Time-domain Multiplexing Research Articles

Optical Crosstalk and Performance in Time Division Multiplexed Derivative Sensing Techniques

In many time-domain multiplexing (TDM) schemes, the demodulation process can fail at relatively high modulation frequencies, often referred to as signal over-scaling. A novel frequency modulated (FM-TDM) derivative sensing scheme has been developed to tackle this problem. The derivative scheme produces simultaneously two types of measurements: (a) a principal phase measurement, corresponding to the phase of the sensor due to a change in optical path length; and (b) a differential measurement of the principal phase, this being the difference of the phase at two times, $\tau $ apart. If signal over-scaling occurs, the differential signal is integrated to recover the primary signal. In this way, over 180 dB of dynamic range can be achieved for frequencies less than 100 Hz. The behaviour of the derivative scheme can be understood in terms of a two-point difference operator – which is directly proportional to frequency and shown to be the basis of optical crosstalk in the measurement of the principal phase. A method of crosstalk reduction was demonstrated for the principal phase measurement. However, crosstalk is independent of frequency for the differential measurements. The benefits of the scheme are discussed, including the capacity to recover an over-scaled signal with a consequent increase in dynamic range.

Bidirectional Viterbi decoding algorithm for OvTDM

Overlapped time domain multiplexing (OvTDM) is an innovative encoding scheme that can obtain high spectral efficiency. However, the intentional inter-symbol interference (ISI) caused by OvTDM will make the decoding process more complex. The computational complexity of maximum likelihood sequence detection increases exponentially with the growth of spectral efficiency in OvTDM. As a consequence of high complexity, the decoding effort for a given spectral efficiency may occasionally exceed the physical limitations of the decoder, leading inevitably to buffer overflows and information erasures. In this paper, we propose a bidirectional Viterbi algorithm (BVA) based on the bidirectional sequence decoding for OvTDM. With the BVA, the decoding operation starts simultaneously from the both ends of the corresponding trellis and stops at the middle of trellis. The simulation results show that compared with Viterbi algorithm (VA), the decoding time of BVA can be reduced by about half. And the memory space of two decoders in BVA are about half of that in VA, which means that the BVA has lower memory requirements for decoder. And the decoding performance of BVA is almost the same as VA.

Time Domain Multiplexing for Efficiency Enhanced Piezoelectric Energy Harvesting in MEMS

The conversion efficiency of piezoelectric energy harvesters (EH) have been improved by several approaches including frequency up-conversion (FUC) techniques that trigger the high-frequency (HF) piezoelectric resonators using low-frequency (LF) mechanical inputs. This work proposes a new time-domain multiplexing technique to further improve the harvesting efficiency for random mechanical impacts using commercially available microfabrication processes. The FUC is implemented by a slowly moving shuttle beam, which represents the LF mechanical inputs, that triggers the free ends of piezoelectric cantilever beams. Mechanical impacts by the LF shuttle lead to the cantilever beams vibrating at their higher natural resonance frequencies. In the proposed approach, resonators are exposed to the LF mechanical input at unequal distances, which results in sequential HF vibrations. As a result, the HF electrical outputs fit sequentially within the long period of the LF input. Analytical and experimental comparisons support the increased electrical output using time domain multiplexing.

Continuous-wave 6-dB-squeezed light with 2.5-THz-bandwidth from single-mode PPLN waveguide

Terahertz (THz)-bandwidth continuous-wave (CW) squeezed light is essential for integrating quantum processors with time-domain multiplexing (TDM) by using optical delay line interferometers. Here, we utilize a single-pass optical parametric amplifier (OPA) based on a single-spatial-mode periodically poled ZnO:LiNbO3 waveguide, which is directly bonded onto a LiTaO3 substrate. The single-pass OPA allows THz bandwidth, and the absence of higher-order spatial modes in the single-spatial-mode structure helps avoid degradation of squeezing. In addition, the directly bonded ZnO-doped waveguide has durability for high-power pump and shows small photorefractive damage. Using this waveguide, we observe CW 6.3-dB squeezing at 20-MHz sideband by balanced homodyne detection. This is the first realization of CW squeezing with a single-pass OPA at a level exceeding 4.5 dB, which is required for the generation of a two-dimensional cluster state. Furthermore, the squeezed light shows 2.5-THz spectral bandwidth. The squeezed light will lead to the development of a high-speed on-chip quantum processor using TDM with a centimeter-order optical delay line.

${N}$ -Continuous Signaling for GFDM

Generalized frequency division multiplexing (GFDM) has been a candidate multicarrier scheme in the 5th generation cellular networks for its flexibility of transmitter filter in time and frequency. However, for the circularly shaped transmitter filter, GFDM provides limited performance gain of sidelobe suppression. In this paper, we propose a scheme, called time-domain N-continuous GFDM (TD-NC-GFDM), to reduce the discontinuities caused by the GFDM transmitter filter and achieve promising sidelobe suppression gain. Based on time-domain N-continuous orthogonal frequency devision multiplexing (TD-NC-OFDM), TD-NC-GFDM signal can be obtained by superposing a smooth signal in the time domain. The smooth signal is linearly combined by basis signals in a new basis set related to GFDM transmitter waveform. To eliminate the interference caused by the smooth signal, two solutions are proposed. Firstly, a signal recovery algorithm for reception is adopted at the cost of high complexity. Thus, secondly, to simplify the TD-NC-GFDM receiver, a low-interference TD-NC-GFDM is proposed by redesigning the basis signals. A soft truncation of the basis signals in TD-NC-GFDM is given to design the basis signals in the low-interference TD-NC-GFDM. Then, the smooth signal is aligned with the beginning of the GFDM symbol and is added in the front part of the GFDM symbol. Moreover, for a big number of GFDM subsymbols, theoretical analysis proves that the signal-to-interference ratio (SIR) in TD-NC-GFDM is much higher than that in TD-NC-OFDM. Simulation results shows that TD-NC-GFDM can obtain significant sidelobe suppression performance as well as the low-interference TD-NC-GFDM, which can achieve the same BER performance as the original GFDM.

Multiuser PSK-Chirp-BOK Communication System Under Co-Channel Interference

A chirp binary orthogonal keying (BOK) system normally transmits 1 bit per symbol and requires two correlators at the receiver. This brief presents a chirp-BOK system incorporating phase shift keying (PSK) modulation, viz. , PSK-chirp-BOK system, to double the transmission rate while using only two correlators. Design of a set of orthogonal chirps is presented to minimize the performance degradation due to non-orthogonality between up-chirps and down-chirps in a PSK-chirp-BOK multiuser system. In an effort to reduce the required total system bandwidth while maintaining the same number of chirps for a multiuser system, a time domain multiplexing of orthogonal chirp sets with filtering using fractional Fourier transform is proposed. Symbol error rate performance of the proposed PSK-chirp-BOK system in a multiuser environment under Nakagami- $m$ fading with the effect of co-channel interference is analyzed. Benefits of the proposed scheme over the existing ones are also outlined.

Channel Estimation for TDS-OFDM Systems in Rapidly Time-Varying Mobile Channels

This paper explores the performance of time-domain synchronous orthogonal frequency-division multiplexing (TDS-OFDM) systems operated under rapidly time-varying mobile channels. Since a rapidly time-varying channel contains more unknown channel coefficients than the number of observations, the mobile channel can conveniently be modeled with the discrete Legendre polynomial basis expansion model to reduce the number of unknowns. The linear minimum mean square error (LMMSE) estimate can be exploited for channel estimation on inter-block-interference-free received signal samples owing to transmitting pseudo-noise (PN) sequences. In conventional TDS-OFDM systems, the channel estimation performance is limited due to estimating channel responses only from the beginning part of the channel. Therefore, a new system model named “ partitioned TDS-OFDM system ” is proposed to improve the system performance by inserting multiple PN sequences to the middle and end parts of the channel as well. In addition to providing the reconstruction error performance, Bayesian Cramer–Rao lower bound is derived analytically. Also, the LMMSE-based symbol detection is employed. To alleviate the negative effects of inter-carrier-interference (ICI) occuring in mobile channels, ICI cancellation is applied to enhance the detection performance. The simulation results demonstrate that the proposed TDS-OFDM system is superior to the conventional system and its corresponding performance is able to approach the achievable lower performance bound.

Acquisition of Neural Action Potentials Using Rapid Multiplexing Directly at the Electrodes.

Neural recording systems that interface with implanted microelectrodes are used extensively in experimental neuroscience and neural engineering research. Interface electronics that are needed to amplify, filter, and digitize signals from multichannel electrode arrays are a critical bottleneck to scaling such systems. This paper presents the design and testing of an electronic architecture for intracortical neural recording that drastically reduces the size per channel by rapidly multiplexing many electrodes to a single circuit. The architecture utilizes mixed-signal feedback to cancel electrode offsets, windowed integration sampling to reduce aliased high-frequency noise, and a successive approximation analog-to-digital converter with small capacitance and asynchronous control. Results are presented from a 180 nm CMOS integrated circuit prototype verified using in vivo experiments with a tungsten microwire array implanted in rodent cortex. The integrated circuit prototype achieves <0.004 mm2 area per channel, 7 µW power dissipation per channel, 5.6 µVrms input referred noise, 50 dB common mode rejection ratio, and generates 9-bit samples at 30 kHz per channel by multiplexing at 600 kHz. General considerations are discussed for rapid time domain multiplexing of high-impedance microelectrodes. Overall, this work describes a promising path forward for scaling neural recording systems to numbers of electrodes that are orders of magnitude larger.

Multifunctional Wavelength-Selective Switch for Nyquist Pulse Generation and Multiplexing

We demonstrate optical Nyquist pulse train generation using a wavelength-selective switch (WSS) without an optical multiplexing circuit. The representative approach generating optical Nyquist pulse trains is based on spectral filtering with a WSS and optical time-domain multiplexing (OTDM) with an optical circuit. The use of the optical circuit causes difficulties when extending optical Nyquist pulse train technology to the near-infrared band because this band has no well-established OTDM modules. To resolve this problem, we assign the role of multiplexing to the WSS, in addition to spectral filtering. Furthermore, the WSS-only approach is expected to reduce optical losses and increase pulse-train parameter flexibility. Using only the WSS, we successfully generate optical Nyquist pulse trains in the near-infrared band and realize flexible control of pulse number, width, and interval. In the case of a 10-pulse train, optical loss due to multiplexing is improved by 8.0 dB compared with the conventional method.

Lifetime-engineered NIR-II nanoparticles unlock multiplexed in vivo imaging.

Deep tissue imaging in the second near-infrared (NIR-II) window holds great promise for physiological studies and biomedical applications1-6. However, inhomogeneous signal attenuation in biological matter7,8 hampers the application of multiple-wavelength NIR-II probes to multiplexed imaging. Here, we present lanthanide-doped NIR-II nanoparticles with engineered luminescence lifetimes for in vivo quantitative imaging using time-domain multiplexing. To achieve this, we have devised a systematic approach based on controlled energy relay that creates a tunable lifetime range spanning three orders of magnitude with a single emission band. We consistently resolve selected lifetimes from the NIR-II nanoparticle probes at depths of up to 8 mm in biological tissues, where the signal-to-noise ratio derived from intensity measurements drops below 1.5. We demonstrate that robust lifetime coding is independent of tissue penetration depth, and we apply in vivo multiplexing to identify tumour subtypes in living mice. Our results correlate well with standard ex vivo immunohistochemistry assays, suggesting that luminescence lifetime imaging could be used as a minimally invasive approach for disease diagnosis.

TPC Together with Overlapped Time Domain Multiplexing System Based on Turbo Structure

Overlapped time domain multiplexing (OvTDM) is a novel technique for utilizing inter-symbol interference (ISI) to benefit a communication system. We implement the OvTDM technique based on turbo structure and associate a turbo product code (TPC) to construct a novel coded turbo-structure OvTDM system. Two schemes of the iterative receiver and soft-input and soft-output (SISO) decoding algorithms are presented. Simulation results show an attractive advantage and performance of designed structures.

Time-domain multiplexed high resolution fiber optics strain sensor system based on temporal response of fiber Fabry-Perot interferometers.

We developed a multiplexed strain sensor system with high resolution using fiber Fabry-Perot interferometers (FFPI) as sensing elements. The temporal responses of the FFPIs excited by rectangular laser pulses are used to obtain the strain applied on each FFPI. The FFPIs are connected by cascaded couplers and delay fiber rolls for the time-domain multiplexing. A compact optoelectronic system performing closed-loop cyclic interrogation is employed to improve the sensing resolution and the frequency response. In the demonstration experiment, 3-channel strain sensing with resolutions better than 0.1 nε and frequency response higher than 100 Hz is realized.

A novel transceiver structure including power and audio amplifiers for Internet of Things applications

Both of the Circuit and System design aspects of the Internet of Things (IoT) are emerging with computation and communication issues. In communication, it utilizes transceivers like wireless sensor network (WSN). In spite of the conventional WSN, a key component of IoT is its direct engagement with humans (animals). Therefore, communication is required in audio–visual ranges. This means an IoT device (IoTD) will be ported with audio amplifiers. This paper presents a novel IoT transceiver with a reconfigurable PA-CDA with time-domain and task-oriented multiplexing. The run-time reconfiguration of an area-power-efficient common part is utilized. The topology and scheduling issues of the Bluetooth Low Energy (BLE) standard are well-suited to the proposed architecture. Time, energy, and protocol concerns are studied and appropriate models presented. The PA and CDA operate in the 2.4 GHz and audio-range frequencies, respectively. The battery life and power consumption are improved by about 40% and 20%, respectively.

Integrated power line and visible light communication system compatible with multi‐service transmission

As the shortage of spectrum is becoming a serious problem to the wireless communication, visible light communication (VLC) is attracting widespread attention. However, the access to the backbone network is likely to completely modify the communication network, thus leading to a great cost. Fortunately, the technology of power line communication (PLC) is a perfect match to VLC, which provides power supply and connection for VLC to the backbone network. In this study, the authors take an integration of PLC and VLC into consideration and mainly focus on a communication framework with multi-service. In the case of not making a big modification to the origin network, a direct re-transmission concept is considered and three implementation schemes which differ in frequency domain, time domain, and bit division multiplexing (BDM) are proposed. The demonstration platform and the simulation results suggest that the proposed schemes have the ability to support multi-service transmission with no big modification of current power line network. Moreover, the BDM scheme can accommodate different requirements of quality of service with better performance than conventional multi-service schemes.

Ultrahigh resolution fiber optic strain sensing system for crustal deformation observation

Due to the advantages of high resolution, low cost, small size, easy deployment and capability of multiplexed sensing, the recent developed optical fiber grating sensors provide powerful tools for crustal deformation monitoring. This paper reviews the development of several types of fiber-optic sensors with ultrahigh resolution in quasi-static domain. The fiber Bragg grating based Fabry-Perot interferometers and the -phase-shifted fiber Bragg gratings which are used as sensing components in the high resolution sensors are introduced. Some novel techniques such as interrogating the sensing components with intensity modulation sideband, dual feedback-loop structure for high bandwidth/large measurement range sensing, and the time-domain multiplexing of the high resolution quasi-static strain sensor are discussed in detail. Each sensing scheme with both operation process and achieved performances are given. The implementation of fiber grating sensors for in-situ measurement of crustal deformation and the results are also introduced. Compared with the traditional methods used in crustal deformation observation, the high-performance fiber optic strain sensors mentioned in the paper shows great potentials in providing wider measurement approaches in geophysical researches.

Multipoint Remote Methane Measurement System Based on Spectrum Absorption and Reflective TDM

An all-optical methane measurement system was developed. The technologies of spectrum absorption and reflective time domain multiplexing (TDM) were applied. It provides a novel and intrinsically safe solution to achieve the multipoint and remote gas measurement. The information of the concentration and position were obtained at the same time by employing of only a single optic fiber, a single laser source, and a single detector. In order to improve detection sensitivity and accuracy, an optimized gas chamber based on reflective mode was implemented. An experimental system with three measurement points and 4-km optical length was built up. The minimum detection limit of 2% lower explosive level was achieved, and the linear correlation coefficient was 0.999, 0.996, and 0.989 at 0, 2, and 4 km, respectively. The experimental results demonstrate that such a system is suitable for the multipoint and remote gas measurement application.

Invited Article: Generation of one-million-mode continuous-variable cluster state by unlimited time-domain multiplexing

In recent quantum optical continuous-variable experiments, the number of fully inseparable light modes has drastically increased by introducing a multiplexing scheme either in the time domain or in the frequency domain. Here, modifying the time-domain multiplexing experiment reported in the work of Yokoyama et al. [Nat. Photonics 7, 982 (2013)], we demonstrate the successive generation of fully inseparable light modes for more than one million modes. The resulting multi-mode state is useful as a dual-rail continuous variable cluster state. We circumvent the previous problem of optical phase drifts, which has limited the number of fully inseparable light modes to around ten thousands, by continuous feedback control of the optical system.

A THz Superconducting Imaging Array Developed for the DATE5 Telescope

Dome A in Antarctica, located at an altitude of 4093 m and with very low temperature in winter down to 83C, is an exceptionally dry site. Measurements of the atmospheric transmission in the range of 0.75-15 THz by a Far-infrared/THz Fourier transform spectrometer (FTS) strongly suggest that Dome A is a unique site for ground-based THz observations, especially for the 200- and 350-micron windows. A 5-m THz telescope (DATE5) is therefore proposed for Chinese Antarctic Kunlun Observatory. We are currently developing a THz superconducting imaging array (TeSIA) for the DATE5. The TeSIA will be working at the 350-m window, with a pixel number of and a sensitivity (NEP) of 10 W/Hz. Ti transition-edge sensors with time-domain multiplexing and TiN microwave kinetic inductance detectors with frequency-domain multiplexing are both developed for the TeSIA. In this paper, detailed system designs and some measurement results will be presented.

Approach to suppress out‐of‐band emission for dual pseudo noise padded time‐domain synchronous‐orthogonal frequency division multiplexing systems

The dual pseudo noise padded (DPNP) time-domain synchronous-orthogonal frequency division multiplexing (TDS-OFDM) which utilises the second pseudo noise (PN) sequence for channel estimation is able to reduce the complexity and improve the accuracy of channel estimation compared to the classical TDS-OFDM. However, because of the duplicate PN sequence, DPNP TDS-OFDM will suffer from severer out-of-band emission, which is a headache issue for the conventional TDS-OFDM. In this study, a novel out-of-band suppression approach based on a new PN design criterion as well as a windowing operation is proposed for the DPNP TDS-OFDM systems. The frame structure and system model are also modified to accommodate to the approach, while the overlap-and-add approach is adopted to guarantee the spectral efficiency. Both simulation and experimental results show that this approach could achieve satisfactory performance with much less complexity and no spectral efficiency loss compared to the classical DPNP TDS-OFDM system. In addition, an iterative channel estimation method is also proposed for the modified frame structure to combat against the long channel delay spread.

PHOTONIC AND OR GATE BY USING FIBER COUPLER

The development of optical logic gate by using fiber coupler can be realized by properly determine the delay in Pulse Position Propagation (PPM) to set a logic bit in Time Domain Multiplexing (TDM) systems. Soliton has been used to maintain the pulse shape for a long haul distance connection up to 100km. In this study, a numerical modeling of secant hyperbolic soliton pulse based on Nonlinear Schrodinger Equation (NLSE) was performed in balance of Self-Phase modulation and Group velocity dispersion by normalized distance, time and field calculated by 1032 iteration step. Two picosecond soliton pulse transmission in anomalous dispersion region served as input generated inside fiber coupler and the code generator will encode signal within the altered time difference of ±0.25t. The signal would move in the fiber coupler and the phase modulator controls the phase of the bisoliton generation from 0 to 2p. The result is the formation of optical logic AND and OR gate at the phase difference of 0.4p and 1.1p with normalized power of ~6 and parameter offset ε=0.25.