Integrated MLL chip-based PAM-4/DMT-16QAM photonic-wireless link in W-band for flexible applications.

To accommodate the demand of exponentially increasing global wireless traffic driven by the coming beyond 5G and 6G, wireless communication has stepped into the millimeter wave (MMW) band to exploit large available bandwidth. The future wireless application scenarios require wireless communication systems with high speed, low cost, a small footprint and simple configuration, and the integrated light source-based intensity modulation and direct detection (IM-DD) photonic-wireless system can better meet the demand than the traditional system based on bulky components. In this paper, we experimentally demonstrate a lens-free pulse-amplitude-modulation with four levels (PAM-4) and discrete multi-tone with 16-quadrature amplitude modulation (DMT-16QAM) MMW photonic-wireless transmission system in the W-band using an integrated mode-locked laser (MLL) chip and a mixer-based receiver, which could be applicable for flexible wireless applications. The integrated MLL as an on-chip single light source is used to generate W-band signals and simplify the transmitter. The signal-to-noise ratio of the generated wireless signal is improved by two coherent optical carriers both modulated with data and then beating in the photodiode. In addition, we investigate the IM-DD configuration by employing an envelope detector (ED) to receive the PAM-4 signal for further simplifying the system. The ED-based photonic-wireless system is more suitable for the applications with lower data rate and low cost. For higher data rate, the mixer-based PAM-4/DMT-16QAM systems with up to 31.75 Gbit/s net data rate are more favorable, although the cost is also higher.

Symmetric carrier assisted differential detection receiver with low-complexity signal-signal beating interference mitigation.

Low-cost, spectrally efficient self-coherent detection capable of optical field recovery is desired for inter-data center connections and metro networks. In this work, a simplified symmetric carrier assisted differential detection (S-CADD) receiver structure is proposed, which removes the single-ended photodiode branch and saves one ADC in standard asymmetric CADD (A-CADD) receivers. To compensate for the signal-signal beating (SSBI) impairment, a low-complexity iterative SSBI mitigation algorithm is put forward as well. The computational complexity is reduced by moving equalization, de-modulation and modulation out of each iteration. Based on single-carrier twin-single sideband (SSB) pulse- and quadrature-amplitude modulation (PAM and QAM) signals, the OSNR sensitivity with different carrier-to-signal power ratios (CSPRs) at back-to-back (BTB) scenario, the bit-error rates (BER) performance after 1000km fiber transmission, and the received optical power sensitivity at BTB are theoretically and numerically compared for both S-CADD and A-CADD receivers, respectively.

224-Gbps single-photodiode PAM-4 transmission with extended transmitter bandwidth based on optical time-and-polarization interleaving.

We have proposed and demonstrated the optical time-and-polarization interleaving (OTPI) technique, which can effectively extend the transmitter bandwidth for an intensity modulation and direct detection (IM/DD) optical system. The 224-Gbit/s line-rate OTPI-PAM-4 signal is successfully transmitted over a 500-m standard single-mode fiber (SSMF) in the C band, using the transmitter with a bandwidth of 25 GHz and the receiver with a single photodiode. By using a 33%-return-to-zero (RZ) pulse train, a bit-error ratio (BER) below 7% hard-decision forward error correction (HD-FEC) threshold is achieved. BER below 20% soft-decision forward error correction (SD-FEC) threshold is also realized using a carrier suppressed return-to-zero (CSRZ) pulse train. The OTPI technique can also be used for more higher-order pulse amplitude modulation (PAM) formats, making it a promising technique for next-generation high-speed optical interconnects.

Time skew enabled vestigial sideband modulation for dispersion-tolerant direct-detection transmission.

Fiber dispersion and square-law detection-induced power fading is a fundamental obstacle in intensity modulation with direct detection links. In this Letter, we propose a hardware-efficient vestigial sideband (VSB) transmitter to suppress such an impairment. By introducing an appropriate time skew between the differential arms of the Mach-Zehnder modulator, a VSB signal can be generated based on a single digital-to-analog convertor without optical filtering. At the receiver, a Volterra equalizer is utilized to mitigate the inter-symbol interference. In the proof-of-concept experiment, 32 Gbaud 4/6/8-ary pulse amplitude modulation signals can be successfully transmitted over an 80 km standard single-mode fiber with bit-error rates below the KP4, 7% hard-decision and 20% soft-decision forward error correction thresholds, respectively. The proposed scheme provides a promising and low-cost solution for high-speed metro and data center interconnect applications.

Machine learning classifier based on FE-KNN enabled high-capacity PAM-4 and NRZ transmission with 10-G class optics.

Modified by special feature engineering, a powerful and low-order equalizer based on K-nearest neighbors (KNN) classifier is applied to improve performance of high-speed system with bandwidth-limited optics. The feature construction and feature weighting are specially designed to conduct an appropriate a feature engineering-based KNN (FE-KNN) scheme, which contains more data characteristics to enhance the equalization performance. Experimental comparisons of KNN classifier with/without feature engineering, decision feedback equalizer (DFE) and feed-forward equalizer (FFE) are implemented to prove the feasibility of our scheme in both 25-Gb/s NRZ and 50-Gb/s PAM-4 transmission experiments with 10-G optics system. The corresponding results show that, without the feature engineering, the performance achieved by the common KNN is not improved even in the case of hard decision (HD). In contrast, compared to the common 11-taps DFE, the performance achieved by FE-KNN with only 5 taps is improved by 1-dB at KP4-FEC threshold (BER=2.2E-4) for 25-Gb/s NRZ transmission. While, for 50-Gb/s PAM-4 case, 0.5-dB sensitivity improvement is achieved by our scheme compared to the common 11-taps DFE under the HD-FEC limit (BER=3.8E-3).

High-fidelity and low-latency mobile fronthaul based on segment-wise TDM and MIMO-interleaved arraying.

In this paper, we firstly demonstrate an advanced arraying scheme in the TDM-based analog mobile fronthaul system to enhance the signal fidelity, in which the segment of the antenna carrier signal (AxC) with an appropriate length is served as the granularity for TDM aggregation. Without introducing extra processing, the entire system can be realized by simple DSP. The theoretical analysis is presented to verify the feasibility of this scheme, and to evaluate its effectiveness, the experiment with ~7-GHz bandwidth and 20 8 × 8 MIMO group signals are conducted. Results show that the segment-wise TDM is completely compatible with the MIMO-interleaved arraying, which is employed in an existing TDM scheme to improve the bandwidth efficiency. Moreover, compared to the existing TDM schemes, our scheme can not only satisfy the latency requirement of 5G but also significantly reduce the multiplexed signal bandwidth, hence providing higher signal fidelity in the bandwidth-limited fronthaul system. The experimental result of EVM verifies that 256-QAM is supportable using the segment-wise TDM arraying with only 250-ns latency, while with the ordinary TDM arraying, only 64-QAM is bearable.

Impact of SOA-induced pattern effect on the filter requirements in vestigial sideband direct detected PAM4 transmission.

In an optical filter based VSB-DD transmission system, semiconductor optical amplifier (SOA) is a promising option to enhance system optical power margin. While, in practical system, the low input saturation power makes the SOA-amplified signal susceptible to the pattern effect, which causes a considerable spectral broadening, thereby influencing the design of VSB filter. In this paper, the relationship between SOA-induced pattern effect and the requirements of the VSB filter is systematically investigated. Firstly, qualitative analysis is given and upper sideband (USB) is proved better than lower sideband (LSB) owing to the suppression of SOA-induced pattern effect. Then, 56Gbps IM/DD PAM4 transmission is experimentally conducted. With respective optimal filter configuration, performance of USB signal is superior to LSB signal in all cases. Results show that USB signal has 1dB sensitivity superiority to LSB signal for 56Gb/s PAM4 after 40km transmission. And in 80km case, only by using USB signal, can HD-FEC limit (3.8 × 10-3) be achieved. Also, we study requirements on other filter parameters, including redundant bandwidth and filter steepness.

Discovery of Heteropolytantalate: Synthesis and Structure of Two 6-Peroxotantalo-4-phosphate Clusters.

Polyoxometalates (POMs) of Nb and Ta are greatly different from those of Mo, W, and V that have been studied extensively and developed well. The latter can be formed simply by acidification of their aqueous monomeric oxoanions and has found application areas from catalysis to magnetism, materials science, medicine, and nanotechnology. Even now the polyoxoniobate (PONb) chemistry has accelerated dramatically over the last 15 years, and a vast expansion of available PONbs has been reported. However, after nearly 200 years of POM research, Ta-based POM chemistry is still at its infant stage and only dominated by the isopolyoxotantalate ions (Ta6 and Ta10) and transition-metal-capped Ta6 species, along with two Ti-substituted polyoxotantalates [Ti2Ta8O28]8- and [Ti12Ta6O44]10- reported very recently. In this study, we discover two novel peroxotantalophosphate clusters [P4(TaO2)6O25]12- (1) and [P4(TaO2)6O24]10- (2) by incorporating phosphorus heteroatom into Ta-oxo framework, which represent the first two examples of heteropolytantalate. Interestingly, two P2Ta3 half-units are cis- and trans-condensed in 1 and 2, leading to "open" and "closed" configurations, respectively. These two chemically and structurally related clusters can be isolated in a controlled manner, and the yields are relatively high. Both compounds were characterized in the solid state by single-crystal X-ray diffraction, 31P MAS NMR, FT-IR, TGA, and elemental analysis as well as by 31P NMR in solution. The results presented here provide a strategy to be applicable to other heteroatom-incorporated polyoxotantalates and further expand the phase space for polyoxotantalate chemistry.

Carbon dioxide and weak magnetic field enhance iron-carbon micro-electrolysis combined autotrophic denitrification.

Combining iron-carbon micro-electrolysis and autotrophic denitrification is promising for nitrate removal from wastewater. In this study, four continuous reactors were constructed using CO2 and weak magnetic field (WMF) to address challenges like iron passivation and pH stability. In the reactors with CO2 + WMF (10 and 35 mT), the increase in total nitrogen removal efficiency was significantly higher (96.2 ± 1.6 % and 94.1 ± 2.7 %, respectively) than that of the control (51.6 ± 2.7 %), and Fe3O4 converted to low-density FeO(OH) and FeCO3, preventing passivation film formation. The WMF application decreased the N2O emissions flux by 8.7 % and 20.5 %, respectively. With CO2 + WMF, the relative enzyme activity and abundance of denitrifying bacteria, especially unclassified_Rhodocyclaceae and Denitratisoma, increased. Thus, this study demonstrates that CO2 and WMF optimize the nitrate removal process, significantly enhancing removal efficiency, reducing greenhouse gas emissions, and improving process stability.

Octamolybdate-supported tricarbonyl metal derivatives: [{H2Mo8O30}{M(CO)3}2]8- (M = Mn(I) and Re(I)).

Two novel octamolybdate-based tricarbonyl metal derivatives have been successfully synthesized and characterized, which represent the first two examples of tricarbonyl metal groups attached to a new {Mo(8)O(30)} building block.