We experimentally demonstrated, for, it is believed, the first time, high-capacity polarization- and mode-division multiplexing free-space optical transmission with adequate strong turbulence resiliency. A compact spatial light modulator-based polarization multiplexing multi-plane light conversion module was employed to emulate strong turbulent links. By employing advanced successive interference cancellation multiple-input multiple-output decoder and redundant receive channels, the strong turbulence resiliency was significantly improved in a mode-division multiplexing system. As a result, we achieved a record-high line rate of 689.2 Gbit/s, channel number of 10, and net spectral efficiency of 13.9 bit/(s Hz) in a single-wavelength mode-division multiplexing system with strong turbulence.

DRL-based max-min fair RIS discrete phase shift optimization for MISO-OFDM systems

In this paper, we investigate a reconfigurable intelligent surface (RIS) assisted downlink orthogonal frequency division multiplexing (OFDM) transmission system. Taking into account hardware constraint, the RIS is considered to be organized into several blocks, and each block of RIS share the same phase shift, which has only 1-bit resolution. With multiple antennas at the base station (BS) serving multiple single-antenna users, we try to design the BS precoder and the RIS reflection phase shifts to maximize the minimum user spectral efficiency, so as to ensure fairness. A deep reinforcement learning (DRL) based algorithm is proposed, in which maximum ratio transmission (MRT) precoding is utilized at the BS and the dueling deep Q-network (DQN) framework is utilized for RIS phase shift optimization. Simulation results demonstrate that the proposed DRL-based algorithm can achieve almost optimal performance, while has much less computation consumption.

Dynamic Pilot Design for Multicast in the Internet of Vehicles Running at Different Speeds

High mobility of vehicles causes time-frequency selective fading over physical channels within the Internet of vehicles (IoV). To improve the resource utilisation efficiency, a novel transmission strategy, based on dynamic pilot design, is proposed in this paper to reduce the pilot consumption in doubly selective channel estimation for the multicast to vehicles running at different speeds. As the channel coherence time is mainly influenced by the receiver mobility in the multicast from a base station to vehicles, we define a multicast block as the channel coherence time of the slowest vehicle in the multicast group, where common pilot symbols are shared. Then, the multicast data destined for different vehicles are loaded into the block according to their own channel coherence times. To evaluate the performance and resource utilisation of our dynamic pilot design, the metrics of overhead rate, spectral efficiency, and energy efficiency are formulated for the IoV multicast using multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) transmissions. In terms of these three metrics, illustrative numerical results on the comparisons between our dynamic pilot design and the conventional counterpart are provided, which not only substantiate that the former outperforms the latter but also present useful tools and specifications for the pilot design in the IoV multicast using MIMO-OFDM transmissions over doubly selective channels.

Multimode OAM Convergent Transmission With Co-Divergent Angle Tailored by Airy Wavefront

Wireless backhaul offers a more cost-effective, time-efficient, and reconfigurable solution than wired backhaul to connect the edge-computing cells to the core network. As the amount of transmitted data increases, the low-rank characteristic of Line-of-Sight (LoS) channel severely limits the growth of channel capacity in the point-to-point backhaul transmission scenario. Orbital Angular Momentum (OAM), also known as vortex beam, is considered a potentially effective solution for high-capacity LoS wireless transmission. However, due to the shortcomings of its energy divergence and the specificity of multi-mode divergence angles, OAM beams have been difficult to apply in practical communication systems for a long time. In this work, a novel multi-mode convergent transmission with co-scale reception scheme is proposed. OAM beams of different modes can be transmitted with the same beam divergent angle, while the wavefronts are tailored by the ring-shaped Airy compensation lens during propagation, so that the energy will converge to the same spatial area for receiving. Based on this scheme, not only is the Signal-to-Noise Ratio (SNR) greatly improved, but it is also possible to simultaneously receive and demodulate OAM channels multiplexed with different modes in a limited space area. Through prototype experiments, we demonstrated that 3 kinds of OAM modes are tunable, and different channels can be separated simultaneously with receiving power increasing. The measurement isolations between channels are over 11 dB, which ensures a reliable 16-QAM multiplexing wireless transmission demo system. This work may explore the potential applications of OAM-based multi-mode convergent transmission in LoS wireless communications.

SFBC Recognition over Orthogonal Frequency Division Multiplexing Schemes in the Presence of Inphase and Quadrature Phase Discrepancies for Cognitive Radio Applications

A radio is adaptive if it can autonomously analyze the communications environment and instantly modify its settings to achieve the best possible efficiency. In orthogonal frequency division multiplexing (OFDM) transmissions, identifying the space frequency block coding (SFBC) category utilized is one of the most important tasks of an adaptive receiver. Previous approaches to this problem did not take into consideration the fact that real systems typically suffer from transmission defects. This study offers a novel maximum likelihood recognizer capable of distinguishing between SFBC OFDM waveforms in the context of inphase and quadrature phase differences (IQDs). The theoretical findings show that IQDs arising from the transmitter and recipient can be combined with channel paths to generate so-called effective channel paths. The conceptual examination demonstrates that the outlined maximum likelihood strategy of the SFBC recognition and effective channel estimation processes is implemented by an expectation maximization tool utilizing the error control decoders’ soft outputs. The simulations results reveal that the suggested strategy delivers a much greater recognition accuracy than the typical approaches outlined in the comparable literature. At a signal-to-noise ratio (SNR) of 14 dB, for example, the proposed approach achieves a bit error rate (BER) of 0.00002, which is very close to the case of perfect estimation and compensation for IQDs, outperforming the previous reported works which achieved BERs of 0.01 and 0.02.

Optimal Design of Eigenvalues for the Full-Spectrum Modulated Nonlinear Frequency Division Multiplexing Transmission System

Nonlinear frequency division multiplexing (NFDM) system is an optional candidate to overcome the fiber nonlinearity limit. A full-spectrum modulated NFDM system, modulating data on combined continuous spectrum (CS) and discrete spectrum (DS) together, was proposed in recent years to improve the data rates and spectral efficiency (SE) by exploiting all the degrees of freedom offered in the nonlinear spectrum. However, the selection of discrete eigenvalues greatly affects the performance of both CS and DS. Designing appropriate eigenvalues of DS is an important issue to ensure the high SE and excellent performance of the system. In this paper, we discussed the selcection principle of eigenvalues and analyzed it from multiple perspectives, 11 eigenvalues with 64-quadrature amplitude modulation (64QAM) are selected for DS. Besides optimizing the eigenvalues at the transmitter, the linear minimum mean-square estimate (LMMSE) method was used at the receiver to furtherly improve the performance of DS. Through the numerical simulation, a 113 Gb/s (SE of 2.8 bits/s/Hz) full-spectrum modulated NFDM system was set up and transmitted 1120 km distance, where the Q-factors of both CS and DS are above the hard-decision forward error correction (HD-FEC) threshold. The results provide a way to design an efficiently full-spectrum modulated NFDM system.

Design of Multilayered Ring-Core Fiber With Low-Level Inter-Group Crosstalk

We propose a novel ring-core fiber (RCF) with multilayered ring core and modulated refractive index profile (RIP). The simulated results indicate that such a special fiber design is able to support 5 orbital angular momentum (OAM) mode groups (18 OAM modes) with crosstalk between any two high-order adjacent mode groups (MGs) less than -28 dB/km over the whole C and L band. At the wavelength of 1550 nm, crosstalk between the two highest-order MGs is below -36 dB/km. Besides, with the limitation of the existing manufacturing restriction, the geometric size and doping concentration of the designed fiber are allowable and reasonable, which is promising for practical manufacturing. The designed RCF also shows good bending characteristics. Moreover, ultra-broadband performance is analyzed over the whole C + L band ranging from 1530 nm to 1625 nm. This kind of fiber design could be employed in small-scale multiple-input multiple-output digital signal processing (MIMO-DSP) assisted intra-group modes multiplexing transmission or MIMO-free inter-group modes multiplexing transmission combined with the mature wavelength-division multiplexing technique.

Channel model for the dual-polarization b-modulated nonlinear frequency-division multiplexing optical transmission systems.

We consider optical transmission systems based on the nonlinear frequency division multiplexing (NFDM) concept, i.e., the systems employing the nonlinear Fourier transform (NFT) for signal processing and data modulation. Our work specifically addresses the double-polarization (DP) NFDM setup that utilizes the so-called b-modulation, the most efficient NFDM method proposed up-to-date. We extend the previously-developed analytical approach based on the adiabatic perturbation theory for the continuous nonlinear Fourier spectrum (b-coefficient) onto the DP case to obtain the leading order of continuous input-output signal relation, i.e., the asymptotic channel model, for an arbitrary b-modulated DP-NFDM optical communication system. Our main result is in deriving the relatively simple analytical expressions for the power spectral density of the components of effective conditionally Gaussian input-dependent noise emerging inside the nonlinear Fourier domain. We also demonstrate that our analytical expressions are in remarkable agreement with direct numerical results if one extracts the "processing noise" arising due to the imprecision of numerical NFT operations.

Bandwidth-Decomposed Analysis of Spatial-Mode Dispersion of Coupled 2-Core Fiber Employing Linear Optical Sampling

The complex impulse responses of two types of coupled 2-core fibers, with homogeneous and heterogeneous cores, were measured by using a measurement system based on linear optical sampling. The measurements were performed over a bandwidth of 20 nm (∼2.5 THz), and the spatial-mode dispersion (SMD) was analyzed. Several lengths of both homogeneous and heterogeneous core fibers were prepared to investigate the length dependence of SMD. The SMDs for divided frequency bands were examined by numerically analyzing the measured broadband impulse response. Consequently, the power impulse response at each subband was obtained, and the variation in the SMDs was examined. In addition, the statistical distribution of the SMD was revealed to be different for each type of the 2-core fiber. The SMD observed at a specific band is handled by the receiver in space-division multiplexing transmission systems. Finally, we present a measurement of mode dependent loss (MDL) of a 2-core fiber, and discuss the impact of time axis inaccuracy to the MDL measurement. These analyses performed are expected to be beneficial for designing such systems.

Photonic integrated multicast switch-based optical wireless data center network

Optical wireless data center networks (OW-DCNs), which employ optical wireless technology and optical wired switching technology, are gaining interest as they promise to eliminate cable complexity, as well as to create high bandwidth interconnections and a low-cost and power-efficient system. In particular, the incorporation of optical tunable transmitters (T-TXs) and passive optical beam steering technologies is a promising way to build an OW-DCN benefitting from the potential of fast optical switching speed, low switch control complexity, and easy reconfiguration. However, the practical deployment of such an OW-DCN remains a challenge as fast (nanosecond) T-TX is required for fast optical switching operation. Implementation of fast T-TX can be realized by an array of lasers and optical gates, which are combined with photonic integration technology to achieve a compact, stable, and efficient nanosecond T-TX. In this paper, we propose an OW-DCN based on arrayed waveguide grating routers and fast T-TXs that exploit photonic integrated circuit multicast switches (PIC-MCSs). This PIC-MCS chip not only offers nanosecond-scale fast optical switching but also plays an essential role in enabling multicast operation, T-TX sharing, and dynamic bandwidth allocation between the intra- and inter-cluster networks. A 4×2 PIC-MCS has been designed, fabricated, and characterized in this proposed OW-DCN system. Experimental results validate that the proposed OW-DCN supports lossless, nanosecond, and multicast switching operation. Moreover, the dynamic bandwidth allocation and optical packet switching capability have been experimentally demonstrated. Finally, system performance with this fabricated PIC-MCS chip in a 4×4 rack OW-DCN is experimentally validated for different transmission scenarios and modulation formats. Wavelength division multiplexing multicast transmission with 50 Gb/s non-return-to-zero on–off-keying signals has been verified with less than 1.5 dB power penalty. 58 Gb/s four-level pulse-amplitude modulation transmission has also shown operation below the forward error correction threshold of 3.84×10−3 for all the different transmission scenarios.