Time Division Multiplexing Research Articles

Underwater Acoustic Positioning for Close Sources Using Time Division and Code Division Multiplexing

Underwater acoustic positioning system (UAPS) is used to know the positions of underwater robots and underwater structures. In ultra-short baseline (USBL) acoustic positioning systems, the three-dimensional position is determined by measuring the time difference of arrival (TDOA). In this paper, we investigate the acoustic positioning system targeting multiple sound sources and propose a simultaneous multi-point measurement method using time division and code division multiplexing (TD-CDM). TD-CDM provides higher position accuracy than code division multiplexing (CDM) and has a much shorter positioning time than time division and multiplexing (TDM). The effectiveness of TD-CDM has been proven by the results of the water tank experiment and simulation.

High-Resolution Optical Fiber Strain Sensor Array Based on Pound-Drever-Hall Technique With Fast Phase Regulation Method

Sensors that use Pound-Drever-Hall (PDH) technology benefit from the good performance and high resolution but are in shortage of large-scale multiplexing. This paper proposed a fast phase regulation method for the hybrid PDH multiplexing scheme based on space division multiplexing and time division multiplexing. Using both the information provided by an I/Q demodulator, the points from the error signal that the close-loop control requires can be rebuilt in the software with a phase rotation algorithm within a time period of less than 20 us. The relationship between the multiplexing scale and parameters selection is also given in this paper, showing that the system can have a larger multiplexing scale up to several tens sensors with appropriate parameters selection. A multi-channel strain array of up to 10 channels with a resolution better than 10 pϵ/√Hz@1 Hz over 50 Hz band per channel has been realized in this paper, and the system works with good channel crosstalk isolation. The experiment results show that the developed instrument has potential usage in the field of large-scale quasi-static strain measurement and earthquake observation.

The fiber ring laser intra-cavity gas sensor for C2H2 and CO2 detection based on photoacoustic spectroscopy

In this paper, a two-component gas sensing system in the resonant cavity of the fiber ring laser based on photoacoustic spectroscopy (PAS) is designed by embedding the PAS cell into the laser resonant cavity. A ring cavity erbium-doped fiber (EDF) laser is built in this system, combined with a longitudinal resonant PAS cell to detect the PAS signal, and a custom-designed wavelength modulator uses the fiber bragg grating (FBG) to paste the piezoelectric transducer (PZT) for scanning and modulating the gas absorption wavelength. The gas concentration is calculated by the second harmonic (2f) PAS signal. Based on time division multiplexing (TDM) technology, the acetylene (C2H2) and carbon dioxide (CO2) is detected respectively by the time-sharing operation of C-band and L-band EDFs for different wavelengths output. The time-sharing operation of the two wavelength laser is realized through optical switches, which effectively avoids the laser power loss and the instability of each wavelength caused by the mode competition phenomenon of multiple wavelengths. The detection performance of C2H2 and CO2 is implemented under normal temperature and pressure. The PAS signal intensity is further improved by optimizing the wavelength modulation amplitude and incident laser power. The linear fit (R2) and minimum detection limit (MDL) of C2H2 and CO2 are 0.99821, 728 ppbv, 0.9994 and 503 ppmv respectively. It shows that the two-component gas sensing system designed in this paper has a good linear response and high sensitivity, and plays an important role on the multi-component gas detection.

Study on Memristor-Based 3-D Convolutional Neural Network Adapting to Non-Ideal Effects

Memristor-based neural network (MNN) has made breakthroughs in image classification, but for complex video recognition, the required deep MNN will inevitably be more sensitive to non-ideal effects. In this work, we evaluate a memristor-based 3-D convolutional neural network (CNN) for action recognition and incorporate various non-ideal effects at the circuit and device level. Hardware-friendly schemes such as time division multiplexing (TDM) and updating with increasing thresholds are also described. Furthermore, a strategy combining forward averaging and mapping back is proposed to alleviate the IR drop effect. By setting the intersection conductance as the differential reference, deep MNNs are able to adapt to the updating asymmetry. Under the above adaptive strategies and in situ training, the network achieves an accuracy of 92.9% higher than the software and requires fewer updating pulses.

Fast gas sensing scheme with multi-component gas measurement capacity based on non-dispersive frequency comb spectroscopy (ND-FCS).

A fast gas sensing scheme based on a non-dispersive frequency comb spectroscopy (ND-FCS) is proposed and experimentally demonstrated. Its capacity for multi-component gas measurement is experimentally investigated as well, by using the time-division-multiplexing (TDM) method to realize specific wavelength selection of the fiber laser optical frequency comb (OFC). A dual-channel optical fiber sensing scheme is established with a sensing path consisting of a multi-pass gas cell (MPGC), and a reference path with a calibrated signal to track the repetition frequency drift of the OFC for a real-time lock-in compensation and system stabilization. The long-term stability evaluation and the simultaneous dynamic monitoring are carried out, with the target gases of ammonia (NH3), carbon monoxide (CO) and carbon dioxide (CO2). The fast CO2 detection in human breath is also conducted. The experimental results show that at an integration time of 10 ms, the detection limits of the three species are evaluated to be 0.0048%, 0.1869% and 0.0467%, respectively. A low minimum detectable absorbance (MDA) down to 2.8 × 10-4 can be achieved and a dynamic response with millisecond time can be realized. Our proposed ND-FCS exhibits excellent gas sensing performance with merits of high sensitivity, fast response and long-term stability. It also shows great potential for multi-component gas monitoring in atmospheric monitoring applications.

Analysis and improvement of dynamic range in a time-division-multiplexing interferometric fiber-optic sensor array.

Dynamic range, the ratio of maximum detectable amplitude, and the noise level, are of great importance in characterizing the performance of an interferometric fiber-optic sensor (IFOS). Here we analyze the dependence of dynamic range on carrier frequency and repetition frequency in a time-division-multiplexing (TDM) IFOS array using a heterodyne detection scheme. A novel, to the best of our knowledge, demodulation method with higher carrier frequency and an upgraded algorithm is proposed to improve the dynamic range. In the simulation and experiment, the carrier frequency was increased from 50 kHz to 20 MHz, and the demodulation method was adjusted correspondingly. The maximum detectable phase amplitude is elevated by more than 6 dB as the theoretical expectation, and the demodulated noise floor is proven to be barely influenced, holding at ∼-100 dB ref rad/H z. Experimental results demonstrate that the average dynamic range over 10 Hz to 20 kHz is enhanced by approximately 10.5 dB. The analysis and optimization presented here provide an effective solution to improve the dynamic range of TDM IFOS without any hardware change, which is promising in various applications.

Non-mechanical Lidar beamforming enabled by combined wavelength-division- and time-division-multiplexing

We propose and demonstrate a Lidar beam-forming scheme without moving parts, enabled by the combination of wavelength-division-multiplexing (WDM) and time-division-multiplexing (TDM). The lights of different wavelength are demultiplexed into N different waveguides to form N WDM channels; each of them is further switched into M waveguides to form M TDM channels with their exiting ends placed in the focal plane of a lens. The lights exiting from the N × M array at different positions in the focal plane of the lens are collimated into different beams pointing to different directions. Two-dimensional beam-forming of N × M beams is realized in the space-wavelength domain (the WDM channels) and space-time domain (the TDM channels), in which the N WDM beams can be simultaneously directed into different direction by the TDM switches sequentially. The scheme has been successfully demonstrated with the off-the-shelf discrete fiber optic components using 4 WDM channels and 1 × 4 optical fiber switches for beam forming, together with the time-of-flight (ToF) technique for distance measurement for obtaining the three dimensional (3D) point cloud data. In particular, we have experimentally realized four different beam scanning modes using different configurations of the time- and wavelength-channels. The addition of the TDM channels helps to reduce the required number of wavelength channels in the pure wavelength-multiplexed scheme, and vice versa, to overcome the technical limitations on the number of WDM or TDM channels. Furthermore, adding the WDM channels helps to relax the speed requirement of the switches and related electronics of the TDM channels. We believe our Lidar beam-forming scheme combining both TDM and WDM may provide an attractive alternative to the MEMS and optical phased array based beam scanning, as well as the pure WDM beam-forming schemes.

Wavelength- and Time-Division Multiplexing via Pump Current Variation of a Pulsed Semiconductor Laser—A Method of Synchronization for Quantum Key Distribution

The dependence of the semiconductor laser wavelength on the pump current is a well-known phenomenon, which is generally attributed to a change in the refractive index of the active layer due to carrier injection. This effect is usually considered to be a drawback as it causes frequency chirping of the pulses produced via direct current modulation. Here, we show that at high values of thermal resistance of a laser diode, the lasing wavelength is red-shifted due to the change of the refractive index caused by a thermal effect and this shift may significantly exceed (in absolute value) the blue shift related to a refractive index change induced by the carrier injection. We propose to benefit from the ability to tune wavelength by the pump current variation and use the same laser to generate qubits and synchronization optical pulses for quantum key distribution at different wavelengths. To demonstrate the proposed method of synchronization, we perform numerical simulations as well as proof-of-principle experiments.

Configurable Hybrid Energy Synchronous Extraction Interface With Serial Stack Resonance for Multi-Source Energy Harvesting

Conventional multi-source energy harvesters use the time-division multiplexing (TDM) scheme, where the inductor operates in discontinuous current mode (DCM) and fails to perform well in continuous current mode (CCM). This article presents a single-inductor hybrid energy synchronous extraction circuit with serial stack resonance (SSR) for multi-source energy harvesting. The proposed circuit incorporates a synchronous electric charge extraction (SECE) circuit for ac energy and two buck–boost circuits for dc energy with a single inductor, which can operate either in a TDM mode to harvest energy from different sources separately or in an SSR mode to extract energy from multiple sources synchronously. The proposed harvester is fabricated in a 0.18- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS process. It is configured with a photovoltaic (PV) cell, a thermoelectric generator (TEG), and two piezoelectric transducers (PZTs), where the number of PZTs is flexible to meet different application needs.

Digital inter-symbol interference mitigation method for Nyquist OTDM signal detection with coherent sampling and KNN classifier

We propose and experimentally demonstrate a digital method to mitigate inter-symbol interference (ISI) in demultiplexing Nyquist optical time-division multiplexing (N-OTDM) signal using a simple but powerfully serviceable k-nearest neighbors (KNN) classifier. The proposed method is evaluated under four types of sampling pulse-width corresponding to various extents of ISI distortion in 160 Gbaud down to 40 Gbaud demultiplexing experiments, and the mitigation performance is found to be better along with the increasement of sampling pulse-width. Compared with the traditional decision-directed least mean square (DD-LMS) equalization method, a 3.9 dB required-OSNR (at a BER of 10−4) improvement is observed under the pulse-width of 5.3 ps. Demultiplexing performance is investigated under the pulse-widths of 3.2 ps and 3.9 ps as well, in which the improvement of required-OSNR are found to be 1.33 dB and 2.95 dB, respectively. The performance improvement clearly indicates the strong ISI mitigation ability of the KNN classifier for N-OTDM signal demultiplexing. In addition, experimental results also confirm that the proposed digital ISI mitigation method could release the pulse-width requirement of the N-OTDM demultiplexing, which paves the way for future practical application of N-OTDM technique.

A Review on Nonlinearity in Semiconductor optical Amplifer as Application in all Optical Signal Processing

The optical signal processing is the future of communication network. High speed all-optical packet routing is one of the essential applications of all-optical networks which is only possible with the development of all-optical logic technology. The development of optical logic elements and circuits are the steps in the growth of this technology and higher speed up to Tbit/sec can be achieved. The optical carrier frequency range 1013 to 1016 Hz provides enormous potential bandwidth with superior information carrying capacity over a long transmission distance. The need of higher capacity is continuing to encourage research in wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) based transmission systems, which need optical demultiplexing and wavelength conversion technology. Therefore, for high-speed optical networks, it is required to develop the all-optical gates to avoid power consumption in opto-electronics conversion. All optical logic gates perform computing operations, storage and transmission of data using light also known as optical computing. Optical technology promises massive upgrades in the efficiency and speed of computers, aswell as significant shrinkage in their size and cost. The non-linear optical device to be used i.e. Semiconductor Optical Amplifier (SOA) has proved to be the promising for all optical functions like wavelength conversion, logic functions, signal representation in all optical domain. Its compact size, high gain, fast response, strong refractive index variation, easy to manufacture and integration, and power efficiency makes it most optimum device for optical signal processing. In this paper, the application of SOA in optical processing is thoroughly reviewed and orient towards the latest application in neural networks.

Design and Implementation of an Efficient Hardware Coprocessor IP Core for Multi-axis Servo Control Based on Universal SoC

The multi-axis servo control system has been extensively used in industrial control. However, the applications of traditional MCU and DSP chips in high-performance multi-axis servo control systems are becoming increasingly difficult due to their lack of computing power. Although FPGA chips can meet the computing power requirements of high-performance multi-axis servo control systems, their versatility is insufficient, and the chip is too costly for large-scale use. Therefore, when designing the universal SoC, it is better to directly embed the coprocessor IP core dedicated to accelerating the multi-motor vector control current loop operation into the universal SoC. In this study, a coprocessor IP core that can be flexibly embedded in a universal SoC was designed. The IP core based on time division multiplexing (TDM) technology could accelerate the multi-motor vector control current loop operation according to the hardware–software coordination scheme proposed in this study. The IP was first integrated into a universal SoC to verify its performance, and then the FPGA prototype verification for the SoC was performed under three-axis servo control systems. Secondly, the ASIC implementation of the IP was also conducted based on the CSMC 90 nm process library. The experimental results revealed that the IP had a small area and low power consumption and was suitable for application in universal SoC. Therefore, the cheap and low-power single universal SoC with the coprocessor IP can be suitable for multi-axis servo control.

Time-domain measurement of twin beams produced by fiber amplifiers with an ultra-fast pulse train as a pump.

A pulsed pumped four-wave mixing process via χ(3) nonlinearity in an optical fiber can generate optical pulses with continuous variable quantum correlation. However, pairwise correlation of the generated pulses in this system has not been demonstrated. Here we report a time-domain measurement of an intensity difference squeezed state generated in a fiber. With a fast response differential detection system, we show the generated twin-beam pulses are pairwisely correlated, and -3.8-dB (-8.1 dB after detection losses correction) intensity difference squeezing degree is measured in the time domain. Our result is beneficial for generating multi-mode entangled state by time-division multiplexing in fiber system.

A high-precision bidirectional time-transfer system over a single fiber based on wavelength-division multiplexing and time-division multiplexing

In this paper, a high-precision bidirectional time-transfer system over a single fiber based on wavelength-division multiplexing and time-division multiplexing (SFWDM-TDM) is proposed, which combines the advantages of wavelength-division multiplexing and time-division multiplexing. It uses two dense wavelength-division channels to effectively suppress the problem of optical fiber reflection. At the same time, the time-division multiplexing method is used in combination with sampling and holding the time to complete the multi-user task. In hardware, we optimized the carrier processing and the high-precision time-delay control module of the SFWDM-TDM system to complete high-precision time-transfer equipment. In software and algorithm, the optical fiber time-interval measurement method and measurement times are optimized, and the SFWDM-TDM system reaches a synchronization accuracy of 8.9 ps at 1 s. Finally, a real-time detection mechanism with self-recovery ability is added to the system. This lays the foundation for a reliable, long-distance, high-precision, and multi-user mode optical fiber time- and frequency-transfer network.

Design and analysis of a compact micro-ring resonator signal emitter to reduce the uniformity-induced phase distortion and crosstalk in orbital angular momentum (OAM) division multiplexing.

To improve the transmission capacity of an optical system, different multiplexing schemes have been proposed, such as optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), polarization division multiplexing (PolDM), spatial division multiplexing (SDM), etc. One kind of SDM technique to boost the capacity is through modifying the spatial phase structure of an optical beam, which is known as the orbital angular momentum (OAM) division multiplexing. Moreover, the OAM signal emitter can be produced by using mature and high-yield silicon photonic (SiPh) technology, without the need of using bulky optical components or expensive spatial light modulator (SLM). The SiPh-based micro-ring resonator is one of the promising OAM signal emitter candidates, since it is simple, compact and easy to fabricate. However, the device performance is highly subjected to the structural design, and the uniformity-induced phase distortion will significantly degrade the purities of OAM beams; hence, introducing severe OAM signal crosstalk during the OAM division multiplexing. In this work, a compact SiPh-based micro-ring resonator type OAM signal emitter with detailed design parameters is presented and the output signal uniformity issue is comprehensively investigated. Two kinds of the structural optimization are performed by adjusting the angular grating width as well as the grating height. The results indicate that a significant improvement in output OAM beam uniformity can be achieved, with the attenuation factor being improved over 88% at the price of acceptable 4 ∼ 5% coupling efficiency reduction. The variations of the transmission and the uniformity induced by the fabrication error are also analyzed.

Simultaneous Discrimination of Multiple Chromophores With Frequency Division Multiplexed Four-Color Functional Near-Infrared Spectroscopy

Herein, we implement a four-color functional near-infrared spectroscopy (fNIRS) system using frequency division multiplexing (FDM) to demonstrate simultaneous discrimination of multiple chromophores. The proposed fNIRS system consists of a four-channel flexible patch with two light-emitting diode (LED) modules, four photodiodes, and a control board for FDM. Unlike the conventional time division multiplexing (TDM) method that turns on/off individual LEDs sequentially, the FDM method can simultaneously turn on/off the entire LEDs using different modulation frequencies. Therefore, the sampling rate of FDM can be increased by eight times compared to the case of TDM under the same condition of two light sources with four wavelengths. First, for validation of the developed fNIRS system, we conducted a comparative experiment with a commercial spectrometer using two types of dyes in a customized phantom. By comparing the absorbance spectra for each wavelength using both instruments, we confirmed that the correlation between the absorbance measured by the two instruments was high with an R-square value of up to 0.99662. Second, three chromophores of oxyhemoglobin (HbO), deoxyhemoglobin (HbR), and the oxidation state of cytochrome-c-oxidase (oxCCO) were discriminated in vivo using four wavelengths for the cerebral blood flow response with correlation coefficients and effect size. The average hemodynamic response during cerebral activity clearly presented an increased HbO and a decreased HbR simultaneously. We also confirmed that the strong correlation between oxCCO (directly involved in oxygen metabolism) and hemoglobin difference (cerebral oxygenation) showed a Pearson’s correlation coefficient of 0.7342. These results demonstrate the validity of the FDM-based fNIRS system and the successful acquisition of brain signals. This reveals that FDM can be applied to various bio-engineering applications in the future.

Simultaneous temperature and magnetic field measurements using time-division multiplexing

Simultaneous temperature and magnetic field measurements using time-division multiplexing

Paper] Light Field Display Using Wavelength Division Multiplexing

A novel light field display based on time-division multiplexing is proposed. The display consists of a high-speed projector and a passive screen unlike conventional time-division multiplexing based methods. The high-speed projector projects monochrome images onto the screen while changing wavelength of a light source at high-speed. The screen has spectroscopic characteristics and delivers images toward different viewing points in horizontal direction corresponding to wavelength of projected images by chromatic dispersion. The proposed display is composed of a completely passive screen and only one projector, which is suitable for making a large display. We verified feasibility of the proposed display by experiments using a prototypes.

The strategic analysis and operation of the multiservice model used for synchronous transmission in communication networks

The core idea of asynchronous transfer mode (ATM) technology was revealed a long time ago. The main technology under development at the time was time-division multiplexing (TDM) technology, which consisted of synchronous switching based on the sequence number of bytes in the integrated frame. The main disadvantage of TDM technology, also known as synchronous transport module (STM) synchronous transmission technology, is the inability to reallocate the bandwidth of the integrated channel between subchannels. During those periods when no user data is sent on the subchannel, the aggregated channel still sends the bytes of this subchannel filled with zeros. Efforts to load sub channels’ idle periods necessitate the introduction of a header for each subchannel’s data. In intermediate statistical time-division multiplexing (STDM) technology, which allows idle periods to be filled by transmitting bursts of traffic from other sub channels, headers that actually have a subchannel number are introduced. In this paper, the strategic analysis and operation of technologies used in multiservice networks were discussed. Simultaneously, the structure of data sets is drawn into sets resembling computer networks. The fact that each packet has an address allows it to be transmitted asynchronously since its location relative to data on other subchannels is not its address. Asynchronous packets from one subchannel are inserted into the free time slots of another subchannel, but they are not mixed with the data of this subchannel because they have their own address.

Time-Division Multiplexing Light Field Display with Learned Coded Aperture.

Conventional stereoscopic displays suffer from vergence-accommodation conflict and cause visual fatigue. Integral-imaging-based displays resolve the problem by directly projecting the sub-aperture views of a light field into the eyes using a microlens array or a similar structure. However, such displays have an inherent trade-off between angular and spatial resolutions. In this paper, we propose a novel coded time-division multiplexing technique that projects encoded sub-aperture views to the eyes of a viewer with correct cues for vergence-accommodation reflex. Given sparse light field sub-aperture views, our pipeline can provide a perception of high-resolution refocused images with minimal aliasing by jointly optimizing the sub-aperture views for display and the coded aperture pattern. This is achieved via deep learning in an end-to-end fashion by simulating light transport and image formation with Fourier optics. To our knowledge, this work is among the first that optimize the light field display pipeline with deep learning. We verify our idea with objective image quality metrics (PSNR, SSIM, and LPIPS) and perform an extensive study on various customizable design variables in our display pipeline. Experimental results show that light fields displayed using the proposed technique indeed have higher quality than that of baseline display designs.