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Frequency division multiplexing (FDM) passive optical network (PON) is considered as a competitive solution for 5G-advanced and 6G mobile fronthaul (MFH) to support multiple radio units (RUs) with shared trunk fiber and distributed unit (DU). However, it is likely to cause uplink signal overlapping by laser wavelength drift, and guard bands should be reserved between FDM channels, which limits the spectral efficiency (SE). In this work, we propose to aggregate multiple uplink FDM channels using power division multiplexing and jointly adopt successive interference cancellation (SIC) at the receiver to improve the uplink SE. We first theoretically derive the SE of such power- and frequency-division multiplexing (PFDM) PON with both guard band and SIC together, and reveal that PFDM PON can achieve higher SE than the pure FDM PON with guard band only. Focusing on the case of two overlapped uplink signals, we propose a pilot-tone assisted SIC to improve the estimation accuracy of frequency offset and phase noise in heavily overlapped uplink signals. In the proof-of-concept experiments, compared to the orthogonal case without considering the guard band, 0.29 bit/s/Hz SE penalty due to the residual interference after SIC is observed when two uplink signals are completely overlapped. The results showed that, in the presence of laser drift, PFDM PON becomes more spectrally-efficient than FDM PON when the relative wavelength drift exceeds 20% of the signal bandwidth. [ABSTRACT FROM AUTHOR]

In this paper, multi-dimensional distribution matching (MDDM) based on bit-level shaping is proposed to implement probabilistic shaping (PS) for high order modulation formats. MDDM is optimized across multiple dimensions including polarizations and different time slots to achieve better performance. The implementation complexity of PS can also be reduced by optimizing the multiple bit-level distribution matchers (DMs) for generating multi-dimensional symbols. To further reduce the complexity, degenerate MDDM is also proposed with almost the same performance as MDDM. In simulations of an additive white Gaussian noise (AWGN) channel, for probabilistically shaped 256-ary quadrature amplitude modulation (256QAM) at a shaping rate of 2.2 bits/amplitude with a block length of 80, four-dimensional distribution matching (4D-DM) and degenerate 4D-DM provide normalized generalized mutual information (NGMI) gains of 0.047 and 0.046 over constant composition distribution matching (CCDM), respectively. The performance of 4D-DM and degenerate 4D-DM is also evaluated by experiments. Compared to CCDM, degenerate 4D-DM achieves a NGMI gain of 0.055. [ABSTRACT FROM AUTHOR]

With the increasing wireless data and massive interconnections in B5G/6G, demands for an intelligent, flexible, and cost-effective fronthaul network have emerged to provide services for a ubiquitous mobile society. This calls for disruptive innovations in both the architecture and transmission technology. In this article, we introduce a point-to-multipoint coherent architecture as a promising solution, and analyze its advantages and challenges compared to other fronthaul solutions. Based on this architecture, we propose a novel joint resource allocation framework that enables the fronthaul to intelligently self-manage and self-optimize in real time. In addition, we propose a low-complexity joint resource allocation algorithm for different scenarios in this framework. Simulations show that the proposed framework and algorithm can effectively utilize the limited resources and improve the performance of the fronthaul. [ABSTRACT FROM AUTHOR]

A nonlinearity-tolerant constant composition distribution matching (CCDM) scheme based on a parallel bisection architecture is proposed. Compared with CCDM based on arithmetic coding (AC), the bisection-based CCDM (BS-CCDM) can achieve the same linear performance and an improved tolerance to fiber nonlinear impairments in unrepeated fiber transmissions. The same linear performance of AC-CCDM and BS-CCDM is evaluated both in simulations and experiments with various modulation formats and shaping rates. The improved fiber nonlinear performance is demonstrated in simulations with probabilistically shaped dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) signals for unrepeated transmission over 250 km of standard single-mode fiber (SSMF). BS-CCDM provides a ∼0.22 dB gain in effective signal-to-noise ratio (SNR) on average over AC-CCDM for a variety of spectral efficiencies with a block length of 64. And a gain in achievable information rate (AIR) of ∼0.11 bits/4D-symbol is obtained correspondingly. In addition, by further optimizing the look-up tables (LUTs) in BS-CCDM, a better trade-off between the linear performance and nonlinearity tolerance is achieved. Compared with AC-CCDM, the optimized BS-CCDM provides the gain of 0.48 dB and 0.7 dB in effective SNR for the block length of 64 and 128, respectively. [ABSTRACT FROM AUTHOR]

Efficient synchronization is of great importance for fiber optical coherent communication system. Here, we propose an accurate and low-complexity timing/frequency synchronization scheme based on sparse fast Fourier transform (S-FFT). The proposed scheme consists of coarse timing/frequency synchronization, fine timing synchronization, and fine frequency synchronization. Inspired by the idea of S-FFT, we take full advantages of the sparse nature of training symbols (TSs) and realize the synchronization in the frequency domain. The proposed scheme enables accurate timing synchronization at low signal-to-noise ratio (SNR) region. A complete frequency offset estimation (FOE) range of [−symbol rate/2, + symbol rate/2] can be realized with high resolution. Finally, the proposed scheme is experimentally verified through 10 Gbaud dual-polarization (DP) 16/32-quadrature amplitude modulation (QAM) transmission, without the BER performance penalty. In particular, the total computation complexity is reduced by nearly 200 times in comparison with that using conventional sliding window correlation scheme. [ABSTRACT FROM AUTHOR]

We design a new kind of fractional Fourier transform superimposed training sequence (TS) to simultaneously estimate linear distortions including chromatic dispersion (CD), timing offset (TO), and frequency offset (FO), etc. Without sacrificing additional spectrum efficiency, the scheme can reduce the computational complexity sharply compared to a regular estimation method. The feasibility of the proposed scheme is verified by simulations of 28 GBaud dual polarization quadrature phase-shift keying (DP-QSPK) and 16-ary quadrature amplitude modulation (DP-16 QAM) system. Simulation results show that the superimposed TS has negligible negative effects on the transmission performance considering 7% forward error correction while the TO, FO, and rough CD estimation perform well after a long distance transmission. The max TO estimation (TOE) errors are, respectively, eight samples and 12 samples for QPSK and 16 QAM, and the max FO estimation errors are, respectively, 5.7 and 3.6 MHz for QPSK and 16 QAM. The rough CD estimation (CDE) error is below 946 ps/nm. The TOE and rough CDE separately decrease the complexity of frame synchronization and CDE. In total, the computation complexity of the estimations decreases by 91.8% compared to a regular blind estimation method. [ABSTRACT FROM AUTHOR]

Low-intensity and low-frequency ultrasound (LILFU) can enhance the bactericidal action of antibiotics against various sensitive bacterial species. The current study investigated the effects of LILFU combined with tobramycin on extended-spectrum beta-lactamases (ESBLs) Escherichia coli biofilms (a multi-drug resistant bacteria). The biofilms of ESBLs E. coli were established and treated with ultrasound (42 kHz and ISATA of 0.66 W/cm2) continuously for 0.5 h with and without tobramycin. The bacterial viability, the morphology and the antibiotic penetration of ESBLs E. Coli biofilms were investigated. The results demonstrated that the bacterial viability of biofilms significantly declined and the diameter of the inhibition zone was significantly increased after treatment with ultrasound combined with tobramycin compared with the controls (P  < 0.05). Confocal laser scanning microscopy showed that the bacterial viability was affected most in the outer layer of ESBLs E. coli biofilms after joint treatment. The morphological structure of the biofilms was altered remarkably after joint treatment based on scanning electron microscopy, especially in regard to reduced thickness and loosened structure. These results suggest that the combination of ultrasound and tobramycin can exert synergistic bactericidal effects against biofilms formed by ESBLs E. coli. [ABSTRACT FROM AUTHOR]

Enabling communication networks with sensing functionality has attracted significant interest lately. The digital subcarrier multiplexing (DSCM) technology is widely promoted in short-reach scenarios for its inherent flexibility of fine-tuning the spectrum. Its compatibility with large-scale as-deployed coherent architectures makes it particularly suited for cost-sensitive integrated sensing and communication applications. In this paper, we propose a scheme of spectrally integrating the digital linear frequency modulated sensing signal into DSCM signals to achieve simultaneous sensing and communication through shared transmitter. Consequently, this cost-effective scheme has been demonstrated to achieve 100-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 200-Gb/s dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) transmission with a distributed acoustic sensing sensitivity of 69 p ε /Hz and 88 p ε /Hz respectively, at a spatial resolution of 4 m.

A low-complexity scheme is proposed to realize irregular uniform quadrature amplitude modulation (QAM) formats with Gray mapping, which are named amplitude-division irregular QAM (AD-Ir-QAM) formats. Compared to conventional probabilistic shaping (PS) with the Maxwell-Boltzmann (MB) distribution (PS-MB), irregular QAM formats show a smaller peak-to-average power (PAPR) and achieve a better performance in systems with the peak-power constraint. Compared with irregular QAM formats realized by PS (PS-Ir-QAM), AD-Ir-QAM formats realize a more flexible rate adaptation and have a lower implementation complexity. Experimental results obtained in an unamplified coherent optical system show that, at a generalized mutual information (GMI) of 4.5 bits/2D-symbol, AD-Ir-100QAM achieves gains of 2.1 and 0.5 dB in the power budget over PS-MB-100QAM and PS-Ir-100QAM, respectively.

Digital pre-distortion (DPD) is a powerful technique to mitigate transmitter nonlinear distortion in optical transmissions. In this Letter, the identification of DPD coefficients based on the direct learning architecture (DLA) using the Gauss-Newton (GN) method is applied in optical communications for the first time. To the best of our knowledge, this is the first time that the DLA has been realized without training an auxiliary neural network to mitigate optical transmitter nonlinear distortion. We describe the principle of the DLA using the GN method and compare the DLA with the indirect learning architecture (ILA) that uses the least-square (LS) method. Extensive numerical and experimental results indicate that the GN-based DLA is superior to the LS-based ILA, especially in a low signal-to-noise ratio scenario.

We propose and experimentally demonstrate a coherent digital-analog radio-over-fiber (DA-RoF) system and achieve the transmission of Tb/s common public radio interface (CPRI)-equivalent data rate for fronthaul. The proposed coherent DA-RoF system includes DA-RoF modulation, demodulation and DA-RoF compatible coherent digital signal processing (DSP) blocks. A theoretical analysis of the DA-RoF scheme together with parameter optimization is accomplished as well. In the experiment, a 25 Gbaud DA-RoF signal with 1 Tb/s CPRI-equivalent data rate is transmitted in the system, satisfying the error vector magnitude (EVM) requirement for 256-quadrature amplitude modulation (QAM) signal transmission. With the symbol rate reduced to 10 Gbaud, an EVM below 2.5% is achieved, which meets the requirement for 1024-QAM transmission. The experimental results show that the coherent DA-RoF system is a promising solution for future fronthaul.

A subcarrier-pairing entropy loading (SubP-EL) scheme with fairly low complexity is proposed for digital subcarrier-multiplexing (SCM) systems with colored signal-to-noise ratio (SNR) distributions. With the constraint of the target entropy, SubP-EL iteratively optimizes the entropy of subcarriers in pair. After convergence, SubP-EL can approach the optimal performance which is evaluated in simulations and experiments by comparison with the brute-force search method. Meanwhile the complexity of SubP-EL is significantly reduced compared with the brute-force search. In particular, in an 8-subcarrier system with five different SNRs, the complexity of SubP-EL is reduced by approximately a factor of 764, 95 and 13 with the entropy granularities of 0.05 bits/2D-symbol, 0.1 bits/2D-symbol and 0.2 bits/2D-symbol, respectively. The performance of SubP-EL is evaluated in simulations and experiments. In the experiments with 345 km fiber transmissions, the average NGMI gain of SubP-EL over the system without entropy loading is 0.0286 for different optical filtering cases.

In recent years, in order to increase the capacity and scalability of intra-datacenter (DC) transmission, the optical frequency comb (OFC) source has been considered promising to replace discrete lasers, aiming to reduce the cost of wavelength division multiplexing (WDM) transmission within DC. In this paper, an OFC based coherent architecture is proposed. An OFC, in the receiver side, is split by a splitter with a uniform power ratio and separately used as local oscillators (LOs) to detect the demultiplexed signals. The signal spectrum is copied onto every tone of the LO-OFC, and a large frequency offset (FO) tolerance is achieved. In addition, the required ADC sampling rate is the same as a system without FO. Extensive simulations are conducted. In the simulated coherent WDM transmission system, a 3-tone-OFC is used to provide 3 carriers, and an 11-tone-OFC is split and used to provide LO-OFCs. For a 64GBd polarization multiplexing 16 quadrature amplitude modulation (PM-16QAM) WDM transmission, the tolerances of FO are up to about ±0.3THz and ±0.374THz for the 1st/3rd signal, and the 2nd signal, respectively, below the pre-forward error correction (FEC) bit error rate (BER) level of 1.25×10 -2 . Moreover, the maximum tolerance of FO linearly increases with the number of effective tones in LO-OFC. Further, extensive experiments with back-to-back connection are conducted to verify the performance. The tolerance of FO is up to >36 GHz for 36GBd PM-16QAM transmission with a 3-tone-LO-OFC below the BER level of 1.25×10 -2 . The proposed OFC based coherent architecture is a promising solution for intra-DC interconnections with a large FO.

Fractional Fourier transform (FrFT) training sequences (TSs) have been proposed to superimpose with signals to achieve time synchronization and polarization mode dispersion (PMD) estimation simultaneously before equalization. The feasibility of the proposed scheme is verified by experiment of a 30 GBaud/s PDM quadrature phase shift keying (QPSK) system. Without sacrificing spectrum efficiency, the proposed PMD estimation is robust against amplifier spontaneous emission (ASE) noise and has steady performance in a wide PMD range from 0 ps to 108 ps. The root mean square error (RMSE) of PMD is 5.6 ps by experimental verification. Further, the PMD estimated before equalization is utilized to adjust the numbers of taps in a subsequently tap-varied equalizer, which achieves feedforward adaptive equalization (FFD-AEQ). The number of the AEQ taps is adjusted to an optimal value with the prior-PMD information estimated by FrFT TSs. The experimental results show that the tap-varied AEQ has 52.6% power consumption decrease compared to the tap-fixed AEQ with 13 taps.

We propose an algorithm to track the rotation of state of polarization (RSOP) for short-distance coherent subcarrier-multiplexing systems. 3 pilot tones are used to estimate RSOP matrices on a block-by-block basis and recover phase noise as well. An ultra-fast RSOP tracking ability using the proposed algorithm is demonstrated by experiment. Specifically, the bit error rate increases from 2.3×10 -3 to 5.6×10 -3 when the RSOP speed increases from 0 rad/s to 50 Mrad/s. We also demonstrate the robustness of the algorithm against polarization mode dispersion and polarization dependent loss.

We propose a blind and fast modulation format identification (MFI) enabled by the digital frequency-offset (FO) loading technique for hitless coherent transceiver. Since modulation format information is encoded to the FO distribution during digital signal processing (DSP) at the transmitter side (Tx), we can use the fast Fourier transformation based FO estimation (FFT-FOE) method to obtain the FO distribution of individual data block after constant modulus algorithm (CMA) pre-equalization at the receiver side, in order to realize non-data-aided (NDA) and fast MFI. The obtained FO can be also used for subsequent FO compensation (FOC), without additional complexity. We numerically investigate and experimentally verify the proposed MFI with high accuracy and fast format switching among 28 Gbaud dual-polarization (DP)-4/8/16/64QAM, time domain hybrid-4/16QAM, and set partitioning (SP)-128QAM. In particular, the proposed MFI brings no performance degradation, in term of tolerance of amplified spontaneous emission (ASE) noise, laser linewidth, and fiber nonlinearity. Finally, a hitless coherent transceiver enabled by the proposed MFI with switching-block of only 2048 symbols is demonstrated over 1500 km standard single mode fiber (SSMF) transmission.