Simple Digital Signal Processing Research Articles

Thin-film lithium niobate-based electro-optic comb cloning for self-homodyne coherent communication.

As the optical communication industry advances, metropolitan area networks (MANs) and radio access networks (RANs) are extensively deployed on a large scale, demanding energy-efficient integrated light sources and simplified digital signal processing (DSP) technologies. The emergence of thin-film lithium niobate (TFLN) has given rise to high-performance, energy-efficient on-chip modulators, making on-chip optical frequency comb (OFC) more appealing. Owing to the phase uniformity and stability of this chip-scale device, it has been possible to eliminate the carrier frequency phase estimation (CPE) in DSP stacks using comb-clone-enabled self-homodyne detection. Here we report the first use, to our knowledge, of a TFLN on-chip electro-optic (EO) frequency comb to realize comb cloning and self-homodyne coherent detection. We transmit three optical pilot tones and eight data channels encoded with 20 Gbaud polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16-QAM) over 10 km and 80 km standard single-mode fibers. The bit error ratios (BERs) of the eight channels reach below 10-3, a result made possible by our on-chip comb. The scalability and mass producibility of on-chip EO combs, combined with the simplified DSP, show potential in our proposed fifth-generation (5G) RAN and MAN transmission scheme.

A coherent MMW-ROF link employing mixed modulations and photonic down-conversion

We propose and experimentally demonstrate a coherent millimeter wave radio-over-fiber (MMW-ROF) link employing mixed modulations with a simple digital signal processing (DSP) unit. At the transmitter, a light wave is intensity and phase-modulated by two microwave vector signals in a dual-parallel Mach–Zehnder modulator (DP-MZM). At the receiver, by using a 3-line optical frequency comb (OFC) signal as the optical local oscillator (OLO), the microwave vector signals can be photonically down-converted to intermediate frequency (IF) signals. Then, instead of a complex digital phase-locked loop (DPLL), simple algorithms are used to remove the laser phase fluctuation between two free-running lasers and do the demodulations. In the 25 km fiber transmission experiments, we successfully down-convert two 50 Msymbol/s 16-quadrature amplitude modulation (16-QAM) signals at 25 GHz and 24 GHz to intermediate frequencies of 2 GHz and 1 GHz. The transmission performance of the proposed link in terms of error vector magnitudes (EVMs) are also investigated. The results show that when the receiver optical power is -20 dBm, the measured EVMs are still less than 12.5% which satisfies the 3rd Generation Partnership Project (3GPP) standard.

Demonstration of all-digital burst clock and data recovery for symmetrical 50 Gb/s/[formula omitted] PON based on low-bandwidth optics

We experimentally demonstrated all-digital burst clock and data recovery (BCDR) for symmetrical single-wavelength 50 Gb/s four-level amplitude modulation (PAM-4) passive optical network (PON) over the same fiber link at O-band, with the using of 10G-class optics and simple digital signal processing (DSP). The uplink burst frame and BCDR algorithm is used for accurately evaluating PON uplink transmission performance. The results show that over 26 dB link loss budget for continuous-mode uplink transmission and over 25 dB for burst-mode uplink transmission at a bit error rate of 3.8 × 10−3 can be achieved, respectively. For symmetrical 50 Gb/s/λ PON uplink burst-mode, there are 1 dB power budget loss less than uplink continuous-mode.

Experimental investigation of VCSEL-based optical heterodyning with PAM 4 and envelop detection for 5G fronthaul systems

Vertical cavity surface-emitting lasers (VCSELs) are power-efficient optical sources for application in passive optical networks. Despite these sources having the potential of generating coherent high-frequency radio frequency signal, their application for high-order amplitude modulation formats is yet to be demonstrated experimentally. In this work, for the first time, we experimentally demonstrate a four-level pulse amplitude modulation (4-PAM) format on a photonically generated RF carrier signal using optical heterodyning of two independent VCSELs. Two low-power VCSELs were combined at an optical coupler and allowed to beat a PIN photodiode to generate a 10-GHz RF carrier. In the first demonstration, an 8.5 Gbps data signal was modulated on the photonically generated 10 GHz RF carrier. In the second demonstration, a 17-Gbps PAM-4 data signal was externally modulated onto the same VCSEL carrier, ultimately doubling the network bit rate. A simple digital signal processing (DSP) receiver was used as an alternative to costly receiver hardware to decode the transmitted PAM-4 data signal. A 24-km fiber transmission penalty of 7-dB was recorded when PAM-4 signal was photonically generated and transmitted. The penalty introduced by moving from binary transmission to PM-4 transmission was recorded to be 3-dB.

Cost-effective and robust DSP scheme for a short-reach coherent system in the presence of transmitter IQ skew and chromatic dispersion.

A cost-effective and robust digital signal processing (DSP) scheme is proposed and demonstrated experimentally in a coherent 61 GBaud PDM 16QAM system. In our scheme, multi-stage DSP blocks are used to deal with channel effects, transceiver in-phase and quadrature (IQ) skew, and phase noise. A 4×4 real-valued multiple-input multiple-output (RV-MIMO) with N1 taps is for polarization recovery and receiver IQ skew calibration. After frequency offset compensation, two 2×2 RV-MIMO with N2 taps are used to compensate for chromatic dispersion (CD), inter-symbol interference, transmitter IQ skew, and phase noise. Finally, the residual phase noise is eliminated by the maximum likelihood (ML) estimator. The experimental results indicate that the proposed scheme provides better received optical power sensitivity and CD tolerance than the existing simplified DSP schemes. In addition, the proposed scheme can tolerate transmitter IQ skew up to 7 ps in a 10 km case, which outperforms both simplified and conventional DSP schemes. Meanwhile, the proposed scheme can keep the same transceiver IQ skew and CD tolerance and has reduced complexity by 25% after 10 km links, compared to 4×4 RV-MIMO followed by a transmitter skew compensator. To the best of our knowledge, the proposed scheme is the most cost-effective solution for a high baud rate datacenter interconnects where transmitter IQ skew and CD have to be dealt with.

Development and Validation of a New Wearable Mobile Device for the Automated Detection of Resting Tremor in Parkinson's Disease and Essential Tremor.

Involuntary tremor at rest is observed in patients with Parkinson’s disease (PD) or essential tremor (ET). Electromyography (EMG) studies have shown that phase displacement between antagonistic muscles at prevalent tremor frequency can accurately differentiate resting tremor in PD from that detected in ET. Currently, phase evaluation is qualitative in most cases. The aim of this study is to develop and validate a new mobile tool for the automated and quantitative characterization of phase displacement (resting tremor pattern) in ambulatory clinical settings. A new low-cost, wearable mobile device, called µEMG, is described, based on low-end instrumentation amplifiers and simple digital signal processing (DSP) capabilities. Measurements of resting tremor characteristics from this new device were compared with standard EMG. A good level of agreement was found in a sample of 21 subjects (14 PD patients with alternating resting tremor pattern and 7 ET patients with synchronous resting tremor pattern). Our results demonstrate that tremor analysis using µEMG is easy to perform and it can be used in routine clinical practice for the automated quantification of resting tremor patterns. Moreover, the measurement process is handy and operator-independent.

100+ Gbps/λ 50 km C-Band Downstream PON Using CD Digital Pre-Compensation and Direct-Detection ONU Receiver

We experimentally demonstrate a single-wavelength 100 Gbps downstream PON transmission aided by chromatic dispersion digital pre-compensation (CD-DPC) using simple digital signal processing (DSP) finite impulse response (FIR) filters in combination with an IQ Mach–Zehnder modulator (IQ-MZM) at the transmitter side and direct-detection receiver at the optical network unit (ONU). A reach of 50 km over standard single-mode fiber in C-band and an optical distribution network (ODN) loss of 28.5 dB are achieved. Transmission of 50 and 125 Gbps over 50 km of fiber is also tested, achieving 32 dB and 24 dB of ODN loss, respectively. The complexity of the filters, the optimization of the main design parameters, and the tolerance of the CD-DPC to the uncertainty of the exact accumulated link dispersion are analyzed in detail.

Beyond 1 Tb/s Intra-Data Center Interconnect Technology: IM-DD OR Coherent?

We discuss technology options and challenges for scaling intra-datacenter interconnects beyond 1 Tb/s bandwidths, with focus on two possible approaches: pulse amplitude modulation (PAM)-based intensity modulation-direct detection (IM-DD) and baud-rate sampled coherent technology. In our studies, we compare the performance of various orders of PAM modulation (PAM4 to 8). In addition to these fixed PAM signaling options, a flexible PAM (FlexPAM) technique leveraging granularity in spectral efficiency (SE) is proposed to maximize link margin. For baud-rate sampled coherent technology, we propose a simplified digital signal processing (DSP) architecture to bring down power consumption of the coherent approach closer to that of IM-DD PAM. We also propose two new phase noise tolerant 2D coherent modulation formats to relax the laser linewidth requirement. In closing, a comparative study of fixed IM-DD PAM versus coherent polarization multiplexed-quadrature amplitude modulation (PM-QAM) is presented for a 1.6 Tb/s solution (200 Gb/s per dimension), with consideration of link loss/reach budget, power consumption, implementation complexity, as well as fan-out granularity.

A Novel Method for Synchronization Parameters Automatic Extraction in Computer Electromagnetic Eavesdropping

Electromagnetic compromising emanations are potential threat to computer security. Computer emits energy in the form of electromagnetic wave which includes the processed information. The electromagnetic wave can be received and decoded in the distance, so the unintended information leakage occurs. In this paper, the automatic information reconstruction for computer electromagnetic eavesdropping is studied. This paper attempts to combine simple digital signal processing method for the first time to extract electromagnetic leakage information. The performance of the new method is presented numerically and experimentally. Compared with other algorithms, this method has strong practicability and reliability. Under complex electromagnetic environment, the synchronization parameters can be extracted and the information can be reconstructed automatically, quickly and reliably.

Transmission of Higher Order Solitons Created by Optical Multiplexing

The nonlinear Fourier transform (NFT) is a promising tool to linearize the inherently nonlinear optical fiber channel. The NFT transforms a time-domain signal into the continuous and the discrete spectrum. The discrete spectrum is composed of an arbitrary number of complex valued discrete eigenvalues and their associated amplitudes. These discrete eigenvalues relate to solitons, which maintain their shape or return to it in an oscillating manner, while passing through the optical channel. Higher order solitons consisting of multiple eigenvalues are complex pulses, which are created and demodulated by sophisticated digital signal processing (DSP) leading to demanding hardware requirements. This paper shows a way to work with higher order solitons in a wavelength division multiplexing such as fashion by using optical-electrical signal processing and presents boundaries of this method. Optical-electrical signal processing decreases the required electrical and electro-optical hardware specifications substantially and enables to use a simplified DSP. The proposed creation method is subsequently employed to transmit higher order solitons consisting of five QPSK modulated eigenvalues. Furthermore, the optical-electrical processing is benchmarked against the Darboux transformation, which creates higher order solitons purely numerically. The results show that for a fifth-order soliton transmission the proposed method can significantly reduce the hardware requirements and DSP complexity.

On the energy and computational cost of message authentication schemes for GNSS

To produce a position, velocity, and time estimate, modern Global Navigation Satellite System (GNSS) receivers expend energy on two very different kinds of computation: one being high-rate, low resolution, and the other being low-rate, high-resolution. The first kind is largely comprised of integer multiply, or multiply-and-accumulate operations, mostly applied to simple digital signal processing operations, such as frequency-translation, digital filtering, and correlation. These operations generally run at a rate commensurate with the receiver's front-end analog-to-digital converters, which will run at rates of some tens of megahertz, but may only require a few bits of integer resolution. The second kind mostly consists of baseband processing, such as detection, estimation, and tracking, orbit propagation, and position computation. These operations will generally require floating point arithmetic and will run at rates of some tens of Hertz to some kilohertz.

Harnessing machine learning for fiber-induced nonlinearity mitigation in long-haul coherent optical OFDM

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has attracted a lot of interest in optical fiber communications due to its simplified digital signal processing (DSP) units, high spectral-efficiency, flexibility, and tolerance to linear impairments. However, CO-OFDM’s high peak-to-average power ratio imposes high vulnerability to fiber-induced non-linearities. DSP-based machine learning has been considered as a promising approach for fiber non-linearity compensation without sacrificing computational complexity. In this paper, we review the existing machine learning approaches for CO-OFDM in a common framework and review the progress in this area with a focus on practical aspects and comparison with benchmark DSP solutions.

A Spectral-Efficient Self-Homodyne-Detected Microwave Photonic Link With an Extended Fiber-Reach

In this letter, a simple and spectral-efficient self-homodyne-detected microwave photonic link is proposed and experimentally demonstrated. At the transmitter, a phase-modulated optical signal and an intensity-modulated optical local oscillator signal are polarization-multiplexed and delivered in a single fiber over long distance. At the receiver, the two signals are coherently detected by a self-homodyne interferometer and then demodulated through simple digital signal processing. The proposed scheme is experimentally validated by 25-km fiber transmission of the 50-Msymbol/s 16-QAM microwave vector signals at 2 and 2.5 GHz. The transmission performance compared with the conventional homodyne detection scheme in terms of error vector magnitude and bit error rate is evaluated. The proposed scheme can be a potential solution to wireless access networks for both high spectral efficiency and low-cost implementation.

Hybrid Processing Design for Multipair Massive MIMO Relaying With Channel Spatial Correlation

Massive multiple-input multiple-output (MIMO) avails of simple transceiver design which can tackle many drawbacks of relay systems in terms of complicated signal processing, latency, and noise amplification. However, the cost and circuit complexity of having one radio frequency (RF) chain dedicated to each antenna element are prohibitive in practice. In this paper, we address this critical issue in amplify-and-forward (AF) relay systems using a hybrid analog and digital (A/D) transceiver structure. More specifically, leveraging the channel long-term properties, we design the analog beamformer which aims to minimize the channel estimation error and remain invariant over a long timescale. Then, the beamforming is completed by simple digital signal processing, i.e., maximum ratio combining/maximum ratio transmission (MRC/MRT) or zero forcing (ZF) in the baseband domain. We present analytical bounds on the achievable spectral efficiency taking into account the spatial correlation and imperfect channel state information at the relay station. Our analytical results reveal that the hybrid A/D structure with ZF digital processor exploits spatial correlation and offers a higher spectral efficiency compared to the hybrid A/D structure with MRC/MRT scheme. Our numerical results show that the hybrid A/D beamforming design captures nearly 95% of the spectral efficiency of a fully digital AF relaying topology even by removing half of the RF chains. It is also shown that the hybrid A/D structure is robust to coarse quantization, and even with 2-bit resolution, the system can achieve more than 93% of the spectral efficiency offered by the same hybrid A/D topology with infinite resolution phase shifters.

Experimental Demonstration of Flexible Hybrid Modulation Formats for Future Adaptive Optical Transmission Systems

The exponentially growing demand for high-speed data transmission is one of the main reasons for the ongoing evolution from fixed and static to link-adaptive systems in fiber-optic communications. The need to maximize the flexibility as well as the capacity of the system, while keeping the optical network infrastructure unchanged, leads to modifications of the system terminals, resulting in so-called flexible transceivers. A highly desired property of such a transceiver is a fine data rate granularity, which allows us to match the transmission capacity to the link budget. In this paper, we experimentally analyze flexible quadrature-division hybrid modulation formats, namely Flex-PAM, as a promising candidate for flexible transceivers. This method works at every integer number of bits-per-symbol, and, compared with other candidates proposed in the literature, requires a very simple digital signal processing scheme. We focus in particular on the effects of the effective number of bits in state-of-the-art digital/analog and analog/digital converters and fiber nonlinearities on the performance of Flex-PAM transmission. We analyze the capabilities of flexible modulation in a $ \text{5} \times \text{32}$ GBd wavelength division multiplexing coherent transmission on a dense $ \text{37.5}$ GHz grid and support the results by thorough numerical simulations. We successfully demonstrate the capability of Flex-PAM modulation to handle a wide range of bit-rates ( $ \text{5}\times \text{128}$ to $ \text{5}\times \text{320}$ Gbit/s), a maximum spectral efficiency of 6.8(b/s)/Hz, and propose a simple and adaptive digital signal processing scheme to compensate for the major linear and nonlinear impairments of the system.

Color and Vector Flow Imaging in Parallel Ultrasound With Sub-Nyquist Sampling.

RF acquisition with a high-performance multichannel ultrasound system generates massive data sets in short periods of time, especially in "ultrafast" ultrasound when digital receive beamforming is required. Sampling at a rate four times the carrier frequency is the standard procedure since this rule complies with the Nyquist-Shannon sampling theorem and simplifies quadrature sampling. Bandpass sampling (or undersampling) outputs a bandpass signal at a rate lower than the maximal frequency without harmful aliasing. Advantages over Nyquist sampling are reduced storage volumes and data workflow, and simplified digital signal processing tasks. We used RF undersampling in color flow imaging (CFI) and vector flow imaging (VFI) to decrease data volume significantly (factor of 3 to 13 in our configurations). CFI and VFI with Nyquist and sub-Nyquist samplings were compared in vitro and in vivo. The estimate errors due to undersampling were small or marginal, which illustrates that Doppler and vector Doppler images can be correctly computed with a drastically reduced amount of RF samples. Undersampling can be a method of choice in CFI and VFI to avoid information overload and reduce data transfer and storage.

Orbital-angular-momentum mode-group multiplexed transmission over a graded-index ring-core fiber based on receive diversity and maximal ratio combining.

An orbital-angular-momentum (OAM) mode-group multiplexing (MGM) scheme using high-order mode groups (MGs) in a graded-index ring-core fiber (GIRCF) is proposed, in which a receive-diversity architecture is designed for each MG to suppress the mode partition noise resulting from random intra-group mode crosstalk. The signal-to-noise ratio (SNR) of the received signals is further improved by a simple maximal ratio combining (MRC) technique on the receiver side to efficiently take advantage of the diversity gain of the receiver. Intensity-modulated direct-detection (IM-DD) systems transmitting three OAM mode groups with total 100-Gb/s discrete multi-tone (DMT) signals over a 1-km GIRCF and two OAM mode groups with total 40-Gb/s DMT signals over an 18.4-km GIRCF are experimentally demonstrated, respectively, to confirm the feasibility of our proposed OAM-MGM scheme.

100 Gb/s to 1 Tb/s Based Coherent Passive Optical Network Technology

Following on from current 1 Gb/s to 10 Gb/s based passive optical networks (PON), IEEE 802.3ca has commenced discussion of the first 100 Gb/s-based PON standard in the form of 100 G Ethernet PON (100G-EPON), in anticipation of growing bandwidth demand by emerging applications such as fixed-mobile convergence for 5G and beyond 5G (B5G) where mobile front-haul and mobile back-haul (MFH/MBF) will require greatly increased capacity of up to 1 Tb/s. In the move toward these 100 Gb/s to 1 Tb/s class PONs, research into the latest coherent technology based PON systems is being pursued due to its high receiver sensitivity at much higher bit rates, superior to current direct-detection based PON systems, aided by the rapid progress of the latest small-scale digital signal processing (DSP) suitable for access spans of less than a few tens of km. In this paper, we review the recent progress of such 100 Gb/s to 1 Tb/s class coherent PON technology. As the key successful factor for realizing 100 Gb/s to 1 Tb/s based coherent PON systems, the latest trends in coherent DSP and the accompanying embedded coherent transceivers are summarized from the point of view of miniaturization. In addition, highlighting the strong progress with the upcoming coherent PON systems, the first demonstration of a 100 Gb/s/λ × 8 wavelengths (800 Gb/s) based real-time wavelength division multiplexing (WDM)-PON system using a simplified DSP suitable for access spans is presented. A 100 Gb/s/λ based time division multiplexing (TDM)-PON system is also introduced, addressing the burst-mode coherent reception technique which will be the main issue for achieving practical TDM-based coherent PON systems.

DSP-enabled reconfigurable and transparent spectral converters for converging optical and mobile fronthaul/backhaul networks.

Dynamically reconfigurable and transparent signal spectral conversion is expected to play a vital role in seamlessly integrating traditional metropolitan optical networks and mobile fronthaul/backhaul networks. In this paper, a simple digital signal processing (DSP)-enabled spectral converter is proposed and extensively investigated, for the first time, which just utilizes a single standard dual-parallel Mach-Zehnder modulator (DP-MZM) driven by SDN-controllable RF signals and DC bias currents. As an important thrust of the paper, optimum operating conditions of the proposed converter are analytically identified, statistically examined and experimentally verified. Optimum operating condition-supported spectral converter performances in IMDD-based network nodes are explored both theoretically and experimentally in terms of frequency detuning range-dependent conversion efficiency, spectral conversion-induced OSNR/power penalty and transparency to input signal characteristics. The proposed spectral converter has unique advantages including low configuration complexity, strict transparency, SDN-controllable performance reconfigurability and flexibility, as well as negligible spectral conversion-induced latency.

DSP-based CSO cancellation technique for RoF transmission system implemented by using directly modulated laser.

We propose and demonstrate a simple composite second-order (CSO) cancellation technique based on the digital signal processing (DSP) for the radio-over-fiber (RoF) transmission system implemented by using directly modulated lasers (DMLs). When the RoF transmission system is implemented by using DMLs, its performance could be limited by the CSO distortions caused by the interplay between the DML's chirp and fiber's chromatic dispersion. We present the theoretical analysis of these nonlinear distortions and show that they can be suppressed at the receiver by using a simple DSP. To verify the effectiveness of the proposed technique, we demonstrate the transmission of twenty-four 100-MHz filtered orthogonal frequency-division multiplexing (f-OFDM) signals in 64 quadrature amplitude modulation (QAM) format over 20 km of the standard single-mode fiber (SSMF). The results show that, by using the proposed technique, we can suppress the CSO distortion components by >10 dB and achieve the error-vector magnitude performance better than 6% even after the 20-km long SSMF transmission.