Nonlinearity mitigation in a fiber-wireless integrated system based on low-complexity autoencoder and BiLSTM-ANN equalizer.

We propose and experimentally demonstrate an intelligent nonlinear compensation method using a stacked autoencoder (SAE) model in conjunction with principal component analysis (PCA) technology and a bidirectional long-short-term memory coupled with ANN (BiLSTM-ANN) nonlinear equalizer for an end-to-end (E2E) fiber-wireless integrated system. The SAE-optimized nonlinear constellation is utilized to mitigate nonlinearity during the optical and electrical conversion process. Our proposed BiLSTM-ANN equalizer is primarily based on time memory and information extraction characteristics, which can compensate for the remaining nonlinear redundancy. A low-complexity 50 Gbps E2E-optimized nonlinear 32 QAM signal is successfully transmitted over a span of 20 km standard single-mode fiber (SSMF) and 6 m wireless link at 92.5 GHz. The extended experimental results indicate that the proposed E2E system can achieve a reduction of up to 78% in BER and a gain in receiver sensitivity of over 0.7 dB at BER of 3.8 × 10-3. Moreover, computational complexity is reduced by more than 10 times compared to the classical training model.

Spectrally efficient multi-service fiber-wireless access in low-cost direct-detection THz system at 300†GHz.

This Letter demonstrates a novel, to the best of knowledge, overlapping single-sideband (OSSB) transmission scheme for spectrally efficient multi-service fiber-wireless (FiWi) access in a low-cost direct-detection (DD) THz system. Utilizing the proposed OSSB scheme, user data from different services can share the same spectrum resource yet can be successfully demodulated via one cost-effective DD THz receiver in conjunction with the Kramers-Kronig (KK) based SSB field reconstruction and look-up table (LUT) enabled signal separation algorithms. A proof-of-principle experiment is conducted. Based on an IQ modulator and a single THz zero-bias diode (ZBD), two independent 10-GBd quadrature phase shift keying (QPSK) signals with an overlapped spectrum are successfully demodulated after 20-km fiber and up to 3-m wireless transmission at the 300-GHz band. To the best of our knowledge, this is the first demonstration of multi-service FiWi access with an OSSB format in a 300-GHz DD THz system.

Adaptive feedback-driven probabilistic shaping for high-order modulation formats in a fiber-THz seamless integration system at 320 GHz.

In this Letter, we propose a novel, to the best of our knowledge, adaptive feedback-driven probabilistic constellation-shaping (FBD-PCS) method based on the robustness evaluation criteria and employ variational autoencoder (VAE)-based equalizers to implement polarization demultiplexing and nonlinear equalization for the recovery of high-order PCS-QAM signals. We experimentally demonstrate the fiber-THz 2 times 2 MIMO system with a net rate of 366.4 Gbit/s using dual-polarization 40 Gbaud PCS-64QAM signal over a 20 km SSMF and 6 m wireless link. Specifically, the feedback mechanism drives the fiber-THz system to solve optimization problems, adaptively matching the optimized distribution of transmitted symbols that maximizes normalized generalized mutual information (NGMI). We also examine six scenarios to explore nonlinear resistances of FBD-PCS symbols and the robustness of VAE-based equalizers. The results demonstrate the superiority of FBD-PCS over the Maxwell-Boltzmann (M-B) distributions in practical nonlinear-dominant systems. Additionally, the FBD-PCS signals can break limitations for ultrahigh rate transmission with the help of advanced equalizers.

Real-time 100-GbE fiber-wireless seamless integration system using an electromagnetic dual-polarized single-input single-output wireless link at the W band.

This Letter demonstrates a real-time 100-GbE fiber-wireless seamless integration system operating at the whole W band (75-110 GHz). Based on a pair of commercial digital coherent optical modules, the real-time transparent transmission of 125-Gb/s dual-polarized quadrature phase-shift keying signal has been successfully achieved over two-spans of 20-km fiber and up to 150-m electromagnetic dual-polarized single-input single-output wireless link. To the best of our knowledge, this is the first real-time demonstration of 100-GbE signal transmission over >100-m wireless distance at the millimeter-wave band based on photonics. We believed this real-time and high-speed fiber-wireless seamless integration system with a wireless coverage up to hundreds of meters can significantly accelerate the progress of upcoming 6G.

Photonics-aided integrated sensing and communications in mmW bands based on a DC-offset QPSK-encoded LFMCW.

The evolution of mobile communications towards millimeter-wave (mmW) bands provides a strong opportunity for the seamless integration of radar and wireless communications. We present a photonics-aided mmW integrated sensing and communications (ISAC) system constructed by photonic up-conversion using a coherent optical frequency comb, which facilitates zero frequency offset of the resulting mmW signal. The sensing and communications functions are enabled by a joint waveform that encodes a DC-offset QPSK signal on a linear frequency-modulated continuous wave (LFMCW) in baseband. The QPSK encoding ensures the constant envelope of the mmW ISAC signal for long-distance radar detection. The optimized DC offset preserves the distinctive chirp phase and good cross-correlation of the original LFMCW, which can achieve high-resolution sensing by radar de-chirping and assist in communication sequence synchronization by pulse compression, respectively. Experimental results show that the single-user detection with less than 20-mm sensing error and dual-user detection with a 10.4-cm ranging resolution are realized at 28-GHz band, respectively. The wireless communication with a 11.5-Gbit/s transmission rate also at 28-GHz band is successfully tested. Moreover, the proof-of-concept experiments demonstrate the good frequency tunability and wavelength tolerance of the proposed ISAC system.

Real-time demonstration of 103.125-Gbps fiber-THz-fiber 2 × 2 MIMO transparent transmission at 360-430 GHz based on photonics.

In this Letter, we experimentally demonstrate the first real-time transparent fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) transmission system with a record line rate of 125.516 Gbps at 360-430 GHz based on photonic remote heterodyning, hybrid optoelectronic down-conversion, and commercial digital coherent modules. The 103.125-Gbps net data rate using dual-polarization quadrature phase-shift keying (DP-QPSK) modulation is successfully transmitted over two spans of 20-km standard single-mode fiber (SSMF) and 60-cm wireless distance under 15% soft-decision forward error correction (SD-FEC) for a pre-FEC bit error ratio (BER) threshold of 1.56 × 10-2 (post-FEC BER < 10-15). The optical signal to noise ratio (OSNR) margin and the stability of the transmission system are extensively investigated. To the best of our knowledge, this is the first time to realize >100-Gbps real-time transparent fiber-THz-fiber link transmission at beyond the 350-GHz band, making it a promising scheme to pave the way towards a practical seamless integration of a fiber-THz-fiber link to the future 6G mobile communication system.

All-optical generation of binary phase-coded microwave pulses without baseband components based on a dual-parallel Mach-Zehnder modulator.

In this paper, we propose an all-optical system for the generation of binary phase-coded microwave pulses without baseband components. The scheme is based on a dual-parallel Mach-Zehnder modulator (DPMZM). By properly applying the coding signals and the microwave signals to the precisely biased DPMZM, accurate π phase shift binary phase-coded microwave pulses without baseband components can be generated. The proposed system has an extremely simple and stable all-optical structure, leading to a large frequency tuning range and a high signal quality. The operation of the system is very easy. The generation of the 2-Gbit/s 14-GHz and 4-Gbit/s 16-GHz binary phase-coded microwave pulses under different coding signal amplitudes and microwave carrier powers are experimental verified. The results show that the proposed binary phase-coded microwave pulses generation system has high quality and performance.

Photonic generation of background-free binary and quaternary phase-coded microwave pulses based on vector sum.

A photonic microwave phase-coded pulse generator is proposed and experimentally demonstrated based on the principle of vector sum. The key component of the proposed pulse generator is an integrated polarization-division multiplexing Mach-Zehnder modulator (PDM-MZM) and a 90° hybrid coupler. By properly setting the data sequences applied to the specially biased PDM-MZM, binary and quaternary phase-coded microwave pulses (PCMPs) that are free from the background signals can be generated. Since no filters and polarization adjustment are involved, the proposed pulse generator is characterized by a simple structure, low-loss, flexible frequency tunability and high long-term stability. The experimental results show that background-free 4 Gb/s Barker and Frank PCMPs at 18 GHz and 2 Gb/s Barker and Frank PCMPs at 24 GHz are successfully generated. The calculated pulse compression ratio and peak-to-side lobe ratio are in good agreement with the theoretical values.

Chromatic dispersion immune microwave photonic phase shifter based on double-sideband modulation.

A chromatic dispersion (CD) immune microwave photonic phase shifter (MPPS) based on double-sideband (DSB) modulation is proposed and demonstrated. An optical spectrum processor introduces the phase shift to the MPPS. The DSB signals along two orthogonal polarizations are demodulated to two RF signals with both quadrature amplitude and phase items, transferring the CD-induced power fading to the phase item of the synthetic RF signals. Experimental results show that the RF signals over 14-25 GHz obtain random phase shift in 360° range without a power fading point (PFP) after passing through a dispersion compensation fiber with CD of -331 ps/nm. The phase variation and power variation of the phase-shifted signal are <±5.7° and <±0.9 dB, respectively, at the original PFP at 16 GHz.

Broadband chromatic-dispersion-induced power-fading compensation for radio-over-fiber links based on Hilbert transform.

A Hilbert-transform-based broadband chromatic dispersion (CD) compensation scheme for radio-over-fiber links is proposed and experimentally demonstrated. By constructing a Hilbert transform path, CD-induced phase shifts, which initially lead to periodic power fading of the output RF signals, are transferred to the phases of the RF signals. As a result, the powers of the output RF signals are free from the effect of CD in a broadband frequency range. Experimental results show that a flat normalized amplitude-frequency response is actualized within 2-24 GHz, with only 3.02 dB/4.27 dB power fluctuation after transmission over an equivalent of a 38.6 km/43.6 km single-mode fiber. Besides, compared with a conventional dispersive path, the proposed CD compensation scheme significantly improves the third-order spurious-free dynamic range by 23.60 dB.

Equivalent photonic switch for microwave frequency shift keying signal generation.

A photonic microwave frequency shift keying (FSK) signal generator is proposed and experimentally demonstrated based on an equivalent photonic switch (EPS). The EPS is constructed using a polarization-multiplexing dual-drive Mach-Zehnder modulator (PM-DMZM). By properly controlling the data sequences and RF signals applied to the PM-DMZM, microwave FSK signals with flexible frequency intervals can be obtained. The proposed FSK signal generator features the advantages of a simple structure, low loss, good stability, and great frequency tunability. In addition, the proposed setup can also be easily reconfigured to generate microwave amplitude shift keying and phase shift keying signals. The experimental results show that 2 Gb/s at 5/14 GHz and 1 Gb/s at 6/20 GHz microwave FSK signals are successfully generated, after transmission over 5 km single-mode fiber. The required received optical power at 7% forward error correction threshold is only -14.48 dBm.

Optical network solution to the synchronization of distributed coherent aperture radar.

Distributed coherent aperture radar (DCAR) is an important direction for next-generation radar due to its high sensitivity. The challenge to realize DCAR is the synchronization among geographically distributed radar units. We propose an optical network for DCAR synchronization. The proposed network achieves functions of phase-coded pulse generation, time synchronization, and phase synchronization with the help of microwave photonics techniques. Proof-of-concept experiments are conducted in fiber and space transmission scenarios. The combined radar beams have negligible energy loss when synchronization is achieved.

2D optically controlled radio frequency orbital angular momentum beam steering system based on a dual-parallel Mach-Zehnder modulator.

An optically controlled system for generating and continuously steering radio frequency (RF) signals with double orbital angular momentum (OAM) modes is proposed and experimentally demonstrated. The optical carrier's utilization efficiency can be doubled through the distinct electro-optical modulation, which is based on two single-sideband modulation operations on a single optical carrier through a customized dual-parallel Mach-Zehnder modulator. A constructive antenna phase feeding method of a circular antenna array for collectively forming and steering an OAM radio beam is proposed and illustrated. A proof-of-concept experiment is conducted to generate and steer a dual-mode RF-OAM beam to two different two-dimensional (2D) directions, independently and simultaneously. One 17 GHz OAM beam with mode L=1 is continuously steered to 2D directions (:, 0°, 0°), (:, 0°, 1.70°), (:, 0°, 3.87°), (:, 0°, 6.17°), and(:, 0°, 7.80°), with vortex properties, where ":" means "any value of." Meanwhile, the 19 GHz OAM beam with mode L=-1 carried is steered from (:, 0°, 0°) to (:, 0°, -6.72°), and the constellations are obtained successfully.

Anti-dispersion phase-tunable microwave mixer based on a dual-drive dual-parallel Mach-Zehnder modulator.

An optically-controlled phase-tunable microwave mixer based on a dual-drive dual-parallel Mach-Zehnder modulator (DDDP-MZM) is proposed, which supports wideband phase shift and immunity to power fading caused by chromatic dispersion. By using carrier-suppressed single side-band (CS-SSB) modulation for the local oscillator (LO) signal and carrier-suppressed double side-band (CS-DSB) modulation for the input signal, no vector superposition for the same output microwave frequency occurs, making the system immune from power fading caused by chromatic dispersion. Phase tuning is achieved by shifting the phase of the LO signal, and direct electrical tuning of the wideband microwave input signal is avoided, thus supporting large working bandwidth. A phase-shifted down-conversion experiment is carried out, where a phase shift with 0 ~390° and down-conversion are achieved with a phase variation of less than 5° and power variation less than 3.5 dBm when the input signal sweeps between 12 ~16 GHz. The mixer is simple and power-efficient since it uses a single compact modulator, and does not require any optical filters. No power notches are observed in the output microwave spectrum, proving that the dispersion-related frequency-selective fading is mitigated.

Demonstration of 144-Gbps Photonics-Assisted THz Wireless Transmission at 500 GHz Enabled by Joint DBN Equalizer.

The THz wireless transmission system based on photonics has been a promising candidate for further 6G communication, which can provide hundreds of Gbps or even Tbps data capacity. In this paper, 144-Gbps dual polarization quadrature-phase-shift-keying (DP-QPSK) signal generation and transmission over a 20-km SSMF and 3-m wireless 2 × 2 multiple-input multiple-output (MIMO) link at 500 GHz have been demonstrated. To further compensate for the linear and nonlinear distortions during the fiber-wireless transmission, a novel joint Deep Belief Network (J-DBN) equalizer is proposed. Our proposed J-DBN-based schemes are mainly optimized based upon the constant modulus algorithm (CMA) and direct-detection least mean square (DD-LMS) equalization. The results indicate that the J-DBN equalizer has better bit error rate (BER) performance in receiver sensitivity. In addition, the computational complexity of the J-DBN-based equalizer can be approximately 46% lower than that of conventional equalizers with similar performance. To our knowledge, this is the first time that a novel joint DBN equalizer has been proposed based on classical algorithms. It is a promising scheme to meet the demands of future fiber-wireless integration communication for low power consumption, low cost, and high capacity.