Frequency Division Multiplexing Research Articles

Quantitative phase image stitching guided by reconstructed intensity images in one-shot double field of view multiplexed digital holographic microscopy

In digital holographic microscopy (DHM), achieving large field of view (FOV) imaging while maintaining high resolution is critical for quantitative phase measurements of biological cell tissues and micro-nano structures. We present a quantitative phase image stitching guided by reconstructed intensity images in one-shot double FOV multiplexed DHM. Double FOVs are recorded simultaneously through frequency division multiplexing; intensity feature pairs are accurately extracted by multi-algorithm fusion; aberrations and non-common baselines are effectively corrected by preprocessing. Experimental results show that even if phase images have coherent noise, complex aberrations, low overlap rate and large size, this method can achieve high-quality phase stitching.

Extending the coverage of user-diversified IM-DD PON by FDM: a mathematical model with experimental verification.

Future passive optical networks (PONs) call for more flexibility to support diversified users with various rate demands and link qualities. Using traditional time-division multiplexing (TDM), the concept of a flexible rate PON was proposed to accommodate more users with link diversity by rate adaptation. In this Letter, we reveal the PON coverage can be further extended through frequency-division multiplexing (FDM) in the presence of multiuser diversity, namely, (i) there exist users with frequency-dependent link conditions and (ii) the link conditions exhibit disparity among users. We build a mathematical model and propose an optimization algorithm based on the binary tree search to optimize diversity gain. We experimentally verify its feasibility by studying the diversity gain concerning chromatic dispersion, optical path loss, and signal-to-noise ratio (SNR) variation in a 200G-class intensity-modulation direct-detection (IM-DD) system.

Beyond 100G three-dimensional flexible coherent PON with time, frequency, and power resource-allocation capabilities

Emerging services such as 8K/16K high-quality video streaming, virtual reality/augmented reality, beyond-5G mobile Internet, and edge computing are driving the need for even higher speed, capacity, and flexibility in optical access networks. Coherent passive optical networks (CPONs) have garnered significant attention in recent years due to their superior receiver sensitivity and high flexibility for 100G speeds and beyond. Research interest has surged in flexible, multi-dimensional multiplexing schemes that go beyond traditional time-division multiplexing (TDM). To further leverage the flexibility offered by CPONs, this paper proposes and experimentally demonstrates what we believe to be a novel three-dimensional flexible coherent PON (3D FLCS-CPON) for downstream applications. This system offers resource-allocation capabilities in the time, frequency, and power domains. By utilizing digital subcarrier multiplexing and power allocation in both the time and frequency domains, we achieve more flexible and adaptable access rates compared to traditional TDM and frequency-division multiplexing (FDM). We present in detail the operating principles of 3D FLCS-CPON in the downstream, along with steps for power calibration between subcarriers. As a proof of concept, we demonstrate the feasibility of 3D FLCS-CPON in the downstream using four subcarriers, achieving a peak data rate of 250Gb/s/λ over a 20-km fiber.

Dual-Channel Metal Debris Signal Differential Detection Based on Frequency Division Multiplexing

Dual-Channel Metal Debris Signal Differential Detection Based on Frequency Division Multiplexing

Individually Addressable Multitouch Sensors Using a Sweep Signal for Minimal Wiring Complexity

Although touch sensing has been developed rapidly, it is difficult to design a simple circuit or a system that contains many sensors; the numbers of wires increase in proportion to the number of sensors. Although efforts have been made to reduce the number of wires, multi-touch measurements are still challenging because it is difficult to decouple multi-touch signals. Here, we apply FDM (Frequency Division Multiplexing) to SFCS (Swept Frequency Capacitive Sensing) when developing a sensor system capable of multi-touch with minimal wiring complexity. The sensor system can be extended in series and arrays by connecting multiple sensors using only two wires. A signal generated via frequency sweeping is expressed as a unique response in a specific frequency domain using a multi-band band-stop filter. Considering such unique touch responses, each capacitive multi-touch sensor is individually addressable, thereby allowing touch to be detected regardless of sensor location.

Scan-less 3D microscopy based on spatiotemporal encoding on a single-cavity dual-comb laser.

Dual-comb microscopy enables high-speed and high-precision optical sampling by simultaneously extracting both amplitude and phase information from the interference signals with frequency division multiplexing. In this Letter, we introduce a spatiotemporal encoding approach for dual-comb microscopy that overcomes previous limitations such as mechanical scanning, low sampling efficiency, and system complexity. By employing free-space angular-chirp-enhanced delay (FACED) and a low-noise single-cavity dual-comb laser, we achieve scan-less 3D imaging with nanometer precision and a 3D distance-imaging rate of 330 Hz, restricted only by the repetition rate difference of the dual-comb laser. Specifically, the FACED unit linearly arranges the laser beam into an array. A grating subsequently disperses this array transversely into lines, facilitating ultrafast spectroscopic applications that are 1-2 orders of magnitude quicker than traditional dual-comb methods. This spatiotemporal encoding also eases the stringent conditions on various dual-comb laser parameters, such as repetition rates, coherence, and stability. Through carefully designed experiments, we demonstrate that our scan-less system can measure 3D profiles of microfabricated structures at a rate of 7 million pixels per second. Our method significantly enhances measurement speed while maintaining high precision, using a compact light source. This advancement has the potential for broad applications, including phase imaging, surface topography, distance ranging, and spectroscopy.

Enhancing spectral efficiency with low complexity filtered‐orthogonal frequency division multiplexing in visible light communication system

AbstractThe filtered‐orthogonal frequency division multiplexing (F‐OFDM) scheme has gained attention as a promising solution in the field of visible light communication (VLC) systems. One crucial aspect in VLC is the conversion of the complex F‐OFDM signal into a real signal that corresponds with direct detection and intensity modulation. Traditionally, achieving a real F‐OFDM signal has involved imposing Hermitian symmetry (HS) on the samples of the Inverse Fast Fourier transform (IFFT), which requires 2N‐point IFFT and obtains an N‐point FFT, thus adding complexity. In this study, a novel approach is presented and implemented, aiming to enhance spectral efficiency and reduce system complexity by generating a real F‐OFDM signal without relying on HS. This approach is then compared with HS‐free (HSF)‐OFDM, direct current biased optical OFDM, and asymmetrically clipped optical OFDM. The suggested method offers a remarkable improvement of ~50% in the required IFFT/FFT volume. Consequently, this method reduces hardware complexity and power usage compared with the traditional F‐OFDM method. Moreover, regarding error rates, the proposed method demonstrates better spectral efficiency than HSF‐OFDM.

Noise-Canceling Channel Estimation Schemes Based on the CIR Length Estimation for IEEE 802.11p/OFDM Systems

This paper investigates methods for noise-canceling channel estimation (NC-CE) to track rapid time-varying channels in IEEE 802.11p/orthogonal frequency division multiplexing (OFDM) systems. To this end, we introduce a novel three-step channel estimation technique based on the estimated length of the channel impulse response (CIR). This approach aims to surpass the performance of conventional designs that rely on constructed data pilots (CDPs). In the first step, we not only eliminate noise components but also estimate the channel frequency responses (CFRs) of virtual subcarriers for long preamble parts. Moving on to the second step, we incorporate a modified CDP method without a frequency-domain reliability test and interpolation, taking into account the CFRs of virtual subcarriers obtained at the previous OFDM symbol time. The final step can be implemented as the operation of the inverse fast Fourier transform (IFFT)/nulling/FFT to reduce noise components from the CFRs obtained in the second step and generate CFRs for virtual subcarriers to be used in the next symbol time. The results of our simulations validate the effectiveness of our proposed channel estimation schemes.

Rotating permanent magnet antenna array for directional communication in pipeline monitoring system

As a novel pipeline monitoring technology, wireless sensor networks (WSN) have the advantages of low power consumption, flexibility, and robustness in aboveground, underground, and underwater pipeline monitoring systems. Using low-frequency (LF) rotating permanent magnet antennas (RPMA) for communications from sensor nodes (SN) to autonomous unmanned vehicles (AUV) has the advantages of low attenuation, low latency, and portable size. However, the communication ranges of different SNs may overlap and information from different SNs should be distinguished. Methods like time-division multiplexing (TDM) and frequency-division multiplexing (FDM) are not suitable in this case. Another way is to form RPMAs into an array so that directional communications from SNs to AUVs can be realized and there is no overlap between SNs. Therefore, this work analyzes and enhances the directivity of linear RPMA arrays in the near-field region. Based on the designed RPMA array prototype, the directional communication is realized at a distance of 5 m across concrete walls. This work paves the way for directional communications from SNs to AUVs in underground pipeline monitoring systems.

A long-reach radio over free space optics (Ro-FSO) system using hybrid orthogonal frequency division multiplexing (OFDM)-multibeam concept with enhanced detection

Abstract This paper focuses on designing of a 10 Gbit/s-10GHz hybrid Orthogonal frequency division multiplexing (FDM) based Radio over Free Space Optics (Ro-FSO) transmission link using optical single sideband modulation format and its performance has been compared using a different number of transmission beams. The proposed link has been simulated and compared using 1-beam, 2-beam, and 4-beam in the system. We show that by using the 4-beam system in the OFDM-Ro-FSO link, a 3000 m range has been achieved reliably, which a notable improvement when compared to previous work is. Also, we show an improved performance of the system by using an enhanced detection mechanism using a Square root module (SRm) at the receiver side.

Two-component intra-cavity photoacoustic spectroscopy sensor using Y-shaped photoacoustic cell with two resonant cavities

In this paper, an intra-cavity photoacoustic spectroscopy (PAS) gas sensing system based on a 3D printed Y-shaped PAS cell with two longitudinal resonant cavities is designed for two-component detection. The sensor embeds the Y-shaped photoacoustic cell in the resonator of fiber ring laser for the intra-cavity gas detection, improving the laser power in the gas detection. The two resonant cavities of the Y-type PAS cell are formed into C-band and the L-band lasers, guaranteeing the laser stable output of C-band and the L-band. Comparing with the conventional dual-wavelength laser, the dual-wavelength laser scheme proposed in the paper eliminates the model competition and possesses much higher laser stability. A dedicated wavelength modulator is used to control the fiber bragger grating (FBG) to generate the stress change, and then scan the absorption peaks of the corresponding gases. The second harmonic (2f) PAS signal is used to calculate the gas concentration. The acetylene (C2H2) and carbon dioxide (CO2) were detected based on the frequency division multiplexing (FDM) technique using two resonant cavities. A microphone is used to simultaneously detect the photoacoustic signals of two resonance cavities. A demodulation frequency with a frequency intervals of 100 Hz ensures there is no cross-interference in demodulation of signals from each resonant cavity. The trace detection of C2H2 and CO2 was realized at normal temperature and pressure. The PAS signal intensity was further improved by optimizing the modulation amplitude and incident laser power. The minimum detection limits (MDLs) and the linear fit R-squared values (R2) of C2H2 and CO2 were 529 ppbv, 0.99971, 282 ppmv and 0.99991, respectively. It indicates that the two-component gas sensing system in this paper has good linearity and high sensitivity. It provides a valuable trace gas detection device for multi-component gas detection.

Analyzing correlation with BER: Constellation diagrams across various modulation modes and FDM/OFDM

Over the last two decades, owing to rapid advancements in digital signal processing and integrated circuit technology, Frequency Division Multiplexing (FDM) and Orthogonal Frequency Division Multiplexing (OFDM) have emerged as pivotal technologies for wireless multimedia communication. Research reveals that coupling OFDM with diverse modulation modes aids in mitigating the Bit Error Rate (BER) during transmission. Nonetheless, a comprehensive elucidation remains absent regarding the interplay between constellation diagrams and BER, as well as the correlation between FDM and BER. Thus, the primary focus of this research is to dissect the correlation between BER and constellation diagrams for distinct modulation methodologies, while also scrutinizing the impact of FDM on BER. The research methodology adopted herein entails two main steps: First, gathering papers pertinent to constellation diagrams, BER, FDM, and OFDM technology. Second, leveraging survey and literature analysis methodologies for thorough examination. The findings highlight that diverse constellation diagram designs yield disparate BER outcomes post demodulation. Foremost among the influential factors on FDM and OFDM technology BER are distinct Inter-Carrier Interference (ICI) patterns and the quantity of subcarriers stemming from signal wave side lobes. To address the BER challenge, a prospective avenue involves refining the selection of constellation diagram styles for demodulation, leveraging an understanding of constellation diagram traits. Furthermore, future research endeavors should delve deeply into co-channel interference and adjacent channel interference to contribute effectively towards BER reduction and augmenting channel reliability.

A Novel Continuous-Variable Quantum Key Distribution Scheme Based on Multi-Dimensional Multiplexing Technology

Dual-polarization division multiplexing (DPDM) is considered to be a potential method to boost the secure key rate (SKR) of the continuous-variable quantum key distribution (CV-QKD) system. In this article, we propose a pilot alternately assisted local local oscillator (LLO) CV-QKD scheme based on multi-dimensional multiplexing, where time division multiplexing and frequency division multiplexing are combined with dual-polarization multiplexing techniques to dramatically isolate the quantum signal from the pilot tone. We establish a general excess noise model for the LLO CV-QKD system to analyze the influence mechanism of various disturbances (e.g., time-domain diffusion, frequency-domain modulation residual, and polarization perturbation) on the key parameters, such as the channel transmittance and excess noise. Specifically, the photon leakage noise from the reference path to the quantum path and that between quantum signals with two different polarization paths are simultaneously analyzed in the dual-polarization LLO CV-QKD scheme for the first time. Furthermore, a series of simulations are established to verify the performance of the proposed scheme. The results show that the maximal isolation degree achieves 84.0 dB~90.4 dB, and the crosstalk between pilot tones and quantum signals can be suppressed to a very small range. By optimizing the system parameters (e.g., modulation variance and repetition frequency), the SKR with 12.801 Mbps@25 km is achieved under the infinite polarization extinction ratio (PER) and 30 dB residual ratio of the frequency modulation in the nanosecond-level pulse width. Moreover, the performance of the proposed DPDM CV-QKD scheme under relatively harsh conditions is simulated; the results show that the SKR with 1.02 Mbps@25 km is achieved under a relatively low PER of 17 dB with the nanosecond-level pulse width and 20 dB residual ratio of the frequency modulation. Our work lays an important theoretical foundation for the practical DPDM LLO CV-QKD system.

A Low-Power Impedance-to-Frequency Converter for Frequency-Multiplexed Wearable Sensors.

We propose a low-power impedance-to-frequency (I-to-F) converter for wearable transducers that change both its resistance and capacitance in response to mechanical deformation or changes in ambient pressure. At the core of the proposed I-to-F converter is a fixed-point circuit comprising of a voltage-controlled relaxation oscillator and a proportional-to-temperature (PTAT) current reference that locks the oscillation frequency according to the impedance of the transducer. Using both analytical and measurement results we show that the operation of the proposed I-to-F converter is well matched to a specific class of sponge mechanical transducer where the system can achieve higher sensitivity when compared to a simple resistance measurement techniques. Furthermore, the oscillation frequency of the converter can be programmed to ensure that multiple transducer and I-to-F converters can communicate simultaneously over a shared channel (physical wire or virtual wireless channel) using frequency-division multiplexing. Measured results from proof-of-concept prototypes show an impedance sensitivity of 19.66 Hz/ Ω at 1.1 kΩ load impedance magnitude and a current consumption of [Formula: see text]. As a demonstration we show the application of the I-to-F converter for human gesture recognition and for radial pulse sensing.

Auxiliary Management and Control Channel (AMCC) for Coherent Frequency-division-multiplexing (FDM) PON

Auxiliary Management and Control Channel (AMCC) for Coherent Frequency-division-multiplexing (FDM) PON

Optimized deep learning based hypernet convolution neural network and long short term memory for joint pilot design and channel estimation in MIMO‐OFDM model

AbstractIn multiple input multiple output‐orthogonal frequency division multiplexing (MIMO‐OFDM) systems, efficient pilot design (PD) and channel estimation (CE) greatly influences the reliability and robustness of pilot‐based CE methods. But, accurate estimationof the channel remains a challenge in a high‐mobility environment with non‐linear channel characteristics. Many techniques have been introduced to overcome the pilot contamination and CE problems in the MIMO‐OFDM system to overcome this issue. However, these techniques take multiple paths at the receiver, resulting in delay spread and interference in the communication. Hence, this study presents a novel deep learning (DL) based technique for channel estimation based on channel state information (CSI). A DL technique based on a hyper convolutional neural network (Hyper‐CNN) is introduced for the optimal pilot design. The selection of the pilot position can be made using the tunicate swarm optimization (TSO) approach. Finally, a DL‐based long short‐term memory (LSTM) model is proposed for the CE in the MIMO‐OFDM system. The proposed method is implemented in the MATLAB platform, and the outcome is compared under different metrics like mean square error (MSE) and bit error rate (BER). In the experimental scenario, the proposed method attains the BER of 0.20 and 0.03 for CP (cyclic prefix) and without CP, respectively. In addition, the proposed method attains the MSE of 1.14, 0.99, 1.08 and 0.97 for 8, 16, 48 and 64 pilots, respectively. The performance of a proposed method is compared with the existing method and proves the efficacy of the proposed method.

Frequency-tunable quartic soliton in nonlinear negative-index metamaterials

An exact quartic soliton solution along with its constraint conditions in metamaterials (MMs) is presented on the basis of the nonlinear Schrödinger equation (NLSE) with second-, third- and fourth-order diffraction. Through analyzing the parameter relations based on the Drude model, it is demonstrated that the quartic soliton can only exist in the negative-index region (NIR) of self-focusing nonlinear MMs. With insight into the underlying parameter relations of quartic soliton, it is found that by selecting suitable frequency of incident wave, the quartic soliton in negative-index MMs (NIMs) exhibits frequency tunability or frequency stability. Particularly, the quartic solitons with different frequencies can propagate in an identical velocity, which provides a theoretical basis for realizing frequency division multiplexing (FDM) technology in nonlinear MMs. Different from pure-quartic soliton (PQS) in ordinary materials exhibiting oscillatory decaying tails in spatial domain and a flat top in frequency domain on logarithmic scale, the quartic soliton reported here possesses triangular distributions in both spatial and frequency domains. The presented results contribute to a fundamental theoretical support for future application of quartic soliton and are beneficial for exploring new solitary waves in MMs.

Efficient and Low-Complex Signal Detection with Iterative Feedback in Wireless MIMO-OFDM Systems.

To solve error propagation and exorbitant computational complexity of signal detection in wireless multiple-input multiple-output-orthogonal frequency division multiplexing (MIMO-OFDM) systems, a low-complex and efficient signal detection with iterative feedback is proposed via a constellation point feedback optimization of minimum mean square error-ordered successive interference cancellation (MMSE-OSIC) to approach the optimal detection. The candidate vectors are formed by selecting the candidate constellation points. Additionally, the vector most approaching received signals is chosen by the maximum likelihood (ML) criterion in formed candidate vectors to reduce the error propagation by previous erroneous decision, thus improving the detection performance. Under a large number of matrix inversion operations in the above iterative MMSE process, effective and fast signal detection is hard to be achieved. Then, a symmetric successive relaxation iterative algorithm is proposed to avoid the complex matrix inversion calculation process. The relaxation factor and initial iteration value are reasonably configured with low computational complexity to achieve good detection close to that of the MMSE with fewer iterations. Simultaneously, the error diffusion and complexity accumulation caused by the successive detection of the subsequent OSIC scheme are also improved. In addition, a method via a parallel coarse and fine detection deals with several layers to both reduce iterations and improve performance. Therefore, the proposed scheme significantly promotes the MIMO-OFDM performance and thus plays an irreplaceable role in the future sixth generation (6G) mobile communications and wireless sensor networks, and so on.

Spectral-Efficient Frequency-Division Photonic Millimeter-Wave Integrated Sensing and Communication System Using Improved Sparse LFM Sub-Bands Fusion

Scheduling radar and communication functions in a frequency-division multiplexing (FDM) fashion is a classic solution to achieve mutual-interference-free integrated sensing and communication (ISAC) system. However, it is haunted by poor spectral efficiency (SE) in comparison to the spectrum-sharing scheme. In this paper, we propose a novel photonics millimeter-wave ISAC system reaping the FDM scheme while offering inspirations for significantly enhancing SE using super-resolution techniques. Using the coherent fusion processing (CFP) of sparse sub-band linear frequency modulated (LFM) radar signals, a small fraction of the total bandwidth can enable a full-band-equivalent radar range resolution and more spectrum resources can be allocated for communications, leading to an ultrahigh SE for ISAC system. Moreover, an improved sparse sub-band fusion algorithm is developed based on particle swarm optimization to enhance the accuracy of synthesized radar range profiles. In experiments, a 60-GHz super-resolution FDM ISAC system using photonics-assisted millimeter-wave signal generation and dual-parallel coherent de-chirping processing is established, wherein the idle spectrum between two widely-spaced LFM sub-bands is loaded with broadband orthogonal frequency division multiplexing (OFDM) communication signals. Our system can simultaneously achieve up to 18-Gbit/s communication data rate and a radar range resolution as high as 2.14 cm, by exploring the globally unlicensed 7-GHz spectrum around 60-GHz with 4.5-GHz communication and 2-GHz radar bandwidths. In addition, a 0.97-cm radar range resolution corresponding to 16-GHz equivalent bandwidth (EB) is experimentally obtained by using two 1.5-GHz LFM sub-bands. This renders only ∼9.4% EB loss (3 GHz/32 GHz), without significant performance tradeoff between sensing and communications.

Multiplexed fiber-optic photoacoustic sensors for simultaneous detection of multi-point gases

A multiplexing scheme of fiber-optic photoacoustic (PA) gas sensors is demonstrated for simultaneous detection of multi-point gases. By sharing the PA demodulation device, the average cost of single point measurement can be significantly reduced. The passive fiber-optic PA gas sensing probe is integrated by a PA tube and a Fabry-Perot (F-P) interferometric (FPI) cantilever. The laser excited the PA signals is simultaneously incident into two sensing probes. The superimposed F-P interference spectrum containing multi-point gas concentration information is received by a high-speed spectrometer. For synchronous measurement of multi-point gases, a white-light interferometry (WLI) based frequency division multiplexing (FDM) spectral demodulation method is exploited. A multi-point acetylene gas remote monitoring system is established by two PA gas sensing probes with 3 km fiber cable. The experimental results show that the sensing probes have achieved the detection limit of sub-ppm. The crosstalk between the two sensors is characterized by the gas concentrations at the two points, indicating a crosstalk of about − 35 dB. This multiplexing scheme of fiber-optic PA sensing probes has the merits of remote monitoring, low crosstalk, high sensitivity, intrinsic safety and low cost. It can be applied for coal spontaneous combustion monitoring, dissolved gas analysis and gas micro-leakage monitoring.