Amplitude Modulation Signals Research Articles

Kalman filtering-based decision threshold dynamic optimization in photonics-aided millimeter-wave systems

We propose a novel, to the best of our knowledge, decision threshold dynamic optimization (DTDO) method based on Kalman filtering to mitigate the nonlinear effect impacts of the time-domain jitter and saturation distortion on a bit error rate (BER) performance. This dynamic optimization method was validated in a photonics-aided W-band millimeter-wave system. Compared to nonlinear algorithms and clustering classification methods, DTDO offers lower complexity and superior dynamic tracking capabilities when optimizing the BER performance in photonics-aided systems. The four-level pulse amplitude modulation (PAM4) signal, after being transmitted over a 50-m W-band link, was equalized using a low-complexity filter, maintaining an exceptionally low BER. This approach effectively leverages the inherent low-complexity advantages of single-carrier transmission over the link.

Real-time DSP-Free 40 Gbit/s PAM4 transmission over 10 km fiber enabled by optical injection locking of directly modulated laser

AbstractWe present a comprehensive performance analysis of injection-locked directly modulated laser (DML) for optical communication systems, focusing on both non-return-to-zero (NRZ) and 4-level pulse amplitude modulation (PAM4) signal transmission. We demonstrate real-time PAM4 40 Gbit/s transmission over 10 km of single-mode fiber enabled by optical injection locking without pre-emphasis or post-equalization, achieving a bit error rate (BER) below $${10^{-6}}$$ 10 - 6 , and doubling capacity compared to unlocked transmission with the same laser. Our study investigates the dependence of system performance on the injected power and frequency offset of the master laser. Results indicate that lower injection powers while maintaining a stable locking regime, yield better performance in terms of extinction ratio and BER. Optimized parameters lead to enhanced transmission performance, providing valuable insights into the design and optimization of injection-locked DML systems for optical communication applications employing direct modulation.

Simplified bit-class probabilistic shaping strategy based on PDM systems.

This study aims to present a simplified bit-class probabilistic shaping (PS) strategy based on power domain multiplexing (PDM) systems. In this scheme, we employ bit-weighted distribution matching (BWDM) to achieve PS. By ingeniously applying the process of signal superposition, the low-complexity bit-class PS scheme requires encoding of only a small number of bits in the low-power two quadrature phase-shift keying (QPSK) signal, enhancing the quality of the transmitted signal with simple operations. At the receiver, to ensure the transmission performance of the system while minimizing computational complexity and enhancing system operational efficiency, we implemented the region decision method for demodulation. The proposed scheme was experimentally validated on a 2 km 7-core fiber, achieving accurate transmission of signals at a net rate of 103.6 Gb/s. The experimental results indicate that, when compared to the uniform 16-quadrature amplitude modulation (16QAM) signal and the PDM-orthogonal frequency division multiplexing (OFDM) signal without PS, the receiving sensitivity of the PS-16QAM signal in our scheme can gain 0.4 dB and 0.33 dB at a bit error rate (BER) of 3.8 × 10-3 under the same net rate.

Impact of Receiver Bandwidth on the Performance of Nonlinear Volterra Equalizer in IM/DD Systems

Nonlinear electrical equalization is an effective means to compensate for nonlinear waveform distortions in short-haul intensity-modulation (IM)/direct-detection (DD) optical transmission systems. Nonlinear distortions contain broader spectral components than the original signal. Thus, a wide receiver bandwidth and high sampling rate are beneficial to effective compensation of nonlinear distortions. However, this leads to poor signal-to-noise ratio at the receiver. In this paper, we investigate the impact of receiver bandwidth on the performance of 2nd-order nonlinear Volterra equalizer in IM/DD transmission systems. Through the computer simulation and the experimental verification performed with 4-ary pulse amplitude modulation signals, we show that an optimum receiver bandwidth for bit-error ratio performance increases with the amount of nonlinear distortions in IM/DD systems. We also show that the sampling rate should also be increased to avoid the aliasing of the nonlinear distortion components. The cost of the receiver increased by the wider receiver bandwidth and higher sampling rate could be offset in part by the non-integer fractionally sampled nonlinear Volterra equalizer.

Analysis of the Flash Phenomenon in Rotor Targets with a Scatter Point Model

This paper thoroughly investigates the occurrence of the flash phenomenon in rotor targets. The flash phenomenon is discernible in rotor targets across the time and frequency domains. This phenomenon is characterized by the modulation of signal amplitude through the Singer function, resulting in periodic peaks in the time domain. The appearance of these peaks in the time domain corresponds to the presence of a corresponding frequency band in the time–frequency domain. The influence of interference between scattering points on the amplitude and phase of echo is investigated by employing the echo scattering point model and the echo analytical expression as analytical foundations. A more comprehensive examination of the mechanisms that contribute to the emergence of the flash phenomenon in the time and time–frequency domains is undertaken. Furthermore, its objective is to verify the validity of the analysis performed on the mechanism of the phenomenon. Based on theoretical investigations and simulation results, it can be concluded that the flash phenomenon observed in rotor targets is due to interference between particular scattering points. Fundamentally, it can be comprehended as a phenomenon of interference. This research achievement has specific theoretical and practical value in detecting and recognizing rotor targets.

SSBI counteraction technology in a single photodetector-based direct detection system receiving an independent dual-single sideband signal

To further meet the large capacity, high spectrum efficiency (SE), reduce the signal-signal beat interference (SSBI) of the independent dual single sideband (ISB) system and the complexity of the receiver, we propose an iterative signal-signal beat interference counteraction (ISSBIC) algorithm to suppress SSBI. The 16-Gbps left sideband and the 16-Gbps right sideband signals in the ISB system are quadrature phase-shift keying (QPSK) modulated. After standard single-mode fiber (SSMF) transmission, the LSB and RSB signals are synthesized to a 16-quadrature amplitude modulation (QAM) signal after conversion through the photodetector (PD) square law. The simulation results show that the ISSBIC with just 2 iterations is superior to Kramers–Kronig (KK) receiver in processing the system error bit. In the meantime, the ISSBIC requires a sampling rate of 20GSa/s, whereas KK requires 40GSa/s. Moreover, the proposed ISSBIC algorithm has a simpler complexity. According to the simulation results, the suggested ISSBIC receiver can lower the ADC requirements and system costs, which opens up a wide range of applications in multiplex and higher SE transmission systems.

Dynamic mode decomposition-based technique for cross-term suppression in the Wigner-Ville distribution

This paper presents a new method for time-frequency representation (TFR) using dynamic mode decomposition (DMD) and Wigner-Ville distribution (WVD), which is termed as DMD-WVD. The proposed method helps in removing cross-term in WVD-based TFR. In the suggested method, the DMD decomposes the multi-component signal into a set of modes where each mode is considered as mono-component signal. The analytic modes of these obtained mono-component signals are computed using the Hilbert transform. The WVD is computed for each analytic mode and added together to obtain cross-term free TFR based on the WVD. The effectiveness of the proposed method for TFR is evaluated using Rényi entropy (RE). Experimental results for synthetic signals namely, multi-component amplitude modulated signal, multi-component linear frequency modulated (LFM) signal, multi-component nonlinear frequency modulated (NLFM) signal, multi-component signal consisting of LFM and NLFM mono-component signal, multi-component signal consisting of sinusoidal and quadratic frequency modulated mono-component signals, and synthetic mechanical bearing fault signal and natural signals namely, electroencephalogram (EEG) and bat echolocation signals are presented in order to show the effectiveness of the proposed method for TFR. It is clear from the results that the proposed method suppresses cross-term effectively as compared to the other existing methods namely, smoothed pseudo WVD (SPWVD), empirical mode decomposition (EMD)-WVD, EMD-SPWVD, variational mode decomposition (VMD)-WVD, VMD-SPWVD, and DMD-SPWVD.

Bi-directional SMF-NDF-optical/5G NR wireless converged systems

This study presents the transmission of fifth-generation (5G) new radio (NR) signals over bi-directional single-mode fiber (SMF)-negative dispersion fiber (NDF)-optical/5G NR wireless converged systems, using two cascaded reflective semiconductor optical amplifiers (RSOAs). Downstream transmission of 5G NR 16-quadrature amplitude modulation (QAM) signals achieved an aggregate net bit rate of 228.037 Gb/s, comprising 59.813, 74.766, and 93.458 Gb/s at 150, 250, and 325 GHz, respectively. The system achieved a low bit error rate (BER) and clear constellation patterns. However, systems without NDF exceed the forward error correction 7% threshold, indicating the importance of NDF in compensating for fiber dispersion. Moreover, for upstream transmission, two cascaded RSOAs are used to transmit 5G 16-QAM-orthogonal frequency division multiplexing signals, achieving a total data rate of 40 Gb/s. The use of two cascaded RSOAs in the system significantly improved upstream performance, resulting in low BERs and clear constellation patterns. This system not only achieved high data rates but also extended transmission coverage to support the deployment of advanced 5G NR applications.

Optimization Strategies for Rydberg Atom-based AM Receiver Operational Parameters

Abstract The Rydberg atom-based receiver, as a novel type of antenna, demonstrates broad application prospects in the field of microwave communications. However, since Rydberg atomic receivers are nonlinear systems, mismatches between the parameters of the received amplitude modulation (AM) signals and the system's linear workspace and demodulation operating points can cause severe distortion in the demodulated signals. To address this, the article proposes a method for determining the operational parameters based on the mean square error (MSE) and total harmonic distortion (THD) assessments, and presents strategies for optimizing the system’s operational parameters focusing on linear response characteristics (LRC) and linear dynamic range (LDR). Specifically, we employ a method that minimizes the MSE to define the system’s linear workspace, thereby ensuring the system has a good LRC while maximizing the LDR. To ensure the signal always operates within the linear workspace, an appropriate carrier amplitude is set as the demodulation operating point. By calculating the THD at different operating points, the LRC performance within different regions of the linear workspace is evaluated, and corresponding optimization strategies based on the range of signal strengths are proposed. Moreover, to more accurately restore the baseband signal, we establish a mapping relationship between the carrier Rabi frequency and the transmitted power of the probe light, and optimize the slope of the linear demodulation function to reduce the MSE to less than 0.8×10-4. Finally, based on these methods for determining operational parameters, we explore the effects of different laser Rabi frequencies on system performance, and provide optimization recommendations. This research provides robust support for the design of high-performance Rydberg atom-based AM receivers.

Ultra-low-complexity weight-sharing trigonometric nonlinear equalizer for beyond net-200-Gb/s/λ short-reach optical interconnects.

An ultra-low-complexity third-order weight-sharing trigonometric nonlinear equalizer (WS-TNLE) is proposed to eliminate nonlinear signal distortions in short-reach optical interconnects exceeding net 200 Gb/s/λ. By replacing the second- and third-order nonlinear terms in a third-order weight-sharing diagonally pruned Volterra nonlinear equalizer (WS-DP-VNLE) with cosine and sine terms, respectively, the required number of real-valued multiplications per symbol of the proposed third-order WS-TNLE is significantly reduced to the same value as the number of weight-sharing kernels. When transmitting probabilistically shaped 16-level pulse amplitude modulation (PS-PAM-16) signals at net rates ranging from 200.4 Gb/s to 300.4 Gb/s over a 1-km standard single-mode fiber (SSMF), the proposed third-order WS-TNLE requires the lowest number of real-valued multiplications per symbol, ranging from 10 to 44, which reduces the computational complexity by up to 96.2% and 52.4% compared to the third-order WS-DP-VNLE and WS-DP-absolute-term nonlinear equalizer (WS-DP-ATNLE), respectively.

Adaptive Measurement and Parameter Estimation for Low-SNR PRBC-PAM Signal Based on Adjusting Zero Value and Chaotic State Ratio

Accurately estimating the modulation parameters of pseudorandom binary code–pulse amplitude modulation (PRBC–PAM) signals damaged by strong noise poses a significant challenge in emitter identification and countermeasure. Traditionally, weak signal detection methods based on chaos theory can handle situations with low signal-to-noise ratio, but most of them are developed for simple sin/cos waveform and cannot face PRBC–PAM signals commonly used in ultra-low altitude performance equipment. To address the issue, this article proposes a novel adaptive detection and estimation method utilizing the in-depth analysis of the Duffing oscillator’s behaviour and output characteristics. Firstly, the short-time Fourier transform (STFT) is used for chaotic state identification and ternary processing. Then, two novel approaches are proposed, including the adjusting zero value (AZV) method and the chaotic state ratio (CSR) method. The proposed weak signal detection system exhibits unique capability to adaptively modify its internal periodic driving force frequency, thus altering the difference frequency to estimate the signal parameters effectively. Furthermore, the accuracy of the proposed method is substantiated in carrier frequency estimation under varying SNR conditions through extensive experiments, demonstrating that the method maintains high precision in carrier frequency estimation and a low bit error rate in both the pseudorandom sequence and carrier frequency, even at an SNR of −30 dB.

Highly Efficient Slow‐Light Mach–Zehnder Modulator Achieving 0.21 V cm Efficiency with Bandwidth Surpassing 110 GHz

AbstractHigh‐speed electro‐optic modulators are key components in modern communication networks and various applications that require chip‐scale modulation with large bandwidth, high modulation efficiency, and compact footprint. However, fundamental trade‐offs make it challenging to achieve these metrics simultaneously, and thus new methodologies must be explored. To this end, a Mach–Zehnder modulator harnessing slow‐light waveguides and capacitively loaded slow‐wave electrodes are presented on silicon‐nitride‐loaded lithium niobate on an insulator platform. The increased group index and reduced microwave loss significantly improve the modulation efficiency. With the 1‐mm‐length modulation section, a low half‐wave voltage length product Vπ·L of 0.21 V cm is obtained, which is one order of magnitude smaller than that of conventional thin film lithium niobate Mach–Zehnder modulators, and a modulation bandwidth of surpassing 110 GHz is achieved. The digital signal processor‐free non‐return‐to‐zero signal and eight‐level pulse amplitude modulation signal of up to 180 and 300 Gbps, respectively, are generated by the modulator, which provides ultra‐large bandwidth, ultra‐high efficiency, and a compact solution for next‐generation electro‐optic systems.

Enhanced Performance Analysis of 120 Gbps Single‐Channel IQ Modulation Based PDM‐FSO Transmission System Using 64‐QAM Signals

ABSTRACTThe ever‐augmenting demand of data channel‐bandwidth by the end users has resulted in the exploration of new transmission techniques for wireless transmission networks. In this work, we have demonstrated the modeling of a single‐channel free space optics (FSO) transmission system by employing polarization‐division‐multiplexing (PDM) technique. Further, in‐phase‐quadrature modulation (IQM) has been used to increase the transmission efficiency of the system. To decrease the baud‐rate of the system, we have proposed the use of 64‐level quadrature amplitude modulation (QAM) signals. The net transmission rate of the system is 120 Gbps with a baud‐rate of 10 Gbps. Furthermore, the overcome the signal degradation of the 120 Gbps signal propagating through atmospheric channel, we have reported the use of advanced digital signal processing (DSP) algorithms. The system's performance is investigated for adverse external climate weather conditions and the results reported show reliable 120 Gbps data transmission along range between 870 m and 16.25 km with good performance of BER and EVM 7.5%.

Kurtosis Control of Amplitude-Modulated non-Gaussian Signals for Fatigue Test

The amplitude modulation method was used to generate non-Gaussian signals that acted as excitation for fatigue tests. The fatigue life of structures under non-Gaussian excitation has been proven to be closely related to the features of the amplitude modulation signal (AMS) and kurtosis of the structural response. In this study, the modelling of the AMS by Beta and Weibull distributions and the resulting kurtosis range problem is first reviewed. To solve this problem, a new method for creating an AMS based on a linear combination of Beta and Weibull distributions is proposed. To ensure that the high kurtosis of the amplitude-modulated non-Gaussian signal is correctly transferred to the structural response, the method is further developed to fulfil the specifications for the fatigue damage spectrum (FDS) by controlling the spectral content of the AMS. Herein, a Gaussian AMS with a low-pass cutoff frequency is first generated and then converted to a Weibull or Beta AMS based on the cumulative distribution function (CDF) transformation. The proposed method is verified using simulated and field-measured data. The results show that the full range of specified kurtosis is achieved with the new AMS modelling method. The high kurtosis of the non-Gaussian input signal can be transferred to the linear system response if the mean value of AMS during the period of the system impulse response is the same as AMS.

A fully integrated GaAs HBT power amplifier for WLAN 802.11ax applications

AbstractA fully integrated gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) power amplifier (PA) for wireless local area network 802.11ax applications is presented in this paper. The structure consists of two‐stage power cells. To satisfy the high linearity requirements of IEEE 802.11ax, we analyzed the distortion of amplitude‐to‐amplitude and amplitude‐to‐phase. Due to the thermal and voltage sensitivity of GaAs HBT, an adaptive bias circuit is designed to ensure linearity. Moreover, an efficient passive output matching network is designed by analyzing the efficiency of the passive network. The design electromagnetic structure is fabricated in a 2‐μm GaAs HBT process. Under continuous wave testing, the output power reaches 27.2 dBm and the maximum efficiency of 28% at 2.4 GHz. Under the excitation of a 40 MHz 1024‐quadratic amplitude modulation signal, the output power meeting error vector magnitude of −35 dB reaches 16.8–17.2 dBm from 2.4 to 2.5 GHz.

Estimate non-Gaussian road roughness from international roughness index

Gaussian model is commonly used to characterize road roughness, while field measured data is usually non-Gaussian. A new method based on the amplitude modulation and International Roughness Index is proposed in this paper to estimate non-Gaussian road roughness. A modified Gamma distribution is proposed to describe the International Roughness Index and to create the amplitude modulation signal. To ensure that the high kurtosis of the amplitude-modulated non-Gaussian profile is correctly transferred to the vehicle response, the method is further developed to fulfill the specifications for the fatigue damage spectrum by controlling the spectral content of the amplitude modulation signal. Herein, a Gaussian amplitude modulation signal with a cutoff frequency is firstly generated, and then converted to a modified Gamma distribution amplitude modulation signal based on the cumulative distribution function transform. Field data are used to verify the proposed method. The results show that a good match is obtained between the estimated and measured non-Gaussian road roughness. The estimated International Roughness Index and fatigue damage potential of vehicle response are well represented using the proposed method.

Bidirectional TFDM 200 G coherent PON with a simplified receiver at the ONU side.

Time and frequency division multiplexing (TFDM) coherent passive optical networks (PONs) are considered as a promising candidate for future optical access networks due to the advantage of high sensitivity, high spectral efficiency, and flexibility. We propose a novel, to our knowledge, bidirectional TFDM 200-Gb/s coherent PON architecture based on the digital subcarrier multiplexing (DSCM) technology. A polarization-insensitive simplified coherent receiver is achieved at the ONU side by Alamouti coding and heterodyne detection. We experimentally demonstrate this bidirectional coherent PON with a 50-Gbaud Alamouti 16 quadrature amplitude modulation (QAM) signal transmitted downstream and a 25-Gbaud dual polarization (DP)-16QAM signal transmitted upstream. By using a single sideband modulated transmitter, only one laser source is required at the optical network unit (ONU) side. The power budgets can reach 30 and 33 dB for the downstream and upstream, respectively, after the 20-km transmission over a standard single-mode fiber (SSMF).

Simplified coherent chaotic optical secure communication scheme based on the Kramers-Kronig receiver.

Enhancing physical layer encryption in fiber-optic networks remains a challenging yet vital task. In this Letter, we propose a simplified coherent chaotic secure optical communication scheme based on the Kramers-Kronig (KK) receiver. This scheme incorporates a semiconductor laser with a phase-conjugated optical feedback serving as a common chaotic source, and its chaotic output is directly injected into the two slave lasers arranged separately at the transmitter and receiver end to achieve high-quality synchronization of chaotic signals, with a corresponding chaotic bandwidth of 30.6 GHz. By virtue of the common-signal-induced broad chaotic synchronization, a proof-of-principle demonstration is successfully conducted. It involves the secure transmission of a 20 Gbaud 16-level quadrature amplitude modulation (16QAM) signal over a 50 km standard single-mode fiber (SSMF) link. At the receiver end, we deploy a KK receiver to reconstruct the field of the optical signal and hence enable signal compensation and recovery with offline digital signal processing (DSP). This method simplifies device requirements in the current chaotic coherent optical secure communication, offering a cost-effective mode and promising path for advancing physical layer encryption in inter-data center communications.

Ultra-low-power consumption silicon electro-optic switch based on photonic crystal nanobeam cavity

Ultra-low-power consumption and high-speed integrated switches are highly desirable for future data centers and high-performance optical computers. In this study, we proposed an ultra-low-power consumption silicon electro-optic switch based on photonic crystal nanobeam cavities on a foundry platform. The proposed switch showed an ultra-low static-tuning power of 0.10 mW and a calculated dynamic switching power of 6.34 fJ/bit, with a compact footprint of 18 μm × 200 μm. Additionally, a 136-Gb/s four-level pulse amplitude modulation signal transmission experiment was carried out to verify the capability of the proposed electro-optic switch to support high-speed data transmission. The proposed device has the lowest static-tuning power consumption among silicon electro-optic switches and the highest data transmission rate. The results demonstrate the potential applications of this switch in high-performance optical computers, data center interconnects, optical neural networks, and programmable photonic circuits.

Dual-polarization carrier-assisted differential detection with asymmetric twin-SSB modulation and simplified receiver structure

Carrier-assisted differential detection (CADD) is a promising solution for high-capacity and cost-sensitive short-reach application scenarios, in which the optical field of a complex-valued double-sideband (CV-DSB) signal is reconstructed without using a local oscillator laser. In this work, we propose a polarization division multiplexed asymmetric twin single-sideband CADD (PDM-ATSSB CADD) scheme to realize the optical field recovery of the PDM CV-DSB signals. The polarization fading is solved by using a pair of optical bandpass filters (OBPFs) to suppress the unwanted other polarized offset carrier and signal, and the dual-polarization optical field is recovered by the CADD receiver. An asymmetric twin-SSB signal is used to relax the sharpness requirement of optical filter edges. We also propose a joint signal-signal beat interference (SSBI) iterative mitigation algorithm, which can effectively mitigate intra- and inter-polarization SSBI. The proposed PDM-ATSSB CADD scheme is validated for 30 Gbaud PDM asymmetric twin-SSB 16-ary quadrature amplitude modulation signals. We illustrate the parameter optimization process for PDM-ATSSB CADD, including optical delay and the number of iterations. The impacts of phase noise and relative intensity noise for laser and polarization impairment on PDM-ATSSB CADD are evaluated through numerical simulation. Compared with the PDM-symmetric-TSSB CADD (PDM-STSSB CADD), the required frequency gap to reach the 7% FEC threshold can be reduced by 1 GHz, and the OSNR sensitivity is improved by about 3 dB. Moreover, two simplified PDM-ATSSB CADD schemes are proposed and discussed, which could be considered hardware-efficient and integrative candidates for metro and inter-data center interconnects.